SATELLITE COMMUNICATION LECTURE NOTES - … COMMUNICATION LECTURE NOTES World Institute Of Technology 8km milestone ,Sohna Palwal Road , NH-71 B ,Sohna Website : E-mail : [email protected]

World Institute Of Technology

8km milestone ,Sohna Palwal Road , NH-71 B ,Sohna , Gurgaon ,Haryana.

Website : www.wit.net.in E-mail : [email protected]

SATELLITE

COMMUNICATION

LECTURE NOTES




Lecture: 1 Principles of satellite comm.

Evolution and growth of comm. satellites










Synchronous Satellite




Lecture 2

Satellite frequency allocation and band spectra







Lecture 3

Advantages of satellite communication










Lecture4




Lecture 5

MODEM

A modem (modulator-demodulator) is a device that modulates an analog carrier signal to

encode digital information, and also demodulates such a carrier signal to decode the transmitted

information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce

the original digital data. Modems can be used over any means of transmitting analog signals, from

driven diodes to radio.

The most familiar example is a voice band modem that turns the digital data of a personal

computer into modulated electrical signals in the voice frequency range of a telephone channel. These

signals can be transmitted over telephone lines and demodulated by another modem at the receiver

side to recover the digital data. Modems are generally classified by the amount of data they can send

in a given time unit, normally measured in bits per second (bit/s, or bps). They can also be classified

by the symbol rate measured in baud, the number of times the modem changes its signal state per

second. For example, the ITU V.21 standard used audio frequency-shift keying, aka tones, to carry 300

bit/s using 300 baud, whereas the original ITU V.22 standard allowed 1,200 bit/s with 600 baud

using phase shift keying. A satellite modem or sat modem is a modem used to establish data

transfers using a communications satellite as a relay. There is a wide range of satellite modems from

cheap devices for home internet access to expensive multifunctional equipment for enterprise use.A

"modem" stands for "modulator-demodulator". A satellite modem's main function is to transform an

input bit stream to a radio signal and vice versa. There are some devices that include only a




demodulator (and no modulator, thus only allowing data to be downloaded by satellite) that are also

referred to as "satellite modems". These devices are used in satellite Internet access (in this case

uploaded data is transferred through a conventional PSTN modem or an ADSL modem

CODEC




Lecture 6:

Unit2: Communication satellite design link

General link design equation:




Consider a transmitting source, in free space, radiating a total power Pt W

uniformly in all directions. At a distance R m from the source, the flux density

crossing the surface of a sphere with radius R is given by

F = Pt / 4 πR2

If the transmitting antenna has a gain Gt ,

Then the flux density becomes F = Pt Gt / 4 πR2

Where Pt Gt is called the effective isotropically radiated power.

If the receiving antenna has an effective aperture Ae , then power received by

the receiving antenna is

Pr = Pt Gt Ae / 4 πR2

W

Gain and area of an antenna are related by

G = 4 π Ae / λ2

where λ is the wavelength at the frequency of operation.

Hence Pr becomes,

Pr = Pt Gt Gr/ ( 4 πR/ λ 2

) W




This expression is known as the link equation.

In decibel terms, Pr = EIRP + Gr - Lp

Where Lp = ( 4 πR/ λ 2

) is the path loss which accounts for the

way energy spreads out as an EM wave travels away from a transmitting source

in three-dimensional space.

More generally, the link equation would be

Pr = EIRP + Gr - Lp – La - Lra - Lta dBW

Where La is the attenuation in atmosphere

Lta is losses associated with the transmitting antenna

Lra is losses associated with the receiving antenna




Lecture 7 and 8

System noise temperature,C/N and G/t ratio




�/N=PrG/KTsBG

C/N= Pr/ KTsBG

Where Ts representsthe several sources of noise in the receiver in the form of

system noise temperature.

Let Rf amplifier ,downconverter,IF amplifier by noiseless units with their

respective equivalent noise temperature as Trf,Tm,Tif.Gr,Gm,Gif be the gains.

Pn =Gif K Tif B + Gif Gm K Tm B + Grf Gif Gm K B (Trf+Tin)

Pn= Grf Gif Gm[KTifB/GifGm+KTmB/Grf+kB((Trf+Tin)

Pn = Grf Gif Gm K Ts B

Final equation that we will get




KTSB=KB[Trf+Tin+Tm/GRF+Tif/GmGrf

Ts=[Trf+Tin+Tin/GRF+Tif/GmGRf)

C/N=PTGTGR(λ/4πd)2/KTSBLA

(C/N0) = 10logPTGT-20log (4πd/λ)+ 10log(GR/TS)-10logLA-10logK

In the above equations GR/TS represents a ratio (dbk-1)that can be used to

specify the quality of an earth station.This GR/TS is called figure of merit.

