[信息与通信]CDMA20001X基础知识.ppt

GNL 070  主要内容 码分多址的基本原理 码分多址的关键技术 CDMA同GSM的比较 目标 不同用户是如何用码区分出来的 三种CDMA码的用途 CDMA逻辑信道的划分 编码，软切换和功率控制 码分多址的基本原理 码分多址的关键技术 CDMA同GSM的比较 CDMA IS-95A+系统结构实例 CDMA 物理信道 CDMA 首选载波分配 联通拥有的频率资源 CDMA的基本原理：CDMA码 PN码 （伪随机噪声序列） 貌似随机，实际上是有规律的周期性二进制序列。在CDMA中用到两个伪随机序列，长码和短码。 Walsh Code 具有正交特性 伪随机码 (PN CODE) 的生成 伪随机码 (PN CODE) Walsh码递归公式 8阶Walsh码 CDMA Codes PN short code Length＝215 －1 ＋1＝ 215 ＝32，768 chips Walsh code 64 PN long code Length＝242-1 4.4× 1012 chips CDMA系统中采用的编码 CDMA系统中采用的编码 PN短码偏置 CDMA系统中采用的编码 CDMA前向信道结构 码分多址的基本原理 码分多址的关键技术 CDMA同GSM的比较 CELP Code Excited Linear Predictive Compressed digital voice between XC and MS Uses much less data to approximate the original signal 8K or 13K 功率控制 反向开环功率控制 反向闭环功率控制 前向闭环功率控制 开环功率控制： Assumes Loss is Similar on Forward and Reverse Paths Receive Power+Transmit Power = -73 All powers in dBm Example: For a Received Power of -85 dBm Transmit Power = (-73) - (-85) Transmit Power = +12 dBm Provides an Estimate of Reverse TX Power for Given Propagation Conditions 反向功率控制： Maximum System Capacity is Achieved if: All Mobiles are Powered Controlled to the Minimum Power for Acceptable Signal Quality As a result, all Mobiles are Received at About Equal Power at the Base Station Independent of Their Location There are Two Types of Reverse Control: Open Loop Power Control Closed Loop Power Control Open Closed Loop Power Control are Always Both Active 闭环功率控制： Directed by Base Station Updated Every 1.25 msec Commands Mobile to Change TX Power in +/-1 dB Step Size Fine Tunes Open Loop Power Estimate Power Control Bits are Punctured over the Encoded Voice Data Puncture Period is two 19.2 kbps Symbol Periods = 103.6 usec CDMA软切换 更软切换 软切换 CBSC 间软切换 分集技术： 信号元素合并(Rake 接收机) 帧选择 Path Diversity Improves Voice Quality Soft Handoff Improves Voice Quality Frequency Diversity Improves Voice Quality 码分多址的基本原理 码分多址的关键技术 CDMA同GSM的比较 Performance Enhancements Reverse Link Pilot for Each Mobile True QPSK Modu