TD-SCDMA系统所采用的技术(二)

３综合采用各种多址方式TD-SCDMA系统综合使用CDMA、TDMA、FDMA和SDMA四种多址方式，充分发挥码分、时分、频分和空分制式技术优势，最大限度地提高系统容量。TD-SCDMA多址技术原理如图1所示，其三维空间分别为CDMA码道、频率和时间。图1是典型的TD-SCDMA信道结构，三个载波带宽为1.6MHz，用TDMA方式将每个载波分为7个业务时隙（5个下行时隙DL和2个上行时隙UL）和1个DL/UL隔离时隙，采用CDMA技术将每个时隙扩频，最多可得16码道（即扩频系数为16），每个码道可传输一个用户信息。TD-SCDMA的基本信元（RU）由时…     

分组自适应分配时隙的RFID防碰撞算法研究

为了解决射频识别(Radio Frequency IDentification,RFID)系统中的多标签防碰撞问题,在分析帧时隙ALOHA算法的基础上,提出一种基于分组自适应分配时隙的RFID防碰撞算法(GAAS).首先让阅读器对标签随机所选的时隙进行扫描统计,并将其发送给每一个标签,标签再进行相应地时隙调整,使阅读器跳过空闲时隙和碰撞时隙,自适应地分配有效时隙,进而对标签进行快速识别.当未识别标签数比较大时,算法采用分组以及动态调整帧长等策略,以减少时隙处理的时间.仿真结果表明:GAAS算法提高了系统的识别效率和稳定性,降低了传输开销.特别是当标签数超过1000时,该算法的吞吐率仍保持在71%以上,比传统的帧时隙ALOHA-256算法和分组动态帧时隙ALOHA算法的系统效率分别提高了300%和97.2%.     

无线电通信

0609442多通道时隙式1-坚持CSMA多址通信系统协议分析〔刊,中〕//计算机工程与应用.—2005,41(35).—106-117(D2)本部分报道了“网络、通信与安全”栏目的3篇文章,内容涉及多通道时隙式1-坚持CSMA多址通信系统协议分析(作者:唐芸,云南大学通信工程系);基于免疫原理的宏病毒防护模型的研究(作者:李媛圆,郑州信息工程大学信息工程学院);Client Puzzle协议在防御资源耗尽型DoS攻击中的应用(作者:邱科宁,广州仲恺农业技术学院计算机与电子工程学院)。06094432.4GHz-波段无线LAN系统采用自适应多级调制的自适应多代码传输=Adaptive multi…     

网络视频传输的自适应调节策略的实现

介绍了一种基于端对端的网络自适应码率传输控制的传输算法。在分析已有的算法基础上,提出了一种改进的网络自适应算法,并将此算法应用在视频流传输系统中。经过端对端的视频流传输实验,通过人主观的观感感受和对客观数据进行对比的方式表明,设计的技术方案能够保证视频流传输的实时、完整和高效,达到网络视频传输自适应码率控制的解决途径。通过此系统实现的监控中视频流的实时传输,能达到公网或者通过移动网络中传输时视频的可靠与稳定,从而达到设计目的。     

多速率CDMA系统自适应接收机

随着移动通信的发展，多速率数据通信业务越来越多。第三代移动通信系统能够支持可变服务质量的不同数据速率业务。因此，多速率接收机的研究也越来越引起重视。多速率系统中，当干扰信号具有不同速率参数时，不同用户的周期平稳性不同，单速率自适应FIR接收机的性能会严重下降。提出了一种适合于多速率多址干扰下的自适应滤波器组接收机，仿真结果表明，自适应滤波器组接收机能够更有效地消除多种模式的多址干扰，比单速率接收机具有更好的性能。     

