K. Ben Letaief

Cooperative Communications for Cognitive Radio Networks

Cognitive radio is an exciting emerging technology that has the potential of dealing with the stringent requirement and scarcity of the radio spectrum. Such revolutionary and transforming technology represents a paradigm shift in the design of wireless systems, as it will allow the agile and efficient utilization of the radio spectrum by offering distributed terminals or radio cells the ability of radio sensing, self-adaptation, and dynamic spectrum sharing. Cooperative communications and networking is another new communication technology paradigm that allows distributed terminals in a wireless network to collaborate through some distributed transmission or signal processing so as to realize a new form of space diversity to combat the detrimental effects of fading channels. In this paper, we consider the application of these technologies to spectrum sensing and spectrum sharing. One of the most important challenges for cognitive radio systems is to identify the presence of primary (licensed) users over a wide range of spectrum at a particular time and specific geographic location. We consider the use of cooperative spectrum sensing in cognitive radio systems to enhance the reliability of detecting primary users. We shall describe spectrum sensing for cognitive radios and propose robust cooperative spectrum sensing techniques for a practical framework employing cognitive radios. We also investigate cooperative communications for spectrum sharing in a cognitive wireless relay network. To exploit the maximum spectrum opportunities, we present a cognitive space-time-frequency coding technique that can opportunistically adjust its coding structure by adapting itself to the dynamic spectrum environment.

Cooperative Spectrum Sensing Optimization in Cognitive Radio Networks

Cognitive radio is being recognized as an intelligent technology due to its ability to rapidly and autonomously adapt operating parameters to changing environments and conditions. In order to reliably and swiftly detect spectrum holes in cognitive radios, spectrum sensing must be used. In this paper, we consider cooperative spectrum sensing in order to optimize the sensing performance. We focus on energy detection for spectrum sensing and find that the optimal fusion rule is the half-voting rule. Next, the optimal detection threshold of energy detection is determined numerically. Finally, we propose a fast spectrum sensing algorithm for a large network which requires fewer than the total number of cognitive radios to perform cooperative spectrum sensing while satisfying a given error bound.

Power Control in Cognitive Radio Systems Based on Spectrum Sensing Side Information

Cognitive radio has been recently proposed as a promising technology to improve the spectrum utilization efficiency by intelligently sensing and accessing some vacant bands of licensed users. In this paper, we consider the coexistence between a cognitive radio and a licensed user in order to enhance the spectrum efficiency. We develop an approach to allow the cognitive radio to operate in the presence of the licensed user. In order to minimize the interference to the licensed user, the transmit power of the cognitive radio is controlled by using the side information of spectrum sensing. Numerical results will show that the quality of service for the licensed user can be guaranteed in the presence of the cognitive radio by the proposed approach.

Space-Time/Frequency Coding for MIMO-OFDM in Next Generation Broadband Wireless Systems

With the advent of next generation (4G) broadband wireless communications, the combination of multiple-input multiple-output (MIMO) wireless technology with orthogonal frequency division multiplexing (OFDM) has been recognized as one of the most promising techniques to support high data rate and high performance. In particular, coding over the space, time, and frequency domains provided by MIMO-OFDM will enable a much more reliable and robust transmission over the harsh wireless environment. In this article we provide an overview of space-time (ST) coding, space-frequency (SF) coding, and space-time-frequency (STF) coding for MIMO-OFDM systems. Performance results show that STF coding can achieve the maximum diversity gain in an end- to-end MIMO-OFDM system over broadband wireless channels. Furthermore, for orthogonal frequency division multiple access (OFDMA), we propose a multiuser SF coding scheme that can achieve the maximum diversity for each user while minimizing the interference introduced from all the other users.

Reliable relay assisted wireless multicast using network coding

We first consider a topology consisting of one source, two destinations and one relay. For such a topology, it is shown that a network coding based cooperative (NCBC) multicast scheme can achieve a diversity order of two. In this paper, we discuss and analyze NCBC in a systematic way as well as compare its performance with two other multicast protocols. The throughput, delay and queue length for each protocol are evaluated. In addition, we present an optimal scheme to maximize throughput subject to delay and queue length constraints. Numerical results will demonstrate that network coding can bring significant gains in terms of throughput.

