Switch-and-stay and switch-and-examine dual diversity for high-speed free-space optics links

Spatial diversity is widely adopted as a major approach towards performance improvement in wireless communications. This study proposes the application of two diversity schemes - switch-and-stay combining (SSC) and switch-and-examine (SEC) combining - in a dual-branch receiving optical wireless system. These schemes, when compared to the selection combining scheme, significantly lower the processing load associated with the monitoring of each branch signal strength, thus reducing the complexity of the required hardware. The reduction of processing load supports the typical high data rate that an optical wireless system is capable of offering. Analytical analyses are conducted to evaluate the dual-branch receiver system operating in a spatially correlated and lognormally distributed optical fading channel. Bit error rate (BER) performance and processing load relative to branch monitoring are characterised, obtained and discussed. The results confirm that BER performance is dependent, among others, on the selected threshold value of signal-to-noise ratio (SNR) below which the receiver switches to a new branch. The results also show that at a branch average SNR of 17-dB, maximum achievable processing load reductions are 91- for SSC and 35- for SEC. Corresponding BER performance of SSC degrades, however, by 1-dB, whereas the degradation is nearly 0-dB for SEC.