下载文档原格式
移动通讯词汇英汉对照Z散射发射 spurious emission散射输出功率 spurious output power散射响应抗扰性 spurious response immunity散射响应频率 spurious response frequency散射窄带（射频）分量 spurious narrow-bandwidth （RF） components 再定时 retiming再起动规程 restart procedure再生 regeneration再生器 regeneration再生中继器 regenerative repeater再生中继站 regenerative relay station再现性 reproducibility在线测试 on-line testing载波 carrier载波电话 carrier telephone载波电话增音机 carrier telephone repeater载波电话终端机 carrier telephone terminal载波跌落 carrier drop载波功率 carrier power载波恢复 carrier recovery， carrier reinsertion载波检测多址 carrier sense multiple access （CSMA）载波降低度 carrier reduction载波频率偏置 carrier frequency offset载波抑制度 carrier suppression载干比 carrier-to-interference ratio载体设备 vehicle equipment载躁比 carrier to noise ratio早期故障期 early fault period躁声带宽 noise bandwidth躁声功率 noise power躁声温度 noise temperature躁声系数 noise factor， noise figure躁声抑制 noise suppression增量调制 delta modulation （DM）增量调制编码 delta modulation code增强型定位报告系统 EPLRS增益控制 gain control增值业务 value added service窄带发射 narrowband emission窄带干扰 narrowband disturbance窄带器件 narrowband device占机信号 seizing signal占机证实信号 seizing-acknowledgement signal占空建立呼叫 on-air-call-set-up占线前标 seizure precursor占线测试 engaged test占用带宽 occupied bandwidth战略密码体制 strategic cipher system战术电台 tactical radio战术密码体制 tactical cipher system战术通信电子对抗系统 tactical communication electronic warfare system 折叠单极天线 folded monopole antenna折叠偶极天线 folded dipole antenna折射指数 refractive index折线编码律 segmented encoding law帧 frame帧定位 frame alignment帧定位时隙 frame alignment time slot帧定位信号 frame alignment signal帧定位恢复时间 frame alignment recovery time帧号码 frame number （FN）帧失位时间 out-of-frame alignment time帧同步码 frame synchronization code真迹电报 telewriting真空电子器件 vacuum electron device真空电容器 vacuum capacitor阵列天线 array antenna振荡 oscillation振荡器 oscillator振动试验 vibration test振幅键控 amplitude shift keying （ASK）振幅压扩单边带 amplitude companded SSB （ACSB）振铃音 ringing tone振鸣 howling整流 rectification正/零/负码速调整 positive/ zero/ negative justification正常检查 normal inspection正常充电 normal charging正常突发 normal burst正反码 positive and inverse code正交部分响应键控 quadrature partial response keying （QPRK）正交调幅 quadrature amplitude modulation （QAM）正交调幅器 quadrature modulator正交调频 quadrature frequency hopping正交相干解调器 quadrature coherent demodulator正码速调整（正脉冲塞入） positive justification （positive pulse stuffing）正态分布 normal distribution正向话终信号 clear-forward signal支线 branch feeder直达线路 direct route直接波 direct wave直接长途拨号网 direct distance dial network直接分配 direct distribution直接呼叫 direct call直接检测 direct detection直接接入 direct access直接耦合放大器 direct-coupled amplifier直接调频 direct frequency modulation直接序列扩频 direct sequence spread spectrum （DS）直流/直流变换器 DC/DC converter直流放大器 DC amplifier直同连接延时 through-connection delay指配频带 assigned band指配频率 assigned frequency指数分布 exponential distribution指数分布随机变量 exponential random variable质量保证 quality assurance （QA）质量测试 quality test质量管理 quality management质量监督 quality surveillance质量控制 quality control， mass control质量体系 quality system智能 intelligence智能控制 intelligent control智能网 intelligent network （IN）置乱 scramble置信度 confidence中波传输 medium wave propagation中和 neutralization中继器 repeater中频 intermediate frequency中频干扰 intermediate frequency jamming （interference）中频抑制比 intermediate frequency rejection ratio中心辐射 center radiation中心频率 center frequency中央控制台 central control post终端 terminal终端不平衡电压 asymmetrical terminal voltage终端平衡电压 symmetrical terminal voltage终端设备 terminal equipment （TE）终端适配功能 terminal adapter （TA）终端透明度 terminal transparency终端网络 terminating network终端移动无线局 terminal mobile services switching center 终端移动性 terminal mobility终结点 destination node终止电压 end voltage （cut-off voltage）重复码 repetition code重建样值 reconstructed sample重置规程 reset procedure重置性 resettability周期 cycle周期（卫星的） period （of a satellite）周期检定 periodic vertification啁啾 chirp主瓣 main lobe主波束宽度 principle half-power beamwidths主呼线识别提供 calling line identification presentation主呼线识别限制 calling line identification restriction （CLIR）主交换机 host change主控站 main control station主钟 master clock助听器 audiphone贮存寿命 storage life驻波 standing wave驻波保护电路 standing wave protection ciruit驻波比 standing wave ratio驻极体 electret驻极体传声器 electret microphone专权（同步网） despotic （synchronized） network专线 private line， dedicated line专用集成电路 application specific integrated circuit （ASIC）专用控制信道 dedicated channel专用数据网 private data network专用线 dedicated line专用线路 tie line专用小交换机 private branch exchange （PBX）专用信道 dedicated channel专用移动通信系统 private mobile radio system专用自动小交换机 private automatic branch exchange （PABX）转发启动抗扰性 repeating attack rejection转发启动灵敏度 repeating attack sensitivity转发器 repeater转发调制灵敏度 repeating modulation sensitivity转发音频失真 repeating distortion装置连线阻抗 installation wiring impedance准传输集群 quasi-peak value准峰值 quasi-peak value准峰值电压表 quasi-peak voltmeter准峰值电压表的脉冲响应特征 pulse response characteristic of a quasi-peak voltmetre准峰值检波器 quasi-peak detector准脉动躁声 quasi-impulsive noise准确度 accuracy准同步 quasi-synchronous浊音 voiced sounds姿态稳定卫星 attitude-stabilized satellite资用功率 available power子层 sublayer子带编码 sub-band coding （SBC）子基地台 subbase station子序列越区规程 subsequent handover procedure子帧 subframe自测试 self-test自动（动态）压缩 automatic （dynamic） compression自动拨号设备 automatic dialing unit （ADU）自动测试 automatic test自动车辆定位系统 automatic vehicle location system自动电话 automatic telephone system自动电话机 automatic telephone set自动发射机识别 automatic transmitter identification自动呼叫 automatic calling自动呼叫识别 automatic call identification自动化 automation自动交换设备 automatic switching equipment自动接入多信道无绳电话 autoaccess multiple channel cordless telephone自动频率公用 automatic frequency sharing自动频率控制 automatic frequency control （AFC）自动频率微调 automatic frequency fine control （AFEC）自动频率微调捕捉范围 automatic frequency fine control pull-in range自动频率微调捕捉时间 automatic frequency fine control pull-in time自动频率微调剩余误差 automatic frequency fine control residential error 自动频率微调同步范围 automatic frequency fine control locking-in range 自动频率微调跟踪速率 automatic frequency fine control maximum tracking rate 自动请求重发 automatic repeat request （ARQ）自动信道选择 automatic channel selection自动寻呼系统 automatic paging system自动功率控制 automatic power control （APC）自动增益控制 automatic gain control （AGC）自动增益控制特性 automatic gain-control （AGC） characteristic自放电 self-discharge自激荡器 self-excited oscillation自举电路 bootstrap circuit自然辐射源 natural radiator自然干扰 natural interference自然空间辐射 natural space radiation自然冷却 natural cooling自然躁声 natural noise自适应 