相干光码分多址系统中拍频噪声的影响

随着相位编解码器关键技术的突破,精确分析相位编码光码分多址(OCDMA)系统误码率性能显得十分必要.本文研究了相干光码分多址系统中拍频噪声、多址干扰、散粒噪声和热噪声对系统性能的影响.在充分考虑了干扰信号输入光偏振影响的情况下,根据系统噪声和干扰的矩母函数.采用高斯近似的方法得到误码率的计算公式,并比较了多址干扰和拍频噪声对系统性能的影响.当所有干扰用户与目标用户平行时得到误码率上界;当所有干扰用户与目标用户垂直时得到误码率下界;干扰用户随机偏振时的误码率介于两者之间.结果表明:拍频噪声是影响相干OCDMA系统性能的主要因素;随着接收光功率的增加,干扰信号偏振对系统误码率的影响趋于明显.当系统误码率小于10-10时,误码率随用户数目的增加迅速增加;当误码率大于10-10时,误码率随用户数目的变化缓慢.     

基于PIC的MC-CDMA系统多级型多用户检测技术

为了抑制MAI和远近效应以提高系统的性能,采用多级型多用户检测技术,充分利用各个用户的扩频序列时延幅度和相位信息对各用户进行联合检测,利用前级判决的信息构造所有用户的干扰信号,然后从接收信号抵消掉干扰信号,从总体上提高各用户的性能。仿真验证表明,PIC的处理延迟小,且在用户增多的情况下误码率大为降低,但随着级数的增加,计算复杂度增大,故实际应用中一般选三级为宜。     

基于最大化容量的SLNR多用户MIMO预编码的研究

在多用户MIMO系统中可以联合预编码和功率分配来提高系统QoS.在重点研究信漏噪比SLNR的基础上,提出一种低复杂度的联合预编码与功率分配的算法.首先利用信道的迹为各个用户分配功率,然后以误码率最小准则为用户的各个空间子信道分配功率,既克服了SLNR只利用单个子信道传输的缺点,同时兼顾系统性能和运算复杂度.理论分析和仿...     

码分多址测控系统的功率调节

远近效应是码分多址测控系统中影响飞行器多址接收性能的重要因素。分析了远近效应在多站测控系统中对飞行器接收性能的影响,提出了在该系统中进行功率控制的必要性,给出了地面测控站功率调节的方法和策略,可作为多站应用的码分多址测控系统功率调节的参考思路。     

一种在TDD系统切换中有效降低掉线率的方法

对于所有类型的切换算法来说,如何减少掉线率和提高切换成功率是一个十分重要的衡量标准。本文主要研究了当目标小区为TDD系统时的切换过程,提出了一种在切换中降低用户掉线率的方法:当系统无线资源不足时,利用TDD系统不连续帧传输特性来临时提供切换用户资源,降低切换用户掉线率;当系统有资源时,恢复用正常的子帧传输,保证QoS。分析表明这是一种行之有效的方案。     

CDMA系统中基于公平性考虑的博弈功率控制算法

为解决CDMA系统中各用户与基站间距离的远近公平问题,提出了一种基于公平性考虑的非合作博弈功率控制算法。该算法综合考虑用户路径增益与发射功率等,设计了一种新的且存在唯一纳什均衡的代价函数,进而能够改善系统中的远近公平问题。仿真结果表明,与传统的功率控制算法相比,提出的功率控制算法具有用户发射功率降低,而且用户获得的SIR是远近公平的,并且用户获得的收益也是远近公平的优势。系统的远近公平问题得到了很好的改善。     

CDMA系统中的自适应反向闭环功率控制

在码分多址(CDMA)移动通信系统中,反向链路功率控制对克服远近效应和增加系统容量是非常重要的。本文提出了一种基于改进的神经网络(MNN)的自适应闭环功率控制算法,该方法平滑了移动信道衰落的影响,使基站接收到的小区中所有用户的信号功率相等。仿真结果表明,由于神经网络能够较好地识别反向链路的时变特性,MNN功率控制方法比传统的固定步长功率控制方法取得了更好的控制性能和系统容量。     

三阶色散效应对光码分多址系统误码率的影响

光码分多址(OCDMA)系统的误码率是评价系统性能的重要指标,由于色散和非线性效应会导致超短光脉冲在光纤传输过程中脉宽的展宽,进而影响OCDMA系统的误码率。在基于理想情况的OCDMA系统误码率定义的基础上,研究了三阶色散和三阶非线性效应对系统误码率的影响,进一步分析了有硬限幅器和二维OCDMA系统的误码率。结果表明,三阶色散和三阶非线性效应都会导致OCDMA系统误码率的增大,但三阶非线性效应对系统性能影响没有三阶色散效应明显;在系统参数相同基础上,接收端引入光硬限幅器和系统采用二维编码均可优化系统性能。     

