Adaptive Performance Improvement of OFDM Radio over Fiber Systems

Nonlinear distortion due to large peak to average power ratio is a major concern with OFDM systems. In this paper, we present an efficient adaptive modulation technique to mitigate nonlinear distortion effects of OFDM radio signals transmitted over optical fiber (radio-over-fiber: ROF). First, we modulate all subcarriers at primary (high) level. Then we identify the subcarriers with high distortion and appropriately reduce the modulation level on those subcarriers to secondary levels. This is done at the transmitter side for each OFDM symbol. This procedure is repeated until the nonlinear distortion is below a predetermined threshold. This technique is shown to improve the BER performance considerably while the reduction in data rate for a system with 64 subcarriers and 16 QAM as primary and 4 QAM as secondary modulation levels is around 4%.