OFDMA中频谱感知的功率控制算法研究

结合认知无线电的特点,将基于频谱感知的功率控制算法应用到OFDMA系统中,建立了PA-OFDMA(Power Adaptation-OFDMA)仿真模型,实现了认知环境下的多址接入技术。该模型基于频谱感知信息,通过一种功率阈值分配算法自适应地为每个子信道分配一个独立的功率阈值,限定使用该信道认知用户的最大发射功率,实现非授权用户与授权用户的频谱共享。与干扰温度模型进行了比较,仿真结果表明,该模型解决了干扰温度模型中的局部干扰问题,提高了系统容量,具有较高的频谱利用率和较低的误比特率。     

认知系统中基于有限感知与混合保护的功率分配策略

研究了频谱共享环境中,一种基于认知用户有限感知区域与主用户混合保护措施的功率分配方法,在考虑主用户占用子信道概率的情况下,通过建立认知系统的多区域模型,提出了适应于主用户存在与否两种不同情况下的干扰避免策略。在满足认知用户自身最大功率约束与主用户混合干扰功率约束的条件下,基于最大化认知系统传输速率的目标,提出了各子信道集合的和功率以及不同集合内功率分配的联合优化方法,且复杂度较低。仿真结果表明,本文所提方法在不同参数设置下均达到了较好的速率性能,且远远超过机会频谱接入方式所获得的最大传输速率。     

衰落信道下基于接收机信标感知的频谱共享

理想的主、次系统频谱共享应尽量使次级用户发射机与主用户接收机处于互为对方的保护边界和干扰边界的临界状态。本文基于此临界状态对信标感知灵敏度和次级用户发射功率的要求,通过分析信道衰落和双向信道非对称性引起的临界干扰概率,首次针对基于接收机信标感知的频谱共享方法,给出了衰落信道下满足主用户干扰约束的次级用户发射功率约束机制。针对单个工作频段存在多个主用户接收机的（广播）场景,以距次级用户发射机最近的主用户接收机为干扰保护目标,通过分析信标的多个衰落副本的累积作用对信标感知的影响,进一步给出了这种场景下的次级用户发射功率约束与主用户临界干扰概率的关系。      

基于非理想感知的认知无线电系统感知时间与频谱分配联合优化算法

针对非理想感知情况下感知时间与频谱分配联合优化问题,同时考虑漏检与主用户重新占用频谱两种场景所造成的主次用户碰撞,并通过量化主用户对认知用户的干扰,给出有无主用户存在时认知系统可获得的吞吐量。在总传输功率约束以及对主用户的最大干扰功率约束两个限制条件下,以最大化系统平均吞吐量为优化目标,给出感知时间与频谱分配联合优化算法。算法首先通过折半法搜索最优感知时间,在既定的感知时间下,将子信道分配给能获得最大平均吞吐量的认知用户,在此基础上,利用凸优化相关理论求得最优功率分配。仿真结果表明,本文所提算法相比于传统频谱分配算法系统平均吞吐量性能提升了10%左右。     

一种非理想感知条件下的双门限机会频谱接入策略

为了有效提高认知网络中次用户的吞吐率性能,提出了一种应用于非理想感知条件下的双门限机会频谱接入策略。该策略能在次用户对信道占用情况感知非理想的条件下,综合考虑次用户的信道质量和非准确的信道占用信息,使次用户在信道感知为空闲和占用时分别以不同的信道质量门限选择接入信道,从而最大程度地利用信道质量较好的传输机会,提高次用户的吞吐率性能。结合本策略,建立了以次用户的有效吞吐率最大化为目标、以对主用户的干扰程度为约束条件的优化问题,并在次用户对信道感知存在误差的条件下给出了最优门限的设计方法。仿真结果表明,所提出的双门限机会频谱接入策略能够在非理想感知条件下显著提高次用户的有效吞吐率。     

基于空间位置的混合频谱共享系统功率分配研究简

针对次用户空间位置分布的随机性,提出空域混合Overlay/Underlay频谱共享模型以提高无线频谱利用率。根据次用户在不同空间位置对主用户的干扰,在主用户干扰容限约束下推导出次用户工作于Overlay状态和Underlay状态的空间区域;以最大化系统容量为准则建立认知系统功率优化分配模型,推导出次用户的最优功率分配方案,进而得到认知系统可获得的最大容量;理论和仿真结果表明,基于空间位置的混合频谱共享系统可以获得比Overlay系统更高的容量。     

