Analytical Study of OFDM Signal Waveform Degradation Due to Power Amplifier Nonlinearity in Nigerian Base Stations

Orthogonal frequency division multiplexing waveforms are highly susceptible to nonlinear distortion introduced by power amplifiers operating near saturation, causing spectral regrowth that generates out-of-band interference and in-band error vector magnitude degradation that reduces system throughput. The extent of this problem at base stations operating in Nigeria's variable environmental and power quality conditions is analytically unexplored. This study analytically investigates OFDM signal waveform degradation caused by power amplifier nonlinearity at base stations in the Nigerian operational context, using field measurements to parameterize the nonlinearity models and validate analytical predictions. Peak-to-average power ratio measurements of live 4G LTE downlink signals were collected at twelve urban base station locations in Rivers State and Abuja using calibrated measurement receivers. The Rapp model and Saleh model for power amplifier nonlinearity were fitted to measured amplifier characteristics obtained through collaboration with network maintenance engineers. Analytical expressions for adjacent channel leakage ratio and error vector magnitude degradation were derived and validated against measurements. Results show that power supply fluctuations causing sub-optimal drain voltage at base station amplifiers increase in-band EVM by 2.8 dB above manufacturer specifications, equivalent to a one-and-a-half-order modulation scheme downgrade. Predistortion system effectiveness was reduced by 34% under grid power instability conditions. The study recommends improved base station power conditioning and argues that Nigerian network performance standards should account for power quality effects in EVM compliance measurements. Keywords: OFDM, power amplifier, nonlinearity, base station, EVM degradation.