Lecture 9:

Atmospheric & Ionospheric effects on link design










Lecture 10:

Complete link design







Lecture11:

Earth Station parameter

�� .� ��.

��

Here d is the range between transmitting and receiving anteena.The antenna

parameters such as its diameter D and antenna beam width Θ3DB affect the

C/N0 as these are linked to the gain by expressions given by

G = ŋ� �� . ��

G = ŋ ��




Where ŋ is the antenna efficiency. The satellite antenna beam width �� is

determined by the required earth coverage. for a given earth station diameter

DES,

�� ŋ� ŋ ��

where ŋ� and ŋ� are the effeciencies of transmmiting and receiving

anteenas respectively.However ,if earth satation anteena beamwidth �� is fixed,C/N0 is given by: �� ŋ��!�� ŋ��"#$� �� %� ��&'�

()*+, ,-.(/0*1 23*42 ��* 01�5,(2,2 40/3 6*4 %,5-.,1�0,2. In the case antenna diameter DSAT and DES are given ,the expression

for C/NO reduces to




Lecture 12:

Unit 3: Analog satellite communication




Baseband signal:

FDM techniques







Lecture 13:

S/N and C/N ratio in frequency modulation in satellite link













Lecture 14

S/N ratio in FM with multiplexed telephone signal In satellite link







Single channel per carrier

Single channel per carrier (SCPC) refers to using a single signal at a

given frequency and bandwidth. Most often, this is used onbroadcast satellites to indicate

that radio stations are not multiplexed as subcarriers onto a single video carrier, but

instead independently share a transponder. It may also be used on other communications

satellites, or occasionally on non-satellite transmissions.

In an SCPC system, satellite bandwidth is dedicated to a single source. This makes sense if

it is being used for something like satellite radio, which broadcasts continuously. Another

very common application is voice, where a small amount of fixed bandwidth is required.

However, it does not make sense for burst transmissions like satellite internet access

or telemetry, since a customer would have to pay for the satellite bandwidth even when

they were not using it.Where multiple access is concerned, SCPC is essentially FDMA.

Some applications use SCPC instead of TDMA, because they require guaranteed,

unrestricted bandwidth. As satellite TDMA technology improves however, the applications

for SCPC are becoming more limited.

Advantages

• simple and reliable technology

• low-cost equipment

• any bandwidth (up to a full transponder)

• usually 64 kbit/s to 50 Mbit/s

• easy to add additional receive sites

Disadvantages

• inefficient use of satellite bandwidth for burst transmissions, typically encountered with packet data transmission

• usually requires on-site control

• When used in remote locations, the transmitting dish must be protected.




• A dish which is moved out of alignment can result in fines as high as $1,100 per minute (as of 2003) from the satellite operator

Lecture no 15:

Companded single sideband systems







Lecture no16:

Analog FM/FDM TV satellite link:

For satellite Tv transmission the baseband video signal frequency modulates a video carrier and the

two audio signals frequency modulate two audio carriers. The details of the video modulation

depends on the transponder BW available. Typical values for network TV are a peak deviation ΔFp of

10.75 MHz and maximum video modulating frequency fv of 4.2 MHZ.

The performance of FM-FDM television channel is expressed in terms of peak to peak luminance

signal to noise ratio .For a sine wave, peak to peak power is (�√�)2 times the rms power. The peak to

peak value of luminance signal is 1/√�) times the peak to peak value of the composite television

channel

S/N= (�√�)2 (1/√��(C/N)i.3/2.

8%9(:(;%<,(=%9(: )

2 pq

= 6(C/N)( 8%9(:) (;%<,(=

%9(: )2 pq

Where p is the pre emphasis factor, q is the weighting factor, fmax is the maximum video modulating

frequency. And modulation index m is the modulation index which is the ratio of ;%<,(=

%9(:

Eq can be written as:

(S/N)pp=6(C/N)I 3(1+m)2pq

Intermodulation products & their effects in FM/FDM system






















Lecture 17

Energy disposal in FM/FDM system




Lecture18:

Advantages of digital communication




Advantages of digital communication




Lecture19:

Elements of Digital satellite communication system






















Lecture20

Digital baseband signals







Digital modulation techniques:

Techniques are as follows:

1.BPSK

2.DPSK

3.QPSK

BPSK
















Lecture 21

Satellite digital link design :










Time Division Multiplexing:




Lecture22

Unit5: Multiple access technique




























‘

LECTURE 23










Lecture 24




TDMA compared to FDMA







Lecture 25

TDMA Burst time plan







Lecture 26

Multibeam TDMA satellite system

This is about the antennas that are used in satellite communication. There may

be a zone beam antenna or spot beam antenna at the satellite. Zone beam

antenna can receive from transmitters anywhere within the coverage zone

whereas the spot beam antennas have limited range. Thus, INTELSAT V there

are 4 beams at c band interconnected by 80MHz BW transponders in which the

beam interconnections are static and switched occasionally to accommodate

major adjustments in traffic patterns.