Toward an Effective Risk-Conscious and Collaborative Vehicular Collision Avoidance System

In this paper, we introduce a cooperative collision-avoidance (CCA) scheme for intelligent transport systems. Unlike contemporary strategies, the envisioned scheme avoids flooding the considered vehicular network with high volumes of emergency messages upon accidental events. We present a cluster-based organization of the target vehicles. The cluster is based upon several criteria, which define the movement of the vehicles, namely, the directional bearing and relative velocity of each vehicle, as well as the inter-vehicular distance. We also design a risk-aware medium-access control (MAC) protocol to increase the responsiveness of the proposed CCA scheme. According to the order of each vehicle in its corresponding cluster, an emergency level is associated with the vehicle that signifies the risk of encountering a potential emergency scenario. To swiftly circulate the emergency notifications to collocated vehicles to mitigate the risk of chain collisions, the medium-access delay of each vehicle is set as a function of its emergency level. Due to its twofold contributions, i.e., the cluster-based and risk-conscious approaches, our adopted strategy is referred to as the cluster-based risk-aware CCA (C-RACCA) scheme. The performance of the C-RACCA system is verified through mathematical analyses and computer simulations, whose results clearly verify its effectiveness in mitigating collision risks of the vehicles arising from accidental hazards.

Joint Beamforming and Scheduling in Cognitive Radio Networks

Cognitive radio has been recently proposed as a promising technology to improve the spectrum utilization. In this paper, we consider the coexistence between a large number of cognitive radio users and a licensed user in order to enhance the spectrum efficiency. With the deployment of M antennas at the cognitive base station, an efficient transmit beamforming technique combined with user selection is proposed to maximize the downlink throughput and satisfy the signal-to-interference- and-noise ratio (SINR) constraint as well as limit interference to the primary user. In the proposed user selection algorithm, cognitive users who are nearly orthogonal to the primary user are first pre-selected so as to minimize the mutual interference. Then, M best cognitive users who are nearly mutual orthogonal to each other are scheduled from those pre-selected cognitive users. Simulation results show that our proposed method is able to achieve high sum-rate throughput, with affordable complexity. Moreover, our proposed technique with equal power allocation suffers a negligible performance loss compared to the one with the optimal power allocation.

Co-channel interference cancellation for space-time coded OFDM systems

Space-time coded orthogonal frequency division multiplexing (OFDM) is a promising scheme for future wideband multimedia wireless communication systems. The combination of space-time coding (STC) and OFDM modulation promises an enhanced performance in terms of power and spectral efficiency. Such combination benefits from the diversity gain within the multiple-input-multiple-output ST coded system and the matured OFDM modulation for wideband wireless transmission. However, STC transmit diversity impairs the systems interference suppression ability due to the use of multiple transmitters at each mobile. We propose an effective co-channel interference (CCI) cancellation method that employs angle diversity based on null-steering beamforming or minimum variance distortion response beamforming. It is shown that the proposed method can effectively mitigate CCI while preserving the space-time structure, thereby, significantly improving the systems interference suppression ability without significant bit-error rate performance degradation. Furthermore, it is demonstrated that the proposed method can significantly combat the delay spread detrimental effects over multipath fading channels without the use of interleaving.

User Cooperation in Heterogeneous Cognitive Radio Networks with Interference Reduction

In cognitive radio systems, secondary users can share the spectrum with the primary user as long as the quality of service (QoS) of the primary system is guaranteed. However, the system throughput of the cognitive system will be limited when the QoS requirement is stringent. Recently cooperative diversity has been proposed as a powerful method that can provide dramatic gains in wireless environments. In this paper, we investigate the problem of spectrum sharing together with adaptive user cooperation in heterogeneous cognitive relay system. To maximize the throughput of the cognitive system, one best relay will be selected and besides, optimal power allocation is performed between the source and the relay. In addition, beamforming is applied to further reduce the interference and improve the system performance. Simulation results show the improvement of the throughput as opposed to the direct transmission.

A Unified Cross-Layer Framework for Resource Allocation in Cooperative Networks

Node cooperation is an emerging and powerful solution that can overcome the limitation of wireless systems as well as improve the capacity of the next generation wireless networks. By forming a virtual antenna array, node cooperation can achieve high antenna and diversity gains by using several partners to relay the transmitted signals. There has been a lot of work on improving the link performance in cooperative networks by using advanced signal processing or power allocation methods among a single source node and its relays. However, the resource allocation among multiple nodes has not received much attention yet. In this paper, we present a unified cross- layer framework for resource allocation in cooperative networks, which considers the physical and network layers jointly and can be applied for any cooperative transmission scheme. It is found that the fairness and energy constraint cannot be satisfied simultaneously if each node uses a fixed set of relays. To solve this problem, a multi-state cooperation methodology is proposed, where the energy is allocated among the nodes state-by-state via a geometric and network decomposition approach. Given the energy allocation, the duration of each state is then optimized so as to maximize the nodes utility. Numerical results will compare the performance of cooperative networks with and without resource allocation for cooperative beamforming and selection relaying. It is shown that without resource allocation, cooperation will result in a poor lifetime of the heavily-used nodes. In contrast, the proposed framework will not only guarantee fairness, but will also provide significant throughput and diversity gain over conventional cooperation schemes.