In this paper, the performance of Multiple-Input Multiple-Output (MIMO) systems based on the fractional Fourier transform (FRFT) and the fast Fourier transform (FFT) in underwater acoustic (UWA) communication channels is evaluated and compared for various conditions including the number of subcarriers, modulation schemes, the number of paths between the transmitter and receiver, Doppler frequencies, and MIMO modes including diversity and multiplexing. The study demonstrates while the computational complexity of the FRFT is in the order of the FFT, the performance of the FRFT-based UWA system is superior than that of the FFT-based system for all multipath scenarios, and for the flat fading channel they achieve the same performance.

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In this paper, we propose and apply two novel schemes of multi-band carrierless amplitude and phase (m-CAP) modulation for spatial modulation (SM) based visible light communications (VLC) systems. In SM, both spatial and signal constellation are utilised to improve the performance of the m-CAP system. Here, we have adopted the non-DC m-CAP which is a power efficient technique with no DC-bias. The bit error rate (BER) performance and the spectral efficiency of the proposed systems are compared with single-input single-output (SISO) and multiplexing multiple-input multiple-output (MIMO) m-CAP systems. Results demonstrate that non-DC m-CAP, with the same spectral efficiency as that of SISO m-CAP, outperforms other schemes in terms of BER due to its power efficiency, however a higher spectral efficiency can be achieved by the SM-MIMO m-CAP scheme.

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