窃听环境下AF中继选择与功率分配的研究

传统的最佳中继选择方案仅参考了合法用户的信道状态信息,在实际的通信系统中由于存在窃听用户而无法保证信息的可靠传输。现有的最佳中继选择方案将窃听用户的信道状态信息纳入考虑后,系统的安全性能得到了一定改善,但是依然采用的是等功率分配。针对放大转发协议,本文在现有最佳中继选择方案的基础上,以降低系统安全中断概率为目标,在系统总功率受限的前提下,根据源节点和中继节点以及中继节点和目的节点间的信道参数引出功率分配因子,对源节点和中继节点间的功率进行适当分配。通过仿真对比,可以发现功率分配能够降低系统的安全中断概率,从而改善系统的安全性能。      

分布式双向中继选择算法及用户功率分配

针对放大转发的瑞利双向中继信道的节点选择问题,提出了基于部分信道信息的分布式双向中继选择算法。算法通过计算双向链路的接收信噪比,推导出满足目标接收信噪比的转发阈值,各中继节点根据该阈值决定是否参与转发,从而实现分布式选择。此外,考虑用户总功率受限的情况,在分布式中继选择基础上提出了优化功率分配策略,使双向信道的接收信噪比更加接近。仿真结果表明,分布式中继选择算法与最优多中继算法的系统传输速率相似,计算复杂度大大降低,尤其是在中继数目增大的情况下更加明显。优化功率分配策略能进一步提高系统能量效率,在相同性能下可节省7%左右的功率。     

基于中断概率的协作通信中继选择与功率分配算法

研究了功率受限情况下多中继协作通信网络的中继选择和功率优化问题。在AF网络中,提出了一种低复杂度中继选择与功率分配算法,其目标是在总功率一定的条件下使系统的中断概率最小。本算法对源节点和所有潜在中继节点进行功率分配,结合当前信噪比选择最优的中继集合,通过最速下降法求出使系统中断概率最低的功率分配因子。该算法不需要知道大量瞬时信道信息、不需要系统在等功率条件下进行中继选择,只需求得中继节点排列矩阵便可根据当前信噪比自适应获得最优中继节点集合。仿真结果表明,在相同条件下,该算法明显优于不同中继节点集合下几种算法的中断性能,并且与传统的SAF及AAF算法相比,有效降低了中断概率,提升了系统性能和功率效率。     

基于萤火虫算法的多中继功率分配

为了充分实现中继协作,降低多中继协作通信系统功率分配优化问题的计算复杂度,提出了基于萤火虫算法的多中继功率分配方案。在一定的总功率和节点功率约束下,以最大化平均信噪比为优化目标函数,建立了多中继协作系统的功率分配最优化模型。选取该目标函数作为萤火虫的适应度函数,用向量表示萤火虫的状态,该向量的维数为待分配源节点和中继节点的个数,通过萤火虫聚集得到种群中最好的萤火虫,即可获得渐进最优功率分配。仿真结果表明,与平均功率分配相比,基于萤火虫算法的功率分配方案能降低2.44%~6.17%的比特差错率,提高了系统性能。     

基于译码-转发的多中继合作分集系统的功率分配

该文研究了衰落信道中基于译码-转发的多中继合作分集系统的功率分配问题。假设源节点有完全的信道状态信息,在总功率一定的条件下,以最小化系统的中断（outage）概率为目标,给出了如何选择中继节点以及如何在源节点和所选中继节点之间进行功率分配的算法。通过在任何时候最大化系统的瞬时容量来最小化系统的中断概率。仿真结果表明,该文提出的算法性能优于平均分配功率时系统的性能。     

基于译码－转发的多中继合作分集系统的功率分配

该文研究了衰落信道中基于译码－转发的多中继合作分集系统的功率分配问题。假设源节点有完全的信道状态信息,在总功率一定的条件下,以最小化系统的中断(outage)概率为目标,给出了如何选择中继节点以及如何在源节点和所选中继节点之间进行功率分配的算法。通过在任何时候最大化系统的瞬时容量来最小化系统的中断概率。仿真结果表明,该文提出的算法性能优于平均分配功率时系统的性能。     

