Primary User Receiver Research Articles

Ergodic capacity and outage probability optimization for secondary user in cognitive radio networks under interference outage constraint

This paper considers a cognitive radio network where a secondary user (SU) coexists with a primary user (PU). The interference outage constraint is applied to protect the primary transmission. The power allocation problem to jointly maximize the ergodic capacity and minimize the outage probability of the SU, subject to the average transmit power constraint and the interference outage constraint, is studied. Suppose that the perfect knowledge of the instantaneous channel state information (CSI) of the interference link between the SU transmitter and the PU receiver is available at the SU, the optimal power allocation strategy is then proposed. Additionally, to manage more practical situations, we further assume only the interference link channel distribution is known and derive the corresponding optimal power allocation strategy. Extensive simulation results are given to verify the effectiveness of the proposed strategies. It is shown that the proposed strategies achieve high ergodic capacity and low outage probability simultaneously, whereas optimizing the ergodic capacity (or outage probability) only leads to much higher outage probability (or lower ergodic capacity). It is also shown that the SU performance is not degraded due to partial knowledge of the interference link CSI if tight transmit power constraint is applied.

Cross-Layer Optimization and Receiver Localization for Cognitive Networks Using Interference Tweets

A cross-layer resource allocation scheme for underlay multi-hop cognitive radio networks is formulated, in the presence of uncertain propagation gains and locations of primary users (PUs). Secondary network design variables are optimized under long-term probability-of-interference constraints, by exploiting channel statistics and maps that pinpoint areas where PU receivers are likely to reside. These maps are tracked using a Bayesian approach, based on 1-bit messages - here refereed to as "interference tweet" - broadcasted by the PU system whenever a communication disruption occurs due to interference. Although nonconvex, the problem has zero duality gap, and it is optimally solved using a Lagrangian dual approach. Numerical experiments demonstrate the ability of the proposed scheme to localize PU receivers, as well as the performance gains enabled by this minimal primary-secondary interplay.

Sum Rate Maximization for Cognitive MISO Broadcast Channels: Beamforming Design and Large Systems Analysis

This paper considers the ergodic weighted sum rate maximization (WSRMax) problem for an underlay cognitive radio multiple input single output (MISO) broadcast channel. In this setting, a secondary network, consisting of a base-station with M transmit antennas and K single-antenna secondary users (SUs), is allowed to share the same spectrum with a primary user (PU), under an average total transmit power (ATTP) constraint and an average interference power (AIP) constraint at the PU receiver. We show that the ATTP constraint always remains active, and as the maximum ATTP P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">av</sub> → ∞, the ergodic WSR approaches infinity similar to conventional non-CR networks. We propose a novel low-complexity suboptimal beamforming scheme termed "Partially-Projected & Regularized Zero-Forcing Beamforming" (PP-RZFBF) with a close-form beamformer, by combining the regularized zero-forcing (RZF) with the channel projection idea, to achieve a tradeoff between maximizing secondary throughput and suppressing secondary multiuser interference as well as the interference on PU. In order to analyze and optimize the performance of this scheme, we employ the large system analysis technique, in the limit as M and K approach infinity with a fixed ratio r=K\M. This allows us to derive deterministic limiting approximations for the PP-RZFBF problem which enables us to determine asymptotically optimal beamformers for PP-RZFBF. In the large system limit, for the PP-RZFBF scheme, we also find that as P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">av</sub> → ∞, the interference on PU caused by the secondary transmission is asymptotically removed. A special suboptimal beamforming scheme called "CZFBF" is also considered, which involves zero forcing all the interference, including the secondary multiuser interference as well as the interference imposed on PU. Various interesting comparisons between PP-RZFBF and CZFBF are provided. Numerical simulations illustrate that the asymptotically optimal beamformers for the PP-RZFBF scheme provide an excellent performance even for finite-sized systems.

