Digital Carrier Systems

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EE 442 – Spring SemesterLecture 12

1 0 1 1 0 1

1 0 0 1 0 1

(4 states)

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Digital Carrier Systems

In the last lecture we studied baseband digital signals; that is, the

modulating signal m(t) have not been frequency shifted.

However, for wireless and satellite communications we must use higherfrequencies to transmit and receive communication signals.

Now we require a modulator and a demodulator – together they form a “modem.”

There are two basic forms of carrier modulation – they are (1) amplitude modulation and (2) angle modulation (phase and frequency modulation).We have already studied both of these under the heading of analog modulation.

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Example of Amplitude Shift Keying (ASK)

( )cosASK Cm t t

This is binary amplitude shift keying (BASK).

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Example of Multilevel ASK with 2-Bit Coding

http://www.tmatlantic.com/encyclopedia/index.php?ELEMENT_ID=10420

This is multilevel amplitude shift keying.

Symbols 00, 01, 10 & 11 translate into four amplitude levels.

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Band Limiting Softens the Edges of ASK Waveforms

http://www.slideshare.net/Zeolite27/dc-ppt-final

You can see the similarity between ASK and analog AM because the amplitude

of the modulated signal is proportional to m(t).

m(t)

This is more realistic case for actual ASK communication systems.In fact, all waveforms are softened by bandwidth limitations.

time

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Next, Phase Shift Keying (PSK)

Angle modulation gives rise to both phase modulation and frequency modulation.

Starting with phase modulation; this is generally known as “phase shift keying.”

http://electronicdesign.com/communications/understanding-modern-digital-modulation-techniques

m(kTb) = +1

m(kTb) = -1

Example:

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Constellation Diagram For PSK

( )cos( )C CA m t t cos( ) for ( ) 1C C bA t m kT

cos( ) for ( ) 1C C bA t m kT

PSK

We can also express as I and Q components.

Q

I

A special case: on-off keying (OOK)

0

I

Q

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Expressing PSK in I and Q Components

cos forPSK C C k b b bA t kT t kT T

For PSK we can write,

cos( )cos sin( )sin

Therefore,

cos sin( ) for

PSK C k C C k C

PSK k C k C b b b

A t A t

a t b t kT t kT T

This is in polar form (I and Q)

For binary PSK we have k = 0 or radians.This is 2-QAM but we don’t generally use this terminology for binary PSK.

Note: Quadrature amplitude modulation (QAM) is a mixture of bothamplitude modulation and phase modulation.

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Binary PSK (BPSK) Transmitter and Receiver

Carrier

cos(ct)Balanced

Modulator AmplifierBPF

LPFNRZ Datainput

PSK

BPSK Modulator:

LPF S&H

+

cos(ct)

PSKd(t)

Comparator

Binary dataoutput

( ) cos[2 ( )] cos[ ( )]Cr t B t t B t

BPSK Demodulator:

Sample atcenter ofsymbol

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Binary PSK (BPSK) Received Waveforms

Without noise With noise

After Lawrence Burns, “Digital Modulation and Demodulation,” Chapter 4in RF and Microwave Circuit Design for Wireless Communications, editedby Lawrence E. Larson, Artech House Publishers, 1996. Pages 99 to 233.Lawrence Burns was an engineer at 3COM.

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Frequency Shift Keying (FSK)

In frequency shift keying each digital symbol has its own unique carrier signal frequency for encoding it. The signal amplitude and phase remain the same, only the frequency is varied. In the figure binary frequency shift keying (BFSK) is illustrated.

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FSK Modulation and Demodulation

FSK Modulator:

VoltageControlledoscillator

AmplifierBPF

NRZ Datainput

FSK

RFOutput

Vcontrol

t

Amplifier LPFBPF

FrequencyDiscriminator

FSK+ m(t)

Comparator

Binary dataoutput

FSK Demodulator:

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Multilevel Frequency Shift Keying (FSK)

This animation shows frequency shift keying of the sinusoidal carrier signal. A two-digit code modulates the carrier signal frequency into four frequencies

Symbol Binary code Frequency

“0” 00 4 kHz

“1” 01 3 kHz

“2” 10 2 kHz

“3” 11 1 kHz

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Comparing PSDs For Binary ASK, PSK and FSK

