1、1,數(shù)字通信 (第十一講) 接收機(jī)的構(gòu)成原理(4) 2007,Yuping Zhao (Professor) 趙玉萍 Department of Electronics Peking University Beijing 100871, China email: ,2,M-ary orthogonal signals,3,The pdf of first correlation is,The rest of correlation,4,M 元正交信號(hào),假設(shè)發(fā)送信號(hào)集為調(diào)制到互不交疊的載頻 f1, f2, f3, f4,5,當(dāng)圖 1 中的接收信號(hào)大 于下 3 圖中其他分支的 接收信號(hào)時(shí)，判決發(fā)送

2、 信號(hào) 1。 誤碼率的計(jì)算 - 注意誤 符號(hào)率和誤比特率的互 算。,6,正確接收信號(hào)概率的求法：當(dāng)發(fā)送信號(hào)為 s1 時(shí)，接受信號(hào)采樣值 r1 大于所有噪聲采樣值,由于信號(hào)之間相互獨(dú)立，其概率密度函數(shù)可以表示為各項(xiàng)的乘積，其中每一項(xiàng)為,7,正確接收概率為,錯(cuò)誤接收概率為,8,For the bit error rate calculation,這里不出現(xiàn)格林編碼的問(wèn)題,每個(gè)符號(hào)錯(cuò)成其他符號(hào)的概率為：,每個(gè)符號(hào)攜帶的平均比特?cái)?shù)為：,平均每符號(hào)的錯(cuò)誤比特?cái)?shù)為：,誤比特率為：,9,Orthogonal signals,10,M-ary biorthogonal signals,Constructed

3、from M/2 orthogonal signals,Demodulation: Use M/2 orthogonal cross correlation or matched filters,Tx: s1(t),11,Cross correlation,The sign used to determine s(t) or -s(t) was transmitted,The correct decision for s1(t) r1 0 r1 rm=nm,Thus, -r1 nmr1, correct,12,The probability of correct decision is,13,

4、Biorthogonal signals,14,差分編碼,設(shè) ak 及 bk 均為二進(jìn)制符號(hào)，則有,15,設(shè) ak 及 bk 均為復(fù)數(shù)相位符號(hào)，則有,表示為相位關(guān)系，則有,16,Differential PSK,The received signal is compared with phase of the preceding signal no carrier information needed,17,No noise, - k - k-1 No noise, - rkr*k is independent of carrier phase,18,Error probability est

5、imation of DPSK,Assumption: k - k-1 = 0,- compared with nk-1 and nk,19,x, y are the uncorrelated Gaussian random variable , N0,Similar to PSK noise is twice larger DPSK has 3 dB poorer performance,20,Binary PSK and DPSK- less than 3dB differences Pb get smaller, differences becomes small,21,BPSK Pm=

6、1 Im=1,發(fā)送的信號(hào),或,34,因?yàn)?所以,判決,35,優(yōu)化雙二進(jìn)制信號(hào): 信號(hào)的譜在 f = 0 為零,36,37,部分響應(yīng)信號(hào)的擴(kuò)展 (M levels PAM),預(yù)編碼:,38,部分響應(yīng)信號(hào)的錯(cuò)誤概率,逐符號(hào)檢測(cè),發(fā)送信號(hào)的分布,Bm=0, 2d, 4d, 2(M - 1)d,39,40,因?yàn)?所以,41,42,性能損失 (/4)2 或 2.1dB,采用ML 序列檢測(cè), 2.1 dB 的損失可以補(bǔ)償,43,關(guān)于中繼的概念,模擬中繼系統(tǒng)：將接收信號(hào)放大后傳輸出去 數(shù)字中繼系統(tǒng)：將接收信號(hào)解碼、判決后，重新調(diào)制再發(fā)射出去,44,特性比較,模擬通信系統(tǒng)：將信號(hào)放大過(guò)程中也放大了噪聲值，多個(gè)中繼導(dǎo)致噪聲信號(hào)的增加 數(shù)字通信