多比特增量-总和调制器中的降噪环路

在单级多比特增量-总和调制器中,量化器的规模是与其比特数成指数增长的,当比特数增加时,量化器会很快变得难以实现.本文提出了一种新结构:降噪环路(Noise-Reducing Loop),能够利用较少比特的量化器获得较多比特量化器的效果,在获得高信噪比的同时,大大地减小了电路规模.在此基础上应用动态量化(Dynamic Quantization)算法,可以使调制器在很宽的工作范围内具有较高的性能.     

直扩数字接收机中AD量化比特数的确定

量化比特数的确定是IF数字接收机设计的关键。从高斯分布函数的角度，得到了不同量化比特数和量化电平下直扩系统的输出信噪比计算公式；对存在窄带干扰情况下AD量化比特数的确定给出了定量的分析。计算机仿真结果表明：无干扰时取4bit量化即可获得较好的接收机性能；干信比为24dB时，量化比特数应至少为6bit。     

基于1比特量化的大规模MIMO雷达系统直接定位算法

1比特量化技术在大规模MIMO雷达系统中的应用使得系统成本、功耗及传输带宽显著降低。但这同时也对如何从1比特量化后的数据中提取目标高精度信息提出了严峻挑战。针对基于1比特量化的二次定位算法在低信噪比下定位精度低、鲁棒性差的问题,该文提出了一种基于1比特量化的大规模MIMO雷达系统目标直接定位算法。首先,通过将接收信号进行1比特量化,并推导基于1比特信号的概率分布,建立了关于目标位置的代价函数;其次,通过证明代价函数的凸性,利用梯度下降算法求解了回波中未知的信号参数;最后,根据最大似然估计实现了目标直接定位。仿真实验分析了所提算法的定位性能,结果表明,所提算法仅需传输相较于高精度采样(16比特为例)直接定位算法6.25%的通信带宽,同时其功耗仅为前者的0.1%。此外,与基于1比特量化的二次定位算法相比,所提算法在低信噪比下便可实现对目标位置的有效估计,并且其定位性能在低信噪比和低MIMO天线数量下均明显优于前者。同时,其性能会随着过采样技术的应用进一步提升。      

雷达数据压缩的实验研究

介绍了一种压缩雷达数据的方法,其主要编码方式是用FFT变换编码加上霍夫曼统计编码。对所采集的雷达数据按不同的量化比特进行了压缩实验,并对数据的压缩性能作了分析。分析表明,量化比特数越大,均方误差越小;而压缩比越小,压缩速度越快。用户应根据需要选取适当的量化比特数(如8位)。     

多次观测信号检测的最优量化

信号检测的最优量化是降低实现复杂度的重要手段.在有限精度量化前提下,本文推导了多次观测信号检测的检测性能(检测概率和虚警概率),并证明了实际信号环境中多次观测积累值是最佳统计量.文中进一步提出了基于遗传算法的最优量化设计方法,可方便地优化等效检测能力因子等各种检测性能指标,离线设计最优量化参数.以红外弱小目标检测多帧图像预积累为例,最优量化设计可将量化比特数由通常的8~14比特减少到2~4比特,有效降低图像处理实现复杂度.     

认知无线电中基于感知信息量化的合作频谱感知

针对认知无线电频谱感知性能的提高与传输开销的矛盾,本文提出了一种基于感知信息量化的合作频谱感知方案。该方法对各认知用户的本地检测结果采用3个判决门限进行2比特位的量化,在融合中心处对量化信息加权处理后,得到最终主用户是否存在的判决。本文详细研究了采用该方案的检测概率,虚警概率和吞吐量,理论分析和仿真结果表明,该方法在每个认知用户只增加1bit传输开销的前提下能够极大的改善检测性能,实现了检测性能与传输开销的较好权衡。     

基于低精度量化的大规模MIMO-OFDM系统可达速率的研究

该文针对大规模MIMO-OFDM系统,研究当基站端仅配备低精度模数转换器且采用最大比合并(MRC)接收算法时系统中用户可达速率的性能。通过采用加性量化噪声模型(AQNM)将非线性量化函数转化为线性量化函数,首先推导得出用户上行可达速率的闭式表达式。然后基于此表达式,将具备低精度模数转换器系统与传统具有无穷精度的模数转换器系统性能进行分析比较。最后将该文所得到的结果进行仿真分析。同时,该文还指出通过增加基站端天线数目可以弥补由于低精度模数转换器所造成的系统性能的损失。     

