64,816 research outputs found
Optimizing the Energy Efficiency of Short Term Ultra Reliable Communications in Vehicular Networks
We evaluate the use of HARQ schemes in the context of vehicle to infrastructure communications considering ultra reliable communications in the short term from a channel capacity stand point. We show that it is not possible to meet strict latency requirements with very high reliability without some diversity strategy and propose a solution to determining an optimal limit on the maximum allowed number of retransmissions using Chase combining and simple HARQ to increase energy efficiency. Results show that using the proposed optimizations leads to spending 5 times less energy when compared to only one retransmission in the context of a benchmark test case for urban scenario. In addition, we present an approximation that relates most system parameters and can predict whether or not the link can be closed, which is valuable for system design
Deconstructing the Blockchain to Approach Physical Limits
Transaction throughput, confirmation latency and confirmation reliability are
fundamental performance measures of any blockchain system in addition to its
security. In a decentralized setting, these measures are limited by two
underlying physical network attributes: communication capacity and
speed-of-light propagation delay. Existing systems operate far away from these
physical limits. In this work we introduce Prism, a new proof-of-work
blockchain protocol, which can achieve 1) security against up to 50%
adversarial hashing power; 2) optimal throughput up to the capacity C of the
network; 3) confirmation latency for honest transactions proportional to the
propagation delay D, with confirmation error probability exponentially small in
CD ; 4) eventual total ordering of all transactions. Our approach to the design
of this protocol is based on deconstructing the blockchain into its basic
functionalities and systematically scaling up these functionalities to approach
their physical limits.Comment: Computer and Communications Security, 201
Random Linear Network Coding For Time Division Duplexing: When To Stop Talking And Start Listening
A new random linear network coding scheme for reliable communications for
time division duplexing channels is proposed. The setup assumes a packet
erasure channel and that nodes cannot transmit and receive information
simultaneously. The sender transmits coded data packets back-to-back before
stopping to wait for the receiver to acknowledge (ACK) the number of degrees of
freedom, if any, that are required to decode correctly the information. We
provide an analysis of this problem to show that there is an optimal number of
coded data packets, in terms of mean completion time, to be sent before
stopping to listen. This number depends on the latency, probabilities of packet
erasure and ACK erasure, and the number of degrees of freedom that the receiver
requires to decode the data. This scheme is optimal in terms of the mean time
to complete the transmission of a fixed number of data packets. We show that
its performance is very close to that of a full duplex system, while
transmitting a different number of coded packets can cause large degradation in
performance, especially if latency is high. Also, we study the throughput
performance of our scheme and compare it to existing half-duplex Go-back-N and
Selective Repeat ARQ schemes. Numerical results, obtained for different
latencies, show that our scheme has similar performance to the Selective Repeat
in most cases and considerable performance gain when latency and packet error
probability is high.Comment: 9 pages, 9 figures, Submitted to INFOCOM'0
Achieving Energy-Efficient Uplink URLLC with MIMO-Aided Grant-Free Access
The optimal design of the energy-efficient multiple-input multiple-output
(MIMO) aided uplink ultra-reliable low-latency communications (URLLC) system is
an important but unsolved problem. For such a system, we propose a novel
absorbing-Markov-chain-based analysis framework to shed light on the puzzling
relationship between the delay and reliability, as well as to quantify the
system energy efficiency. We derive the transition probabilities of the
absorbing Markov chain considering the Rayleigh fading, the channel estimation
error, the zero-forcing multi-user-detection (ZF-MUD), the grant-free access,
the ACK-enabled retransmissions within the delay bound and the interactions
among these technical ingredients. Then, the delay-constrained reliability and
the system energy efficiency are derived based on the absorbing Markov chain
formulated. Finally, we study the optimal number of user equipments (UEs) and
the optimal number of receiving antennas that maximize the system energy
efficiency, while satisfying the reliability and latency requirements of URLLC
simultaneously. Simulation results demonstrate the accuracy of our theoretical
analysis and the effectiveness of massive MIMO in supporting large-scale URLLC
systems.Comment: 14 pages, 9 figures, accepted to appear on IEEE Transactions on
Wireless Communications, Aug. 202
- …