无源RFID自适应帧时隙防碰撞算法研究

射频识别RFID作为一种重要的物联网终端数据采集技术,系统的吞吐率直接影响着数据采集终端的性能,但目前广泛应用于无源RFID系统的帧时隙类防碰撞算法吞吐率普遍较低.本文着重分析了影响无源RFID帧时隙类ALOHA防碰撞算法性能两类因素：帧长和碰撞时隙的处理方式,通过构建和求解帧长调整和标签碰撞的数学模型,给出了无源RFID帧时隙类ALOHA防碰撞算法的具体优化途径和方案：帧长自适应调整和碰撞实时散列.在此基础上提出了自适应二进制散列帧时隙ALOHA防碰撞算法-ABSFSA.实验结果表明ABSFSA算法在同等条件下可以有效减少无效时隙,明显将RFID系统的吞吐率稳定提高到45%.本文的研究工作为无源RFID帧时隙类防碰撞算法的优化提供了可供参考的数学模型,同时对提升物联网数据采集终端的性能具有一定的应用价值.     

流媒体文件多码率自适应处理系统研究

为了应付流媒体系统应用网络带宽不足问题,设计了基于多码率文件传输技术和可变编解码参数Filters播放链路动态重连机制的自适应流媒体传输系统。该系统在网络带宽发生波动的条件下,能够自适应地选择与可用带宽相匹配码率的视频数据进行动态传输,进而减少视频数据缓冲等待次数和等待时间,有效提高网络带宽利用率和媒体内容服务质量,同时采用"最短"的链路进行媒体内容的动态重连,避免Filters动态重连中出现"卡塞感",有效提高流媒体系统的服务质量。     

改进的分组预约多址协议性能分析

分级预约多址（PRMA）协议已被广泛地应用于短距离无线通信系统中,它不仅适用了数据传输,同样也适用于话音传输。但由于PRMA协议的帧长是固定的,竞争的产生使部分时隙被浪费,信道容易不能更好地被提高,而且在重负荷情况下系统会发生不稳定问题。提出了一种改进的分组预约多址协议（IPRMA）。进行了理论分析和计算机仿真,由于采用排队等待方式接入时隙信道,降低了分组时,减少了分组丢弃率,从而提高了系统容量。     

可变速率可变功率的V-BLAST系统的性能分析

对采用V-BLAST算法的多输入多输出无线通信系统的功率控制进行了研究,提出了一种新的基于相邻时隙平均功率控制的功率控制算法,通过对不同发射天线的连续时隙功率的综合控制达到提高系统发射功率利用率的目的,将发射功率的变化反映到天线发射端数据速率的变化,提高了系统的频谱利用率,从而提高了系统的性能。文章提出的速率分阶的方法,为多输入多输出无线通信系统提供了一种可行的自适应调制方法。仿真结果表明,采用相邻时隙平均功率控制算法不仅可以提高系统的发射功率利用率的10％,而且能够改善系统性能0．5dB左右。     

TDS-OFDMA系统的Simulink仿真

搭建了TDS-OFDMA系统的Simulink仿真平台.仿真系统采用基于SNR门限的子带自适应编码调制算法(SBLA),根据各用户子带的信道估计结果自适应地选择编码速率和调制方式,进而在保证BER性能的情况下获得更高的传输吞吐率.仿真表明,该多址接入系统可以支持40个用户同时进行信息传输,最大净荷速率达到40.8 Mb...     

Design of spread spectrum multicode CDMA transport architecture formultimedia services

We investigate a new application of the well-known spread spectrum code division multiple access (SS-CDMA) techniques to multimedia services related to the development of the next-generation wireless mobile networks interconnecting with a wireline ATM-based broadband network. Such services allow users to share novel multimedia applications without any geographical restrictions. However, since the mobile radio channel has a fixed limited bandwidth, the traditional SS-CDMA system may not be sufficient to accommodate the variable bit rate (VBR) multimedia services requested by multiple mobile users simultaneously. Moreover, the traffic load at the base station can change dynamically due to the time-varying throughput requirement of these requested multimedia services. To tackle this difficulty, a multicode CDMA (MC-CDMA) technique is proposed to provide multirate multimedia services by varying the number of spreading codes assigned to each user in order to meet its throughput requirement. In MC-CDMA, a spreading code can be used to transmit information at a basic bit rate. Users (video or data) who need higher transmission rates can use multiple codes in parallel. Meanwhile, the maximum available number of codes in the MC-CDMA system is still limited. Hence, a cost-effective dynamic code allocation scheme has then been proposed to dynamically assign an appropriate number of codes to each user for achieving the maximum resource utilization for multiuser multimedia services via the mobile radio channel. Finally, a number of real multimedia titles generated from the well-known MacroMind Director are conducted to evaluate performance     

