Chapter 4Chapter 4

Baseband Data TransmissionBaseband Data Transmission

& &

Digital Modulation TechniquesDigital Modulation Techniques

Chapter OverviewChapter Overview Baseband transmissionBaseband transmission Line codingLine coding Intersymbol Interference (ISI)Intersymbol Interference (ISI) Nyquist WaveNyquist Wave Digital Modulation techniquesDigital Modulation techniques

Baseband TransmissionBaseband Transmission

The original band of frequencies of a signal before it is The original band of frequencies of a signal before it is modulated for transmission at a higher frequency. modulated for transmission at a higher frequency.

A type of data transmission in which digital or analog A type of data transmission in which digital or analog data is sent over a single unmultiplexed channel, such as data is sent over a single unmultiplexed channel, such as an Ethernet LAN. an Ethernet LAN.

Baseband transmission use TDM to send simultaneous Baseband transmission use TDM to send simultaneous bits of data along the full bandwidth of the transmission bits of data along the full bandwidth of the transmission channel.channel.

Cont’d...Cont’d... Baseband Center Point DetectionBaseband Center Point Detection

The detection of digital signals involve two processes:The detection of digital signals involve two processes:

1.1. Reduction of each received voltage pulseReduction of each received voltage pulse

(i.e. symbol) to a single numerical value.(i.e. symbol) to a single numerical value.

2.2. Comparison of this value with a reference Comparison of this value with a reference voltage (or for multisymbol signaling, a voltage (or for multisymbol signaling, a set of reference voltages) to determine set of reference voltages) to determine which symbol was transmitted.which symbol was transmitted.

Line CodingLine Coding

Pulse modulation applied to binary symbol, Pulse modulation applied to binary symbol, the resulting binary waveform is called PCM the resulting binary waveform is called PCM waveform.waveform. In telephony applications : In telephony applications : line codesline codes

Pulse modulation is applied to nonbinary Pulse modulation is applied to nonbinary symbol, the resulting waveform called M-ary symbol, the resulting waveform called M-ary pulse modulation waveform.pulse modulation waveform.

Some basic nomenclatureSome basic nomenclature Information source: can be either analog or digitalInformation source: can be either analog or digital binary sequence: sequence of {1, 0} to describe information binary sequence: sequence of {1, 0} to describe information

sourcesource Bit symbol: a symbol represents k bits (M=2Bit symbol: a symbol represents k bits (M=2kk)) Symbol alphabet: sSymbol alphabet: s00,s,s11,…,s,…,sM-1M-1 (alphabet size M) (alphabet size M) symbol stream: sequence of symbols selected from the symbol stream: sequence of symbols selected from the

alphabetalphabet Data rate Data rate

Bit rate RBit rate Rbb in bits/sec (bps). in bits/sec (bps). Symbol rate RSymbol rate Rs s in symbols/sec (sps) in symbols/sec (sps) Bit interval TBit interval Tbb: duration of a bit (sec/bit): duration of a bit (sec/bit) Symbol interval TSymbol interval Tss: duration of a symbol (sec/symbol): duration of a symbol (sec/symbol)

Important relationImportant relation

k

sbsb

ss

bb

M

k

TTkRR

TR

TR

2)5(

)4(,)3(

1)2(,

1)1(

Illustration and exampleIllustration and example

Cont’d...Cont’d...

Line codesLine codes Converting standard logic level to a form more Converting standard logic level to a form more

suitable to telephone line transmission.suitable to telephone line transmission. Four main groups Four main groups

Non return to zero (NRZ)Non return to zero (NRZ) Return to Zero (RZ)Return to Zero (RZ) Phase encodedPhase encoded Multilevel binaryMultilevel binary

Cont’d...Cont’d...1.1. Non Return To Zero (NRZ)Non Return To Zero (NRZ)

Subgroups :Subgroups : NRZ-L NRZ-L

Used extensively in digital logic circuitUsed extensively in digital logic circuit Binary 1 one represented by one voltage levelBinary 1 one represented by one voltage level Binary 0 is represented by another voltage level. Binary 0 is represented by another voltage level.

