The RAKE receiver is essentially a diversity receiver designed specifically for CDMA, where the diversity is provided by the fact that the multipath components are practically uncorrelated from one another when their relative propagation delays exceed a chip period.
An M branch (M-finger) RAKE receiver implementation:
Figure 12.2 Block diagram of a RAKE receiver
- In the Above figure 12.1 each correlator detects a time shifted version of the original CDMA transmission, and each finger of the RAKE correlates to a portion of the signal which is delayed by at least one chip in time from the other fingers.
- A RAKE receiver utilizes multiple correlators to separately detect the M strongest multipath components
- The M decision statistics are weighted to form an overall decision statistic as shown in Figure 12.1. The outputs of the 1 M correlators are denoted as Z1, Z2 ,... and ZM
- They are weighted by a1,a2,, ...asid respectively. The weighting coefficients are based on the power or the SNR from each correlator output. If the power or SNR is small out of a particular correlator, it will be assigned a small weighting fact
- Just as in the case of a maximal ratio combining diversity scheme, the overall signal Z' is given by
The weighting coefficients, am, are normalized to the output signal power of the correlator in such a way that the coefficients sum to unity