An adaptive reservation scheme for VBR voice traffic in wireless ATM networks

A reservation scheme, named dynamic hybrid partitioning, is proposed for the Medium Access Control (MAC) protocol of wireless ATM (WATM) networks operating in Time Division Duplex (TDD) mode. The goal is to improve the performance of the real-time Variable Bit Rate (VBR) voice traffic in networks with mixed voice/data traffic. In most proposed MAC protocols for WATM networks, the reservation phase treats all traffic equally, whether delay-sensitive or not. Hence, delay-sensitive VBR traffic sources have to compete for reservation each time they wake up from idle mode. This causes large and variable channel access delays, and increases the delay and delay variation (jitter) experienced by ATM cells of VBR traffic. In the proposed scheme, the reservation phase of the MAC protocol is dynamically divided into a contention-free partition for delay-sensitive idle VBR traffic, and a contention partition for other traffic. Adaptive algorithms dynamically adjust the partition sizes to minimize the channel bandwidth overhead. Simulation results show that the delay performance of delay-sensitive VBR traffic is improved while minimizing the overhead.     

An access protocol for supporting multiple classes of service in alocal wireless environment

This paper presents a time split token-based contention resolution (TS-TBCR) protocol for providing multiple classes of traffic access to a shared wireless media. Since the protocol requires immediate response from the base station, it is best suited for short-range wireless applications. The key feature of the protocol is that it allows multiplexing of various services with high throughput even under heavy traffic loads. The protocol provides near perfect scheduling of information transfer by reserving a small part of the bandwidth for contention resolution. This resolves contention in an a priori manner, thus preventing collisions and leading to high channel bandwidth utilization. This protocol allows the system to support a high load while keeping packet loss probability within acceptable limits. Using a token-based mechanism, the protocol implements an access scheme, as well as a service (scheduling) discipline. The protocol is analytically modeled and extensive simulations performed to evaluate its performance. Results indicate that for multimedia traffic transport, the protocol can dynamically, and without any centralized control, realize any service discipline from first-in first-out (FIFO: complete fairness) to head-of-line (HOL: complete biasing), merely based on the relative service requirements of the traffic classes. This brings in fairness since no service class is excessively penalized     

Measurement-Based Achievable Throughput Estimation in IEEE 802.11a WLANs

Knowledge of dynamically changing achievable throughput is important in the design of an adaptive QoS-provisioning system over WLANs. In this letter, we introduce a novel measurement-based throughput estimation scheme and its practical realization for IEEE 802.11a WLAN environment. By utilizing traffic statistics measurements at the wireless AP (access point) in a timely manner, the proposed scheme estimates idle (i.e., remaining) channel time and converts it into available min/max throughput guidelines. Prototype realization in a Linux-based testbed verifies that the proposed scheme can estimate the available throughput with precision.     

