Performance of quasisynchronous scale time code division multiple access for AWGN channel

We analyze bit error rate performance of a recently proposed multiple access system called scale time code division multiple access (STCDMA) for quasisynchronous communication over an AWGN channel. STCDMA depends on code, time and scale orthogonality introduced by spreading sequences and wavelets. Wavelets are employed as an orthogonal set of symbols for signaling, and their orthogonality over scale and time is exploited. The channel is partitioned into different scales, and each scale into different time slots. Each user is assigned a specific scale, time slot, and spreading code. Information symbols of each user are encoded by the Haar wavelet in its scale and time slot, and then they are spread by its spreading code. Complex-valued Hadamard sequences are used as spreading sequences and conventional detector (i.e. matched filter) is used at the receiver. Results show that the performance of STCDMA gets much better than that of CDMA over the quasisynchronous AWGN channel as the number of scales increases.     

Topology-transparent reservation time division multiple access in multihop ad hoc networks

In this paper, an optimal topology-transparent reservation time division multiple access (OTTR-TDMA) protocol is proposed for multihop ad hoc networks. Our MAC protocol is based on a slotted TDMA architecture in which every slot is divided into reservation phase and data transmission phase. Every node not only transmits data in its assigned slots that are computed by Chlamtac’s algorithm, but it also utilizes free slots among its non-assigned slots through reservation. Through theoretical analysis, we derive the proper parameter for the best performance. We compare our protocol with other TDMA protocols and find that it gives better performance. Simulation results show that they are consistent with our analytical results.     

基于码分多址复用的双重加密可逆信息隐藏

针对多数密文域可逆信息隐藏算法嵌入容量小、加密算法单一的问题,提出一种双重加密的方法,并利用码分多址复用（CDMA）的思想嵌入秘密信息。加密时将图像分块,先对像素块进行多粒度置乱加密,再对块中每个像素的中间2位用流密码加密。信息嵌入采用码分多址的思想,选取k个长为4的相互正交的矩阵嵌入k层秘密信息,利用矩阵的正交性实现秘密信息多层嵌入,在提高嵌入容量的同时保证了对像素点的较小改变。对不满足嵌入条件的像素块嵌入伪比特,可避免使用位置图。拥有信息提取密钥的合法接收者可以提取秘密信息;拥有图像解密密钥可以近似恢复原始图像;拥有两种密钥既可提取秘密信息又可无损恢复原始图像。实验结果表明,512×512灰度图像Lena在峰值信噪比（PSNR）大于36 dB时最大嵌入容量133 313 bit。所提算法增强了加密图像安全性,在保证可逆性的同时大大提高密文域可逆信息隐藏嵌入容量。     

Scheduling multihop CDMA networks in the presence of secondary conflicts

This paper presents an algorithm for producing near-optimal conflict-free schedules for networks operating under code division multiple access (CDMA). A procedure for finding a lower bound on the length of such schedules is also presented. The presence of both primary and secondary conflicts (due to imperfectly orthogonal CDMA codes) are accounted for by these algorithms. The complexity of both algorithms is analyzed and computational experience with both procedures is presented. Using the lower bound, it is shown that the heuristic is effective. The complexity analysis demonstrates that it is efficient enough to use in networks of realistic size, even when the schedules must be produced in real time.This work was sponsored in part by the Naval Research Laboratory under Contract N00014-84-K-2011 and by the New York State Foundation for Science and Technology as part of its Centers for Advanced Technology Program.     

Adaptive rate coding using convolutional codes for asynchronous code division multiple access communications over slowly fading channels

This paper presents a method of code rate adaptation using punctured convolutional codes for direct sequence spread spectrum communication systems over slowly fading channels. A blind channel estimation technique is used to estimate the nature of the multi-user channel at the detector (before the decoder). The path gains obtained from the channel estimation technique are used to adapt the code rates. Punctured codes derived from a specific rate 1/2 (M = 4) mother code are used to provide error protection corresponding to the actual channel state. The upper and lower bounds on the bit error probability and the upper bound on the error event probability are derived for hard-decision and soft-decision decoding over Rayleigh and Rician fading channels. The throughput gains obtained using the adaptive scheme and the performances of the punctured codes are studied.     