adaptation自适应差分脉冲编码调制 adaptive differential pulse code modulation （ADPCM）自适应接收机 adaptive receiver自适应均衡 adaptive equalization自适应控制系统 adaptive control system自适应天线 adaptive antenna自适应通信 adaptive communication自适应短波电台 adaptive HF station自适应预测 adaptive prediction自适应预测编码 adaptive prediction coding自适应增量调制 adaptive delta modulation （ADM）自学习系统 self-learning system自由空间 free space自由空间传播 free space propagation自由空间光通信 free space optical communication自由空间基本传输损耗 free space basic transmission loss自组织无线电通信网 self-organization radio communication network 字长 word length字段 field字符 character字符长度 character size字符串 character string字符差错率 character error rate字符速率 character rate字符信号 character signal字节 octet字块 block综合办公室自动化系统 integrated automated office system综合测试仪 general-purpose tester综合环境实验 combined environment test综合数字网 integrated digital network综合相位控制 generated phase control综合业务数字网 ISDN， integrated services digital network综合业务网 integrated services network （ISN）总失真系数 total distortion factor总损耗（无线线路的） total loss （of a radio link）阻断/解阻 blocking /deblocking阻塞 blocking阻塞（四分之一波长）滤波器 stop （quarter-wave） filter组合干扰 combination interference组合音 combination tone组呼 talkgroup call阻尼振荡 damped oscillation组装效率 packaging efficiency副/主瓣比 maximum relative side lobe level可用幅偏 maximum usable amplitude deviation似然译码 maximum likelihood decoding调整率（塞入率） maximum justification rate （maximum stuffing rate）有用功率 maximum useful power允许频（相）偏 maximum permissible frequency （phase） deviation最低可用频率 lowest usable frequency最低位 least-significant bit （LSB）可用频率 maximum usable frequency （LUF）振荡频率 maximum frequency of oscillation最小可用场强，最小可用功率通量密度minimum usable field-strength （Emin），minimum usable power flux density （Pmin）最小码距 minimum distance最小频移键控 minimum shift keying （MSK）最终检验 final inspection工作频率 optimum working frequency （OWF）左旋极化波（或逆时针极化波）left-hand （or anti-clockwise）-polarized wave。
通信行业英语中英对照手册(F)F-CAPICH Forward-Common Auxiliary PIlot CHannel 前向公共辅助导频信道F-CPHCH Forward Common PHysical CHannel 前向公共物理信道F-DAPICH Forward Dedicated Auxiliary PIlot CHannel 前向专用辅助导频信道F-DCCH Forward Dedicated Control CHannel 前向专用控制信道F-DPHCH Forward Dedicated PHysical CHannel 前向专用物理信道F-EDFA Forward pumped EDFA 前向泵激励掺铒光纤放大器F-FCH Forward Fundemental CHannel 前向基本信道F-PCH Forward-Paging CHannel 前向寻呼信道F-PICH Forward-PIlot CHannel 前向导频信道F-RAMA Fair Resource Assignment Multiple Address 合理资源分配多址访问F-SCH Forward Supplemental CHannel 前向辅助信道F-SYNC Frame SYNChronizer 帧同步器F-SYNCH Forward-SYNchronous CHannel 前向同步信道FA Fiber Adaption 光纤适配FA Frame Aligner 帧定位器FA Frame Alignment 帧定位FAB Fiber Array Block 光纤阵列块FABM Fiber Amiplifier Booster Module 光纤放大器增强模块FAC Forward Acting Code 前向作用码FACCH Fast Associated Control CHannel 快速相关控制信道FACH Forward Access CHannel 前向接入信道FAITH Fiber Almost Into The Home 准光纤到家FAL Frame Alignment Loss 帧定位丧失FAL Frequency Allocation List 频率分配表FAM Frame Alignment Module 帧定位模块FAN Fiber in the Access Network 接入网光纤FAQ Frequently Asked Questions 常遇到的问题FART Frame Alighment Recovery Time 帧同步恢复时间FAS Fiber Access System 光纤接入系统FAS Flexible Access System 灵活接入系统FAS Frame Alignment Signal 帧定位信号FAST Fiber At Subscriber Terminal 用户端光纤FAT File Allocation Table 文件分配表FAT Flexible Access Termination 灵活接入终端FATDDL Frequency And Time Division Data Link 频分与时分数据链路FAW Frame Alignment Word 帧定位字FAXIWF FAX InterWorking Function 互通功能FB Fiber Booster 光纤增强器FB Fiber Bundle 光纤束FBB Fiber BackBone 光纤干线FBCN Fuzzy Backward Congestion Notification 模糊反向拥塞通知FBG Fiber Bragg Grating 光纤布拉格光栅FBGLS FBG Laser Sensor FBG激光传感器FBR Fiber Bragg Reflector 光纤布拉格反射器FBR Fixed Bit Rate 固定比特率FBS Flexible Bandwidth Sharing 灵活带宽共享FC Fiber Channel 光纤通道FC Forward Compatibility 前向兼容性FCA Fixed Channel Allocation 固定信道分配FCAL Fiber Channel Arbitrated Loop 光纤信道仲裁环路FCB File Control Block 文件控制块FCC Facsimile Conrol Channel 控制信道FCC Federal Communications Commission (美国)联邦通信委员会FCC Frequency Channel Code 频道编码FCCCH Forward Common Control CHannel 前向公共控制信道FCFS First Come First Served 先来先服务FCH Facsimile Channel Handling 信道处理FCLS First Come Last Served 先来后服务FCM Fuzzy Cognitive Map 模糊认知图FCN Frequency-Converting Network 变频网络FCN Full Connected Network 全连接网络FCP Frequency Control Program 频率控制程序FCS Fast Circuit Switching 快速电路交换FCS Fiber Channel Standard 光纤信道标准FCS Frame Check Sequence 帧校验序列FCT Fixed Celullar Terminal 固定蜂窝终端FD Fiber Duct 光纤管道FD Frame Disassembler 帧分解器FD-SS Frequency-Diversity Spread Spectrum 频率分集扩频FDCT Forward Discrete Consine Transform 前向离散余弦变量FDD Frequency Division Duplex 频分双工FDDI Fiber Distributed Data Interface 分布式光纤数据接口FDF Full-duplex Data Flow 全双工数据流FDI Feeder Distribution Interface 馈线分配接口FDL Fiber Delay Line 光纤延迟线FDM Frequency Division Mutiplexing 频分复用FDMA Frequency Division Multiple Access 频分多址接入FDP Fiber Distribution Point 光纤分布点FDR Forward Deflection Routing 前向改向路由选择FDX Full DupleX 全双工FE Function Element 功能单元FE Function Entity 功能实体FE-CDMA Frequency-Encoded CDMA 频率编码CDMAFEA Function Entity Action 功能实体作用FEAM Functional Entity Access Management 功能实体接入管理FEC Forward Error Conrol 前向过失控制FEC Forward Error Correction 前向纠错FEC Forwarding Equivalence Class 转发等价类型FECC-F Forward Error Correction Count-Fast data 前向纠错快速计数数据FECN Forward Explicit Congestion Notification 前向显式拥塞通知FED Forward Error Detection 前向检错FEFO First Ended First Out 先结束先送FEP Front-End Processor 前端处理器FER Frame Erasure Rate 帧删除率FER Frame Error Rate 误帧率FERF Far End Receive Failure 远端接收失效FES Fixed Earth Station 固定地球站FEXT Far-End CrosST alk 远端串音FFH Fast Frequency-Hopping 快速跳频FFP Fiber Fabry-Perot 光纤法布里-珀罗FFP-TF Fiber Fabry-Perot Tunable Filter 光纤法布里-珀罗可调滤波器FFPF Fiber Fabry-Perot Filter 光纤法布里-珀罗滤波器FFPI Fiber Fabry-Perot Interferometer 光纤法布里-珀罗干预仪FFRN Four-Fiber Ring Node 四纤环节点FFT Fast Fourier Transform 快速傅立叶变换FH Frame Handler 帧处理器FH Frame Header 帧头FH Frequency Hopping 跳频FH-CDMA Frequency-Hopped CDMA 跳频CDMAFHR Fixed Hierarchical Routing 固定等级选路FHSP Frame Handler SubPort 帧处理程序子端口FHSS Frequency Hopping Spread Spectrum 跳频扩频FI Format Identifier 格式标识符FIC Fiber Interface Card 光纤接口卡FICS Facsimile Intelligent Communication System 智能通信系统FIFO First In First Out 先进先出FIFS First In First Served 先进先服务FILO Firs In Last Out 先进后出FIM Fiber Interface Module 光纤接口模块FIM/CM Feature Interactive Management & Calling Management 特征交互管理与呼叫管理FIMS Feature Interaction in Multimedia System 多媒体系统特征交互FIN Full Interconnection Network 全互联网络FITL Fiber In The Loop 环路光纤FIU Facilities Interface Unit 设备接口单元FLAG