Optimal Power Allocation for MIMO Underwater Wireless Optical Communication Systems Using Channel State Information at the Transmitter

For multiple-input–multiple-output underwater wireless optical communication (UWOC) systems with on–off keying modulation, we obtain the optimal power allocation policies that minimize the bit error rate (BER), subject to average power constraints. In this article, we consider a comprehensive channel model that takes into account absorption, scattering, and weak turbulence-indeed lognormal fading. The optimal power allocation policies are obtained using the perfect channel state information available at the receiver (CSIR) and transmitter (CSIT) for both MISO and MIMO systems. In the MISO UWOC link, the optimal policies result in allocating the whole available power to the optical source with highest instantaneous SNR, i.e., maximum fading coefficient. In the MIMO UWOC link, the equivalent fading coefficient of the $i$th optical source is defined as the sum of the fading coefficient associated with the $i$th optical source and other receiver apertures. In this case, the total available power should be allocated to the optical source with the highest equivalent fading coefficient to have the minimum BER. Furthermore, we analytically obtain the exact BER expressions for both MISO and MIMO systems assuming the optimal power allocation. Simulation results are further provided to verify our analytical results. The results demonstrate that MIMO techniques with optimal power allocation provide better performance compared to other alternatives such as equal power allocation.