On the Capacity of Secondary Networks Over Opportunistic Spectrum Access in Rayleigh Fading Channels

In this paper the effect of the opportunistic spectrum access on the spectrum utilization is studied in terms of the secondary network capacity measured at the secondary receiver. A mathematical model is developed to represent the secondary network capacity in Rayleigh fading channel. An exact analytical solution for the capacity is derived for both sensing and accessing fading channels. A numerical evaluation of the channel capacity is presented for different channel sensing and accessing schemes. The effects of detection and accessing channel parameters on the capacity are investigated. The analytical results that are validated by substantial simulations showed how the utilization of the network can be increased significantly by the suggested opportunistic spectrum accessing technique. It was found that when having a good sensing system with a high secondary user signal to noise ratio, accessing the licensed band increases and drives the spectrum utilization to its maximum. In addition, this work shows how the capacity can be positively affected by three factors: the secondary accessing channel, the primary user interference and the desired quality of service (QoS) of primary user. The awareness of a proper sensing scheme can maximize the spectrum utilization without degrading the QoS of primary users.     

Q学习算法在机会频谱接入信道选择中的应用

针对“先听后传”的机会频谱接入中认知用户的信道选择问题,本文提出了一种基于Q学习的信道选择算法。在非理想感知的条件下,通过建立认知用户的信道选择模型并设计恰当的奖励函数,使智能体能够与未知环境不断交互和学习,进而选择长期累积回报最大的信道接入。在学习过程中,本文引入了Boltzmann实验策略,运用模拟退火思想实现了资源探索与资源利用之间的折衷。仿真结果表明,所提算法能够在未知环境先验知识条件下可以快速选择性能较好的信道接入,有效提高认知用户的接入吞吐量和系统的平均容量。      

Optimal energy-delay tradeoff for opportunistic spectrum access in cognitive radio networks

Cognitive radio (CR) has been considered as a promising technology to enhance spectrum efficiency via opportunistic transmission at link level. Basic CR features allow secondary users (SUs) to transmit only when the licensed channel is not occupied by primary users (PUs). However, waiting for an idle time slot may lead to large packet delays and high energy consumption. We further consider that the SU may decide, at any moment, to use another dedicated way of communication (4G) in order to transmit his packets. Thus, we consider an Opportunistic Spectrum Access (OSA) mechanism that takes into account packet delay and energy consumption. We formulate the OSA problem as a Partially Observable Markov Decision Process (POMDP) by explicitly considering the energy consumption as well as packets’ delay, which are often ignored in existing OSA solutions. Specifically, we consider a POMDP with an average reward criterion. We derive structural properties of the value function and we show the existence of optimal strategies in the class of the threshold strategies. For implementation purposes, we propose online learning mechanisms that estimate the PU activity and determine the appropriate threshold strategy on the fly. In particular, numerical illustrations validate our theoretical findings.     

动态频谱接入MAC协议研究

机会频谱接入（OSA,opportunistic spectrum access）允许未授权用户在空间域和时域上共享授权频谱,但仅当授权用户没有占用这些频谱时认知用户才能接入。另外频谱环境的动态性使MAC协议设计面临着几个重要的问题,即认知用户需要确定何时以什么方式接入哪一个信道发送/接收数据而不影响主用户的通信。从OSA-MAC设计面临的技术挑战出发,对近年来国内外在该方向的研究成果做了总结和分析,并阐述了OSA-MAC设计亟待解决的问题和今后的研究方向。     

Hopping control channel MAC protocol for opportunistic spectrum access networks

Opportunistic spectrum access （OSA） is considered as a promising approach to mitigate spectrum scarcity by allowing unlicensed users to exploit spectrum opportunities in licensed frequency bands. Derived from the existing channel-hopping multiple access （CHMA） protocol,we introduce a hopping control channel medium access control （MAC） protocol in the context of OSA networks. In our proposed protocol,all nodes in the network follow a common channel-hopping sequence; every frequency channel can be used as control channel and data channel. Considering primary users＇ occupancy of the channel,we use a primary user （PU） detection model to calculate the channel availability for unlicensed users＇ access. Then,a discrete Markov chain analytical model is applied to describe the channel states and deduce the system throughput. Through simulation,we present numerical results to demonstrate the throughput performance of our protocol and thus validate our work.     

