Mohammed M. A. Mohammed, Cuiwei He and Jean Armstrong,"Mitigation of Side-Effect Modulation in Optical OFDM VLC Systems," IEEE Access, Vol.6, PP.58161 - 58170, 2018
Abstract-Side-effect modulation (SEM) has the potential to be a significant source of interference in future visible light communication (VLC) systems. SEM is a variation in the intensity of the light emitted by a luminaire and is usually a side effect caused by the power supply used to drive the luminaires. For LED luminaires powered by a switched mode power supply, the SEM can be at much higher frequencies than that emitted by conventional incandescent or fluorescent lighting. It has been shown that the SEM caused by commercially available LED luminaires is often periodic and of low power. In this paper, we investigate the impact of typical forms of SEM on the performance of optical OFDM VLC systems; both ACO-OFDM and DCO-OFDM are considered. Our results show that even low levels of SEM power can significantly degrade the bit-error-rate performance. To solve this problem, an SEM mitigation scheme is described. The mitigation scheme is decision-directed and is based on estimating and subtracting the fundamental component of the SEM from the received signal. We describe two forms of the algorithm; one uses blind estimation, while the other uses pilot-assisted estimation based on a training sequence. Decision errors, resulting in decision noise, limit the performance of the blind estimator even when estimation is based on very long signals. However, the pilot system can achieve more accurate estimations, and thus a better performance. Results are first presented for typical SEM waveforms for the case where the fundamental frequency of the SEM is known. The algorithms are then extended to include a frequency estimation step and the mitigation algorithm is shown also to be effective in this case.
M. M. A. Mohammed, C. He and J. Armstrong, "Diversity Combining in Layered Asymmetrically Clipped Optical OFDM," Journal of Lightwave Technology, Vol. 35, Iss. 11, PP.2078 - 2085, 2017
Abstract - We describe a new receiver for layered asymmetrical clipped optical orthogonal frequency-division multiplexing (LACO-OFDM) in which diversity combining is used at each layer to improve the performance. In each layer of LACO-OFDM, some subcarriers carry data and others carry only the components which result from clipping. In the new receiver at each layer, a nonlinear process is used to recover this clipped signal and use it for diversity combining. Simulation results show that the new receiver outperforms a conventional receiver by up to 2 dB when two layers are used. The performance of LACO-OFDM with diversity combining is compared with other forms of optical OFDM. The results show that at high bit rates LACO-OFDM with diversity combining is the most power efficient system, while ACO-OFDM with diversity combining but without layering is the best for low bit rates. In addition, the diversity combining receiver is compared to the recently proposed receiver for LACO-OFDM based on the noise cancellation. Results show that the diversity combining and noise cancellation have the same performance over flat channels. However, over frequency selective channels, the diversity combining receiver has better performance.
C. He and J. Armstrong, "Clipping Noise Mitigation in Optical OFDM Systems," IEEE Communications Letters, Vol. 21 , Iss. 3, PP.548 - 551, 2017
Abstract-This letter describes a new non-linear algorithm for clipping noise mitigation in intensity modulation/direct detection dc biased optical orthogonal frequency division multiplexing (DCO-OFDM) systems. Clipping noise is often the major limitation in DCO-OFDM. In this letter, we show that extra information about the clipped signal can be extracted using a non-linear process and then used to mitigate the clipping noise. The effectiveness of the new algorithm is demonstrated by simulation and in an optical wireless experiment.
Professor Jean Armstrong