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1、Signal Encoding Techniques Chapter 6 Reasons for Choosing Encoding Techniques nDigital data, digital signal nEquipment less complex and expensive than digital-to-analog modulation equipment nAnalog data, digital signal nPermits use of modern digital transmission and switching equipment Reasons for C
2、hoosing Encoding Techniques nDigital data, analog signal nSome transmission media will only propagate analog signals nE.g., optical fiber and unguided media nAnalog data, analog signal nAnalog data in electrical form can be transmitted easily and cheaply nDone with voice transmission over voice- gra
3、de lines Signal Encoding Criteria nWhat determines how successful a receiver will be in interpreting an incoming signal? nSignal-to-noise ratio nData rate nBandwidth nAn increase in data rate increases bit error rate nAn increase in SNR decreases bit error rate nAn increase in bandwidth allows an in
4、crease in data rate Factors Used to Compare Encoding Schemes nSignal spectrum nWith lack of high-frequency components, less bandwidth required nWith no dc component, ac coupling via transformer possible nTransfer function of a channel is worse near band edges nClocking nEase of determining beginning
5、 and end of each bit position Factors Used to Compare Encoding Schemes nSignal interference and noise immunity nPerformance in the presence of noise nCost and complexity nThe higher the signal rate to achieve a given data rate, the greater the cost Basic Encoding Techniques nDigital data to analog s
6、ignal nAmplitude-shift keying (ASK) nAmplitude difference of carrier frequency nFrequency-shift keying (FSK) nFrequency difference near carrier frequency nPhase-shift keying (PSK) nPhase of carrier signal shifted Basic Encoding Techniques Amplitude-Shift Keying nOne binary digit represented by prese
7、nce of carrier, at constant amplitude nOther binary digit represented by absence of carrier nwhere the carrier signal is Acos(2fct) Amplitude-Shift Keying nSusceptible to sudden gain changes nInefficient modulation technique nOn voice-grade lines, used up to 1200 bps nUsed to transmit digital data o
8、ver optical fiber Binary Frequency-Shift Keying (BFSK) nTwo binary digits represented by two different frequencies near the carrier frequency nwhere f1 and f2 are offset from carrier frequency fc by equal but opposite amounts Binary Frequency-Shift Keying (BFSK) nLess susceptible to error than ASK n
9、On voice-grade lines, used up to 1200bps nUsed for high-frequency (3 to 30 MHz) radio transmission nCan be used at higher frequencies on LANs that use coaxial cable Multiple Frequency-Shift Keying (MFSK) nMore than two frequencies are used nMore bandwidth efficient but more susceptible to error nf i
10、 = f c + (2i 1 M)f d nf c = the carrier frequency nf d = the difference frequency nM = number of different signal elements = 2 L nL = number of bits per signal element Multiple Frequency-Shift Keying (MFSK) nTo match data rate of input bit stream, each output signal element is held for: Ts=LT second
11、s nwhere T is the bit period (data rate = 1/T) nSo, one signal element encodes L bits Multiple Frequency-Shift Keying (MFSK) nTotal bandwidth required 2Mfd nMinimum frequency separation required 2fd=1/Ts nTherefore, modulator requires a bandwidth of Wd=2L/LT=M/Ts Multiple Frequency-Shift Keying (MFS
12、K) Phase-Shift Keying (PSK) nTwo-level PSK (BPSK) nUses two phases to represent binary digits Phase-Shift Keying (PSK) nDifferential PSK (DPSK) nPhase shift with reference to previous bit nBinary 0 signal burst of same phase as previous signal burst nBinary 1 signal burst of opposite phase to previo
13、us signal burst Phase-Shift Keying (PSK) nFour-level PSK (QPSK) nEach element represents more than one bit Phase-Shift Keying (PSK) nMultilevel PSK nUsing multiple phase angles with each angle having more than one amplitude, multiple signals elements can be achieved nD = modulation rate, baud nR = d
14、ata rate, bps nM = number of different signal elements = 2L nL = number of bits per signal element Performance nBandwidth of modulated signal (BT) nASK, PSK BT=(1+r)R nFSKBT=2DF+(1+r)R nR = bit rate n0 r 1; related to how signal is filtered n DF = f2-fc=fc-f1 Performance nBandwidth of modulated sign
15、al (BT) nMPSK nMFSK nL = number of bits encoded per signal element nM = number of different signal elements Quadrature Amplitude Modulation nQAM is a combination of ASK and PSK nTwo different signals sent simultaneously on the same carrier frequency Quadrature Amplitude Modulation Reasons for Analog
16、 Modulation nModulation of digital signals nWhen only analog transmission facilities are available, digital to analog conversion required nModulation of analog signals nA higher frequency may be needed for effective transmission nModulation permits frequency division multiplexing Basic Encoding Techniques nAnalog data to analog signal nAmplitude modulation (AM) nAngle modulation nFrequency modulation (FM) nPhase modulation (PM)
