50 research outputs found
Vector Broadcast Channels: Optimal Threshold Selection Problem
Threshold feedback policies are well known and provably rate-wise optimal
selective feedback techniques for communication systems requiring partial
channel state information (CSI). However, optimal selection of thresholds at
mobile users to maximize information theoretic data rates subject to feedback
constraints is an open problem. In this paper, we focus on the optimal
threshold selection problem, and provide a solution for this problem for finite
feedback systems. Rather surprisingly, we show that using the same threshold
values at all mobile users is not always a rate-wise optimal feedback strategy,
even for a system with identical users experiencing statistically the same
channel conditions. By utilizing the theory of majorization, we identify an
underlying Schur-concave structure in the rate function and obtain sufficient
conditions for a homogenous threshold feedback policy to be optimal. Our
results hold for most fading channel models, and we illustrate an application
of our results to familiar Rayleigh fading channels.Comment: Submitted to IEEE International Symposium on Information Theory, St.
Petersburg, Russia, Aug 201
Vector Broadcast Channels: Optimality of Threshold Feedback Policies
Beamforming techniques utilizing only partial channel state information (CSI)
has gained popularity over other communication strategies requiring perfect CSI
thanks to their lower feedback requirements. The amount of feedback in
beamforming based communication systems can be further reduced through
selective feedback techniques in which only the users with channels good enough
are allowed to feed back by means of a decentralized feedback policy. In this
paper, we prove that thresholding at the receiver is the rate-wise optimal
decentralized feedback policy for feedback limited systems with prescribed
feedback constraints. This result is highly adaptable due to its distribution
independent nature, provides an analytical justification for the use of
threshold feedback policies in practical systems, and reinforces previous work
analyzing threshold feedback policies as a selective feedback technique without
proving its optimality. It is robust to selfish unilateral deviations. Finally,
it reduces the search for rate-wise optimal feedback policies subject to
feedback constraints from function spaces to a finite dimensional Euclidean
space.Comment: Submitted to IEEE International Symposium on Information Theory, St.
Petersburg, Russia, Aug 201
Outage Capacity of Opportunistic Beamforming with Random User Locations
This paper studies the outage capacity of a network consisting of a multitude
of heterogenous mobile users, and operating according to the classical
opportunistic beamforming framework. The base station is located at the center
of the cell, which is modeled as a disk of finite radius. The random user
locations are modeled using a homogenous spatial Poisson point process. The
received signals are impaired by both fading and location dependent path loss.
For this system, we first derive an expression for the beam outage probability.
This expression holds for all path loss models that satisfy some mild
conditions. Then, we focus on two specific path loss models (i.e., an unbounded
model and a more realistic bounded one) to illustrate the applications of our
results. In the large system limit where the cell radius tends to infinity, the
beam outage capacity and its scaling behavior are derived for the selected
specific path loss models. It is shown that the beam outage capacity scales
logarithmically for the unbounded model. On the other hand, this scaling
behavior becomes double logarithmic for the bounded model. Intuitive
explanations are provided as to why we observe different scaling behavior for
different path loss models. Numerical evaluations are performed to give further
insights, and to illustrate the applicability of the outage capacity results
even to a cell having a small finite radius.Comment: To appear in Globecom 2013, Atlanta, US
Optimality of binary power-control in a single cell via majorization
This paper considers the optimum single cell power-control maximizing the
aggregate (uplink) communication rate of the cell when there are peak power
constraints at mobile users, and a low-complexity data decoder (without
successive decoding) at the base station. It is shown, via the theory of
majorization, that the optimum power allocation is binary, which means links
are either "on" or "off". By exploiting further structure of the optimum binary
power allocation, a simple polynomial-time algorithm for finding the optimum
transmission power allocation is proposed, together with a reduced complexity
near-optimal heuristic algorithm. Sufficient conditions under which
channel-state aware time-division-multiple-access (TDMA) maximizes the
aggregate communication rate are established. Finally, a numerical study is
performed to compare and contrast the performance achieved by the optimum
binary power-control policy with other sub-optimum policies and the throughput
capacity achievable via successive decoding. It is observed that two dominant
modes of communication arise, wideband or TDMA, and that successive decoding
achieves better sum-rates only under near-perfect interference cancellation
efficiency.Comment: 24 pages, 11 figure