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Download HIGH SPEED DESIGN TECHNIQUES
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QPSK MODULATION 01 Q 11 2-BITS/SYMBOL I 10 00 I OR Q CHANNEL SAMPLING CLOCK t a 5.56 In the QPSK system, the magnitude of each symbol is equal, and only the phase is modulated. More complex modulation schemes such as QAM (Quadrature Amplitude Modulation), use more symbols on the constellation and thereby transmit more bits of information per symbol (at the expense of more sensitivity to noise and more complex digital signal processing). Figure 5.57 shows a 16-QAM constellation which contains 4-bits of information per symbol. Note that the I and Q channel receiver DSP must now identify the signal as representing one of the four possible levels. Although the 16-QAM signal carries more bits per symbol, it is more sensitive to noise, and the ADC requires more resolution (typically 8-bits) than for QPSK modulation (typically 4, 5, or 6 bits). 50
