ISI mitigation and M-branch RAKE receiver

Signal processing to minimize ISI

Equalization minimizes ISI. An equalizer is a receiver filter with roughly the inverse frequency response of the channel, so that channel + equalizer gives a flat, linear-phase response. Since the mobile channel is random and time-varying, adaptive equalizers are used: a known training sequence first sets the tap weights, and then the equalizer tracks the channel using its own decisions.

RAKE receiver

A RAKE receiver is a CDMA receiver with several correlators (fingers), each locked to a different multipath copy of the signal. Since the PN code's autocorrelation is very small beyond one chip, copies delayed by more than one chip can be separated and then combined. This gives multipath diversity instead of ISI.

Working of 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]-> m'

1. A searcher finds the delays of the M strongest paths.
2. Each correlator multiplies the input by the PN code shifted to its path delay τ_m and integrates, giving output Z_m.
3. Each output is weighted by α_m, proportional to that path's strength, e.g. α_m = Z_m² / Σ Z_m².
4. The outputs are co-phased and summed: Z' = Σ α_m Z_m.
5. Z' is integrated over the bit and compared with a threshold to decide the bit.

If one path fades deeply, the others usually do not, so the combined output is much more reliable. IS-95 mobiles use 3 fingers; base stations use 4.