Impact of HPA nonlinearity on the performance of power domain OFDM-NOMA system

Non-orthogonal multiple access (NOMA) is expected to be used in beyond fifth-generation (B5G) and sixth-generation (6G) mobile networks to support ultra-massive connectivity. Since the two preceding mobile networks generations used orthogonal frequency division multiplexing (OFDM), NOMA is expected to be combined with OFDM. Unfortunately, the OFDM signal suffers from a high peak to average power ratio (PAPR) that limits its performance as it passes through the nonlinear high power amplifier (HPA). In literature, few works have studied the effect of nonlinear distortion on OFDM-NOMA. Furthermore, the HPA models used in previous works to describe the impact of nonlinear distortion on OFDM-NOMA in downlink (DL) were impractical or inaccurate. In contrast, this work uses the well-known soft limiter (SL) model with input back-off (IBO) as a practical controlling parameter. Also, instead of investigating the effect of nonlinear distortion on OFDM-NOMA in DL only, this work investigated this effect in both DL and uplink (UL). In particular, during this work, the performance of the OFDM-NOMA system in the presence of nonlinear distortion in both UL and DL is investigated in terms of users’ achievable data rate, sum rate capacity, system fairness, and the bit error rate (BER) of each user. Results showed that, in DL, the NU is the most affected by the nonlinear distortion, while, in UL, the nonlinear distortion caused by the NU’s HPA is more severe than the nonlinear distortion caused by other users.