NRZ-M NRZ-M Used in magnetic tape recording.Used in magnetic tape recording. The 1 (mark) is represented by change in levelThe 1 (mark) is represented by change in level The 0 (space) is represented by no change in levelThe 0 (space) is represented by no change in level Differential encodingDifferential encoding

NRZ-SNRZ-S Complement of NRZ-MComplement of NRZ-M 1 is represented by no change in level 1 is represented by no change in level 0 is represented by a change in level0 is represented by a change in level

Cont’d...Cont’d...2.2. Return To Zero (RZ)Return To Zero (RZ)

Subgroups:Subgroups: Unipolar RZUnipolar RZ

1 is represented by a half bit wide pulse.1 is represented by a half bit wide pulse. 0 is represented by the absence of pulse.0 is represented by the absence of pulse.

Bipolar RZBipolar RZ 1 & 0 are represented by opposite level pulses that are one 1 & 0 are represented by opposite level pulses that are one

half bit wide.half bit wide. Pulse present in each bit interval.Pulse present in each bit interval.

RZ-AMIRZ-AMI 1 is represented by equal amplitude alternating pulses.1 is represented by equal amplitude alternating pulses. 0 is represented by the absences of pulse.0 is represented by the absences of pulse.

Cont’d...Cont’d...3.3. Phase EncodedPhase Encoded

Subgroups:Subgroups: Bi-phase-level @ Manchester codingBi-phase-level @ Manchester coding Bi-phase-markBi-phase-mark Bi-phase-spaceBi-phase-space Delay modulation @ Miller codingDelay modulation @ Miller coding

4.4. Multilevel BinaryMultilevel Binary Used three levels to encode the binary data.Used three levels to encode the binary data. Dicode and duo binaryDicode and duo binary

Cont’d...Cont’d...

Parameters :Parameters :• DC componentsDC components• Self-clockingSelf-clocking• Error detectionError detection• Bandwidth CompressionBandwidth Compression• Differential encodingDifferential encoding• Noise immunityNoise immunity

Cont’d...Cont’d... PCM Word sizePCM Word size

Magnitude of the quantization Magnitude of the quantization distortion errordistortion error

|e| |e| << pV pVpp

|emax| = q/2 = Vpp

2(L-1)

= Vpp

2L

L = number of quantization L = number of quantization levels, large enoughlevels, large enough

Number of bits/sample, Number of bits/sample, ιι

22ι = L ≥ (1/2p) = L ≥ (1/2p) levelslevels

ιι ≥ ≥ log log 2 (1/2p) (1/2p) bitsbits

Cont’d...Cont’d... M-ary Pulse Modulation waveformsM-ary Pulse Modulation waveforms

Three basic ways to modulate information on to a Three basic ways to modulate information on to a sequence of pulsesequence of pulse

PAMPAM PPMPPM PWM @ pulse duration modulation (PDM)PWM @ pulse duration modulation (PDM)

M-ary pulse modulation M-ary pulse modulation When the information samples are first quantized, When the information samples are first quantized,

yielding symbols from an M-ary alphabet set, then yielding symbols from an M-ary alphabet set, then modulated on to pulses, the resulting pulse modulation is modulated on to pulses, the resulting pulse modulation is digital.digital.

Cont’d...Cont’d... M-ary PAM M-ary PAM

one of M allowable amplitude levels are assigned one of M allowable amplitude levels are assigned to each of the M possible symbol values.to each of the M possible symbol values.

M-ary PPM M-ary PPM modulation is effected by delaying or advancing a modulation is effected by delaying or advancing a

pulse occurrence.pulse occurrence. M-ary PWM M-ary PWM

modulation is effected by varying the pulse width modulation is effected by varying the pulse width by an amount that corresponds to the value of the by an amount that corresponds to the value of the symbols.symbols.

M-ary pulse modulation block-M-ary pulse modulation block-diagramdiagram

k-bittuple

M-ary Pulse Modulator

Binary

PCMsequence

SymbolSequence

M-ary

Pulse-modulationWaveform

Illustration of PCM signalsIllustration of PCM signals

Bandwidth efficiencyBandwidth efficiency How much date rate can be supported by the How much date rate can be supported by the

system with each unit frequency bandsystem with each unit frequency band The higher bandwidth efficiency, the better.The higher bandwidth efficiency, the better.

bit rateBandwidth Efficiency= bits/sec/Hz

Signal bandwdithbR

W

Intersymbol Interference (ISI)Intersymbol Interference (ISI)

Tx – the information symbols characterized as Tx – the information symbols characterized as impulse or voltage levels, modulate pulses that are impulse or voltage levels, modulate pulses that are then filtered to comply with some bandwidth then filtered to comply with some bandwidth constraint.constraint.

Baseband system – the channel has distributed Baseband system – the channel has distributed reactance that distorts the pulses.reactance that distorts the pulses.