认知频谱共享系统最大比例合并分集的性能分析

研究了一个认知频谱共享系统,该系统是由一个次用户发射机、一个次用户接收机和一对主用户发射机和接收机组成,次用户接收机利用最大比合并策略提高系统性能。首先分析了来自主用户发射机的干扰对频谱共享系统中断概率的影响。经过一些运算得到了中断概率确切的封闭表达式,为评价系统关键参数的联合作用提供了有效手段,如次用户接收机天线的数目、次用户最大的发射功率、干扰温度限制以及主用户发射功率等。另外,还对系统中断概率的渐近性能进行了研究,揭示了次用户系统中断概率具有饱和现象。最后,分析了该系统分集复用增益。仿真结果表明,该中断概率确切封闭表达式与Monte Carlo仿真结果非常吻合,干扰温度约束和主用户的干扰对中断概率具有显著的不利影响,增加次用户接收机的天线数目可提高系统性能。     

主用户活跃性下的多功率分配策略

考虑到认知用户在信息传输过程中主用户的状态可能随时变化,提出了一种新的功率分配模式——多功率分配策略。在基于频谱感知的系统模型中,以认知系统的吞吐量为目标函数,得出了主用户感知过程的多种状态,并分配三种不同的功率,最大化认知系统的容量。仿真结果表明,随着主用户活跃指数的逐渐提高,所提新模型的功率分配策略要优于传统方法。同时分析了新的功率分配下平均干扰功率与主用户接收端的信噪比对系统吞吐量和最优感知时间的影响,进一步验证了所提出新策略的有效性。 〖HT5H〗关键词：〖HT5K〗认知无线电;主用户活跃;频谱感知分配;多功率分配;吞吐量最大化     

基于干扰温度的多用户MIMO频谱共享系统容量分析

在无线通信认知无线电中,频谱共享系统被证实能够提供更高的频谱利用率。考虑到多用户多天线的频谱共享系统中次用户的发射功率受限于主用户的干扰温度,介绍了单天线系统和3种不同MIMO传输机制,并根据SNR分别推导出具体的容量表达式。最后通过仿真得出ST/SC方式能够获得最佳容量,而STBC方式应用于频谱共享系统没有提升容量的优势。     

一种基于OFDM的多用户认知无线电系统联合资源分配算法

针对频谱共享环境,研究了采用OFDM调制方式的多用户认知无线电系统中,综合考虑自身发射功率约束与主用户干扰功率约束的资源分配问题.基于最大化认知用户和速率的优化目标,给出了连续比特条件下最优的多水位注水功率分配表达式;针对更实际的整数比特要求情况,通过基于相对增益因子RG的子信道选择机制与每个认知用户内多约束的贪婪比特与功率分配策略,来最大限度地提高频谱利用率.仿真结果表明:联合的子信道、功率与比特分配算法在不同的参数设置下均达到了良好的性能,在不影响主用户通信的前提下有效地提高了认知系统的频谱利用率.     

On the Impacts of Channel Estimation Errors and Feedback Delay on the Ergodic Capacity for Spectrum Sharing Cognitive Radio

Spectrum sharing cognitive radio aims to improve the spectrum efficiency via sharing the spectrum band licensed to the primary user (PU) with the secondary user (SU) concurrently provided that the interference caused by the SU to the PU is limited. The channel state information (CSI) between the secondary transmitter (STx) and the primary receiver (PRx) is used by the STx to calculate the appropriate transmit power to limit the interference. We assume that this CSI is not only having channel estimation errors but also outdated due to feedback delay, which is different from existing studies. We derive closed-form expressions for the ergodic capacities of the SU with this imperfect CSI under the average interference power (AIP) constraint and the peak interference power (PIP) constraint. Illustrative results are presented to show the effects of the imperfect CSI. It is shown that the ergodic capacity of the SU is robust to the channel estimation errors and feedback delay under high feedback delay. It is also shown that decreasing the distance between STx and secondary receiver (SRx) or increasing the distance between STx and PRx can mitigate the impact of the imperfect CSI and significantly increase the ergodic capacity of the SU.     

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.     