基于级联信息素更新蚁群算法的多用户检测

给出了基本蚁群算法在多用户检测中的具体实现,同时针对基本蚁群算法易陷入局部最优解的缺点,提出了一种改进蚁群算法的多用户检测方法。该算法对蚁群算法的信息素更新采用串联式的多级更新策略,首先进行有选择性的信息素更新,然后引进随机扰动因子进一步修改信息素,最后采用最大门限确定信息素的范围。最终的仿真结果表明:所提出的级联信息素更新蚁群算法的多用户检测(UCP-ACO-MUD)算法具有较强的跳出局部最优解的性能,效果良好。     

CDMA系统中多用户检测技术的分析

多用户检测技术是第三代移动通信系统的关键技术,具备良好的抗多址干扰和远近效应性能。从移动通信系统抗干扰的角度出发,分析了CDMA系统中多址干扰(MAI)的产生原因,对多用户检测(MUD)技术的概念系统模型以及常用的MUD算法进行了阐述,并指出其局限性和研究方向。     

CDMA体制中的地址码选型

论文结合扩频码分多址通信的基本原理,详细讨论了m序列、Gold序列、Kasami序列以及M序列作为地址码的优缺点,并结合实际系统分析了它们的应用场合,为日益发展的军、民用移动通信系统的地址码提供选型参考。     

基于位置与标志分离的访问控制列表优化

在前期工作的LISA体系结构下,提出了IBAC模型,提供了更加精确和高效的网络访问控制,然而,IBAC增大了访问控制列表的处理开销。为此,对访问控制列表的规则组织结构进行了分析和优化,模拟实验证明了优化方法的有效性。     

基于CDMA技术的FBG传感系统研究

基于码分多址(CDMA)技术,采用格雷互补序列对光源进行调制,使用相关运算对接收信号进行解调.该方法具有传感器间串音小和系统信噪比高等优点.基于该方法搭建了光纤Bragg光栅(FBG)传感实验平台,对其温度传感特性进行了实验研究.结果表明:系统具有温度响应线性度好和串音低的优点.     

Quality of service estimation for soft handoff region ratio and call admission control in CDMA cellular systems

In the operational stage of code division multiple access (CDMA) systems, the soft handoff region ratio (SHRR) is among the important parameters that guarantee the quality of service (QoS) and resource allocation. This paper analytically investigates the effects of the SHRR on QoS. In doing so, we simultaneously take into account the capacity increasing factor and traffic load variation. For this purpose, we propose a new traffic load estimation model using a semi-Markov chain in order to consider the call arrivals influenced by the SHRR under general resident time condition. We also present the calculation algorithm of the QoS measures such as the new call blocking probability (NBP) and forced termination probability (FTP) by applying the proposed model to the prioritized call admission control (CAC). The numerical result illustrates the effects of various soft handoff system parameters. Likewise, it shows that the value of SHRR has a significant impact of the QoS under the normal propagation conditions. Sensitivity analysis is also performed to show the effect of exponential cell resident time assumption. The proposed modeling is expected to provide a practical guide to improve the QoS of CDMA soft handoff systems.     

基于多符号检测和Turbo乘积码的PCM/FM遥测系统性能

多符号检测(MSD)和Turbo乘积码(TPC)技术联合应用可以大幅提高脉冲编码调制/调频(PCM/FM)遥测系统性能。针对MSD算法计算复杂度高的问题提出了一种改进的MSD算法,可以有效降低计算复杂度;在TPC的传统Chase译码算法中通过简化软输入信息计算可以降低系统存储量。仿真结果表明,改进方法和传统的两种技术联合使用相比,虽然损失了约1.7dB解调增益,但仍提高PCM/FM信号解调性能约8dB,且算法复杂度低,存储量小,更适合硬件实现。     

基于LEAP神经网络的同步DS-CDMA伪码序列盲估计*

针对特征分解方法在实现非等功率同步直接序列码分多址（DS-CDMA）信号伪码序列盲估计时存在的处理数据向量不能太长以及不能工作于非平稳环境中的问题,引入了一种由主分量分析实现自适应特征提取的在线无监督学习（LEAP） 神经网络（NN）。首先将已分段的一周期DS-CDMA信号作为NN的输入信号,用NN各权值向量的符号函数代表DS-CDMA信号各用户的伪码序列,然后通过不断输入信号来反复训练权值向量直至收敛,最终DS-CDMA信号各用户的伪码序列就可以通过各权值向量的符号函数重建出来。此外,采用变步长以提高收敛速度。理论分析与仿真实验表明,LEAP NN至少可以实现-20dB信噪比下10个用户的非等功率同步DS-CDMA伪码序列盲估计,并且比传统的Sanger NN具有更快的收敛速度。     

