CDM&SS
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Highlights of this part
• • • •
正交编码 Orthogonal code 码分多址通信CDMA Code division multiple access 伪随机序列Pseudonoise (PN) sequence 扩频通信 Spread spectrum
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Orthogonal code
– Better suited for digital – Often gets higher capacity – Relaxes need for high g Q filters
•
Cons
– Strict synchronization and – guard time needed – Still susceptible to jamming, other cell interference – Often requires equalizer
Alternative to FDMA and TDMA?
• What if we could allow users to share time and frequency?
• Eliminates need for tight synchronization among many different users • Eliminates need for expensive analog filters • May have favorable impact on capacity (?)
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m序列的性质(cont’d)
– m序列的自相关特性
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m序列的性质(cont’d)
– m序列的功率谱
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扩频通信(spread spectrum)
– 定义：用来传输的信号带宽远远大于信息本身带宽的一种通信方式。 – 分类： 直接序列扩频DS-SS——Direct Sequence Spread Spectrn 跳频扩频 FH-SS——Frequency Hopping Spread Spectrum 线性调频——Chirp Modulation – 理论基础：香农信道容量公式（用带宽换取信噪比） – 特点： 1、抗窄带干扰能力强； 2、防截获能力强； 3、抗多径衰落能力强； 4、可实现码分多址； 5、有测距能力。
• But: B t
• How do we separate the users? • Won’t they interfere with each other?
Code Division Multiple Access (CDMA)
Transmitter of CDMA
J
Tb ，扩频增益 Tc
• 正交的概念
互相关系数：
互相关系数：
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Orthogonal code（cont’d）
• Hadamard矩阵
– 特点：是一个由元素+1和-1 1构成的方阵，每一行(或列)都是正交 码组，可以用递推法生成高阶矩阵。
1 1 1 1 1 1 1 1 H4 1 1 1 1 1 1 1 1
Freq. Division Multiple Access (FDMA)
• AMPS (analog), the First Generation (1G) used 30KHz for each user. • Pros
– – – – Very Simple to design Narrowband (no ISI) Synchronization is easy N i No interference t f among users in a cell Narrowband interference static spectrum allocation Freq. reuse is a problem High Q analog filters or large guard band required
通信原理
邢慧玲
xinghl@ Dalian University of Technology
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Syllabus
• Introduction 绪论：基本的概念，信号基础(CH1、CH2、 CH3) • Information theory基础信息论：信源部分、信道部分 (CH1、CH4) • Digital baseband system数字基带传输(CH6) • Analog g to digital g conversion模拟信号的数字传输(CH9) • Modulation technology调制技术： Analog modulation模拟调制(CH5) Digital modulation数字调制(CH7、CH8) • Optimal receiver数字最佳接收(CH10) • Channel coding信道编码(CH11) • CDM and spread spectrum码分复用与扩频通信(CH12) • Synchronization同步(CH13)
there are 4 Walsh codes In general there are always J Walsh codes, as long as J = 2, 4, 8, 16, 32, 64, 128, ...
How CDMA Work
• Assume a noiseless y channel with unity gain • Walsh codes allow f four users to be b transmitted at the same time and frequency using four times the number of bits
Spreading code for CDMA
•The spreading code Ck must be unique u que for o each eac user. use . • Ideally, they are orthogonal to one another, i.e. < Ci, Ck > = 0, unless i = k < Ci, Ck > = J, if i = k • Example: Walsh Codes For a sprea-SS system
抗窄带干扰
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DS-SS system(cont’d) 抗截获能力强
可实现码分多址
RAKE receiver
扩频系统利用PN序列良好的自相关特性，进行RAKE接收，具有很 强的抗多径衰落的能力。 ？？相关器如何设计
PN序列的其他应用
– – – – – –
分离多径 误码率测量 时延测量 噪声产生器 通信加密 数据加扰
• Cons
– – – – –
Time Division Multiple Access (TDMA) • Besides freq., can also partition time: users take turns using the channel • IS-54 (2G) used same 30 KHz channels, but with three users sharing them (3 slots) • GSM has 8 slots/270 KHz • Pros
• PN序列：类似随机序列的基本特性的确定性序列(可重复产生) • 常用PN序列：
– m序列 – Gold码 – M序列 – 二次剩余序列
*
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m序列
• m序列:
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m序列的性质
• 产生m序列的充要条件:
• m序列的性质
– 均衡性:m序列一个周期中”0””1” 0 1 数目基本相等( (“1” 1 比“0 0”多一个)。 – 游程分布特性 – 移位相加特性：
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Orthogonal code（cont’d）
• Walsh矩阵
– 特点：是一个由元素+1和-1构成的方阵，每一行(或列)都是正交 码组，可以用Hadamard矩阵的行(或列)构造，按+1和-1交变的次 数由少到多排列。
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码分多址CDMA
• 正交编码的应用——码分多址CDMA通信
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What is Multiple Access? • Multiple users want to communicate in a common geographic area • Cellular Example: Many people want to talk on their cell phones. phones phone must communicate with • Each p a base station. • Imagine if only one person could talk on their cell phone at a time! Problem: How should we share our resources so that as many users as possible can communicate simultaneously.
Sgn()为判决， 相乘求和为相关接收
正交，相关输出为0
A numerical example for CDMA
Pseudo Noise(PN) Sequence
• 随机序列的三个基本特性
– 均衡性：序列中“0”“1”等概 – 游程特性：序列中长度为1的游程占1/2，长度为k的游程约占1/2k – 白噪声特性：自相关函数为δ(τ)，功率谱平坦
