456 research outputs found
Multi-User Diversity vs. Accurate Channel State Information in MIMO Downlink Channels
In a multiple transmit antenna, single antenna per receiver downlink channel
with limited channel state feedback, we consider the following question: given
a constraint on the total system-wide feedback load, is it preferable to get
low-rate/coarse channel feedback from a large number of receivers or
high-rate/high-quality feedback from a smaller number of receivers? Acquiring
feedback from many receivers allows multi-user diversity to be exploited, while
high-rate feedback allows for very precise selection of beamforming directions.
We show that there is a strong preference for obtaining high-quality feedback,
and that obtaining near-perfect channel information from as many receivers as
possible provides a significantly larger sum rate than collecting a few
feedback bits from a large number of users.Comment: Submitted to IEEE Transactions on Communications, July 200
Low-complexity user selection for rate maximization in MIMO broadcast channels with downlink beamforming
We present in this work a low-complexity algorithm to solve the sum rate maximization problem in multiuser MIMO broadcast
channels with downlink beamforming. Our approach decouples the user selection problem from the resource allocation problem
and its main goal is to create a set of quasiorthogonal users. The proposed algorithm exploits physical metrics of the wireless
channels that can be easily computed in such a way that a null space projection power can be approximated efficiently. Based on the
derived metrics we present a mathematical model that describes the dynamics of the user selection process which renders the user
selection problem into an integer linear program. Numerical results show that our approach is highly efficient to form groups of
quasiorthogonal users when compared to previously proposed algorithms in the literature. Our user selection algorithm achieves
a large portion of the optimum user selection sum rate (90%) for a moderate number of active users
Antenna Combining for the MIMO Downlink Channel
A multiple antenna downlink channel where limited channel feedback is
available to the transmitter is considered. In a vector downlink channel
(single antenna at each receiver), the transmit antenna array can be used to
transmit separate data streams to multiple receivers only if the transmitter
has very accurate channel knowledge, i.e., if there is high-rate channel
feedback from each receiver. In this work it is shown that channel feedback
requirements can be significantly reduced if each receiver has a small number
of antennas and appropriately combines its antenna outputs. A combining method
that minimizes channel quantization error at each receiver, and thereby
minimizes multi-user interference, is proposed and analyzed. This technique is
shown to outperform traditional techniques such as maximum-ratio combining
because minimization of interference power is more critical than maximization
of signal power in the multiple antenna downlink. Analysis is provided to
quantify the feedback savings, and the technique is seen to work well with user
selection and is also robust to receiver estimation error.Comment: Submitted to IEEE Trans. Wireless Communications April 2007. Revised
August 200
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