Impact of Primary Network on Secondary Network with Generalized Selection Combining

We propose cognitive spectrum sharing with generalized selection combining (GSC) at the secondary user (SU) in the presence of multiple primary transceivers with outdated channel information. Our main motivation is to determine the impact of GSC and outdated channel information on the outage probability of cognitive spectrum sharing subject to two practical power constraints: 1) maximum transmit power at the SU transmitter and 2) peak interference temperature at the primary user (PU) receiver. We derive new closed-form expressions for the exact and asymptotic outage probability in Rayleigh fading. Our expressions provide concise representations of the diversity order and the array gain. We confirm that the diversity order of GSC is entirely dependent on the secondary network and is equal to the available number of receiver antennas at the SU. This result is consistent with those of maximal-ratio combining (MRC) and selection combining (SC) in cognitive spectrum sharing. More importantly, our results show that the outage probability decreases with the increasing correlation coefficient of the outdated channel.

The Cognitive Ad-hoc Network Spectrum - Routing Selection Method

Based on cognitive self-organizing network transmission delay larger and larger interference to primary user receiver problem, put forward a method of spectrum joint routing selection, this method is effective to consider the transmission delay and the main user interference at the receiving end, mainly from bandwidth available probability; sent via link the number of bits of variance; spread spectrum characteristic; primary user receiver protection; spectrum sensing five aspects to consider. Cognitive users of the information interaction with on-demand routing discovery process started, and to establish collaborative perception nodes along the route cluster, using the routing maintenance message to maintain the nodes in the cluster. At the same time on the cognitive network capacity maximization as the goal, put forward to jointly design spectrum collaborative perception and channel allocation. The simulation results prove the effectiveness of the method.

Joint Subcarrier and Bit Allocation for Secondary User with Primary Users’ Cooperation

Interference between primary user (PU) and secondary user (SU) transceivers should be mitigated in order to implement underlay spectrum sharing in cognitive radio networks (CRN). Considering this scenario, an improved joint subcarrier and bit allocation scheme for cognitive user with primary users’ cooperation (PU Coop) in CRN is proposed. In this scheme, the optimization problem is formulated to minimize the average interference power level at the PU receiver via PU Coop, which guarantees a higher primary signal to interference plus noise ratio (SINR) while maintaining the secondary user total rate constraint. The joint optimal scheme is separated into subcarrier allocation and bit assignment in each subcarrier via arith-metric geo-metric (AM-GM) inequality with asymptotical optimization solution. Moreover, the joint subcarrier and bit optimization scheme, which is evaluated by the available SU subcarriers and the allocated bits, is analyzed in the proposed PU Coop model. The performance of cognitive spectral efficiency and the average interference power level are investigated. Numerical analysis indicates that the SU’s spectral efficiency increases significantly compared with the PU non-cooperation scenario. Moreover, the interference power level decreases dramatically for the proposed scheme compared with the traditional Hughes-Hartogs bit allocation scheme.

Optimal Power Allocation for Bi-Directional Cognitive Radio Networks with Fading Channels

In this paper, we consider a spectrum-sharing cognitive radio network where a bi-directional secondary network is allowed to access the spectrum allocated to a primary user (PU) on condition that the interference power received at the PU receiver is below a predefined threshold. Under such an interference power constraint (IPC), we investigate the optimal power allocation strategies to achieve the fundamental capacity limits of a bi-directional secondary network with fading channels. In particular, both the ergodic capacity and the outage capacity are considered. Closed-form solutions are obtained for both scenarios. Besides, a key parameter referred to as Interference-Transmission-Ratio (ITR) is proposed. It is shown that under a given IPC, ITR is the only factor that affects the transmission policies of the secondary network. In general, the transceiver with lower ITR value has the priority to transmit. We also show that unlike the conventional bi-directional communications network where concurrent transmission is always optimal, concurrent transmission is not always the first choice for bi-directional CRNs in terms of maximizing the ergodic capacity.

Spectrum Sharing Single-Carrier in the Presence of Multiple Licensed Receivers

In this paper, maximal-ratio combining (MRC) and selection combining (SC) are proposed in spectrum sharing single-carrier networks with multiple primary user receivers (PU-Rxs). Taking into account the peak interference power at the PU-Rx's and the maximum transmit power at the secondary user (SU), the impact of multiple PU-Rx's on the secondary network is characterized when the secondary user receiver (SU-Rx) is equipped with multiple antennas. In doing so, exact and asymptotic expressions are derived for the cumulative distribution function, taking into account two realistic scenarios: non-identical frequency selective fading between the secondary user transmitter (SU-Tx) and the PUs, and frequency selective fading between the SU-Tx and the SU-Rx. Based on these, exact and asymptotic expressions for the outage probability and average bit error rate are derived. Furthermore, an exact closed-form expression for the ergodic capacity is derived. It is shown that the asymptotic diversity gain depends only on the number of receive antennas and the number of multipath channels. It is further shown that the number of PU-Rx's and fading severities between the SU-Tx and the PU-Rx's have no impact on the asymptotic diversity gain.

