Evelio Martin Garcia Fernandez

Short Channel Hopping Sequence Approach to Rendezvous for Cognitive Networks

The rendezvous process allows cognitive users to find an available channel and establish a communication link. The implementation of rendezvous strategies to minimize the time to rendezvous (TTR) is a major challenge in this context. In this paper we present a comprehensive analysis of the benefits of periodic channel hopping sequences in the rendezvous process. We also introduce a novel short-sequence-based (SSB) rendezvous algorithm which works under symmetric and asymmetric channel models. Theoretical analysis and simulations show that the proposed SSB outperforms other recently developed rendezvous protocols in terms of expected TTR and maximum TTR.

Rate and Energy Efficient Power Control in a Cognitive Radio Ad Hoc Network

Location awareness allows to define a concurrent transmission region where a primary and a secondary networks can coexist. We investigate the impact of joint rate and power control on the performance of a cognitive radio ad hoc network overlaying a primary system. The proposed strategies adequately adjust the secondary user transmit power to increase the concurrent transmission probability while respecting a given constraint on the achievable primary rate. Two power control strategies are described: Rate-Efficient Power Control to maximize the secondary capacity, and Energy-Efficient Power Control to minimize the secondary energy consumption. Numerical results show that the proposed schemes expand the concurrent transmission region and that it is possible to increase the average rate and save energy.

On the impact of active queue management on VoIP quality of service

In this paper, we assess the impact of active queue management (AQM) on the quality of service of voice over IP (VoIP). Some of the most representative AQM schemes are analyzed through extensive simulations; evaluating its effect on the perceived quality of voice calls. We investigate different network scenarios, changing network loads and scheme control parameters. Network impairments are related to user perception by means of well known algorithmic models, expressing user satisfaction in the MOS scale. Our main results shows that the use of active queue management schemes significantly improve the perceived quality of voice calls, offering adequate user satisfaction.

Cooperative overlay secondary transmissions exploiting primary retransmissions

We introduce an overlay cooperative cognitive radio scheme, in which the secondary network transmits only during the retransmissions of the primary network. The secondary network makes use of multiple relays in order to increase its performance compared to a non-cooperative scenario. Moreover, the secondary operates without harming the performance of the primary network. Different cooperative protocols are employed and associated with hybrid automatic repeat request mechanisms. Our results show that the incremental decode-and-forward technique allows the secondary network to achieve the highest throughput among the considered methods, at the cost of a very small degradation in the performance of the primary network.

Ultrareliable Short-Packet Communications With Wireless Energy Transfer

We analyze and optimize a wireless system with energy transfer in the downlink and information transfer in the uplink, under quasi-static Nakagami-m fading. We consider ultrareliable communication scenarios representative of the fifth generation of wireless systems, with strict error and latency requirements. The error probability and delay are investigated, and an approximation for the former is given and validated through simulations. The numerical results demonstrate that there are optimum numbers of channels uses for both energy and information transfer for a given message length.

Impact of Rate Control on the Performance of a Cognitive Radio Ad-Hoc Network

Location awareness allows to define a concurrent transmission region where a primary and a secondary networks can coexist. In this paper we investigate the impact of channel rate control on the performance of a cognitive radio ad hoc network overlaying an infrastructure-based system. The proposed algorithm adequately adjusts the secondary user rate to be as large as possible while respecting a given constraint on the achievable primary rate. As a consequence, the rate control scheme expands the concurrent transmission region and increases the average rate of the secondary users, without affecting the primary. Numerical results show that it is possible to increase the throughput of the cognitive ad-hoc network and therefore increase the efficiency of the spectrum utilization.

Voice over IP quality of service using active queue management

This paper presents a comprehensive study about the impact of active queue management (AQM) on voice over Internet protocol (VoIP) quality of service. One of the most representative AQM scheme is analyzed through extensive simulation and its effect on the perceived quality of voice calls is evaluated. Different network scenarios, changing network loads and different scheme control parameters are investigated. Network impairments are related to user perception by means of well known algorithmic models, expressing user satisfaction in the MOS scale. The main results obtained show that the use of active queue management schemes like adaptive random early detection (ARED) significantly improve the perceived quality of voice calls.

Energy Efficient Power Allocation Schemes for a Two-User Network-Coded Cooperative Cognitive Radio Network

This paper investigates the outage behavior and power allocation problem in a two-user network-coded cooperative cognitive radio network over Rayleigh fading channels. First, we derive an exact expression for the outage probability of the secondary transmissions, and then we propose an approximate expression whose validity limits are later evaluated. To improve the power efficiency of the secondary network, two power allocation schemes are proposed. In the first scheme, the transmit power is adjusted as a function of the average channel gains and the power control is formulated as a convex optimization problem where the objective is to minimize the total transmit power subject to outage probability constraints. In the second scheme, the transmit power is adjusted as a function of the instantaneous channel gains to guarantee a target signal-to-interference-plus-noise ratio at the receiver. We evaluate the performance of both schemes under cooperative and non-cooperative scenarios considering different network geometries. Our results indicate that the benefits of network coding cooperation are substantial for both power allocation schemes.

Combining diversity and multiplexing in wireless communications systems

This paper presents a simple multiple-input multiple output (MIMO) hybrid system for wireless channels trying to achieve near optimal values of diversity-multiplexing tradeoff. The hybrid systems studied combine well known space time codes and full multiplexing schemes. A decoding procedure for the hybrid systems using linear combinatorial nulling and symbol cancellation is also presented. Through computer simulation upon different digital modulation constellations, the error performance and spectral efficiency of the hybrid systems are compared to the diversity-only and multiplexing-only schemes.

Ultra reliable short message relaying with wireless power transfer

We consider a dual-hop wireless network where an energy constrained relay node first harvests energy through the received radio-frequency signal from the source, and then uses the harvested energy to forward the sources information to the destination node. The throughput and delay metrics are investigated for a decode-and-forward relaying mechanism at finite blocklength regime and delay-limited transmission mode. We consider ultra-reliable communication scenarios under discussion for the next fifth-generation of wireless systems, with error and latency constraints. The impact on these metrics of the blocklength, information bits, and relay position is investigated.