637 research outputs found
Finite-Blocklength and Error-Exponent Analyses for LDPC Codes in Point-to-Point and Multiple Access Communication
This paper applies error-exponent and dispersion-style analyses to derive
finite-blocklength achievability bounds for low-density parity-check (LDPC)
codes over the point-to-point channel (PPC) and multiple access channel (MAC).
The error-exponent analysis applies Gallager's error exponent to bound
achievable symmetrical and asymmetrical rates in the MAC. The dispersion-style
analysis begins with a generalization of the random coding union (RCU) bound
from random code ensembles with i.i.d. codewords to random code ensembles in
which codewords may be statistically dependent; this generalization is useful
since the codewords of random linear codes such as random LDPC codes are
dependent. Application of the RCU bound yields improved finite-blocklength
error bounds and asymptotic achievability results for i.i.d. random codes and
new finite-blocklength error bounds and achievability results for LDPC codes.
For discrete, memoryless channels, these results show that LDPC codes achieve
first- and second-order performance that is optimal for the PPC and identical
to the best-prior results for the MAC.Comment: 32 pages, 2 figure
A Simple Technique for the Converse of Finite Blocklength Multiple Access Channels
A converse for the Discrete Memoryless Multiple Access Channel is given. The
result in [13] is refined, and the third order term is obtained. Moreover, our
proof is much simpler than [13]. With little modification, the region can be
further improved.Comment: Results are refine
NOMA in the Uplink: Delay Analysis with Imperfect CSI and Finite-Length Coding
We study whether using non-orthogonal multiple access (NOMA) in the uplink of
a mobile network can improve the performance over orthogonal multiple access
(OMA) when the system requires ultra-reliable low-latency communications
(URLLC). To answer this question, we first consider an ideal system model with
perfect channel state information (CSI) at the transmitter and long codewords,
where we determine the optimal decoding orders when the decoder uses successive
interference cancellation (SIC) and derive closed-form expressions for the
optimal rate when joint decoding is used. While joint decoding performs well
even under tight delay constraints, NOMA with SIC decoding often performs worse
than OMA. For low-latency systems, we must also consider the impact of
finite-length channel coding, as well as rate adaptation based imperfect CSI.
We derive closed-form approximations for the corresponding outage or error
probabilities and find that those effects create a larger performance penalty
for NOMA than for OMA. Thus, NOMA with SIC decoding may often be unsuitable for
URLLC
Short-Packet Communications in Non-Orthogonal Multiple Access Systems
This work introduces, for the first time, non-orthogonal multiple access
(NOMA) into short-packet communications to achieve low latency in wireless
networks. Specifically, we address the optimization of transmission rates and
power allocation to maximize the effective throughput of the user with a higher
channel gain while guaranteeing the other user achieving a certain level of
effective throughput. To demonstrate the benefits of NOMA, we analyze the
performance of orthogonal multiple access (OMA) as a benchmark. Our examination
shows that NOMA can significantly outperform OMA by achieving a higher
effective throughput with the same latency or incurring a lower latency to
achieve the same effective throughput targets. Surprisingly, we find that the
performance gap between NOMA and OMA becomes more prominent when the effective
throughput targets at the two users become closer to each other. This
demonstrates that NOMA can significantly reduce the latency in the context of
short-packet communications with practical constraints.Comment: 6 pages, 4 figures. This paper has already been submitted to IEEE ICC
201
Cross-layer Optimization for Ultra-reliable and Low-latency Radio Access Networks
In this paper, we propose a framework for cross-layer optimization to ensure
ultra-high reliability and ultra-low latency in radio access networks, where
both transmission delay and queueing delay are considered. With short
transmission time, the blocklength of channel codes is finite, and the Shannon
Capacity cannot be used to characterize the maximal achievable rate with given
transmission error probability. With randomly arrived packets, some packets may
violate the queueing delay. Moreover, since the queueing delay is shorter than
the channel coherence time in typical scenarios, the required transmit power to
guarantee the queueing delay and transmission error probability will become
unbounded even with spatial diversity. To ensure the required
quality-of-service (QoS) with finite transmit power, a proactive packet
dropping mechanism is introduced. Then, the overall packet loss probability
includes transmission error probability, queueing delay violation probability,
and packet dropping probability. We optimize the packet dropping policy, power
allocation policy, and bandwidth allocation policy to minimize the transmit
power under the QoS constraint. The optimal solution is obtained, which depends
on both channel and queue state information. Simulation and numerical results
validate our analysis, and show that setting packet loss probabilities equal is
a near optimal solution.Comment: The manuscript has been accepted by IEEE transactions on wireless
communication
Benefit of Delay on the Diversity-Multiplexing Tradeoffs of MIMO Channels with Partial CSI
This paper re-examines the well-known fundamental tradeoffs between rate and
reliability for the multi-antenna, block Rayleigh fading channel in the high
signal to noise ratio (SNR) regime when (i) the transmitter has access to
(noiseless) one bit per coherence-interval of causal channel state information
(CSI) and (ii) soft decoding delays together with worst-case delay guarantees
are acceptable. A key finding of this work is that substantial improvements in
reliability can be realized with a very short expected delay and a slightly
