340 research outputs found
Optimal Power Control in Decentralized Gaussian Multiple Access Channels
We consider the decentralized power optimization problem for Gaussian
fast-fading multiple access channel (MAC) so that the average sum-throughput is
maximized. In our MAC setup, each transmitter has access to only its own fading
coefficient or channel state information (CSI) while the receiver has full CSI
available at all instants. Unlike centralized MAC (full CSIT MAC) where the
optimal powers are known explicitly, the analytical solution for optimal
decentralized powers does not seem feasible. In this letter, we specialize
alternating-maximization (AM) method for numerically computing the optimal
powers and ergodic capacity of the decentralized MAC for general fading
statistics and average power constraints. For illustration, we apply our AM
method to compute the capacity of MAC channels with fading distributions such
as Rayleigh, Rician etc.Comment: 4 pages, 4 figures, accepted for publication to IEEE Communication
letter
Downlink SDMA with Limited Feedback in Interference-Limited Wireless Networks
The tremendous capacity gains promised by space division multiple access
(SDMA) depend critically on the accuracy of the transmit channel state
information. In the broadcast channel, even without any network interference,
it is known that such gains collapse due to interstream interference if the
feedback is delayed or low rate. In this paper, we investigate SDMA in the
presence of interference from many other simultaneously active transmitters
distributed randomly over the network. In particular we consider zero-forcing
beamforming in a decentralized (ad hoc) network where each receiver provides
feedback to its respective transmitter. We derive closed-form expressions for
the outage probability, network throughput, transmission capacity, and average
achievable rate and go on to quantify the degradation in network performance
due to residual self-interference as a function of key system parameters. One
particular finding is that as in the classical broadcast channel, the per-user
feedback rate must increase linearly with the number of transmit antennas and
SINR (in dB) for the full multiplexing gains to be preserved with limited
feedback. We derive the throughput-maximizing number of streams, establishing
that single-stream transmission is optimal in most practically relevant
settings. In short, SDMA does not appear to be a prudent design choice for
interference-limited wireless networks.Comment: Submitted to IEEE Transactions on Wireless Communication
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