Performance analysis of an MC-CDMA broadcasting system under high-power amplifier nonlinearities

Multi-carrier modulation (MCM) schemes have been under development as an effective way to improve channel capacity utilization under multi-path interference and selective fading reception for next generation two-way wireless communication systems. However, the application of oneway broadcasting is one where there is a great need for a new system that can not only operate in a multi-path environment, but can reuse scarce spectral resources and employ cost-effective high-power amplifiers. For this research, an MCM system referred to as Multi-carder Code Division Multiple Access (MC-CDMA) was developed to characterize the primary broadcasting concern of operating under high power amplifier (HPA) non-linearities. This new broadcasting system was evaluated under a host of multi-path conditions, amplifier characteristics, and spreading architectures. Results indicate that for a single user broadcasting system where processing gain is 64, pseudo-random noise (PN) sequences outperformed the classic Walsh-Hadamard sequences and transforms by as much as 4 dB at an uncoded bit error rate (BER) of 1e-2 in multipath environments under non-linear amplification (NLA). Amplifier characterization confirms that the amplitude characteristics (AM/AM) are the dominant variable in reducing BER performance under HPAs. Simulation and analysis under the most severe travelling wave tube amplifier (TWTA) model indicates that a 3 dB increase in input back-off (IBO) of the amplifier results in an equivalent 3 dB improvement in performance at an uncoded BER of 1e-3. The power spectral density of the HPA output waveforms confirmed these results where the instance of spectral regrowth outside the signal bandwidth decreased with increasing IBO. Concurrently, analysis was performed to determine the dominant metric in reducing performance when the system operates under both HPA and multipath channels. Investigation into two small-scale multipath fading environments common to mobile reception, Rayleigh and Ricean fading, confirm that the HPA characteristics dominate the performance degradation experienced. The overall results confirm that the MC-CDMA system developed here has application to the realworld broadcasting environment where HPA non-linear effects dominate performance limits.