This work proposes a dual pairwise coding (DPWC) technique for multiple-input multiple-output (MIMO) orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) systems. DPWC is a technique designed to mitigate the signal-to-noise ratio (SNR) imbalance created in the received information symbols due to the light emitting diodes' bandwidth limitations and the strong correlation of typical indoor VLC MIMO channels. With singular value decomposition precoding, the MIMO VLC channel is decomposed into independent parallel sub-channels. The DPWC technique is then applied to further improve the system performance by pairing and interleaving OFDM subcarriers and MIMO sub-channels with imbalanced SNR. The error performance of the proposed DPWC technique is investigated in detail via simulation and experimental demonstration. Its effectiveness with different degrees of bandwidth-limitation (data rate) and channel correlation is presented, and the results are compared with other variants of the pairwise coding technique. DPWC provides performance improvement with a significant reduction in the SNR required to achieve a specific bit error rate (BER).

This work investigates the impact of timing offset on the performance of an optical spatial modulation (OSM) technique termed spatial pulse position modulation (SPPM). SPPM combines optical spatial shift keying (OSSK) with pulse position modulation (PPM), exploiting the spatial domain to improve spectral the efficiency of PPM. In most works related to OSM, a perfect synchronization between the transmit and receive units is assumed. However, timing offset can result from clock jitter and variation in propagation delay of the transmit paths. Timing offset introduces intersymbol interference which can degrade system performance, hence the need to investigate its effect. Using the union bound technique, we derive the analytical upper bound on the symbol error rate (SER) of an SPPM-based optical wireless communication system that is impaired by timing offset. The analysis is validated via closely matching simulation results. Results show that error performance degrades as timing offset increases. For a small range of timing offset values, OSSK shows a better tolerance compared to SPPM.

In this paper, the performance of optical spatial modulation (OSM) in the indoor multipath optical wireless channel is investigated. Multipath propagation of the transmitted signal results in the temporal dispersion which causes intersymbol interference (ISI) especially in high-speed communications. The OSM schemes have been investigated in line-of-sight (LOS) channels. However, given the recent trend in Gigabits/s communication, there is a need to investigate the impact of the neglected higher-order reflections. Two variants of OSM are explored as case studies: optical space shift keying and spatial pulse position modulation. The multipath-induced ISI is modeled to account for the spreading of the transmitted signal, and the analytical upper bound on the symbol error rate of both OSM schemes in the multipath channel is derived. The derived analytical bounds are validated by closely matching simulation results. Furthermore, using the spatial distributions of the multipath-induced power penalty, the LOS channel response and the delay spread across the room, we demonstrate how the interaction between these parameters impacts error performance. Multipath-induced ISI has a significant adverse impact on the performance of OSM schemes, particularly on the detection of the activated transmitter as the incurred ISI can alter the channel gain of the received symbol.

This paper examines the effect of synchronization error on the performance of spatial modulation (SM) technique in optical wireless communication (OWC) systems. SM exploits the deployment of multiple transmitters by encoding user information on their spatial domain. In most works related to SM, a perfect synchronization among these multiple transmitters is assumed. However, synchronization error can result from multipath propagation in OWC channel, and clock jitter and variation in propagation delay of each transmitter. Synchronization error degrades system performance and hence the need to investigate its effect. Using union bound technique, and defining synchronization errors as timing offsets in the received signals, we derive the symbol error rate for space shift keying (SSK), generalized SSK (GSSK), SM, and generalized SM (GSM) schemes, and we validate our analysis with tightly matched simulation results. Results show that degradation in performance increases with synchronization error. While SSK is tolerant for a small range of synchronization error, GSSK, SM, and GSM are significantly impaired. Our results also demonstrate the dependence of SM on channel gain values. We observe that the lower the channel gain of the transmitter in which synchronization error occurs, the lesser the impact of the synchronization error on the system performance.

In this paper, we investigate the performance of a spatial modulation scheme that combines the energy efficiency of pulse position modulation (PPM) with the spectral efficiency of generalized space shift keying (GSSK) for optical wireless communication system. The combined scheme is called generalized spatial pulse position modulation (GSPPM). We present the error performance analysis and validate the results through simulation. Error performance results show that the upper bound derived in the theoretical analysis is tight for all cases when the symbol error rate is below 0.1. The influence of parameters such as channel path gain and number of PPM slots on GSPPM's performance is illustrated. GSPPM is compared with some of the existing spatial modulation techniques, and results show that GSPPM provides an attractive trade-off between energy and spectral efficiency.