Adaptive Bit Loading for BICM-OFDM with Square Lattice QAM Constellations

We consider a transmission system employing orthogonal frequency-division multiplexing (OFDM) with bit- interleaved coded modulation (BICM) and perfect channel state information (CSI) at both transmitter and receiver. For exploiting the transmit CSI, we present adaptive bit loading (ABL) schemes that minimize the bit-error rate at the decoder output. A simple low complexity and a generalized ABL procedure for square lattice quadrature amplitude modulation signal constellations and Gray encoding scheme are proposed. Simulation results show that at low and medium signal-to-noise ratios (SNRs) the simple low complexity ABL scheme performs as well as the general ABL scheme. The benefits of using the generalized procedure are only visible at high SNR.