Chuyen T. Nguyen

Spectral Efficiency of Full-Duplex Multi-user System: Beamforming Design, User Grouping, and Time Allocation

Full-duplex (FD) systems have emerged as an essential enabling technology to further increase the data rate of wireless communication systems. The key idea of FD is to serve multiple users over the same bandwidth with a base station (BS) that can simultaneously transmit and receive the signals. The most challenging issue in designing an FD system is to address both the harmful effects of residual self-interference caused by the transmit-to-receive antennas at the BS as well as the co-channel interference from an uplink user (ULU) to a downlink user (DLU). An efficient solution to these problems is to assign the ULUs/DLUs in different groups/slots, with each user served in multiple groups. Hence, this paper studies the joint design of transmit beamformers, ULUs/DLUs group assignment, and time allocation for each group. The specific aim is to maximize the sum rate under the ULU/DLU minimum throughput constraints. The utility function of interest is a difficult nonconcave problem, and the involved constraints are also nonconvex, and so this is a computationally troublesome problem. To solve this optimization problem, we propose a new path-following algorithm for computational solutions to arrive at least the local optima. Each iteration involves only a simple convex quadratic program. We prove that the proposed algorithm iteratively improves the objective while guaranteeing convergence. Simulation results confirm the fast convergence of the proposed algorithm with substantial performance improvements over existing approaches.

Energy Harvesting for Throughput Enhancement of Cooperative Wireless Sensor Networks

We consider an energy harvesting for cooperative wireless sensor networks with a nonlinear power consumption model. The information transmission from the source node to the destination node is assumed to occur via several clusters of decode-and-forward relay nodes. We assume that all the source and relay nodes have the ability to harvest energy from the environment and use that harvested energy to transmit and forward the information to the next hop. Under such assumption, the objective of our design is to improve the total throughput of the end-to-end link over a number of transmission blocks subject to constraints on energy causality, battery overflow, and time duration for energy harvesting. The optimization problem is found to be a nonconvex maximin fractional program, which is difficult to solve in general. We present an efficient iterative algorithm to solve the optimization problem. Specifically, by introducing novel transformations, we apply an approximate convex technique to obtain a convex problem at each iteration. We then propose an iterative power allocation algorithm which converges to a locally optimal solution at a Karush-Kuhn-Tucker point. Numerical results are provided to evaluate the proposed scheme.

A novel approach to security enhancement of chaotic DSSS systems

In this paper, we propose a novel approach to the enhancement of physical layer security for chaotic direct-sequence spread-spectrum (DSSS) communication systems. The main idea behind our proposal is to vary the symbol period according to the behavior of the chaotic spreading sequence. As a result, the symbol period and the spreading sequence vary chaotically at the same time. This simultaneous variation aims at protecting DSSS-based communication systems from the blind estimation attacks in the detection of the symbol period. Discrete-time models for spreading and despreading schemes are presented and analyzed. Multiple access performance of the proposed technique in the presence of additional white Gaussian noise (AWGN) is determined by computer simulations. The increase in security at the physical layer is also evaluated by numerical results. Obtained results show that our proposed technique can protect the system against attacks based on the detection of the symbol period, even if the intruder has full information on the used chaotic sequence.

Tweaked binary tree algorithm to cope with capture effect and detection error in RFID systems

This paper proposes a new RFID binary tree-based identification protocol, namely Tweaked Binary Tree (TBT), to cope with hidden tag problem caused by capture effect and detection error phenomena. In TBT, the whole identification process is divided into multiple binary tree cycles, and the hidden tags in a cycle are checked and re-transmitted in the first slot of the next one. The average number of slots for a successful detection of a tag, and the tag loss rate, defined as a ratio between the number of missing tags and the whole tag cardinality, are theoretically analyzed. Computer simulations are also performed to validate the theoretical analysis. We also confirm the superiority of the proposed method in comparison with a conventional General Binary Tree (GBT) one.

