Welcome to the World of CDMA
QPSK Spreading - Receiver Statistics
The QPSK version of the modulator duplicates the BPSK structure, but with a quadrature RF carrier applied (Figure 1).
The channel model is the same as the BPSK case (Figure
We can shortcut most of the math here by noting that each of the QPSK channels, I and Q, is exactly like the BPSK model, except that half the signal energy appears in each channel, so we replace with /2. The noise and interference, on the other hand is the same in each channel, so those expressions are unchanged. The detection amplitude means and variances are now
is unchanged. The reason for the quadrature spreading is really to make sure that the mutual interference between users and between stations is uniformly distributed in phase. It otherwise contributes nothing to performance.
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