Diversity and RAKE receiver operation

How diversity improves network quality

Diversity is a receiver technique that gives the receiver two or more independently faded copies of the same signal and then selects or combines them. Because deep fades on independent branches rarely happen at the same time, the chance that all copies are weak is very small.

• If one branch is in a fade with probability p, the probability that all M independent branches fade together is about pᴹ (e.g. 0.1 → 0.001 for M = 3).
• The combined signal has a higher and more stable average SNR, so the bit error rate falls sharply and fewer calls drop.
• Unlike equalization, diversity needs no training sequence and adds little delay.
• Result: better coverage at cell edges, lower required transmit power, higher capacity and better voice/data quality.

RAKE receiver

A RAKE receiver is a diversity receiver used in CDMA (IS-95, WCDMA). In CDMA the chip duration is very short, so multipath copies that arrive more than one chip (Tc) apart are resolvable. Instead of treating these delayed copies as interference, the RAKE receiver collects them, like the fingers of a garden rake gathering leaves.

             +------------+   +----+   w1
        +--->| Correlator |-->|Z1  |--(x)--+
        |    |  code c(t) |   +----+       |
        |    +------------+                |
 r(t)   |    +------------+   +----+   w2  v   +-------+
 ------>+--->| Correlator |-->|Z2  |--(x)-(+)->|Integr.|
        |    | c(t - τ2)  |   +----+       ^   | & dec.|
        |    +------------+                |   +-------+
        |    +------------+   +----+   wM  |      |
        +--->| Correlator |-->|ZM  |--(x)--+      v
             | c(t - τM)  |   +----+          data bits
             +------------+
      (one "finger" per resolvable multipath)

Operation

1. The received signal r(t) is fed to M correlators, called fingers.
2. Each finger multiplies r(t) by the user's PN code delayed by the delay τₖ of one multipath component, and integrates over one bit. This despreads that one path; other paths look like noise to it because the PN code has low autocorrelation for shifts ≥ Tc.
3. A searcher (path finder) estimates the delays τₖ and strengths of the strongest paths and assigns them to the fingers.
4. Finger outputs Z₁ … Z_M are weighted by w₁ … w_M, proportional to the SNR (amplitude) of each path, so strong paths count more. This is maximal-ratio style combining: Z′ = Σ wₖ·Zₖ
5. The combined output Z′ is integrated and passed to a decision device to recover the bits.

Benefit: multipath, which harms narrowband systems, becomes a source of multipath (time) diversity, improving SNR and reducing fading in CDMA systems.