Dr. Mohamed Elalemmelalem.com/docs/ECE568_Notes.pdf · Advanced Communication Systems by Dr. Mohamed Elalem Lectures on Advanced Communication Systems Electrical and Computer Engineering

Advanced Communication Systemsby

Dr. Mohamed Elalem

Lectures on Advanced Communication Systems

Electrical and Computer Engineering

Elmergib - University, Libya, 2018

Contents

1 Introduction Review 5

1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

1.2 Definition of Communication Requirements . . . . . . . . . . . . . . . . . . . 7

1.3 Examples of Communication Systems . . . . . . . . . . . . . . . . . . . . . . 8

1.4 Analog Communication Systems . . . . . . . . . . . . . . . . . . . . . . . . . 8

1.5 Review of Digital Communication Systems . . . . . . . . . . . . . . . . . . . 10

1.6 Bandwidth and Probability of Error of the Main Digital Modulations . . . . 12

2 Multiplexing 16

2.1 Frequency Division Multiplexing (FDM) . . . . . . . . . . . . . . . . . . . . 16

2.1.1 Group (G) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.1.2 Supergroup (SG) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.1.3 Mastergroup (MG) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

2.1.4 Super Mastergroup (SMG) . . . . . . . . . . . . . . . . . . . . . . . . 19

2.1.5 FDM Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.2 Time Division Multiplexing (TDM) . . . . . . . . . . . . . . . . . . . . . . . 23

2.2.1 ITU-T Modulation Plan . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.2.2 Digital Hierarchy in USA and Japan . . . . . . . . . . . . . . . . . . 29

1

2.2.3 CCITT Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

2.3 Space Division Multiplexing (SDM) . . . . . . . . . . . . . . . . . . . . . . . 31

2.4 Code Division Multiplexing (CDM) . . . . . . . . . . . . . . . . . . . . . . . 31

3 High Frequency System (HF) 32

3.1 Sky Wave Propagation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

3.1.1 Reflective Index in Ionized Region . . . . . . . . . . . . . . . . . . . . 36

3.1.2 National Phenomena Affecting Ionospheric Propagation . . . . . . . . 38

3.2 Radio Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.2.1 Emission Types: Formats . . . . . . . . . . . . . . . . . . . . . . . . 40

3.2.2 Examples of HF Systems . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.2.3 Single Sideband Transmitter . . . . . . . . . . . . . . . . . . . . . . . 41

3.2.4 Single Sideband Receiver . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.2.5 HF-SSB Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

3.3 HF Transmitter Design Considerations . . . . . . . . . . . . . . . . . . . . . 44

3.3.1 Frequency Modulation Transmitter . . . . . . . . . . . . . . . . . . . 44

3.3.2 Intermodulation Distortion(IM) . . . . . . . . . . . . . . . . . . . . . 46

3.4 Radio Link System (Line of Sight System (LOS)) . . . . . . . . . . . . . . . 48

3.4.1 Characteristics of LOS . . . . . . . . . . . . . . . . . . . . . . . . . . 48

3.4.2 Engineering of Radio Links . . . . . . . . . . . . . . . . . . . . . . . 49

3.4.3 Atmospheric Effects on free space Transmission . . . . . . . . . . . . 51

3.4.4 Terrain Effects on LOS Propagation . . . . . . . . . . . . . . . . . . . 54

3.4.5 Path Profile . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

3.4.6 Link Reliability and Performance . . . . . . . . . . . . . . . . . . . . 59

3.4.7 FM Radio System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Fall 2018-2019, 2 M.Elalem

3.4.7.1 Important parameters of FM . . . . . . . . . . . . . . . . . 61

3.5 Repeaters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

3.5.0.1 Sections of Repeater . . . . . . . . . . . . . . . . . . . . . . 64

3.5.1 Loading of Radio Relay System . . . . . . . . . . . . . . . . . . . . . 66

3.6 Digital Microwave System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

3.6.1 Performance of Digital Microwave System . . . . . . . . . . . . . . . 68

3.6.1.1 Radio System performance Density guide lines . . . . . . . . 68

3.6.1.2 Link Calculation Procedures . . . . . . . . . . . . . . . . . . 70

4 Satellite Communications 75

4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

4.1.1 Terminologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

4.1.2 Categories of orbits . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

4.1.3 Satellite Axial Motions . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4.1.4 Advantages of GEO . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

4.2 Spectrum Allocations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

4.2.1 VHF and UHF Frequency Ranges . . . . . . . . . . . . . . . . . . . . 80

4.2.2 Microwave Bands: L and S . . . . . . . . . . . . . . . . . . . . . . . . 81

4.2.3 Microwave Bands: C, X, and Ku . . . . . . . . . . . . . . . . . . . . 81

4.2.4 Millimeter Wave and Higher: Ka−, Q−, and V−Bands . . . . . . . . 82

4.2.5 Station Segments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

4.2.6 Satellite Subsystem . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.2.6.1 Nile Sat 102 . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

4.2.7 Input and Output Back Off . . . . . . . . . . . . . . . . . . . . . . . 86

4.2.8 Transponder Gain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87


4.2.9 Azimuth Angle, Elevation Angle and Slant Range . . . . . . . . . . . 87

4.3 The Earth Station . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

4.3.1 Block Diagram of the Earth Station . . . . . . . . . . . . . . . . . . . 91

4.3.2 Azimuth and Elevation Angles for Some Libyan Cities . . . . . . . . 94

4.3.3 Very Small Aperture Terminal (VSAT) . . . . . . . . . . . . . . . . . 94

4.3.4 Access Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95

4.4 Capacity in Digital Satellite . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

5 Transmission Media 99

5.1 Twisted Pair . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

5.2 Coaxial Cable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

5.2.1 Coaxial Repeater Design . . . . . . . . . . . . . . . . . . . . . . . . . 108

5.2.2 The Choice Between Coaxial and Microwave Link . . . . . . . . . . . 110

5.2.3 Coaxial Versus Waveguide . . . . . . . . . . . . . . . . . . . . . . . . 111

5.3 Optical Fiber . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111


Lecture 1

Introduction Review

1.1 Introduction

Today, communication enters our daily lives in so many different ways that it is very easy to

overlook the multitude of its facets. The telephones at our hands, the radios and televisions

in our living rooms, the computer terminals with access to the Internet in our offices and

homes, and our newspapers are all capable of providing rapid communications from every

corner of the globe. Communication provides the senses for ships on the high seas, aircraft in

flight, and rockets and satellites in space. Communication through a wireless telephone keeps

a car driver in touch with the office or home miles away. Communication keeps a weather

forecaster informed of conditions measured by a multitude of sensors. Indeed, the list of

applications involving the use of communication in one way or another is almost endless.

In the most fundamental sense, communication involves the transmission of information

from one point to another through a succession of processes, as described here:

1- The generation of a message signal: voice, music, picture, or computer data.

2- The description of that message signal with a certain measure of precision, by a set of

5

symbols: electrical, aural, or visual.

3- The encoding of these symbols in a form that is suitable for transmission over a physical

medium of interest.

4- The transmission of the encoded symbols to the desired destination.

5- The decoding and reproduction of the original symbols.

6- The re-creation of the original message signal, with a definable degradation in quality;

the degradation is caused by imperfections in the system.

There are three basic elements to every communication system, namely, transmitter, chan-

nel, and receiver, as depicted in Figure 1.1.

Figure 1.1: General communication system.

The transmitter is located at one point in space, the receiver is located at some

other point separate from the transmitter, and the channel is the physical medium that

connects them. The purpose of the transmitter is to convert the message signal produced


by the source of information into a form suitable for transmission over the channel. How-

ever, as the transmitted signal propagates along the channel, it is distorted due to channel

imperfections. Moreover, noise and interfering signals (originating from other sources) are

added to the channel output, with the result that the received signal is a corrupted version

of the transmitted signal. The receiver has the task of operating on the received signal so as

to reconstruct a recognizable form of the original message signal for a user.

1.2 Definition of Communication Requirements

To design any communication link, the following requirements have to be considered

1- Communication objectives

2- Trafficable requirements

3- Criteria od acceptability

4- Operating requirements

5- Initial survey

6- Facility survey

7- Communication techniques

8- Service quality and reliability vs cost

9- Serviceability and personal requirements

10- Growth and future expansion


1.3 Examples of Communication Systems

Telephone land and mobile

Telex

Fax

Data terminal

Video Telephone

Cable TV

Telemetry

TV

Radio

1.4 Analog Communication Systems

m(t) = message signal of bandwidth BW = ω

Amplitude Modulation (AM): The transmission BW = BT = 2ω,

SNR =2〈m2(t)〉

1 + 〈m2(t)〉 ·SR

N

N = 2ηω; noise at the IF output,〈m2(t)〉 is the signal power, and SR is the received power.

η = KT ; T : Temperature in Kelvin, and K is Boltzmann’s constant,

K = 1.38× 10−23 J/K


For sinusoidal message (single tone),

SNR =1

3· SR

ηω⇒ SR = 3ηω · SNR

SR|thN

= 10; threshold value. OrSR

ηω= 20

N0 = KTBT ; BT transmission bandwidth.

Single side band (SSB): The transmission BW = BT = ω,

SNR =SR

ηω⇒ SR = ηω · SNR

Angle Modulation

v(t) = Ac cos(ωct + k

∫

m(t)dt), Frequancy Modulation(FM)

v(t) = Ac cos(ωct+ km(t)), Phase Modulation(PM)

BT = 2(β + 1)ω = 2βω + 2ω = 2∆f + 2ω, [Carlson Rule]

where ∆f is known as frequency deviation. There are two versions of FM signal:

Wide Band Frequency Modulation (WBFM) (Large β), BT = 2∆f .

Narrow Band Frequency Modulation (NBFM) (small β), BT = 2ω.

More general FM bandwidth

BT = 2(β + α)ω

The Signal to Noise ratio measured at the receiver side is

SNR = 3β2〈m2(t)〉 · SR

ηω

SR = ηωSNR · 1

3β2〈m2(t)〉SR|thN

= 10; at threshold,SR|thηω

= 10BT

ω


Example:

Design an FM system with no bandwidth constraints, given SNR = 50dB and 〈m2(t)〉 = 12,

η = 10−8 W/Hz, and ω = 10KHz.

Solution:

SR|thηω

=10BT

ω⇒ SR|th =

10BT

ω· ω ⇒ SR|th = 10−7BT → (1)

SR = ηω · SNR · 1

3β2〈m2(t)〉 ; SNR = 50dB = 105

SR =ηω · SNR

32β2

=20

3β2→ (2)

SR ≥ SR|th → 20

3β2≥ 10−7BT = 10−7 × 2(β + 1)ω

20

3β2≥ 2× 10−3β → β ≤ 14

BT = 2βω = 280 KHz. For more conservative BT = 300 KHz

SR =20

3× 142= 34.01 mW = 15.3 dBm

1.5 Review of Digital Communication Systems

As a general review, there is a lot of modulation scheme versions.

• Analog Pulse Transmission: the information is just sampled. No encoding is performed.

The samples are not digitalized.

– Pulse Amplitude Modulation (PAM)

– Pulse Width Modulation (PWM)

– Pulse Position Modulation (PPM)


• Digital Pulse Transmission. The information is sampled, quantized and encoded. It is

completely in digital form.

– Pulse Code Modulation (PCM)

– Delta Modulation (DM)

– Adaptive Delta Modulation (ADM)

– Differential Pulse Code Modulation (DPCM)

Each of above transmission could be

• Transmission as baseband

• Transmission with carrier (Paseband)

When carrier is used, there are other classifications of digital modulation

• Amplitude Shift Keying (ASK)

• Frequency Shift Keying (FSK)

• Phase Shift Keying (PSK)

• Differential Phase Shift Keying (DPSK)

• M-ary

• · · · etc.

The main pulse shapes that are used to represent the symbol could be: ”Return to Zero”

(RZ) format, or ”Not Return to Zero” (NRZ) format. The bandwidth that is needed for RZ

is greater than that of NRZ.


If a roll-off filter with a factor alpha is user, then the transmission bandwidth is

BT =1

Tb(1 + α)

In M-ary case, M = 2k and Ts = Tbk, where k is the number of bits for each sample

BTmin=

1

Ts=

1

2kTb, BT =

1

2kTb(1 + α) =

BTbinary

k

The probability of error in M-ary for PAM is give as

Pe = 2M − 1

MQ(√

6

m2 − 1

Eb

η

)

Eb = SavTs = average energy per bit

Pe = Q(√

Eb

η

)

, bipolar

Pe = Q(√

Eb

2η

)

, unipolar

1.6 Bandwidth and Probability of Error of the Main

Digital Modulations

ASK or OOK

BT = 2rb

Pe = Q(√

Eb

2η

)

, Coherent BASK (rarly used)

Pe =1

2exp−

Eb8η , Non-Coherent BASK (rarly used)

Pe = 2Q(√

2π2Eb log2M

M2η

)

, M-ary PSK

• In ASK probability of error (Pe) is high and SNR is less.


