Randomized Broadcast Channel Access Algorithms for Ad Hoc Networks

The problem of broadcast channel access in single-hop and multihop ad hoc networks is considered. Two novel randomized and distributed channel access algorithms are developed and analyzed for single-hop and multihop networks, respectively. These algorithms are designed based on maximizing the worst-case channel efficiency, by optimizing some key parameters, including the backoff probability distribution and the slot length. The proposed algorithm for single-hop networks offers significantly higher throughput than the CSMA methods when the traffic load is heavy, while still achieving good performance when the load is light or medium. The proposed algorithm for multihop networks can flexibly adapt to the traffic load, and offers much better throughput performance than the existing broadcast scheduling algorithms in light or medium load.     

Random Network Coding Based Schemes for Perfect Wireless Packet Retransmission Problems in Multiple Channel Networks

Solving WPRTPs (wireless packet retransmission problems) using NC (network coding) is increasingly attracting research efforts. However, no work on NC based schemes for WPRTPs in MCNs (multiple channel networks) has been found. In this paper, RNC (random network coding) based schemes for P-WPRTPs (perfect WPRTPs) in MCNs, denoted as MC-P-WPRTPs (multiple channel perfect WPRTPs), are studied by transforming MC-P-WPRTPs into ILP (integer linear programming) problems. The ILP problems corresponding to MC-P-WPRTPs with four typical configurations are derived. Then the corresponding packet retransmission schedule schemes for MC-P-WPRTPs are proposed based on the solutions to the ILP problems and random network coding. To solve the ILP problems efficiently, an algorithm named as progressively fixing algorithm is proposed which recursively reduce the size of the ILP problem by fixing some of the variables according to some criteria. The criteria are related to the solution to the LP problem obtained by relaxing the integral constraints on the variables in the original ILP problem. Simulation results show that the NC based schemes for MC-P-WPRTPs are effective in saving packet retransmissions. In some situations, NC based schemes can save about 50 % packet retransmissions.     

A Sequential Approach for Optimal Broadcast Scheduling in Packet Radio Networks

An important problem that arises in the design of packet radio networks is that of scheduling access to the high speed communications channel in such a way as to avoid interference while keeping the frame length to a minimum. The broadcast scheduling problem is known to be NP-hard and to date, this problem has been formulated as a nonlinear discrete optimization problem for a given frame length, and solved via heuristic approaches by parametrically varying the length of the frame. This paper presents a linear integer programming formulation for the composite problem of maximizing channel utilization while minimizing the length of the frame. It then introduces a solution approach based on solving two relatively easier (though still NP-complete) integer programs in succession. Computational experiments are conducted on a set of benchmark cases and additional randomly generated problem instances. Results show that this sequential integer programming approach is very effective, solving all the problems optimally within a few seconds. These results imply that optimal solutions can be identified in very little time for problems of realistic size, and that heuristic approaches will be needed only when problems get much larger than those considered in the literature to date.     

Modeling Queueing and Channel Access Delay in Unsaturated IEEE 802.11 Random Access MAC Based Wireless Networks

In this paper, we present an analytic model for evaluating the queueing delays and channel access times at nodes in wireless networks using the IEEE 802.11 Distributed Coordination Function (DCF) as the MAC protocol. The model can account for arbitrary arrival patterns, packet size distributions and number of nodes. Our model gives closed form expressions for obtaining the delay and queue length characteristics and models each node as a discrete time $G/G/1$ queue. The service time distribution for the queues is derived by accounting for a number of factors including the channel access delay due to the shared medium, impact of packet collisions, the resulting backoffs as well as the packet size distribution. The model is also extended for ongoing proposals under consideration for 802.11e wherein a number of packets may be transmitted in a burst once the channel is accessed. Our analytical results are verified through extensive simulations. The results of our model can also be used for providing probabilistic quality of service guarantees and determining the number of nodes that can be accommodated while satisfying a given delay constraint.      