Thus narrowing the satellite beam antenna beam width increases

communications capacity .It provides a high gain for the coverage area and

hence permits power savings in both uplink and downlinks. The use of spot

beam has further advantage that the same frequency can be spatially reused

many times. Thus, if India is covered by N non overlapping spot beam, satellite

capacity will increase N fold over the achieved using one beam.

The main problem in the interconnection of UBS and DBS in the satellite

communication system such that of INTELSAT VI is their interconnectivity. This is

achieved by introducing an onboard switching technique called SATELLITE

SWITCH. The corresponding system would therefore called satellite switched

TDMA system. This satellite switch is actually the dynamic satellite switching

that uses a microwave switch matrix onboard the satellite. The satellite switch

can interconnect the six can interconnect the six up beams to six down beams in

accordance with the desired single point connectivity matrix pattern for dwell

interval of programmable location and duration. For rf case switching matrix




mentioned above is utilized. The output signals from the matrix are amplified

eventually fed to the antennas .The switching matrix is steered by an inboard

Distribution control unit and acquisition and synchronization unit provides the

time refrences necessary for TDMA operation.

Beam hopping(Transponder Hopping)TDMA

In the beam hopping TDMA the switching is achieved by each station hopping

from one frequency to another according to the destination of the transmitted

burst.Each earth station selects its transmission time on the corresponding

frequency so as to transmit its burst within the correct time slot. It should be

noted that the beam hopping is also carried out along with the on-board signal

regeneration .This helps in avoiding the interburst interferences associated with

conventional TDMA .

CDMA and hybrid access technique

In CDMA access the whole BW of the transponder is used all the time and

signals from the users are encoded so that the information from an individual

transmitter can be detected and recovered only by a properly synchronized

receiving station that knows the code being used. Thus while the capacity of the

communication is increased by the multiple access, the secrecy of transmission

is maintained.This has the potential applications in military satellite

communication.




Lecture 27

Unit6: Satellite Orbits

Introduction

The orbit of a satellite in use for communication purposes has special significance.Its

geostationary and has to be maintained geostationary at all costs.

Thus there are two main problems involved in communication satellites regarding

orbit, namely (i)Launching and putting the satellite into geostationary

orbit(ii)maintaining it

Once the satellite is placed in geostationary orbit,it has to be parked in desired slot till

its whole life. Communication satellite move around the earth as planets do around

the sun and therefore three keplers law apply on them




1. Kepler’s first law states that, the path followed by a satellite around a primary is

elliptical with the center of masses at one of the foci.

2. Kepler’s second law states that, for equal time intervals, a satellite will sweep

equal areas in its orbital plane.

3. Kepler’s third law states that, the square of the periodic time of orbit is

proportional to the cube of the mean distance between the two bodies.

Synchronous orbit

A geostationary satellite is synchronous and has an equatorial circular and direct

orbit. At geostationary altitude and orbital velocity of satellite equals the velocity

of a point on earth’s equator. Let M and m be the mass of earth and satellite

respectively. similarly, let RE be the radius of synchronous circular orbit of satellite.

If w is the angular velocity of the circular motion of satellite in radians per second

then centripetal force acting on the satellite would be mRw2.

This centripetal force equal to the geostationary force b/w the satellite and earth

expressed by GmM/R2.

94� � 9>�

At the earth’s surface the gravitational force in mg where g is the acceleration due

to the gravity.

From above two equations

R=[ ?��4� ]1/3

Where f=w/2π is the rotation rate n revolution per second. Putting f=1 revolution per

day and RE =6370Km,g=9.8,one obtains R=42,208Km.Subtracting RE form R one gets

35,838 km as the orbital height above the equator which is quite close to the precise

value of 35,860.




Lecture 28

Orbital parameters:

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The satellite’s movement in an orbit follows the three kepler’s law.The first kepler’s lkaw gives that the satellite moves along a conic on the orbit plane

Where p is a parameter,e is the eccentricity and v is the vertical angle.