• ASK has lowest noise immunity against noise.

• ASK is a bandwidth efficient system but it has lower power efficiency.

FSK

BT = 2∆f + 2Bbb, Bbb =1

Tb= rb

- NBFM: BT = 2Bbb =2Ts

= 2rb

- WBFM: BT = 2∆f

Pe = Q(√

1.217Eb

η

)

, (BFSK)

Pe =1

2exp−Eb/4η, (NFSK)

• In case of FSK, (Pe) is less and SNR is high.

• This technique is widely employed in modem design and development.

• It has increased immunity to noise but requires larger bandwidth compare to

other modulation types.

PSK & DPSK

BT = 2rb

Pe = Q(√

2Eb

η

)

, PSK (using matched filter)

Pe =2

kQ(

sinπ

M

√

2Eb

η

)

, M-PSK

Pe =1

2e−A2Tb/2η =

1

2exp−Eb/η, (Non-coherent PSK and DPSK)

• In case of PSK probability of error is less. SNR is high.

• It is a power efficient system but it has lower bandwidth efficiency.

• PSK modulation is widely used in wireless transmission.


• The variants of basic PSK and ASK modulations are QAM, 16-QAM, 64-QAM

and so on.

Example 1

Binary data is transmitted over a telephone line with usable bandwidth of 3.4 KHz using

FSK signalling scheme. The transmit frequencies are 2025 Hz and 2225 Hz. The data rate

is 300 b/s, and the average SNR of channel output is 6dB. Calculate the probability of error

Pe for coherent and non-coherent cases.

f1 = 2025, f2 = 2225, , so ∆f = 200 Hz

rb =1

Tb= 300 b/s


S

N=A2/2

ηB=

A2/2

3400η= 4 → A2

η= 27200

A2Tbη

= 90.67

• For coherent FSK

Pe = Q(√

1.217A2Tbη

)

= 5× 10−14

• For non-coherent FSK

Pe =1

2e−A2Tb/8η = 6× 10−6

FSK is more complex (Less Pe), while NFSK is simple (high Pe).

Example 2

Binary data is to transmitted over a microwave channel at rate of 3 Mb/s. assuming the

channel noise is AWGN with η/2 = 10−14 W/Hz. Find the power and bandwidth required

by QPSK to maintain Pe = 10−4.

M = 4 = 2k, then k = 2

Pe =2

kQ(

sinπ

M

√

2Eb

η

)

= Q(√

Eb

η

)

Ts = 2Tb, then Tb =1

3×106= 0.33× 10−6 sec

10−4 = Q(x), → x2 =Eb

η=A2Ts2η

From Q- Function Table x = 3.719

A2/2 = x2η × Rb/2 = 3.7192 × 10−14 × 3× 106/2 = 4.149× 10−7 W = −33.82dBm.

Bt = 2rs =2rbk

= rb = 3 Mb/s.


Lecture 2

Multiplexing

Multiplexing is the set of techniques that allows sending of multiple signals simultaneously

across a single transmission link.

• Analog Mutliplexing

– Frequency Division Multiplexing (FDM)

– Wave Division Multiplexing (WDM)

– Space Division Multiplexing (SDM)

• Digital Multiplexing

– Time Division Multiplexing (TDM)

– Code Division Multiplexing (CDM)

2.1 Frequency Division Multiplexing (FDM)

FDM has three main processing:

• Mixing Process

16

• Filtering Process

• Combining Process

The following example illustrates these processes.

Example 1

Assume that a voice channel occupies a bandwidth of 4 kHz. We need to combine three voice

channels into a link with a bandwidth of 12 kHz, from 20 to 32 kHz. Show the configuration,

using the frequency domain. Assume there are no guard bands. Indicate the proper carriers.

We shift (modulate) each of the three voice channels to a different bandwidth, as shown

in Figure 2.1. We use the 20 to 24 kHz bandwidth for the first channel, the 24 to 28 kHz

bandwidth for the second channel, and the 28 to 32 kHz bandwidth for the third one. Then

we combine them.

Figure 2.1: Illustration of FDM example.


2.1.1 Group (G)

one Group consists of 12 voice channels. One voice channel (0.3− 3.4 KHz≡ 4 KHz). The

bandwidth of Group is (60− 108 KHz). The hierarchy of Group is shown in Figure 2.2 We

Figure 2.2: Standard Group.

can note that Group consists of:

• 12 modulators

• 12 filters

• 12 demodulators

2.1.2 Supergroup (SG)

one Supergroup consists of 60 voice channels. The bandwidth of Supergroup is (60 × 4 =

240 KHz). The frequency band extends from (312 − 552 KHz). Five Groups from One

Supergroup (1SG = 5G). The hierarchy of Supergroup is shown in Figure 2.3.


Figure 2.3: Standard Supergroup.

2.1.3 Mastergroup (MG)

one Mastergroup consists of five Supergroup; (1MG = 5SG = 300v.c.) The bandwidth of

Mastergroup is (300× 4 = 1200 KHz). The frequency band extends from (812− 2044 KHz).

There are guard bands of 8 KHz between multiplexed Supergroups. Note that 2044− 812 =

1232 KHz, and 1232 − 1200 = 32 KHz. The hierarchy of Mastergroup is shown in Figure

2.4.

2.1.4 Super Mastergroup (SMG)

one Super Mastergroup consists of three Supergroup; (1SMG = 3MG = 900v.c.) The

bandwidth of Super Mastergroup is (900×4 = 3600 KHz). The frequency band extends from

(8516−12388 KHz). There are guard bands of 136 KHz between multiplexed Mastergroups.

Note that 12388 − 8516 = 3872 KHz, and 3872 − 3600 = 272 KHz. Figure 2.5 shows


Figure 2.4: Standard Mastergroup.

the bandwidth of Super Mastergroup. As a final recall of the above discussion, Figure 2.6

Figure 2.5: Standard Supper Mastergroup.

displays a review of different FDM standards.

Figure 2.7 illustrates the standard FDM hierarchy.


Figure 2.6: Different FDM Standards.

Figure 2.7: Standard FDM hierarchy.

2.1.5 FDM Specifications

Pilot it is used for

• - Level regulation

• - Frequency synchronization

• - Alarm system

• Maintenance monitoring.


Frequency generation Usually a master oscillator is used. All carrier frequencies are

generated by the same Synthesizer. This can be seen from Figure 2.9.

Figure 2.8: Frequency Synthesizer.

Other FDM Systems: Direct to Line (DTL)

If crystal filter are used which operate at the 8 MHz range from a channel group of 12

channels, 48 KHz bandwidth in the range 8140 to 8188 KHz by selecting carriers in the

range 8140 to 8188 KHz selecting upper sideband this group can then be translated

to the standard range 60 to 108 KHz by a carrier 8248KHz. Such systems are used in

Direct to Line (DTL) and Direct Formed Supergroup (DFSG).

132 voice channels (2SG+ 1G) can be formed using DTL. This eliminates Group and

Supergroup equipment.


Figure 2.9: Direct to Line (DTL).

2.2 Time Division Multiplexing (TDM)

Time Division Multiplexing (TDM) is the means by which multiple digital signals (or ana-

logue signals carrying digital data) can be carrier on a single transmission link by interleaving

portions of each signal in time as shown in Figure 2.10. Interleaving can be done at bits

or blocks of bytes. This enables digitally speach signals to be transmitted and switched

optimally within a circuit-switching networks.

Figure 2.10: TDM for two signals.


2.2.1 ITU-T Modulation Plan

TDM can be divided in two types; Plesiochronous Digital Hierarchy (PDH) and Synchronous

Digital Hierarchy (SDH)

a)- Plesiochronous Digital Hierarchy is a technology used in telecommunication networks

to transmit large quantities of data over digital transport equipment such as fiber

optic and microwave radio systems. As the term ”Plesiochronous” refers in Greek,

PDH networks run in a state where different parts of the network are almost, but nor

quite perfectly synchronized.

The European and American versions of PDH system differ slightly in the details of

their working, but the principles are the same.

(i)- North American standards (D1 System): in which 24 telephone channels are mul-

tiplexed using fs = 8 KHz

1 frame = (7 + 1)× 24 + 1 = 193 bits

Ts =1

fs= 125 µsec

∴ The transmission bit rate =193

125× 10−6= 1.544 Mbps

(ii) - European Standards (EX Systems) It is also known as Conference European

Post Telephone (CEPT). It consists of 32 channels, 30 channels for voice and 2

channels for synchronization and signalling.

Total number of bits per frame = 32× 8 = 256 in 125µsec.

∴ number of bits per second =256

125= 2.048Mbps


Figure 2.11: One frame of Digital Service 1 (DS1). Some times known as Telecommunication-

1 Carrier (T1).

1 frame=125 µ sec, so 1 Time slot=12532

= 3.9 µ sec. Time per bit=3.98

= 488 n sec.

Figure 2.12: One frame of CEPT system.

The whole 2 Mbps may be used for non speech purposes for example, data transmission.


Table 2.1: North America Standard

Voice/Data Channel Bit rate (MBps)

DS1 24 1.544

DS1C 48 3.152

DS2 96 6.312

DS3 672 44.736

DS4E 1920 139.264

DS4 4032 274.176

Table 2.2: European Standards

Voice/Data Channel Bit rate (MBps)

E1 30 2.048

E2 120 8.448

E3 480 34.368

E4 1920 139.264

E5 7680 565.148


In order to move multiple 2 Mbps data streams from one place to another, they are

combined together or ”multiplexed” in groups of four. This is done by taking 1 bit from

stream 1, followed by bit 1 from stream 2, followed by bit 1 from stream 3, followed by

bit 1 from stream 4. Then transmitting multiplexer also adds additional bits in order

to allow the far end receiving multiplexer to decode which bits belong to which 2 Mega

data stream and so correctly reconstitute the original data streams. These additional

bits are called ”Justification” or ”Stuffing” bits.

The resulting data stream form the above process runs at 8.448 Mbps (about 8 Mbps).

Similar techniques are used to combine four × 8 Mbps together, giving 34 Mbps.

Four × 34 Mbps gives 140 Mbps. Four × 140 Mbps gives 565 Mbps. See Table 2.2.

565 Mbps is the rate typically used to transmit data over a fiber optic system for long

distance transport.

TDM Hierarchy & Standards

Figure 2.13: General TDM Hierarchy


For Figure 2.13, N = 24 for T1 System, and N = 32 for CEPT System

The CEPT Hierarchy is shown in Figure 2.14.

Figure 2.14: CEPT Hierarchy

b)- Synchronization Digital Hierarchy (SDH): It is fully Synchronization approach. The

basic of SDH is synchronous multiplexing. In SDH, the multiplexed channels are in

fixed locations relative to the framing byte. Demultiplexing is achieved by gating out

the required bytes from the digital stream. This allows a single channel to be dropped

from data stream without multiplexing intermediate rates as is required in PDH.

SDH rates is a transport hierarchy based on multiples of 155.52 Mbps. The basic unite

of SDH is STM − 1, as follows:

STM − 1 = 155.52 Mbps

STM − 4 = 622.08 Mbps

STM − 16 = 2588.32 Mbps

STM − 64 = 9953.28 Mbps


2.2.2 Digital Hierarchy in USA and Japan

Table 2.3: Digital Hierarchy in USA

Name Bit rate (Mbps) no. of Channels

1ry PCM (T1-Carrier) 1.544 24

M1−2 MUX (T2-Carrier) 6.312 96

M2−3 MUX (T3-Carrier) 44.736 672

M3−4 MUX (T4-Carrier) 274.176 4032

M4−5 MUX (T5-Carrier) 400.352 5760

Table 2.4: Digital Hierarchy in Japan

Name Bit rate (Mbps) no. of Channels

1ry PCM 1.544 24

M1−2 MUX 6.312 480

M2−3 MUX 32.064 1440

M3−4 MUX 96.728 5760

M4−5 MUX 565.148 8192

2.2.3 CCITT Standards

• Bell System (T1-Carrier), 24 vc, · · · etc.

• CEPT System (E1), 30 vc, · · · etc.

• Modified Bell System (introduction of changing in synchronization and signalling.

– Framing bit is the first, not the last in the frame.


– To deal with multiframe (12 frames)

– 8-bits for voice, one used in each codeword.

There two possible versions:

Version 1 Common signalling for all channels. This is shown in Figure 2.15.

Figure 2.15: Common signalling for all channels

Version 2 Signalling is associated with each channel as shown in Figure 2.16.