Fiber Link Around the Globe 环球光纤链路FLL Fiber in the Local Loop 局域环路光纤FLM Fiber Loop Mirror 光环路镜像FM Facilities Management 设施管理FM Fault Management 故障管理FM Flexible Multiplexer 灵活复用器FM Forward Monitoring 前向监控FM Frequency Modulation 调频FM/FDMA Frequency Midulation / FDMA 调频/频分多址FM/TDMA Frequency Midulation / TDMA 调频/时分多址FMBS Frame Mode Bearer Service 帧模式承载业务FMC Fixed-Mobile Covergence 固定移动融合FMD Follow-Me-Diversion 跟我转移FMD Frame Mode Data 帧模式数据FMD Function Management Data 功能管理数据FMDI Function Management Data Interpreter 功能管理数据解释程序FMMS Fixed Media Mass Storage 固定媒体大容量存储器FMS File Management Subsystem 文件管理子系统FMS File Management System 文件管理系统FMSR FP-Mode Suppression Ratio 法布里-珀罗特模式抑制比FMUX Flexible MUltipleXer 灵活复用器FMV Full-Motion Video 全运动视频FN Fiber Node 光纤节点FN Functional Network 功能网络FNA Flexible Networking Architecture 灵活的网络结构FNA Free Network Address 空闲网络地址FNAE Free Network Address Element 空闲网络地址元素FNAS Frame relay Network Access Subsystem 帧中继网络接入子系统FNN Fuzzy Neural Network 模糊神经网络FNP Frontend Network Processor 前端网络处理机FO Fiber Optics 光纤FOA Fiber Optic Amplifier 光纤放大器FOAN Fiber Optic Access Network 光纤接入网络FOB FDM Output Buffer FDM输出缓冲器FOC Fiber Optic Cable 光缆FOC Fiber Optic Communication 光纤通信FOCC FOrward Control Channel 前向控制信道FOCN Fiber Optic Communication Network 光纤通信网FOCUS Fiber Optic Connection Universal System 光纤连接通用系统FOE Fiber Optic Extender 光纤延长器FOF Fluorescent Optical Fiber 发光光纤FOI Fiber Optic Isolator 光纤隔离器FOID Fiber Opitc Interface Device 光纤接口设备FoIP Fax over IP IPFOIRL Fiber Optic Inter-Repeater Link 中继器间光纤链路FOL Fiber Optic Laser 光纤激光器FOLAN Fiber Optic LAN 光纤局域网FOM Fiber Optic Modem 光纤调制解调器FOMAU Fiber Optic Medium Attachment Unit 光纤媒介附属单元FOP Failure Of Protocol 协议失效FOPMA Fiber Optic Physical Medium Attachment 光纤物理媒体装置FOS Fiber Optic Sensor 光纤传感器FOTC Fiber Optic Trunk Cable 干线光缆FOTIC Fiber Optic Transmitter Integrated Circuit 光纤发射机集成电路FOTN Fiber Optic Transmission Network 光纤传输网络FOTS Fiber Optic Temperature Sensor 光纤温度传感器FOX Fiber Optic eXtender 光纤扩展器FP Function Processor 功能处理机FPAD Facsimile Packet Assembly / Disassembly 分组组合/拆卸FPBS Fiber Polarization Beam Spliter 光纤偏振分束器FPGA Field Programmable Gate Array 现场可编程门阵列FPH FreePHone 免费(被叫集中付费)FPLL Frequency and Phase Locked Loop 锁频/锁相环FPLMTS Future Public Land Mobile Telecommunication Systems 未来公用陆地移动通信系统FPM Four Photon Mixing 四光子混合FPS Fast Packet Switching 快速分组交换FPSLA Fabry-Perot Semiconductor Laser Amplifier 法布里-珀罗半导体激光放大器FPT/FPS Fast Packet Transfer / Switching 快速分组传送/交换FR Frame Relay 帧中继FR Full Rate 全速率FRA File Relative Address 文件相关地址FRA Fixed Radio Access 固定无线接入FRAD Frame Relay Access Device 帧中继接入设备FRDTS Frame Relay Data Transmission Services 帧中继数据传输业务FRI Frame Relaying Information 帧中继信息FRMR FRaMe Reject 帧拒绝FRP Fast Reservation Protocol 快速保留协议FRP Fast Resolution Protocol 快速分辨协议FRP/DT Fast Reservation Protocol with Delayed Transmission 具有延迟传输的快速保留协议FRPH Frame Relay Packet Handler 帧中继分组处理程序FRS Frame Relay Service 帧中继业务FRS Frame Relay Switch 帧中继交换机FRSE Frame-Relay Switching Equipment 帧中继交换设备FRSF Frame Relay Service Function 帧中继业务功能FRT Frame Relay Terminal 帧中继终端FRTE Frame-Relay Terminal Equipment 帐中继终端设备FRTT Fixed Round-Trip Time 固定往返时间FS Fax Server 服务器FS Fiber Sensor 光纤传感器FS Frame Start signal 帧起始信号FS Frame State 帧状态FS Frame Switching 帧交换FS Frame Synchronizer 帧同步器FSAN Full Service Access Network 全业务接入网FSK Frequency Shift Keying 频移键控FSL Flexible System Link 灵活系统链路FSM FDDI Switching Module FDDI交换模块FSM FDM-channel Selector Module FDM信道选择器模块FSN Full Service Network 全业务网FSS Fixed Satellite Service 固定卫星业务FSS Flying Spot Scanner 飞点扫描器FSS Frame Synchronous Scrambling 帧同步扰码FSU Fixed Subscriber Unit 固定用户单元FSW Frame Synchronization Word 帧同步字FSYN Frame SYNchronization signal 帧同步信号FT Fiber Termination 光纤终端FT Fixed radio Terminal 固定式无线电终端FTAM File Transfer Access and Management 文件传送存取和管理FTAMS File Transfer Access and Management Services 文件传递访问及管理服务FTC Facsimile Transport Channel 传送信道FTC Fault Tolerant Computer 容错电脑FTC Fault-Tolerant Computing 容错计算FTF Fiber Termination Frame 光纤终端架FTLA Fiber-T o-the-Last Amplifier 光纤到末级放大器FTM FDM Transmitter Module FDM发送器模块FTM Fiber Terminal Module 光纤终端模块FTM Fiber Transfer Module 光纤传送模块FTM File Transfer Manager 文件传送管理器FTN Facsimile Transmission Network 传输网FTN Four-Terminal Network 四端网络FTP File Transfer Protocol 文件传送协议FTR Full Text Retrieval 全文检索FTS Fast Track Selector 快速磁道选择器FTS Frame Transport System 帧传送系统FTSA Fiber-To-the-Service Area 光纤到服务区FTSMSTR Frame Transport System MaSTeR 帧传送系统主程序FTTA Fiber To The Apartment 光纤到公寓FTTB Fiber To The Bridge 光纤到桥梁FTTB Fiber To The Building 光纤到楼宇FTTC Fiber To The Curb 光纤到路边FTTCa Fiber To The Cabinet 光纤到机箱FTTD Fiber To The Desk 光纤到桌面FTTF Fiber To The Feeder 光纤到馈送器FTTF Fiber To The Floor 光纤到楼层FTTH Fiber To The Home 光纤到户FTTK Fiber To The Kerb 光纤到路边FTTN Fiber To The Node 光纤到节点FTTO Fiber To The Office 光纤到办公室FTTP Fiber To The Pedestal 光纤到人行道FTTR Fiber To The Remote module 光纤到远端模块FTTR Fiber To The Rural 光纤到农村FTTS Fiber To The Subscriber 光纤到用户FTTSA Fiber To The Service Area 光纤到服务区FTTV Fiber To The Village 光纤到村FTTx Fiber To The… 光纤到…FTTZ Fiber To The Zone 光纤到小区FUNI Frame User Network Interface 帧用户网络接口FWA Fixed Wireless Access 固定无线接入FWAN Fixed Wireless Access Network 固定无线接入网FWC Frequency and optical Wavelength Converter 频率和光波长变换器FWM Four Wave Mixing 四波混频FWPCS Future Wireless PCS 未来无线个人通信系统FWS Fast-Wavelength-Switched 快速波长交换。
扩频通信(spread spectrum communication)是近几年内迅速发展起来的一种通信技术。
在早期研究这种技术的主要目的是为提高军事通信的保密和抗干扰的性能，因此这种技术的开发和应用一直是处于保密状态。
美国在20世纪50 年代中期，就开始了对扩频通信的研究，当时主要侧重在空间探测、卫星侦察和军用通信等方面。
以后，随着民用通信的频带拥挤日益严重，又由于近代微电子技术、信号处理技术、大规模集成电路和计算机技术的快速发展，与扩频通信有关的器件的成本大大地降低，从而进一步推动了扩频通信在民用领域的发展金额应用，而且也使扩频通信的理论和技术也得到了进一步的发展。
目前在军事上，它已经广泛应用于各种战略和战术通信的系统中，成为电子战中反干扰的一种重要的手段。
扩频技术在军事应用上的最成功的范例可以以美国和俄国的全球卫星定位系统(GPS和GLONASS)以及美军的联合战术分布系统(JTIDS)为代表；GPS和GLONASS在民用上也都得到了广泛的应用，这些系统的技术基础就是扩频技术。
扩频的码分多址技术应用于蜂窝移动通信中时，大大降低了噪声和衰落的影响，同时还避免了复杂的频率分配和时隙划分等技术上的困难，并可以省去保护频带或时隙，极大地提高了蜂窝通信系统中小区的频率复用度，使信号频谱利用率得到提高。
1990年1月，国际无线电咨询委员会(CCIR,现为ITUR)在研究未来民用陆地移动通信系统的计划报告中已明确地建议采用扩频通信技术[5]。
美国已制定出了基于CDMA蜂窝技术的IS-95标准，Samsung、Motorola等公司也已相继推出了各自的CDMA移动通信商用实验网已开通运行，并取得了良好的效果。
扩频技术由于其本身具备的优良性能而得到广泛应用，到目前为止，其最主要的两个应用领域仍是军事抗干扰通信和移动通信系统，而跳频系统与直扩系统则分别是在这两个领域应用最多的扩频方式。
一般而言，跳频系统主要在军事通信中对抗故意干扰，在卫星通信中也用于保密通信，而直扩系统则主要是一种民用技术。
扩展频谱通信（Spread Spectrum Communication），简称扩频通信，是一种信息传输方式，其信号所占有的频带宽度远大于所传信息必需的最小带宽；频带的扩展是通过一个独立的码序列（一般是伪随机码）来完成，用编码及调制的方法来实现的，与所传信息数据无关；在接收端则用同样的码进行相关同步接收、解扩及恢复所传信息数据。