A NEW RANDOM PACKET ACCESS PROTOCOL IN WCDMA

In this paper, a new Random Packet Access Protocol (RPAP) is proposed in WCDMA systems. The new proposed RPAP can efficiently prevent unnecessary interference by stopping a transmission if it is bound to be collided with others. Throughput about the new RPAP is deliberated and analyzed. Computer simulation shows that this protocol has better throughput performance compared with conventional one currently used in WCDMA.     

Packet Switching in a Multiaccess Broadcast Channel: Performance Evaluation

In this paper, the rationale and some advantages for multiaccess broadcast packet communication using satellite and ground radio channels are discussed. A mathematical model is formulated for a "slotted ALOHA" random access system. Using this model, a theory is put forth which gives a coherent qualitative interpretation of the system stability behavior which leads to the definition of a stability measure. Quantitative estimates for the relative instability of unstable channels are obtained. Numerical results are shown illustrating the trading relations among channel stability, throughput, and delay. These results provide tools for the performance evaluation and design of an uncontrolled slotted ALOHA system. Adaptive channel control schemes are studied in a companion paper.     

Device-to-Device Assisted Downlink Broadcast Channel in Cellular Networks

In this paper, two protocols of device-to-device (D2D) communication are proposed to assist multi-input multi-output downlink broadcast channel in cellular networks. Devices in a D2D group are arranged to extract their own data originated from a base station (BS) in an order according to their decoding performance. At each intermediate hop, a best-decoding node is selected and responsible for the next hop transmission. In the first protocol, the selected user forwards its own data stream after decoding, then a proper interference cancellation is performed at each receiving node to eliminate this interference from the buffered reception of the broadcast channel in the first hop from the BS. A closed form representation of the outage probability of each node is derived. In the second protocol, the selected user re-constructs its own signal and cancels this signal from its received vector and forwards the resultant vector. This forwarded signal contains less interference and preserves the data information for the users who have not yet decoded their data. The receiving nodes decode their data streams by jointly considering the received vectors from all the previous hops. The comparison of the two proposed protocols in terms of computational complexity, achieved diversity order and performance are provided. We show that the proposed protocols provide promising diversity gain and avoid error propagations as compared to traditional schemes.     

用于MIMO广播信道的带多波束选择的正交随机波束成形算法

本文提出一种基于有限反馈的波束和用户选择方案,在这种方案中,用户端利用一个随机正交码本对其信道方向信息(CDI)进行量化,计算其最大信号与干扰加噪声功率比(SINR),并把这些信息反馈给基站;基站根据接收到的这些反馈信息,按照和容量最大的准则选择出多个正交波束以及相应的多个用户.和sharif等人最近提出的方案相比,我们提出的方案能根据系统参数,如用户数和信噪比(SNR),对选择的波束成形矢量及其对应的用户的数量和集合进行调整,当用户数量较小时,和容量性能得到了很大的提升,同时避免了选择波束成形矢量时的用户冲突,另外,基站也不需要广播波束成形矢量给各个用户.     

Improving TDMA Channel Utilization in Random Access Cognitive Radio Networks by Exploiting Slotted CSMA

This paper aims to improve the spectrum efficiency of the licensed time division multiple access (TDMA) channel by exploiting the unused periods of primary users (PUs) in cognitive radio networks. A wireless network that consists of two classes of users, PUs and CR users, accessing a time slotted based common communication channel is considered. PUs employ TDMA and have always high priority over the CR users to access the channel. CR users utilize slotted Carrier Sense Multiple Access and can access the channel when it is not occupied by the PUs. New expressions for the throughput of both CR network and overall network have been derived in order to evaluate the channel utilization. Besides, an example network have been developed, modeled and simulated by using the OPNET Modeler simulation software with the purpose of verifying the analytical throughput results. The simulation results obtained under various network load conditions are consistent with the analytical results. This study has also proposed that the overall channel utilization can be improved by well exploiting the spectrum holes without interfering with the PU transmissions.     