Figure 2.16: Signalling is associated with each channel


2.3 Space Division Multiplexing (SDM)

It is a multiplexing technique in which physical separation of transmitting antennas is

used to deliver simultaneously different data streams. SDM is an approach to Multi

Input Multi Output (MIMO) system. It approves capacity by increasing the number

of antennas in the fading channels.

Figure 2.17: Space Division Multiplexing concept

2.4 Code Division Multiplexing (CDM)

Figure 2.18: Code Division Multiplexing concept


Lecture 3

High Frequency System (HF)

Radio frequency (RF) transmission between 3 and 30 MHz by ITU convention is called high

frequency (HF) or shortwave. Three types of HF transmission are known:

• Groundwave propagation which is used up to 100 Km in Land area and up to 300 Km

over seas.

• Line of Sight (LOS) propagation which depends on terminal separation and operating

frequency. This transmission is limited to about 30− 60 Km.

• Skywave propagation which can be used for any distance.

3.1 Sky Wave Propagation

The skywave transmission phenomenon of HF depends on ionospheric refraction. Transmit-

ted radio waves hitting the ionosphere are bent or refracted. When they are bent sufficiently,

the waves are returned to earth at a distant location. Often at the distant location they are

reflected back to the sky again, only to be returned to earth still again, even further from

the transmitter.

32

The ionosphere is the key to HF skywave communication. Look at the ionosphere as

a layered region of ionized gas above the earth. The amount of refraction varies with the

degree of ionization. The degree of ionization is primarily a function of the sun?s ultraviolet

(UV) radiation. Depending on the intensity of the UV radiation, more than one ionized layer

may form (see Figure 3.1. The existence of more than one ionized layer in the atmosphere

Figure 3.1: Atmospheric layers as a function of height above the earth’s surface

is explained by the existence of different (UV) frequencies in the sun?s radiation. The

lower frequencies produce the upper ionospheric layers, expending all their energy at high

altitude. The higher frequency UV waves penetrate the atmosphere more deeply before

producing appreciable ionization.

Four layers of the ionosphere have been identified and labeled as follows:

D-Layer Not always present, but when it does exist, it is a daytime phenomenon. It is the

lowest of the four layers. When it exists, it occupies an area between 50 and 90 km

above the earth. The D-Layer is usually highly absorptive due to its high collision

frequency. The electron density ranges of this layer are as follow:


102 ele/cm3 at 70 km



E-Layer A daylight phenomenon, existing between 90 and 140 km above the earth. It

depends directly on the sun?s UV radiation and hence it is most dense directly under

the sun. The layer all but disappears shortly after sunset. Layer density varies with

seasons. The electron density range of this layer is 104 − 105 ele/cm3

F1-Layer A daylight phenomenon existing between 140 and 250 km above the earth. At

sunset the F1-layer rises, merging with the next higher layer, the F2-layer.

F2-Layer This layer exists day and night between 150 and 250 km (night) and 250 and

300 km above the earth (day). The electron density range of this layer is 106 ele/cm3.

At night, Layers D, E, F1, become very much depleted of free electrons leaving the layer F2

available for communication.

Figure 3.2 displays the electron densities of the these layers at different heights.

Height(Km)

Figure 3.2: Electron density as a function of altitude and various ionospheric layers.

Consider these layers as mirrors or partial mirrors, depending on the amount of ionization

present. Thus transmitted waves striking an ionospheric layer, particularly the F-layer,


Table 3.1

Number of Hops Path Length (km)

1 < 4000

2 4000− 7000

3 7000− 12000

may be refracted directly back to earth and received after their first hop, or they may be

reflected from the earth back to the ionosphere again and repeat the process several times

before reaching the distant receiver. The latter phenomenon is called multihop transmission.

Single and multihop transmission are illustrated in Figure 3.3

Figure 3.3: Single and multihop HF skywave transmission.

To obtain some idea of the estimated least possible number of F-layer hops as related to

path length, Table 3.1 may be used as a guide:

HF propagation above about 8 MHz encounters what is called a skip zone. This is an

”area of silence” or a zone of no reception extending from the outer limit of groundwave

communication to the inner limit of skywave communication (first hop). The skip zone is


shown in Figure 3.4 The region of coverage from a HF transmitter can be extended through

the skip zone by a subset of skywave transmission called ”near-vertical incidence” (NVI) or

”quasi-vertical incidence” (QVI).

Figure 3.4: Skip zone. A = limit of groundwave communication; B = skip zone. Note:

Good HF coverage is possible in the skip zone using near-vertical incidence skywave (NVIS)

transmission.

3.1.1 Reflective Index in Ionized Region

The approximated equation of the first order phase reflective index in ionoshere region can

be written as

nRF =

√

1− Ne2

ǫ0mω2,

where: N : the electron density (ele/cm3), e: the electron charge (1.6×10−19 C), m: electron

mass (m = 9.11× 10−31 Kg), ǫ0: the free space permittivity (8.85× 10−12 F/m), and ω: is

the radian frequency.

From Figure 3.5 and using the law of refraction (Snell’s Law), we get

n0 sin θ1 = nRF sin θ2


with each θ as the angle measured from the normal of the boundary. The condition for

Figure 3.5: Reflection and refraction on the ionosphere layer.

the wave to return to the earth is to have total internal reflection, which begins when the

reflection angle is θ2 = 90

nRF = n0 sin θ1 = sin θ1, since n0 = 1 for free space.

For vertical incident (θ1 = 0),

∴ nRF = sin(0) = 0 =

√

1− Ne2

ǫ0mω2

N = 1.24× 104f 2 ⇒ f0 = 9× 10−3√N f0 in MHz

This is know as the critical frequency for vertical incident.

For incident angle (oblique incident) and refereing to Figure 3.6, we get

f cos θ1 = f0 f =f0

cos θ1= f0 sec θ1

As it can be seen that as θ1 increases, f increases. The value of frequency f is called

maximum usable frequency (MUF). Omitting the subscript form theta, we get

fMUF = f0 sec θ


Figure 3.6: Oblique incident and maximum usable frequency.

Note that each angle has its own MUF. The operating frequency is in the order of between

60% to 85% of MUF.

Lower Usable Frequency(LUF ) ≤ f ≤ (MUF ) ⇒ f = kfMUF , k < 1

Optimal Working Frequency

fOWF =MRF × fMUF

Multipath Reduction Factor (MRF) is the lowest percent of the MUF to the operating

frequency for which the range of multipath propagatin time difference is less than a specific

value. The MRF thus defines the frequency above which a specified minimum protection

against multipath is provided.

3.1.2 National Phenomena Affecting Ionospheric Propagation

Solar disturbances are responsible for many of the major changes in the ionosphere. As a

result a knowledge of when and how long they happening and their size can help in predicting

what ionosphere radio conditions may be like.

a)- Sunspot: areas on the surface of the sun that are a littel cooler than the surrounding


areas. Their presence leads to higher levels of radiation being emitted and therefore

affects HF propagation.

b)- Sudden Ionosphere Disturbance (SID): it occurs without warning for any length of time

(few minutes to several hours. When it occurs, long distance propagation is almost

blanked out.

c)- Sporadic-E: it is irregular cloud forming at heights near E-Layer, harmful or helpful.

d)- Magnetic storms: it is a temporary disturbance of the earthmagnetsphere caused by a

solar wind.

e)- Multipath.

f)- Fading.

3.2 Radio Systems

1- One way radio system, illustrated in Figure 3.7, e.g.Broadcasting.

Figure 3.7: One way radio system.

2- Simplex radio system as illustrated in Figure 3.8.

3- Duplex radio system as illustrated in Figure 3.9.


Figure 3.8: Simplex radio system.

Figure 3.9: Duplex radio system.

3.2.1 Emission Types: Formats

• AX → X is a digital, for AM, SSB, DSB.

• FX → X is a digital, for FM, PM.

• PX → X is a digital, for PAM, PCM, etc.

For examples

A3 → Telephony

A3J → SSB Telephony

A5 → TV

F3 → Telephony

F4 → FAX

P3 → PCM


3.2.2 Examples of HF Systems

1- Broadcasting Systems

- AM Broadcasting (Radio)

⋆ Signal bandwidth = 5 KHz

⋆ Transmission bandwidth = 5 KHz

- FM Broadcasting (Radio)

⋆ Signal bandwidth = 15 KHz

⋆ Frequency deviation f = 75 KHz

• FM transmission (For telephony of low data rates)

• Independent sideband transmission (ISB), bandwidth up to 12 KHz.

3.2.3 Single Sideband Transmitter

Figure 3.10 demonstrates the SSB modulator in which local oscillator is used to translate

the transmission bandwidth to HF band. fif is in the range of 100 KHz.

fLO is in the range of 3− 30 MHz.

3.2.4 Single Sideband Receiver

Figure 3.11 demonstrates the SSB demodulator.

3.2.5 HF-SSB Operations

1- Pilot carrier: to ensure synchronization. The power of the pilot tone is about 10

to 20 dB below the message power. To detect the pilot, very sharp filter is needed.


Figure 3.10: Single sideband modulator.

Figure 3.11: Single sideband demodulator.

Pilot is also used to feed oscillators at the demodulator to generate the same reference

frequency.

2- Synthesizer

f2 = f1 +f, f is the increment.


Figure 3.12: Synthesizer

3- Frequency stability

frequency Stability =allowable frequency variation (absolute)

workingfrequency

As an example: if ±0.5 Hz is allowed, and the operating frequency is 10 MHz, then

frequency Stability =±0.5

10× 106= ±5 × 10−8 or 5 parts in 108

Figure 3.13: Synthesizer


3.3 HF Transmitter Design Considerations

[1 -] Operating frequency

[2 -] Attenuation of radio waves (depends on layers)

[3 -] Antenna gains and space requirements

[4 -] Transmission line losses

[5 -] Complexity repair

[6 -] Accuracy requirement.

Link calculation:

SR = Pt − Lft +Gt − L− Lit +Gr − Lfr(dB),

where

Lft feeder transmission line loss in transmitter,

Lfr feeder transmission line loss in receiver,

Lit total ionosphere loss, it can be calculated from data table,

L free space loss

Gt, Gr transmitter and receiver antenna gain respectively,

Pt transmitter power,

SR receiver power.

For SSB, SR = ηω SNR.

3.3.1 Frequency Modulation Transmitter

The following figure shows a block diagram of FSK modulator

The power amplifier should be:


Figure 3.14: FM Transmitter

1)- Linear amplifier

2)- Higher power

3)- Automatic Logical Control (ALU)

4)- to manimize intermodulation effect to an acceptable level.

Figure 3.15: Automatic Logical Control (ALU)


3.3.2 Intermodulation Distortion(IM)

Intermodulation (IM) Distortion is the result of the presence of intermodulation products. If

two signals with frequencies f1 and f2 are passed through a nonlinear device, the result will

contain IM products of frequency energy components. These components may be present

either inside and/or outside the band of interest for a particular device. IM products may

be produced from harmonics of the desired signals in question, either as products between

harmonics or as one of the signals and the harmonic of the other(s) or between both signals

themselves. The products result when two (or more) signals beat together or mix. Look at

the mixing possibilities when passing f1 and f2 through a nonlinear device. The coefficients

indicate the first, second, or third harmonics.

Second-order products f1 ± f2

Third-order products 2f1 ± f2, 2f2 ± f1

Fourth-order products 2f1 ± 2f2, 3f1 ± 1f2.

Intermodulation noise may result from a number of causes:

• Improper level setting. If the level of input to a device is too high, the device is driven

into its nonlinear operating region (overdrive).

• Improper alignment causing a device to function nonlinearly.

• Nonlinear envelope delay.

IM distortion may be measured in two different ways:

• Two-tone test: the two-tone test is carried out by applying two tones simultaneously

at the audio input of the SSB transmitter. A 3 : 5 frequency ratio between the


two tones is desirable so that the IM products can be identified easily. For a 3 kHz

input audio channel, a 3 : 5 frequency ratio could be tones of 1500 and 2500 Hz.

The test tones are applied at equal amplitude, and their gains are increased to drive

the transmitter to full power output. Exciter or transmitter output is sampled and

observed on a spectrum analyzer. The amplitudes of the undesired products and the

carrier products are measured in terms of decibels below either of the equal-amplitude

test tones as they appear in the exciter or transmitter output. The decibel difference

is the signal-to-distortion ratio (S/D).. This should be 40 dB or better. As one might

expect, the highest level product is the third-order product. This product is two times

the frequency of one tone minus the frequency of the second tone. For example, if the

two test tones are 1500 and 2500 Hz, then

2× 1500− 2500 = 500 Hz or 2× 2500− 1500 = 3555 Hz

and, consequently, the third-order products are 500 and 3500 Hz. The presence of IM

products numerically lower than 40 dB indicates maladjustment or deterioration of one

or several transmitter stages, or overdrive.

