CDMA通信技术3直接序列扩频【荐】.ppt

第3章 直接序列扩频 Prof. Li Ping’an Wuhan Univ. of Tech. DS-SS Baseband Waveform Chips Demodulating DS Signals (1/2) Demodulating DS Signals (2/2) 扩频运算 模2加（异或实现）实现扩频 d(t) 0 1 -1 1 C(t) 1 0 1 1 0 1 0 1 -1 1 1 -1 1 -1 0 1 0 0 1 0 1 1 -1 1 1 -1 1 -1 1 -1 1 1 -1 1 -1 BPSK扩频发射机 BPSK扩频发射机 BPSK扩频发射机 发射的信号 发射信号 BPSK DS 接收机 BPSK DS 接收机 数据解调 等效基带接收模型 DS-SS系统干扰抑制 假设存在窄带干扰 DS-SS系统干扰抑制 假设存在宽带干扰 DS-CDMA通信 Duplex Methods of Radio Links Frequency Division Duplex (FDD) Forward link frequency and reverse link frequency is different In each link, signals are continuously transmitted in parallel. Time Division Duplex (TDD) Forward link frequency and reverse link frequency is the same. In each link, signals are incontinuously transmitted by turns just like a ping-pong. Example of FDD systems Example of TDD Systems Multiple Access Methods FDMA Overview TDMA Overview What is CDMA ? Summary of Multiple Access DS-CDMA System Overview (Forward link) Multiplexing using Walsh Code Synchronous DS-CDMA Asynchronous DS-CDMA Mobile Propagation Environment ??? Multi-path Fading Fading in non-CDMA System Fading in CDMA System ... Fading in CDMA System (continued) RAKE Near-Far Problem Power Control... Power Control (continued) Effect of Power Control Frequency Allocation (1/2) Frequency Allocation (2/2) Soft Handover (1/2) Soft Handover (2/2) The peaks and bottoms of received power appear, in proportion to Doppler frequency. Base Station (BS) Mobile Station (MS) multi-path propagation Path Delay Power path-2 path-2 path-3 path-3 path-1 path-1 Time Power Path Delay Power path-1 path-2 path-3 With low time-resolution, different signal paths cannot be discriminated. These signals sometimes strengthen, and sometimes cancel out each other, depending on their phase relation. ??? This is “fading”. In this case, sign