In this paper, we propose two digital signal processing (DSP) techniques, the orthogonal circulant matrix transform (OCT) technique and the singular value decomposition (SVD)-based adaptive loading, to reduce the bit error rate (BER) of multiple-input-multipleoutput orthogonal frequency division multiplexing (MIMO-OFDM)-based visible light communication (VLC) systems, without and with using the channel state information (CSI), respectively. A gigabit/s 2 × 2 MIMO-OFDM VLC system under ~100-MHz system bandwidth, with both symmetrical and asymmetrical MIMO setups, is demonstrated. It is shown that both techniques can attain outstanding BER reduction regardless of the transceivers' geometrical distributions. The SVD-based adaptive loading exhibits the best performance but requires the CSI. The OCT technique can achieve suboptimal performance without the needs of CSI. In both the 1.6-Gbit/s symmetrical MIMO setup and the 1.2-Gbit/s asymmetrical setup, we achieved more than one and two orders of magnitude reductions in the BER by using the OCT technique and the SVD-based adaptive loading, respectively.
To alleviate the error propagation issue of non-orthogonal multiple access (NOMA) based visible light communication (VLC) systems, we propose and experimentally demonstrate the subcarrier pairwise coding (PWC) based NOMA-VLC. Without extra coding overhead, the computation-efficient subcarrier PWC can significantly improve the overall system performance by pairing the subcarriers of high signal-to-noise ratios (SNRs) with those of low SNRs. Under different transmitted baud rate from 150 MSa/s to 225 MSa/s, the bit error rate (BER) performance with and without PWC is investigated in a NOMA-VLC system. Experimental results show that with the optimized power ratio between the NOMA users, up to 3.3-dB Q 2-factor improvement can be achieved by the proposed scheme.
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