• Tests using white noise loading: The white noise test for IM distortion more nearly

simulates operating conditions of a complex signal such as voice. The 3 kHz audio

channel is loaded with uniform amplitude white noise and a slot is cleared. The signal-

to distortion ratio is the ratio of the level of the white noise signal outside the slot to

the level of the distortion products in the slot.


3.4 Radio Link System (Line of Sight System (LOS))

Very High frequency (VHF), Ultra High Frequency (UHF), and Microwave Frequency are

the band that are used in Telephone channel, data transmission and TV program channels.

The key term here is line-of-sight. It implies that the antenna of the radio link on one end

has to be able to ”see” the antenna on the other end.

3.4.1 Characteristics of LOS

The main characteristics of Line to Sight are:

1)- Signal follow straight line

2)- free space attenuates the sighnals

3)- uses frequencies higher than 30 MHz

4)- can allow the use of large bandwidth signals.

5)- signals can be predicted at the receiver with very good accuracy.

A series of LOS links is shown in Figure 3.16.

Figure 3.16: A sketch of an LOS microwave radio relay system.


3.4.2 Engineering of Radio Links

To design a radio link, the following steps have to be taken

1)- Initial planning and site selection A LOS microwave route consists of one, several, or

many hops. It may carry analog or digital traffic. The design engineer will want to

know if the LOS subsystem to be installed is an isolated system on its own, such as a

private microwave radio link system or studio-to-transmitter link or is part of a larger

telecommunication network where the link may be part of a backbone route or a ”tail”

from the backbone. (Equipments and tower antenna requirements)

2)- Operating frequency

3)- Path profile: a path profile is a graphical representation of a path between two adjacent

radio link sites in two dimensions. From the profile, tower heights are derived, and,

subsequently, these heights can be adjusted on paper so that the ray beam reflection

point will avoid reflective surfaces. The profile essentially ensures that the proper

clearances of path obstructions are achieved.

4)- Path calculations: to calculate transmission loss for over-the-horizon paths, for LOS

paths the calculation is simple it is just free space loss and adding certain fade margin.

To compute the propagation loss between to points separated by distance D Km, the

formula is:

LdB = 32.44 + 20 log10D + 20 logdB f

where f is the operating frequency in MHz and D is in Km

At f = 455 MHz, and D = 32 Km, then L = 112 dB. For f = 4 GHz, and D = 32 Km,

then L = 132 dB.


5)- Path survey

6)- Equipment configuration

Figure 3.17: Auxiliary link.

7)- Establishing frequency plan

Figure 3.18: Overcome interference.


8)- Installation and testing of equipments

9)- Beam alignment

Figure 3.19: Beam alignment.

3.4.3 Atmospheric Effects on free space Transmission

If a radio beam is propagated in free space, where there is no atmosphere, the path followed by

the beam is a straight line. However, a radio ray propagated through the earths atmosphere

encounters variations in the atmospheric refractivity index along its trajectory that causes

the ray path to become curved. Atmospheric gases will absorb and scatter the radio path

energy, the amount of absorption and scattering being a function of frequency and altitude

above sea level. Absorption and scattering do become serious contributors to transmission

loss above 10 GHz.

(1)- Atmospheric Absorption This amount of absorption can be neglected in operating

frequency below 5 GHz. Figure 3.20 shows the amount of attenuation as a function of

frequency.

(2)- Scattering due to rain and fog:

Rain and fog cause small amount of attenuation in frequencies below 5 GHz. Figure

3.21 shows this amount for different values of rainfall rates.


Figure 3.20: Attenuation due to atmospheric gases

Figure 3.21: Attenuation due to different values of rainfall rates

(3)- Atmospheric refraction: The K-factor is a scaling factor that helps quantify curvature

of an emitted ray path. Common radio links, which are described as line-of-sight

incorrectly suggest that effective communications are limited by the optical horizon

(i.e., K = 1). In most cases radio links are not restricted to LOS propagation. In fact,


we often can achieve communications beyond the optical horizon by some 15% (i.e.,

K = 1.33) Figure 3.22 shows this concept in a simplified fashion.

Figure 3.22: Optical line-of-sight versus radio line-of-sight.

Figure 3.23 shows the effects of various K-factors on the bending of the radio ray beam.

This bending is due to angular refraction. Angular refraction through the atmosphere

occurs because radio waves travel with differing velocities in different parts of a medium

of varying dielectric constant. The K-factor can now be defined as the ratio of the

radius, R, of the ray beam curvature to the true radius of the earth, R0 ≃ 6370 Km ,

or

K ≃ R

R0

where K is often called the effective earth radius factor and R is the effective earth

radius.

K-factor can also be calculated as

K ≃[

1 +R0∆n

∆h

]−1


Figure 3.23: Ray beam bending for various K-factors.

where n is the atmospheric index of refraction, h is the height above mean sea level,

∆n∆h

is the rate of change of n with respect to h, and R0 is the true radius of the earth

(6370 Km).

K = 43is taken to represent the refractive index with height.

K > 1; the ray bends toward the earth (reduce the tower length). This is because

∆n∆h

= −ve.

K < 1; the ray bends outside the earth. For certain unnatural phenomena ∆n∆h

= +ve.

3.4.4 Terrain Effects on LOS Propagation

Figure 3.24 illustrates many terrain cases. Figure 3.24-(a) the transmitter, the receiver and

the obstacle are all with same height. Figure 3.24-(c,b) the obstacle is below and above the

LOS path respectively.


Figure 3.24: Different terrain cases.

If we define the ratio h/hi as

h

hi=

Clearance

Radius of the sst fresnel zone

Looking to Figure ??

if the Path ABC - Path AB = nλ2

, satisfies, then the

region is known as Fresnel Zone.

• n = 1 is known as 1st Fresnel zone,

• n = 2 is known as 2nd Fresnel zone,· · · etc.

The nth Fresnel zone radius Rn is given by

Rn =

√

nλ( d1d2d1 + d2

)= 17.3

√

n

fGHz

( d1d2d1 + d2

),

where where d1 is the distance to the near-end antenna and d2 is the distance to the far-end

antenna from the obstacle.

Figure 3.25 shows path attenuation versus path clearance.


Figure 3.25

3.4.5 Path Profile

to calculate the path profile, the following steps should be taken

(1)- Obtain obstacles’ information from topographical information maps, the scale may be

taken as 1 : 2500 cm. ”Tabloid” size might be used (11in× 17in). So scale of 2 miles

for one inch (max. of 30 inches).

(2)- On a graph paper, indicate the locations of all the obstructs as shown in Figure 3.26

(3)- Obtain Earth curvature by

EC(feet) =0.667d1d2

K=

d1d21.5K

, d1, d2 in mile

EC(meter) =0.078d1d2

K=

d1d212.75K

, d1, d2 in Km


Figure 3.26

d1 is the distance between 1st location to the obstruct, and d2 is the distance between

the obstruct and 2nd location. K is the effective earth radius to the true earth radius

ratio.

(4)- Obtain the first fresnel zone radius by

R1 = 547

√

d1d2fMHzD

= 17.3

√

d1d2fGHzD

, d1, d2 and D = d1 + d2 in Kms

or

R1 = 2280

√

d1d2fMHzD

= 72.1

√

d1d2fGHzD

, d1, d2 and D = d1 + d2 in miles

The optimum clearance x = 0.6R1

(5)- Setup table as follows

obstacle location clearance d1 d2 EC

A 0.6R1A

√ √ECA

B 0.6R1B

√ √ECB

......

......

...

E 0.6R1E

√ √ECE


Figure 3.27

(6)- Put this figures on graph paper as shown

(7)- If there are tree areas, add about 10− 12 m and about 3 m for average growth.

Example 1

Two sites A and B at altitudes of 60 m and 85 m above the sea level respectively and

separated by 40 km. Assume that there is no vegetation between them and there is one

major obstacle of height 100 m and at distance 15 Km from site A. If the transmission

frequency is 2 GHz. Find the height at site B if the tower height at site A is 60 m.

Solution:

EC =0.078d1d2

k=

0.078× 15× 25

4/3= 21.9 m

R1 = 547

√

d1d2fMHzD

= R1 = 547

√

15× 25

2000× 40= 37.45 m


F1 = 0.6× R1 = 22.47 m

tanα =144.37− 120

15000=

24.37

15000=

x

40000→ x = 65 m

∴ the height tower at site B is 120 + x = 185 m from sea level and it is 100 m height from

Figure 3.28: Layout of Example 1

the earth level.

3.4.6 Link Reliability and Performance

The reliability of a link is defined as

ℜ =MTBF

MTBF +MTTR

MTBF : Mean Time Between Failure, and MTTF : Mean Time To Repair. 99% reliability

for the link (14min/day or 7h/month).


99.99% reliability for the link means (4.3min/month or 8.6sec/day).

Input signal level at receiver:

Ci =C

N

∣∣∣iKTBF

K = 1.36× 10−23 jK−1 (Boltzman’s constant.) T = Noise temperature, B = BW , and F :

Noise Figure (generated inside the system itself)

F =S/N

∣∣∣i

S/N∣∣∣o

=Si

So· N0

KTB=

Si

GSi· N0

KTB=

N + 0

KTBG

where So = SiG.

For ideal system F = 1 = 0dB

For practical system F > 1 > 0dB

Ci = 10 logKT + 10 logB +NF + (C

N)dB dBw

NF = 10 logF

Ci = −204(dBw) + 10 logB +NF + 10 for the case ofC

N

∣∣∣i= 10 and T = 290 K

As an example

If Ci = −114 dBw, and L = 140 DB, then Pt = −114 + 140 = 26dBw = 400 W without

antenna gains.

with antenna gains (Gt = Gr = 20dB) and cable losses (Lft = Lfr = 2dB)

Pt − 2 + 20− 140 + 20− 2 = −114 → Pt = 10 dBw = 0.1W

Typical Transmission power: 0.1 W or −10 dBw for HF band

1 W or 0 dBw for HF band

10 W or 10 dBw for lower frequency band.


melalem

Sticky Note

No

melalem

Sticky Note

dB

3.4.7 FM Radio System

Figure 3.29

3.4.7.1 Important parameters of FM

(1)- Modulation Index (β):

β =ffm

, f frequency deviation from the carrier

BT = 2(β + 1)fm = 2βfm + 2fm

(2)- Deviation sensitivity (volts/MHz), i.e., 0.05 Vrms/MHz.

(3)- For deviation/channel (i.e.voice channel).

• RMS deviation/channel, for number of channels = 120 ch, the RMS deviation at

the test tone level may be taken 50 KHz, 100 KHz 200 KHz

• RMS deviation/channel, for number of channels = 300 ch or more, the RMS

deviation = 200 KHz


The peak frequency deviation (Dp) is given by

Dp =

4.47drms

[

log−1(

−15+10 logN20

)]

, N ≥ 240 vc;

4.47drms

[

log−1(

−1+4 logN20

)]

, 12 ≤ N ≤ 240 vc

where

Dp : Peak deviation in KHz,

drms : the RMS deviation/ch in KHz,

N : the number of channels.

If N = 300 vc, drms = 200 KHz, then Dp = 2753 KHz.

BT = 2Dp + 2Bbb where Bbb is the baseband bandwidth = 300× 4 = 1200 KHz

∴ BT = 2× 2753 + 2× 1200 = 7906 KHz

f in single tone (sinusoidal) is know, but in microwave signal is not easy to be known.

3.5 Repeaters

Figure 3.30: Terminal and Superheterodyne repeaters

There are two types of repeaters

1- Terminal repeaters: in which Drop and Insert is possible.


Figure 3.31: Main functions of terminal repeater

Figure 3.32: Drop and Insert functions of terminal repeater

2- Superheterodyne repeaters: No Drop nor Insert.

⋆ RF Repeater: Radio link repeaters amplify the signal along the radio route, pro-

viding gain on the order of 110 dB.

With a RF repeater amplification is carried out directly at radio frequencies. The

incoming RF signal is amplified, translated in frequency, and then amplified again.

⋆ IF Repeater: IF repeater may be used when there are no drops and inserts at


Figure 3.33: RF Repeater

the repeater facility. An IF repeater inserts less noise into the system because

there are less modulation/demodulation steps required to carry out the repeating

process

Figure 3.34: IF Repeater

Note that fLO − f1 = in the range of 63 MHz

3.5.0.1 Sections of Repeater

(1)- Antenna: Dish antenna is usually used


Figure 3.35

Figure 3.36

The antenna gain is given by:

G = η(πD

λ

)2

where: η is the antenna efficiency, D is the antenna diameter, and λ the wavelength of

the operating frequency. The gain of the antenna increases as frequency and diameter

increase.

Typical diameters are: 3.7 m, 2.4 m, 1.8 m, 1.4 m, and 1.2 m.