Design and Performance Analysis of a Novel Distributed Queue Access Protocol for Cellular-Based Massive M2M Communications

This paper proposes a novel access protocol based on the distributed queue (DQ) mechanism to effectively tackle the massive access issue in the cellular-based machine-to-machine (M2M) communications. To fully take the advantage of the DQ mechanism, we newly propose a method to avoid the DQ’s inherent over-division problem by letting the base station first roughly probes the number of colliding devices in a random access opportunity. Based on the probing result, the base station then randomly divides these devices into a determined number of groups and “pushes” these groups to the end of a logical access queue. In addition, we develop an analytic model to accurately estimate the average access delay of the proposed protocol in the massive scenarios. Computer simulations are also performed to validate the correctness of the analytic model as well as the effectiveness of the proposed protocol in comparison with the LTE standard and conventional DQ access schemes.

Modified tree-based identification protocols for solving hidden-tag problem in RFID systems over fading channels

Hidden-tag problem is one of the most important issues in the implementation of radio-frequency identification (RFID) systems. Due to effects of imperfect wireless channels, RFID tags can be hidden during the identification process by either another tag or an unsuccessful detection. The former is known as the capture effect (CE) while the latter is the detection error (DE). This study newly proposes two modified tree-based identification protocols, namely tweaked binary tree (TBT) and tweaked query tree (TQT), which are able to tackle the hidden-tag problem caused by both the CE and DE. The performance of the proposed TBT and TQT protocols, in terms of the average number of slots required to detect a tag, and the tag-loss rate, is evaluated in comparison with that of previously proposed ones. Computer simulations and numerical results confirm the effectiveness of the proposed protocols.

Performance evaluation of missing-tag algorithms in CDMA-based RFID systems

This paper Investigates designing efficient communication schemes to monitor and to identify missing tags in CDMA-based RFID systems in a short period of time. In our considered systems, each CDMA tag is assigned with an unique Gold code, and randomly responds to the RFID reader thanks to the Frame Slotted ALOHA-based protocol. The de-correlating detector is implemented at the reader to eliminate impacts of Multiple Access Interference (MAI). We, then, propose and analyze two protocols, namely CDMA-based Two-Phase Protocol (CTPP) and CDMA-based Two-Phase Protocol with Tag Removal (CTPP/TR), in which optimal protocol parameters are determined to minimize the execution time. Computer simulations are performed to validate the theoretical analysis considering effects of fading phenomenon. In the numerical results, we evaluate the performance of the missing-tag protocols with different system parameters. From the results, adequate system settings are selected to fulfill requirements of the so-called false alarm rate and the false detection rate. We also confirm the superiority of the proposed protocols in comparison with conventional ones.

TCP over free-space optical links with ARQ and AMC: A cross-layer performance analysis

This paper presents a comprehensive cross-layer analysis framework on the performance of transmission control protocol (TCP) over free-space optical (FSO) links, which employ the automatic repeat request (ARQ) and adaptive modulation and coding (AMC) schemes. From the framework, we quantify the impacts of different parameters/settings of ARQ, AMC and FSO links on TCP throughput performance. We also discuss several optimization aspects for TCP performance.

Performance analysis of hybrid ALOHA/CDMA anti-collision scheme for RFID systems over fading channels

This paper proposes and theoretically investigates the efficiency of a hybrid ALOHA/CDMA anti-collision scheme for passive radio frequency identification (RFID) systems. The decorrelating detector is supposed to be used at RFID reader, while each tag is assumed to assign with a Gold code and responds to the reader in a randomly selected time slot. The tag identification is performed considering the influence of both code collisions and detection errors due to fading channels. The system efficiency, which is defined as the average number of successfully detected tags over a slot, is theoretically derived. Computer simulations are also performed to validate the theoretical analysis. In the numerical results, we evaluate the performance of the tag identification procedure with system parameters that maximize the system efficiency. We also confirm the superiority of the proposed system in comparison with conventional ALOHA-based ones.

A simple method for localization in cognitive radio networks

In this paper, we study and propose an efficient method to accurately estimate the position and the transmit power of a primary user for spatial resource utilization in cognitive radio networks (CRNs). Not similar to conventional methods where positions of at least four secondary users are assumed to be known and then, complex techniques such as least square and constrained optimization algorithms are used, we handle the estimation problem with only three known positions of secondary users. Moreover, we give closed forms for all estimated parameters and study the problem in practical scenarios by considering a received signal threshold at secondary users. Computer simulations show the effectiveness of the proposed method.