Hela Hakim

Spectrum Sharing Techniques for Broadcast Cognitive Radio Networks

In this paper, we consider a secondary broadcast network where a multi-antenna transmitter broadcasts the same data toward a large number of secondary receivers (SRs) in the presence of a primary communication. Thanks to its multi-antenna capabilities, the secondary transmitter (ST) uses an orthogonal beamforming technique to broadcast its data while controlling the interference perceived by the primary receiver. We develop and investigate three broadcast transmission schemes. The first one is simple and operates in underlay mode where the ST broadcasts its data simultaneously as the primary transmission. The second scheme operates in overlay mode where ST helps the primary transmission by means of cooperative diversity transmission. SRs exploit a post-transmission interference cancellation techniques to cancel the interference caused by the primary transmission. The third scheme operates also in overlay mode and the secondary network exploits also the cooperative diversity technique. The metric used to evaluate the performance of secondary broadcast network is the rate of served SRs. We compare the performances of the three schemes by simulations. Also, analytical expressions of the outage probability for the first and second schemes are provided. Simulations along with analytical results proved that our two overlay proposed schemes ensure low secondary outage probability.

Performance Comparison between Adaptive and Fixed Transmit Power in Underlay Cognitive Radio Networks

In this paper, we compare the performance in terms of symbol error probability, data rate and power consumption of the use of fixed transmit power (FTP) and adaptive transmit power (ATP) in underlay cognitive radio networks. The use of FTP alleviates the signaling requirements of underlay cognitive radio networks compared to the ATP. Nevertheless, the use of FTP influences the performances of the underlay cognitive radio networks. To study this influence, we consider three relay selection schemes using FTP: opportunistic decode and forward with FTP (O-DF with FTP), opportunistic amplify and forward with FTP (O-AF with FTP) and partial relay selection with FTP (PR with FTP). We compare the performances of these schemes in terms of symbol error probability, data rate and power consumption with three relay selection schemes using ATP: opportunistic decode and forward with ATP (O-DF with ATP), opportunistic amplify and forward with ATP (O-AF with ATP) and partial relay selection with ATP (PR with ATP). We provide exact and/or lower bound expressions of the symbol error probabilities of O-DF, O-AF and PR with FTP. The analytical study for the data rate and the power consumption is also provided. Our comparison study shows that FTP has a positive impact on the data rate and power consumption performance while it deteriorates the symbol error probability performance.

Energy efficient adaptive transmission strategy using cooperative diversity for wireless sensor networks

Energy saving strategies pose challenges for wireless Sensor Networks communication, as the nodes have limited energy resources and are battery powered. Thus, there is a strong interest in developing energy efficient transmission techniques. In this paper, we present a new approach to minimize the energy consumption of clustered wireless sensor networks, while achieving the required level of reliability. We consider clusters where sensors transmit monitored data to nearby cluster-head using cooperative communication. The goal of our approach is to minimize the transmit power energy based on Bit Error Probability (BEP) criterion for selective digital relaying (SDR) schemes whose closed form expression was recently derived in previous works. Therefore, the proposed scheme can efficiently allocate the least transmission power among the source and the relays that satisfy the predefined level of reliability while decreasing the energy consumption of the node. Simulation results show that our relay selection algorithm achieves significant energy savings (more than 50% compared with direct transmissions for moderate SNR values and up to 90% in high SNR regions).

Spectrum sharing for bi-directional communication in cognitive radio networks

In this paper, we propose a spectrum sharing protocol where a pair of secondary users and a pair of primary users bidirectionally communicate. A secondary relay equipped with two antennas is deployed to ensure two-way relaying for primary users and thus they give the chance for secondary users to access the spectrum. To transmit its data, the secondary transmitter does not take into account the interference caused to primary receiver. It adjusts its transmit power so that the secondary required data rate is attained. In return, the relay uses zero forcing precoding to make two way relaying for primary users with no interference. We analytically derive the average outage probability and bit error probability of secondary transmissions. Analytical results are verified through simulations. Our proposed spectrum sharing scheme is proved to significantly outperform the conventional underlay scheme while respecting the performances requirement of the primary users.

