51 research outputs found
Ultra-Reliable Short-Packet Communications: Fundamental Limits and Enabling Technologies
The paradigm shift from 4G to 5G communications, anticipated to enable ultra-reliable low-latency communications (URLLC), will enforce a radical change in the design of wireless communication systems. Unlike in 4G systems, where the main objective is to provide a large transmission rate, in URLLC, as implied by its name, the objective is to enable transmissions with low latency and, simultaneously, very high reliability. Since low latency implies the use of short data packets, the tension between blocklength and reliability is studied in URLLC.Several key enablers for URLLC communications have been designated in the literature. Of special importance are diversity-enabling technologies such as multiantenna systems and feedback protocols. Furthermore, it is not only important to introduce additional diversity by means of the above examples, one must also guarantee that thescarce number of channel uses are used in an optimal way. Therefore, it is imperative to develop design guidelines for how to enable reliable detection of incoming data, how to acquire channel-state information, and how to construct efficient short-packet channel codes. The development of such guidelines is at the heart of this thesis. This thesis focuses on the fundamental performance of URLLC-enabling technologies. Specifically, we provide converse (upper) bounds and achievability (lower) bounds on the maximum coding rate, based on finite-blocklength information theory, for systems that employ the key enablers outlined above. With focus on the wireless channel, modeled via a block-fading assumption, we are able to provide answers to questions like: howto optimally utilize spatial and frequency diversity, how far from optimal short-packet channel codes perform, how multiantenna systems should be designed to serve a given number of users, and how to design feedback schemes when the feedback link is noisy. In particular, this thesis is comprised out of four papers. In Paper A, we study the short-packet performance over the Rician block-fading channel. In particular, we present achievability bounds for pilot-assisted transmission with several different decoders that allow us to quantify the impact, on the achievable performance, of imposed pilots and mismatched decoding. Furthermore, we design short-packet channel codes that perform within 1 dB of our achievability bounds. Paper B studies multiuser massive multiple-input multiple-output systems with short packets. We provide an achievability bound on the average error probability over quasistatic spatially correlated Rayleigh-fading channels. The bound applies to arbitrary multiuser settings, pilot-assisted transmission, and mismatched decoding. This makes it suitable to assess the performance in the uplink/downlink for arbitrary linear signal processing. We show that several lessons learned from infinite-blocklength analyses carry over to the finite-blocklength regime. Furthermore, for the multicell setting with randomly placed users, pilot contamination should be avoided at all cost and minimum mean-squared error signal processing should be used to comply with the stringent requirements of URLLC.In Paper C, we consider sporadic transmissions where the task of the receiver is to both detect and decode an incoming packet. Two novel achievability bounds, and a novel converse bound are presented for joint detection-decoding strategies. It is shown that errors associated with detection deteriorates performance significantly for very short packet sizes. Numerical results also indicate that separate detection-decoding strategies are strictly suboptimal over block-fading channels.Finally, in Paper D, variable-length codes with noisy stop-feedback are studied via a novel achievability bound on the average service time and the average error probability. We use the bound to shed light on the resource allocation problem between the forward and the feedback channel. For URLLC applications, it is shown that enough resources must be assigned to the feedback link such that a NACK-to-ACK error becomes rarer than the target error probability. Furthermore, we illustrate that the variable-length stop-feedback scheme outperforms state-of-the-art fixed-length no-feedback bounds even when the stop-feedback bit is noisy
Short-packet Transmission via Variable-Length Codes in the Presence of Noisy Stop Feedback
We present an upper bound on the error probability achievable using
variable-length stop feedback codes, for a fixed size of the information
payload and a given constraint on the maximum latency and the average service
time. Differently from the bound proposed in Polyanskiy et al. (2011), which
pertains to the scenario in which the stop signal is sent over a noiseless
feedback channel, our bound applies to the practically relevant setup in which
the feedback link is noisy. By numerically evaluating our bound, we illustrate
that, for fixed latency and reliability constraints, noise in the feedback link
can cause a significant increase in the minimum average service time, to the
extent that fixed-length codes without feedback may be preferable in some
scenarios.Comment: Submitted to a Transactions on Wireless Communication
Joint source-channel coding with feedback
This paper quantifies the fundamental limits of variable-length transmission
of a general (possibly analog) source over a memoryless channel with noiseless
feedback, under a distortion constraint. We consider excess distortion, average
distortion and guaranteed distortion (-semifaithful codes). In contrast to
the asymptotic fundamental limit, a general conclusion is that allowing
variable-length codes and feedback leads to a sizable improvement in the
fundamental delay-distortion tradeoff. In addition, we investigate the minimum
energy required to reproduce source samples with a given fidelity after
transmission over a memoryless Gaussian channel, and we show that the required
minimum energy is reduced with feedback and an average (rather than maximal)
power constraint.Comment: To appear in IEEE Transactions on Information Theor
Active sequential hypothesis testing
Consider a decision maker who is responsible to dynamically collect
observations so as to enhance his information about an underlying phenomena of
interest in a speedy manner while accounting for the penalty of wrong
declaration. Due to the sequential nature of the problem, the decision maker
relies on his current information state to adaptively select the most
``informative'' sensing action among the available ones. In this paper, using
results in dynamic programming, lower bounds for the optimal total cost are
established. The lower bounds characterize the fundamental limits on the
maximum achievable information acquisition rate and the optimal reliability.
