DOI: 10.25549/usctheses-c127-621298
title: Physical layer multicasting with opportunistic multicast scheduling

description: Physical layer multicasting with optimized opportunistic scheduling is studied for cellular networks, where the problem of efficiently transmitting a common set of fountain-encoded data from a single base station to multiple users over quasi-static fading channels is examined. The proposed opportunistic multicast scheduling (OMS) scheme attempts to achieve a better tradeoff between multiuser diversity and multicast gain by transmitting to a subset of users in each time slot using the maximal data rate that ensures successful decoding by these users. The research work consists of three major parts. ❧ In the first part, we consider OMS in the single-antenna downlink scenario. We analyze the system delay in homogeneous networks by capitalizing on extreme value theory and derive the optimal selection ratio ({\em i.e.,} the portion of users selected in each time slot) that minimizes the delay. Then, we extend results to heterogeneous networks where users are subject to different channel statistics. By partitioning users into multiple approximately homogeneous rings, we turn a heterogeneous network into a composite of smaller homogeneous networks and derive the optimal selection ratio for heterogeneous networks. Computer simulations confirm theoretical results and illustrate that the proposed OMS can achieve significant performance gains in both homogeneous and heterogeneous networks as compared with the conventional unicast and broadcast scheduling.  ❧ In the second part, we extend the results to the multi-antenna downlink scenario where a multi-antenna base-station transmits a common message to a set of users, each with a single receive antenna.  First, we propose the use of OMS for two of the most popular conventional multi-antenna multicast solutions - the spatial multiplexing and the transmit beamforming schemes.  Capitalizing on extreme value theory, we derive analytical expressions for the average system throughput and utilize this result to compute the optimal user selection ratio for both spatial multiplexing and transmit beamforming scenarios.  To improve upon these conventional schemes furthermore, we propose an optimized space-time transmission (OST) scheme, where an arbitrary number of signal dimensions can be used and the statistics of the space-time codeword can be chosen to maximize the rate of the worst user in the selected group. An iterative user selection (IUS) algorithm is also proposed to improve the multicast group selection in each time slot. The proposed OST with IUS is a generalization of transmit beamforming and spatial multiplexing and, thus, outperforms both conventional schemes.  ❧ In the third part, we investigate the use of OMS for systems with multiple multicast groups.  Here, we consider a general downlink scenario where a single base-station transmits independent data streams to multiple groups of users. Within each group, the common information is transmitted, yet the source information is independent between groups.  To achieve this goal, the use of OMS is extended to systems with multiple multicast groups and the so-called multicast throughput region is defined to characterize the performance of the multigroup OMS scheme.  The analytical results based on extreme value theory are utilized to accurately predict the optimal multicast group-sizes and the optimal power allocation policy when maximizing the weighted sum throughput. By choosing the weights appropriately, the method can be further utilized to ensure proportional fairness among the multiple multicast groups. The efficacy of the proposed OMS schemes is shown through numerical simulations.