longer (but bounded) worst-case decoding delay guarantee in communication
systems where the transmitter has access to even one bit per coherence interval
of causal CSI. While similar in spirit to the recent work on communication
systems based on automatic repeat requests (ARQ) where decoding failure is
known at the transmitter and leads to re-transmission, here transmit
side-information is purely based on CSI. The findings reported here also lend
further support to an emerging understanding that decoding delay (related to
throughput) and codeword blocklength (related to coding complexity and delays)
are distinctly different design parameters which can be tuned to control
reliability.Comment: 5 pages, 2 figures, to appear in the Proceedings of the IEEE
International Symposium on Information Theory, Nice, France, 24-29 June, 200
On Capacity Region of Wiretap Networks
In this paper we consider the problem of secure network coding where an
adversary has access to an unknown subset of links chosen from a known
collection of links subsets. We study the capacity region of such networks,
commonly called "wiretap networks", subject to weak and strong secrecy
constraints, and consider both zero-error and asymptotically zero-error
communication. We prove that in general discrete memoryless networks modeled by
discrete memoryless channels, the capacity region subject to strong secrecy
requirement and the capacity region subject to weak secrecy requirement are
equal. In particular, this result shows that requiring strong secrecy in a
wiretap network with asymptotically zero probability of error does not shrink
the capacity region compared to the case of weak secrecy requirement. We also
derive inner and outer bounds on the network coding capacity region of wiretap
networks subject to weak secrecy constraint, for both zero probability of error
and asymptotically zero probability of error, in terms of the entropic region
The Arbitrarily Varying Multiple-Access Channel with Conferencing Encoders
We derive the capacity region of arbitrarily varying multiple-access channels
with conferencing encoders for both deterministic and random coding. For a
complete description it is sufficient that one conferencing capacity is
positive. We obtain a dichotomy: either the channel's deterministic capacity
region is zero or it equals the two-dimensional random coding region. We
determine exactly when either case holds. We also discuss the benefits of
conferencing. We give the example of an AV-MAC which does not achieve any
non-zero rate pair without encoder cooperation, but the two-dimensional random
coding capacity region if conferencing is possible. Unlike compound
multiple-access channels, arbitrarily varying multiple-access channels may
exhibit a discontinuous increase of the capacity region when conferencing in at
least one direction is enabled.Comment: 12 pages, accepted for publication in IEEE Transaction on Information
Theor
Finite Blocklength Performance of Multi-Terminal Wireless Industrial Networks
This work focuses on the performance of multi-terminal wireless industrial
networks, where the transmissions of all terminals are required to be scheduled
within a tight deadline. The transmissions thus share a fixed amount of
resources, i.e., symbols, while facing short blocklengths due to the
low-latency requirement. We investigate two distinct relaying strategies,
namely best relay selection among the participating terminals and best antenna
selection at the access point of the network. In both schemes, we incorporate
the cost of acquiring instantaneous Channel State Information (CSI) at the
access point within the transmission deadline. An error probability model is
developed under the finite blocklength regime to provide accurate performance
results. As a reference, this model is compared to the corresponding infinite
bocklength error model. Both analytical models are validated by simulation. We
show that the average Packet Error Rate (PER) over all terminals is convex in
the target error probability at each single link. Moreover, we find that: (i)
The reliability behavior is different for the two strategies, while the
limiting factors are both finite blocklengths and overhead of acquiring CSI.
(ii) With the same order of diversity, best antenna selection is more reliable
than best relay selection. (iii) The average PER is increasing in the number of
participating terminals unless the terminals also act as relay candidates. In
particular, if each participating terminal is a candidate for best relay
selection, the PER is convex in the number of terminals
Asymptotic Estimates in Information Theory with Non-Vanishing Error Probabilities
This monograph presents a unified treatment of single- and multi-user
problems in Shannon's information theory where we depart from the requirement
that the error probability decays asymptotically in the blocklength. Instead,
the error probabilities for various problems are bounded above by a
non-vanishing constant and the spotlight is shone on achievable coding rates as
functions of the growing blocklengths. This represents the study of asymptotic
estimates with non-vanishing error probabilities.
In Part I, after reviewing the fundamentals of information theory, we discuss
Strassen's seminal result for binary hypothesis testing where the type-I error
probability is non-vanishing and the rate of decay of the type-II error
probability with growing number of independent observations is characterized.
In Part II, we use this basic hypothesis testing result to develop second- and
sometimes, even third-order asymptotic expansions for point-to-point
communication. Finally in Part III, we consider network information theory
problems for which the second-order asymptotics are known. These problems
include some classes of channels with random state, the multiple-encoder
distributed lossless source coding (Slepian-Wolf) problem and special cases of
the Gaussian interference and multiple-access channels. Finally, we discuss
avenues for further research.Comment: Further comments welcom
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