Compression of Clipped OFDM IQ Samples for Cloud Radio Access Network

In this paper, we analyze the impact of reducing the Peak to Average Power Ratio (PAPR) of a time domain Orthogonal Frequency Division Multiplexing (OFDM) signal before its compression, as required for the transmission of IQ samples in a Centralized Radio Access Network architecture. The analysis is performed considering a clipping function for PAPR reduction. First, the distribution of the clipped OFDM IQ samples is obtained which in turn allows to derive the threshold and codebook levels of a non-uniform quantizer optimized for clipped IQ samples and the number of representation bits for each quantized levels in the entropy coding stage. Then, we provide a closed-form expression for the Modulation Error Ratio of the compressed and clipped OFDM signal. The simulation results show that the Gaussian quantizer applied to a clipped signal is not quite robust compared to a quantizer that has been optimized w.r.t. the distribution of clipped IQ samples, especially in the low-rate quantization region.