ISI mitigation and space diversity techniques

Signal processing operation that minimizes ISI: equalization

Equalization is the signal processing used at the receiver to reduce inter-symbol interference (ISI) caused by multipath delay spread and band-limited channels. When the delay spread is larger than the symbol period, each symbol spreads into the next. An equalizer is a filter whose response is roughly the inverse of the channel response, so that

H_eq(f) · H_ch(f) ≈ constant (flat), i.e. h_ch(t) * h_eq(t) ≈ δ(t)

Because the mobile channel changes with time, the equalizer is adaptive: it is first trained with a known training sequence, then tracks the channel in decision-directed mode using algorithms such as LMS or RLS. Common types are linear transversal (FIR) equalizers, decision feedback equalizers (DFE) and MLSE (Viterbi) equalizers.

Space diversity techniques

Space (antenna) diversity uses two or more antennas spaced far enough apart that their fading is uncorrelated: about λ/2 at the mobile and 10λ or more at the base station. The branch signals are then used in one of these ways:

1. Selection diversity – The receiver monitors the SNR of all M branches and connects the branch with the highest instantaneous SNR to the demodulator. It is simple and needs one receiver after the switch.

2. Feedback (scanning) diversity – The branches are scanned in a fixed order until one is found above a preset threshold; that branch is used until it falls below the threshold, then scanning restarts. Only one receiver is needed, but it performs a little worse than selection.

3. Maximal ratio combining (MRC) – Every branch is co-phased, weighted in proportion to its signal amplitude (gain ∝ signal voltage / noise power), and summed. The output SNR is the sum of branch SNRs (γ_M = Σ γᵢ), which is the best possible. It needs a receiver and phase correction on every branch.

4. Equal gain combining (EGC) – Branches are co-phased and added with equal weights. It performs only slightly worse than MRC and is easier to build because no amplitude estimation is needed.

 Ant1   Ant2   ...  AntM
  |      |           |
 [G1]   [G2]  ...   [GM]   gains / co-phasing
  |      |           |
  +------+-----+-----+
               |
      Select / Sum (combiner)
               |
          Demodulator

Selection and feedback pick one branch; MRC and EGC add all branches.