扩展频谱技术具有以下特点：l、很强的抗干扰能力由于将信号扩展到很宽的频带上，在接收端对扩频信号进行相关处理即带宽压缩，恢复成窄带信号。
对干扰信号而言，由于与扩频用的伪随机码不相关，则被扩展到一很宽的频带上，使之进入信号通频带内的干扰功率大大降低，相应的增加了相关器的输出信号/干扰比，因此具有很强的抗干扰能力。
其抗干扰能力与其频带的扩展倍数成正比，频谱扩展得越宽，抗干扰的能力越强。
2、可进行多址通信扩展频谱通信本身就是一种多址通信方式，称为扩频多址（SSMA-Spread Specrum Multiple Access），实际上是码分多址（CDMA）的一种，用不同的扩频码组成不同的网。
虽然扩展频谱系统占用了很宽的频带，但由于各网在同一时刻共同一频段，其频段利用率甚至比单路单载波系统还要高。
CDMA是未来全球各人通信的一种主要的多址通信方式。
3、安全保密由于扩频系统将传送的信息扩展到很宽的频带上去，其功率密度随频谱的展宽而降低，甚至可以将通信信号淹没在噪声中。
因此，其保密性很强，要截获或窃听、侦察这样的信号是非常困难的，除非采用与发送端所用的扩频码且与之同步后进行相关的检测，否则对扩频信号是无能为力的。
由于扩频信号功率谱密度很低，在许多国家，如美、日、欧洲等国家对专用频段，如ISM频段，只要功率谱密度满足一定的要求，就可以不经批准使用该频段。
4、抗多径干扰在移动通信、室内通信等通信环境下，多径干扰是非常严重的，系统必须具有很强的抗干扰的能力，才能保证通信的畅通。
扩展频谱技术具有很强的抗多径能力，它是利用利用扩频所用的扩频码的相关特性来达到抗多径干扰，甚至可以利用多径能量来提高系统的性能。
扩展频谱通信，简称扩频通信，是一种信息传输方式，其信号所占有的频带宽度远大于所传信息必需的最小带宽；频带的扩展是通过一个独立的码序列（一般是伪随机码）来完成，用编码及调制的方法来实现的，与所传信息数据无关；在接收端则用同样的码进行相关同步接收、解扩及恢复所传信息数据。
中文名扩频通信外文名spread sprectrum communications定义扩展频谱通信与光纤通信、卫星通信一同被誉为进入信息时代的三大高技术通信传输方式。
理论基础根据香农（C.E.Shannon）在信息论研究中总结出的信道容量公式，即香农公式：C=W×Log2（1+S/N）式中：C--信息的传输速率S--有用信号功率W--频带宽度N--噪声功率由式中可以看出：为了提高信息的传输速率C，可以从两种途径实现，既加大带宽W或提高信噪比S/N。
换句话说，当信号的传输速率C一定时，信号带宽W和信噪比S/N是可以互换的，即增加信号带宽可以降低对信噪比的要求，当带宽增加到一定程度，允许信噪比进一步降低，有用信号功率接近噪声功率甚至淹没在噪声之下也是可能的。
扩频通信就是用宽带传输技术来换取信噪比上的好处，这就是扩频通信的基本思想和理论依据。
工作原理在扩频发信机中，射频载波通常经过两次调制过程：一次同常规调制一样，被信息信号所调制；另一次由码序列进行扩频调制，相应地在收信机中先用约定的码序列做相关处理（解扩），然后再进行信息信号的解调。
在发端输入的信息先经信息调制形成数字信号，然后由扩频码发生器产生的扩频码序列去调制数字信号以展宽信号的频谱。
展宽后的信号再调制到射频发送出去。
在接收端收到的宽带射频信号，变频至中频，然后由本地产生的与发端相同的扩频码序列去相关解扩。
再经信息解调、恢复成原始信息输出。
由此可见，—般的扩频通信系统都要进行三次调制和相应的解调。
一次调制为信息调制，二次调制为扩频调制，三次调制为射频调制，以及相应的信息解调、解扩和射频解调。
中文2800字毕业设计英文翻译专业电子信息工程班级2010级学生姓名学号课题码分多址通信系统的建模、仿真和设计——初始化模块、基站接收模块指导教师2014 年06 月10 日译文原文1.1 The basic concept of spread-spectrum communicationSpread spectrum communication’s basic characteristics, is used to transmit information to the signal bandwidth(W) is far greater than practical required minimum(effective) bandwidth (F∆),as the radio of processing gain P G.=/G P∆FWAs we well know,the ordinary AM,FM,or pulse code modulation,GP value in the area more than 10 times,collectively,the “narrow-band communication”,and spread-spectrum communication GP values as hundred or even thousands of times, can be called “broadband communication”.Due to the spread-spectrum signal,it is very low power transmitters,transmission space mostly drowned in the noise,it is difficult to intercepted by the other receiver ,only spreading codes with the same (or random PN code) receiver, Gain can be dealt with ,and despreading resume the original signal.1.2 The technology superiority of spread-spectrum communication.Strong anti-interference, bit error rate is low. As noted above, the spread spectrum communication system due to the expansion of the transmitter signal spectrum, the receiver despreading reduction signal produced spreading gain, thereby greatly enhancing its interference tolerance. Under the spreading gain, or even negative in the signal-to-noise ratio conditions, can also signal from the noise drowned out Extraction, in the current business communications systems, spread spectrum communications systems, spread spectrum communication is only able to work in a negative signal-to-noise ratio under the conditions of communication .Anti-multi-path interference capability, increase the reliability of system. Spread-spectrum systems as used in the PN has a good correlation, correlation is very weak. Different paths to the transmission signal can easily be separated and may intime and re-alignment phase, formation of several superimposed signal power, thereby improving the system’s performance to receive increased reliability of the system.Easy to use the same frequency, improving the wireless spectrum utilization. Wireless spectrum is very valuable,although long-wave microwave have to be exploited, and still can not meet the needs of community. To this end, countries around the world are designed spectrum management, users can only use the frequency applications,rely on the channel to prevent the division between the channel interference.Due to the use of spread-spectrum communication related receive this high-tech,low signal output power(“a W,as a general-100mW),and will work in the channel noise and thermal noise in the background,easy to duplicate in the same area using the same frequency,can now all share the same narrow-band frequency communication resources.Spread-spectrum communication is digital communication,particularly for digital voice and data transmission with their own encryption, only in the same PN code communication between users, is good for hiding and confidential in nature, facilitating communication business. Easy to use spread-spectrum CDMA communications, voice compression and many other new technologies, more applicable to computer networks and digitization of voice,image information transmission.Communication in the most digital circuits, equipment, highly integrated, easy installation, easy maintenance, but also very compact and reliable. The average failure rate no time was very long.1.3 Spread spectrum communication systemSpread spectrum communication,namely, spread spectrum communications (Spread spectrum communication), with fiber-optic communications,satellite communications,with access to the information age as the three major high-tech communications transmission. Spread spectrum communication is to send the information to be pseudo-random data is coded(Spread spectrum sequence: spread sequence) modulation, spread spectrum and then the realization of transmission; thereceiving end is using the same modem code and related processing, the restoration of the original data. Spread spectrum communication system has three main characteristics.(1) Carrier is an unpredictable, or so-called pseudo-random broadband signal.(2) Carrier data bandwidth than the modulation bandwidth is much wilder.