Security-oriented transmission based on cooperative relays in cognitive radio

In this paper, we propose a security-oriented transmission scheme with the help of multiple relays in Cognitive Radio (CR). To maximise the Secrecy Capacity (SC) of the source-destination link in CR, both beamforming and cooperative jamming technologies are used to improve the performance of the Secondary User (SU) and protect the Primary User (PU). The effectiveness of the proposed scheme is demonstrated using extensive simulation. Both theoretical analyses and simulation results reveal that the proposed scheme contributes to the secure transmission of the SU with acceptable attenuation of the Signal-to-Noise Ratio (SNR) at the PU receiver, and the upper bound of the SC at the SU receiver is able to exploit the power allocation strategy.

Resource Allocation for OFDMA Cognitive Radios Under Channel Uncertainty

A resource allocation problem for a cognitive radio base station (CR-BS) communicating with multiple CR mobile stations (MSs) is considered in the downlink, which relies on orthogonal frequency-division multiple access (OFDMA). To protect the incumbent primary user (PU) system operating over the same frequency band, the interference inflicted to the PU receiver must be regulated. Since the channel gain estimates from the CR-BS to the PU receiver are typically uncertain in practice, a robust interference power constraint is advocated. Although the latter translates to a second-order cone constraint, the overall optimization problem for joint transmit-power and subcarrier allocation is non-convex, and coupled over all subcarriers in general. To circumvent the resulting computational hurdle, a tight polyhedral approximation of the second-order cone is employed. Practical finite-alphabet constellations are adopted and total weighted achievable rate is maximized. It is shown that a near-optimal solution can be obtained based on the Lagrangian dual.

Beamforming in a Multi-User Cognitive Radio System with Partial Channel State Information

This paper focuses on a downlink beamforming problem in the cognitive radio (CR) communication system where primary users (PUs) coexist with secondary users (SUs). It is assumed that the SU transmitter has multiple antennas and transmits data to another SU's single-antenna receiver by employing the beamforming. By properly designing a beamforming vector, a SU can maximize its channel gain while maintaining an interference with PU below a predefined level. Designing a beamforming vector includes steering beamforming vector direction as well as adjusting transmit power. In order to construct the optimal beamforming vector, the SU transmitter requires knowledge of the downlink channel information of both PU and SU receivers sent on the feedback channel. However, it is impractical to assume that the transmitter has the perfect channel state information (CSI) considering the tremendous feedback overhead. One way to tackle this feedback overhead problem is to use a finite number of feedback bits indicating the index of a predefined codeword in the codebook. We analyze interference to the PU receiver and propose an algorithm to design a beamforming vector considering an error attributed to the partial CSI. The simulation shows that the analysis is quite accurate and that the beamforming vector, designed according to the proposed algorithm, keeps interference to PU below a predefined level while taking into account the error owing to the partial CSI. In addition, we propose a feedback bit allocation mechanism, in order to maximize the gain of the SU link. By using the proposed scheme, the CR system become robust against errors attributed to the partial CSI.

The Effects of Limited Channel Knowledge on Cognitive Radio System Capacity

We examine the impact of limited channel knowledge on the secondary user (SU) in a cognitive radio system. Under a minimum signal-to-interference-and-noise ratio (SINR) constraint for the primary user (PU) receiver, we determine the SU capacity under five channel knowledge scenarios. We derive analytical expressions for the capacity cumulative distribution functions and the probability of SU blocking as a function of allowable interference. We show that imperfect knowledge of the PU-PU channel gain by the SU-Tx often prohibits SU transmission or necessitates a high interference level at the PU. We also show that errored knowledge of the PU-PU channel is more beneficial than statistical channel knowledge and imperfect knowledge of the SU-Tx to PU-Rx link has a limited impact on SU capacity.