Value of eccentricity e determines the type of conic



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r=p/(1+ecosv)

Where p is a parameter,e is the eccentricity and v is the vertical angle.

Value of eccentricity e determines the type of conic

PHYSICAL LAWSKepler’s 1st Law: Law of Ellipses

The orbits of the planets are ellipses with the sun at one focus

PHYSICAL LAWSKepler’s 1st Law: Law of Ellipses

The orbits of the planets are ellipses with the sun at one focus

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e = @ *— �05�6,< 1— DEEFGHD�— <(5()*6(I but if e>1 then its hyperbola.

The elliptical orbit has been depicted.Here a is the semi-major axis and is linked to the orbital parameter by the expression

T = 2πJ(�K�

Where µ is the quantity equal to GM,where G being the gravitional constant.eqaul to 6.6728*10-11 m3kg-1s-2. And M be earth’s mass equals to the 5974*1024kg

Satellite velocity for the circular orbit is JK( where a is the orbit radius.

L2 � = 2

K5 -

K(

So following should be remembered:

x=rcosv =a(cos.εεεε-e)

y=rsinv = a sinεεεε(1-e2)-1/2

r = p/(1+ecosv)

e= (ra - rp) /(ra + rp )

Area of the ellipse = ππππab.

Semi major axis = a = (ra+rp)/2

Apogee distance = ra =a (1+e)

Perigee distance = rp =a (1-e)




Locus parameter = a (1-e2)

A synchronous satellite has T as sidereal period of rotation of its primary body T P.

T = TP = M1 N OPQRSTU* 24h = 86163,44 s = 23h 56 m 4 s

Satellite location wrt Earth:







Lecture 29:

Look angles










Earth coverage & slant range,







ECLIPSE EFFECT













Lecture30

Satellite placement in geostationary orbit:










Lecture 31:




station keeping




Satellite stabilization:







Lecture32:

Unit 7: Special purpose satellite

BDS:

Direct broadcast satellite (DBS) refers to satellite television (TV) systems in which the subscribers, or end users, receive signals directly from geostationary satellites. Signals are broadcast in digital format at microwave frequencies. DBS is the descendant of direct-to-home (DTH) satellite services. A DBS subscriber installation consists of a dish antenna two to three feet (60 to 90 centimeters) in diameter, a conventional TV set, a signal converter placed next to the TV set, and a length of coaxial cable between the dish and the converter. The dish intercepts microwave signals directly from the satellite. The converter produces output that can be viewed on the TV receiver. A number of companies provide DBS and DTH service throughout the world. In the United States, DirecTV and EchoStar are the main vendors. DIGITAL DBS TV Digital TV signals are transmitted a s a 20 Mbps QPSK




signal occupying about 27 MHz of transponder bandwidth. The 40 Mbps signal has a data rate between 23 and 27 Mbps with the remaining bits used for error control and system operation. DBS-TV signals make extensive use of error correction and error detection techniques in the form of a double layer of error control coding with interleaving. Each DBS-TV satellite carries up to 32 high power transponders covering part of the 12.2 to 12.7 GHz broadcast satellite band (BSS), and the satellites at each orbit location transmit in opposite hands of circular polarization (CP). Signals with opposite hands of circular polarization are orthogonal, and a suitably designed earth station antenna can separate two signals with opposite hands of circular polarization. In the simplest DBS- TV receiving terminal, an electronically controlled polarizer is used immediately behind the antenna feed. The polarizer can be set to receive LHCP (left-hand circular polarization) or RHCP by changing the voltage supplied to the low noise block converter (LNB) unit at the antenna. The polarizer converts the circularly received polarization signal to a linearly polarized signal in a section of waveguide, and a linear probe in the waveguide converts the signals to currents that drive the LNA input. Customers wanting to receive signals from more than one orbital location need an antenna with two feeds. DBS-TV receiving antennas are typically an offset parabolic reflector design with the feed below the antenna aperture. The offset feed design eliminates blockage of aperture by the feed which occurs in symmetrical reflector antenna designs, and improves the aperture efficiency of the antenna, and therefore increases its gain. Offset fed parabolic reflectors have a beam squint effect in the plane of symmetry when operated in opposite hands of circular polarizations.

