数据与计算机通信05-SignalEncodingTechniques 2014.ppt

Binary Frequency Shift Keying Most common form is binary FSK (BFSK) Two binary values represented by two different frequencies (near carrier) Less susceptible to error than ASK Up to 1200bps on voice grade lines High frequency radio Even higher frequency on LANs using coax Multiple FSK More than two frequencies used More bandwidth efficient More prone to error Each signalling element represents more than one bit FSK on Voice Grade Line Phase Shift Keying Phase of carrier signal is shifted to represent data Binary PSK Two phases represent two binary digits Differential PSK Phase shifted relative to previous transmission rather than some reference signal Differential PSK Quadrature PSK More efficient use by each signal element representing more than one bit e.g. shifts of ?/2 (90o) Each element represents two bits Can use 8 phase angles and have more than one amplitude 9600bps modem use 12 angles , four of which have two amplitudes Offset QPSK (orthogonal QPSK) Delay in Q stream QPSK and OQPSK Modulators Examples of QPSF and OQPSK Waveforms Performance of Digital to Analog Modulation Schemes Bandwidth ASK and PSK bandwidth directly related to bit rate FSK bandwidth related to data rate for lower frequencies, but to offset of modulated frequency from carrier at high frequencies In the presence of noise, bit error rate of PSK and QPSK are about 3dB superior to ASK and FSK Quadrature Amplitude Modulation QAM used on asymmetric digital subscriber line (ADSL) and some wireless Combination of ASK and PSK Logical extension of QPSK Send two different signals simultaneously on same carrier frequency Use two copies of carrier, one shifted 90° Each carrier is ASK modulated Two independent signals over same medium Demodulate and combine for original binary output QAM Modulator QAM Levels Two level ASK Each of two streams can be in one of two states Four state system Essentially QPSK Four level ASK Combined stream in one of 16 states 64 and 256 state systems have been implement