Cyril Leung

An achievable rate region for the multiple-access channel with feedback

An achievable rate region R_{1} \leq I(X_{1};Y|X_{2},U), R_{2} \leq I(X_{2}; Y|X_{1},U), R_{1}+R_{2} \leq I(X_{1}, X_{2};Y) , where p(u,x_{l},x_{2},y)= p(u)p(x_{l}|u)p(x_{2}|u)p(y|x_{l},x_{2}) , is established for the multiple-access channel with feedback. Time sharing of these achievable rates yields the rate region of this paper. This region generally exceeds the achievable rate region without feedback and exceeds the rate point found by Gaarder and Wolf for the binary erasure multiple-access channel with feedback. The presence of feedback allows the independent transmitters to understand each others intended transmissions before the receiver has sufficient information to achieve the desired decoding. This allows the transmitters to cooperate in the transmission of information that resolves the residual uncertainty of the receiver. At the same time, independent information from the transmitters is superimposed on the cooperative correction information. The proof involves list codes and block Markov encoding.

Wireless Energy Harvesting and Spectrum Sharing in Cognitive Radio

A wireless energy harvesting protocol is proposed for a decode-and-forward relay- assisted secondary user (SU) network in a cognitive spectrum sharing paradigm. An expression for the outage probability of the relay-assisted cognitive network is derived subject to the following power constraints: 1) the maximum power that the source and the relay in the SU network can transmit from the harvested energy, 2) the peak interference power from the source and the relay in the SU network at the primary user (PU) network, and 3) the interference power of the PU network at the relay-assisted SU network. The results show that as the energy harvesting conversion efficiency improves, the relay- assisted network with the proposed wireless energy harvesting protocol can operate with outage probabilities below 20% for some practical applications.

Forward Error Correction for an ARQ Scheme

This paper examines the use of BCH error-correcting codes in improving the performance of a stop-and-wait automatic repeat-request (ARQ) scheme over random error and Rayleigh fading channels. Two models are analyzed. The first model considers the effect of forward error correction on the mean wasted time per message. The second model assumes a Poisson arrival process for the messages and examines the effect of forward error correction on the mean time between the arrival of a message and its successful transmission. In both models, our results indicate that the performance of the ARQ scheme can be substantially improved by the use of forward error correction.

Lifetime Analysis of a Two-Hop Amplify-and-Forward Opportunistic Wireless Relay Network

An expression is derived for the probability mass function (PMF) of the relay transmit power in a variable gain amplify-and-forward (VG-AF) opportunistic wireless relay network (OWRN). The PMF is used to calculate the average relay transmit power. An expression is also obtained for evaluating the transition probabilities between energy states in a Markov chain model of the OWRN. This model is used to compute the average OWRN lifetime for a small number of relays, allowable transmit power levels, and low initial relay energy levels. Unfortunately, the computational complexity of this approach becomes prohibitive as the number of relays, transmit power levels, and initial energy levels increase. A low-complexity method, based on an existing expression and the average relay transmit power, is used to estimate the average network lifetime. The method is shown to yield very accurate results for practical initial relay energy levels.

Energy Efficient Collaborative Spectrum Sensing Based on Trust Management in Cognitive Radio Networks

An energy efficient collaborative spectrum sensing (EE-CSS) protocol, based on trust management, is proposed. The protocol achieves energy efficiency by reducing the total number of sensing reports exchanged between the honest secondary users (HSUs) and the secondary user base station (SUBS) in a traditional collaborative spectrum sensing (T-CSS) protocol. It is shown that the minimum total number of sensing reports required to satisfy a target global false alarm (FA) and missed detection (MD) probabilities in T-CSS is higher than that in EE-CSS. Expressions for the steady-state average SU trust value τ̅ and total number N̅ of SU sensing reports transmitted are derived, as is an expression for the energy consumption, in EE-CSS and T-CSS. The global FA and detection probabilities Qf and Qd are obtained for a commonly used decision fusion technique. The impact of link outages on τ̅, N̅, Qf, and Qd is also analyzed. The results show that the energy consumption in EE-CSS can be much lower compared to that in T-CSS for long range communications where the transmit energy is dominant.

A predictive strategy for lifetime maximization in selective relay networks

Two algorithms based on an energy conserving dynamic transmit power threshold are proposed for improving the lifetime in relay networks utilizing selective relay strategies with Amplify-and-Forward (AF) relays. the lifetime of the relay network is defined as the maximum number of successfully received messages satisfying a desired SNR at the destination under probability of outage constraints. In the first algorithm, the predicted outage probability, calculated based on the energy conserving dynamic threshold, is constrained at each transmission. In this case, when the number of relays is large, the improvement is substantial. As the number of relays decreases, the method improves the lifetime under the condition of high initial energy levels at the relays. In the second method, targeting applications which are not sensitive to the distribution of outage throughout the lifetime of the relay network, the predicted probability of outage, calculated based on laws-of-physics limitations only, is constrained at each transmission. Using the second method, greater lifetime improvements are achieved and average outage constraints are maintained at the expense of a few instantaneous outage probability violations. Both algorithms are implemented in conjunction with previously proposed energy greedy relay selection strategies such as Minimum Power Transmission (MPT), Maximum Residual Energy (MRE), Minimum Energy Index (MEI), and Maximum Outage Probability (MOP).

On the undetected error probability of triple-error-correcting BCH codes

The probability of undetected error P/sub u/( epsilon ) for the primitive triple-error-correcting BCH codes of blocklength 2/sup m/-1 used solely for error detection on a binary symmetric channel with crossover probability epsilon >

Secrecy capacity and wireless energy harvesting in amplify-and-forward relay networks

The secrecy capacity (SC) from the source to a destination in a dual-hop amplify-and-forward (AF) relay network using cooperative jamming (CJ) is investigated. Conditions for achieving a positive SC are derived. An energy constrained untrusted cooperative relaying protocol is considered in which the relay forwards information from the source to the destination while harvesting energy from the source signal and the destination jamming signal. An optimization problem is proposed to maximize the SC subject to the energy constraints. A method is proposed to find the optimal power split ratio (PSR) which maximizes the SC. An approximate expression for the optimal PSR is also derived. Results show that a mutually beneficial relationship exists between source-destination pair and the relay. This allows the relay to achieve a net energy gain while the secrecy of the confidential message from the source to the destination is protected.

On the undetected error probability of binary expansions of Reed-Solomon codes

It has been shown by Kasami and Lin (see IEEE Trans. Commun., vol.32, p.998, 1984) that (n,k) Reed-Solomon codes used over a q-ary symmetric channel are proper. In this correspondence, it is shown that the binary expansions of these codes and their extensions, when used on the binary-symmetric channel, are not necessarily proper. In particular, certain codes of rate less than [1-log/sub 2/m+{(m-1)/m}log/sub 2/(m-1)] where m=log/sub 2/ q are not proper.

Lifetime maximization with predictive power management in selective relay networks

A diversity scheme which exploits the existence of a source-destination path to improve the lifetime and outage probability of a wireless selective relay network with Amplify-and-Forward (AF) relays is proposed and studied. The destination obtains a portion of the required signal to noise ratio (SNR) during the broadcast phase (Phase I) from the source and the remainder of the required SNR from a selected relay in the second phase (Phase II) of the transmission protocol. An algorithm based on an energy conserving dynamic transmit power threshold is proposed to improve the network lifetime. It is shown that the proposed scheme improves network lifetimes for the following four different relay selection strategies: Minimum Transmit Power (MTP), Maximum Residual Energy (MRE), Maximum residual-Energy Index (MEI), and Minimum Outage Probability (MOP).