The 3-dB Beam width of the antenna is defined as

Bw = 70λ

D

As an example, a 2-meter parabolic dish operates at 6 GHz, then its 3 -dB beamwidth

is 1.15

(2)- Branching Filter


Figure 3.37

Port 1 receiver input or transmitter output.

Port 2 pass channel port to the next branching filter.

Port 3 dummy load (for balancing).

Port 4 Drop channel port for Rx or Insert channel port for Tx.

3.5.1 Loading of Radio Relay System

(1)- Noise Power Ratio (NPR), defined as NPR is the decibel ratio of the noise level in a

measuring channel with the baseband fully loaded to the level in that channel with all

of the baseband noise loaded except the measuring channel.

The composite noise power for N Channels:

P (dBmo) =

−1 + 4 logN, N < 240;

−15 + 10 logN, N ≥ 240.

NPR can be calculated by

NPR = P − noise power in the slot (test channel)

(2)- Bandwidth Ratio (BWR) is defined as

BWR = 10 log(occuipied baseband bandwidth

voice channel bandwidth

)


(3)- Noise Load Ratio (NLR), which is defined as

NLR = 10 log( Equivelant noise test power

voice channel test tone power

)

Now

SNR = NPR +BWR−NLR

Example 2

In 120 channel system having baseband in 60− 552 KHz band, the noise power ratio should

achieve 58 dB. Calculate the signal to noise ratio (SNR).

Solution:

BWR = 10 log552− 60

3.1= 20

NLR = −1 + 4 log 120 = 7.3

SNR = NPR +BWR−NLR = 58 + 20− 7.3 = 70.7dB

3.6 Digital Microwave System

• Low capacity system (LCS)≤ 10 Mbps.

• Medium capacity system (MCS) 10 ≤MCS ≤ 100 Mbps.

• High capacity system (HCS) > 100 Mbps.

A general block diagram for digital microwave system is shown in Figure 3.38

Phase shift Keying (PSK) scheme and its versions (QPSK, 8PSK, 16PSK · · · ) are used

in microwave links


Figure 3.38: Digital microwave system

3.6.1 Performance of Digital Microwave System

It is measured in terms of bit error rate (BER). Probability of error Pe is a function of the

energy bit per noise (Eb/N0).

Eb

N0

=C

N· W (IF BW)

R(bit rate)

The spectrum efficiency is defined as

η =R

Wbit/sec/Hz =

1

WTlog2M, Rb =

1

Tlog2M

3.6.1.1 Radio System performance Density guide lines

(1)- Determine CN

requirements, this is obtained from specific Pe, R, and BW

C

N=

[Eb

N0

]

from Pe

· R

BW

(2)- Get receiver noise figure and noise power

N = KTBF

As an example, if 8PSK is used:

spectrum efficiency 3n/s, and R = 90 Mbps, then B = 903= 30 MHz.

(3)- Determine

Cmin =(C

N+N

)

dBw


(4)- Get transmit power or assume it, or use available value. Pt range (20 dBm - 35 dBm)

(5)- Get system gain (Gs)

Gs = (Pr − Cmin) ≥ Ls = Lf + Lb −Gt −Gr +Mf

where

Mf = Fade Margin

Ls = free space loss, Ls = 62.4 + 20 log d(km) + 20 log f(GHz)

Lf = feeder loss, depending on the feeder type (tabulated)

Lb = branching loss

Gr = receiver antenna gain

Gt = transmitter antenna gain.

Table 3.2 lists the typical values for the feeder loss for different feeder types

Table 3.2: Feeder loss in dB/Km

Feeder 2 GHz 4 GHz 6 GHz 8 GHz

Rectangular waveguide - 0.027 0.068 0.087

Elliptical waveguide - 0.028 0.039 0.058

Circular waveguide - 0.013 0.03 0.036

Coaxial cable 0.062 - - -

The fade margin Mf is calculated by the formula:

Mf = 30 log d+ 10 log 6ABf − 10 log(1− ℜ)− 70 dB,

where ℜ is the reliability objective. If ℜ = 90%, then (1 − ℜ) = 0.01. d is the link

distance in Km, and f is the frequency in GHz. The factor A is known as the roughness

factor, and B is the factor to consist worst month probability to annual probability.


A and B can take the following values

A =

4, for very smooth terrain including over water ;

1, for average terrain with some roughness;

0.25, for mountainous, very rough, or very dry terrain.

B =

1, worst case;

12, Gulf Coast or similar hot, humid areas;

14, for average in land area;

18, for mountanous and dry area

The above formula can be rearranged as

(1− ℜ)% = 6× 10−5ABfd3 × 10−Mf10

Fade margin increases by about 10 dB increase by additional 9 in the reliability objective.

ℜ1 = 99%, ℜ2 = 99.9% and ℜ3 = 99.99% increase the fade margin.

3.6.1.2 Link Calculation Procedures

The following steps can be followed:

1. Increase Pt,

2. Choose lossless feeder,

3. Choose receiver with low noise figure,

4. Increase antenna diameter (to increase gain)

5. Increase spacing (space or frequency) if diversity is used,

6. Introduce diversity if not used,


7. Increase tower height.

Example 3

The reliability requirement of one hop no-diversity link is 99.99%. Assuming that the system

operates over an average terrain with some roughness, the carrier frequency is 1.8 GHz.

Calculate the maximum distance between the towers if the system gain is 105 dB, and

receiver and transmitter diameter is 3 m. Use A = B = 1, Lf = 10.8 dB, and Lb = 2 dB.

Solution:

Gs = Pr − Cmin ≥Mf + Ls + Lf + Lb −Gt −Gr

Mf = 30 log d+ 10 log(6ABf)− 10 log(1− ℜ)− 70

Gt = Gr = η(π × 3

λ

)2

assume η = 1 → Gt = Gr = (18π)2 = 20 log(18π)

105 ≥ 30 log d+ 10 log(6× 1× 1× 1.8)− 10 log(1× 10−4)− 70

+ 10.8 + 2 + (32.44 + 20 log(1800) + 20 log d− 40 log(18π))

∴ 105 ≥ 50 log d+ 20.58 → log d ≤ 84.42

50= 48.79 Km

As f ⇑, Ls ⇑, as f ⇑, G ⇑ and feed loss ⇑, you may reduce d.

Table 3.3 shows some example of frequency allocation to digital radio links as proposed by

CCIR.

Example 4

Three sites A, B, and C are separated as shown in Figure 4.9, and connected using 630 Mbps

at 2 GHz band utilization as shown. The system has the following specifications.

(1)- The radio gives 20 dBm output power and utilize 8PSK modulator technique with 4 dB

noise figure.


Table 3.3

Frequency

(GHz)

Freq. rangechannel

spacing

modulation

Tech

PCM ch

Capacity

bit

rate(Mbps)

2 1.9− 2.3 29 8PSK 960 Ch 70

4 3.7− 4.2 29 8PSK 960 Ch 70

8 7.725− 8.275 40 16QAM 960 Ch 200

8 7.725− 8.275 29& 65 8PSK 960 Ch 70

10 10.7-11.7 70 PSK 960 Ch 140

10 10.7-11.7 80 8PSK 960 Ch 140

10 10.7-11.7 84 16QAM 960 Ch 140

(2)- The radio equipment are installed next to 3.5 m antenna at the top of the towers.

(3)- The target probability of error for the two links is 10−9.

Figure 3.39

(4)- The area has average terrain, worst case climate conditions.

(5)- Only one 15 m obstacle exist at 4 Km from site B toward site A.

(6)- Adaptive equalizer is used with 2.7dB improvement factor.


(a)- Find the tower hight site A.

(b)- What is the maximum achievable link availability.

Solution:

(a)- The tower heights calculation should be started from B −C link, since it is the largest.

Since the area is flat and no obstacles between the two sites and the heights effective obstacle

is the earth curvature at mid-point which is 422= 21 Km.

EC = 0.078d1d2K

= 0.07821× 21

1.3325.8 m

R1 = 17.3

√

d1d2fGHzD

= 17.3

√

21× 21

2× 42= 39.6 m → 0.6R1 = 23.8 m

Total effective obstacle height = 49.6 m. Take the antenna height at both sites B and C as

hb = hc = 50 m.

Now for the link A− B link, follow Table 3.4 contents.

Table 3.4

Obstacle mid-point

distance from B (Km) 4 17.5

Obstacle height 15 0

EC (m) 7.3 17.9

R1 (m) 23.02 36.2

0.6R1 (m) 13.8 21.7

Total effective height (m) 36.1 39.6

Very clear that the existing obstacle has no effect and the dominant height is the midway

height. Since B tower is 50 m height and the effective is 40 m at mid-way, this tower height

at A is 30 m.


(b)- The path loss general equation

Prmin= Pt +Gr +Gt − Ls + EQIF (Equalizatin gain)

G = 18.5 + 20 logD(m) + 20 log fGHz = 35.4 dB

Ls = 32.4 + 20 logDKm + 20 log fMHz = 129.3 dB

Pr = 20 + 35.4× 2− 129.3 + 2.7 = −35.8 dB

From the curve for 8PSK at 10−9 gives Eb

N0

= 20.8 dB. You may also use the formula

Pe =1

kerfc

(

sinπ

M

√Eb

N0

)

=2

kQ(

sinπ

M

√

2Eb

N0

)

, M = 2k = 8, k = 3

Cmin = Ntotal+C

N

∣∣∣dB+NF = Ntotal+

Eb

N0

+10 logR+NF = −174+4+20.8+10 log(630×106) = −61.2 dB

Mf = Pr−Cmin = −35.8−(−61.2) = 25.4 = 30 log dKm+10 log 6ABfGHz−10 log(1−ℜ)−70

10 log(1−ℜ) = 30 log dKm+10 log 6ABfGHz−70−Mf = 30 log 42+10 log 6×1×1×2−70−25.4 = −36 dB

→ ℜ = 99.97%


Lecture 4

Satellite Communications

4.1 Introduction

A communications satellite is a microwave repeater station that permits two or more users

with appropriate earth stations to deliver or exchange information in various forms. A

satellite in a Geostationary Earth orbit (GEO) revolves around the Earth in the plane of the

equator once in 24 hours, maintaining precise synchronization with the Earth’s rotation.

It is well known that a system of three satellites in GEO each separated by 120 degrees

of longitude, as shown in Figure 4.1, can receive and send radio signals over almost all the

inhabited portions of the globe. (The small regions around the North and South Poles above

81 NL and below 81 are not covered.)

The range from user to satellite is a minimum of 36, 000 km, which makes the design

of the microwave link quite stringent in terms of providing adequate received signal power.

Also, that distance introduces a propagation delay of about one-quarter of a second for a

single hop between a pair of users.

The key dimension of a GEO satellite is its ability to provide coverage of an entire

75

Figure 4.1: A system of three geostationary communication satellites provides nearly world-

wide coverage.

hemisphere at one time. If the satellite has a specially designed communications beam

focused on certain areas, then any receiving antennas within the footprint of the beam

(the area of coverage) receives precisely the same transmission. Locations well outside the

footprint generally are not able to use the satellite effectively.

Orbits that are below a mean altitude of about 36, 000 km have periods of revolution

shorter than 24 hours and hence are termed non-GEO.

Low Earth orbit (LEO), in which satellites are at an altitude of approximately 780 km

and each passes a given user in only a few minutes. The advantage to using a non-GEO

satellite network is that the range to the user is shorter; hence, less radiated power is required

and the propagation delay is reduced as well.

4.1.1 Terminologies

⋆ Transponder is the term used in the industry to identify one complete microwave

channel of transmission from a satellite.


⋆ Footprint of the beam is the the area of coverage that receives precisely the same

transmission. Locations outside the footprint generally are not able to use the satellite

effectively.

⋆ Elevation angle of satellite is the angle between horizontal axis and line of sight between

Earth station and the satellite. Minimum Elevation angle is 5. [0− 90 degree]

⋆ Azimuth angle is the angle measured clockwise from the north to the intersection of

local horizontal plane and the plane passing through Earth station, the satellite and

Earth center. [0− 360 degree]

4.1.2 Categories of orbits

The orbits may be cataloged by:

⋆ their altitude

Low Earth Orbit (LEO)

Medium Earth Orbit (MEO)

Geostationary Earth Orbit (GEO)

⋆ their orbits

Polar

Equational

Inclined

GEO is equational (circular orbit) [23h, 56min, 4.1 sec]


4.1.3 Satellite Axial Motions

Figure 4.2 provides the general view of the three classical axes of attitude: roll, pitch and

yaw. The roll axis, which typically is in the orbit plane, is pointed in the direction of orbital

flight; pitch corresponds to the up-down motion of the nose of the spacecraft along an axis

perpendicular to flight and in the orbit plane; and yaw is a twisting motion of the antenna

about the axis pointed directly down from the spacecraft (e.g., sideways motion of the nose).

