DC-Balanced Even-Dimensional CAP Modulation for Visible Light Communication

Carrierless amplitude and phase (CAP) is a high spectral efficiency (SE) modulation scheme for visible light communication (VLC) systems. For N-dimensional (N-D) CAP, the
N
pulse shaping filters (PSFs) should satisfy zero inter-symbol interference (ISI) and zero inter-channel interference (ICI) conditions. In a practical VLC system, direct-current-balanced (DC-balanced) CAP PSFs are required to avoid severe low-frequency distortion. In this paper, we discuss the possibility of designing perfect DC-balanced N-D PSFs based on zero ISI and zero ICI conditions. Perfect PSFs should be spectrally efficient and flattened. We first prove that it's impossible to design perfect DC-balanced PSFs if the dimension is odd, but it's possible to design perfect DC-balanced PSFs if the dimension is even. Based on our proposed theories, we use a modified minimax optimization method to design a group of perfect DC-balanced 4-D PSFs. A simulation based on the bandpass VLC channel is set up to compare the bit error rate (BER) performance of odd-dimensional and even-dimensional PSFs. The simulation shows that even-dimensional PSFs outperform odd-dimensional PSFs. Moreover, a VLC system based on a laser diode is demonstrated. The experimental results show that the BER performance of even-dimensional CAP (2-D or 4-D CAP) is much better than that of odd-dimensional CAP (3-D CAP) in the bandpass VLC system. We show that the novel 4-D CAP has the nearly identical BER performance as classical 2-D CAP and it can be a good candidate multiuser modulation scheme for bandpass VLC systems.