Multiuser MIMO Relay Networks in Nakagami-m Fading Channels

This paper proposes a low complexity protocol that preserves full diversity in multiuser amplify-and-forward relay networks with N_S antennas at the source, N_R antennas at the relay, and N_D antennas at each of the K destinations. In the proposed protocol, a two-fold diversity is guaranteed: 1) multi-antenna diversity via transmit antenna selection with maximal-ratio combining (TAS/MRC), and 2) multiuser diversity via opportunistic scheduling. Under perfect feedback with precise channel state information (CSI), we derive new exact and asymptotic symbol error rate (SER) expressions in closed-form for the general case of Nakagami-m fading. We prove that the full diversity order of N_SN_DKm_X + min{N_SN_Rm_Y, N_RN_DKm_Z} is guaranteed, where m_X, m_Y, and m_Z denote the fading parameters of the source-destination, source-relay, and relay-destination links, respectively. To examine the impact of delayed feedback, we next derive new exact and asymptotic SER expressions in closed-form. We prove that in the presence of delayed feedback, outdated CSI degrades the diversity order to N_Dm_X + min{N_Rm_Y, N_Dm_Z}. In addition, based on our asymptotic expressions, we determine the optimal power allocation between the source and the relay such that the SER is minimized. We show that optimal power allocation offers superior performance over uniform power allocation; highlighting a pivotal design choice for maximizing network performance without investing additional resources.