Bandpass system – characterized by fading channels Bandpass system – characterized by fading channels that behave like undesirable filters manifesting signal that behave like undesirable filters manifesting signal distortion.distortion.

Rx – Rx – equalizing filterequalizing filter is configured to compensate for is configured to compensate for the distortion caused by Tx & channel.the distortion caused by Tx & channel.

Cont’d...Cont’d...

Equivalent system transfer functionEquivalent system transfer function

H(f) = HH(f) = Htt(f) H(f) Hcc(f) H(f) Hrr(f)(f)

WhereWhere

HHtt(f) – transmitting filter(f) – transmitting filter

HHcc(f) – filtering within the channel(f) – filtering within the channel

HHrr(f) – equalizing filter(f) – equalizing filter

Cont’d...Cont’d...

Due to effect of system filtering, the received Due to effect of system filtering, the received pulses can overlap one another. pulses can overlap one another.

Tail of pulse can smear into adjacent symbol Tail of pulse can smear into adjacent symbol intervals , thereby interfering with the intervals , thereby interfering with the detection process and degrading the error detection process and degrading the error performance - performance - Intersymbol interference (ISI)Intersymbol interference (ISI) effects of filteringeffects of filtering Channel-induced distortionChannel-induced distortion

ISI measured by eye patternISI measured by eye pattern

Cont’d...Cont’d...

Nyquist filterNyquist filter is one whose frequency transfer is one whose frequency transfer function can be represented by a rectangular function function can be represented by a rectangular function convolved with any real even-symmetric frequency convolved with any real even-symmetric frequency functionfunction

Nyquist pulseNyquist pulse is one whose shape can be represented is one whose shape can be represented by a sinc by a sinc (t/T)(t/T) function multiplied by another time function multiplied by another time functionfunction

Most popular of Nyquist filterMost popular of Nyquist filter Raised-cosine filterRaised-cosine filter Root-raised cosine filterRoot-raised cosine filter

Cont’d...Cont’d... Pulse Shaping to reduce ISIPulse Shaping to reduce ISI

Pulse that spread in time will degrade the system’s Pulse that spread in time will degrade the system’s error performance due to increase ISI.error performance due to increase ISI.

Reduce the required system bandwidth. Reduce the required system bandwidth. Compress the bandwidth of the data impulse to Compress the bandwidth of the data impulse to

some reasonably small bandwidth greater than the some reasonably small bandwidth greater than the Nyquist minimum – Nyquist minimum – pulse shaping with Nyquist pulse shaping with Nyquist filter.filter.

Zero ISI is only when the sampling is performed at Zero ISI is only when the sampling is performed at exactly the correct sampling time when the tails of exactly the correct sampling time when the tails of pulses are large.pulses are large.

Cont’d...Cont’d...

Raised-cosine filterRaised-cosine filter One frequently used One frequently used H(f)H(f) transfer function transfer function

belonging to the Nyquist class (zero ISI at the belonging to the Nyquist class (zero ISI at the sampling time).sampling time).

It can be expressed asIt can be expressed as

Amplitude response of raised-cosine filter with Amplitude response of raised-cosine filter with various roll-off factors various roll-off factors

Impulse response of raised-cosine filter with Impulse response of raised-cosine filter with various roll-off factors various roll-off factors

Consecutive raised-cosine impulses, Consecutive raised-cosine impulses, demonstrating zero-ISI property demonstrating zero-ISI property

Cont’d...Cont’d... Impulse response for the H(f)Impulse response for the H(f)

Minimum required bandwidthMinimum required bandwidth

DSB bandwidth DSB bandwidth

Cont’d...Cont’d...

Two types of error-performance degradationTwo types of error-performance degradation1.1. Due to a decrease in received signal power or an Due to a decrease in received signal power or an

increase in noise or interference power, giving increase in noise or interference power, giving rise to a loss in signal-to-noise ratio, rise to a loss in signal-to-noise ratio, EEbb/N/Noo

2.2. Due to signal distortion such as ISIDue to signal distortion such as ISI

Nyquist’s WaveNyquist’s Wave

Nyquist pulse Nyquist pulse Impulse response of the raised-cosine filterImpulse response of the raised-cosine filter Faster transitionFaster transition

Square-root Nyquist pulseSquare-root Nyquist pulse Impulse response of a root-cosine filterImpulse response of a root-cosine filter Does not exhibit zero ISIDoes not exhibit zero ISI

Digital Modulation TechniquesDigital Modulation Techniques

Digital modulationDigital modulation The process by which digital symbols are The process by which digital symbols are

transformed into waveforms that are compatible transformed into waveforms that are compatible with the characteristic of the channel.with the characteristic of the channel.

