Sonia Aïssa

Capacity and power allocation for spectrum-sharing communications in fading channels

This paper investigates the fundamental capacity limits of opportunistic spectrum-sharing channels in fading environments. The concept of opportunistic spectrum access is motivated by the frontier technology of cognitive radio which offers a tremendous potential to improve the utilization of the radio spectrum by implementing efficient sharing of the licensed spectrum. In this spectrum-sharing technology, a secondary user may utilize the primary users licensed band as long as its interference to the primary receiver remains below a tolerable level. Herein, we consider that the secondary users transmission has to adhere to limitations on the ensuing received power at the primarys receiver, and investigate the capacity gains offered by this spectrum-sharing approach in a Rayleigh fading environment. Specifically, we derive the fading channel capacity of a secondary user subject to both average and peak received-power constraints at the primarys receiver. In particular, considering flat Rayleigh fading, we derive the capacity and optimum power allocation scheme for three different capacity notions, namely, ergodic, outage, and minimum-rate, and provide closed-form expressions for these capacity metrics. Numerical simulations are conducted to corroborate our theoretical results.

Fundamental capacity limits of cognitive radio in fading environments with imperfect channel information

In this paper, we analyze the capacity gains of opportunistic spectrum-sharing channels in fading environments with imperfect channel information. In particular, we consider that a secondary user may access the spectrum allocated to a primary user as long as the interference power, inflicted at the primars receiver as an effect of the transmission of the secondary user, remains below predefined power limits, average or peak, and investigate the capacity gains offered by this spectrum-sharing approach when only partial channel information of the link between the secondaryiquests transmitter and primarys receiver is available to the secondary user. Considering average received-power constraint, we derive the ergodic and outage capacities along with their optimum power allocation policies for Rayleigh flat-fading channels, and provide closedform expressions for these capacity metrics. We further assume that the interference power inflicted on the primaryiquests receiver should remain below a peak threshold. Introducing the concept of interference-outage, we derive lower bounds on the ergodic and outage capacities of the channel. In addition, we obtain closedform expressions for the expenditure-power required at the secondary transmitter to achieve the above-mentioned capacity metrics. Numerical simulations are conducted to corroborate our theoretical results.

Closed-form expressions for the outage and ergodic Shannon capacity of MIMO MRC systems

Transmit-beamforming (TB) over multiple-input multiple-output (MIMO) fading channels steers the transmit power in the receivers direction, so as to maximize the output signal-to-noise ratio (SNR) after maximal ratio combining (MRC) at the receiver. This letter proposes a simple algorithm that allows evaluating an exact and tractable expression for the probability density function of the SNR at the output of the TB receiver, subject to Rayleigh fading. The latter enables the derivation of closed-form expressions for the outage and ergodic capacity of MIMO MRC systems under Rayleigh fading, thereby avoiding the need for time-consuming numerical integrations or Monte Carlo simulations.

Cooperative dual-hop relaying systems with beamforming over nakagami-m fading channels

In this paper, we investigate the end-to-end performance of dual-hop relaying systems with beamforming over Nakagami-m fading channels. Our analysis considers semi-blind (fixed-gain) relays with single antennas, and source and destination nodes equipped with multiple antennas. Closed-form expressions for the outage probability (OP), moment generating function (MGF), and generalized moments of the end-to-end signal-to-noise ratio (SNR) are derived. The proposed expressions apply to general operating scenarios with distinct Nakagami-m fading parameters and average SNRs between the hops. The influence of the power imbalance, fading parameters, and antenna configurations on the overall system performance are analyzed and discussed through representative numerical examples. Furthermore, the exactness of our formulations is validated by means of Monte Carlo simulations.

Effective capacity for interference and delay constrained cognitive radio relay channels

This paper investigates delay constrained performance of a cognitive radio relay network when the cognitive (secondary) user transmission is subject to satisfying spectrum-sharing restrictions imposed by a primary user. The primary user allows a secondary user to gain access to its allocated spectrum band as long as certain thresholds on the interference power, on the peak or average values, inflicted on the primary receiver are not exceeded by the transmission of the secondary users. In addition, we assume that the secondary transmitter benefits from an intermediate node, chosen from K terminals, to relay its signal to the destination. Considering that the transmission of the secondary user is subject to satisfying a statistical delay quality-of-service (QoS) constraint, we study the maximum arrival rate of the secondary users relay link while the interference limitations required by the primary user are satisfied. Particularly, we obtain the effective capacity of the secondary network and determine the power allocation policies that maximize the effective capacity of the secondary users relaying channel. In addition, we derive closed-form expressions for the effective capacity of the channel in Rayleigh block-fading environment under peak or average interference-power constraints. Numerical simulations are provided to endorse our theoretical results.