一种针对协作分集的分布式功率分配算法

 本文研究在解码转发协作分集中的功率分配问题.本文首先建立了功率分配问题的数学模型,其能够在满足一定平均误比特率的前提下,使源节点和伙伴节点的发射功率之和最小化,从而不仅能够开发多用户无线系统所固有的空间分集能力,而且能够降低移动节点的能耗和网络中的同道干扰.本文还推导了伙伴节点和目的节点的平均误比特率公式,用于刻画在多径衰落中协作分集系统的性能.利用平均误比特率公式,本文提出了一种分布式的功率分配算法,能够使源节点、伙伴节点和目的节点逐个消息地计算最优的功率分配方案.本文所提出的算法不仅具有较小的控制协议开销,而且对信道状态的变化具有自适应能力.因此,该算法能够适应分布式的和动态的无线网络环境.最后,本文通过仿真验证了所提出的算法.     

无线协作网络中基于数据传输需求的分布式功率控制

在未来无线通信网络中,协作通信的性能依赖于通信资源的有效分配,比如中继选择和功率控制等.在本文中,我们建议了一个分布式买者和卖者博弈理论框架,以满足用户链路质量需求为基础,解决多用户协作通信中最优化中继选择和功率控制.本文联合考虑了源节点和中继节点的功率分配,进而优化源节点和中继节点的收益.这里提出的方法不仅有助于源节点找到相对位置较好的中继节点以及在源和中继之间进行最优化功率分配从而最小化源节点的支付,而且有助于相互竞争的中继节点提供优化的价格以最大化它们各自的收益.此外,这里的优化价格可以仅由局部信道状态信息和其他节点的能量价格决定.如果获得的中继节点总数增加,全网的能量消耗会更低.     

协作通信中基于DSTC的自适应能量分配方案

基于协作AF(Amplify and Forward)通信模型,提出了一种基于分布式空时分组码的自适应能量分配方案。该方案首先以最小化中断概率为准则,在源节点与中继节点之间根据信道状态信息情况,决定是否放大转发中继节点接收信号。然后,在中继节点采用最优功率分配策略,实现整个系统的最优化传输。仿真显示了所提出的两种自适应方案与传统自适应方案、非自适应方案,以及在不同调制制度下的性能比较。仿真结果表明,与传统非自适应方案相比,所提方案1的误码率性能提高了约5 d B。     

一种改进的中继节点选择方案

3GPP标准组织在2004年底启动了其长期演进(LTE)技术的标准化工作.上行传输方案采用带循环前缀的单载波频分多址接入(SC-FDMA)技术.对LTE上行链路进行仿真,实现无中继节点的直接传输.引入中继节点,通过传统最大路径损耗最小(LMP,least maximum pathloss)方案选取最优中继参与协作传输.重点分析并指出了传统LMP算法的缺点和不足,在此基础上提出了一种改进的中继节点选择算法,并结合考虑了源节点和中继节点的功率分配.从系统误码率和用户消耗功率两个方面对各传输方案进行了仿真分析.仿真结果表明LTE上行链路引入中继节点实现协作传输可明显提高系统的误码率性能.协作传输还节约了功率的使用,提高了用户的待机时间和功率效率.相比传统中继选择方案,改进后的LMP算法无论是从误码率还是功率消耗都得到明显改善.     

A coalitional game‐based relay load balancing and power allocation scheme in decode‐and‐forward cellular relay networks

In this paper, a game theoretic relay load balancing and power allocation scheme is proposed for downlink transmission in a decode‐and‐forward orthogonal frequency division multiple access‐based cellular relay network. A system with a base station communicating with multiple users via multiple relays is considered. The relays have limited power, which must be divided among the users they support. In traditional scheme, each relay simply divides its transmit power equally among all its users. Moreover, each user selects the relay with the highest channel gain. In this work, we do not apply the traditional relay scheme. It is because the users are distributed randomly, and by applying the traditional relay selection scheme, it may happen that some relays have more users connected to them than other relays, which results in having unbalanced load among the relays. In order to avoid performance degradation, achieve relay load balancing, and maximize the total data rate of the network, a game theoretic approach is proposed, which efficiently assigns the users to relays. The power of each relay is wisely distributed among users by the efficient power allocation scheme. Simulation results indicate that the proposed game‐based scheme can considerably improve the average sum‐spectral efficiency. Moreover, it shows that by applying the game, users who can connect to uncongested relays join them as opposed to connecting to congested relays. Copyright © 2015 John Wiley & Sons, Ltd.     