Packet Switching in a Multiaccess Broadcast Channel: Dynamic Control Procedures

In a companion paper [1], the rationale for multiaccess broadcast packet communication using satellite and ground radio channels has been discussed. Analytic tools for the performance evaluation and design of uncontrolled slotted ALOHA systems have been presented. In this paper, a Markovian decision model is formulated for the dynamic control of unstable slotted ALOHA systems and optimum decision rules are found. Numerical results on the performance of controlled channels are shown for three specific dynamic channel control procedures. Several practical control schemes are also proposed and their performance compared through simulation. These dynamic control procedures have been found to be not only capable of preventing channel saturation for unstable channels but also capable of achieving a throughput-delay channel performance close to the theoretical optimum.     

A Multibeam Packet Satellite Using Random Access Techniques

The performance of a multibeam packet satellite using ALOHA type random access techniques is studied. The satellite provides either no buffer or an infinite buffer for successful packets on the uplink channels. A TDMA frame is organized in accordance with the traffic demand from each area. System throughput and packet delay have been successfully obtained. Aiming at improving the performance of the system, three different protocols are introduced and studied in the zero buffer case. It is demonstrated, through examples, that satisfactory system performance can be obtained using an appropriate protocol. However, system performance can also be severely degraded if the transition time for the satellite transponder to switch from one zone to another is not negligible. In this case, the performance can be restored via the use of a buffer of sufficiently large size.     

光接入网络中的数字信号处理技术

采用先进的数字信号处理(DSP)技术,在发射机和接收机分别引入预处理和后处理,以提高光接入网络的频谱效率并延长传输距离。研究了一种基于光超奈奎斯特(Super-Nyquist)滤波的类9状态正交振幅调制(9QAM)信号多模均衡(MMEQ)后端DSP算法,使用这种方案,能够有效提高频谱效率,实现了频谱效率高达4 bps/Hz的正交移相键控(QPSK)信号传输;还研究了一种基于数字SuperNyquist信号前端预处理的方案,此方案的优点是不需要光预滤波即可达到相同的频谱效率。使用一个采用直接调制激光器(DML)、直接探测和数字均衡技术的高速无载波幅度相位-64状态正交振幅调制(CAP-64QAM)系统,在20 km标准单模光纤(SSMF)上实现了创纪录的60 Gbit/s CAP-64QAM信号传输;使用相干探测,实现了速率高达100 Gbit/s的64状态正交振幅调制-正交频分复用(16QAM-OFDM)实时传输系统,解决了实时OFDM信号处理中的关键问题。     

Digital Signal Processing for Optical Access Networks

In this paper, we investigate advanced digital signal process ing （DSP） at the transmitter and receiver side for signal pre equalization and postequalization in order to improve spec trum efficiency （SE） and transmission distance in an optical access network. A novel DSP scheme for this optical super Nyquist filtering 9 Quadrature Amplitude Modulation （9 QAM） like signals based on muhimodulus equalization with out post filtering is proposed. This scheme recovers the Ny quist filtered Quadrature PhaseShift Keying （QPSK） signal to a 9QAMlike one. With this technique, SE can be increased to 4 b/s/Hz for QPSK signals. A novel digital superNyquist signal generation scheme is also proposed to further suppress the Nyquist signal bandwidth and reduce channel crosstalk without the need for optical prefiltering. Only optical cou plers are needed for superNyquist wavelengthdivisionmulti plexing （WDM） channel multiplexing. We extend the DSP for shorthaul optical transmission networks by using highorder QAMs. We propose a highspeed Can＇ierless Amplitude/Phase 64 QAM （CAP64 QAM） system using directly modulated la ser （DML） based on direct detection and digital equalization. Decisiondirected least mean square is used to equalize the CAP64QAM. Using this scheme, we generate and transmit up to 60 Gbit/s CAP64QAM over 20 km standard single mode fiber based on the DML and direct detection. Finally, several key problems are solved for real time orthogonalfre quencydivisionmultiplexing （OFDM） signal transmission aml processing. With coherent detection, up to 100 Glfit/s 16 QAMOFDM realtime transmission is possible.     

