Wireless Transmission - SJTUxue-gt/wireless/Online Course/Modules.W/W.tra/W.tra.1.pdf · Wireless transmission & modulation Transmission on media From data to waves at physical layer

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Wireless Transmission Module W.tra.1

Dr.M.Y.Wu@CSEShanghai Jiaotong University

Shanghai, China

Dr.W.Shu@ECEUniversity of New Mexico

Albuquerque, NM, USA

© by Dr.Wu@SJTU & Dr.Shu@UNM

W.tra.1-2

Wireless transmission & modulation

Transmission on mediaFrom data to waves at physical layer

Digital modulationsModulation performance

Modulation schemesASK, FSK, PSK

MSK, QAMMCM/OFDM

End of module W.tra.1


W.tra.1-3

Transmission on mediaDigital transmission’s 3 categories:

Pulse transmission, used inIR (Impulse radio)UWB (Ultra wideband)

Basic modulation, used inTDMA cellular networkFDMA cable TV

Spread spectrum modulation, used inCDMAWLANs


W.tra.1-4

Data wavesAnalog modulation

Carrier wave to be used, c(t) carrier signalData to be transmitted, x(t) modulating signalSignal is superimposed on a carrier signal

Three basic schemesAmplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)

analogmodulation

radiocarrier

analogbasebandsignal

radio transmitter


W.tra.1-5

Data wavesDigital modulation

Two steps1) Digital modulation: digital signal analog baseband signal

Three basic schemesAmplitude shift keying (ASK)Frequency shift keying (FSK)Phase shift keying (PSK)

2) Analog modulation: analog baseband signal carrier signal

digitalmodulation

digitaldata

101101001analog

modulation

radiocarrier

analogbasebandsignal

radio transmitter


W.tra.1-6





MSK, QAMMCM/OFDM



W.tra.1-7

Digital modulation

Why digital modulation?Better characteristics

Greater noise immunityRobust to channel impairments

Richer applicationsEasier multiplexing of various forms of information (e.g. voice, data, video)

Better performance & costWith digital error-control codes, better for detection and correctionWith encryption, better securityWith equalization, better channel performanceEmbedded software allows more flexibility


W.tra.1-8

Digital modulation

Modulating signalsRepresented as a time sequence of symbolsEach symbol has m finite states,

represents n=log2 m bits of information

RatesSymbol rateCoding rate (e.g. how much spent on FEC, usually ≤ 1)Bit rate (mbps) depending on

Symbol rateCoding rate


W.tra.1-9

Digital modulationPerformance requirements

Provides low bit error rates at low SNRPerform well in multi-path and fading conditionsOccupy a minimum of bandwidthEasy and cost-effective to implement

How to measure?In terms of

Power efficiency (energy efficiency)Bandwidth efficiency


W.tra.1-10

Digital modulation

Power efficiencyPerform well at low power levelNot critical for wireless equipment using AC powerTwo factors of the power requirement,

the operating powerthe radiated signal power, which translates directly into signal coverage

A function of the data rateA function of the receiver’s complexity


W.tra.1-11

Digital modulation

Bandwidth efficiencyη = Data-rate (bps) / RF signal bandwidth

Moving from analog to digital, and then from TDMA to CDMA to increase the bandwidth efficiencyIn a cellular industry, the bandwidth efficiency is directly linked into increase of revenues.Due to the bursty nature of data, WLAN rarelyapproaches system capacity, the bandwidth-efficient modulation is relatively non critical.


W.tra.1-12

Digital modulation

A side-effect: out-of-band radiationACI (adjacent-channel interference)

The interference that a transmitting radio presents to the channels immediately above and below its own channel.

Specification: -40 or -60 dB below the main lobeISI (Inter-symbol interference)

The energy intended for one symbol spills over to the adjacent symbol

The higher the symbol rate, the worse ISIISI limits channel bandwidth with multi-path propagation: signals of different symbols can cancel each other misinterpretation error


W.tra.1-13

Link performance improvement

EqualizationSender transmit a training sequence An equalizer within receiver will be trained to

Know avg range of delay characteristicsBe programmed to compensate ISI

DiversityUse two or more receiving/sending antennas which are strategically spaced

Channel codingAdd redundant data bits in the transmitted messagesUsed to detect or correct some of errors


