141 research outputs found
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
Opportunistic Routing in Ad Hoc Networks: How many relays should there be? What rate should nodes use?
Opportunistic routing is a multi-hop routing scheme which allows for
selection of the best immediately available relay. In blind opportunistic
routing protocols, where transmitters blindly broadcast without knowledge of
the surrounding nodes, two fundamental design parameters are the node
transmission probability and the transmission spectral efficiency. In this
paper these parameters are selected to maximize end-to-end performance,
characterized by the product of transmitter density, hop distance and rate. Due
to the intractability of the problem as stated, an approximation function is
examined which proves reasonably accurate. Our results show how the above
design parameters should be selected based on inherent system parameters such
as the path loss exponent and the noise level.Comment: 5 pages, 8 figures, Submitted to IEEE GLOBECOM 201
Delay Constrained Scheduling over Fading Channels: Optimal Policies for Monomial Energy-Cost Functions
A point-to-point discrete-time scheduling problem of transmitting
information bits within hard delay deadline slots is considered assuming
that the underlying energy-bit cost function is a convex monomial. The
scheduling objective is to minimize the expected energy expenditure while
satisfying the deadline constraint based on information about the unserved
bits, channel state/statistics, and the remaining time slots to the deadline.
At each time slot, the scheduling decision is made without knowledge of future
channel state, and thus there is a tension between serving many bits when the
current channel is good versus leaving too many bits for the deadline. Under
the assumption that no other packet is scheduled concurrently and no outage is
allowed, we derive the optimal scheduling policy. Furthermore, we also
investigate the dual problem of maximizing the number of transmitted bits over
time slots when subject to an energy constraint.Comment: submitted to the IEEE ICC 200
Optimum Pilot Overhead in Wireless Communication: A Unified Treatment of Continuous and Block-Fading Channels
The optimization of the pilot overhead in single-user wireless fading
channels is investigated, and the dependence of this overhead on various system
parameters of interest (e.g., fading rate, signal-to-noise ratio) is
quantified. The achievable pilot-based spectral efficiency is expanded with
respect to the fading rate about the no-fading point, which leads to an
accurate order expansion for the pilot overhead. This expansion identifies that
the pilot overhead, as well as the spectral efficiency penalty with respect to
a reference system with genie-aided CSI (channel state information) at the
receiver, depend on the square root of the normalized Doppler frequency.
Furthermore, it is shown that the widely-used block fading model is only a
special case of more accurate continuous fading models in terms of the
achievable pilot-based spectral efficiency, and that the overhead optimization
for multiantenna systems is effectively the same as for single-antenna systems
with the normalized Doppler frequency multiplied by the number of transmit
antennas.Comment: Submitted to IEEE Trans. Wireless Communication
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
Limited Feedback-based Block Diagonalization for the MIMO Broadcast Channel
Block diagonalization is a linear precoding technique for the multiple
antenna broadcast (downlink) channel that involves transmission of multiple
data streams to each receiver such that no multi-user interference is
experienced at any of the receivers. This low-complexity scheme operates only a
few dB away from capacity but requires very accurate channel knowledge at the
transmitter. We consider a limited feedback system where each receiver knows
its channel perfectly, but the transmitter is only provided with a finite
number of channel feedback bits from each receiver. Using a random quantization
argument, we quantify the throughput loss due to imperfect channel knowledge as
a function of the feedback level. The quality of channel knowledge must improve
proportional to the SNR in order to prevent interference-limitations, and we
show that scaling the number of feedback bits linearly with the system SNR is
sufficient to maintain a bounded rate loss. Finally, we compare our
quantization strategy to an analog feedback scheme and show the superiority of
quantized feedback.Comment: 20 pages, 4 figures, submitted to IEEE JSAC November 200
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