Service-Outage Capacity Maximization in Cognitive Radio for Parallel Fading Channels

This paper focuses on a cognitive radio network consisting of a secondary user (SU) equipped with orthogonal frequency-division multiplexing (OFDM) technology able to access N randomly fading frequency bands for transmitting delay-insensitive (e.g. data) as well as delay-sensitive (e.g. voice or video) data. Each band is licensed to a distinct delay-sensitive primary user (PU) interested in meeting a minimum rate guarantee for delay-sensitive services with a maximum allowable primary outage probability or a primary outage constraint (POC) . Typically, a PU is oblivious to the SU's existence and has its own power policy based on the channel side information (CSI) of its direct gain between the PU transmitter and the PU receiver only. Under the assumption that the SU knows PUs' power policies and CSI of the entire network, we solve the SU's ergodic capacity maximization problem subject to SU's average transmit power and outage probability constraints (SOC) and all POCs or the so-called service-outage based capacity maximization for SU with POCs. We use a rigorous probabilistic power allocation technique that allows us to derive optimal power policies applicable to both continuous and discrete fading channels. Also, a suboptimal power control policy is proposed in order to avoid the high computational complexity of the optimal policy when N is large. Numerical results are presented to illustrate the performance of the power allocation algorithms.

Ergodic and outage capacities of relaying channels in spectrum‐sharing constrained systems

This study investigates the capacity of multiple relay channels in different fading and shadowing environments under spectrum-sharing constraints. The authors consider that a secondary user (SU) is allowed to share the spectrum band with a primary user (PU) provided that the SU's transmit power remains below an interference power threshold set by the PU. Considering a scenario where the SU's transmitter and receiver cannot communicate directly, a relay node, chosen among a set of K terminals, helps transmitting data from the SU's transmitter to the destination. The SU's transmitter and chosen relay node adapt their corresponding transmission parameters so as to satisfy the interference-power constraint at the PU's receiver. The authors derive closed-form expressions for the ergodic capacity of the SU's channel in Rayleigh fading, Nakagami-m fading and lognormal shadowing environments. They further obtain the outage capacity assuming the aforementioned environments and the above-mentioned spectrum-sharing limitations. Numerical results are provided to reinforce our theoretical derivations.

Cognitive Amplify-and-Forward Relaying with Best Relay Selection in Non-Identical Rayleigh Fading

This paper investigates several important performance metrics of cognitive amplify-and-forward (AF) relay networks with a best relay selection strategy and subject to non-identical Rayleigh fading. In particular, assuming a spectrum sharing environment consists of one secondary user (SU) source, K SU relays, one SU destination, and one primary user (PU) receiver, closed-form expressions for the outage probability (OP), average symbol error probability (SEP), and ergodic capacity of the SU network are derived. The correctness of the proposed analysis is corroborated via Monte Carlo simulations and readily allows us to evaluate the impact of the key system parameters on the end-to-end performance. An asymptotic analysis is also carried out and reveals that the diversity gain is defined by the number of relays pertaining to the SU network (i.e., K), being therefore not affected by the interference power constraint of the PU network.

BER Performance of Walsh Hadamard Like Codes Based on Complementary Sequence Sets in a CDMA and Cognitive Underlay System

In this paper, a method of constructing a sequence set similar to Walsh Hadamard (WH) codes is described. In general, two complementary sequence set of length 2 is used as a basic kernel to construct WH codes. In this work, four complementary sequence set of length 3 is used as a basic kernel. The proposed code set have a nearly non linear phase sequences with a more evenly spread frequency spectrum when compared with WH codes. It is shown that the proposed spreading codes perform comparable to WH in a Direct Sequence code division multiple access (DS-CDMA) system in an Additive White Gaussian Noise and Rayleigh channel at a reduced code length. In this work, bit error rate performance of the Underlay Cognitive Radio system implemented using DS-CDMA system is simulated. Primary and Secondary Users are distinguished by assigning spreading codes from different families and it is assumed that the channel information is known by both the Primary and Secondary User receivers. This new code set offers good performance in a Cognitive Radio Underlay system.

Capacity and power allocation for spectrum sharing in cognitive radio systems under unknown channel state information and imperfect spectrum sensing

Cognitive radio is a technology with tremendous potential for more efficient utilisation of the radio spectrum. Based on the available network side information to the secondary user, three transmission models are proposed in the literature: the `underlay` method, the `overlay` method and the `hybrid` method. The authors study here the capacity and power allocation for the hybrid method. In particular, the authors investigate the ergodic capacity of the secondary user under joint average transmission power and average interference to the primary user receiver constraints when no channel state information is available to the secondary user except its probability density function. An algorithm based on the barrier method is proposed to achieve this. Finally, simulation results are presented and analysed to validate the authors` theoretical results.

