56 research outputs found
A Rate-Compatible Sphere-Packing Analysis of Feedback Coding with Limited Retransmissions
Recent work by Polyanskiy et al. and Chen et al. has excited new interest in
using feedback to approach capacity with low latency. Polyanskiy showed that
feedback identifying the first symbol at which decoding is successful allows
capacity to be approached with surprisingly low latency. This paper uses Chen's
rate-compatible sphere-packing (RCSP) analysis to study what happens when
symbols must be transmitted in packets, as with a traditional hybrid ARQ
system, and limited to relatively few (six or fewer) incremental transmissions.
Numerical optimizations find the series of progressively growing cumulative
block lengths that enable RCSP to approach capacity with the minimum possible
latency. RCSP analysis shows that five incremental transmissions are sufficient
to achieve 92% of capacity with an average block length of fewer than 101
symbols on the AWGN channel with SNR of 2.0 dB.
The RCSP analysis provides a decoding error trajectory that specifies the
decoding error rate for each cumulative block length. Though RCSP is an
idealization, an example tail-biting convolutional code matches the RCSP
decoding error trajectory and achieves 91% of capacity with an average block
length of 102 symbols on the AWGN channel with SNR of 2.0 dB. We also show how
RCSP analysis can be used in cases where packets have deadlines associated with
them (leading to an outage probability).Comment: To be published at the 2012 IEEE International Symposium on
Information Theory, Cambridge, MA, USA. Updated to incorporate reviewers'
comments and add new figure
Using Channel Output Feedback to Increase Throughput in Hybrid-ARQ
Hybrid-ARQ protocols have become common in many packet transmission systems
due to their incorporation in various standards. Hybrid-ARQ combines the normal
automatic repeat request (ARQ) method with error correction codes to increase
reliability and throughput. In this paper, we look at improving upon this
performance using feedback information from the receiver, in particular, using
a powerful forward error correction (FEC) code in conjunction with a proposed
linear feedback code for the Rayleigh block fading channels. The new hybrid-ARQ
scheme is initially developed for full received packet feedback in a
point-to-point link. It is then extended to various different multiple-antenna
scenarios (MISO/MIMO) with varying amounts of packet feedback information.
Simulations illustrate gains in throughput.Comment: 30 page
Feedback Communication Systems with Limitations on Incremental Redundancy
This paper explores feedback systems using incremental redundancy (IR) with
noiseless transmitter confirmation (NTC). For IR-NTC systems based on {\em
finite-length} codes (with blocklength ) and decoding attempts only at {\em
certain specified decoding times}, this paper presents the asymptotic expansion
achieved by random coding, provides rate-compatible sphere-packing (RCSP)
performance approximations, and presents simulation results of tail-biting
convolutional codes.
The information-theoretic analysis shows that values of relatively close
to the expected latency yield the same random-coding achievability expansion as
with . However, the penalty introduced in the expansion by limiting
decoding times is linear in the interval between decoding times. For binary
symmetric channels, the RCSP approximation provides an efficiently-computed
approximation of performance that shows excellent agreement with a family of
rate-compatible, tail-biting convolutional codes in the short-latency regime.
For the additive white Gaussian noise channel, bounded-distance decoding
simplifies the computation of the marginal RCSP approximation and produces
similar results as analysis based on maximum-likelihood decoding for latencies
greater than 200. The efficiency of the marginal RCSP approximation facilitates
optimization of the lengths of incremental transmissions when the number of
incremental transmissions is constrained to be small or the length of the
incremental transmissions is constrained to be uniform after the first
transmission. Finally, an RCSP-based decoding error trajectory is introduced
that provides target error rates for the design of rate-compatible code
families for use in feedback communication systems.Comment: 23 pages, 15 figure
Optimizing HARQ and relay strategies in limited feedback communication systems
One of the key challenges for future communication systems is to deal with fast changing channels due to the mobility of users. Having a robust protocol capable of handling transmission failures in unfavorable channel conditions is crucial, but the feedback capacity may be greatly limited due to strict latency requirements. This paper studies the hybrid automatic repeat request (HARQ) techniques involved in re-transmissions when decoding failures occur at the receiver and proposes a scheme that relies on codeword bundling and adaptive incremental redundancy (IR) to maximize the overall throughput in a limited feedback system. In addition to the traditional codeword extension IR bits, this paper introduces a new type of IR, bundle parity bits, obtained from an erasure code across all the codewords in a bundle. The type and number of IR bits to be sent as a response to a decoding failure is optimized through a Markov Decision Process. In addition to the single link analysis, the paper studies how the same techniques generalize to relay and multi-user broadcast systems. Simulation results show that the proposed schemes can provide a significant increase in throughput over traditional HARQ techniques
When all information is not created equal
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2008.Includes bibliographical references (p. 191-196).Following Shannon's landmark paper, the classical theoretical framework for communication is based on a simplifying assumption that all information is equally important, thus aiming to provide a uniform protection to all information. However, this homogeneous view of information is not suitable for a variety of modern-day communication scenarios such as wireless and sensor networks, video transmission, interactive systems, and control applications. For example, an emergency alarm from a sensor network needs more protection than other transmitted information. Similarly, the coarse resolution of an image needs better protection than its finer details. For such heterogeneous information, if providing a uniformly high protection level to all parts of the information is infeasible, it is desirable to provide different protection levels based on the importance of those parts. The main objective of this thesis is to extend classical information theory to address this heterogeneous nature of information. Many theoretical tools needed for this are fundamentally different from the conventional homogeneous setting. One key issue is that bits are no more a sufficient measure of information. We develop a general framework for understanding the fundamental limits of transmitting such information, calculate such fundamental limits, and provide optimal architectures for achieving these limits. Our analysis shows that even without sacrificing the data-rate from channel capacity, some crucial parts of information can be protected with exponential reliability. This research would challenge the notion that a set of homogenous bits should necessarily be viewed as a universal interface to the physical layer; this potentially impacts the design of network architectures. This thesis also develops two novel approaches for simplifying such difficult problems in information theory. Our formulations are based on ideas from graphical models and Euclidean geometry and provide canonical examples for network information theory. They provide fresh insights into previously intractable problems as well as generalize previous related results.by Shashibhushan Prataprao Borade.Ph.D
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