Multi-User Diversity Gain for Oblivious and Informed Users in Downlink Channels

The absolute gain of multi-user diversity in the context of a downlink channel is the focus of this contribution. Enjoying multi-user diversity gain first requires the channel information availability at the transmitter (CSIT) in a normal downlink (DL) system. Although multi-user diversity gains have been specified well, but the burden of exchange of information required is not rigorously accounted for. We analyze a time-division duplex (TDD) broadcast channel with initial assumption of channel information neither at the base station (BS) nor at the users´ side. We propose two different but simple transmission strategies which make necessary channel state information (CSI) available at both communicating ends. We give approximate analytical expressions for both strategies which capture well the resource utilization (the cost) and the gain associated to multi-user diversity. This helps us to analyze the absolute gain of the multi-user diversity. The two schemes, the one with oblivious users and the other with informed users, are compared for the sum rate and interesting conclusions are drawn.