基于单个自适应神经元的CDMA反向功率控制算法

CDMA(码分多址)是现代移动通信中的一项重要技术。文章从分析CDMA移动通信的“远近效应”问题着手，提出了一种基于单个自适应神经元(Adaptive Neuron)的反向闭环功率控制算法。该方法能够平滑信道衰落的影响，使基站接收到的小区中所有用户的信号功率基本相等，从而克服反向链路功率控制的“远近效应”并增加系统容量。与传统的固定步长功率控制方法进行仿真比较的结果显示，这种算法具有更快的响应时间，更小的超调量和跟踪误差。     

半导体激光器辐射损伤对空间光通信误码率的影响

为了提高空间光通信系统的抗辐射性能,研究了半导体激光器的辐射损伤效应对空间光通信系统误码率的影响.在此基础上,进一步推导了半导体激光器辐射损伤对空间光通信系统误码率的影响模型.通过仿真研究,其结果表明:辐射对半导体激光器的输出光功率线性下降.误码率在1000Gy之前可以忽略影响,辐射至3000Gy之后对误码率的影响迅速...     

Optimal power allocation in DS-CDMA with adaptive SIC technique

This paper proposes an optimal power allocation in direct sequence-code division multiple access (DS-CDMA) system. The objective is to minimize total transmit power, while simultaneously meeting the certain sum channel capacity (data transmission rate) and outage probability constraints on Rayleigh fading channel. Then a weighted correlator with an adaptive successive interference cancelation (SIC) scheme is developed using neural network (NN) for an improvement in receiver performance. A closed mathematical form of joint probability of error (JPOE) is derived. This determines the number of active users’ interfering effect that needs to be canceled in order to achieve a desired bit error rate (BER) value. Mathematical analysis shows that better receiver performance can be achieved through large change in weight up-gradation (w) for the strong users with a particular change in learning rate (η). Simulation results in terms of sum capacity as well as weak user’s (users with poor channel gain) capacity, outage probability and BER performance duly support the effectiveness of the proposed scheme over the existing works.     

Direct-detection optical synchronous CDMA systems with doubleoptical hard-limiters using modified prime sequence codes

An optical code-division multiple-access (CDMA) system with double optical hard-limiters is proposed where the optical hard-limiters are placed before and after an optical correlator. Moreover, the-effect of the optical hard-limiter on the performance of the optical synchronous CDMA systems using modified prime sequence codes as signature codes is analyzed under the assumption of a Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. We evaluate the performance under average power and bit rate constraints. Our results show that using the single optical hard-limiter slightly degrades the performance of the optical CDMA systems under the assumption of Poisson shot noise model for the receiver photodetector where the noise due to the detector dark currents exists. Moreover, we show that the optical CDMA systems with double optical hard-limiters have better performance than other conventional CDMA systems with and without the optical hard-limiter when the number of simultaneous users is not so large     

Code division multiple-access techniques in optical fiber networks.II. Systems performance analysis

For pt.I see ibid., vol.37, no.8, p.824-33 (1989). In Part I a technique based on optical orthogonal codes was presented to establish a fiber-optic code-division multiple-access (FO-CDMA) communications system. The results are used to derive the bit error rate of the proposed FO-CDMA system as a function of data rate, code length, code weight, number of users, and receiver threshold. The performance characteristics for a variety of system parameters are discussed. A means of reducing the effective multiple-access interference signal by placing an optical hard-limiter at the front end of the desired optical correlator is presented. Performance calculations are shown for the FO-CDMA with an ideal optical hard-limiter, and it is shown that using an optical hard-limiter would, in general, improve system performance     

全双工协作多播业务在NOMA系统中的应用和性能

研究了多播业务在协作非正交多址接入（non-orthogonal multiple access,NOMA）系统中的应用及其性能。在一个单播和多播混合业务的场景中,多播用户组中选择某个信道状态最好的用户作为中继进行解码转发单播用户的信号,并在中继用户支持全双工的模式下,分析和推导了单播用户的中断概率和平均可达速率的闭式表达式。蒙特卡洛仿真结果和理论推导高度一致,表明提出的协作多播应用可以极大改善单播用户的中断性能,且多播用户的容量性能在中继发射功率不是很高的情况下不会受到影响。     

对准误差对前置光放大卫星激光通信系统性能的影响

发射机及接收机的对准误差都会引起前置光放大卫星激光通信系统信号的衰落,在同时考虑发射机、接收机对准误差的条件下优化系统性能非常重要。将接收机对准误差引起的空间光耦合损耗用一个高斯函数近似,并同时考虑发射机对准误差引起的对准损耗,推导出了接收光功率概率密度的近似解析表达式,应用该概率密度函数,建立了基于平均误码率原则的前置光放大卫星激光通信系统的优化模型。仿真结果表明,在给定平均误码率要求及对准误差一定时,存在一个最佳发射光束宽度、接收天线直径及空间光耦合参数,使所需的发射功率最小,采用更大的接收天线并不能降低对发射功率的要求。     

