Importance and types of diversity

Importance of diversity

The mobile channel causes multipath (Rayleigh) fading: the received signal can drop by 20–40 dB in a fraction of a wavelength of movement. Diversity is important because:

• It gives several independently faded copies, so the chance of all being in a deep fade together (≈ pᴹ) is very small.
• It greatly reduces BER and call drops without extra transmit power.
• Many forms (space, polarization) need no extra bandwidth.
• It needs no training overhead and is cheap compared with the improvement it gives (often 10–20 dB gain at low BER).

Types of diversity techniques

1. Space (antenna) diversity

• Two or more antennas separated so that fading is uncorrelated (≈ λ/2 at mobile, ≥ 10λ at base station).
• Combining: selection, feedback, maximal ratio and equal gain combining.

2. Polarization diversity

• Horizontally and vertically (or ±45°) polarized antennas at one location. Scattering de-correlates the two polarizations.
• Compact; widely used at base stations; gain is a little less than space diversity.

3. Frequency diversity

• The same information is sent on carriers separated by more than the coherence bandwidth (B_c ≈ 1/(5σ_τ)).
• Disadvantage: needs extra spectrum and a receiver per frequency. Frequency hopping and OFDM use this idea.

4. Time diversity

• Repeats the information at intervals greater than the coherence time (T_c ≈ 0.423/f_d).
• Implemented with channel coding plus interleaving, or with a RAKE receiver in CDMA.

5. Angle (pattern) diversity

• Directional antennas pointing in different directions pick up differently faded multipath components.