Performance Analysis of the Primary User in the Secondary User Relay Assisted Spectrum Sharing Networks

In this paper, inaccurate spectrum detecting by the secondary user (SU) is taken into account. The impact of the interference caused by the SUs due to miss detection on the primary user (PU) in a spectrum sharing network is analyzed, and those SU nodes of correct detection are assumed to act as potential relays to assist the PU transmission process based on two proposed cooperative transmission schemes, referred to as, the distance based and the signal-to-noise-ratio (SNR) based schemes. We utilize stochastic geometry to analyze the impact of the secondary network parameters and cooperative transmission schemes on a typical primary source–destination (S–D) pair for the SU relay assisted spectrum sharing networks in Rayleigh fading environment. Using this approach, closed-form expressions for the primary system success probabilities with those cooperative transmission schemes as well as the PU direct re-transmission scheme are derived respectively. Simulations confirm our analytical derivations and results demonstrate that significant improvement on the PU success probability by using SU cooperative transmission schemes, and the SNR based scheme is superior to the distance based scheme.     

Optimal precoder design for non‐regenerative multiple‐input multiple‐output cognitive relay systems with perfect and imperfect channel state information

This paper studies optimal precoder design for non‐regenerative multiple‐input multiple‐output (MIMO) cognitive relay systems, where the secondary user (SU) and relay station (RS) share the same spectrum with the primary user (PU). We aim to maximize the system capacity subject to the transmit power constraints at the SU transmitter (SU‐Tx) and RS, and the interference power constraint at the PU. We jointly optimize precoders for the SU‐Tx and RS with perfect and imperfect channel state information (CSI) between the SU‐Tx/RS and PU, where our design approach is based on the alternate optimization method. With perfect CSI, we derive the optimal structures of the RS and SU‐Tx precoding matrices and develop the gradient projection algorithm to find numerical solution of the RS precoder. Under imperfect CSI, we derive equivalent conditions for the interference power constraints and convert the robust SU‐Tx precoder optimization into the form of semi‐definite programming. For the robust RS precoder optimization, we relax the interference power constraint related with the RS precoder to be convex by using an upper bound and apply the gradient projection algorithm to deal with it. Simulation results demonstrate the effectiveness of the proposed schemes. Copyright © 2013 John Wiley & Sons, Ltd.     

Secrecy outage analysis in a hybrid cognitive relay network with energy harvesting

In this paper, we investigate the physical layer security of a hybrid cognitive relay network using an energy harvesting relay in presence of an eavesdropper. In the hybrid scheme, a secondary user (SU) as well as a cognitive relay works either in underlay or in overlay mode. In underlay, the transmit power of the SU as well as the relay is limited by the maximum acceptable interference at primary user (PU) receiver as required by an outage constraint of PU, a quality of service for PU. The secondary network consists of a decode and forward relay that harvests energy from radio frequency signal of secondary transmitter as well as PU transmitter to assist the SU in forwarding the information signal to the destination. A time switching relaying protocol is used at the relay. We evaluate the secrecy outage probability of secondary relay network assuming that channel state information of the interfering links from both the SU and relay transmitter to PU receiver is imperfect. Our results reveal the impact of imperfect channel state information, energy harvesting time, tolerable interference threshold, and PU outage constraint on the secrecy outage probability of SU.     

Performance of cognitive relay network with energy harvesting relay under imperfect CSI

This paper evaluates the performance of an underlay cognitive relay network under imperfect channel state information (CSI) where a secondary user (SU) transmits using a secondary relay (SR) based on decode and forward scheme. The outage probability (OP) of SU is investigated in a scenario where the decode and forward relay harvests energy from radio frequency signal of SU. The relay uses a fraction of time for harvesting in time switching–based relaying (TSR) while a fraction of received power is used for harvesting in power splitting–based relaying (PSR) scheme. The SU and relay control their transmit power using a scaling factor, based on CSI of the interfering links (ie, links from SU transmitter and SR to the primary user [PU] receiver) to protect the quality of service of PU. The available CSI at the SU and SR are imperfect due to practical limitation. Analytical expressions of the OP are derived for TSR‐ and PSR‐based schemes. The impact of harvesting time, power splitting ratio, imperfect CSI, PU outage constraint and interference threshold on the OP of the SU network, and average transmit power of SR is indicated. Further, the impact of multiple SRs is also shown.     

