The Impact Of Clipping On Typical Digital Modulation System

Besides the constant envelope modulation, wireless signals are always subjected to the nonlinear distortions from amplifiers or A/D and D/A converters which is mainly characterized by the peak clipping. The problem of clipping is more severe when the peak-to-average power ratio (PAPR) is large. Clipping will result in two adverse effects:the re-growth of the side-band spectrum which will interferer communications in adjacent channels and the distortion of the transmitted signal which will degrade the performance of the desired communication. This thesis mainly focuses on the latter.As Orthogonal Frequency Division Multiplexing (OFDM) system generally has higher PAPR, we first investigate the statistical properties of the clipping noise in OFDM systems both with theoretical analysis and simulation, including mean, variance, cumulative distribution and the correlation coefficients. The results show that PAPR may not be an effective metric to reflect the clipping noise. Reducing PAPR does not mean a direct reduction of clipping noise. Moreover, the performance loss caused by clipping noise can be insignificant for moderate or high clipping threshold. Our observation is consistent with those published in literatures.Next, the thesis investigates the impact of clipping to the single carrier systems since the problem also exists when the single carrier system adopts high order modulation and raised cosine pulse shaping. The Power Spectrum Density (PSD) and Bit Error Rate (BER) indicate that clipping will lead to spectrum spreading as well as BER degradation when the clipping threshold is low. However, the problem seems less severe in single carrier system than in OFDM system.