Impacts of Residual Frequency Offset on the Sum Rate Performance of OFDM based NOMA Systems

Orthogonal frequency division multiplexing (OFD-M) system is extremely sensitive to the inter-carrier interference (ICI) caused by residual frequency offset (RFO). Yet the impact of RFO on the sum rate performance of OFDM based non-orthogonal multiple access (OFDM-NOMA) systems hasn't been discussed in the existing works. Due to this fact, based on the assumption of perfect successive interference cancellation (SIC) at the receiver, the ergodic sum rates of uplink and downlink OFDM-NOMA systems with RFO are derived and presented as integral forms. Moreover, the loss ratio is adopted to evaluate the impact of RFO on the ergodic sum rate performance, which can also be regarded as a reference for RFO mitigation to maintain the ergodic sum rate loss at a certain level. The numerical results indicate that, the ergodic sum rate loss ratio induced by RFO is considerable with a high RFO value, e.g., for both uplink and downlink with the input signal to noise ratio (SNR) of 10 dB, the ergodic sum rate suffers a 30% loss when the normalized RFO value is 0.45. In particular, it is recommended to control the normalized RFO value within 0.05 through RFO mitigation such that the ergodic sum rate loss may be neglectable. Besides, the power allocation proportion between the paired users is shown to have little impact on the sum rate loss ratio of OFDM-NOMA systems.