Moreover, upper bounds are obtained via an analysis of two heuristic policies
for dynamic selection of actions. It is shown that the first proposed heuristic
achieves asymptotic optimality, where the notion of asymptotic optimality, due
to Chernoff, implies that the relative difference between the total cost
achieved by the proposed policy and the optimal total cost approaches zero as
the penalty of wrong declaration (hence the number of collected samples)
increases. The second heuristic is shown to achieve asymptotic optimality only
in a limited setting such as the problem of a noisy dynamic search. However, by
considering the dependency on the number of hypotheses, under a technical
condition, this second heuristic is shown to achieve a nonzero information
acquisition rate, establishing a lower bound for the maximum achievable rate
and error exponent. In the case of a noisy dynamic search with size-independent
noise, the obtained nonzero rate and error exponent are shown to be maximum.Comment: Published in at http://dx.doi.org/10.1214/13-AOS1144 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Nonasymptotic coding-rate bounds for binary erasure channels with feedback
We present nonasymptotic achievability and converse bounds on the maximum coding rate (for a fixed average error probability and a fixed average blocklength) of variable-length full-feedback (VLF) and variable-length stop-feedback (VLSF) codes operating over a binary erasure channel (BEC). For the VLF setup, the achievability bound relies on a scheme that maps each message onto a variable-length Huffman codeword and then repeats each bit of the codeword until it is received correctly. The converse bound is inspired by the meta-converse framework by Polyanskiy, Poor, and Verdú (2010) and relies on binary sequential hypothesis testing. For the case of zero error probability, our achievability and converse bounds match. For the VLSF case, we provide achievability bounds that exploit the following feature of BEC: the decoder can assess the correctness of its estimate by verifying whether the chosen codeword is the only one that is compatible with the erasure pattern. One of these bounds is obtained by analyzing the performance of a variable-length extension of random linear fountain codes. The gap between the VLSF achievability and the VLF converse bound, when number of messages is small, is significant: 23% for 8 messages on a BEC with erasure probability 0.5. The absence of a tight VLSF converse bound does not allow us to assess whether this gap is fundamental
Reliable Transmission of Short Packets through Queues and Noisy Channels under Latency and Peak-Age Violation Guarantees
This work investigates the probability that the delay and the peak-age of
information exceed a desired threshold in a point-to-point communication system
with short information packets. The packets are generated according to a
stationary memoryless Bernoulli process, placed in a single-server queue and
then transmitted over a wireless channel. A variable-length stop-feedback
coding scheme---a general strategy that encompasses simple automatic repetition
request (ARQ) and more sophisticated hybrid ARQ techniques as special
cases---is used by the transmitter to convey the information packets to the
receiver. By leveraging finite-blocklength results, the delay violation and the
peak-age violation probabilities are characterized without resorting to
approximations based on large-deviation theory as in previous literature.
Numerical results illuminate the dependence of delay and peak-age violation
probability on system parameters such as the frame size and the undetected
error probability, and on the chosen packet-management policy. The guidelines
provided by our analysis are particularly useful for the design of low-latency
ultra-reliable communication systems.Comment: To appear in IEEE journal on selected areas of communication (IEEE
JSAC
Low-Latency Short-Packet Transmissions: Fixed Length or HARQ?