End-to-End Adaptive QoS Provisioning over GPRS Wireless Mobile Network

The General Packet Radio Service (GPRS) offers performance guaranteed packet data services to mobile users over wireless frequency-division duplex links with time division multiple access, and core packet data networks. This paper presents a dynamic adaptive guaranteed Quality-of-Service (QoS) provisioning scheme over GPRS wireless mobile links by proposing a guaranteed QoS media access control (GQ-MAC) protocol and an accompanying adaptive prioritized-handoff call admission control (AP-CAC) protocol to maintain GPRS QoS guarantees under the effect of mobile handoffs. The GQ-MAC protocol supports bounded channel access delay for delay-sensitive traffic, bounded packet loss probability for loss-sensitive traffic, and dynamic adaptive resource allocation for bursty traffic with peak bandwidth allocation adapted to the current queue length. The AP-CAC protocol provides dynamic adaptive prioritized admission by differentiating handoff requests with higher admission priorities over new calls via a dynamic multiple guard channels scheme, which dynamically adapts the capacity reserved for dealing with handoff requests based on the current traffic conditions in the neighboring radio cells. Integrated services (IntServ) QoS provisioning over the IP/ATM-based GPRS core network is realized over a multi-protocol label switching (MPLS) architecture, and mobility is supported over the core network via a novel mobile label-switching tree (MLST) architecture. End-to-end QoS provisioning over the GPRS wireless mobile network is realized by mapping between the IntServ and GPRS QoS requirements, and by extending the AP-CAC protocol from the wireless medium to the core network to provide a unified end-to-end admission control with dynamic adaptive admission priorities.     

A novel self‐adaptive transmission scheme over an IEEE 802.11 WLAN for supporting multi‐service

The IEEE 802.11 wireless local area network (WLAN) media access control (MAC) specification is a hybrid protocol of random access and polling when both distributed coordination function (DCF) and point coordination function (PCF) are used. Data traffic is transmitted with the DCF, while voice transmission is carried out with the PCF. Based on the performance analysis of the MAC protocol for integrated data and voice transmission by simulation, this paper puts forward a self‐adaptive transmission scheme to support multi‐service over the IEEE 802.11 WLAN. The simulation results show that, on the premise of satisfying the maximum allowable delay of packet voice, the self‐adaptive transmission scheme can improve the data traffic performance and increase the WLAN capacity through dynamic and appropriate adjustment of the protocol parameters. Especially, voice traffic is sensitive to delay jitter, and the self‐adaptive scheme can effectively decrease it. Finally, it is worth noting that the adaptive scheme is easy to be realized, whereas no change in the MAC protocol is needed. Copyright © 2006 John Wiley & Sons, Ltd.     

Efficient protocols for multimedia streams on WDMA networks

This paper introduces a new approach to integrate different types of medium access control (MAC) protocols into a single wavelength-division-multiplexing (WDM) network system. The WDM network is based on a passive star coupler, and the purpose of integrating different MAC protocols is to efficiently accommodate various types of multimedia traffic streams with different characteristics and quality of service demands. Our integrated MAC protocol is termed multimedia wavelength-division multiple-access (M-WDMA). Three types of multimedia traffic streams are considered in this paper: constant-bit-rate traffic and two classes of variable-bit-rate traffic. Accordingly, three tunable transmitters and one fixed home channel receiver are used in the design of each WDM node. The transmitters transmit the three types of multimedia traffic streams in a pipeline fashion so as to overcome the tuning time overhead and to support parallel transmissions of traffic streams that emerge simultaneously. We further incorporate a dynamic bandwidth allocation scheme that dynamically adjusts the portions of bandwidth occupied by the three types of traffic streams according to their demands. Consequently the M-WDMA protocol achieves high utilization and efficiently adapts to the demands of the multimedia streams so as to guarantee their QoS. The performance of the M-WDMA is evaluated through a simple analytical model and extensive discrete-event simulations. It is shown that the M-WDMA can satisfy the QoS requirements of various mixes of multimedia traffic streams even under very stringent requirements. Moreover, we show that the M-WDMA outperforms conventional MAC protocols for WDM networks. As a result, we expect M-WDMA to be a good multimedia MAC candidate protocol for future-generation WDM networks.     

