A beamformed alamouti amplify-and-forward scheme in multigroup multicast cloud-relay networks

In this paper, we consider a cloud relay network (C-RN) which provides reliable communication between long-distance users. Specifically, we study the amplify-and-forward (AF) schemes in C-RNs. In our scenario setting, with the cloud processor units fully coordinating in the network, the C-RN can be treated as an MIMO relay system. We therefore propose the beamformed (BF) Alamouti AF scheme to provide multigroup multicast information delivery in this network. By applying an Alamouti space-time code structure, the relays adopt two rank-one weights to AF the received signals in two time slots. Then, one more degree of freedom is available compared to the traditional BF AF scheme, and a new fractional semidefinite relaxation (SDR) is obtained from a max-min-fair quality-of-service (QoS) perspective. We prove that the Gaussian randomization algorithm based on the new fractional SDR has the same approximation quality-i.e., on the order of √M-as the traditional rank-two SDR approximation in multigroup multicast networks without relays, where M is the number of users served in the network. This result is verified by our numerical experiments.