扩频技术 扩频信号的检测.pdf

2006-10-18 2/30 引言?扩频接收机的结构 RF MF A/D 时间频率同步时间同步基准 RAKE 解交织信道译码本节重点讨论内容? 2006-10-18 3/30 m TT >> 平坦衰落信道(Flat fading channel), 信道中只有一条径。 bit or symbolbit or symbol T m = 多径信道的时延扩展 T = 一个比特或符号的长度无线系统带宽 TW ∝ 1. 窄带系统: Three kinds of systems (1) 2006-10-18 4/30 相邻符号或比特间无干扰。但是:衰落还是存在的。在判决处没有干扰。 Received replicas of same symbol overlap in multipath channel 1. 窄带系统(续): No intersymbol interference (ISI) 2006-10-18 5/30 ... mm TT TT ≈<< 2. 宽带系统(TDM, TDMA): 频率选择性衰落(Selective fading channel), transversal filter channel model, good performance possible through adaptive equalization (future lecture). T = bit or symbol duration Intersymbol interference causes signal distortion T m= delay spread of multipath channel Three kinds of systems (2) 2006-10-18 6/30 S/N BER Frequency-selective channel (no equalization) Flat fading channel AWGN channel (no fading) Frequency-selective channel (with equalization) “BER floor” BER 性能 2006-10-18 7/30 mc TTT>> >> 3. 宽带系统(DS-CDMA): 频率选择性衰落(Selective fading channel), transversal filter channel model, good performance possible through use of Rake receiver (this lecture). Bit (or symbol)Bit (or symbol) T m= delay spread of multipath channel T = bit (or symbol) duration T c= Chip duration ... c TW ∝ Three kinds of systems (3) 2006-10-18 8/30 ? Rake 接收机<=> 一个简单的信号处理例子, 可以说明一些重要的要领, 特别是信道的要领。? Rake 用于 DS-CDMA (Direct Sequence Code Division Multiple Access) 系统。? Rake 的每一指路(fingers) 同步于宽带信道中的 Rake 接收机能区分的每一条径。? Rake 接收机的很重要一项任务就是信道估计。? Rake 接收机的每一指路的输出进行合并,一般使用最大比合并方式(Maximum bining: MRC) Rake receiver structure and operation 2006-10-18 9/30 in which case the received (equ ivalent low-pass) signal is of the form Suppose a signal s ( t )is transmitted. A multipath channel with M physical paths can be presented (in equivalent low-pass signal domain) in form of its Channel Impulse Response (CIR) () 1 0 () () () m M j mm m rt st ht ae st φτ?= =?= ?∑() 1 0 () m M j mm m ht ae t φδτ?= =?∑. To start with: multipath channel 2006-10-18 10/3