(3) Receiving process is generated by local broadband carrier signal and receiving a copy of the signal to the broadband signal to achieve.The main way of spread spectrum are as follows: Direct Sequence Spread Spectrum(DSSS) using high-speed pseudo-random code on to the low-speed data transmission spread spectrum modulation; Frequency-hopping system using pseudo-random code to control the carrier frequency in a wider band of the change; TH is the data transmission time slot is a pseudo-random; chirp frequency system is a linear extension of the process of change. Combination of a number of ways of hybrid systems are often applied.The most important measure pf spread-spectrum system is an indicator of spreading gain, also known as processing gain. It is precisely because of the spread spectrum system itself with its performance characteristics with a series of advantages.1.4 Code division multiple accessCode division multiple access (CDMA) is a channel access method used by various radio communication technologies. It should not be confused with the mobile phone standards called cdmaOne, CDMA2000(the 3G evolution of cdmaOne) and WCDMA (the 3Gstandard used by GSM carrier), which are often referred to as simply CDMA, and use CDMA as an underlying channel access method.One of the concepts in data communication is the idea of allowing several transmitters to send information simultaneously over a signal communication channel. This allows several users to share a band of frequencies (see bandwidth). This concept is called multiple access. CDMA employs spread-spectrum technology and a special coding scheme( where each transmitter is assigned a code) to allow multiple user to be multiplexed over the same physical channel. By contrast, time division multipleaccess (FDMA) divides it by frequency. CDMA is a form of spread-spectrum signaling, since the modulated coded signal has a much higher data bandwidth than the data being communicated.1.5 Spread-spectrum characteristic of CDMAMost modulation schemes try to minimize the bandwidth of this signal since bandwidth is a limited resource. However, spread spectrum use a transmission bandwidth that is several orders of magnitude greater than the minimum required signal bandwidth. One of the initial reasons for doing this was military applications including guidance and communication systems. These system were designed using spread spectrum because if its security and resistance to jamming. Asynchronous CDMA has some level of privacy built in because the signal is spread using a pseudo-random code; this code makes the spread spectrum signals appear random or have noise-like properties. A receiver cannot demodulate this transmission without knowledge of the pseudo-random sequence used to encode the data. CDMA also resistant to jamming. A jamming signal only has a finite amount of power available to jam the signal. The jammer can either spread its energy over the entire bandwidth of the signal or jam only part of the entire signal.CDMA can also effectively reject narrow band interference. Since narrow band interference affects only a small portion of the spread spectrum signal, it can easily be removed through notch filtering without much loss of information. Convolution encoding and interleaving can be used to assist in recovering this lost data. CDMA signal are also resistant to multipath fading. Since the spread spectrum signal occupies a large bandwidth only a small portion of this will undergo fading due to multipath at any give time. Like the narrow band interference this will result in only a small loss of data and can be overcome.Another reason CDMA is resistant to multipath interference is because the delayed versions of the transmitted pseudo-random code, and will thus appear as another user, which is ignored at the receiver. In other words, as long as the multipath channel induces at least one chip of delay, 天the multipath channel induces at least one chip of delay,the multipath signals will arrive at the receiver.in other words, as long as the multipath channel induces at least one chip of delay, the multipath signalswill arrive at the receiver such that they are shifted in time by at least one chip from the intended signal. The correlation properties of the pseudo-random codes are such that this slight delay causes the multipath to appear uncorrelated with the intended signal, and it is thus ignored.Some CDMA devices use a rake receiver, which exploits multipath delay components to improve the performance of the system. A rake receiver combines the information from several correlators, each one tuned to a different path delay, producing a stronger version of the signal than a simple receiver with a signal correlation tuned to the path delay of the strongest signal.Frequency reuse is the ability to reuse the same radio channel frequency at other cell sites within a cellular system. In the FDMA and TDMA systems frequency planning is and important consideration. The frequencies used in different cells must be planned carefully to ensure signals from different cells do not interfere with each other. In a CDMA system, the same frequency can be used in every cell, because channelization is done using the pseudo-random codes. Reusing the same frequency in every cell eliminates the need for frequency planning in a CDMA system; however, planning of the different pseudo-random sequences must be done to ensure that the received signal from one cell does not correlate with the signal from a nearby cell.Since adjacent cell use the same frequencies, CDMA systems have the ability to perform soft handoffs. Soft handoffs allow the mobile telephone to communication simultaneously with two or more cells. The best signal quality in selected until the handoff is complete. This is different from hard handoffs utilized in other cellular systems. In a hard handoff situation, as the mobile telephone approaches a handoff, signal strength may vary abruptly. In contrast, CDMA systems use the soft handoff, which is undetectable and provides a more reliable and higher quality signal.Concluding remarksspread-spectrum technology in the initial stages of development, it has become a theory and a major technological breakthrough. Later in the development process is the improvement and hardware performance improved. Development to thepresent,spread-spectrum technology and the theory has been almost perfect,mainly from the point of view of overall performance, and the other new technology applications. Therefore, the application has been driven by the development of spread-spectrum technology is a power driving force, the future wireless communication systems, such as mobile communication. Wireless LAN, global personal communications, spread-spectrum technology will certainly play an important role.译文正文1.扩频通信系统概述扩频通信，即扩展频谱通信（Spread spectrum communication)，它与光纤通信、卫星通信，一同誉为进入信息时代的三大高技术通信传输方式，扩频通信是将待传送的信息数据被伪随机码调制，实现频谱扩展后再传输；接收端则采用相同的编码进行解调及相关处理，恢复原始信息数据。
