24 research outputs found
Short Packets over Block-Memoryless Fading Channels: Pilot-Assisted or Noncoherent Transmission?
We present nonasymptotic upper and lower bounds on the maximum coding rate
achievable when transmitting short packets over a Rician memoryless
block-fading channel for a given requirement on the packet error probability.
We focus on the practically relevant scenario in which there is no \emph{a
priori} channel state information available at the transmitter and at the
receiver. An upper bound built upon the min-max converse is compared to two
lower bounds: the first one relies on a noncoherent transmission strategy in
which the fading channel is not estimated explicitly at the receiver; the
second one employs pilot-assisted transmission (PAT) followed by
maximum-likelihood channel estimation and scaled mismatched nearest-neighbor
decoding at the receiver. Our bounds are tight enough to unveil the optimum
number of diversity branches that a packet should span so that the energy per
bit required to achieve a target packet error probability is minimized, for a
given constraint on the code rate and the packet size. Furthermore, the bounds
reveal that noncoherent transmission is more energy efficient than PAT, even
when the number of pilot symbols and their power is optimized. For example, for
the case when a coded packet of symbols is transmitted using a channel
code of rate bits/channel use, over a block-fading channel with block
size equal to symbols, PAT requires an additional dB of energy per
information bit to achieve a packet error probability of compared to
a suitably designed noncoherent transmission scheme. Finally, we devise a PAT
scheme based on punctured tail-biting quasi-cyclic codes and ordered statistics
decoding, whose performance are close ( dB gap at packet error
probability) to the ones predicted by our PAT lower bound. This shows that the
PAT lower bound provides useful guidelines on the design of actual PAT schemes.Comment: 30 pages, 5 figures, journa
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MIMO block-fading channels with mismatched CSI
YesWe study transmission over multiple-input multiple-output (MIMO) block-fading channels with
imperfect channel state information (CSI) at both the transmitter and receiver. Specifically, based on
mismatched decoding theory for a fixed channel realization, we investigate the largest achievable rates
with independent and identically distributed inputs and a nearest neighbor decoder. We then study the
corresponding information outage probability in the high signal-to-noise ratio (SNR) regime and analyze
the interplay between estimation error variances at the transmitter and at the receiver to determine
the optimal outage exponent, defined as the high-SNR slope of the outage probability plotted in a
logarithmic-logarithmic scale against the SNR. We demonstrate that despite operating with imperfect
CSI, power adaptation can offer substantial gains in terms of outage exponent.A. T. Asyhari was supported in part by the Yousef Jameel Scholarship, University of Cambridge, Cambridge, U.K., and the National Science Council of Taiwan under grant NSC 102-2218-E-009-001. A. Guillén i Fà bregas was supported in part by the European Research Council under ERC grant agreement 259663 and the Spanish Ministry of Economy and Competitiveness under grant TEC2012-38800-C03-03
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
URLLC with Massive MIMO: Analysis and Design at Finite Blocklength
The fast adoption of Massive MIMO for high-throughput communications was
enabled by many research contributions mostly relying on infinite-blocklength
information-theoretic bounds. This makes it hard to assess the suitability of
Massive MIMO for ultra-reliable low-latency communications (URLLC) operating
with short blocklength codes. This paper provides a rigorous framework for the
characterization and numerical evaluation (using the saddlepoint approximation)
of the error probability achievable in the uplink and downlink of Massive MIMO
at finite blocklength. The framework encompasses imperfect channel state
information, pilot contamination, spatially correlated channels, and arbitrary
linear spatial processing. In line with previous results based on
infinite-blocklength bounds, we prove that, with minimum mean-square error
(MMSE) processing and spatially correlated channels, the error probability at
finite blocklength goes to zero as the number of antennas grows to
infinity, even under pilot contamination. On the other hand, numerical results
for a practical URLLC network setup involving a base station with
antennas, show that a target error probability of can be achieved
with MMSE processing, uniformly over each cell, only if orthogonal pilot
sequences are assigned to all the users in the network. Maximum ratio
processing does not suffice.Comment: 30 pages, 5 figure
