On broadcast approach for multiple access channel

A broadcast strategy for multiple access communication over slowly fading channels is introduced. It is an extension of the existing broadcast approaches to multiple access communication, in which the channel state information is known to the receiver only. In this strategy, the transmitters split their information streams into multiple independent coded information layers, each adapted to a specific actual channel realization. The major distinction between the proposed strategy and the existing one is that in the existing approach, each transmitter adapts its transmission strategy only to the fading process of its direct channel to the receiver, hence directly adopting a single-user strategy previously designed for the single-user channels. However, the contribution of each user to a network-wide measure (e.g., sum-rate capacity) depends not only on the users' direct channel to the receiver, but also on the qualities of other channels. Driven by this premise, an alternative broadcast strategy is proposed here in which the transmitters adapt their transmissions to the combined state resulting from both users' channels. This leads to generating a larger number of information layers by each transmitter and a different decoding strategy by the receiver. The fundamental trade-off among the rates of different information layers is established. Capacity region characterized by sum rates of two users' equivalent layers is derived for independent and finite state channel realization. This region turns out to be larger than the equivalent capacity region for existing broadcast strategy which in turn provides a larger average rate. This approach is extended to continuous channels with infinite states and corresponding average rate is expressed as a function of joint distribution function of channel states and power density function among the transmission layers.;Proposed approach is further extended to multiple access channels subjected to lower uncertainty level due to the availability of partial state information. Each transmitter is aware of its own channel state while the receiver is assumed to possess complete knowledge of the network. Existing approach that implements fixed number of information layers and successive decoding of all layers during each transmission cycle, is outperformed by proposed broadcast strategy in terms of achievable rate region and average rate. Finally, this approach is extended to channels with multiple transmitting antennas and multiple receiving antennas. Relation between multiple and single antenna scenarios are derived for certain special cases where the multiple antenna system can be converted into an equivalent single antenna network with modified channel coefficients. Numerical evaluation is provided to demonstrate performance gain over existing approach for similar networks.