扩展频谱通信（Spread Spectrum Communication），简称扩频通信，是一种信息传输方式，其信号所占有的频带宽度远大于所传信息必需的最小带宽；频带的扩展是通过一个独立的码序列（一般是伪随机码）来完成，用编码及调制的方法来实现的，与所传信息数据无关；在接收端则用同样的码进行相关同步接收、解扩及恢复所传信息数据。
[1]扩展频谱通信与光纤通信、卫星通信一同被誉为进入信息时代的三大高技术通信传输方式根据香农（C.E.Shannon）在信息论研究中总结出的信道容量公式，即香农公式：C = W×Log2（1+S/N）式中：C--信息的传输速率 S--有用信号功率 W--频带宽度 N--噪声功率由式中可以看出：为了提高信息的传输速率C，可以从两种途径实现，既加大带宽W或提高信噪比S/N。
换句话说，当信号的传输速率C一定时，信号带宽W和信噪比S/N是可以互换的，即增加信号带宽可以降低对信噪比的要求，当带宽增加到一定程度，允许信噪比进一步降低，有用信号功率接近噪声功率甚至淹没在噪声之下也是可能的。
扩频通信就是用宽带传输技术来换取信噪比上的好处，这就是扩频通信的基本思想和理论依据。
[4]在发端输入的信息先经信息调制形成数字信号，然后由扩频码发生器产生的扩频码序列去调制数字信号以展宽信号的频谱。
展宽后的信号再调制到射频发送出去。
在接收端收到的宽带射频信号，变频至中频，然后由本地产生的与发端相同的扩频码序列去相关解扩。
再经信息解调、恢复成原始信息输出。
由此可见，—般的扩频通信系统都要进行三次调制和相应的解调。
一次调制为信息调制，二次调制为扩频调制，三次调制为射频调制，以及相应的信息解调、解扩和射频解调。
与一般通信系统比较，扩频通信就是多了扩频调制和解扩部分。
[6]由于扩频通信能大大扩展信号的频谱，发送端用扩频码序列进行扩频调制，以及在接收端用相关解调技术，使其具有许多窄带通信难以替代的优良性能，能在民用后，迅速推广到各种公用和专用通信网络之中，主要有以下几项特点：（1）易于重复使用频率，提高了无线频谱利用率无线频谱十分宝贵，虽然从长波到微波都得到了开发利用，仍然满足不了社会的需求。
扩频通信系统的介绍摘要：本应用笔记概述了扩频技术的原理，讨论了涵盖直接序列和快速跳频的方法。
相关理论方程的性能估算。
以及讨论直接序列扩频（DSSS）和跳频（FHSS）这两种扩频方式。
简介扩频技术越来越受欢迎，就连这一领域以外的电器工程师都渴望能够深入理解这一技术。
很多书和网站上都有关于这方面的书，但是，很多都很难理解或描述的不够详尽。
（例如，直接序列扩频技术广泛关注的是伪随机码的产生）。
下面讨论扩频技术（双关语意）。
简史一名女演员和一名音乐家首次以书面形式描述了扩频通信技术。
1941年，好莱坞女星Hedy Lamarr和钢琴家George Antheil描述一个安全的无线链路来控制鱼雷。
他们获得了美国专利#2.292.387。
但这一技术被遗忘了，没有在当时受到美军的重视，直到20世纪80年代它才开始活跃起来。
从那时起，这一技术在有关恶劣环境中的收音机链接方面越来越受欢迎。
最典型的扩频技术应用是数据收发器包括卫星定位系统（GPS）、3G移动通信、无限局域网(符合IEEE®802.11a,IEEE 802.11b,IEEE 802.11g标准)，还有蓝牙技术也帮助了那些通讯落后和无线电通信条件有限的地方，因此，它是一种昂贵的资源。
扩频通信的原理扩频是香农定理的典型：C=B×log2(1+S/N) 公式（1）在公式中，C为信道容限，单位是比特/秒(bps),意指单位时间内信道中无差错传输的最大信息量。
B为信号频带宽度，单位是Hz，S/N为信噪比。
也就是说，C为信道允许通过的信息量，也代表了扩频的性能。
带宽(B)是代价，因为频率是一个有限的资源。
信噪比体现了环境条件或物理特性（如障碍、干扰器、干扰等）。
上式说明，的情况下，在无差错传输的信息速率C不变时，如果信噪比很低，则可以用足够宽的带宽来传输信号，即使信号功率密度低于噪音水平。
（公式可用！）改变公式（1）中对数的底数，2改为e，则为In=loge。
因此，C/B=(1/ln2)×ln(1+S/N)=1.443×ln(1+S/N)公式（2）根据MacLaurin扩展公式ln(1+x)=x-x2/2+x3/3-x4/4+…+(-1)k+1xk/k+…:C/B=1.443×(S/N-1/2×(S/N)2+1/3×(S/N)3-…) 公式（3）在扩频应用中，通常S/N很低。
（正如刚才提到的，信号功率密度甚至低于噪音水平。
）假定噪音水平即S/N<<1，香农公式可简单表示为：C/B≈1.443×S/N公式（4）简化为：C/N≈S/N 公式（5）或者：N/S≈B/C 公式（6）向固定了信噪比的信道发送错误的信息，只要执行基本扩频信号的传播操作：增加传输带宽。
尽管这一原则看起来很简单明确，但实现她却很复杂，主要是因为展宽基带的电子设备必须同时存在展宽和解扩的操作过程。
定义不同的扩频技术都有一个共同之处：密钥（也称为代码或序列）依附于传输信道。
以插入代码的形式准确地定义扩频技术，术语“频谱扩展”是指扩频信号的几个数量级的带宽在有密钥的传输信道中的扩展。
以传统的方式定义扩频更为精确：在射频通信系统中，将基带信号扩展为比原有信号的带宽宽得多的高频信号（如图1）。
在此过程中，传输宽带信号产生的损耗，表现为噪声。
扩频信号带宽与信息带宽之比称为处理增益。
扩频过程的处理增益大都在10dB 到60dB 之间。
要应用扩频技术，只需在天线（接收器）之前加入相应的扩频码。
相反，你可以删除一个点的扩频码（称为解扩操作）接收发射链路数据恢复。
解扩过程是重新恢复原始带宽的过程。
很明显，同样的代码必须在事先知道在传输通道两端的信息。
（在某些情况下，在调制和解调的过程中代码应该是知道的）。
图1.扩频通信系统传播工作带宽的影响图2说明了信号带宽的通信链路评估图2.扩频操作遍及一个更宽的频率带宽的信息能量扩频调制是一种适用于如BPSK 或直接转换。
传统的调制可以证明所有其他信号接收不到扩频代码将保持它们原有的信息，极没有被扩展。
解扩过程中带宽的影响同样，解扩过程如图3。
输入的扩频码频率数据的处理增益数据输入宽度 扩频调制 数据输入 能量能量PF 载体 输电链 扩频代码接收链扩频代码数据输入射频输出射频输入 射频连接 相同的配置序列 数据输出能量数据输入宽度数据输入 解扩调制能量输入的扩频码 数据的处理增益PF 载体 频率图3,在解扩过程中恢复的原有信号在这里，解扩调制已经取得了正常解调操作，也表明了干扰或干扰信号在解扩传输过程中被扩展！由于带宽的浪费抵消了传播的多用户扩频结果直接在一个更宽的频带使用，完全对应之前的“处理增益”。
因此扩频并没有节约有限的频率资源。
过度的使用虽然得到了补偿，但是可能有很多用户共享这一扩大频率波段（如图4）。
图4.在相同的频带多个用户共享扩频技术。
扩频是宽带技术相对于常规窄带技术，扩频过程是一种宽带技术。
例如，W - CDMA 和UMTS 都是宽带技术，与窄带广播相比，它需要一个比较大的频率带宽。
扩频的优点抗干扰性能和抗干扰的影响扩频技术有很多优点。
.抗干扰性是最重要的一个优点。
有意或无意的干扰和干扰信号都是不希望存在的因为它们不包含扩频密钥。
只有期望信号才有密钥，在解扩过程中才会被接收器接收，如图5。
图5.扩频通信系统。
注意，解扩链路中数据信号被传输的同时干扰能源也被传输。
输电链 扩频代码 接收链扩频代码数据输入射频输出射频输入 射频连接 数据输出数据 干扰 数据扩展和干扰扩展数据扩展 数据扩展和干扰 用户1+用户2+用户3+…+用户N数据输入获得的扩频增益无论在窄带或宽带中，如果它不涉及解扩过程，你几乎可以忽略干扰。
这种抑制反应也适用于其他没有正确密钥的扩频信号。
因此不同的扩频通信系统可以工作在同一频段，例如CDMA 。
值得注意的是，扩频是宽带技术，但反之则不然：宽带技术不涉及扩频技术。
抗截获抗截获是扩频通信技术的第二个优势。
由于非法的听众没有密钥用于原始信号传播，这些听众无法解码。
没有合适的钥匙，扩频信号会出现噪音或干扰。
（扫描方法可以打破的这些密钥，但是密钥是短暂的。
）甚至更好，信号电平可以低于噪声水平，因为扩频传输降低了频谱密度，如图6。
（总能量是相同的，但它是广泛存在于频率的。
）因此信息是无形的，这一影响在直接序列扩频（DSSS ）技术上有充分的体现。
（在下文的DSSS 作更详细说明。
）其他接收机无法“看到”这种传输，它们只能出现在整体噪音水平略有增加的情况下！图6.在被噪音水平之下的扩频频谱信号。
在没有正确的扩频传输密钥的情况下，接收器不能“看到”传输过程。
抗衰落（多径效应）无线信道通常具有多径传播，即有一个以上的信号从发射机传到接收器（如图7）。
这种多路径可以通过空气的反射或折射以及从地面反射或物体如这些路径建筑物引起。
图7.信号是如何通过多个路径到达接收器的。
这种反射路径（R ）可干扰直接路径（D ）的现象称为解扩过程的同步衰落。
因为解扩过程使信号D 与信号R 的同步被拒绝，即使它们包含了相同的密钥。
将反射路径的信号应用于解扩是个有用的方法。
扩频技术在CDMA 的应用请注意，扩展频谱不是一个扩频调制方案，不应与其他调制方式相混淆。
例如我们可以使用扩频技术发射一个由PSK 或BPSK 的已调信号。
.感谢调制的信号的编码基础，使RxRDTx 噪声基准 扩展后的数据噪声基准数据传播之前扩频频谱也可用于其他类型的多址实现（即可以同时进行多个通讯联系和实际或表面上相同的物理介质共存）。
到目前为止，有三个主要的方法可用。
FDMA-频分多址FDMA 分配一个特定的载波频率给通信信道。
不同用户使用频谱的切片数是受到限制的（如图8）。
在已有的三种多路存取方法中，FDMA 在频带利用方面是效率最低的。
FDMA 的方法包括Methods 包括无线电广播，电视，高级移动电话系统AMPS 等。
图8. FDMA 系统中不同的用户的载波频率分配。
TDMA-时分多址TDMA 的不同用户彼此间发言和听取信息时，是根据定义的时隙分配来处理的（如图9）。
不同的通信信道可以建立一个唯一的载波频率。
TDMA 的例子有全球移动通信系统GSM ，DECT ，TETRA 和IS - 136。
图9.在TDMA 系统中不同用户的时隙分配。
CDMA-码分多址CDMA 的传播是由密钥或代码决定的（如图10）。
在这个意义上说，扩频就是一种CDMA 。
在发射器和接收器密钥必须提前被定义和确定。
它的例子有IS - 95(DS)，IS- 98，蓝牙和无线局域网。
用户1 用户2 用户3 用户N 用户1 用户2 用户3 用户N时间段 时间段 时间(ms,us) 用户1 用户2 用户3 用户N频率(kHz,MHz,GHz)Fc1 Fc2 Fc3 FcN图10.CDMA 系统中相同频带有独特的钥匙或代码。
当然，人们可以结合上述存取方法，例如，全球移动通信系统GSM 结合了TDMA 和FDMA 。
GSM 定义了不同的载波频率（细胞）的拓扑领域，并设定时段内每一个细胞。
扩频和（的）编码密钥在这一点上，值得重申的是扩频的主要特点是一个代码或密钥必须在发射器和接收器之前就是已知的。
现代通讯的代码是数字序列必须长期存在和随机出现的，尽可能地显示为“噪音像”。
在任何情况下，代码必须确保是可再生的。
或者接收器不能提取已发出去的消息。
因此，该序列是几乎是随机的 。
这样的代码被称为伪随机数（PRN ）或序列。
最常用的方法来产生伪随机是基于反馈移位寄存器的。
许多书籍都在介绍伪随机码的发展与特征，但是，实际的发展已超出了这些教材所叙述的。
注意的是，建立或选择适当的序列或序列集并不是微不足道的。
为了保证有效的扩频通信，伪随机序列必须尊重一定的规律如长度、自相关、互相关、正交。
比较受欢迎伪随机序列有Barker 码，M 序列码，Gold 码，Walsh 码等。
考虑到存在更复杂的序列集，给它提供了一个更强大的扩展频谱链路。
但是这产生了成本问题：扩频和解扩都需要在速度和性能都更复杂的电子产品，数字扩频解扩芯片包含几百万个等效的2输入与非门在几十兆赫间切换。