URLLC with Massive MIMO: Analysis and Design at Finite Blocklength
The fast adoption of Massive MIMO for high-throughput communications was enabled by many research contributions mostly relying on infinite-blocklength information-theoretic bounds. This makes it hard to assess the suitability of Massive MIMO for ultra-reliable low-latency communications (URLLC) operating with short-blocklength codes. This paper provides a rigorous framework for the characterization and numerical evaluation (using the saddlepoint approximation) of the error probability achievable in the uplink and downlink of Massive MIMO at finite blocklength. The framework encompasses imperfect channel state information, pilot contamination, spatially correlated channels, and arbitrary linear spatial processing. In line with previous results based on infinite-blocklength bounds, we prove that, with minimum mean-square error (MMSE) processing and spatially correlated channels, the error probability at finite blocklength goes to zero as the number M of antennas grows to infinity, even under pilot contamination. However, numerical results for a practical URLLC network setup involving a base station with M-100 antennas, show that a target error probability of 10^¿5 can be achieved with MMSE processing, uniformly over each cell, only if orthogonal pilot sequences are assigned to all the users in the network. Maximum ratio processing does not suffice.The work of Johan Östman, Alejandro Lancho, and Giuseppe Durisi was supported in part by the Swedish Research Council under grant 2016-03293 and in part by the Wallenberg AI, Autonomous Systems, and Software Program. The work of Luca Sanguinetti was supported in part by the Italian Ministry of Education and Research (MIUR) in the framework of the CrossLab Project (Departments of Excellence)
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
Power-Controlled Feedback and Training for Two-way MIMO Channels
Most communication systems use some form of feedback, often related to
channel state information. The common models used in analyses either assume
perfect channel state information at the receiver and/or noiseless state
feedback links. However, in practical systems, neither is the channel estimate
known perfectly at the receiver and nor is the feedback link perfect. In this
paper, we study the achievable diversity multiplexing tradeoff using i.i.d.
Gaussian codebooks, considering the errors in training the receiver and the
errors in the feedback link for FDD systems, where the forward and the feedback
are independent MIMO channels.
Our key result is that the maximum diversity order with one-bit of feedback
information is identical to systems with more feedback bits. Thus,
asymptotically in , more than one bit of feedback does not
improve the system performance at constant rates. Furthermore, the one-bit
diversity-multiplexing performance is identical to the system which has perfect
channel state information at the receiver along with noiseless feedback link.
This achievability uses novel concepts of power controlled feedback and
training, which naturally surface when we consider imperfect channel estimation
and noisy feedback links. In the process of evaluating the proposed training
and feedback protocols, we find an asymptotic expression for the joint
probability of the exponents of eigenvalues of the actual
channel and the estimated channel which may be of independent interest.Comment: in IEEE Transactions on Information Theory, 201
Generalized HARQ Protocols with Delayed Channel State Information and Average Latency Constraints
In many wireless systems, the signal-to-interference-and-noise ratio that is
applicable to a certain transmission, referred to as channel state information
(CSI), can only be learned after the transmission has taken place and is
thereby delayed (outdated). In such systems, hybrid automatic repeat request
(HARQ) protocols are often used to achieve high throughput with low latency.
This paper put forth the family of expandable message space (EMS) protocols
that generalize the HARQ protocol and allow for rate adaptation based on
delayed CSI at the transmitter (CSIT). Assuming a block-fading channel, the
proposed EMS protocols are analyzed using dynamic programming. When full
delayed CSIT is available and there is a constraint on the average decoding
time, it is shown that the optimal zero outage EMS protocol has a particularly
simple operational interpretation and that the throughput is identical to that
of the backtrack retransmission request (BRQ) protocol. We also devise EMS
protocols for the case in which CSIT is only available through a finite number
of feedback messages. The numerical results demonstrate that the throughput of
BRQ approaches the ergodic capacity quickly compared to HARQ, while EMS
protocols with only three and four feedback messages achieve throughputs that
are only slightly worse than that of BRQ.Comment: 19 pages, 5 figure