Читать книгу Spatial Multidimensional Cooperative Transmission Theories And Key Technologies - Lin Bai - Страница 41
2.3.2.1Reception diversity through selective combining
ОглавлениеAmong MR received signals, the combiner selects a branch having the largest SNR (or the highest absolute power, bit error rate, etc.) for signal detection. Suppose that MR channels are subject to unit Rayleigh energy independent and identical distribution and the noise level is equal on each antenna. At this time, the selection algorithm compares the instantaneous amplitude of each channel sn(n = 1, . . . , MR) and selects the branch having the largest amplitude smax = max(s1, . . . , sMn}. The probability that smax is below a certain threshold S2 is given by
The distribution corresponding to smax can be obtained by the differentiation of Eq. (2.93).
The average SNR of the combiner output is3
when MR is large, the array gain is approximately
where γ ≈ 0.577 215 66 is the Euler constant.
The diversity obtained by the selective combining can be estimated by calculating the bit error rate using the fading distribution given by Eq. (2.94). For the system using BPSK modulation and owning a two-branch diversity, the bit error rate as a function of the average SNR can be expressed corresponding to each channel ρ4 as follows:
when the SNR is high,
The slope of the bit error rate curve is 2. In general, the diversity gain of the MR-branch selection diversity scheme is equal to MR, which indicates that the selection diversity collects all possible diversity from the channel.
