Ultra-Reliable Short-Packet Communications: Fundamental Limits and Enabling Technologies
Doktorsavhandling, 2020

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 the

scarce 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: how

to 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.


Block-fading channels

multiuser massive MIMO

joint detection and decoding

variablelength stop-feedback

ultra-reliable low-latency

short packets

channel estimation

imperfect CSI

Opponent: Marios Kountouris, EURECOM, Sophia Antipolis, France


Johan Östman

Chalmers, Elektroteknik, Kommunikation, Antenner och Optiska Nätverk

Short-packet Transmission via Variable-Length Codes in the Presence of Noisy Stop Feedback

IEEE Transactions on Wireless Communications,; Vol. 20(2021)p. 214-227

Artikel i vetenskaplig tidskrift

J. Östman, A. Lancho, G. Durisi, and L. Sanguinetti. URLLC with massive MIMO: analysis and design at finite blocklength

J. Östman, A. Lancho, and R. Devassy. Short-packet transmission with imperfect detection

Short Packets over Block-Memoryless Fading Channels: Pilot-Assisted or Noncoherent Transmission?

IEEE Transactions on Communications,; Vol. 67(2019)p. 1521-1536

Artikel i vetenskaplig tidskrift

The limits of language

The human language is perhaps the most eminent example of wireless communications. Many ingredients in the way we interact with each other have analogs in wireless communication systems. For example, the rate at which we speak can be measured by the number of syllables per second and the information content as the number of bits per syllable. Clearly, the rate and information per syllable dier signicantly among dierent languages.

Naturally, the faster one speaks, the harder it is to comprehend the content. To alleviate this issue, people typically employ alternative ways to clarify what is being said via so-called diversity-exploiting techniques. Hand gestures provide diversity in space and by repeating what was being said, time-diversity is exploited. If the information is also of a sensitive nature, it can be a good idea to employ a feedback protocol and ask the recipient to acknowledge what he/she heard. If the intended message was misunderstood, it can then be repeated.

Now, equipped with all available human languages and diversity-exploiting techniques, we ask the following fundamental question: for a given probability of a misunderstanding and a given time duration, what is the fastest rate at which a given piece of information can be communicated? This is the topic studied in this thesis but in the context of wireless communication systems.

Teori och praktik för optimal spektral effektivitet i trådlösa ad-hoc nät med strikta krav på latens och tillförlitlighet.

Vetenskapsrådet (VR) (2014-6066), 2015-01-01 -- 2018-12-31.


Informations- och kommunikationsteknik







Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4856




Opponent: Marios Kountouris, EURECOM, Sophia Antipolis, France

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