Power loading for OFDM in tactical packet radio systems

In orthogonal frequency division multiplexing (OFDM) systems, transmitting with different power levels on each subcarrier, known as power loading, can improve performance. Power loading algorithms process channel state information to determine the power distribution that optimizes or improves the performance. Most previous investigations of power loading are for OFDM systems that do not use error-control coding. However, their objective of minimizing the bit error probability at the demodulator output does not minimize the packet error probability when error-control codes are used. Because modern OFDM systems use error-control coding, the utility of those previous results is questionable. Furthermore, the power loading algorithms that minimize the bit error probability use approximations for the error probability that are not accurate for the signal-to-noise ratios of interest in these systems. We consider half-duplex tactical packet communications with OFDM modulation, error-control coding, and iterative decoding. We also investigate the adaptation of the code rate and subcarrier modulation without power loading, as an alternative to power loading. Adaptive coding and modulation is shown to provide higher throughput and more robust performance than power loading. Using a combination of analysis and simulation, we determine the effect of power loading on the binary symbol error probability at the demodulator output, the packet error probability at the decoder output, and the throughput of the packet radio system.