1) d(t) Re 5e
j [u(t)fe]6 fe f f ^
FIGURE 4.1 General receiver block diagram.
MODULATION DETECTION TECHNIQUES 175
If we assume that QPSK was transmitted then we would receive the following constellation diagram
(see Fig. 4.3). The received constellation is rotated by the amount equal to the phase error (assumed
constant for this example).
I-Channel
Q-Channel
fe
= Transmitted State
= Received State
FIGURE 4.3 QPSK constellation offset due to phase error.
As you can clearly see from Fig. 4.3, the residual/uncompensated phase offset will push the phase
states closer to the decision boundary. This will make states more susceptible to noise and thus
degrade BER performance.
In the next subsections we will begin by introducing some IF techniques used to perform coherent
detection and then present some baseband techniques which perform with a higher level of accuracy
and are more commonly deployed in commercial products. These are applicable to both time
division multiple access (TDMA) and code division multiple access (CDMA) systems.
The IF techniques are discussed first for historical reasons in that the initial techniques used for
carrier recovery in the digital cellular area were more focused on the IF signal (analog section).
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