用户1 用户5用户4用户3 用户2An Introduction to Spread-Spectrum CommunicationsAbstract:This application note is a tutorial overview of spread-spectrum principles.The discussion covers both direct-sequence and fast-hopping methods.Theoretical equations are given to allow performance estimates.Relation direct-sequence spread-spectrum(DSSS) and frequency-hopping spread-spectrum(FHSS) methods.IntroductionAs spread-spectrum techmiques become increasingly popular,electrical engineers outside the field are eager for understandable explanations of the technology.There are books and websites on the subject,but many are hard to understand or describe some aspects while ignoring others(e.g.,the DSSS technique with extensive focus on PRN-code generation).The following discussion covers the full spectrum(pun intended).A Short HistorySpread-spectrum communications technology was first described on paper by an actress and a musician!In 1941 Hollywood actress Hedy Lamarr and pianist George Antheil described a secure radio link to control torpedos.They received U.S.Patent #2.292.387.The technology was not taken seriously at that time by the U.S.Army and was forgotten until the 1980s,when it became active.Since then the technology has become increasingly popular for application that involve radio links in hostile environments.Typical applications for the resulting short-range data transceivers include satellite-positioning systemsGPS，3G mobile telecommunications,W-LAN(IEEE®802.11a,IEEE 802.11b,IEEE 802.11g),and Bluetooth®.Spread-spectrum techniques also aid in the endless race between communication needs and radio-frequency availability-situations where the radiospectrum is limited and is,therefore,an expensive resource.Theoretical Justification for Spread SpectrumSpread-spectrum is apparent in the Shannon and Hartley channel-capacity theorem: C=B×log2(1+S/N) (Eq.1)I n this equation,C is the channel capacity in bits per second(bps),which is the maximum data rate for a theoretical bit-error rate(BER).B is the required channel bandwidth in Hz,and S/N is the signal-to-nosie power ratio.To be more explicit,one assumes that C,which represents the amount of information allowed by the communication channel,also represents the desired performance.Bandwidth (B) is the price to be paid,bacause frequency is a limited resource.The S/N ratio expresses the environmental conditions or the physical characteristics (i.e., obstacles ,presence of jammers ,interferences,etc.).There is an elegant interpretation of this equation,applicable for difficult environments,for example,when a low S/N ratio is caused by noise and interference.This approach says that one can maintain or even increase communication performance (high C) by allowing or injecting more bandwidth (high B),even when signal power is below the noise floor. (The equation does not forbid that condition!)Modify Equation 1 by changing the log base from 2 to e (the Napierian number) and by noting that In=loge.Therefore:C/B=(1/ln2)×ln(1+S/N)=1.443×ln(1+S/N) (Eq.2)Applying the MacLaurin series development forln(1+x)=x-x2/2+x3/3-x4/4+…+(-1)k+1xk/k+…:C/B=1.443×(S/N-1/2×(S/N)2+1/3×(S/N)3-…) (Eq.3)S/N is usually low for spread-spectrum applications. (As just mentioned, the signal power density can even be below the noise level.) Assuming a noise level such that S/N <<1,Shannon's expression becomes simply:C/B≈1.443×S/N (Eq.4)Very roughly:C/N≈S/N (Eq.5)Or:N/S≈B/C (Eq.6)To send error-free information for a given noise-to-signal ratio in the channel,therefore,one need only perform the fundamental spread-spectrum signal-spreading operation:increase the transmitted bandwidth.That principle seems simple and evident.Nonetheless,implementation iscomplex,mainly because spreading the baseband (by a factor that can be several orders of magnitude) forces the electronics to act and react accordingly,which,in turn,makes the spreading and despreading operations necessary.DefinitionsDifferent spread-spectrum techniques are available,but all have one idea in common:the key (also called the code or sequence) attached to the communication channel.The manner of inserting this code defines precisely the spread-spectrum technique.The term "spread spectrum" refers to the expansion of signal bandwidth,by several orders of magnitude in some cases,which occurs when a key is attached to the communication channel.The formal definition of spread spectrum is more precise:an RF communications system in which the baseband signal bandwidth is intentionally spread over a larger bandwidth by injecting a higher frequency signal (Figure 1).As a direct consequence,energy used in transmitting the signal is spread over a wider bandwidth,and appears as noise.The ratio (in dB) between the spread baseband and the original signal is called processing gain.Typical spread-spectrum processing gains run from 10dB to 60dB.To apply a spread-spectrum technique,simply inject the corresponding spread-spectrum code somewhere in the transmitting chain before the antenna (receiver).Conversely,you can remove the spread-spectrum code (called a despreading operation) at a point in the receive chain before data retrieval.A despreading operation reconstitutes the information into its original bandwidth.Obviously,the same code must be known in advance at both ends of the transmission channel. (In some circumstances,the code should be known only by those two parties.)Figure 1.Spread-spectrum communication systemBandwidth Effects of the Spreading OperationFigure 2 illustrates the evaluation of signal bandwidths in a communication link.Figure 2.Spreading operation spreads the signal energy over a wider frequency bandwidth.Spread-spectrum modulation is applies on top of a conventional modulation such as BPSK or direct conversion.One can demonstrate that all other signals not receiving the spread-spectrum code will remain ad they are,that is,unspread.Bandwidth Effects of the Despreading OperationSimilarly,despreading can be seen in Figure 3.Figure 3. The despreading operation recovers the original signal.Here a spread-spectrum demodulation has been made on top of the normal demodulation operations.One can also demonstrate that signals such as an interferer or jammer added during the transmission will be spread during the despreading operation!Waste of Bandwidth Due to Spreading Is Offset by Multiple UsersSpreading results directly in the use of a wider frequency band by a factor that corresponds exactly to the "processing gain" mentioned earlier.Therefore spreading does not spare the limited frequency resource.That overuse is well compensated,however,by the possibility that many users will share the enlarged frequency band (Figure 4).Figure 4. The same frequency band can be shared by multipleusers with spread-spectrum techniques.Spread Spectrum Is a Wideband TechnologyIn contrast to regular narrowband technology,the spread-spectrum process is a wideband technology.W-CDMA and UMTS, for example,are wideband technologies that require a relatively large frequency bandwidth, compared to narrowband radio.Benefits of Spread