Abstract
In this work, we consider a multi-antenna channel with orthogonally multiplexed non-cooperative users, and present its achievable information rate regions with and without channel knowledge at the transmitter. With an informed transmitter, we maximize the rate for each user. With an uninformed transmitter, we consider the optimal power allocation that causes the fastest convergence to zero of the fraction of channels whose mutual information is less than any given rate as the transmitter channel knowledge converges to zero. We assume a deterministic space and time dispersive multipath channel with multiple transmit and receive antennas, generating an orthogonally multiplexed Multiple-Input Multiple-Output (MIMO) broadcast system. Under limited transmit power; we consider different user specific space-time modulation formats that represent assignments of signal dimensions to transmit antennas. For the two-user orthogonally multiplexed MIMO broadcast channels, the achievable rate regions, with and without transmitter channel knowledge, evolve from a triangular region at low SNR to a rectangular region at high SNR. We also investigate the maximum sum rate for these regions and derive the associated power allocations at low and high SNR. Furthermore, we present numerical results for a two-user system that illustrate the effects of channel knowledge at the transmitter, the multi-dimensional space-time modulation format and features of the multipath channel.
Highlights
Multiple-Input Multiple-Output (MIMO) systems, employing multiple antennas at the transmitter and receiver, have been shown to yield significant capacity gains for single-user channels [1]
The capacity region of the two-user scalar orthogonal broadcast channel (BC) is shown in [7] to be a rectangle generated by the set of jointly achievable mutual information rate pairs
Superposition coding does not apply to non-degraded broadcast channels because users may employ different rates making successive decoding quite difficult if not impossible [11]. This reference shows that a capacity region for broadcast channels can be achieved by using a coding technique, nicknamed dirty paper coding (DPC) [12], where the interference is non-causally known to the transmitter and unknown to the receiver
Summary
Multiple-Input Multiple-Output (MIMO) systems, employing multiple antennas at the transmitter and receiver, have been shown to yield significant capacity gains for single-user channels [1]. As in [23,24], we assume a non-fading space and time dispersive multipath environment These schemes model the downlink of cellular communication systems with orthogonal user multiplexing. We investigate the achievable rate region of such orthogonally multiplexed broadcast schemes with multi-dimensional space-time modulation, where a transmitter attempts simultaneously to transfer information to several users without mutual interference. We consider the power allocation for each user that causes the fastest convergence to zero of the fraction of channels whose mutual information is less than any given rate, as the transmitter channel knowledge goes to zero For both cases, we investigate the maximum sum rate. Illustrative numerical results are provided for users having different propagation channels, using different multi-dimensional space-time modulation schemes and employing different number of antennas.
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