Figure 4.2: Definition of the three axes of satellite change: roll, pitch, and yaw.

4.1.4 Advantages of GEO

(1)- Satellite is stationary with resect to Earth (no need for tracking system).

(2)- There is no need to switch from one satellite to another.

(3)- Global coverage is possible.

(4)- There is no doppler shift of the operating frequency.


(5)- No handover problems.

while there are some drawback of GEO

(1)- High power loss due to long distance.

(2)- Difficult to cover locations at poles.

(3)- Hight transmission delay.

The following formula is used to calculate the distance from a point on the earth to the

satellite position:

d2 = r20 + (h+ r0)2 − 2r0(h+ r0) · cos(θ)

where θ is the latitude, r0 equational radius (r0 = 6378 Km), and h is the height of the

satellite above the earth (h = 35785 km).

Equational radius is = 6378.16 Km, while the polar radius is = 6356.78 Km. The mean

radius 6371.03 Km.

4.2 Spectrum Allocations

The spectrum of RF frequencies is shown in Figure 4.3, which indicates on a logarithmic

scale the abbreviations that are in common usage. The bottom end of the spectrum from

0.1 to 100 MHz has been applied to the various radio broadcasting services and is not used

for space communication. The frequency bands of interest for satellites lie above 100 MHz,

where we find the V HF , UHF , and super high frequency (SHF ) bands. The SHF range

has been broken down further by common usage into subbands with letter designations, the

familiar L−, S−, C−, X−, Ku−, and Ka− bands being included.


Figure 4.3: The radio frequency spectrum identifying commonly used frequency bands and

their designations

A typical satellite band is divided into separate halves, one for ground-to-space links

(the uplink) and one for space-to-ground links (the downlink). Uplink frequency bands are

typically slightly above the corresponding downlink frequency band, to take advantage of

the fact that it is easier to generate RF power within an Earth station than it is onboard a

satellite.

4.2.1 VHF and UHF Frequency Ranges

The VHF and UHF bands are defined to be in the ranges 30 to 300 MHz and 300 to

3, 000 MHz, respectively. This range already is highly contested for terrestrial wireless ap-

plications, notably cellular; mobile data; and various radio systems used for government and

emergency services. In the past, this range was used by the early scientific satellites and

for telemetry and command purposes. More recently, some of the small non-GEO systems

adopted this range because of the case of generating power and the convenience of using

simple antennas on the satellites and user terminals.


4.2.2 Microwave Bands: L and S

The frequency of 1 GHz represents the starting point for microwave applications that involve

communication between Earth stations and satellites.

• Signals travel nearly by line-of-sight propagation and are less hampered by the iono-

sphere.

• these frequencies have greater bandwidth than VHF/UHF but less than C- or Ku-bands

• more stable propagation under most conditions than do frequencies below 1 GHz

• L− and S− bands are particularly effective for providing rapid communications by

way of mobile and transportable Earth stations.

• The Global Positioning Satellite (GPS) system operates in a segment of L−band and

employs non-GEO satellites at approximately 26, 500 km altitude. Each satellite trans-

mits in the same bandwidth using a different CDMA spreading code. Thus, a receiver

such as a car navigation system with its single omnidirectional antenna can recover

data from multiple satellites and from this data compute a precise location.

• S-band is centered at 2.5 GHz (just below C−band). The bandwidth is much less than

that afforded by C− and Ku−bands. Satellite Digital Audio Radio Service (S-DARS)

employs the S-band.

4.2.3 Microwave Bands: C, X, and Ku

The most popular microwave bands exploited for satellite communication are the C−, X−,

and Ku−bands. These bands, which lie between 3 and 15 GHz, offer more bandwidth.


• C-band is very popular for international telephone services for thin routes to small

cities (and countries) and to reach rural areas.

• C-band is used extensively for commercial satellite communication. Another common

designation we use is 6/4 GHz, which identifies the nominal center of the uplink fre-

quency band (5.925 to 6.425 GHz) followed after the slash (/) by the nominal center

of the downlink frequency band (3.700 to 4.200 GHz).

• Government and military satellite communications systems employ X−band and, on

a limited basis, Ka−band. With an uplink range of 7.90 to 8.40 GHz and a downlink

range of 7.25 to 7.75 GHz.

4.2.4 Millimeter Wave and Higher: Ka−, Q−, and V−Bands

Beginning at around 20 GHz and extending to suboptical frequencies. Ka-band, with its

20/30 GHz allocations and about 2.5 GHz of available spectrum. The frequencies above

about 40 GHz, designated as Q- and V-band, are now being evaluated for space and ground

applications because of the wide bandwidths.

In general,

• C-band (4/6)(3.7− 6.4) for commercial . The minimum space angle for C-band is 4.

• Ku-band (12/14) Ghz is commercial.

• (7/8, 20/30) for military.

4.2.5 Station Segments

Any Satellite consists of two segments:


• Earth segment (Earth station)

• Space segment (Satellite subsystem)

Figure 4.4 shows the uplink and down link (from Earth to Satellite and satellite to Earth

stations).

Figure 4.4: The most basic two-way satellite link provides point-to-point connectivity

4.2.6 Satellite Subsystem

Figure 4.5 shows the main functions of the space segment

• Transponder is the term used in the industry to identify one complete microwave

channel of transmission from a satellite, typically of 36 MHz bandwidth (it may differ

form satellite to another. See Table ??). The number of transponders also differs from

satellite to another. Figure 4.7 illustrates the block diagram of the transponder. LNA

(Low Noise Amplifer) and LPA (Linear Power Amplifier).

• Power generation subsystem which uses solar panels. They may produce 7 Kw/wing.

Panels have 300 w/Kg or 300 w/m2. In 2016 one panel can produce 100 Kw.

The power generation system has also batteries for storage which has life time of around

(15− 20) years. Also the system should have power conditioning system.


Figure 4.5: Main parts of the satellite station

• Command of control subsystem.

• Thrusters which receive signal for the earth station to align the satellite antenna.

• Antenna subsystem


Figure 4.6: Block diagram of the transponder

Table 4.1: No. of transponders in different satellites.

Satellite no. of transponders bandwidth (MHz)

RCA(Radio Corporation of America) 24 36

SBS (Satellite Broadband System) 10 40

Arab Sat 25 33

4.2.6.1 Nile Sat 102

• Launched: 17/8/2000

• launched mass: 1827 Kgm

• Orbit position: 7 west

• power consumption: 3 Kw

• expected life time: 15 years

• Downlink frequency: 11.8-12.3 GHz

• Uplink frequency: 17.4-17.9 GHz


• Transmission bandwidth: 33 MHz

• Output power: 100 Kw

• Polarization: Linear

• Receiving dish antenna: 50cm-75cm

4.2.7 Input and Output Back Off

Input back-off When a number of carriers are present simultaneously in an amplifer, the

operating point must be backed off to a linear portion of the transfer characteristic to

reduce the effects of intermodulation distortion. Such multiple carrier operation occurs

with frequency division multiple access (FDMA). The point to be made here is that

backoff (BO) must be allowed for in the link budget calculations. If the saturation

flux density for single-carrier operation is known. Input IBO will be specified for

multiple-carrier operation, referred to the single-carrier saturation level. The earth-

station EIRP will have to be reduced by the specified BO, resulting in an uplink value

of

(EIRP )U = (EIRPS)U − (IBO)

Output back-off Where IBO is employed a corresponding output (OBO) must be allowed

for in the satellite EIRP . As the curve of Figure ??, OBO is not linearly related to

IBO. A rule of thumb, frequently used, is to take the OBO as the point on the curve

which is 5 dB below the extrapolated linear portion. Since the linear portion gives a

1 : 1 change in decibels, the relationship between input and output BO is

(OBO) = (IBO)− 5 dB

. For example, with an IBO = 11 dB, the corresponding OBO is OBO = 11−5 = 6 dB.


Figure 4.7: Input and output Back off concept

4.2.8 Transponder Gain

The transponder gain is given by

GT (dB) =output EIRP of the satellite

Input flux from Earth= EIRPsat(dB)− ψearth(dB/m

2)

In general beam width of an antenna is given by

Bw =70λ

D, in degree

If f = 6 GHz and D = 10 m, then Bw = 0.35. The larger the antenna diameter, the smaller

of the beam width. The signal from antenna of Bw = 0 is stronger from that of the earth

coverage antenna (Global) by square of the inverse.

P1

P2

=((Bw)2(Bw)1

)2

=(17.34)2

12= 301

4.2.9 Azimuth Angle, Elevation Angle and Slant Range

(1)- As it was defined above, the azimuth angle A is the angle measured clockwise from the

north to the intersection of local horizontal plane and the plane passing through Earth


station, the satellite and Earth center. [0− 360 degree]. It depends on the location of

the earth station with respect to the satellite. It can be calculated as below

For Northern Hemisphere

A = 180− A, for earth station east to the Sat;

A = 180 + A, for earth station west to the Sat.

For southern Hemisphere

A = A, for earth station east to the Sat;

A = 360− A, for earth station west to the Sat.

where

A = tan−1(tan |θs − θl|

sin θ1

)

,

– θs = Satellite longitude

– θl = earth station longitude

– θ1 = earth station latitude

Note that θs is +ve if the satellite is east of Greenwich line, and it is −ve if the satellite

is at the west of Greenwich.

(2)- The Elevation angle E or Look angle is give as

E = tan−1

[r − Re cos θ1 cos(θs − θl)

Re sin(

cos−1(θ1 cos(θs − θl)))

]

− cos−1(cos θ1 cos(θs − θl))

where Re = 6378 Km is the earth radius, and r is the Geostationary orbit radius from

the center of the earth.

r = Re +H = 6378 + 35786 = 42164 Km

(3)- The Slant range (d) is given by

d =

[

(Re +H)2 +R2e − 2Re(Re +H) sin

E + sin−1( Re

Re +HcosE

)]0.5


4.3 The Earth Station

The flux at distance R from the source

ψ =Pt

4πR2

Power received at some point x

PRx = ψAe, where Ae is the effective areaAe = ηA

PRx = ψηA =Pt

4πR2ηA

The antenna gain is

G =(πD

λ

)2

=4πAe

λ2

Or Ae = Gλ2

4π→ PRx =

Pt

4πR2·Gλ

2

4π

∴ PRx =PtGt

4πR2·Gr

λ2

4πIntroducing the gain of transmitting antenna

∴ PRx = PtGtGrλ2

(4πR)2=PtGtGr

L

where L =(4πR)2

λ2= Loss

The power received at satellite

Prsat =PtesGtes

Lup

C

T

∣∣∣up

=PtesGtes

LupT

where C = carrier power= Prsat

T = noise temperature at the satellite.

C

T

∣∣∣up

=EIRPes(G/T )sat

Lup(4.1)

Lup = 92.45 + 20 log fG + 20 logRKm

(G/T )sat = Figure of merit


Also

Pres =PtsatGtsat

Ldown

=ERIRsat

Ldown

C

T

∣∣∣down

=EIRPsat(G/T )es

Ldown

(4.2)

Lup = 92.45 + 20 log fG + 20 logRKm

(G/T )es = Figure of merit

For standard Intelsat earth station

G/T = 40.7 dB/K

In general G/T = 40.7 + 20 logfG4dB/K

Carrier to Noise Ratio CN

C

N

∣∣∣total

=

C

N

∣∣∣

−1

up+C

N

∣∣∣

−1

dwon+C

I

∣∣∣

−1

−1

C

N

∣∣∣up

=C

T

∣∣∣up−10 logK − 10 logB −NF︸︷︷︸

sat

C

N

∣∣∣down

=C

T

∣∣∣down

−10 logK − 10 logB −NF︸︷︷︸

es

CI= Carrier to Intermodulation noise.

Figure of meritG(gain of amplifer)

Te(effective Temp)

Example 1

If R = 35881 Km, EIRP = 5 dB, f = 4 GHz, Des = 4.5 m.

Ges = η(πD

λ

)2

= 109.7ηf 2GHzD

2m

Gr = 43.29 dB

if Tsys = 476 K

G

T= GdB − 10 log Tsys = 16.52 dB/K


4.3.1 Block Diagram of the Earth Station

Figure 4.8 demonstrates the main communication functions of the earth station.

Figure 4.8: Block Diagram of the Earth Station

Example 2

Assume there are three earth stations in three cites (Tripoli, Damascus and Doha) each with

20 MHz bandwidth. The center frequencies are shown in Table 4.2.

Figure 4.9 demonstrates the possible configuration to send and receive one super group

between the three cites. Also Figure 4.10 shown the Tripoli earth station. The different

frequencies are labled on the graph.