W.tra.1-14





MSK, QAMMCM/OFDM



W.tra.1-15

Digital modulation: ASK

Amplitude shift keying (ASK)Simple, low bandwidth requirement

S(t) = Ac cos(2πfc t) for binary 1S(t) = 0 for binary 0

Very susceptible to interferenceMulti-path, noise, path-loss influence amplitude

Not used in wirelessUsed in opticalAlso used in infrared

1 0 1

t


W.tra.1-16

Digital modulation: FSKFrequency shift keying (FSK)

Simple binary FSK (BFSK), k is a small offsetS(t) = Ac cos(2π(fc+k)t) for binary 1S(t) = Ac cos(2π(fc-k)t) for binary 0

Need large bandwidthFrequency spacing is

directly proportional to the required bandwidth

Less susceptible to errorsOften used for wireless4-level FSK

S(t) = Ac cos(2π(fc+3k)t) for binary 10S(t) = Ac cos(2π(fc+ k)t) for binary 11 S(t) = Ac cos(2π(fc- k)t) for binary 01 S(t) = Ac cos(2π(fc-3k)t) for binary 00

1 0 1

t


W.tra.1-17

Digital modulation: PSK (BPSK)Phase shift keying (PSK)

Simple Binary PSK (BPSK), S(t) = Ac cos(2πfct + π ) for binary 1S(t) = Ac cos(2πfct) for binary 0

More resistantto interferenceComplex transmitterand receiverUsed for cable TV4-level Quadrature PSK (QPSK)

S(t) = Ac cos(2πfct + π/4) for binary 10S(t) = Ac cos(2πfct + 3π/4) for binary 11S(t) = Ac cos(2πfct + 5π/4) for binary 01S(t) = Ac cos(2πfct + 7π/4) for binary 00

1 0 1

t


W.tra.1-18

Digital modulation: PSK (QPSK)QPSK double bit rate by coding two bits into one phase sift

The phase shift is relative to a reference signal (with the samefrequency)

Require to produce the reference signal at receiver

BPSK & QPSK

An alternative is to use Differential QPSK (DQPSK)Phase shift is relative to the phase of the previous two bits.

Q

I01

Q

I

11

01

10

00


W.tra.1-19

Digital modulation: MSKMinimum shift keying (MSK)

Is basically a simple Binary PSK (BPSK) without abrupt phase changes

Data bits are separated into even & odd bits, the duration of each bit being doubledUse two frequencies:

f1 (the lower frequency), and f2 = 2 f1 (the higher frequency)

GMSK: add a Gaussian low-pass filter to the MSK

f211f101

inverted f110inverted f200

SignalsEven & odd bits


W.tra.1-20

Example of MSK

data

even bits

odd bits

1 1 1 1 000

t

low frequency

highfrequency

MSKsignal

biteven 0 1 0 1odd 0 0 1 1

signal h n n hvalue - - + +

h: high frequencyn: low frequency+: original signal-: inverted signal

No phase shifts!

By Dr. Schiller,


W.tra.1-21

Digital modulation: QAM

Advanced Phase shift keying: PSK&ASKQuadrature Amplitude Modulation (QAM)Both the amplitude & phase are variedPossible to code n bits using one symbol

2n discrete levels, n=2 identical to QPSKExample: 16-QAM (4 bits = 1 symbol)

3 amplitudes & 12 anglesSymbols 0011 and 0001 have the same phase φ, but different amplitude a. Symbols 0000 and 1000have different phase, but same amplitude.

Used in standard 9600 bit/s modems

0000

0001

0011

1000

Q

I

0010

φ

a


W.tra.1-22

Digital modulation: QAM

64-QAM: hierarchical modulation, used in the digital TV standard Modulates two separate data streams onto a single DVB-T (Digital Video Broadcast TV) stream

6 bit per QAM symbol, 2 most significant bits determine QPSKHigh Priority (HP) embedded within a Low Priority (LP) streamgood reception: resolve the entire 64QAM constellationpoor reception, mobile reception: resolve only QPSK portionHP service coded in QPSK (2 bits), LP uses remaining 4 bits

Q

I

00

10

000010 010101


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Modulation: OFDM

MCM (Multi-Carrier Modulation)OFDM (Orthogonal Frequency Division Multiplexing)Split the high bit rate stream into many lower bit rate streamsEach stream being sent using an independent carrier frequency

N symbol/s to be transmittedC sub carriersEach sub carrier transmit n/c symbols per second

Good at ISI mitigation (due to lower rate)Used in IEEE 802.11a, c = 48


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MSK, QAMMCM/OFDM


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Wireless Transmission - SJTUxue-gt/wireless/Online Course/Modules.W/W.tra/W.tra.1.pdf · Wireless transmission & modulation Transmission on media From data to waves at physical layer