DBS TV RECEIVER: Fig below shows a block diagram of a DBS-TV receiver. The entire front end of the receiver is located at the antenna feed in the form of an LNB to minimize loss of signal and hence to maintain the lowest possible system noise temperature. The electronic polarizer is switched by changing the voltage supplied to the LNB via the cable that interconnects the antenna and set-top receiver. The entire 12.2 to 12.7 GHz band is downconverted by the LNB to the 900-1400 MHz band, where cable losses are much lower than at Ku band. The down-converter consists of a dielectric resonator local oscillator and mixer, followed by an IF



amplifier and band-pass filter. The high gain LNB can drive 100 m of coaxial cable without any reduction in signal quality. Where longer cable amplifiers for the 900-1400 MHz band can be used to boost the signal strength. The set-top box accepts the entire 500 MHz band and separates the individual transponder frequencies. Any one of these frequencies can be selected on demand by the user.

The user enters a desired channel number into the setcontrol, for example, channel 362, which is converted via a stored lookthe receiver to an RF-channel frequency and polarization. The signal from the required transponder is then selected by the receiver by setting the correct polarization at the antenna and tuning the setIF channel frequency. The QPSK signal is then demodulated. The result is a multiplexed bit stream, typically at a rate up to 40 Mbps, which contains the bits for




pass filter. The high gain LNB can drive 100 m of coaxial cable without any reduction in signal quality. Where longer cable runs are needed,

1400 MHz band can be used to boost the signal strength. The top box accepts the entire 500 MHz band and separates the individual

transponder frequencies. Any one of these frequencies can be selected on demand by

The user enters a desired channel number into the set-top box using an IR remote control, for example, channel 362, which is converted via a stored look

channel frequency and polarization. The signal from the equired transponder is then selected by the receiver by setting the correct

polarization at the antenna and tuning the set-up local oscillator to the appropriate IF channel frequency. The QPSK signal is then demodulated. The result is a

eam, typically at a rate up to 40 Mbps, which contains the bits for



pass filter. The high gain LNB can drive 100 m of coaxial cable runs are needed,

1400 MHz band can be used to boost the signal strength. The top box accepts the entire 500 MHz band and separates the individual

transponder frequencies. Any one of these frequencies can be selected on demand by

top box using an IR remote control, for example, channel 362, which is converted via a stored look-up table in

channel frequency and polarization. The signal from the equired transponder is then selected by the receiver by setting the correct

up local oscillator to the appropriate IF channel frequency. The QPSK signal is then demodulated. The result is a

eam, typically at a rate up to 40 Mbps, which contains the bits for




channel 362 and several other video signals. The bit stream is encrypted and contains error control coding bits and data bits. The bit stream is processed to correct and detect errors, de-interleaved, and decrypted. A digital demultiplexer then extracts the bits for the wanted channel, 362 in this example, sends them to a MPEG 2 decoder, and finally generates analog video and audio signals with D/A converters to drive the TV set.

INMARSAT

INMARSAT is an acronym of International Maritime Sa tellite Organization with headquarters at London. It is an international organization that controls satellite systems for communication between ships and coasts so that emergency life saving services may provide to the distressed people. It came into being at midnight on 15th/16th July 1979.INMARSAT has 85 member states. The USA maintains its lead in INMARSAT by holding maximum share in it. Signatures from member countries own INMARSAT, they contribute to the costs of organization & develop its policies.




INTELSAT

VSAT:



A VSAT consists of two parts, a transceiver that is placed outdoors in direct line of sight to the satellite and a device that is placed indoors to with the end communications device, such as a PC. The transceiver receives or sends a signal to a satellite transponder in the sky. The satellite sends and receives signals from a ground station computer that acts as a hub for the

Each end user is interconnected with the hub station via the satellite, forming a star topology. The hub controls the entire operation of the network. For one end user to communicate with another, each transmission has to first go to the hub statithen retransmits it via the satellite to the other end user's VSAT.

The fig above shows a schematic of a VSAT/WLL network. The Geostationary satellite is used to link a large number of VSATs with the main switching centre in a




A VSAT consists of two parts, a transceiver that is placed outdoors in direct line of sight to the satellite and a device that is placed indoors to interface the transceiver with the end communications device, such as a PC. The transceiver receives or sends a signal to a satellite transponder in the sky. The satellite sends and receives signals from a ground station computer that acts as a hub for the system.

Each end user is interconnected with the hub station via the satellite, forming a star topology. The hub controls the entire operation of the network. For one end user to communicate with another, each transmission has to first go to the hub statithen retransmits it via the satellite to the other end user's VSAT.




A VSAT consists of two parts, a transceiver that is placed outdoors in direct line of interface the transceiver

with the end communications device, such as a PC. The transceiver receives or sends a signal to a satellite transponder in the sky. The satellite sends and receives signals

system.