We study short-packet communications, subject to latency and reliability
constraints, under the premises of limited frequency diversity and no time
diversity. The question addressed is whether, and when, hybrid automatic repeat
request (HARQ) outperforms fixed-blocklength schemes with no feedback (FBL-NF)
in such a setting. We derive an achievability bound for HARQ, under the
assumption of a limited number of transmissions. The bound relies on
pilot-assisted transmission to estimate the fading channel and scaled
nearest-neighbor decoding at the receiver. We compare our achievability bound
for HARQ to stateof-the-art achievability bounds for FBL-NF communications and
show that for a given latency, reliability, number of information bits, and
number of diversity branches, HARQ may significantly outperform FBL-NF. For
example, for an average latency of 1 ms, a target error probability of 10^-3,
30 information bits, and 3 diversity branches, the gain in energy per bit is
about 4 dB.Comment: 6 pages, 5 figures, accepted to GLOBECOM 201
Systematic Transmission With Fountain Parity Checks for Erasure Channels With Stop Feedback
In this paper, we present new achievability bounds on the maximal achievable
rate of variable-length stop-feedback (VLSF) codes operating over a binary
erasure channel (BEC) at a fixed message size . We provide new bounds
for VLSF codes with zero error, infinite decoding times and with nonzero error,
finite decoding times. Both new achievability bounds are proved by constructing
a new VLSF code that employs systematic transmission of the first bits
followed by random linear fountain parity bits decoded with a rank decoder. For
VLSF codes with infinite decoding times, our new bound outperforms the
state-of-the-art result for BEC by Devassy \emph{et al.} in 2016. We also give
a negative answer to the open question Devassy \emph{et al.} put forward on
whether the backoff to capacity at is fundamental. For VLSF
codes with finite decoding times, numerical evaluations show that the
achievable rate for VLSF codes with a moderate number of decoding times closely
approaches that for VLSF codes with infinite decoding times.Comment: 7 pages, double column, 4 figures; comments are welcome! changes in
v2: corrected 2 typos in v1. arXiv admin note: substantial text overlap with
arXiv:2205.1539
Short-Packet Communications: Fundamental Performance and Key Enablers
The paradigm shift from 4G to 5G communications, predicted to enable new use cases such as ultra-reliable low-latency communications (URLLC), will enforce a radical change in the design of communication systems. Unlike in 4G systems, where the main objective is to have a large transmission rate, in URLLC, as implied by its name, the objective is to enable transmissions with low latency and, simultaneously, very high reliability. Since low latency implies the use of short data packets, the tension between blocklength and reliability is studied in URLLC.\ua0Several key enablers for URLLC communications have been designated in the literature. A non-exhaustive list contains: multiple transmit and receive antennas (MIMO), short transmission-time intervals (TTI), increased bandwidth, and feedback protocols. Furthermore, it is not only important to introduce additional diversity by means of the above examples, one must also guarantee that the scarce number of channel uses are used in an optimal way. Therefore, protocols for how to convey meta-data such as control information and pilot symbols are needed as are efficient short-packet channel codes.\ua0This thesis focuses on the performance of reliable short-packet communications. Specifically, we provide converse (upper) bounds and achievability (lower) bounds on the maximum coding rate, based on finite-blocklength information theory, for systems that employ the key enablers outlined above. With focus on the Rician and Rayleigh block-fading channels, we are able to answer, e.g., how to optimally utilize spatial and frequency diversity, how far from optimal short-packet channel codes perform, and whether feedback-based schemes are preferable over non-feedback schemes.\ua0More specifically, in Paper A, we study the performance impact of MIMO and a shortened TTI in both uplink and downlink under maximum-likelihood decoding and Rayleigh block-fading. Based on our results, we are able to study the trade-off between bandwidth, latency, spatial diversity, and error probability. Furthermore, we give an example of a pragmatic design of a pilot-assisted channel code that comes within 2.7 dB of our achievability bounds. In Paper B, we partly extend our work in Paper A to the Rician block-fading channel and to practical schemes such as pilot-assisted transmission with nearest neighbor decoding. We derive achievability bounds for pilot-assisted transmission with several different decoders that allow us to quantify the impact, on the achievable performance, of pilots and mismatched decoding. Furthermore, we design short-packet channel codes that perform within 1 dB of our achievability bounds. Paper C contains an achievability bound for a system that employs a variable-length stop-feedback (VLSF) scheme with an error-free feedback link. Based on the results in Paper C and Paper B, we are able to compare non-feedback schemes to stop-feedback schemes and assess if, and when, one is superior to the other. Specifically, we show that, for some practical scenarios, stop-feedback does significantly outperform non-feedback schemes
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