Variable Persistence Sensing Schemes for Opportunistic Spectrum Access

Cognition technologies can significantly enhance spectrum utilization through secondary usage. Secondary users （SUs） search for spectrum holes which are not used by Primary users （PUs） and communicate through them. The design and analysis of simple opportunistic spectrum access schemes for maximizing utilization efficiency and minimizing delay of SUs under limited sensing bandwidth and energy is still at infancy. In this work, we consider PU＇s frequency occupancy patterns and propose three simple variable persistence sensing schemes for maximum exploitation of idle spectrum chunks by SUs, and perform their theoretical analysis. Numerical case studies are presented to verify the theoretical analysis and illustrate the performance of the schemes proposed.     

Performance Analysis of the IEEE 802.15.3 CSMA/CA in Unsaturated Traffic Conditions

In IEEE 802.15.3 the traffic communicated by Carrier sense multi access/Collision avoidance （CSiVIA/CA） mechanism are unsaturated in most cases that are different in IEEE 802.11. This paper presented an extension analytical model based on the Bianchi model in IEEE 802.11 considering unsaturated traffic conditions in error-prone channel. By using this model we analyzed the throughput, frame discard probability and average frame delay performance of IEEE 802.15.3 CSMA/CA. This model is validated through extensive simulation results. Effects of unsaturated situation and channel error condition on the performance were analyzed.     

接入与回传一体化小基站的接入控制与资源分配联合优化算法

针对全双工无线接入与回传一体化小基站场景下长期的频谱效率和能效同时最大化问题,该文提出一种基于近似动态规划理论的接入与回传一体化小基站接入控制与资源分配联合优化算法。该算法首先联合考虑当前基站的资源使用和功率配置情况,在任一用户需求动态到达以及平均时延、小基站回传速率和传输功率约束下,使用受限马尔科夫决策过程(CMDP)建立频谱效率最大化和功率消耗最小化的多目标优化模型,其次运用切比雪夫理论将多目标优化问题转化为单目标问题,并使用拉格朗日对偶分解法进一步转化为非受限的马尔科夫决策过程(MDP)问题。最后,为了解决其求解时存在的“维度灾”爆炸问题,该文提出基于近似动态规划的无线接入与回传一体化小基站资源动态分配算法进行求解,得到此时的接入与资源分配策略。仿真结果表明,所提算法能在保证平均时延约束、小基站回传速率约束和传输功率约束的同时最大化长期平均频谱效率和能效。     

TD-SCDMA： Spectrum Modeling, Experimental Verification of Power Amplifier Nonlinearity

RF Power amplifiers （PA） are critical components in Time division-Synchronous code division multiple access （TD-SCDMA） systems, and PA nonlinearity is one of the main concerns in RF power amplifier designs. This paper presents experimental verification of the spectrum modeling of a RF power amplifier in TD-SCDMA system based on our previous work. The results verify the theoretical spectrum model we derived closely fits the experimental measurements.     

A Decision-Theoretic Framework for Opportunistic Spectrum Access

Built on a hierarchical access structure with primary and secondary users, opportunistic spectrum access improves spectrum efficiency while maintaining compatibility with legacy wireless systems. The basic idea is to allow secondary users to exploit instantaneous spectrum availability while limiting the interference to primary users. In this article, we identify basic components, fundamental trade-offs, and practical constraints in opportunistic spectrum access. We introduce a decision-theoretic framework based on the theory of partially observable Markov decision processes. This framework allows us to systematically tackle the optimal integrated design and quantitatively characterize the interaction between signal processing for opportunity identification and networking for opportunity exploitation. A discussion of open problems, potential applications, and recent developments is also provided.     

An Effective Gated Scheduling with Pre-defined QoS Cross-layer Design for Wireless Networks

The proposed M-gated scheduling algorithm rules the server in wireless networks stays with a station for at most M times gated services. By the method of embedded Markov chain and probability generating function we developed a model with vacations and acquired the theoretical value of average queue length and packet delay. Like E-limlted scheduling algorithm, its performance is close to that of the optimal polling scheme, exhaustive scheduling, but the inherent unfairness problem for the latter is avoided. And it also does not require the server to know the status of stations, queues information. Compared with E-limited, M-gated has less jitter and is more robust. We then use the derived quality of service parameter as the basis for cross-layer design. Simulations results show the network is more stable and can keep the queue length and the packets delay under QoS level. When putting E- limited and M-gated scheduling in the same cross-layer design scheme, the latter has less overhead especially when the system is heavily loaded.     