FSK

PSK

ASK

Pow

er s

pec

tral

de

nsi

ty [

wat

ts/H

z)

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BPSK Waveforms and Noise

Sampled data points

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Quadrature Phase Shift Keying (QPSK)

Sometimes this is known as quadri-phase PSK, 4-PSK, or 4-QAM. QPSK uses four points on the constellation diagram, equi-spaced around a circle. With four phases, QPSK can encode two bits per symbol,

Q

I

I = -1; Q = -1 I = +1; Q = -1

I = +1; Q = +1I = -1; Q = +1

cos sinQPSK C CI t Q t

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i(t)

q(t)

Digital I/Q ModulationAnticipating our coverage of digital communication systems

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Simple QPSK Modulator

QPSK modulator using delay lines to set phase delay:

+45

+135

-135

-45

Delay lines (depend upon fC)

Switch Decoder and Driver

RF Input RF Output

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Widely-Used QPSK Modulator

QPSK Modulator

AmplifierBPF

LPF

NRZ Datainput

PSK

LPF

Serial-to-ParallelParser

cos( )Ct

sin( )Ct

I

Qt

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Basic Building Block: Quadrature Modulator

cos( )Ct

sin( )Ct

I

Q

I and Q can beeither analog or

digital signals

2 2

1

( ) cos( ( ))

( )where ( ) tan

( )

Ct I Q t t

Q tt

I t

( )t

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QPSK Time Domain Waveforms

QPSK

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Data Demultiplexer (Serial to Parallel) For QPSK

Demodulator uses three D-type flip-flops and is driven by clockand clock/2 rates.

Q

I

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QPSK Demodulator

C/R = clock/carrier recovery

STR = symbol timing recovery

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M-ary Signaling With Quadrature Amplitude Modulation (QAM)

Quadrature Amplitude Modulation, QAM is a form of modulation that is a combination of phase modulation and amplitude modulation. The QAM scheme represents bits as points in a quadrant grid know as a constellation map.

16-ary QAM

APSK definitionDefinition: Amplitude and Phase-Shift Keying, APSK, is a digital modulation scheme that uses both the amplitude and the phase changes of on the carrier signal to provide the data transport mechanism for the information. Also called QAM.

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Number-Bases in M-ary Constellations

Variants of QAM are also used for many wireless and cellular technology applications. In addition, 64-QAM and 256-QAM are commonly used in digital cable television and cable modem applications. In the US, 64-QAM and 256-QAM are the mandated modulation schemes for digital cable as standardized by the SCTE in the standard ANSI/SCTE 07 2000.

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Bits/Symbol and Symbol Rates

ModulationBits per Symbol

Symbol Rate

BPSK 1 1 bit rate

QPSK 2 1/2 bit rate

8-PSK 3 1/3 bit rate

16-QAM 4 1/4 bit rate

32-QAM 5 1/5 bit rate

64-QAM 6 1/6 bit rate

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http://farhek.com/jd/i1t1154/up-to/7i45u1/

Greater Number of States Leads to Greater Demand Upon Communication System

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Bit Error Rate versus Energy/Noise Ratio

0/ ( )bE N dB

energy per bit-to-noise power ratio

BER = Bit Error Rate

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Signal-to-Noise Ratio vs. Energy/Bit-to-Noise Ratio

In analog and digital communications, signal-to-noise ratio, usually written S/Nor SNR, is a measure of signal strength relative to background noise strength. The ratio is usually expressed in decibels (dB) and equals 10log10[S/N].

Another metric that is often more useful in digital systems is the energy perbit-to-noise power ratio, denoted by Eb/N0.

Define: Rb = bit rate (in bits per second)S = total signal power (watts)Eb = energy per bit (in joules/bit)N = total noise power (over entire bandwidth B in Hz)N0 = noise spectral density (N = N0B where B = bandwidth)

Then,

Increasing the data rate Rb increases the SNR. However, in general it also increases the noise in the denominator, which lowers the SNR.

0

and andb b bb

b b

E R ES SE SNR

R N R N N B

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WiFi systems use two primary radio transmission techniques.

802.11b (≤ 11 Mbps) − The 802.11b radio link uses a direct sequence spread spectrum technique (DSSS) called complementary coded keying (CCK). The bit stream is processed and then modulated using Quadrature Phase Shift Keying (QPSK).