使用混沌扩频序列增强DS/SS系统性能

基于Logistic映射,利用交织器设计扩频序列发生器,其产生的混沌序列具有良好的相关特性和旁瓣特性,并且是不必寻优而自身平衡的。将这种序列作为扩频序列应用于直接序列扩频系统中,能够有效地减少多址干扰,提高系统的容量,增强系统性能,信息也能够被加密。将这种序列和传统的扩频序列以及普通的混沌序列应用于扩频系统,仿真结果表明,设计的混沌序列性能优于其他的扩频序列。     

大规模无线网络中(X,S)-Bottleneck 区域性能分析

基于二层QBDs(quasi birth-death models),提出了一个(X,S)-bottleneck区域性能分析模型,(X,S)-bottleneck区域的特征通过第1层QBD模型描述,而经过(X,S)-bottleneck区域的每条流的特征由第2层QBD模型描述.基于该性能分析模型,对(X,S)-bottleneck区域的性能进行了一些概率分析,如(X,S)-bottleneck区域的平衡状态概率、经过该区域通信流的成功转发概率和吞吐量等,确定了上述概率及吞吐量的取值范围,仿真实验结果表明了该性能分析模型对于分析(X,S)-bottleneck区域性能的有效性.结论有助于无线网络通信性能的改进和管理.     

Preparation of PCLT/P(VDF-TrFE) pyroelectric sensor based on plastic film substrate

Nanosized lead titanate doped with calcium and lanthanum (PCLT) powder obtained by the sol–gel method was homogeneously mixed with vinylidene fluoride-trifluoroethylene [P(VDF-TrFE)]. The nanocomposite PCLT/P(VDF-TrFE) film was used as the sensing film for pyroelectric sensors, whose detective merit was about 22.4% higher than that of sensors using pure P(VDF-TrFE). A 35 nm ITO film was deposited as the bottom electrode on PET plastic film substrate, on which PCLT/P(VDF-TrFE) was prepared by spin-coating, and Ni–Cr film was chosen as the upper electrode. The PET plastic film substrate could effectively decrease the thermal conductivity of the element and the ITO bottom electrode could reflect the infrared irradiation. The voltage responsivity was increased and the thermal fluctuation noise of the pyroelectric element was decreased significantly. The experimental results indicated that the specific detectivity of PCLT/P(VDF-TrFE) pyroelectric sensors based on PET film substrate reached 3.4E7 cm Hz^1/2 W⁻¹, more than two orders of magnitude higher than that of the sensors on bulk silicon substrate formed under the same conditions.     

Surface characterisation of the Dome Concordia area (Antarctica) as a potential satellite calibration site, using Spot 4/Vegetation instrument

A good calibration of satellite sensors is necessary to derive reliable quantitative measurements of the surface parameters or to compare data obtained from different sensors. In this study, the snow surface of the high plateau of the East Antarctic ice sheet, particularly the Dome C area (75°S, 123°E), is used first to test the quality of this site as a ground calibration target and then to determine the inter-annual drift in the sensitivity of the VEGETATION sensor, onboard the SPOT4 satellite. Dome C area has many good calibration site characteristics: The site is very flat and extremely homogeneous (only snow), there is little wind and a very small snow accumulation rate and therefore a small temporal variability, the elevation is 3200 m and the atmosphere is very clear most of the time. Finally, due to its location, it is frequently within view of many satellites. VEGETATION visible blue channel data (0.43-0.47 μm) of a 716×716 km² area centred on the French-Italian Dome Concordia station, during the 1998-1999, 1999-2000, 2001-2001, and 2001-2002 austral summers were cloud masked and atmospherically corrected. The snow surface Bidirectional Reflectance Distribution Function is very high with little spatial and seasonal variability, which is a major advantage for sensor calibration. The inter-annual variation is found to be very small, proving that the stability of the site is very good.