Joint Design and Separation Principle for Opportunistic Spectrum Access in the Presence of Sensing Errors

Opportunistic spectrum access (OSA) that allows secondary users to independently search for and exploit instantaneous spectrum availability is considered. The design objective is to maximize the throughput of a secondary user while limiting the probability of colliding with primary users. Integrated in the joint design are three basic components: a spectrum sensor that identifies spectrum opportunities, a sensing strategy that determines which channels in the spectrum to sense, and an access strategy that decides whether to access based on potentially erroneous sensing outcomes. This joint design is formulated as a constrained partially observable Markov decision process (POMDP), and a separation principle is established. The separation principle reveals the optimality of myopic policies for the design of the spectrum sensor and the access strategy, leading to closed-form optimal solutions. Furthermore, it decouples the design of the sensing strategy from that of the spectrum sensor and the access strategy, and reduces the constrained POMDP to an unconstrained one. Numerical examples are provided to study the tradeoff between sensing time and transmission time, the interaction between the physical layer spectrum sensor and the MAC layer sensing and access strategies, and the robustness of the ensuing design to model mismatch.     

非理想信道检测的认知无线电网络中能量感知频谱接入

Due to the problem of spectrum underutilization and energy inefficiency in wireless communications, the research on energy efficient Cognitive Radio Networks (CRNs) has received significant attention in both industry and academia. In this paper, we consider the problem of optimal spectrum selection and transmission parameters design with the objective of minimizing energy consumption in CRNs. Since the system state cannot be directly observed due to miss detections and estimation errors, we formulate the optimal spectrum access problem as a Partially Observable Markov Decision Process (POMDP). In particular, the proposed scheme selects the optimal spectrum, modulation and coding scheme, transmission power, and link layer frame size in each time slot according to the belief state, which captures all the history information of past actions and observations. The optimal policy can be acquired by solving POMDP problem with linear programming based algorithm. Simulation results show that significant energy savings can be achieved by the proposed scheme.     

认知无线电网络的MAC层关键技术

认知无线电作为一种智能的频谱共享技术,已成为无线通信领域的研究热点。为达到在不干扰授权用户的条件下有效地实现机会式频谱利用,认知无线电网络的媒体接入控制（MAC）层不仅需要提供传统的服务,还要求能支持一套全新的功能。频谱检测管理通过对检测模式的选取、检测周期及检测时长的设置、检测信道的选取和检测静默期的设置等实现检测策略和参数的选取及优化。接入控制主要采用与授权用户协调接入和透明接入两种方式避免与授权用户的接入产生碰撞。动态频谱分配针对二进制干扰模型和累积干扰模型进行不确定频谱资源的优化分配。安全机制通过增加MAC帧的认证和保密以防御MAC层的安全攻击。跨层设计结合物理层和网络层、传输层等上层信息设计和实现全局优化的MAC层技术。     

基于预留最优信道机制的CR动态频谱接入研究

在认知无线电网络中,如何提高次用户的性能是当前研究的热点。针对等级覆盖式频谱接入模型进行研究分析,考虑了动态频谱切换过程,建立三维的持续时间马尔可夫链模型,计算出切换概率、阻塞概率、掉线概率,并综合分析各个参数对这些概率的影响。同时提出为主用户预留最优信道数的方案,数值结果表明此方案能有效降低次用户的切换概率和掉线概率,使次系统的切换概率、阻塞概率、掉线概率更加均衡,且提高了次用户系统的接入容量。     

The Euclidean Distance Spectra of FDMA-CPM： Algorithms and Applications

The performance of multiuser Continuous phase modulation（CPM） over the additive white Gaussian noise channel with coherent maximum likelihood detection is considered. Algorithms are developed to calculate the Euclidean distance spectra employing tree-search and Astar algorithm. The complexity of proposed algorithms are further reduced using trellis minimization. The distance spectrum is then used to evaluate the performance of mul- tiuser CPM systems, which reveals that the performance of multiuser CPM can be significantly improved by using op- timized parameters. Both equally-powered and non-equally powered systems are considered. Numerical and simulated results confirm that the proposed algorithms can generate the distance spectra of all systems with lower complexity relative to previous methods and are particularly suited for multi user CPM systems.     

三通道有线接入网的构建

在已有的双向HFC和EPON+EoC接入网基础上,提出了三通道有线接入网络的概念.分析了三通道有线接入网的频谱规划、结构形式、关键设备.并描述了多种业务在三通道有线接入网上的应用.     

