338 research outputs found
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
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
Random Beamforming with Heterogeneous Users and Selective Feedback: Individual Sum Rate and Individual Scaling Laws
This paper investigates three open problems in random beamforming based
communication systems: the scheduling policy with heterogeneous users, the
closed form sum rate, and the randomness of multiuser diversity with selective
feedback. By employing the cumulative distribution function based scheduling
policy, we guarantee fairness among users as well as obtain multiuser diversity
gain in the heterogeneous scenario. Under this scheduling framework, the
individual sum rate, namely the average rate for a given user multiplied by the
number of users, is of interest and analyzed under different feedback schemes.
Firstly, under the full feedback scheme, we derive the closed form individual
sum rate by employing a decomposition of the probability density function of
the selected user's signal-to-interference-plus-noise ratio. This technique is
employed to further obtain a closed form rate approximation with selective
feedback in the spatial dimension. The analysis is also extended to random
beamforming in a wideband OFDMA system with additional selective feedback in
the spectral dimension wherein only the best beams for the best-L resource
blocks are fed back. We utilize extreme value theory to examine the randomness
of multiuser diversity incurred by selective feedback. Finally, by leveraging
the tail equivalence method, the multiplicative effect of selective feedback
and random observations is observed to establish the individual rate scaling.Comment: Submitted in March 2012. To appear in IEEE Transactions on Wireless
Communications. Part of this paper builds upon the following letter: Y. Huang
and B. D. Rao, "Closed form sum rate of random beamforming", IEEE Commun.
Lett., vol. 16, no. 5, pp. 630-633, May 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
Coordinated Multicasting with Opportunistic User Selection in Multicell Wireless Systems
Physical layer multicasting with opportunistic user selection (OUS) is
examined for multicell multi-antenna wireless systems. By adopting a two-layer
encoding scheme, a rate-adaptive channel code is applied in each fading block
to enable successful decoding by a chosen subset of users (which varies over
different blocks) and an application layer erasure code is employed across
multiple blocks to ensure that every user is able to recover the message after
decoding successfully in a sufficient number of blocks. The transmit signal and
code-rate in each block determine opportunistically the subset of users that
are able to successfully decode and can be chosen to maximize the long-term
multicast efficiency. The employment of OUS not only helps avoid
rate-limitations caused by the user with the worst channel, but also helps
coordinate interference among different cells and multicast groups. In this
work, efficient algorithms are proposed for the design of the transmit
covariance matrices, the physical layer code-rates, and the target user subsets
in each block. In the single group scenario, the system parameters are
determined by maximizing the group-rate, defined as the physical layer
code-rate times the fraction of users that can successfully decode in each
block. In the multi-group scenario, the system parameters are determined by
considering a group-rate balancing optimization problem, which is solved by a
successive convex approximation (SCA) approach. To further reduce the feedback
overhead, we also consider the case where only part of the users feed back
their channel vectors in each block and propose a design based on the balancing
of the expected group-rates. In addition to SCA, a sample average approximation
technique is also introduced to handle the probabilistic terms arising in this
problem. The effectiveness of the proposed schemes is demonstrated by computer
simulations.Comment: Accepted by IEEE Transactions on Signal Processin
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