Performance of cooperative multiple-input multiple-output NOMA in Nakagami-m fading channels with channel estimation errors

The performance of a downlink non-orthogonal multiple access based cooperative network with maximum ratio transmission/receive antenna selection (MRT/RAS) over Nakagami-m fading channels in the presence of channel estimation errors (CEEs) is studied. In the system, a base station communicates with multiple mobile users through a half-duplex channel state information-based amplify-and-forward relay. All nodes are equipped with multiple antennas and the hybrid diversity technique MRT/RAS is employed in both hops. The outage behaviour of the system is investigated by driving closed-form expression for outage probability (OP). In addition, the corresponding lower and upper bounds of the derived OP are obtained. Moreover, the behaviour of the system is studied in high signal-to-noise ratio region by obtaining an error floor value in the presence of CEE as well as achieving diversity and array gains in the absence of CEE. Finally, the analytical results in the presence and absence of the CEEs are verified by the Monte Carlo simulations. Results show that the MRT/RAS scheme enhances the OP significantly and is much more robust to the CEEs in comparison with the single antenna case.