16 research outputs found
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
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
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
How URLLC can Benefit from NOMA-based Retransmissions
Among the new types of connectivity unleashed by the emerging 5G wireless
systems, Ultra-Reliable Low Latency Communication (URLLC) is perhaps the most
innovative, yet challenging one. Ultra-reliability requires high levels of
diversity, however, the reactive approach based on packet retransmission in
HARQ protocols should be applied carefully to conform to the stringent latency
constraints. The main premise of this paper is that the NOMA principle can be
used to achieve highly efficient retransmissions by allowing concurrent use of
wireless resources in the uplink. We introduce a comprehensive solution that
accommodates multiple intermittently active users, each with its own HARQ
process. The performance is investigated under two different assumptions about
the Channel State Information (CSI) availability: statistical and
instantaneous. The results show that NOMA can indeed lead to highly efficient
system operation compared to the case in which all HARQ processes are run
orthogonally