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Space diversity techniques with block diagrams

Short answer

In space diversity, multiple antennas provide independent fading copies. Selection diversity chooses the branch with highest SNR. Scanning diversity holds the first branch above a threshold. Maximal ratio combining co-phases and weights each branch by its SNR, summing to yield output SNR equal to the sum of individual SNRs. Equal gain combining simply co-phases with equal gains.

Explain various space diversity techniques along with block diagrams.

Space diversity uses M receiving antennas separated enough (about λ/2 at the mobile, 10λ or more at the base station) that each receives an independently faded copy of the signal. The copies are then used in one of four ways.

1. Selection diversity

 Ant1-[Rx1]-+
 Ant2-[Rx2]-+--> [select max SNR] --> demodulator
 AntM-[RxM]-+

The branch with the highest instantaneous SNR is connected to the output. Simple; output SNR = max(γᵢ).

2. Feedback (scanning) diversity

 Ant1-+
 Ant2-+--[scan switch]--[Rx]--> output
 AntM-+        ^          |
               +--[SNR < threshold?]

Branches are scanned in sequence; the first branch above a threshold is held until it falls below it. Needs only one receiver.

3. Maximal ratio combining (MRC)

 Ant1-[co-phase]-[×G1]-+
 Ant2-[co-phase]-[×G2]-+--(Σ)--> detector
 AntM-[co-phase]-[×GM]-+
       Gi ∝ amplitude / noise

Each branch is co-phased and weighted by its SNR, then summed. Output SNR = Σ γᵢ (the best result).

4. Equal gain combining (EGC)

Same as MRC but all gains Gᵢ = 1 after co-phasing. Performance is close to MRC and the circuit is simpler.

✓ Answer checked by Claude Opus 5.5 (high effort)

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