Compare different diversity combining techniques and their performance.

Diversity combines multiple independently fading signal copies to improve reliability. Techniques: Selection combining (selects branch with highest SNR, simplest, diversity gain d=N), Equal gain combining (coherently sums equal-weighted branches, 1dB less than MRC), Maximal ratio combining (MRC, weights proportional to SNR, optimal, maximizes output SNR). For Rayleigh fading, MRC with N branches achieves BER improvement of SNR^(-N). Implementation: Requires channel estimation for MRC weights, selection needs only branch SNR monitoring. Space, time, frequency, and polarization domains provide diversity. Coding also provides diversity through redundancy across fades.