Each end user is interconnected with the hub station via the satellite, forming a star topology. The hub controls the entire operation of the network. For one end user to communicate with another, each transmission has to first go to the hub station that





large city. Each VSAT acts as a link to the local switching centre in the village or rural community, with the final mile of the telephone link being carried over a wireless local loop.

Specifications of VSAT systems:

• They operate in Ku band.

• Earth station diameters range from 1-2 m.

• Transmitter power: 1-2 W.

• Data rate: Few thousand bps to 256 kbps.

VSAT systems do not transmit all the time. Hence a dedicated satellite need not be used. Also DAMA (Demand Assigned Multiple access) is employed in some systems in which channel capacity is assigned in response to fluctuating demands of the VSATs in the network.

Network Architectures

VSAT systems can be implemented in 3 basic architectures.

• One-way implementation.

• Split-two-way implementation.

• Two way implementation.

One-way implementation



This is the mode of a satellite used in the broadcast satellite service (BSS). The introduction of digital telephony allows the provider and user much greater flexibility in the operation of a broadcast network. Fig 1 shows the schematic of BSS.

Split-two-way implementation

This implementation is used when there is no normal return channel (E.g.: with Ku band broadcast satellite service systems that carry internet traffic). The relatively high capacity downlink stream is not the user terminal. If the BSS downlink is used as the download channel from an internet service provider, the only option the user has for a return link is via another telecommunications channel, such as a standardInternet Protocol (IP) is therefore split between a satellite downlink (outbound) channel and a terrestrial telephone (inbound) channel: hence the term split IP for this implementation. The advantage of this approach is that the VSAT tnot require a transmit capability, which significantly reduces its cost and complexity. The disadvantage is that the telephone line connection must usually be through a modem, with a bit rate generally restricted to 56 kbps.

Two way implementation

The service includes a return link so as to establish twothe same satellite, from the hub to the user and from the user back to the hub. Two topologies are implemented: Star topology and Mesh topology.




This is the mode of a satellite used in the broadcast satellite service (BSS). The introduction of digital telephony allows the provider and user much greater flexibility in the operation of a broadcast network. Fig 1 shows the schematic of

way implementation

This implementation is used when there is no normal return channel (E.g.: with Ku band broadcast satellite service systems that carry internet traffic). The relatively high capacity downlink stream is not complemented by an uplink capability from the user terminal. If the BSS downlink is used as the download channel from an internet service provider, the only option the user has for a return link is via another telecommunications channel, such as a standard telephone line. The Internet Protocol (IP) is therefore split between a satellite downlink (outbound) channel and a terrestrial telephone (inbound) channel: hence the term split IP for this implementation. The advantage of this approach is that the VSAT tnot require a transmit capability, which significantly reduces its cost and complexity. The disadvantage is that the telephone line connection must usually be through a modem, with a bit rate generally restricted to 56 kbps.

The service includes a return link so as to establish two-way communication over the same satellite, from the hub to the user and from the user back to the hub. Two topologies are implemented: Star topology and Mesh topology.



This is the mode of a satellite used in the broadcast satellite service (BSS). The introduction of digital telephony allows the provider and user much greater flexibility in the operation of a broadcast network. Fig 1 shows the schematic of a

This implementation is used when there is no normal return channel (E.g.: with Ku band broadcast satellite service systems that carry internet traffic). The relatively

complemented by an uplink capability from the user terminal. If the BSS downlink is used as the download channel from an internet service provider, the only option the user has for a return link is via

telephone line. The Internet Protocol (IP) is therefore split between a satellite downlink (outbound) channel and a terrestrial telephone (inbound) channel: hence the term split IP for this implementation. The advantage of this approach is that the VSAT terminal does not require a transmit capability, which significantly reduces its cost and complexity. The disadvantage is that the telephone line connection must usually be through a modem, with a bit rate generally restricted to 56 kbps.

way communication over the same satellite, from the hub to the user and from the user back to the hub. Two



Star topology:Here, all of the traffic is routed via the master control station, or hub. If a VSAT wishes to communicate with another VSAT, they have to go via the hub, thus necessitating a “double hop” link via the satellite. Since all of the traffic radiates at one time or anothnetwork. Disadvantage: (1) Communication between two VSATs requires a hop thus increasing the time delay. (2) Since the low receive G/T of the VSATs , coupled with their limited transmit EIRPwith high G/T and EIRP, the cost of the hub used is very high.

Mesh topology:Here, each of the VSATs has the capability to directly communicate with any of the other VSATs. The network control and the duties of the handled by one of the VSATs or the master control station functions are shared among the VSATs. Since the traffic can go to or from any of the VSATs, this architecture is referred to as Mesh network.