基于ATM的 ADSL局端接入系统的设计

本文介绍了所设计的基于ATM的ADSL局端接入系统，阐述了其功能组成和工作过程，并对ATU－C电路进行了全面分析。     

On Policy Evolution in Centralized Secure Interoperation of Multiple Domains

Many works have addressed secure inter-operation in multi-domain environments in a centralized way. In these works, there is a global policy integrated or composed by a mediator for mediation of inter-domain resource access. During policy evolution, the global secure policy is recomposed or reintegrated, which is not appropriate for the dynamic environments where policy evolutes frequently. We present a scheme for policy evolution in centralized secure interoperation. In our scheme, policy evolution is gradual; that is, the global policy changes partly and therefore need not be reformed wholly. Besides, for inter-domain resource accesses to be totally mediated in the local domains, the part of global policy for the mediation of inter-domain access of each domain＇s resources is isolated and copied to the domain; we make the isolated policies updated gradually upon gradual policy evolution too. Our scheme makes centralized interoperation appropriate for the dynamic environments.     

Modified 802.11‐Based Opportunistic Spectrum Access in Cognitive Radio Networks

In this letter, a modified 802.11‐based opportunistic spectrum access is proposed for single‐channel cognitive radio networks where primary users operate on a slot‐by‐slot basis. In our opportunistic spectrum access, control frames are used to reduce the slot‐boundary impact and achieve channel reservation to improve throughput of secondary users. An absorbing Markov chain model is used to analyze the throughput of secondary users. Simulation results show that the analysis accurately predicts the saturation throughput.     

Performance Analysis of WLAN Medium Access Control Protocols in Simulcast Radio-Over-Fiber-Based Distributed Antenna Systems

The performance of three wireless local-area network（WLAN） media access control（MAC） protocols is investigated and compared in the context of simulcast radioover-fiber-based distributed antenna systems（RoF-DASs） where multiple remote antenna units（RAUs） are connected to one access point（AP） with different-length fiber links.The three WLAN MAC protocols under investigation are distributed coordination function（DCF） in basic access mode,DCF in request/clear to send（RTS/CTS） exchange mode,and point coordination function（PCF）.In the analysis,the inter-RAU hidden nodes problems and fiber-length difference effect are both taken into account.Results show that adaptive PCF mechanism has better throughput performances than the other two DCF modes,especially when the inserted fiber length is short.     

An Optimal Weight Method for CT Image Denoising

This paper proposes a novel exemplar- based method for reducing noise in computed tomography （CT） images. In the proposed method, denoising is performed on each block with the help of a given database of standard image blocks. For each noisy block, its denoised version is the best sparse positive linear combination of the blocks in the database. We formulate the problem as a constrained optimization problem such that the solution is the denoised block. Experimental results demonstrate the good performance of the proposed method over current state-of-the-art denoising methods, in terms of both objective and subjective evaluations.     

State Estimation for Linear Constrained Systems with Unknown Input

This paper deals with the filtering problem for linear discrete constrained dynamic systems with unknown input. The constraint matrix and constraint vector in this system are allowed to vary in the value and in the dimension. The original full state is separated into two parts, and the estimate of the state is reduced to find the optimal estimate of a singular system. The estimable condition is researched and a recursive estimator for the original full state is presented. A rigorous mathematical induction is given to compare the performance of our approach to that of the existing method without constraint. A numerical example is presented to demonstrate the effectiveness of the new method.     

基于竞争双深度Q网络的频谱感知和接入

认知用户通过频谱感知和接入过程识别频谱状态并占用空闲频谱,可有效利用频谱资源。针对频谱感知中存在感知错误和频谱接入中存在用户碰撞的问题,首先建立多用户多信道模型,设计频谱感知和频谱接入过程;然后通过结合双深度Q网络和竞争Q网络,设计竞争双深度Q网络,解决过估计问题的同时优化网络结构;最后通过智能体与所设计模型中状态、观测、回报和策略的交互,完成使用竞争双深度Q网络解决频谱感知和接入问题的一体化研究。仿真结果表明,相比于已有深度强化学习方法,使用竞争双深度Q网络得到的数值结果更稳定且感知正确率和信道利用率都提高了4%。     

Quick System-Level DDR3 Signal Integrity Simulation Research

Abstract---Double data rate synchronous dynamic random access memory （DDR3） has become one of the most mainstream applications in current server and computer systems. In order to quickly set up a system-level signal integrity （SI） simulation flow for the DDR3 interface, two system-level SI simulation methodologies, which are board-level S-parameter extraction in the frequency-domain and system-level simulation assumptions in the time domain, are introduced in this paper. By comparing the flow of Speed2000 and PowerSI/Hspice, PowerSI is chosen for the printed circuit board （PCB） board-level S-parameter extraction, while Tektronix oscilloscope （TDS7404） is used for the DDR3 waveform measurement. The lab measurement shows good agreement between simulation and measurement. The study shows that the combination of PowerSI and Hspice is recommended for quick system-level DDR3 SI simulation.