Clustering Based Diversity Improving Transmit Laser Selection Schemes Using Quantized Feedback for FSO System

In a free-space optical (FSO) communication system, a conventional transmit laser selection (CTLS) scheme selects the best transmit aperture to send the information bits and offers the best possible error performance. However, its performance is highly dependent on the feedback link, and even a slight error in the feedback information can cause a significant loss to the diversity order and bit error rate (BER). To mitigate this problem, in this research, a novel clustering-based TLS scheme, named two TLS (TTLS), is proposed and analyzed. Note that the word two in TTLS corresponds to the transmission corresponding to the two transmit apertures. TTLS scheme divides the arbitrary number of transmit apertures/lasers into two clusters and then selects the best transmit aperture from each cluster. It is observed by analysis that the TTLS scheme offers a lesser loss in the diversity order as compared to CTLS under imperfect feedback link. Therefore, a significant performance gain can be achieved by TTLS as compared to CTLS under a practical (imperfect/erroneous feedback link) scenario. Further, to retain the full diversity order under the imperfect feedback link, a modified error-tolerant weighting scheme (METWS) is also proposed. It is noticed from the asymptotic analysis that METWS offers full diversity order and outperforms CTLS and repetition coding schemes under both perfect and imperfect feedback links.