一种基于码本轮转的有限反馈量化方法

大规模多输入多输出（Multiple-Input Multiple-Output,MIMO）系统中随着天线数目的增加,其反馈比特数将随之大幅度增加。为此,提出了一种基于码本轮转的有限反馈量化方法。该方法中,用户在前一时刻得到最佳码字之后,在码本中选取轮转区域构成虚拟码本,判断当前时刻的信道向量,满足轮转条件的用户将虚拟码本轮转到码本起始位置,在虚拟码本中进行量化;不满足轮转条件的用户在原始码本中进行量化,选出当前时刻的最优码字。反馈比特数分析与仿真结果表明,所提方法可轮转的虚拟码本减少了量化码字的数量,从而减少了反馈比特数,是系统性能与反馈比特数的一种折中。     

量化精度对数字化接收机性能的影响

随着数字化电子侦察接收机宽带化和实时性要求的日益提高，高速高精度采样和庞大数据处理间的相互制约已经成为影响其性能的主要因素。目前，适当降低量化精度以缓和大带宽和实时处理矛盾的技术途径已经出现，但量化精度对数字化接收机综合性能的影响却并未得到系统的分析。本文通过数值仿真，分析了不同量化精度下数字化接收机的测频精度、灵敏度、动态范围等技术性能的特点，并指出了信噪比对低比特数字化接收机信号功率谱和双音动态范围的影响。     

不同量化比特数对超高清图像质量损伤的影响研究

通过挑选比较严格的超高清视频测试序列,采用x264压缩编码软件,计算量化比特数为10和8的各序列在不同码率下的PSNR值,以此来研究不同量化比特数对超高清图像质量损伤的影响。由实验数据得出,同一序列在同一码率下,量化比特数为10相比量化比特数为8的序列的PSNR高0.7 dB左右,也即量化比特数为10的序列相对量化比特数为8的序列码率节省5%~20%左右。     

Cooperative communication based access technique for sensor networks

ABSTRACT

The collaboration of users in communication systems is defined as cooperative communication. The cognitive radio, i.e. dynamic spectrum access technique, is a wireless communication technology that provides a great chance for unlicensed users to exploit the frequency bands in an opportunistic way. A wireless sensor network is a widely used communication technology composing of spatially distributed independent sensors in order to monitor physical or environmental circumstances. In this work, a new technique that unlicensed users become a cooperative relay when they are in idle mode is proposed. Along with the proposed technique, unlicensed users help sensor nodes as a cooperative relay when they are in idle mode. Similarly, sensor nodes help unlicensed users for detecting idle frequency bands while in sleep mode. By preventing any disruption that remote users may be exposed owing to signal attenuation, the proposed cooperative relay utilises amplify and forward based cooperative communication protocol. Thanks to this approach; the overall network has greater performance than wireless sensor network that does not use cooperative communication based dynamic access technique in terms of throughput, energy, and delay.     

多中继协作通信系统的中断概率分析

协作分集不仅能提升数据传输速率,还可以提高数据传输的稳定性。协作通信方法使用带有一根天线的移动台,在多用户环境中可以共享其他移动用户的天线,这样可产生多根虚拟发射天线,进而得到相应的分集增益,改善移动通信系统的性能。现在通过研究发现,在多中继协作通信中,非对称信道采用放大转发方式下的中继传输,在AF重复转发和机会转发2种方式下表现出了较小的链路中断概率,系统性能得到明显的提升。     

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。      

Cooperative relay to improve diversity in cognitive radio networks

Recent studies demonstrated that dynamic spectrum access can improve spectrum utilization significantly by allowing secondary unlicensed users to dynamically share the spectrum that is not used by the primary licensed users. Cognitive radio was proposed to promote the spectrum utilization by opportunistically exploiting the existence of spectrum ?holes.? Meanwhile, cooperative relay technology is regarded widely as a key technology for increasing transmission diversity gain in various types of wireless networks, including cognitive radio networks. In this article, we first give a brief overview of the envisioned applications of: cooperative relay technology to CRNs, cooperative transmission of primary traffic by secondary users, cooperative transmission between secondary nodes to improve spatial diversity, and cooperative relay between secondary nodes to improve spectrum diversity. As the latter is a new direction, in this article we focus on this scenario and investigate a simple wireless network, where a spectrum-rich node is selected as the relay node to improve the performance between the source and the destination. With the introduction of cooperative relay, many unique problems should be considered, especially the issue for relay selection and spectrum allocation. To demonstrate the feasibility and performance of cooperative relay for cognitive radio, a new MAC protocol was proposed and implemented in a universal software radio peripheral-based testbed. Experimental results show that the throughput of the whole system is greatly increased by exploiting the benefit of cooperative relay.     

