Impact of linear misalignment on a spatial OFDM based pixelated system

This paper investigates the effect of fractional linear misalignment between the transmitter and the receiver for a pixelated MIMO optical wireless system using spatial orthogonal frequency division multiplexing (OFDM). The novelty of this work is in the modeling and analysis of fractional misalignment for spatial OFDM. It is shown that when the receiver is perfectly focused and when fractional misalignment is the only impairment, the received constellation points are phase shifted and attenuated. MATLAB simulations show that the impact of misalignment is the greatest when the fractional offset is equal to half of a pixel. Unlike integral misalignment, the use of equalizers cannot fully compensate the effect of fractional misalignment. It is revealed that when the fractional offset is half of a pixel, the offset causes bit error rate (BER) floor and the degradation is greater for a system with lower number of subcarriers than a system with larger number of subcarriers. We show that the BER degradation caused by fractional offset can be mitigated when a number of higher spatial frequency subcarriers are not used for data transmission.