7.2 RAKE RECEIVER PRINCIPLE
Let us consider a simple BPSK-based CDMA system. The transmitter example is given in Fig. 7.4,
where the input data bits b(t) are multiplied by a chip sequence c(t), in order to produce the spectrally
spread signal y(t). This spread signal is then spectrally up-converted to its allowed frequency band
centered at a carrier frequency denoted by fc. The block diagram in Fig. 7.4 highlights the two operations
performed at the transmitter, namely, spreading and spectral up-conversion.
We can immediately draw the receiver architecture with the assumption of ideal carrier recovery,
chip-time synchronization, and the knowledge of the actual PN chip sequence used at the transmitter.
The receiver block diagram shown in Fig. 7.5 highlights the two operations performed at the receiver,
namely, spectral down conversion and despreading.
The spectral down-conversion operation involves the multiplication of a sinusoid followed by the
low-pass filter (LPF) to remove the unwanted spectral product. This baseband (ZIF) signal becomes
despread by first multiplying by the PN chip sequence c(t) and then accumulating the chips across the
SF interval. In this example, the SF interval is equivalent to the bit time duration.
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