Equalization and diversity; space diversity methods

Need for equalization and diversity

• Equalization is needed to remove inter-symbol interference (ISI) caused by time-dispersive (frequency-selective) multipath. When the delay spread is a large part of the symbol period, earlier symbols overlap with later ones; an adaptive equalizer reverses the channel's effect.
• Diversity is needed to fight deep fading in flat-fading channels. Because the received signal randomly falls 20–40 dB, receiving multiple independently fading copies lets the receiver avoid or combine around deep fades, reducing BER without more transmit power.
• Both together raise link quality, allow higher data rates and lower outage probability.

Space diversity reception methods

1. Selection diversity
2. Feedback (scanning) diversity
3. Maximal ratio combining
4. Equal gain combining

Selection diversity (explained)

M antennas, spaced at least about λ/2 (at the mobile) apart, feed M receivers. The receiver continuously selects the branch with the highest instantaneous SNR and connects it to the demodulator.

 Ant1 ->[Rx1]-SNR1-+
 Ant2 ->[Rx2]-SNR2-+->[Select max]-> demod
 AntM ->[RxM]-SNRM-+

Probability that all M branches are below threshold γ (Rayleigh, mean SNR Γ):

P[γ₁..γ_M ≤ γ] = (1 − e^(−γ/Γ))^M

Mean output SNR improvement:

γ̄/Γ = Σ_(k=1)^(M) 1/k

Example: 2 branches give 1.5 times (≈ 1.76 dB) average SNR improvement, and much larger gain at low outage levels. Simple to build, but only one branch is used at a time.