RAKE receiver and M-branch RAKE

RAKE receiver

A RAKE receiver is a CDMA receiver that collects the energy of several multipath components of the same signal and combines them, instead of treating them as interference. It has several correlators, called fingers, like the teeth of a garden rake. Each finger is locked to one multipath copy that is delayed by at least one chip, so the copies are nearly uncorrelated. Combining them gives multipath (time) diversity, which improves the SNR and reduces fading.

Working of an M-branch RAKE receiver

       +->[Corr. 1, τ1]->Z1->(x)α1-+
       |                           |
r(t) --+->[Corr. 2, τ2]->Z2->(x)α2-(Σ)-> Z'
       |                           |    |
       +->[Corr. M, τM]->ZM->(x)αM-+    v
               ^                  [∫ + decide]
     [PN code + path searcher]          |
                                        v  m'(t)

1. Path searching: a searcher finds the M strongest multipath delays τ₁ … τ_M from the channel's power delay profile using the PN code's sharp autocorrelation.
2. Correlation: correlator m multiplies r(t) by the PN code delayed by τ_m and integrates over the bit. Its output Z_m is the despread data from path m. Other paths, being more than one chip apart, appear as low-level noise.
3. Weighting: each output is multiplied by a weight α_m proportional to the signal strength (SNR) of that path, so strong paths count more:

α_m = Z_m² / Σ_(m=1..M) Z_m²

4. Combining: the weighted outputs are co-phased and summed:

Z' = Σ_(m=1..M) α_m Z_m

5. Decision: the combined output is integrated and compared with a threshold to give the data bit.

Why it works

If one path fades, others are likely strong, because fading on paths separated by more than one chip is independent. With maximal ratio weights the output SNR approaches the sum of the finger SNRs. IS-95 uses 3 fingers in the mobile and 4 in the base station; WCDMA also uses RAKE receivers.