Table 4.2: The center frequencies of Uplink and Downlink for the three cities

Link City Bandwidth Center frequency

Uplink Tripoli 5930− 5950 5940

Uplink Damascus 5990− 6010 6000

Uplink Doha 6220− 6240 6230

Downlink Tripoli 3705− 3725 3715

Downlink Damascus 3765− 3785 3775

Downlink Doha 3995− 4015 4005

Example 3

Calculate the Azimuth and Elevation angles and the distance from a point 40N , 105W for

a satellite where the sub satellite point is located at 119W .

Solution:

Note that θs = 119W

θl = 105W

θ1 = 40N

Since the satellite is at west of Greenwich line

A = tan−1(tan |θs − θl|

sin θ1

)

= tan−1(tan |14|sin40

)

= 21.2

Since it is at north hemisphere

A = 180 + 21.2 = 201.2

To find Elevation angle

E = tan−1

[r − Re cos θ1 cos(θs − θl)

Re sin(

cos−1(θ1 cos(θs − θl)))

]

−cos−1(cos θ1 cos(θs−θl)) = tan−1

[r − ReZ

Re sinW

]

−W


Figure 4.9: Earth station to space station links of the three cities

where cosZ = cos θ1 cos(θs − θl) and W = cos−1 Z

From the give data

Z = cos 40cos14 = 0.7433 → cos−1 = 42 =W

E = tan−1

[r − ReZ

Re sinW

]

−W = 41.49

And

d =

[

(Re +H)2 +R2e − 2Re(Re +H) sin

E + sin−1( Re

Re +HcosE

)]0.5

Re = 6378 Km, r = 42164 Km, and H = 35786 Km

∴ d =

[

(6378 + 35786)2 + 63782 − 2× 6378(6378 + 35786)

× sin

41.49 + sin−1( 6378

6378 + 35786cos 41.49

)]0.5

= 37607 Km


Figure 4.10: Earth station at Tripoli for the above example. Note how the frequencies are

set. The difference beween the coming frequency and the fLO is always 70 MHz

4.3.2 Azimuth and Elevation Angles for Some Libyan Cities

The following table demonstrates the azimuth and elevation angles for some cities in Libya

for both Nile sat and Arab sat (Tripoli, Misurata, Sirte, Benghazi, and Sebha are chosen).

4.3.3 Very Small Aperture Terminal (VSAT)

VSAT is a digital satellite terminal that serves home and business users. So it is a private

network that can bypass the local and long-distance telephone companies. The majority of

VSAT antennas range from 74 cm to 1.2 m. Data rates, in most cases range from 4Kbps up

to 16 Mbps. VSAT can be used in Telephone, Fax, data.


City ArabSat NileSat

Elevation Azimuth Elevation Azimuth

Tripoli 47.17 149.37 45.99 214.11

Misurata 48.58 151.94 45.45 217.21

Sirte 50.48 153.59 45.57 220.11

Benghazi 50.77 160.10 42.57 223.89

Sebha 53.48 146.8 50.50 220.81

There are many types of network configurations:

(1)- Star Configuration: which has main earth station and several hub stations. This kind

the most used configuration. It has two kinds:

(a)- One way transmission: this application basically involves data distribution from the

hub outward to VSAT receive-only terminals.

(b)- Two way transmission: the most widely used application of VSAT communications.

Such a network provides complete flexibility for data exchanges.

(2)- Mesh Configuration: In this topology, all nodes are connected directly to each other

as shown in Figure 4.11. They are self organized and self configured which reduce insulation

overhead.

4.3.4 Access Techniques

Demand-Assigned Multiple Access (DAMA) DAMA is a satellite access method based

on the concept of the traffic channels that can be assigned on demand. When a VSAT

user has traffic, the hub is petitioned for a channel. If a channel is available, the


Figure 4.11: A VSAT network based on a mesh architecture.

hub assigns the channel to the VSAT. When the traffic transaction is completed, the

channel is turned back into the pool of available channels.

Pre Assignment Multiple Access (PAMA) When a nearly continuous traffic flow is

expected from a VSAT to a hub, SCPC (Signal Carrier per Channel) operation may

be an attractive alternative. In this case, each VSAT is assigned a frequency slot on a

full period basis. The bandwidth of the slot should be sufficient to accommodate the

traffic flow.

4.4 Capacity in Digital Satellite

Digital satellite capacity systems can be bandwidth limited or power limited:

Bandwidth Limited

Rb = ω +B − Cw dB

where:

ω = the bandwidth of satellite transponder(dB)

B = the bit rate to symbol rate ratio (dB)


Cw = ratio of the transponder bandwidth to the possible band-limited symbol rate

through the transponder (if no other value available, use 0.8 dB)

Example 4

A typical transponder has a bandwidth of 36 MHz, and QPSK modulation is employed.

What is the satellite link transmission bit rate?

Solution:

Rb = 75.6 + 3− 0.8 = 77.2 dB = 60.26Mbps

Power Limited If a satellite channel is power limited on the downlink, the following ex-

pression may be used to determine Rp:

Rp = (ERIP )dBw = PL +G

T−K − Eb

N0

−M dB

where:

PL = path loss of the down link

GTearth station figure of merit

K = Boltzman constant (−228.6 dBw/Hz/K)

Eb

N0

is the value of the required BER.

M = total system link margin (dB)

Example 5

Given an EIRP from a satellite transponder of 22.5 dBw, GTof the earth terminal of

40.7 dB/K, coherent QPSK modulation, an 8dB margin, which includes modulation

implementation loss, and an Eb

N0

of 9.6 dB for a BER of 10−5, what is the bit rate for

the power-limited case?


Solution:

Rp = 22.5dBw−197+40.7dB/K+228.6dBw−9.6dB−8dB = 77.2dB = 52.48Mbps


Lecture 5

Transmission Media

In a data transmission system, the transmission medium is the physical path between trans-

mitter and receiver. There are two main medium for electromagnetic waves to be transmit-

ted:

• Guided media, or (solid medium), such as copper twisted pair, copper coaxial cable,

and optical fiber.

• Unguided media wireless transmission occurs through the atmosphere, as antennas,

terrestrial microwave, Satellite, broadcast radio, Infrared, or water.

For guided transmission media, the transmission capacity, in terms of either data rate or

bandwidth, depends critically on the distance and on whether the medium is point-to-point

or multipoint. The table below indicates the characteristics typical for the common guided

media for long-distance point-to-point applications.

The three guided media commonly used for data transmission are twisted pair, coaxial

cable, and optical fiber (Figure 5.1). We briefly examine each of these in turn.

The characteristics and quality of a data transmission are determined both by the char-

acteristics of the medium and the characteristics of the signal. In the case of guided media,

99

Figure 5.1: Guided transmission media

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the medium itself is more important in determining the limitations of transmission. For

unguided media, the bandwidth of the signal produced by the transmitting antenna is more

important than the medium in determining transmission characteristics. One key property

of signals transmitted by antenna is directionality. In general, signals at lower frequencies

are omnidirectional; that is, the signal propagates in all directions from the antenna. At

higher frequencies, it is possible to focus the signal into a directional beam. In considering

the design of data transmission systems, key concerns are:

• Date rate

• Distance

The greater the data rate and distance the better.A number of design factors relating to the

transmission medium and the signal determine the data rate and distance:

Bandwidth: All other factors remaining constant, the greater the bandwidth of a signal,

the higher the data rate that can be achieved.

Transmission impairments: Impairments, such as attenuation, limit the distance. For

guided media, twisted pair generally suffers more impairment than coaxial cable, which

in turn suffers more than optical fiber.

Interference: Interference from competing signals in overlapping frequency bands can dis-

tort the signal. Interference is of particular concern for unguided media but is also a

problem with guided media. Proper shielding of a guided medium can minimize this

problem.

Number of receivers: A guided medium can be used to construct a point-to-point link or

a shared link with multiple attachments. In the latter case, each attachment introduces

some attenuation and distortion on the line, limiting distance and/or data rate.

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5.1 Twisted Pair

The least expensive and most widely used guided transmission medium is twisted pair.

Physical Description A twisted pair consists of two insulated copper wires arranged in a

regular spiral pattern. A wire pair acts as a single communication link. Typically, a

number of these pairs are bundled together into a cable by wrapping them in a tough

protective sheath. Over longer distances, cables may contain hundreds of pairs. The

twisting tends to decrease the crosstalk interference between adjacent pairs in a cable.

Neighboring pairs in a bundle typically have somewhat different twist lengths to reduce

the crosstalk interference. On long-distance links, the twist length typically varies from

5 to 15 cm. The wires in a pair have thicknesses of from 0.4 to 0.9 mm.

Applications By far the most common transmission medium for both analog and digital

signals is twisted pair. It is the most commonly used medium in the telephone network

These facilities can handle digital data traffic at modest data rates. Twisted pair is also

the most common medium used for digital signaling. Twisted pair is also commonly

used within a building for local area networks supporting personal computers. Data

rates for such products are typically in the range of 10 Mbps. However, twisted-pair

networks with data rates of to 1 Gbps have been developed, although these are quite

limited in terms of the number of devices and geographic scope of the network. For

long-distance applications, twisted pair can be used at data rates of 4 Mbps or more.

Twisted pair is much less expensive than the other commonly used guided transmission

media (coaxial cable, optical fiber) and is easier to work with.

Transmission Characteristics Twisted pair may be used to transmit both analog and

digital transmission. For analog signals, amplifiers are required about every 5 to 6 km.

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For digital transmission, repeaters are required every 2 or 3 km. Twisted pair is limited

in distance, bandwidth, and data rate.

Unshielded and Shielded Twisted Pair Twisted pair comes in two varieties: unshielded

and shielded. Unshielded twisted pair (UTP) is ordinary telephone wire. This is the

least expensive of all the transmission media commonly used for local area networks

and is easy to work with and easy to install. UTP is subject to external electromagnetic

interference, including interference from nearby twisted pair and from noise generated

in the environment. A way to improve the characteristics of this medium is to shield the

twisted pair with a metallic braid or sheathing that reduces interference.This shielded

twisted pair (STP) provides better performance at higher data rates. Figure 5.3 shows

the unshielded twisted pair. However, it is more expensive and more difficult to work

Figure 5.2: Unshield Twisted Pair (UTP)

with than UTP.

UTP Categories Most office buildings are prewired with a type of 100-ohm twisted pair

cable commonly referred to as voice grade. The new standard reflects advances in cable

and connector design and test methods. It covers 150-ohm STP and 100-ohm UTP.

Figure ?? shows the shielded twisted pair. There are three categories of UTP cabling:

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Figure 5.3: Shield Twisted Pair (STP)

• Category 3: UTP cables and associated connecting hardware whose transmission

characteristics are specified up to 16 MHz

• Category 5 (CAT5): It is a multi-pair (usually 4 pair) high performance cable

that consists of twisted pair conductors, used mainly for data transmission. Basic

CAT5 cable was designed for characteristics of up to 100 MHz. CAT5 cable is

typically used for LAN Ethernet networks running at 10 or 100 Mbps. Unshielded

Twisted Pair (UTP) construction makes the cable highly cost-effective for data

networks.

A key difference between Category 3 and Category 5 cable is the number of twists

in the cable per unit distance. Category 5 is much more tightly twisted, with a

typical twist length of 0.6 to 0.85 cm, compared to 7.5 to 10 cm for Category 3.

The tighter twisting of Category 5 is more expensive but provides much better

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performance than Category 3.

• Category 5e Cable Enhanced Category 5 (CAT5e) is designed to support full-

duplex Fast Ethernet operation and Gigabit Ethernet. Like CAT5, CAT5e is a

100 MHz standard, but it has the capacity to handle bandwidth superior to that

of CAT5.

• Category 6 Cable CAT6 provides higher performance than CAT5e and features

more stringent specifications for crosstalk and system noise.

• Category 6a Cable CAT6a, also known as Augmented Category 6, requires a cable

to operate at a minimum of 500 Mhz and provide up to 10 Gigabits of bandwidth.

• Category 7 Cable CAT7 requires a cable to operate at a minimum of 600 Mhz and

provide up to 10 Gigabits of bandwidth. To further reduce interference, CAT7

cable requires individually fully shielded twisted pairs.

The table below summarize these new cabling schemes and compare them to the ex-

isting standards.

The strength of a signal falls off with distance over any transmission medium. For

guided media attenuation is generally exponential and therefore is typically expressed

as a constant number of decibels per unit distance.

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5.2 Coaxial Cable

Coaxial cable is braided-grounded wire that can provide some shielding and noise immunity;

however, the installation and the termination of the cable itself can be costly. Coaxial

cabling, uses connectors called BNC (Bayonet Nut Connector).

Physical Description Coaxial cable, like twisted pair, consists of two conductors, but is

constructed differently to permit it to operate over a wider range of frequencies. It

consists of a hollow outer cylindrical conductor that surrounds a single inner wire

conductor (Figure 5.1-b). The inner conductor is held in place by a solid dielectric

material. The outer conductor is covered with a jacket or shield. A single coaxial cable

has a diameter of from 1 to 2.5 cm. Coaxial cable can be used over longer distances

and support more stations on a shared line than twisted pair.