802.11a and 802.11g (≤ 54 Mbps) − The 802.11a and g systems use 64-channel orthogonal frequency division multiplexing (OFDM). The transmitter encodes the bit streams onto 64 subcarriers using Binary Phase Shift Keying (BPSK), Quadrature Phase Shift Keying (QPSK), or one of two levels of Quadrature Amplitude Modulation (16-QAM, or 64-QAM).

What Modulation Schemes Does Wi-Fi Use?

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Bandwidth Efficiency (aka Spectral Efficiency)

Given: Eb = energy per bit (joules or ergs)Rb = bit rate (bits/second)B = bandwidth of baseband signal (Hz)N0 = noise spectral density (watts/Hz)N = noise power = N0B (watts)

Therefore, EbRb = total signal power

We define the Bandwidth Use Efficiency as

In general,

bits/second

HzbR

B

2log 1b b bR E R

B NB

Example:GSM Digital Cellular

Data rate = 270 kb/sB = 200 kHz, thus

Bandwidth efficiency =1.35 bits/sec/Hz

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Circuit Switched Networks vs. Packet-Switched Network

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Circuit-Switched Network

TelephoneSwitch

TelephoneSwitch

TelephoneSwitch

TelephoneSwitch

TelephoneSwitch

TelephoneSwitch

TelephoneSwitch

Many paths are possible, but only one is selected per

call.

Once a connection is established, this

connection is maintained until call

is terminated.

Caller

= Dedicated connection (point-to-point)

Subscriber lines(or local loops)

Trunks(links between

Exchanges)

Central Office

Central Office

Central Office

PSTN = public switched telephone network

Full Duplex

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Packet Switched Network

Internet

Many paths possible for a single message as packets are routed to

the destination.

Packets are routed according to the best path available at the

time.

Receiver(destination)

Sender(source)

Message broken into packets andeach addressed

Packets sequentiallyreassembled

to revealmessage

= Packet

Routeror Switch

(Data Packet or “Datagram”)

Large array of routers and data links.

Packet route

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Network Organization

Centralized Network Decentralized Network

(e.g., PSTN)

Distributed Network

(e.g., Internet)

In 1962, Paul Baran (RAND Corp.) envisioned a network of unmanned nodes using intelligent switches to route data node to node to their final destinations. Baran called this "hot-potato routing" or distributed communications. This was implemented in ARPANET which became the Internet.

Concept of hardened networks to deal with disasters.

A networkof routers

A highly vulnerablenetwork

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Packet-Switched Network Operation

• Adaptive routing – routers chose the best path by examining traffic loading along available paths. Routers create a “routing table” for the packet travel.

• All users share the same network resources.

• Packet-switching is more efficient than circuit-switching in networks when data is bursty (i.e., variable delays interspersed with periods of data transmission). More “efficient” means a better utilization of the network resources.

This is an example of

“bursty” data

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An Internet Packet and its Headers

• In IPv4, each packet is restricted to 1,500 bytes of data (i.e., payload)

• Each packet consists of the application data and headers

• The headers contain control and routing information such as:

– Source IP address and destination IP address

– Packet numbering for reconstruction at destination

• Every computer on the Internet has the TCP/IP program. The client/server model is used on the Internet.

• TCP (Transmission Control Protocol) puts the data or message into packets at the source and reassembles the data or message at the destination

• IP (Internet Protocol) does the packet addressing for the routing over the Internet

Application DataIP header TCP/UDP header

Internet Packet

The rules that govern communication – any form – are called “protocols.”

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TCP versus UDP Transmission

TCP is “reliable” because it has flow & congestioncontrol, retransmission, &uses acknowledgements.

UDP does not use these because it is focused onlyupon sending packets.

UDP

TCP and UDP Analogies:

Post OfficeVerifies deliveryRegistered

Letter

TCP

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Layer Pictorial View of Protocol Data Unit Entity

ApplicationData or

Message

Transport Segments

Internet or Network

Packets orDatagrams

Network Access

Frames

Data

DataTransport

Header

DataTransport

HeaderNetwork Header

DataTransport

HeaderNetwork Header

Frame Header

Frame Trailer

Protocol

SMTPHTTP, DNS

TCPUDP

IP

EthernetModem

FDDI

Number of segments 1

Bits transmitted over channel medium

TCP/IP Protocol Architecture Model