Samuel Baraldi Mafra

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.

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.

Outage performance of a network coding aided multi-user cooperative secondary network

We evaluate the performance of a network-coded cooperative secondary network in a cognitive radio system under spectrum sharing constraints. The secondary network is composed of multiple sources that cooperate to transmit their own information to a common secondary destination. The outage probability is analysed under a given maximum interference constraint set by the primary network as well as to the maximum transmit power limit of the secondary sources. Moreover, we also obtain a closed-form equation for the ϵ-outage capacity, that is, the maximum information rate achieved by the secondary sources given a target outage probability. Also, we resort to the Dinkelbach algorithm in order to find the optimum number of parity packets that maximises the ϵ-outage capacity. The influence of the number of source nodes and another system parameters in the secondarys performance is also evaluated, and we show through theoretical and numerical results that cooperative communication with network coding can provide significant gains in terms of outage probability and diversity order when compared with non-cooperative or traditional cooperative. Copyright

Power-rate control with directional transmission and reception in a cognitive radio network

We study the benefits that antenna directivity and power control can bring to a cognitive radio ad hoc network, in terms of concurrent transmission probability, throughput and energy efficiency. The proposed strategies adequately adjust antenna beam pattern and the transmit power of the secondary users to increase the spectrum reuse while respecting a given constraint on the achievable primary rate. Two power control strategies are described: Rate-Efficient Power Control with Directional Transmission and Reception to maximize the secondary channel rate, and Energy-Efficient Power Control with Directional Transmission and Reception to maximize the energy efficiency of the secondary link. Numerical results show that the proposed schemes increase the concurrent transmission probability and that it is possible to increase the average rate and save energy with a lower cost than that of other proposals.

Cooperative cognitive radio protocol exploiting primary retransmissions in Nakagami-m fading

We consider a cooperative overlay cognitive network, where the secondary exploits the primary retransmissions. We show that by using cooperation, the secondary rate can be considerably increased when compared to other schemes, without causing major impact on the primary performance. The best configuration for the proposed scheme is that in which the secondary nodes are close to the primary transmitter.

Power Control and Relay Selection in Cognitive Radio Ad Hoc Networks Using Game Theory

This paper employs game theory to optimize the performance of multiple secondary users (SUs) in a cognitive radio ad hoc network where the primary and SUs have quality of service (QoS) constraints. The proposed algorithm utilizes a distributed noncooperative game theory mechanism among SUs that compete to maximize their individual capacities. We propose two utility functions, UcLog and UcLin, with logarithmic and linear cost functions of the perceived interference in the primary receiver, respectively. The value of the utility function varies according to the transmit power and the relay selection strategy. Simulation results show that both utility functions obtain similar results in most scenarios. However, UcLin causes less interference to the primary receiver and has better convergence than UcLog. The two consecutive games mechanism used in the proposed algorithm allows us to achieve rates above 95% in all simulated scenarios without QoS constraints in secondary links.

Energy-efficient outage-constrained power allocation based on statistical channel knowledge for dual-hop cognitive relay networks

Summary This paper investigates the power allocation problem in decode-and-forward cognitive dual-hop systems over Rayleigh fading channels. In order to optimize the performance of the secondary network in terms of power consumption, an outage-constrained power allocation scheme is proposed. The secondary nodes adjust their transmit power subject to an average interference constraint at the primary receiver and an outage probability constraint for the secondary receivers while having only statistical channel knowledge with respect to the primary nodes. We compare this approach with a power allocation scheme based on instantaneous channel state information under a peak interference constraint. Analytical and numerical results show that the proposed approach, without requiring the constant interchange of channel state information, can achieve a similar performance in terms of outage probability as that of power allocation based on instantaneous channel knowledge. Moreover, the transmit power allocated by the proposed approach is considerably smaller than the power allocated by the method based on instantaneous channel knowledge in more than 50% of the time. Copyright

Network-coded secondary communication with opportunistic energy harvesting

We consider a cognitive wireless network with two energy-constrained secondary users (SUs) and a common secondary destination, where the SUs harvest energy from the transmitted signals of primary user (PU). We obtain closed-form approximations for the outage probability, the outage capacity and the optimum energy harvesting time of the proposed scheme. The analytical derivations are validated by numerical results.

Using multiple co-channel femtocells as relays to increase the performance of the outdoor user

The deployment of femtocells comes as a solution to increase indoor coverage although it may also increase the interference level in the network. In this paper we evaluate the use of multiple co-channel femtocells as relays to the outdoor user in a cognitive radio context, where the outdoor user can be seen as a primary user and the indoor users are seen as secondary users. We show that multiple co-channel femtocells, besides allowing several secondary users to access the spectrum, may considerably increase the performance of the primary user if the femtocells act as relays to the primary signal. Moreover, results show that with the increase of the number of cooperative co-channel femtocells the capacity constraints on the backhaul can be relaxed.