Performance analysis of an opportunistic multi‐user cognitive network with multiple primary users

Consider a multi‐user underlay cognitive network where multiple cognitive users concurrently share the spectrum with a primary network with multiple users. The channel between the secondary network is assumed to have independent but not identical Nakagami‐m fading. The interference channel between the secondary users (SUs) and the primary users is assumed to have Rayleigh fading. A power allocation based on the instantaneous channel state information is derived when a peak interference power constraint is imposed on the secondary network in addition to the limited peak transmit power of each SU. The uplink scenario is considered where a single SU is selected for transmission. This opportunistic selection depends on the transmission channel power gain and the interference channel power gain as well as the power allocation policy adopted at the users. Exact closed form expressions for the moment‐generating function, outage performance, symbol error rate performance, and the ergodic capacity are derived. Numerical results corroborate the derived analytical results. The performance is also studied in the asymptotic regimes, and the generalized diversity gain of this scheduling scheme is derived. It is shown that when the interference channel is deeply faded and the peak transmit power constraint is relaxed, the scheduling scheme achieves full diversity and that increasing the number of primary users does not impact the diversity order. Copyright © 2014 John Wiley & Sons, Ltd.     

Prediction based channel allocation performance for cognitive radio

The interdependency, in a cognitive radio (CR) network, of spectrum sensing, occupancy modelling, channel switching and secondary user (SU) performance, is investigated. Achievable SU data throughput and primary user (PU) disruption rate have been examined for both theoretical test data as well as data obtained from real-world spectrum measurements done in Pretoria, South Africa. A channel switching simulator was developed to investigate SU performance, where a hidden Markov model (HMM) was employed to model and predict PU behaviour, from which proactive channel allocations could be made. Results show that CR performance may be improved if PU behaviour is accurately modelled, since accurate prediction allows the SU to make proactive channel switching decisions. It is further shown that a trade-off may exist between achievable SU throughput and average PU disruption rate. When using the prediction model, significant performance improvements, particularly under heavy traffic density conditions, of up to double the SU throughput and half the PU disruption rate were observed. Results obtained from a measurement campaign were comparable with those obtained from theoretical occupancy data, with an average similarity score of 95% for prediction accuracy, 90% for SU throughput and 70% for PU disruption rate.     

Outage performance of cognitive radio networks with multiple decode‐and‐forward relays

This paper considers the cognitive radio network with one primary user (PU), one secondary user (SU), and multiple decode‐and‐forward relays. We propose a relaying scheme to ensure the priority of primary transmission, where the relays are used to forward PU's message and sometimes also SU's message. First, SU is allowed to use the spectrum to transmit only when its transmission would not affect the decoding status of PU's message at all relays. Second, once the secondary transmission happens, the relays that successively decode SU's message are allowed to retransmit this message when it would not affect the decoding status of PU's message at primary receiver. The interference from PU to SU and the interference from SU to PU are both considered. By analyzing the decoding status of primary message and secondary message at different relays, we formulate the outage probabilities of both primary transmission and secondary transmission. When all channels follow Rayleigh distributions, we derive the analytical expressions for the general case of any number of relays, which are validated by means of Monte Carlo simulations. Copyright © 2016 John Wiley & Sons, Ltd.     

Beamforming and power allocation in the downlink of MIMO cognitive radio systems based on multiobjective optimization

In this paper, power allocation and beamforming are considered in a multiple input multiple output (MIMO) downlink cognitive radio (CR) communication system, which a base station (BS) serves one primary user (PU) and one secondary user (SU). In order to design the CR system, a constrained multiobjective optimization problem is presented. Two objectives are the signal to noise plus interference ratios (SINRs) of PU and SU. Since PU has a spectrum license for data communication, a constraint in the optimization problem is that the SINR of PU must be greater than a predefined threshold based on the PU demand requirement. Another constraint is a limitation on power in BS. By considering the mentioned model, three iterative algorithms are proposed. At each iteration of all algorithms, the receiver beamforming vectors are derived based on the maximization of PU and SU SINRs, by assuming that the allocated powers and BS beamforming vectors are known. Also, power is assigned to users such that the constraint of power limitation is satisfied. The difference between the algorithms is in the obtaining of transmitter beamforming parameters. We evaluate the performance of the proposed algorithms in terms of bit error rate (BER) in simulations. Also, the computational complexity of the proposed algorithms is obtained.