Optimal Relay Selection and Power Allocation for Cognitive Two-Way Relaying Networks

In this paper, we present an optimal scheme for power allocation and relay selection in a cognitive radio network where a pair of cognitive (or secondary) transceiver nodes communicate with each other assisted by a set of cognitive two-way relays. The secondary nodes share the spectrum with a licensed primary user (PU), and each node is assumed to be equipped with a single transmit/receive antenna. The interference to the PU resulting from the transmission from the cognitive nodes is kept below a specified limit. We propose joint relay selection and optimal power allocation among the secondary user (SU) nodes achieving maximum throughput under transmit power and PU interference constraints. A closed-form solution for optimal allocation of transmit power among the SU transceivers and the SU relay is presented. Furthermore, numerical simulations and comparisons are presented to illustrate the performance of the proposed scheme.

End-to-end performance of dual-hop semi-blind relaying systems with partial relay selection

The end-to-end performance of dual-hop cooperative links using semi-blind (fixed gain) relays and partial relay selection is investigated. The selection scheme considers that the source monitors the connectivity among the nodes (relays) of the first hop only. This scheme is interesting in practical ad-hoc and sensor networks. In our analysis, compact closed-form expressions are obtained for the outage probability, probability density function, moment generating functions, and generalized moments of the end-to-end signal-to-noise ratio(SNR), from which other relevant statistics that well-describe the distribution of the end-to-end SNR, such as mean, variance, kurtosis, skewness, and amount of fading, can also be deduced. Furthermore, the dynamic behavior of the end-to-end envelope is investigated, and the corresponding level crossing rate and average fade duration are obtained in an exact manner. Also, tight lower and upper bounds for these second-order statistics are presented in closed-form. Numerical results illustrating the systems performance in terms of the above metrics are provided, and the influence of the relay selection on performance is analyzed and discussed. For instance, it is shown that the power imbalance between the hops may have positive or negative effects on the overall system performance irrespective of the number of selected relays.

Ergodic and Outage Capacities of Spectrum-Sharing Systems in Fading Channels

In this fast growing technology world, where communications play a major rule for connecting people and machines together, the growth in wireless applications have caused an increasing demand for gaining access to the radio spectrum. However, the outdated spectrum utilization policies, imposed by the regulatory bodies in the past century, have caused the spectrum to look over-saturated. Recently, the concept of opportunistic spectrum access has been introduced as a tool to overcome the scarcity of the spectrum. The latter technology offers a tremendous potential to improve the utilization of the radio spectrum by implementing an efficient sharing of the licensed spectrum, whereby a secondary user may utilize the primary users licensed band as long as its interference to the primary receiver remains below a tolerable level. In this paper, we investigate the capacity gains offered by this spectrum-sharing approach in Rayleigh fading environments. In particular, we derive the fading channel capacity of a secondary user subject to both average and peak received-power constraints at the primarys receiver. Considering both constraints, we derive the ergodic and outage capacities along with their optimum power allocation policies for Rayleigh flat-fading channel, and provide closed-form expressions for these capacity metrics. Furthermore, numerical simulations are conducted to corroborate our theoretical results.

Adaptive Rate and Power Transmission in Spectrum-Sharing Systems

In this paper, we investigate the capacity gains offered by cognitive radio in a spectrum-sharing system where the transmit power and rate of the secondary user are adjusted based on the channel variations of the secondary link and spectrum-sensing information pertaining to the activity of the licensed user. We assume a primary/secondary spectrum-sharing system where the secondary users may have access to the spectrum band originally assigned to the primary (licensed) user, as long as the interference power inflicted on the primary receiver is considered unharmful. In this context, considering joint average interference-power and peak transmit-power constraints, we first obtain the optimal power allocation scheme, namely variable power, for maximizing the achievable capacity of the secondary user over fading channels. Thereafter, we look into the variable rate and power adaptation policy by maximizing the achievable capacity under said power constraints and bit error rate requirements in multilevel quadrature amplitude modulation (M-QAM). Finally, the benefits of using soft-sensing information about the primary users activity on the power and rate adaptation strategies are assessed, and numerical results and comparisons illustrating the performance of our spectrum-sharing system in different operating scenarios are provided.

Effective capacity of delay-constrained cognitive radio in Nakagami fading channels

In this paper, we consider coexistence of secondary and primary users who share particular portions of the spectrum and propose a delay-constrained power and rate allocation scheme for the secondary user link. Secondary users are allowed to access the spectrum occupied by a primary user subject to satisfying interference-power limitations imposed by the primary user. Applying this limitation, we obtain the maximum arrival-rate supported by the secondary channel in Nakagami-m block-fading environment subject to satisfying a given statistical delay quality-of-service (QoS) constraint. In this respect, we derive the optimal rate and power adaptation policy that maximizes the effective capacity of the channel, and provide closed-form expressions for the power allocation and the effective capacity. In addition, we obtain closed-form expressions for the expenditure-power that is required at the secondary transmitter to achieve the above-mentioned capacity metric. Moreover, for comparison purposes, we consider two widely deployed power allocation strategies, namely, optimal power and rate allocation (opra) and channel inversion with fixed rate (cifr), and investigate the effective capacity of the channel under these power transmission techniques. Numerical simulations are conducted to corroborate our theoretical results.