信道编码与交织

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57 57 57 57 A57
A57 A57 A57 B57 B57 B57 B57
3 3 3 3 3
3 3 3 3 3 3 3
13
0
0
0
0
0
0 0 0 0 0 0
0
0 0 0 0 0 0
26 26
26 26 26
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Channel Encoding for Control messages in the CCCH (1)控制信道消息包含184比特/20ms,首先进行分组编码,生成多 项式为,P=T39+T26+T23+T17+T13+1。这会使184比特增加40个监督比 特,变为224比特。由于加入更多的监督比特,纠错能力比语音帧强。 (2)加入4个尾比特,进行卷积编码。卷积编码器的生成多项式语音 卷积编码器相同,即,D’(even)=T4+T3+1;D’(odd)=T4+T3+T+1。 (3)CCCH的比特交织与语音帧相同。
132b
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D0 D1…D49 D50… D181 D182… D259
78b
5
D0 D1…D49, D50… D181, D182… D259
50b
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CHANNEL
ENCODING AND DECODING
1 GSM Channel Codec for Speech
2 GSM FEC Codec for CCCH 3 GSM FEC Codec for Data traffic 4 GSM Loop Type & BERs
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19…448 449…455 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19…448 449…455
57 bits
57
57
57
57
57 bits
To odd bits positions
Minimum Distance d0 = n-k+1
The error bits can be corrected = (d0-1)/2
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184b
Block Encoder 224b (n/k=224/184)
Tail bit
228b
Convolutional 456b Encoder 1/2
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116 bits
116 bits
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12
3 3 3 3 3
3 3 3 3 3 3 3
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A57 A57 A57 A57 B57
B57 B57 B57 C57 C57 C57 C57
0 0 0
26
26 26 26 26 26 26
0 0 0
8 blocks of 57 bits
8 bits
D0 D8
57 bits
D1 D9
D2 D10
D3
D4
D5
D6
D7
D11 D12 D13 D14 D15
D438 D439 D450 D451 D452 D453 D454 D455
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1) 给50 Class Ia比特插入3个监督比特。 D0,D1,…D49,P0,P1,P2,D50,…D181,D182,…D259
Linear binary Block Code or (n,k) code: The ratio of the code = k/n (50/53)
Encoding
260 bits are emerged from speech encoder,the most important first:
The first 50 bits are class Ia: The second 132 bits are class Ib: The remaining 78 bits are class II:
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Block Interleaving and Diagonal Interleaving
对卷积编码器输出的456比特进行编号和排序:
D0 D1 D2 D3 D4 D5 D6 D7 D8 … D453 D454 D455 首先,通过块交织将456输入比特分为8块,每块57比特。每块中 的相邻比特相距8比特,这表示交织深度为8比特。
The odd output bits are derived from:D’(o)=1+T+T3+T4
Where + means EXOR,T means delay of one clock period.
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而后进行所谓对角线交织交织,相邻20ms帧的(8×2)块数据 按对应块进行奇偶比特交织,组成114比特的输出数据。这样, (8×2)块数据构成8个114比特块,分为8帧发送出去。当丢失 一帧,只丢失20ms帧的1/8比特,视情况仍有可能纠错。
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Ia:50b Ib:132b
Block Encoder
53b
Convolution alEncoder R=½ + 4 Tail bits
106b 264b 378b
8b
456b
78b
II:78b
Fig.1 GSM Channel Encoding for Speech Signal
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2) Class 1 比特重新排序,首先是偶数比特,接着是监督比特,而后 是倒序的奇数比特,在Class I比特之后插入4个尾比特“0000”,使 卷积编码器清零。最后是78个Class II比特。 D0,D2,D4,…D180,P0,P1,P2,D181,D179,…D1,0,0,0,0,D182,…D259 91 Even bits 91 odd bits 78 class II bits 3) 对包括监督比特在内的Class I比特进行卷积编码,编码比率1/2。 The even output bits are derived from:D’(e)=1+T3+T4
57 bits
57
57
57
57
57 bits
bits bits
To even bits positions
bits bits
bits bits
bits bits
To even bits positions
114 bits bursts
2 stealing flags added per burst
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语音编码器输出260比特/20ms,按照它们的重要性分为3类: • Class Ia bits:50比特,很重要,经分组码和卷积码前向纠错; • Class Ib bits:132比特,比较重要,经卷积码前向纠错; • Class II bits:不重要,不经前向纠错。
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D’(e) )=1+T3+T4
+
Ia bits in 189b/20ms T T T T T
+
D’(o) = 1+T+T3+T4
Fig. 2 GSM Convolutional Encoder for Speech Signal
Channel CODEC for Speech (FORWARD ERROR CORRECTION ENCODER) 在无线和移动通信系统中,信道编码技术,即前向纠错(FEC) 技术,是十分重要的技术措施。信道编码的概念并不复杂,它是 借助于在所要发送的信息数据中有选择地引入一定数目的冗余比 特的方法,为接收端提供检测和/或纠正信息在无线传播环境中 可能发生的各种差错(如比特、帧、逻辑数据块的差错)的能力, 从而提高信息数据的传输质量。用以检测差错的信道编码叫做差 错检测码,如用作帧质量指示的循环校验(CRC)编码;用以纠 正差错的信道编码叫做差错校正码,如分组码、卷积码等。
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