MSAT




f the traffic is routed via the master control station, or hub. If a VSAT wishes to communicate with another VSAT, they have to go via the hub, thus necessitating a “double hop” link via the satellite. Since all of the traffic radiates at one time or another from the hub, this architecture is referred to as Star network. Disadvantage: (1) Communication between two VSATs requires a

increasing the time delay. (2) Since the low receive G/T of the VSATs , coupled with their limited transmit EIRP is compensated for by using a large hub with high G/T and EIRP, the cost of the hub used is very high.

Here, each of the VSATs has the capability to directly communicate with any of the other VSATs. The network control and the duties of the handled by one of the VSATs or the master control station functions are shared among the VSATs. Since the traffic can go to or from any of the VSATs, this architecture is referred to as Mesh network.

fig a fig b.



f the traffic is routed via the master control station, or hub. If a VSAT wishes to communicate with another VSAT, they have to go via the hub, thus necessitating a “double hop” link via the satellite. Since all of the traffic

er from the hub, this architecture is referred to as Star network. Disadvantage: (1) Communication between two VSATs requires a double

increasing the time delay. (2) Since the low receive G/T of the VSATs , is compensated for by using a large hub

Here, each of the VSATs has the capability to directly communicate with any of the other VSATs. The network control and the duties of the hub is either handled by one of the VSATs or the master control station functions are shared among the VSATs. Since the traffic can go to or from any of the VSATs, this

fig a fig b.







Lecture 33:

SARSAT:

Low earth orbit satellite(LEO)




Satellite communication wrt fibre optics:

Characteristics Fiber optic System Satellites Bandwidth Bw of it is

1terahertz

Bw of 36,54,72 MHz

Immunity to interference

Immune to Em interference

Transmission subject to interference from various sources including microwaves.

Durability Storms can knock down overhead lines

Storms can disable individual anteenas but leave network intact.

Security Difficult to tap without detection

Signals are encoded for security.

Multipoint capabilities

Primarily a point to point media

Point to multipoint

Flexibility Difficult to Easy to reconfigure




Lecture 34:

LANDSAT:

LANDSAT-I was first satellite launched in 1972 by NASA followed by INTELSAT-2 ,3,4,5,6 and 7.

reconfigure to meet changing demands.

if hardware has been apprx designed.

Connectivity to customer size

Local loop required.

With antennas installed on customer premises ,as with 14/12 GHz band, local loops not required.




LANDSAT is one of the earth exploration satellite service .This system involves observation of the earth for various purposes,e.g. meterological,geodesy,data collection.




Defense satellite : Mission:The Defense Satellite Communications Systems (DSCS) is an important part of the

comprehensive plan to support globally distributed military users.

Features:Air Force Space Command operates ten Phase III DSCS satellites that orbit the earth

at an altitude of more than 22,000 miles. Each satellite uses six super high frequency

transponder channels capable of providing secure voice and high rate data communications.

DSCS III also carries a single-channel transponder for disseminating emergency action and

force direction messages to nuclear-capable forces.

The system is used for high priority command and control communication such as the

exchange of wartime information between defense officials and battlefield commanders. The

military also uses DSCS to transmit space operations and early warning data to various

systems and users.Background:The Air Force began launching the

DSCS IIIs in 1982. The system is built with single, multiple-

beam antennas that provide more flexible coverage than its

predecessor. The single steerable dish antenna provides an increased

power spot beam which can be tailored to suit the needs of different

size user terminals. DSCS III satellites can resist jamming and are

expected to operate twice as long as the previous generation.

DSCS users operate on the ground, at sea or in the air. Members of the 50th Space Wing's 3rd

Space Operations Squadron at Schriever Air Force Base, Colo., provide satellite bus command

and control for all DSCS satellites.

Air Force Materiel Command's Space and Missile Systems Center at Los Angeles AFB, Calif., is

responsible for development and acquisition of DSCS satellites and ground systems.




Unit 8:Laser satellite communication:

Introduction:

LINK ANALYSIS:

(i)Atmospheric Effects: While analyzing the satellite cross-link,the effects of earth’s atmosphere on the laser signals should b ecarefully studied.The earth’s atmosphere affects the optical signals in a variety of ways such as attenuation due to the energy




absorption,beam spreading due to the scattering of light waves,beam bending due to the refocussing of the optical beams & beam breakup due to the loss of the field coherence over

the beamfront.