Trust establishment in cooperative wireless relaying networks

In cooperative wireless networks, relay nodes are employed to improve the performance of the network in terms of throughput and reliability. However, the presence of malicious relay nodes in the network may severely degrade the performance of the system. When a relay node behaves maliciously, there exists a possibility that such a node refuses to cooperate when it is selected for cooperation or deliberately drops the received packets. Trust establishment is a mechanism to detect misbehaving nodes in a network. In this paper, we propose a trust establishment method for cooperative wireless networks by using Bayesian framework. In contrast with the previous schemes proposed in wireless networks, this approach takes the channel state information and the relay selection decisions into account to derive a pure trust value for each relay node. The proposed method can be applied to any cooperative system with a general relay selection policy whose decisions in each cooperative transmission are independent of the previous ones. Moreover, it does not impose additional communication overhead on the system as it uses the available information in relay selection procedure. Copyright © 2012 John Wiley & Sons, Ltd.     

Cooperative Relaying Scheme for Orthogonal Frequency and Code Division Multiple Access Uplink System

Orthogonal frequency and code division multiplexing (OFCDM) which can be extended orthogonal frequency and code division multiple access (OFCDMA) has a good performance, because it can mitigate multi path interference while maintaining high data rate. Furthermore, cooperative OFCDM relaying system provides these advantages, and this system can effectively use the limited resource with respect to data rate and each terminal’s energy efficiency. Therefore we proposed cooperative OFCDMA relaying uplink system because of the user’s requirement of the high data rate at uplink. However, a throughput performance decreases when this system employs many relays in case of each relay allocated code channel to separate each relay’s signal. Therefore, in this paper, we present that the proposed system combines cyclic delay diversity due to complement throughput performance problem. DF (Decode-and-Forward) is assumed in this paper. It is considered that the users which want to send data to BS only decode the data from other users. In addition to that, we propose the adaptive cooperative communication for the OFCDMA uplink system. The proposed adaptive system can easily select the cooperative relay.     

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.     

Analysis and determination of cooperative MAC strategies from throughput perspectives

In recent years, cooperative communication has been developed as a new communication strategy that incorporates a relay node to assist direct point-to-point transmission. By exploiting cooperative diversity, different types of techniques have been proposed to improve transmission reliability from the physical layer perspective. However, owing to the longer transmission time resulting from the cooperative schemes, there is no guarantee to enhance network throughput in view of the medium access control (MAC) performance. In this paper, system throughput of combined direct/cooperative communication is evaluated by exploiting the proposed analytical model based on the IEEE 802.11 MAC protocol. The feasibility of adopting either cooperative or direct communication is also studied in the analytical model. In terms of network throughput, whether to adopt cooperative schemes depends on the tradeoff between cooperative transmission delay and channel quality of direct communication. Moreover, two cooperative MAC protocols are proposed to determine the circumstances to activate cooperative communication according to the channel quality. The full-channel quality indicator based cooperative (FCC) MAC protocol is introduced to choose both the transmission scheme and the relay node according to the full channel quality information. However, the overhead caused by the FCC scheme can degrade the throughput performance as the number of available relays is significantly increased. Therefore, the bitwise competition based cooperative (BCC) MAC protocol is utilized to efficiently determine a feasible relay node for data transmission. Simulations are performed to validate the effectiveness of proposed analytical models and cooperative MAC protocols. It is observed that the proposed BCC scheme can outperform both the FCC protocol and conventional direct transmission with enhanced system throughput.     

Research on improving the QoE of relay users in cooperative communication systems

Customers'satisfaction with services is reflected by quality of experience (QoE). Insofar, most studies on cooperative communication have been focused on improving the QoE of source users. However, the improvement of a source user's QoE is obtained at cost of degradation of the relay user's QoE. On the other hand, cooperative communications can achieve performance similar to that of a conventional multiple-input multiple-output (MIMO) system by forming virtual M1M0 arrays. Hence, to improve the QoE performance of relay users, this article proposes the concept of a belief threshold at the destination user and a new cooperative scheme based on the belief threshold destination (BTD) technique, while without decaying the BER performance of the communication system.     

Optimal power allocation for cognitive radio networks with relay‐assisted directional transmission

In this paper, the problem of power allocation in a cooperative cognitive radio (CR) network is investigated. An optimal power allocation is proposed to maximize efficiency of the secondary network in which secondary users transmit simultaneously over a spectral band assigned to the primary users. The CR network employs directional relays to improve efficiency of the communication links and minimize interference introduced to the primary users. Unlike the conventional cooperative relay‐assisted network, the directional relays are grouped in clusters. This clustering technique along with directional transmission can significantly reduce interference to the primary links and improve the system performance. Two algorithms are also developed on the basis of the penalty method to determine unknown transmission powers. Some corroborant numerical examples are provided to illustrate quick convergence behavior of the proposed algorithms and great superiority of employing clustered directional relays in cooperative CR networks. Copyright © 2013 John Wiley & Sons, Ltd.