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Diversity and two types of diversity

Short answer

Diversity combats fading by combining independently faded signal copies. Space diversity uses multiple antennas spaced λ/2 (mobile) or 10λ (base), with combining techniques like maximal ratio combining. Time diversity transmits the same information at intervals greater than the coherence time, typically using channel coding and interleaving to recover faded bits.

What is diversity? Explain any two types of diversity techniques in detail.

Diversity

Diversity is a technique in which the receiver gets two or more copies of the same information over independently fading paths (branches) and selects or combines them. Since all branches are unlikely to fade deeply at the same time, the combined signal is much more reliable. It is a powerful, low-cost way to fight small-scale (Rayleigh) fading.

1. Space (antenna) diversity

  • Uses M receiving antennas separated so their signals are uncorrelated: about λ/2 at the mobile and 10λ or more at the base station (where scatterers are far away).
  • No extra bandwidth or transmit power is needed.
  • Branch signals are combined by:
    • Selection – choose the branch with the highest SNR.
    • Feedback/scanning – stay on a branch until it falls below a threshold, then switch.
    • Maximal ratio combining – co-phase, weight each branch by its SNR and add; output SNR = Σ γᵢ (best).
    • Equal gain combining – co-phase and add with equal weights; close to MRC.
 Ant1 --[co-phase ×G1]--+
 Ant2 --[co-phase ×G2]--+--(Σ / select)--> detector
 AntM --[co-phase ×GM]--+
  • Example: two antennas on a base-station mast several metres apart.
  • With MRC, M branches give M times the mean SNR of one branch; with selection the gain is Σₖ₌₁ᴹ 1/k.

2. Time diversity

  • The same information is sent at time intervals greater than the coherence time T_c (≈ 0.423/f_d), so the copies fade independently.
  • Implementations:
    • Channel coding with interleaving – coded bits are spread in time, so a fade causes scattered errors that the decoder corrects (GSM, IS-95, LTE).
    • RAKE receiver – in CDMA, multipath copies separated by more than one chip are despread separately and combined.
    • Repetition coding and ARQ.
  • Drawback: adds delay; useless when the mobile is stationary (T_c becomes very long).
 data->[coder]->[interleaver]->fading channel
     ->[de-interleaver]->[decoder]->data

(Other types: frequency diversity – carriers separated by more than the coherence bandwidth; polarization diversity – orthogonally polarized antennas.)

✓ Answer checked by Claude Opus 5.5 (high effort)

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