Distributed power control algorithm for cognitive radios with primary protection via spectrum sensing under user mobility

Substantial spectrum gains have been demonstrated with the introduction of cognitive radio however; such gains are usually short lived due to the increased level of interference to licensed users of the spectrum. The interference management problem is herein tackled from the transmitter power control perspective so that transmissions by cognitive radio network does not violate the interference threshold levels at the primary users as well as maintain the QoS requirements of cognitive radio users. We model the cognitive radio network for mobile and immobile users and propose algorithms exploiting primary radio environment knowledge (spectrum use), called power control with primary protection via spectrum sensing. The algorithm is briefly introduced for time invariant systems and demonstrated that it has the ability to satisfy tight QoS constraints for cognitive radios as well as meet the interference constraints for licensed users. We, however, further show that such assumption of terminal immobility in the power control algorithm would fail in cases where user mobility is considered, resulting in increased levels of interference to the primary as well as increased outages in cognitive radio network. We model the link gain evolution process as a distance dependent shadow fading process and scale-up the target signal to interference ratio to cope with user mobility. Since mobility driven power control algorithms for cognitive radios have not been investigated before, we therefore, propose a mobility driven power control framework for cognitive radios based on spectrum sensing, which ensures that the interference limit at the primary receiver is unperturbed at all times, while concurrently maintaining the QoS within the cognitive radio network as compared to static user cases. We also corroborate our algorithms with proof of convergence.     

Channel access protocols for multihop opportunistic networks: challenges and recent developments

Heavy traffic over the unlicensed portion of the spectrum along with inefficient usage of the licensed spectrum gave impetus for a new spectrum allocation policy, the main purpose of which is to improve spectrum efficiency through opportunistic spectrum access. Cognitive radios have been proposed as a key enabling technology for such an opportunistic policy. One of the key challenges to enabling multihop CR communications is how to perform opportunistic medium access control while limiting the interference imposed on licensed users. In this article we highlight the unique characteristics of multihop cognitive radio networks, discuss key MAC design challenges specific to such networks, and present some of the work that has been done on MAC design for CRNs.     

Two-stage decision making policy for opportunistic spectrum access and validation on USRP testbed

“Recently, various paradigms, for instance, device-to-device communications, LTE-unlicensed and cognitive radio based on an opportunistic spectrum access (OSA) are being envisioned to improve the average spectrum utilization. In OSA, secondary (unlicensed) users (SUs) need decision making policies (DMPs) to identify and transmit over optimum frequency bands without any interference to the primary (licensed) users as well as minimize the number of collisions among SUs. In this paper, we have proposed a two-stage DMP consisting of Bayesian Multi-armed Bandit algorithm to accurately characterize the frequency band statistics independently at each SU and frequency band selection scheme for orthogonalization of SUs. The analytical and simulation results show that the proposed DMP leads to 45% improvement in the average spectrum utilization compared to 36–39% in the existing DMPs. Furthermore, the number of collisions are 58.5% lower in the proposed DMP making SU terminals energy-efficient. The performance of the proposed DMP has been verified on the proposed USRP testbed in real radio environment and the experimental results closely match the simulated results .”     

Decentralized dynamic spectrum access for cognitive radios: cooperative design of a non-cooperative game

We consider dynamic spectrum access among cognitive radios from an adaptive, game theoretic learning perspective. Spectrum-agile cognitive radios compete for channels temporarily vacated by licensed primary users in order to satisfy their own demands while minimizing interference. For both slowly varying primary user activity and slowly varying statistics of ?fast? primary user activity, we apply an adaptive regret based learning procedure which tracks the set of correlated equilibria of the game, treated as a distributed stochastic approximation. This procedure is shown to perform very well compared with other similar adaptive algorithms. We also estimate channel contention for a simple CSMA channel sharing scheme.     

Coexistence of primary users and secondary users under interference temperature and SINR limit

Spectrum sharing offers the opportunity for promising efficiency gains,and it will be a valuable tool in the era of future radios.To address this issue,we develop a multi-user cognitive radio sharing network based on opportunistic spectrum access and propose a new spectrum sharing strategy.The objective of our interest is to obtain the number of secondary users who can coexist peacefully with primary users to improve the utility of licensed spectrum,at the same time maximize the total goodput under the interference temperature and SINR constraints.Through analysis and simulation,the new strategy of spectrum sharing does improve the goodput performance as well as guarantee the Quality of Service (QoS) of primary and secondary users.     