A Cyclic Shift Relaying Scheme for Asynchronous Two-way Relay Networks

Perfect time synchronization among multiple relay nodes is quite difficult to realize in distributed relay networks. In this paper, we proposed a cyclic prefix (CP) assisted cyclic shift relaying (CFR) scheme for asynchronous two-way amplify-and-forward (AF) relay networks over flat fading channels. In the proposed scheme, a CP is inserted at the two source nodes to combat the asynchronous delays. Each relay amplifies the received mixed asynchronous signals after CP removal, and a cyclic delay is introduced to further improve the system performance. With the CP and the cyclic delay, the multiple flat fading relay channels are transformed into a multipath fading channel. As a result, low complexity frequency domain equalizers, such as zero-forcing and minimum mean square error (MMSE) equalizer, can be used to recover the transmit signal. Furthermore, the performance of the proposed CFR scheme with MMSE equalizer is analyzed and closed-form expression for the lower bound of uncoded bit error rate (BER) performance is derived. Based upon this lower bound, we also investigate the power allocation among the sources and the relays to improve the system performance. Finally, extensive numerical results are provided to show the BER and frame error rate performance of the proposed CFR scheme.     

Nelder‐Mead–based power optimization for secrecy enhancement in amplify‐and‐forward cooperative relay networks

Cooperative communication based on relaying nodes has been considered as a promising technique to increase the physical layer security (PLS) performance in wireless communications. In this paper, an optimal power allocation (OPA) scheme based on Nelder‐Mead (NM) algorithm is proposed for improving the secrecy rate of amplify‐and‐forward (AF) cooperative relay networks employing cooperative jamming (CJ) scheme. The proposed hybrid jamming scheme allows the source and selected relay to transmit the jamming signal along with the information to confound the eavesdropper. The path selection probability of ant colony optimization (ACO) algorithm is used for selecting the relay for transmission. The performance based on secrecy rate is evaluated for “n” trusted relays distributed dispersedly between the source and destination. Gradient‐based optimization and three‐dimensional exhaustive search methods are used as benchmark schemes for comparison of the proposed power optimization algorithm. The secrecy performance is also compared with conventional AF scheme and CJ scheme without power optimization (EPA). The impact of single and multiple relays on secrecy performance is also evaluated. Numerical results reveal that, compared with the gradient method and exhaustive search algorithm, the proposed power allocation strategy achieves optimal performance. Also, the derived OPA results show a significantly higher secrecy rate than the EPA strategy for both CJ and AF schemes.     

Adaptive linearly constrained minimum variance beamforming for multiuser cooperative relaying using the kalman filter

In this paper, we consider a wireless communication scenario with multiple source-destination pairs communicating through several cooperative amplify-and-forward relay terminals. The relays are equipped with multiple antennas that receive the source signals and transmit them to the destination nodes. We develop two iterative relay beamforming algorithms that can be applied in real-time. In both algorithms, the relay beamforming matrices are jointly designed by minimizing the received power at all the destination nodes while preserving the desired signal at each destination. The first algorithm requires the existence of a local processing center that computes the beamforming coefficients of all the relays. In the second algorithm, each relay can compute its beamforming coefficients locally with the help of some common information that is broadcasted from the other relays. This is achieved at the expense of enforcing the desired signal preservation constraints non-cooperatively. We provide two extensions of the proposed algorithms that allow the relays to control their transmission power and to modify the quality of service provided to different sources. Simulation results are presented validating the ability of the proposed algorithms to perform their beamforming tasks efficiently and to track rapid changes in the operating environment.     

Cooperative power allocation with partial channel information in multi‐cell multi‐user dual‐hop MIMO relay systems

This paper considers cooperative power allocation with the use of partial channel state information (CSI) in a multi‐user dual‐hop relay system with multiple antennas. The end‐to‐end capacity can be improved by dynamically allocating the transmit power of the base station and relay according to co‐channel interference caused by the adjacent relays. The proposed scheme allocates the transmit power in association with the eigenvalues and angle difference between the eigenvectors of transmit correlation matrices of the desired and interference channel. It is shown by means of upper‐bound analysis that the end‐to‐end capacity of the proposed scheme can be maximized in highly correlated channel environments when the principal eigenvectors of transmit correlation matrices of the desired and interference channel are orthogonal to each other. It is also shown that the proposed scheme is robust to the channel estimation error. Finally, the performance of the proposed scheme is verified by the computer simulation. Copyright © 2010 John Wiley & Sons, Ltd.     