Bandpass modulationBandpass modulation The shaped pulses modulate a carrier wave The shaped pulses modulate a carrier wave

Radio transmission – the carrier is converted to EM Radio transmission – the carrier is converted to EM field for propagation to the desired destinationfield for propagation to the desired destination

Cont’d...Cont’d...

)2sin()( ftVtv

•If the amplitude, V of the carrier is varied proportional to the information signal, a digital modulated signal is called Amplitude Shift Keying (ASK)

•If the frequency, f of the carrier is varied proportional to the information signal, a digital modulated signal is called Frequency Shift Keying (FSK)

Carrier Signal

Cont’d...Cont’d...

If the phase, θ of the carrier is varied proportional to the information signal, a digital modulated signal is called Phase Shift Keying (PSK)

If both the amplitude,V and the phase, θ of the carrier are varied proportional to the information signal, a digital modulated signal is called Quadrature Amplitude Modulation (QAM)

Cont’d...Cont’d...

Amplitude Shift Keying (ASK)Amplitude Shift Keying (ASK)

A binary information signal directly modulates the amplitude A binary information signal directly modulates the amplitude of an analog carrier.of an analog carrier.

)cos()](1[)( 2 ttvtv cA

mask

Where vask (t) = amplitude shift keying wave

vm(t) = digital information signal (volt)

A/2 = unmodulated carrier amplitude (volt)

ωc = analog carrier radian frequency (rad/s)

1)(,'0'logic0

1)(,'1'logic)cos()(

tvfor

tvfortAtv

m

mcask

Frequency Shift Keying (FSK)Frequency Shift Keying (FSK) Called as BFSKCalled as BFSK The phase shift in carrier frequency (∆f) is proportional to the The phase shift in carrier frequency (∆f) is proportional to the

amplitude of the binary input signal (vamplitude of the binary input signal (vmm(t)) and the direction of the (t)) and the direction of the

shift is determined by the polarityshift is determined by the polarity

tftvfVtv mccfsk ])([2cos)(

Where vfsk(t) = binary FSK waveform

Vc = peak analog carrier amplitude (volt)

fc = analog carrier center frequency (Hz)

∆f = peak shift in analog carrier frequency (Hz)

vm(t) = binary input signal (volt)

1)(,'0'logic][2cos

1)(,'1'logic][2cos)(

tvfortffV

tvfortffVtv

mcc

mccfsk

(Hz)frequency space &mark between difference absolute

(Hz)deviation frequency

,2

sm

sm

ff

f

where

fff

FSK BandwidthFSK Bandwidth

)(22)()( bbmsbmbs fffffffffB

FSK TransmitterFSK Transmitter

Phase Shift Keying (PSK)Phase Shift Keying (PSK)

BPSK TransmitterBPSK Transmitter

BPSK ReceiverBPSK Receiver

BPSKBPSK

Bit value 1 – sine wave Bit value 1 – sine wave Bit value 0 – inverted sine waveBit value 0 – inverted sine wave Very simple PSKVery simple PSK Low spectral efficiencyLow spectral efficiency Robust , used in satellite systemRobust , used in satellite system

Output phase vs time relationship Output phase vs time relationship for a BPSK modulatorfor a BPSK modulator

Binary input

BPSK output

Time

Time

10

1 10 0

QPSKQPSK

QPSKQPSK

2 bits coded as one symbol2 bits coded as one symbol Symbol determines shift of sine waveSymbol determines shift of sine wave Needs less bandwidth compared to BPSKNeeds less bandwidth compared to BPSK More complexMore complex

QPSK signal in the time domainQPSK signal in the time domain

The binary data that is conveyed by this The binary data that is conveyed by this waveform is: 1 1 0 0 0 1 1 0.waveform is: 1 1 0 0 0 1 1 0.

The odd bits, highlighted here, contribute to The odd bits, highlighted here, contribute to the in-phase component: the in-phase component: 11 1 1 00 0 0 00 1 1 11 0 0

The even bits, highlighted here, contribute to The even bits, highlighted here, contribute to the quadrature-phase component: 1 the quadrature-phase component: 1 11 0 0 00 0 0 11 1 1 00

End of Chapter 4End of Chapter 4