341 research outputs found
Tiny Codes for Guaranteeable Delay
Future 5G systems will need to support ultra-reliable low-latency
communications scenarios. From a latency-reliability viewpoint, it is
inefficient to rely on average utility-based system design. Therefore, we
introduce the notion of guaranteeable delay which is the average delay plus
three standard deviations of the mean. We investigate the trade-off between
guaranteeable delay and throughput for point-to-point wireless erasure links
with unreliable and delayed feedback, by bringing together signal flow
techniques to the area of coding. We use tiny codes, i.e. sliding window by
coding with just 2 packets, and design three variations of selective-repeat ARQ
protocols, by building on the baseline scheme, i.e. uncoded ARQ, developed by
Ausavapattanakun and Nosratinia: (i) Hybrid ARQ with soft combining at the
receiver; (ii) cumulative feedback-based ARQ without rate adaptation; and (iii)
Coded ARQ with rate adaptation based on the cumulative feedback. Contrasting
the performance of these protocols with uncoded ARQ, we demonstrate that HARQ
performs only slightly better, cumulative feedback-based ARQ does not provide
significant throughput while it has better average delay, and Coded ARQ can
provide gains up to about 40% in terms of throughput. Coded ARQ also provides
delay guarantees, and is robust to various challenges such as imperfect and
delayed feedback, burst erasures, and round-trip time fluctuations. This
feature may be preferable for meeting the strict end-to-end latency and
reliability requirements of future use cases of ultra-reliable low-latency
communications in 5G, such as mission-critical communications and industrial
control for critical control messaging.Comment: to appear in IEEE JSAC Special Issue on URLLC in Wireless Network
HARQ in relay-assisted transmission for machine type communications
This letter describes the impact of unknown channel access delay on the timeline of the hybrid automatic repeat request (HARQ) process in the 3rd generation partnership project long term evolution (3GPP LTE) system when a relay node (RN) is used for coverage extension of machine type communication (MTC) devices. A solution is also proposed for the determination of unknown channel access delay when the RN operates in the unlicensed spectrum band. The proposed mechanism is expected to help MTC operation in typical coverage holes areas such as smart meters located in the basement of buildings
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
Hybrid Automatic Repeat Request for Downlink Rate-Splitting Multiple Access
This work investigates the design of Hybrid Automatic Repeat Request (HARQ)
strategies for downlink Rate-Splitting Multiple Access (RSMA). The existence of
private and common stream as well as their conditioning for Successive
Interference Cancellation (SIC), gives rise to an expanded set of opportunities
for retransmission of failed packets. Specifically, we devise a scheme in which
the retransmissions are scheduled through the common stream, which offers a
higher success probability. With this, the common stream needs to carry both
new and retransmitted bits, which leads to a layered HARQ (L-HARQ) strategy
which is capable of trading off throughput and reliability. Simulation results
demonstrate that the devised HARQ scheme outperforms RSMA with conventional
HARQ, where each retransmission is handled independently through its own
stream. It also helps in closing the throughput gap between HARQ and Adaptive
Modulation and Coding (AMC) in the high Signal-to-Noise Ratio (SNR) regime
while also achieving a decreased Packet Error Rate (PER) and a lower latency
- …