无线通信网络-第6章 信号编码技术

1、Signal Encoding TechniquesChapter 6Reasons for Choosing Encoding TechniquesnDigital data, digital signalnEquipment less complex and expensive than digital-to-analog modulation equipmentnAnalog data, digital signalnPermits use of modern digital transmission and switching equipmentReasons for Choosing

2、 Encoding TechniquesnDigital data, analog signalnSome transmission media will only propagate analog signals nE.g., optical fiber and unguided medianAnalog data, analog signalnAnalog data in electrical form can be transmitted easily and cheaplynDone with voice transmission over voice-grade linesSigna

3、l Encoding CriterianWhat determines how successful a receiver will be in interpreting an incoming signal?nSignal-to-noise rationData ratenBandwidthnAn increase in data rate increases bit error ratenAn increase in SNR decreases bit error ratenAn increase in bandwidth allows an increase in data rateFa

4、ctors Used to CompareEncoding SchemesnSignal spectrumnWith lack of high-frequency components, less bandwidth requirednWith no dc component, ac coupling via transformer possiblenTransfer function of a channel is worse near band edgesnClockingnEase of determining beginning and end of each bit position

5、Factors Used to CompareEncoding SchemesnSignal interference and noise immunitynPerformance in the presence of noisenCost and complexitynThe higher the signal rate to achieve a given data rate, the greater the costBasic Encoding TechniquesnDigital data to analog signalnAmplitude-shift keying (ASK)nAm

6、plitude difference of carrier frequencynFrequency-shift keying (FSK)nFrequency difference near carrier frequencynPhase-shift keying (PSK)nPhase of carrier signal shiftedBasic Encoding TechniquesAmplitude-Shift KeyingnOne binary digit represented by presence of carrier, at constant amplitudenOther bi

7、nary digit represented by absence of carriernwhere the carrier signal is Acos(2fct) tstfAc2cos01binary 0binary Amplitude-Shift KeyingnSusceptible to sudden gain changesnInefficient modulation techniquenOn voice-grade lines, used up to 1200 bpsnUsed to transmit digital data over optical fiberBinary F

8、requency-Shift Keying (BFSK)nTwo binary digits represented by two different frequencies near the carrier frequencynwhere f1 and f2 are offset from carrier frequency fc by equal but opposite amounts tstfA12costfA22cos1binary 0binary Binary Frequency-Shift Keying (BFSK)nLess susceptible to error than

9、ASKnOn voice-grade lines, used up to 1200bpsnUsed for high-frequency (3 to 30 MHz) radio transmissionnCan be used at higher frequencies on LANs that use coaxial cableMultiple Frequency-Shift Keying (MFSK)nMore than two frequencies are usednMore bandwidth efficient but more susceptible to errornf i =

10、 f c + (2i 1 M)f dnf c = the carrier frequencynf d = the difference frequencynM = number of different signal elements = 2 LnL = number of bits per signal element tfAtsii2cosMi 1Multiple Frequency-Shift Keying (MFSK)nTo match data rate of input bit stream, each output signal element is held for:Ts=LT

11、 secondsnwhere T is the bit period (data rate = 1/T)nSo, one signal element encodes L bitsMultiple Frequency-Shift Keying (MFSK)nTotal bandwidth required 2MfdnMinimum frequency separation required 2fd=1/TsnTherefore, modulator requires a bandwidth ofWd=2L/LT=M/TsMultiple Frequency-Shift Keying (MFSK

12、)Phase-Shift Keying (PSK)nTwo-level PSK (BPSK)nUses two phases to represent binary digits tstfAc2costfAc2cos1binary 0binary tfAc2costfAc2cos1binary 0binary Phase-Shift Keying (PSK)nDifferential PSK (DPSK)nPhase shift with reference to previous bitnBinary 0 signal burst of same phase as previous sign