TD-SCDMA随机接入信道的分析

给出了TD-SCDMA(时分双工同步码分多址)系统中一种新的随机接入方案,对该方案进行了理论分析 和计算机仿真,得到了随机接入信道吞吐率和时延公式,并将其数值计算结果与计算机仿真结果以及原方案计算 机仿真结果进行了对比。     

中继蜂窝网络中增强型随机接入算法

提出一种适用于蜂窝中继网络的增强型随机接入算法。为了保证不同用户的QoS需求,实时业务通过提前预约随机接入信道(Random Access Channel,RACH)实现无冲突的接入,而非实时业务则利用中继节点通过共享剩余的随机接入信道实现两跳接入。为了解决非实时业务在时域接入发生碰撞的问题,中继节点根据当前的非实时业务到达状况和剩余的RACH信道数目,综合地确定所采取的退避策略以及最佳频域退避窗口大小。理论分析和实验仿真表明,该算法能更好地保证非实时业务的接入过程,进一步提高了接入资源的利用率。     

Optimal Relaying Beamforming in Multiple Access Broadcast Channel (MABC) Bidirectional Cognitive Radio Networks in Presence of Interferers

In this paper, a general cognitive radio system consisting of a set of users with different level of spectrum access including two primary transceivers and several types of secondary users is considered. It is assumed that two secondary users operate based on an underlay model at the same frequency bandwidth and at the same time as the primary users based on a multiple access broadcast channel bidirectional beamforming scheme. Other secondary users provide a relaying service to the primary users in exchange for the opportunity to send their messages towards their own destinations for a fixed portion of the communication cycle. In addition, it is assumed that some interferers are active during the communication cycle and cause interference for the network. Furthermore, it is assumed that only partial channel state information (CSI) between interferers and other nodes in the network is available. We provide a robust optimization method against imperfection on the interferers’ CSI to maximize the joint primary and secondary signal-to-interference-plus-noise-ratio with the assumption of limited available power at the secondary relays. An amplify-and-forward relaying scheme is deployed at the secondary relays and the optimal beamforming is obtained using second order convex programming method. The simulation results show the performance of the proposed beamforming method against the existence of interferers, and demonstrate the effectiveness of our robust method against uncertainty in knowledge of interferers’ CSIs.     

TETRA分组数据业务随机接入延迟计算

数字集群系统的参数设计和网络负荷对分组数据业务的延迟具有很大的影响。首先详细分析了陆地集群无线通信(TETRA)分组数据业务在空中接口的发送过程,然后分析了TETRA协议的随机接入算法,最后对分组数据采用离散接入帧进行随机接入的延迟进行了推导,得到了随机接入延迟与相关参数的关系表达式,并对接入延迟与参数的关系进行了分析,对TETRA集群系统的设计具有指导意义。     

多媒体多播组播单频网中免干扰动态信道分配

为了避免多媒体多播组播单频网(MBSFN)区域内部和区域之间的干扰,进一步提高频谱效率,该文提出一种改进的基于噪声调节时滞噪声混沌神经网络(NHNCNN)的动态信道分配方法。首先,根据MBSFN区域的特殊拓扑结构,重新定义了4种电磁兼容限制函数,在此基础上精心构建了免干扰的NHNCNN能量函数。其次对NHNCNN的稳态判定进程加以改进以提高系统的收敛速度。特别地,采用类二分法联合NHNCNN去搜索最小信道分配总数。仿真结果表明,利用富足的NHNCNN时滞、噪声和混沌神经动力,所提算法能有效地搜索到合理解,并最终找到全局最优解,提高了频谱效率。与现有方法相比,所提算法能够实现更好的收敛速度和合理解率。