  Light-emitting diode (LED)-based communications, such as visible light
communications (VLC) and infrared (IR) communications, are candidate techniques
to provide short-range and high-speed data transmission. In this paper, M-ary
pulse amplitude modulation (M-PAM), used as a high bandwidth efficiency scheme,
is compared with an optimized DC-biased optical orthogonal frequency division
multiplexing (DCO-OFDM) scheme. Considering the bandwidth limit and constrained
peak transmitted power characteristics of LEDs, a bit loading algorithm with an
optimized modulation index is used for the DCO-OFDM. To reduce the inter-symbol
interference caused by LEDs, a waveform design algorithm with a minimum mean
squared error (MMSE) equalizer is applied to the M-PAM system. From numerical
results, M-PAM with the optimized signal processing can provide a substantially
higher data rate than the optimally designed DCO-OFDM for the same performance.