Robust Worst-Case Interference Control in Underlay Cognitive Radio Networks

We investigate the problem of power allocation to secondary users (SUs) in underlay cognitive radio networks (CRNs), where the channel state information (CSI) pertaining to the link between an SU's transmitter and a primary user (PU) receiver is uncertain. To keep the interference of SUs to PUs below a given threshold under any realization of uncertainty in CSI, we utilize the robust optimization theory where uncertainty in CSI is defined by a bounded distance between its estimated and exact values, demonstrate that the convexity of allocating power to SUs is preserved, and show that it can be solved in a computationally efficient manner. Considering worst-case interference is a very conservative approach that, although protects PUs to the maximum extent, may cause undesirable and significant reductions in the SUs' throughput as well, which at many instances may not be necessary. Hence, when permissible, it is worthwhile to tradeoff between the robust worst-case interference control to PUs and the SUs' throughput. In doing so, we maintain the probability of violating the interference to PUs below a given threshold by applying the differential norm and the chance constrained approaches, both of which can be solved very efficiently via a water-filling-like formula. Simulation results show the effectiveness of our proposed approach.

Optimizing the Coexistence Performance of Secondary-User Networks Under Primary-User Constraints for Dynamic Spectrum Access

In this paper, we address the following two types of coexistence between wireless networks: 1) coexistence among secondary user (SU) networks and 2) coexistence between primary user (PU) and SU networks. The former type addresses the sharing of a common spectrum opportunity among SU networks to achieve an efficient spectrum usage, whereas the latter type addresses the interference from SUs to PUs. Following the trend of spectrum regulations and industrial standardization, we postulate a potential deployment scenario and describe the management procedure of SU networks under the interference constraints to the PU receivers. For the quantitative evaluation of coexistence, we first propose a new parameter called the quality of coexistence ( QoC). Then, we introduce a novel coexistence scheme called interference-constrained time slot splitting and allocation. In addition, we derive the distribution of the interference to the PU receiver caused by SU transmitters weighted by the time slot fraction assigned to each SU network. After optimizing individual SU transmission under the PU interference constraint, we use the analytical model as a basis to obtain the optimal time slot allocation ratios of multiple SU networks by maximizing the aforementioned QoC. This leads to an efficient spectrum usage by SUs without incurring harmful interference to the PUs. Finally, we compare the proposed scheme with other schemes that select a single SU network to operate based on simplistic criteria. It is shown that when the SU network employs adaptive transmit power control, the medium value of QoC is doubled using the optimal time slot allocation, resulting in significant improvement in coexistence.

Transmission strategies with interference coordination in spectrum sharing cognitive radio networks under outage probability constraint

In this article, we consider a cognitive radio (CR) communication system based on spectrum sharing schemes, where we have a secondary user (SU) link with multiple transmitting antennas and a single receiving antenna, coexisting with a primary user (PU) link with a single receiving antenna. At the SU transmitter (SU-Tx), the channel state information (CSI) of the SU link is assumed to be perfectly known; while the interference channel from the SU-Tx to the PU receiver (PU-Rx) is not perfectly known due to less cooperation between the SU and the PU. As such, the SU-Tx is only assumed to know that the interference channel gain can randomly take values from a finite set with certain probabilities. Considering a SU transmit power constraint, our design objective is to determine the transmit covariance matrix that maximizes the SU rate, while we protect the PU by enforcing both a PU average interference constraint and a PU outage probability constraint. This problem is first formulated as a non-convex optimization problem with a non-explicit probabilistic constraint, which is then approximated as a mixed binary integer programming (MBIP) problem and solved with the branch and bound (BB) algorithm. The complexity of the BB algorithm is analyzed and numerical results are presented to validate the effectiveness of the proposed algorithm. A key result proved in the article is that the rank of the optimal transmit covariance matrix is one, i.e., CR beamforming is optimal under PU outage constraints. Finally, a heuristic algorithm is proposed to provide a suboptimal solution to our MBIP problem by efficiently (in polynomial time) solving a particularly-constructed convex problem.