CDMA基础知识教案分析.ppt

IS-95 CDMA 系统信道及信号 CDMA 物理信道 CDMA前向信道结构 发射机结构 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 Path Diversity Improves Voice Quality Frequency Diversity Improves Voice Quality Effects of Using Variable Length Walsh Codes for Spreading Using Shorter Walsh Codes Precludes Using all Longer Codes Derived from the Original Shorter Codes on a Branch map into Longer Codes 谢谢！ Walsh 码 在CDMA前向逻辑信道中使用,以区分同一小区或扇区 用户 PN 短码 区分不同扇区或小区 伪随机序列 215 = 32,768 unit (period 26.67ms) 用于前向及反向物理信道扩频 64 chips 32,768 chips / 64 = 512 PN offsets .. . ... PN chips 长码 用于反向逻辑信道区分不同用户 伪随机序列 242-1 unit 在前向链路上对业务及寻呼信道进行扰码 Rake接收机 2、RAKE接收技术 Receiver Receiver Receiver Mobile Rake Receiver Analog TDMA Calls Fading D/A Audio Fading D/A Mobile Receiver CDMA Calls Effects of fading are mitigated through use of multiple receivers that sum radio signals from different cells and from reflected rays. Fading is more likely to cause loss of information = lower speech quality Outgoing Waveform -80 dB -90 db -110 db -120 db 1.25 Mhz 1.25 Mhz This 400 Khz fade of 12 db only affects 1/3 of CDMA bandwidth. Therefore, total fade to CDMA user is 2 db The entire 30 KHz Channel of TDMA is affected by this same fade. Therefore, total fade to end user is 12 db. CDMA Offers More Protection Against Frequency Selective Fading Received Waveform -80 db -90 db -110 db -120 db 30 Khz Outgoing Waveform -80 db -90 db -110 db -120 db 30 Khz Received Waveform -80 db -90 db -110 db -120 db 12 dB Fade 2 dB Loss 12 dB Fade 12 dB Loss! CDMA TDMA IS-95 CDMA关键技术—RAKE 接收 Correlator 1 Correlator 2 Correlator 3 Search Correlator Combiner De-Interleaver, Viterbi Decoder Multipath Delay Components