An Enhanced Bit Loading Scheme with Pairwise Coding for Low-pass VLC Systems

We consider DC-biased optical orthogonal frequency division multiplexing (DCO-OFDM) low-pass visible light communication (VLC) systems in which water-filling and uniform power loading strategies have almost the same rate performance. However, conventional bit loading based on either water-filling or uniform power loading cannot reach the maximum achievable rate because the number of bits loaded per subcarrier can only be integers. In this paper, we apply pairwise coding (PWC) to solve this problem by performing joint coding on pairs of subcarriers to balance their symbol error rate (SER) performance. We present a closed-form expression for the optimal parameter of PWC and an approximation of its theoretical performance, and propose an enhanced bit loading scheme with PWC to improve the system rate. Numerical results suggest that the enhanced scheme has a significant rate improvement and approaches the maximum achievable rate for moderate to high signal-to-noise ratio (SNR).