Synchronous fiber-optic CDMA using hard-limiter and BCH codes

In this paper, synchronons code division multiple access (S/CDMA) for fiber-optic local area networks is considered. The performance of the fiber-optic S/CDMA network with negligible thermal and shot noises is interference limited. Here we derive the bit error rate of the S/CDMA system as a function of code length and number of active users, and the performance characteristics are also discussed. Furthermore, we analyze the performance of the fiber-optic S/CDMA system with an ideal optical hard-limiter, and the error probability with error control coding is also derived. In addition, the optimization between the S/CDMA and BCH codes of a constant bandwidth system is presented. The results show that by using an ideal hard-limiter in conjunction with BCH codes in this system, the influence of interference arising from other users can be greatly reduced, and the number of active users can also be increased significantly     

Cellular multicode CDMA capacity for integrated (voice and data)services

This paper analyzes the capacity of a multicode direct sequence-code division multiple access (DS-CDMA) cellular architecture supporting integrated (voice and data) traffic. The capacity estimate (on the uplink) is the number of voice users and data users, at different data rates, that the system can support with quality-of-service (QoS) guarantees (frame error rates (FER), outage probability). This estimate is based on an interference analysis that considers both perfect and imperfect power control, different user distributions in the cell, and the coverage trade-off resulting from hand-set power limitations. Localized interference from high speed data (HSD) users, combined with the effects of power control, adversely impacts voice capacity. The analysis investigates the effect of two important factors on capacity: (1) received power levels for the different classes of users and (2) data user activity. The results obtained are useful in designing power allocation and burst-level admission control strategies to optimize the capacity     

Opportunistic power allocation strategies and fair subcarrier allocation in OFDM-based cognitive radio networks

In this paper we have studied the subcarrier and optimal power allocation strategy for OFDM-based cognitive radio (CR) networks. Firstly, in order to protect the primary user communication from the interference of the cognitive user transmissions in fading wireless channels, we design an opportunistic power control scheme to maximize the cognitive user capacity without degrading primary user’s QoS. The mathematical optimization problem is formulated as maximizing the capacity of the secondary users under the interference constraint at the primary receiver and the Lagrange method is applied to obtain the optimal solution. Secondly, in order to limit the outage probability within primary user’s tolerable range we analyze the outage probability of the primary user with respect to the interference power of the secondary user for imperfect CSI. Finally, in order to get the better tradeoff between fairness and system capacity in cognitive radio networks, we proposed an optimal algorithm of jointing subcarrier and power allocation scheme among multiple secondary users in OFDM-based cognitive radio networks. Simulation results demonstrate that our scheme can improve the capacity performance and efficiently guarantee the fairness of secondary users.     

Retransmission control and fairness issue in mobile slotted ALOHA networks with fading and near‐far effect

The effects of different retransmission control policies on the performance of a mobile data system employing the slotted ALOHA protocol are investigated, with the emphasis on the unfairness between close-in and distant users due to the near-far effect. An analytical multi-group model is developed to evaluate both the user and the network performance of the mobile slotted ALOHA network under two classes of retransmission control policies, namely the Uniform Policy and the Nonuniform Policy. The Uniform Policy requires that all users adopt the same retransmission probability, whereas the Nonuniform Policy allows more distant users to have larger retransmission probability in order to compensate for the unfairness caused by the near-far effect. The performance of a slotted ALOHA network with a linear topology in a Rician fading channel under the two policies is compared by the multi-group model and simulation. The Nonuniform Policy is found to be more effective in alleviating the unfairness of user throughput over a wider range of the data traffic load than the Uniform Policy, which is effective only when the data traffic load is very light. Thus, a mobile data network can enjoy the network performance improvement derived from the near-far effect while the unfairness between close-in and distant users can be greatly mitigated without resorting to power control. This revised version was published online in June 2006 with corrections to the Cover Date.     

Performance analysis of two-way relay cooperation underlay cognitive radio networks based on WIPT

The two-way relay cooperation for underlay cognitive radio networks based on WIPT was presented,where secondary system adopted the two-way relay cooperative transmission.First,the relay adopted a power splitting protocol to harvest energy and to decode information transmitted by secondary users,then the relay forwarded information to two secondary users by using the harvested energy.Subsequently the exact expression of the outage probability,the ergodic capacity and energy efficiency of the secondary user system were derived.Finally,numerical simulation discusses the impact on the system performance with regard to secondary transmission power.The results reveal that the two-way relay cooperation effectively reduces the outage probability of data transmission.The capacity and energy efficiency of the system reaches optimal level when the power allocation coefficient is 0.5.