B. P. Lathi, Zhi Ding - Modern Digital and Analog Communication Systems-Oxford University Press (2009)

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SPREAD SPECTRUM COMMUNICATIONS
Figure 11 • 1 5
Minimum mean
square error
MUD receiver.
Noise
C1 (l) t
t=kTs
® (1)
rk
t=kTs
(0
tf
k
Decision Feedback Receiver
We note that both the decorrelator and the MMSE MUD receivers apply linear matrix processing.
Hence, they are known as linear receivers with low complexity. On the other hand,
the optimum MUD receiver is nonlinear but requires much higher complexity. There is also a
very popular suboptimum receiver that is nonlinear. This method is based on the concept of
successive interference cancellation, known as the decision feedback MUD receiver. 14 • 15
The main motivation behind the decision feedback MUD receiver lies in the fact that in
a near-far environment, not all users suffer equally. In a near-far environment, the stronger
signals are actually winners, whereas the weaker signals are losers. In fact, when a particular
user has a strength ,./Peg e that is stronger than those of al I other users, its conventional matched
filter receiver can in fact deliver better performance than is possible in an environment of equal
strength. Hence, it would make sense to rank the received users in the order of their individual
strength measured by {P;g;}. The strongest user QAM symbols can then be detected first, using
only the conventional matched filter receivers designed for single users. Once the strongest
user symbols is known, its interference effects on the remaining user signals can be canceled.
By canceling the strongest user symbol from the received signal vectors, there are only M - I
unknown user symbols for detection. Among them, the next strongest user signal can be
detected more accurately after the strongest interference has been removed. Hence, its effect
can also subsequently be canceled from received signals, to benefit the M - 2 remaining user
symbols, and so on. Finally, the weakest user signal will be detected last, after all the MAI has
been canceled.
Clearly, the decision feedback MUD receiver relies on the successive interference cancellation
of stronger user interferences for the benefit of weaker user signal detection. For this
reason, the decision feedback MUD receiver is also known as the successive interference cancellation
(SIC) receiver. The block diagram of the decision feedback MUD receiver appears
in Fig. 11.16. Based on Eq. (11.31), the following steps summarize the SIC receiver:
Decision Feedback MUD
Step 1. Rank all user signal strengths {P;g;}. Without loss of generality, we assume that
Let
(i) (i)
Y1 = rk