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Need for diversity; maximum ratio combining

Short answer

Diversity counters deep fading by providing independent signal paths, so the probability of simultaneous fade is low. Maximal ratio combining co-phases the signals from M antennas, weights each by its instantaneous SNR, and sums them, yielding output SNR equal to the sum of branch SNRs — the optimal linear combining method.

Why is there a need to implement diversity? Describe the maximum ratio combining space diversity technique.

Need for diversity

In a fading channel the received signal often falls into deep fades (20–40 dB). Increasing transmit power to cover these fades is costly. Diversity sends or receives the signal over two or more independently fading paths; the chance that all paths fade at the same time is very small. It improves the average SNR and reduces the error rate, with low cost and no extra bandwidth (for space diversity).

Maximal ratio combining (MRC)

In MRC, the signals from all M antenna branches are co-phased, weighted in proportion to their own signal amplitude (SNR), and summed.

 Ant1 ->[x G1]-+
 Ant2 ->[x G2]-+->(Σ)-> output, SNR = Σ γ_i
 AntM ->[x GM]-+
   (co-phased, G_i ∝ r_i/N_i)
  • Output SNR = sum of the branch SNRs: γ_M = Σ γ_i, the best possible for a linear combiner.
  • Gives an acceptable SNR even when no single branch is good.
  • Needs a separate receiver and phase tracking per branch, so it is the most complex method.

✓ Answer checked by Claude Opus 5.5 (high effort)

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