Exploiting non-orthogonal multiple access in downlink coordinated multipoint transmission with the presence of imperfect channel state information
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The downlink coordinated multipoint (CoMP) transmission system with non-orthogonal multiple access (NOMA) is presented to enhance downlink capacity. Since perfect knowledge of a channel is not always available in practice, imperfect channel state information (CSI) needs to be considered in downlink CoMP with NOMA. In this paper, imperfect CSI is modeled with channel estimation error, where a priori of variance of the error estimation is known. Instead of a specific number of coordinated base stations (BSs), a downlink CoMP NOMA with B coordinated BSs is proposed. Further, the ergodic capacity of the cell-center user (CCU), and the cell-edge user (CEU), as well as their sum capacity are analyzed, derived as closed forms, and validated by simulations in both perfect CSI and imperfect CSI with channel estimation error. The result shows that CoMP NOMA outperforms the CoMP orthogonal multiple access (OMA) capacity, where channel estimation error degrades the performance of both CoMP NOMA and CoMP OMA. In CoMP NOMA, it is noted that the impact of channel estimation error is less significant at CEU compared to that at CCU, due to the utilization of the incoming signals from all the coordinated BSs. Additionally, by exploiting non-orthogonal channel, the sum capacity of CoMP NOMA still can be improved with an increase in coordinated BSs. It is also demonstrated that CEU needs to be allocated with much higher power than CCU to maintain their capacity simultaneously. Finally, the performance analysis is successfully validated through the close accordance between the analytical and simulation results.
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