(ii) Complete link analysis: Satellite optical cross link analysis given below is suitable to the case when the effects of atmosphere are considered to be the negligible in satellite cross links. here it has been assumed that the RF link is used for the uplink and downlink transmission to the satellite & the crosslink b/w two satellites is the optical link that uses direct detection system.Thus,when the satellite receives uplink, it directly intensity modulates the uplink carrier BW onto the optical carrier for the crosslink. For this it is essential that the upper limit of the uplink frequency band is within the receiving photodetector BW otherwise then the uplink BW is firstly down connected before Laser modulation.

Thus the satellite RF uplink waveform may be represented

S(t) = u(t)+ nu(t)




Where u(t) is the uplink carrier & n u(t) is the uplink noise and interference.Since the laser power at the optical receiver is the intensity modulated so,

P(t) = Pr (1+βs(t))

Where the Pr is the average power and β is the intensity modulation index.

Lecture 36:

Optical satellite link transmitter:

The transmitter part of an optical satellite link consists orf a laser source, modulator and

antenna along with some data handling electronics.

Laser source: A variety of LASER source may be used for optical satellite communication

systems. These may be gas lasers, solid stata lasers and semi conductor laser.Selection of

Laser source is dependent upon a no. of factors that include link range,propogation media,

data rate and platform limitation.

Modulators: In laser space communication the most preferable modulation is direct intensity

modulation. Various methods that can be used to modulate laser can be as:

Modulation type Analog Pulse Digital

Information Signal Time continuous Time continuous Time sampling

Carrier parameter Continuous Continuous Quantized

Example Intensity Modulation Pulse intensity

modulation

Pulse code

modulation




Modulation rates of 1Gbits/sec with the laser diodes have also been achieved.

Antennas : As with the RF communication system, the Laser satellite communication system

also utilize antennas to direct the transmitted energy. Here antennas are nothing more than

the conventional design telescopes where the size and geometry are dictated by wavelength

and requirement.

Optical satellite link Receiver: Optical receiver consists of an anteena, filter,

photo detector and the conventional receive electronic system. Receive

antennas are also the telescopes whose main purpose is to focus the optical

signal onto the photo detector and to reject as much as of background radiation

as is practical. The receive optical filters are employed to eliminate background

radiation that is not of the same wavelength as the optical signal. Infact the

range of wavelengths around the laser wavelength allowed by optical fibre is

called Optical bandwidth.




Δλ/λ0 =BO/fo

Typical optical fiber BW at 1 micron generally ranges from 1-100 angstroms.

Optical detection are of two types:

1. Direct detection system :It responds to the signal intensity and are most

widely used in optical communication systems.

2. Hetrodyne detection systems: Here the optical signal is combined with a

local oscillator and then both signal are focused on to the same detector.

An optical receiver field of view is defined as the field arrival angles over

which the lens will focus the impinging field onto the photo detector

surface. This depends on the detector focal length fc.Thus the optical field

of view is given by:

Ώfv = Ad/fc2 =Ad/Ar = (Ad/Ar)(λ

2/ Ar)

This equation is termed diffraction limited of view.

Photo detectors normally used in optical detection may be PIN diode or

avalanche photodiode. These phodetector are governed by their

characteristics properties such as detection efficiency,gain,responsitivity

and BW.Infact these are the parameters which a communication

engineer has to know while using photodetectors in the optical receiver

design. The detection efficiency indicates the fraction of the received

power which is actually detected.

Thus the detected count rate of the optical receiver given by:

ns=(V �53%*�

The gain of photo detector is increased by cascading the photoemmisive

surface but this increases noise.




The BW of photo detector determines the rate of power variation that

can be detected.

Shot noise Dark current Thermal current

Pr(t) detector o/p

Pb

Here shot noise , dark current and thermal noise current are taken into

account. In addition to the some background noise power is also taken into

account. all these together form photo detector noise model.

Noise processes are as indicated in fig may be expressed as:

N0 = Nsn+Ndc+Nt

N0 represents the total receiver current processes in terms of 2 sided spectral

level.

In heterodyne detection, a strong local field relative to the received power is

added to the receive field prior photodetection.Thus here photo detector

responds to the intensity of combined field.

R + + +




Lecture 37:

Satellite beam acquisition ,tracking and pointing:







Lecture 38:

Deep space optical communication link:

This shows a typical; example of deep space optical communication link by

NASA.here main space craft communication instrument is the optical

transreceiver package called optranspac.The optranspac carries out various

functions of the optical transmitter and receiver .For example,receives beacon

signal and performs beam aquisition.Tracking and pointing functions along with

the plink optical commands and ranging signals.




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