BEP and Throughput Analysis of Incremental Selective Relaying in DS-CDMA Systems

In this paper, we derive exact form expressions for the Bit Error Probability (BEP) and throughput of cooperative Direct Sequence-Code Division Multiple Access (DS-CDMA) systems using incremental selective relaying which combines selective relaying with incremental relaying protocols in the presence of multipath propagation. The derived results are valid for any multipath intensity profile of the channel and any path delays. They also consider the correlation of the multipath gains. Simulation results along with analytical studies of BEP and throughput prove that the combination of incremental relaying with selective relaying in cooperative DS-CDMA systems improve significantly the throughput performance and can achieve the maximum possible spatial diversity when it is required by destination.

Adaptive WSNs Based on HW/SW Implementation of Selective Relaying Communication Technique

Power consumption poses a challenge issue in Wireless Sensor Networks as the sensor node is power-constrained. Since Wireless communication is among the most energy expensive tasks, many power efficient transmission strategies are reported in the literature. However, most of them are purely software-based implemented. Since relaying algorithms are intensive-computing tasks, a purely software-based implementation is not the wise choice. This work comes to propose an alternative choice for implementing our earlier proposed relaying algorithm, which is a HW/SW implementation. To do so, we have identified the greedy CPU time and energy consuming tasks and implemented their dedicated hardware accelerators. We prototype our work using an FPGA board. Design details and interfacing of the different accelerators show important time execution and power consumption reduction of our algorithm.

Spectrum sharing for bidirectional communication exploiting zero-forcing and singular value decomposition beamforming

In this paper, we propose a dynamic spectrum sharing protocol where a pair of secondary users and a pair of primary users bidirectionally communicate. A secondary relay equipped with multiple antennas is deployed to assist the secondary transmissions and improve the secondary access to the spectrum. We employ a new time division access so that no interference may exist between primary and secondary users. The proposed scheme is then compared to the axiomatic and simple scheme where the secondary users communicate with each other with the assistance of the relay in underlay mode. We study and compare the performances of the two schemes in terms of outage probability. An upper bound for the secondary outage probability of the proposed scheme is derived. Our simulation results prove that the proposed scheme significantly outperforms the underlay spectrum sharing while in both schemes, the primary outage probability is kept identical to the case where secondary users are absent.

Scheduling schemes for N-node cooperative networks

In this paper, we consider an amplify-and-forward cooperative network composed of N nodes. Here we focus on the scheduling problem which consists in assigning the roles {source, relay} to the N nodes. Our target is to investigate and develop relay selection policies that can be used in amplify-and-forward cooperative systems. For fairness and simplicity issues, we assume that a node is chosen as a source in a Round Robin fashion. On the other hand, two relay selection schemes are proposed. In the first scheme, the relay selection problem is addressed under the knowledge of nodes positions and the relay is chosen to be the nearest neighbor of the source. This relay selection method combined with the RR source selection method will be called Network Topology based scheduling (NT-based scheduling). In the second scheme, the relay selection algorithm is based on the instantaneous channels qualities and the relay is the node with the best direct link in term of instantaneous signal-to-noise ratio (SNR). Since this second method requires much signaling which degrades the scheme performance, a channel feedback reduction method is proposed with the use of a threshold T. The relay selection method which use the proposed feedback reduction technique combined with the RR source selection method will be called semi Channels Qualities based scheduling (semi CQ-based scheduling). Simulation results will be provided in order to compare the performance of these scheduling schemes and to show the high performance provided by our proposed algorithms.

Exact BEP of cooperative MC-CDMA systems using selective threshold digital relaying

In this paper, we derive the end-to-end (e2e) Bit Error Probability (BEP) of cooperative Multi Carrier Code Division Multiple Access (MC-CDMA) systems using selective threshold digital relaying (STDR). In STDR, a set of potential relays whose received Signal-to-Noise Ratio (SNR) exceeds a threshold value γt, called reliable relays, is formed. Then, only the best relay among reliable relays is allowed to retransmit the received signal. We activate the relay with the largest SNR in relay-destination link. The derived BEP results are valid for any multipath intensity profile of the channel.