An Investigation of Performance and Diversity Property of Optical Space Shift Keying-Based FSO-MIMO System

In this paper, the bit error rate (BER) and the channel capacity of a multiple-input multiple-output (MIMO) system employing optical space shift keying (OSSK) scheme (termed as MIMO-OSSK) are investigated under combined effect of atmospheric turbulence (AT) and pointing errors (PEs). The analysis reveals that the diversity order of MIMO-OSSK remains intact over all AT regimes and under PE severity. This behavior is contrary to traditional pulse-based modulation schemes, where diversity order depends upon AT conditions and PE severity. In addition, the analysis also demonstrates that the MIMO-OSSK utilizes receiver diversity efficiently as compared to existing pulse-based modulations. In order to examine the effect of all ranges of AT along with PE severity on MIMO-OSSK, three different channel models are considered, i.e., log normal, gamma–gamma, and negative exponential. The probability density functions of the sum of squares of difference between two channel coefficients under these channel models are evaluated, to develop analytical expressions of average BER and ergodic channel capacity for the considered setup. On comparing MIMO-OSSK with the same rate MIMO-pulse amplitude modulation (MIMO-PAM) scheme employing repetition coding, it is observed that the MIMO-OSSK scheme outperforms the MIMO-PAM for a spectral efficiency of more than 2 b/s/Hz, provided that the number of receivers is sufficiently large.