SpectrumResistance to Interference and Antijamming EffectsThere are many benefits to spread-spectrum technology.Resistance to interference is the most important advantage.Intentional or unintentional interference and jamming signals are rejected because they do not contain the spread-spectrum key.Only the desired signal,which has the key, will be seen at the receiver when the despreading operation is exercised.See Figure 5.Figure 5. A spread-spectrum communication system.Note that the interferer’s energy is spread while the data signal is despread in the receive chain.You can practically ignore the interference,narrowband or wideband,if it does not include the key used in the dispreading operation.That rejection also applies to other spread-spectrum signals that do not have the right key.Thus different spread-spectrum communications can be active simultaneously in the same band,such as CDMA.Note that spread-spectrum is a wideband technology,but the reverse is not true:wideband techniques need not involve spread-spectrum technology.Resistance to InterceptionResistance to interception is the second advantage provided by spread-spectrum techniques.Because nonauthorized listeners do not have the key used to spread the originalsignal,those listeners cannot decode it.Without the right key,the spread-spectrum signal appears as noise or as an interferer.(Scanning methods can break the code,however,if the key is short.) Even better,signal levels can be below the noise floor,because the spreading operation reduces the spectral density.See Figure 6.(Total energy is the same,but it is widely spread in frequency.) The message is thus made invisible,an effect that is particularly strong with the direct-sequence spread-spectrum (DSSS) technique.(DSSS is discussed in greater detail below.) Other receivers cannot “see” the transmission;they only register a slight increase in the overall noise level!Figure 6.Spread-spectrum signal is buried under noise level.The receiver cannot “see”the transmission without the right spread-spectrum keys.Resistance to Fading (Multipath Effects)Wireless channels often include multiple-path propagation in which the signal has more that one path from the transmitter to the receiver (Figure 7).Such multipaths can be caused by atmospheric reflection or refraction, and by reflection from the ground or from objects such as buildings.Figure 7.Illustration of how the signal can reach the receiver over multiple paths.The reflected path (R) can interfere with the direct path (D) in a phenomenon called fading.Because the dispreading process synchronizes to signal D,signal R is rejected even though it contains the same key. Methods are available to use the reflected-path signals by dispreading them and adding the extracted results to the main one.Spread Spectrum Allows CDMANote that spread spectrum is not a modulation scheme,and should not be confused with other types of modulation.One can,for example,use spread-spectrum techniques to transmit a signal modulated by PSK or BPSK.Thanks to the coding basis,spread spectrum can also be used as another method for implementing multiple access (i.e.,the real or apparent coexistence of multiple and simultaneous communication links on the same physical media).So far,three mainmethods are available.FDMA-Frequency Division Multiple AccessFDMA allocates a specific carrier frequency to a communication channel.The number of different users is limited to the number of “slices” in the frequency spectrum (Figure 8).Of the three methods for enabling multiple access,FDMA is the least efficient in term of frequency-band usage.Methods of FDMA access include radio broadcasting,TV,AMPS,and TETRAPOLE.Figure 8.Carrier-frequency allocations among different users in a FDMA system.TDMA-Time Division Multiple AccessWith TDMA the different users speak and listen to each other according to a defined allocation of time slots (Figure 9).Different communication channels can then be established for a unique carrier frequency.Examples of TDMA are GSM,DECT,TETRA,and IS-136.Figure 9. Time-slot allocations among different users in a TDMA system.CDMA-Code Division Multiple AccessCDMA access to the air is determined by a key or code (Figure 10).In that sence,spread spectrum is a CDMA access.The key must be defined and known in advance at the transmitter and receiver ends.Growing examples are IS-95 (DS),IS-98,Bluetooth,and WLAN.Figure 10.CDMA systems access the same frequency band with unique keys or codes.One can,of course,combine the above access methods.GSM,for instance,combines TDMA and FDMA.GSM defines the topological areas (cells) with different carrier frequencies,and sets time slots within each cell.Spread Spectrum and (De) coding “Keys”At this point,it is worth restating that the main characteristic of spread spectrum is the presence of a code or key,which must be known in advance by the transmitter and receiver (s).In modern communications the codes are digital sequences that must be as long and as random as possible to appear as “noise-like”as possible.But in any case,the codes must remain reproducible.or the receiver cannot extract the message that has been sent.Thus,the sequence is “nearly random”.Such a code is called a pseudo-random number (PRN) or sequence.The method most frequently used to generate pseudo-random codes is based on a feedback shift register.Many books are available on the generation of PRNs and their characteristics,but that development is outside the scope of this basic tutorial.Simply note that the construction or selection of proper sequences,or sets of sequences,is not trivial.To guarantee efficient spread-spectrum communications,the PRN sequences must respect certain rules,such as length, autocorrelation,cross-correlation,orthogonality,and bits balancing.The more popular PRN sequences have names:Barker,M-Sequence,Gold,Hadamard-Walsh,etc.Keep in mind that a more complex sequence set provides a more robust spread-spectrum link.But there is a cost to this: more complex electronics both in speed and behavior,mainly for the spread-spectrum despreading operations.Purely digital spread-spectrum despreading chips can contain more than several million equivalent 2-input NAND gates,switching at several tens of megahertz.。
