In other words, we basically have two DDs that are operating in a serial
fashion with the third DD computing the sum of the individual DD outputs. Alternatively stated,
there are two conventional DDs (N2) operating on symbols of consecutive time intervals. The third
detector is basically calculating a metric that is the sum of the individually calculated metrics. All the
possible combinations are exhaustively compared and the sequence with the largest projection on the
real axis is chosen.
A note to make here is that this block decoder performs an exhaustive search over all possible
DQPSK sequences of length N, to provide the MLE of the transmitted symbol. Since this is QPSK
the number of searches is 4N??“1. For 3-symbol observation window this equals 16 combinations. As we
will soon see at the end of this chapter, increasing N will improve performance at the expense of
increase complexity. As complexity grows the system designer must evaluate system performance to
determine if rather a coherent detector is required. A block diagram for this MLDD using 3-symbol
observation window is given in Fig. 4.30.
4.3.3 Decision Feedback Differential Detection (DF-DD)
In this noncoherent technique we will make use of past detected symbols in order to improve the
detection of the present symbol [53??“55].
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