Cross-Layer Enhancement to Support TCP-Based Traffics in WLANs

Widespread deployment of wireless local area networks and a gradual increase in streaming applications have brought about a demand for improved quality of service (QoS) in wireless networks. However, increasing user datagram protocol based high priority multimedia traffic and the class differentiation introduced in QoS protocols, has resulted into transmission control protocol (TCP) starvation and increased spurious timeouts. While today’s Internet traffic is still dominated by TCP based applications, the negative effects of IEEE 802.11e enhanced distributed coordination function (EDCF) scheme on TCP performance in the presence of high priority traffic have not been extensively explored. In this paper, the performance of TCP in 802.11e WLAN competing with high priority traffic is examined. The prioritised adaptive enhanced scheme (PAD_EDCF) is proposed. The proposed scheme gives priority to TCP control packets in order to improve the low traffic transmission flow and acquires additional capability of adjusting the MAC parameters based on the traffic load condition. Simulation results demonstrate that the proposed scheme significantly improves TCP performances in terms of traffic efficiency, throughput and reduces delay.     

An OFDM-TDMA/SA MAC Protocol with QoS Constraints for Broadband Wireless LANs

Orthogonal frequency division multiplexing (OFDM) is an important technique to support high speed transmission of broadband traffic in wireless networks, especially broadband wireless local area networks (LANs). Based on OFDM, a new multiple access scheme, called OFDM-TDMA with subcarrier allocation (OFDM-TDMA/SA), is proposed in this paper. It provides more flexibility in resource allocation than other multiple access schemes such as OFDM-TDMA, OFDM-frequency division multiple access (OFDM-FDMA), and orthogonal frequency division multiple access (OFDMA). With OFDM-TDMA/SA, a medium access control (MAC) is designed for broadband wireless LANs. It optimizes bit allocation in subcarriers so that maximum bits are transmitted in each OFDM symbol under a frequency selective fading environment. The OFDM-TDMA/SA MAC protocol also supports three classes of traffic such as guaranteed, controlled-load, and best effort services. Based on the optimum subcarrier bit-allocation algorithm and considering heterogeneous QoS constraints of multimedia traffic, a hierarchical scheduling scheme is proposed to determine the subcarriers and time slots in which a mobile terminal can transmit packets. In such a way, the OFDM-TDMA/SA MAC protocol significantly increases system throughput in a frequency selective fading environment and guarantees QoS of multimedia traffic. Computer simulation is carried out to evaluate the performance of the OFDM-TDMA/SA MAC protocol. Results show that the new MAC protocol outperforms other MAC protocols for OFDM-based wireless LANs. This work was supported by the State of Georgia Yamacraw Project (E21-105).     

An Energy-Efficient Self-Adaptive Duty Cycle MAC Protocol for Traffic-Dynamic Wireless Sensor Networks

A plenty of Medium Access Control (MAC) protocols deal with static traffics with low traffic load. The performance of these protocols drops significantly when network traffic become dynamic or in high traffic load. In this paper, we propose a new MAC protocol called Self-Adaptive Duty Cycle MAC (SEA-MAC) by introducing dual adaptive mechanisms: (1) An Adaptive Scheduling (AS) mechanism which makes the nodes’ active duration adaptive to variable traffic load, thus enabling SEA-MAC to resiliently schedule data transmission for the Sleep period. The algorithm is designated to schedule more data transmission in bursty and high traffic load, thus enabling rapid dissemination of data and reduction of latency. While under the light traffic load, nodes enter the Sleep mode timely, mitigating idle listening and saving energy. (2) A Self-Adaptive duty cycle mechanism which further adjusts the duty cycle and makes SEA-MAC adaptive to the dynamic traffic loads. When network experience with very low or very high traffic load, the protocol further adjusts duty cycle dynamically based on the traffic load, thus reducing inefficient duty cycle or reduce end-to-end delay. Experiment results show that the SEA-MAC is a great advancement compared with AS-MAC and RI-MAC protocols on the performance for unicast scenarios, especially under heavy unicast traffic load. SEA-MAC can reduce average and max end-to-end delay up to 50.90 and 68.20% respectively, lower energy consumption up to about 16.28%, and increase PDR up to about 16% compared with AS-MAC. SEA-MAC superiors to RI-MAC in average end-to-end delay by up to 80% and in maximum end-to-end delay up to 90% for all the traffic load in question. SEA-MAC outperforms RI-MAC significantly in average duty cycle under medium and heavy traffic load.