Performance Evaluation of Secondary Users Operating on a Heterogeneous Spectrum Environment

Radio spectrum is a limited natural resource and with the increasing number of wireless devices, an efficient spectrum management concept to make a better utilization of this resource is essential. Opportunistic spectrum access (OSA) concept is a solution to increase the spectrum capacity and thus reducing the data collision for wireless ad hoc networks. Cognitive radio (CR) technology is developed to realize OSA. Based on CR, the secondary users access opportunistically the spectrum owned by primary users. However, the consequence appearance of primary users affects greatly the performance of secondary users within OSA. Thus, a new spectrum management scheme is a must to reduce such effect. In this paper, a new spectrum management scheme over a heterogeneous spectrum environment is proposed. The proposed scheme is based on using channels from both licensed and unlicensed bands as spectrum environment for ad hoc networks. An analytical model based on Markov chains is developed to evaluate the proposed scheme.     

Elastic bandwidth allocation scheme with softened peak interference power constraint for the dynamic cognitive radio systems

The popularity of diverse wireless communication systems has led to increased strains on the unlicensed spectrum. However, investigations have shown that vast portions of the licensed spectrum remain underutilized across frequency, space and time. To improve the utilization of the existing radio spectrum, cognitive radio (CR) allows a secondary system to access the licensed spectrum as long as the primary system’s operation is not compromised. Two main CR transmission modes, spectrum overlay and underlay have been proposed. In the spectrum overlay mode, challenges in quality-of-service (QoS) provisioning arise due to the necessity for secondary users to vacate the channels when a primary user appears. In the underlay model, interference caused to the primary system has to be carefully managed resulting in a constraint of the secondary system’s transmit power, which causes difficulty in QoS provisioning. In this paper, we propose an elastic bandwidth allocation scheme to make concurrent use of both spectrum overlay and underlay transmission modes. Different from existing hybrid transmission strategy, our scheme employ a novel softened peak interference power constraint to improve the performance of the secondary system while still granting the superior protection to the primary system transmissions. This allows the proposed scheme to achieve a superior transmission capacity in the CR network while avoiding the weaknesses of the both spectrum overlay and spectrum underlay transmission modes.     

认知 mesh 网络服务区分的动态频谱接入策略

动态频谱接入策略是实现认知无线电网络高效利用频谱的关键。与传统认知无线电网络不同,认知mesh网络中不同QoS需求的多类型业务共同接入,为适应这一特点,提出服务区分的动态频谱接入策略。策略依据业务的QoS需求确立优先级,针对不同优先级业务采取不同的信道接入方案,实时业务依据最优传输延迟期望选择接入信道集合,在减小传输延迟的同时降低数据传输过程授权用户出现的概率,普通业务选择最优理想传输成功概率的信道,降低信道切换概率。理论与实验结果表明,与传统的认知网络频谱接入策略相比,提出的策略能提供不同业务的服务区分,满足实时业务的低延迟需求,降低数据传输的中断率,同时在授权信道空闲率与网络负载较大时吞吐量性能较优。     

Dynamics of Multiple-Seller and Multiple-Buyer Spectrum Trading in Cognitive Radio Networks: A Game-Theoretic Modeling Approach

We consider the problem of spectrum trading with multiple licensed users (i.e., primary users) selling spectrum opportunities to multiple unlicensed users (i.e., secondary users). The secondary users can adapt the spectrum buying behavior (i.e., evolve) by observing the variations in price and quality of spectrum offered by the different primary users or primary service providers. The primary users or primary service providers can adjust their behavior in selling the spectrum opportunities to secondary users to achieve the highest utility. In this paper, we model the evolution and the dynamic behavior of secondary users using the theory of evolutionary game. An algorithm for the implementation of the evolution process of a secondary user is also presented. To model the competition among the primary users, a noncooperative game is formulated where the Nash equilibrium is considered as the solution (in terms of size of offered spectrum to the secondary users and spectrum price). For a primary user, an iterative algorithm for strategy adaptation to achieve the solution is presented. The proposed game-theoretic framework for modeling the interactions among multiple primary users (or service providers) and multiple secondary users is used to investigate network dynamics under different system parameter settings and under system perturbation.     

