Shifts of 2 90o eg each element represents two bits

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Unformatted text preview: of 9600bps which have two amplitudes which Quadrature PSK (QPSK) Quadrature QPSK and OQPSK Modulators Modulators Performance of Digital to Analog Modulation Schemes Analog bandwidth ASK/PSK bandwidth directly relates to bit rate multilevel PSK gives significant improvements FSK bandwidth related to data rate for lower FSK frequencies, but to offset of modulated frequency from carrier at high frequencies from in presence of noise: bit error rate of PSK and QPSK are about 3dB bit superior to ASK and FSK superior for MFSK & MPSK have tradeoff between for bandwidth efficiency and error performance Quadrature Amplitude Modulation Modulation QAM used on asymmetric digital subscriber line QAM (ADSL) and some wireless (ADSL) combination of ASK and PSK logical extension of QPSK send two different signals simultaneously on send same carrier frequency same 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 QAM Variants QAM two level ASK each of two streams in one of two states four state system essentially QPSK four level ASK combined stream in one of 16 states have 64 and 256 state systems have improved data rate for given bandwidth but increased potential error rate Analog Data, Digital Signal Analog digitization is conversion of analog data digitization into digital data which can then: into be transmitted using NRZ-L be transmitted using code other than NRZ-L be converted to analog signal analog to digital conversion done using a analog codec codec pulse code modulation delta modulation Digitizing Analog Data Digitizing Pulse Code Modulation (PCM) Pulse sampling theorem: “If a signal is sampled at regular intervals at a If rate higher than twice the highest signal frequency, the samples contain all information in original signal” in eg. 4000Hz voice data, requires 8000 sample eg. per sec per strictly have analog samples strictly Pulse Amplitude Modulation (PAM) so assign each a digital value Pulse Code Modulation(PCM) 4 bit system gives 16 levels Quantized Quantizing error or noise Approximations mean it is impossible to recover Approximations original exactly original 8 bit sample gives 256 levels Quality comparable with analog transmission 8000 samples per second of 8 bits each 8000 gives 64kbps gives SNR = 6.02n + 1.76dB PCM Example PCM PCM Block Diagram PCM Non-Linear Coding Non-Linear Companding Companding Delta Modulation Delta analog input is approximated by a analog staircase function staircase can move up or down one level (δ) at each at sample interval sample has binary behavior since function only moves up or dow...
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