13、al burstnBinary 1 signal burst of opposite phase to previous signal burstPhase-Shift Keying (PSK)nFour-level PSK (QPSK)nEach element represents more than one bit ts42costfAc11432costfAc432costfAc42costfAc010010Phase-Shift Keying (PSK)nMultilevel PSKnUsing multiple phase angles with each angle having

14、 more than one amplitude, multiple signals elements can be achievednD = modulation rate, baudnR = data rate, bpsnM = number of different signal elements = 2LnL = number of bits per signal elementMRLRD2logPerformancenBandwidth of modulated signal (BT)nASK, PSKBT=(1+r)RnFSKBT=2DF+(1+r)R nR = bit raten

15、0 r 1; related to how signal is filteredn DF = f2-fc=fc-f1PerformancenBandwidth of modulated signal (BT)nMPSKnMFSKnL = number of bits encoded per signal elementnM = number of different signal elementsRMrRLrBT 2log11RMMrBT2log1Quadrature Amplitude ModulationnQAM is a combination of ASK and PSKnTwo di

16、fferent signals sent simultaneously on the same carrier frequency tftdtftdtscc2sin2cos21Quadrature Amplitude ModulationReasons for Analog ModulationnModulation of digital signalsnWhen only analog transmission facilities are available, digital to analog conversion requirednModulation of analog signal

17、snA higher frequency may be needed for effective transmissionnModulation permits frequency division multiplexingBasic Encoding TechniquesnAnalog data to analog signalnAmplitude modulation (AM)nAngle modulationnFrequency modulation (FM)nPhase modulation (PM)Amplitude Modulation tftxntsca2cos1nAmplitu

18、de Modulationncos2fct = carriernx(t) = input signalnna = modulation indexnRatio of amplitude of input signal to carrierna.k.a double sideband transmitted carrier (DSBTC)Spectrum of AM signalAmplitude ModulationnTransmitted powernPt = total transmitted power in s(t)nPc = transmitted power in carrier2

19、12actnPPSingle Sideband (SSB)nVariant of AM is single sideband (SSB)nSends only one sidebandnEliminates other sideband and carriernAdvantagesnOnly half the bandwidth is requirednLess power is requirednDisadvantagesnSuppressed carrier cant be used for synchronization purposesAngle ModulationnAngle mo

20、dulationnPhase modulationnPhase is proportional to modulating signalnnp = phase modulation index ttfAtscc2cos tmntpAngle ModulationnFrequency modulationnDerivative of the phase is proportional to modulating signalnnf = frequency modulation index tmntfAngle ModulationnCompared to AM, FM and PM result

21、 in a signal whose bandwidth:nis also centered at fcnbut has a magnitude that is much differentnAngle modulation includes cos( (t) which produces a wide range of frequenciesnThus, FM and PM require greater bandwidth than AMAngle ModulationnCarsons rulewherenThe formula for FM becomesBBT12BFBT22FMfor

22、 PMfor 2BAnBFAnmfmpBasic Encoding TechniquesnAnalog data to digital signalnPulse code modulation (PCM)nDelta modulation (DM)Analog Data to Digital SignalnOnce analog data have been converted to digital signals, the digital data:ncan be transmitted using NRZ-Lncan be encoded as a digital signal using

23、 a code other than NRZ-Lncan be converted to an analog signal, using previously discussed techniquesPulse Code ModulationnBased on the sampling theoremnEach analog sample is assigned a binary codenAnalog samples are referred to as pulse amplitude modulation (PAM) samplesnThe digital signal consists

24、of block of n bits, where each n-bit number is the amplitude of a PCM pulsePulse Code ModulationPulse Code ModulationnBy quantizing the PAM pulse, original signal is only approximatednLeads to quantizing noisenSignal-to-noise ratio for quantizing noisenThus, each additional bit increases SNR by 6 dB, or a factor of 4dB 76. 102. 6dB 76. 12log20SNRdBnnDelta ModulationnAnalog input i