Secrecy analysis of a cooperative NOMA network using an EH untrusted relay

In this paper, a down-link non-orthogonal multiple access (NOMA) system with imperfect successive interference cancellation (SIC) using Energy-Harvesting untrusted relays is investigated. These relaying nodes use in this study use a power-switching architecture to harvest energy from the sources signals and apply an amplify-and-forward protocol to forward the signals. In addition, transmit jamming or artificial noise, is generated by a source node to improve the security of the system and protect confidential source information from untrusted relays. Likewise, three relaying selection strategies are employed to examine the secrecy performance of the proposed system. In order to evaluate the performance evaluation of the proposed system, closed-form expressions of the Secrecy Outage Probability (SOP) are studied over Rayleigh fading channels and a Monte Carlo simulation is used to confirm the analytical results. Furthermore, we study the effects of various parameters, such as power allocation factors, relay node selection, the number of relays, energy harvesting efficiency and the location of relay nodes on the secure outage performances for two users of NOMA system and conventional orthogonal multiple access (OMA). These results show that NOMA offers the better security performance with multiple users.     

全双工MIMO中继的自干扰抑制

在全双工多输入多输出(MIMO)中继系统中,中继发送端和接受端之间的自干扰降低了系统的信道容量,严重影响了中继系统的传输性能。从理论上来讲,全双工中继方案的核心问题就是自干扰抑制。通过在中继处设计收发矢量来抑制自干扰信号,但与已有的基于奇异值分解(SVD)的抑制方案不同,研究了一种旨在放大中继收发端有用信号与自干扰信号功率比(SIR)的抑制方法,将收发抑制矢量进行分开设计,通过公式推导,求出最优的收发抑制矢量。仿真结果表明该方案改善了中继系统的信道容量,自干扰抑制效果明显。      

Duality and Rate Optimization for Multiple Access and Broadcast Channels With Amplify-and-Forward Relays

In this paper, we consider multihop multiple access (MAC) and broadcast channels (BC) where communication takes place with the assistance of relays that amplify and forward (AF) their received signals. For a two-hop parallel AF relay MAC, assuming a sum power constraint across all relays we characterize optimal relay amplification factors and the resulting optimal rate regions. We find the maximum sum rate and the maximum rate for each user in closed form and express the optimal rate pair (R₁, R₂) that maximizes mu₁R₁+mu₂R₂ as the solution of a pair of simultaneous equations. We find that the parallel AF relay MAC with total transmit power of the two users P₁+P₂=P and total relay power P_R is the dual of the parallel AF relay BC where the MAC source nodes become the BC destination nodes, the MAC destination node becomes the BC source node, the dual BC source transmit power is P_R and the total transmit power of the AF relays is P. The duality means that the rate region of the AF relay MAC with a sum power constraint P on the transmitters is the same as that of the dual BC. The duality relationship is found to be useful in characterizing the rate region of the AF relay BC as the union of MAC rate regions. The duality is established for distributed multiple antenna AF relay nodes and multiple (more than 2) hops as well.     

拥有多天线源节点的放大转发双向中继信道机会中继策略研究

We consider a two-way relay network where the Amplify-and-Forward (AF) protocol is adopted by all relays in this paper. The network consists of two multi-antenna source nodes and multiple distributed single-antenna relays. Two opportunistic relaying schemes are proposed to efficiently utilize the antennas of the source nodes and the relay nodes. In the first scheme, the best relay is selected out by a max-min-max criterion before transmitting. After that, at each source, only the antenna with the largest channel gain between itself and the best relay is activated to transmit and receive signals with full power. In the second scheme, assisted by the best relay which is selected by the typical max-min criterion, both source nodes use all their antennas to exchange data, and match filter beamforming techniques are employed at both source nodes. Further analyses show that all schemes can achieve the full diversity order, and the conclusions are not only mathematically demonstrated but numerically illustrated. System performance comparisons are carried out by numerical methods in terms of rate sum and outage probability, respectively. The beamforming assisted scheme can be found to be superior to the antenna selection scheme when accurate Channel State Information (CSI) is available at the transmitters. Otherwise, the latter is very suitable.