Coalitional game‐based behavior analysis for spectrum access in cognitive radios

The core of cognitive radio paradigm is to introduce cognitive devices able to opportunistically access the licensed radio bands. The coexistence of licensed and unlicensed users prescribes an effective spectrum hole‐detection and a non‐interfering sharing of those frequencies. Collaborative resource allocation and spectrum information exchange are required but often costly in terms of energy and delay. In this paper, each secondary user (SU) can achieve spectrum sensing and data transmission through a coalitional game‐based mechanism. SUs are called upon to report their sensing results to the elected coalition head, which properly decides on the channel state and the transmitter in each time slot according to a proposed algorithm. The goal of this paper is to provide a more holistic view on the spectrum and enhance the cognitive system performance through SUs behavior analysis. We formulate the problem as a coalitional game in partition form with non‐transferable utility, and we investigate on the impact of both coalition formation and the combining reports costs. We discuss the Nash Equilibrium solution for our coalitional game and propose a distributed strategic learning algorithm to illustrate a concrete case of coalition formation and the SUs competitive and cooperative behaviors inter‐coalitions and intra‐coalitions. We show through simulations that cognitive network performances, the energy consumption and transmission delay, improve evidently with the proposed scheme. Copyright © 2016 John Wiley & Sons, Ltd.     

On co-channel and adjacent channel interference mitigation in cognitive radio networks

Cognitive radio (CR) is an emerging wireless communications paradigm of sharing spectrum among licensed (or, primary) and unlicensed (or, CR) users. In CR networks, interference mitigation is crucial not only for primary user protection, but also for the quality of service of CR user themselves. In this paper, we consider the problem of interference mitigation via channel assignment and power allocation for CR users. A cross-layer optimization framework for minimizing both co-channel and adjacent channel interference is developed; the latter has been shown to have considerable impact in practical systems. Cooperative spectrum sensing, opportunistic spectrum access, channel assignment, and power allocation are considered in the problem formulation. We propose a reformulation–linearization technique (RLT) based centralized algorithm, as well as a distributed greedy algorithm that uses local information for near-optimal solutions. Both algorithms are evaluated with simulations and are shown quite effective for mitigating both types of interference and achieving high CR network capacity.     

一种基于传输成功率的自适应传输策略

在认知无线电网络中,识别用户可以机会接入分配给主用户的频谱。在满足感知时间需求的情况下,如何能让识别用户跟踪主用户的活动状况,从而减少对主用户的干扰,并同时最大化识别用户的数据吞吐量。针对这个问题提出了一种基于传输成功率的自适应传输策略(ATS-SR),识别用户以自适应帧为单位去监测信道状况和传输数据。仿真结果表明,与文献[2]相比,ATS-SR在不降低吞吐量的情况下可以显著减少对主用户的干扰。     

认知Ad Hoc网络能量有效频谱接入策略

针对认知无线Ad Hoc网络中次用户能量受限问题,提出一种基于能量有效性的机会频谱接入策略。联合考虑信道状态的时变性和次用户的频谱感知准确性,基于部分可观测马尔科夫决策过程（POMDP）建立了一种最大化能量有效性的分析架构,指导次用户选择能效最佳信道,并根据信念状态、信道增益和检测概率,自适应控制传输功率。仿真结果表明,该策略能够有效提高次用户传输的能量有效性,通过对传输功率的有效控制,实现了传输性能和能量开销的有效折中。      

机会频谱接入中的最优资源分配

感知无线电(CR)网络中的机会频谱接入(OSA)可以有效提高频谱的利用率。研究了OSA中的信道分配和功率控制问题,通过将Overlay模式和Underlay模式相结合,求解了次用户吞吐量的最优化问题。在满足干扰温度约束的前提下,优化目标为所有次用户的带权速率之和,同时可提供QoS支持和最小速率保障。此优化问题可转化为二部图最大带权匹配问题并使用匈牙利方法进行求解。仿真结果显示,采用混合模式的信道分配和功率控制算法较单一模式可以获得更大的频谱资源利用率。