1,464 research outputs found
Capacity per Unit Energy of Fading Channels with a Peak Constraint
A discrete-time single-user scalar channel with temporally correlated
Rayleigh fading is analyzed. There is no side information at the transmitter or
the receiver. A simple expression is given for the capacity per unit energy, in
the presence of a peak constraint. The simple formula of Verdu for capacity per
unit cost is adapted to a channel with memory, and is used in the proof. In
addition to bounding the capacity of a channel with correlated fading, the
result gives some insight into the relationship between the correlation in the
fading process and the channel capacity. The results are extended to a channel
with side information, showing that the capacity per unit energy is one nat per
Joule, independently of the peak power constraint.
A continuous-time version of the model is also considered. The capacity per
unit energy subject to a peak constraint (but no bandwidth constraint) is given
by an expression similar to that for discrete time, and is evaluated for
Gauss-Markov and Clarke fading channels.Comment: Journal version of paper presented in ISIT 2003 - now accepted for
publication in IEEE Transactions on Information Theor
Stability of Scheduled Message Communication over Degraded Broadcast Channels
We consider scheduled message communication over a discrete memoryless
degraded broadcast channel. The framework we consider here models both the
random message arrivals and the subsequent reliable communication by suitably
combining techniques from queueing theory and information theory. The channel
from the transmitter to each of the receivers is quasi-static, flat, and with
independent fades across the receivers. Requests for message transmissions are
assumed to arrive according to an i.i.d. arrival process. Then, (i) we derive
an outer bound to the region of message arrival vectors achievable by the class
of stationary scheduling policies, (ii) we show for any message arrival vector
that satisfies the outerbound, that there exists a stationary
``state-independent'' policy that results in a stable system for the
corresponding message arrival process, and (iii) under two asymptotic regimes,
we show that the stability region of nat arrival rate vectors has
information-theoretic capacity region interpretation.Comment: 5 pages, Submitted to 2006 International Symposium on Information
Theor
Optimization of Information Rate Upper and Lower Bounds for Channels with Memory
We consider the problem of minimizing upper bounds and maximizing lower
bounds on information rates of stationary and ergodic discrete-time channels
with memory. The channels we consider can have a finite number of states, such
as partial response channels, or they can have an infinite state-space, such as
time-varying fading channels. We optimize recently-proposed information rate
bounds for such channels, which make use of auxiliary finite-state machine
channels (FSMCs). Our main contribution in this paper is to provide iterative
expectation-maximization (EM) type algorithms to optimize the parameters of the
auxiliary FSMC to tighten these bounds. We provide an explicit, iterative
algorithm that improves the upper bound at each iteration. We also provide an
effective method for iteratively optimizing the lower bound. To demonstrate the
effectiveness of our algorithms, we provide several examples of partial
response and fading channels, where the proposed optimization techniques
significantly tighten the initial upper and lower bounds. Finally, we compare
our results with an improved variation of the \emph{simplex} local optimization
algorithm, called \emph{Soblex}. This comparison shows that our proposed
algorithms are superior to the Soblex method, both in terms of robustness in
finding the tightest bounds and in computational efficiency. Interestingly,
from a channel coding/decoding perspective, optimizing the lower bound is
related to increasing the achievable mismatched information rate, i.e., the
information rate of a communication system where the decoder at the receiver is
matched to the auxiliary channel, and not to the original channel.Comment: Submitted to IEEE Transactions on Information Theory, November 24,
200
Event-Driven Optimal Feedback Control for Multi-Antenna Beamforming
Transmit beamforming is a simple multi-antenna technique for increasing
throughput and the transmission range of a wireless communication system. The
required feedback of channel state information (CSI) can potentially result in
excessive overhead especially for high mobility or many antennas. This work
concerns efficient feedback for transmit beamforming and establishes a new
approach of controlling feedback for maximizing net throughput, defined as
throughput minus average feedback cost. The feedback controller using a
stationary policy turns CSI feedback on/off according to the system state that
comprises the channel state and transmit beamformer. Assuming channel isotropy
and Markovity, the controller's state reduces to two scalars. This allows the
optimal control policy to be efficiently computed using dynamic programming.
Consider the perfect feedback channel free of error, where each feedback
instant pays a fixed price. The corresponding optimal feedback control policy
is proved to be of the threshold type. This result holds regardless of whether
the controller's state space is discretized or continuous. Under the
threshold-type policy, feedback is performed whenever a state variable
indicating the accuracy of transmit CSI is below a threshold, which varies with
channel power. The practical finite-rate feedback channel is also considered.
The optimal policy for quantized feedback is proved to be also of the threshold
type. The effect of CSI quantization is shown to be equivalent to an increment
on the feedback price. Moreover, the increment is upper bounded by the expected
logarithm of one minus the quantization error. Finally, simulation shows that
feedback control increases net throughput of the conventional periodic feedback
by up to 0.5 bit/s/Hz without requiring additional bandwidth or antennas.Comment: 29 pages; submitted for publicatio
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