Transmission Schemes and Power Allocation for Multiuser Massive MIMO Relaying

This paper proposes and analyzes the performance of two simple transmission protocols for a multiuser massive multiple input multiple-output relaying system, where K single-antenna users transmit data to a massive-antenna destination through an N-antenna relay node. In the first transmission protocol, the relay does not need to know the CSI. It just amplifies and forwards the received signals to the destination. In the second protocol, the relay first estimates the channels from all users. It then uses the maximum-ratio combining (MRC) technique to combine all received signals and forwards them to the destination. In both protocols, the destination estimates the channels and employs MRC to decode the signals. We propose an efficient channel estimation method at the destination in which the destination estimates only the effective channels gains. As a consequence, the channel estimation overhead does not depend on the numbers of relay and destination antennas. We derive closed-form expressions for the spectral efficiency of the two transmission protocols. These results allow us to further analyze the system performance and to allocate the transmit powers. Particularly, a max-min power control algorithm is proposed which selects the transmit powers at the relay and users to maximize the lowest spectral efficiency of all users. We show that, by using the max-min power allocation algorithm, the spectral efficiency can be increased significantly, compared to uniform power allocation. Furthermore, if the distance between the users and the relay is large, the first transmission protocol is better and vice versa if this distance is small.