Applications Coaxial cable is perhaps the most versatile transmission medium and is en-

joying widespread use in a wide variety of applications.The most important of these

are

• Television distribution

• Long distance telephone transmission.

• Local area networks.

A cable TV system can carry dozens or even hundreds of TV channels at ranges up to

a few tens of kilometers. Coaxial cable has traditionally been an important part of the

long-distance telephone network.Today, it faces increasing competition from optical

fiber, terrestrial microwave, and satellite. Using frequency division multiplexing FDM,

a coaxial cable can carry over 10, 000 voice channels simultaneously. Coaxial cable is

also commonly used for short-range connections between devices.

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Transmission Characteristics Coaxial cable is used to transmit both analog and digital

signals. Coaxial cable has frequency characteristics that are superior to those of twisted

pair, and can hence be used effectively at higher frequencies and data rates. Because

of its shielded, coaxial cable is much less susceptible to interference and crosstalk than

twisted pair. The principal constraints on performance are attenuation, thermal noise,

and intermodulation noise.The latter is present only when several channels (FDM)

or frequency bands are in use on the cable. For long-distance transmission of analog

signals, amplifiers are needed every few kilometers, with closer spacing required if

higher frequencies are used. The usable spectrum for analog signaling extends to

about 500 MHz. For digital signaling, repeaters are needed every kilometer or so, with

closer spacing needed for higher data rates.

There are different sized of coaxial cables: 0.07/0.179 inches (1.2/4.4 mm), and 0.104/0.379

inches (2.6/9.5 mm). The ratio of outer diameter of the inner conductor to inner diameter

of the outer conductor is (a/b).

The attenuation constant of the coaxial cable α is given by:

α = 1.325× 10−4√

f1a+ 1

b

log(b/a)dB/Km

where a and b are in cm and f in Hz.

frequency

α (d

B/K

m)

2.6/9.5 1.2/4.4

f1

α1

α2

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The characteristic impedance is 75 Ω for Polyethylene (ǫr = 1.43). It is given as

Z0 =138√ǫr

log10b

a= 138 log10

b

a(for air)

ba= 3.6 gives minimum attenuation constant and better Z0.

5.2.1 Coaxial Repeater Design

For microwave noise accumulation should not be more 3 pwp/km. Pico watt Psophometric

weight. PWP = PW × 0.56, or, 1PWP = −90dBm.

There are two types of noise that arise in coaxial cable systems: thermal noise and

intermodulation noise.

Thermal noise: noise generated from an active devices (as repeaters) is KTBF , in dB:

Pn = 10 logKT + 10 logB + 10 logF

where B is the baseband bandwidth. If T = 298K, then Pn = −174dBm/Hz +

10 logB +NF .

For N repeaters;

Pt = Pn +G+ 10 logN − 2.5 + L+Mg dBmp

where L is operating level in dB below 0dB with reference to the minimal operating

point, and Mg is a certain added margin. Assume a coaxial cable of length 100Km,

2pwp/Km

pt = 100× 2 = 200 pwp = 10 log200× 10−12

10−3= −67 dBmp

−67 = Pn +G+ 10 logN + L+Mg − 2.5

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assume N = 20, the space between repeaters d = 10020

= 5 Km. If the attenuation

constant α = 5dB/Km, then L = α · d = 5× 5 = 25dB = G(usually).

Pn = −174dBm/Hz + 10 logB +NF, B = 3KHz

L+Mg = 15dB needsNF = 20dB which possible to be implemented. WhileNF = 1dB

is not possible.

Increasing cable diameter by 25% leads to

L =25

1.25= 20 dB

Intermodulation Noise It has been studied before.

Cascade Noise Figure Every amplifier amplifies both the signal and the noise delivered

to the input. Since an amplifier is never ideal, it also adds some self-noise during the

amplifying process and therefore in the amplifier output there is a sum of amplified

input noise and amplifier self-noise in addition to the amplified signal. Thus, the signal-

to-noise ration always decreases between amplifier input and output. This decrease is

expressed by Noise Figure (NF) and is calculated in decibels:

NF = 10 log[ Signal to noise ratio at input

Signal to noise ratio at output

]

The lower the noise figure, the lower the amplifier self-noise is. Noise figure is different

for different frequencies, therefore for wideband devices (CATV amplifiers) many NF

values are often given. Typically NF = 4 to 9 dB, in low noise amplifiers (LNA)

NF = 0.5 to 2 dB.

For cascading amplifiers, noise figure is expressed by the formula:

F = F1 +F2 − 1

G1

+F3 − 1

G1G2+

F4 − 1

G1G2G3

+ · · ·

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in which Fn is the amplifier noise figure, Gn is the amplifier gain and n is the amplifier

order number in a cascaded circuit. It could be seen that the first amplifier (F1) has

the greatest effect for overall noise figure. Thus, it is very important to use an amplifier

with low noise figure at the beginning of a cascaded circuit. It is also significant to keep

a cable between an antenna and the first amplifier as short and lossless as possible.

5.2.2 The Choice Between Coaxial and Microwave Link

Table 5.1 illustrates comparison between coaxial and microwave

Table 5.1: Comparison between coaxial and waveguide

Coaxial link Microwave link

Land acquisition complicated simple

Drop and insert better less

Fading better (no effect) more

noise accumulation less more

RF interference none major

Limitation on No. of carriers more limited

Repeater spacing 1.5, 4.5, 9 km 30− 50 km

Maintenance simpler more involved

Repeater cost cheaper more expensive

Power requirement less feeding point more feeding point

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5.2.3 Coaxial Versus Waveguide

Two types of transmission line are used in radio link terminals and repeaters: coaxial cables

and waveguides

Coaxial cable it is easier to install, its loss increases exponentially with frequency, and so

it is upper limit of application is in the range of 2− 3 GHz.

• Attenuation or loss is the most parameter.

• VSWR (Voltage Stand Wave Ratio); return loss or mismatch loss.

• Power rating of the line is a function of material and structure of the inner and

outer connectors, it may cause voltage breakdown (not function of frequency)

Waveguide it is superior to coaxial cable in attenuation characteristics at higher frequencies

and it will handle higher power level.

• for lower frequencies (below 3 GHz, the choice between coaxial and waveguide is

economic.

• three types of waveguide are commonly used:

– rectangular: common associated with microwave installation

– elliptical (flex): used for frequency > 20 GHz

– circular: it has minimum loss

The last two types are favored because of the low cost.

5.3 Optical Fiber

An optical fiber cable has a cylindrical shape and consists of three concentric sections: the

core, the cladding, and the jacket (Figure 5.1-c). The core is the innermost section and

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consists of one or more very thin strands, or fibers, made of glass or plastic; the core has a

diameter in the range of 8 to 100µm. Each fiber is surrounded by its own cladding, a glass

or plastic coating that has optical properties different from those of the core.The interface

between the core and cladding acts as a reflector to confine light that would otherwise

escape the core. The outermost layer, surrounding one or a bundle of cladded fibers, is

the jacket.The jacket is composed of plastic and other material layered to protect against

moisture, abrasion, crushing, and other environmental dangers.

Physical Description An optical fiber is a thin (2 to 125 µm ), flexible medium capable

of guiding an optical ray.Various glasses and plastics can be used to make optical

fibers. The lowest losses have been obtained using fibers of ultrapure fused silica.

Ultrapure fiber is difficult to manufacture; higher-loss multicomponent glass fibers are

more economical and still provide good performance. Plastic fiber is even less costly

and can be used for short-haul links, for which moderately high losses are acceptable.

Applications Optical fiber enjoys considerable use in long-distance telecommunications,

and its use in military applications is growing. The continuing improvements in per-

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formance and decline in prices, together with the inherent advantages of optical fiber,

have made it increasingly attractive for local area networking.The following character-

istics distinguish optical fiber from twisted pair or coaxial cable:

• Greater capacity: Data rates of hundreds of Gbps over tens of kilometers have

been demonstrated. Compare this to the practical maximum of hundreds of Mbps

over about 1 km for coaxial cable and just a few Mbps over 1 km or up to

100 Mbps to 1 Gbps over a few tens of meters for twisted pair. ]item Smaller size

and lighter weight: Optical fibers are considerably thinner than coaxial cable or

bundled twisted-pair cables at least an order of magnitude thinner for comparable

information transmission capacity. The corresponding reduction in weight reduces

structural support requirements.

• Lower attenuation: Attenuation is significantly lower for optical fiber than for

coaxial cable or twisted pair and is constant over a wide range.

• Electromagnetic isolation: Optical fiber systems are not affected by external elec-

tromagnetic fields. Thus the system is not vulnerable to interference, impulse

noise, or crosstalk. By the same token, fibers do not radiate energy, so there is

little interference with other equipment and there is a high degree of security.

• Greater repeater spacing: Fewer repeaters mean lower cost and fewer sources of

error. The performance of optical fiber systems from this point of view has been

steadily improving. Repeater spacing in the tens of kilometers for optical fiber

is common, and repeater spacings of hundreds of kilometers have been demon-

strated. Coaxial and twisted-pair systems generally have repeaters every few

kilometers.

Five basic categories of application have become important for optical fiber:

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• Long-haul trunks

• Metropolitan trunks

• Rural exchange trunks

• Subscriber loops

• Local area networks

Long-haul fiber transmission is becoming increasingly common in the telephone net-

work. Long-haul routes average about 1500 km in length and offer high capacity (typ-

ically 20, 000 to 60, 000 voice channels). These systems compete economically with

microwave and have so underpriced coaxial cable in many developed countries that

coaxial cable is rapidly being phased out of the telephone network in such countries.

Undersea optical fiber cables have also enjoyed increasing use.

Metropolitan trunking circuits have an average length of 12 km and may have as

many as 100, 000 voice channels in a trunk group. Most facilities are installed in under-

ground conduits and are repeaterless, joining telephone exchanges in a metropolitan or

city area. Included in this category are routes that link long-haul microwave facilities

that terminate at a city perimeter to the main telephone exchange building downtown.

Rural exchange trunks have circuit lengths ranging from 40 to 160 km and link

towns and villages. In the United States, they often connect the exchanges of different

telephone companies. Most of these systems have fewer than 5000 voice channels. The

technology used in these applications competes with microwave facilities.

Subscriber loop circuits are fibers that run directly from the central exchange to

a subscriber. These facilities are beginning to displace twisted pair and coaxial cable

links as the telephone networks evolve into full-service networks capable of handling

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not only voice and data, but also image and video. The initial penetration of optical

fiber in this application is for the business subscriber, but fiber transmission into the

home will soon begin to appear.

A final important application of optical fiber is for local area networks. Standards

have been developed and products introduced for optical fiber networks that have a

total capacity of 100 Mbps to 10 Gbps and can support hundreds or even thousands of

stations in a large office building or a complex of buildings. The advantages of optical

fiber over twisted pair and coaxial cable become more compelling as the demand for

all types of information (voice, data, image, video) increases.

Transmission Characteristics Optical fiber transmits a signal-encoded beam of light by

means of total internal reflection.Total internal reflection can occur in any transparent

medium that has a higher index of refraction than the surrounding medium. The

optical fiber acts as a waveguide for frequencies in the range of about 1014 to 1015 Hz;

this covers portions of the infrared and visible spectra.

The propagation are:

• single-mode

• step-index multimode

• graded-index multimode.

Two different types of light source are used in fiber optic systems:

• Light Emitting Diode (LED): It is less costly, operates over a greater temperature

range, and has a longer operational life

• Injection Laser Diode (ILD): It operates on the laser principle, is more efficient

and can sustain greater data rates.

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References

[1 ]- Roger L. Freeman “Radio System Design for Telecommunications”, Published by

John Wiley & Sons, Third Edition, 2007.

[2 ]- Roger L. Freeman “Telecommunication System Engineering”, Published by John

Wiley & Sons, Fourth Edition, 2004.

[3 ]- Bruce R. Elbert, “Introduction to Satellite Communication”, Artech House , Inc.

Third Edition, 2008

[4 ]- Thiagarjan Viswanathan, “Telecommunication Switching Systems and Networks”,

Twenty Sixth Edition, 2006.

[5 ]- John C. Bellam, “Digita Telephony”, John Wiley & Sons Inc, Third Edition, 2000.

[6 ]- Bruce A. Carlson, “Communication Systems: An Introduction to Signals and Noise

in Electrical Communication”, McGraw Hill, Fourth Edition, 2002.

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Dr. Mohamed Elalemmelalem.com/docs/ECE568_Notes.pdf · Advanced Communication Systems by Dr. Mohamed Elalem Lectures on Advanced Communication Systems Electrical and Computer Engineering