信道编码与交织

1、April 9, 2022信息产业部电信传输研究所1+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layersCHANNEL ENCODING AND DECODING1 GSM Channel Codec for Speech2 GSM FEC Codec for CCCH3 GSM FEC Codec for Data traffic 4 GSM Loop Type & BERsApril 9, 2

2、022信息产业部电信传输研究所2+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layersChannel 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

3、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。April 9, 2022信息产业部电信传输研究所3+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layers语音编码器输出260比特/20ms，按照它们的重要性分为3类：

4、 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。April 9, 2022信息产业部电信传输研究所4+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layersBlock EncoderIa：50b53bII：78bIb：132b106b 8b78bConvoluti

5、onalEncoder R= + 4 Tail bits378b456bFig.1 GSM Channel Encoding for Speech Signal264bApril 9, 2022信息产业部电信传输研究所5+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layersEncoding260 bits are emerged from speech encoder,the most important

6、first:The first 50 bits are class Ia: D0 D1D49The second 132 bits are class Ib: D50 D181The remaining 78 bits are class II: D182 D259D0 D1D49, D50 D181, D182 D25950b132b78bApril 9, 2022信息产业部电信传输研究所6+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + C

7、opyright ritt7layers1) 给50 Class Ia比特插入3个监督比特。 D0,D1,D49,P0,P1,P2,D50,D181,D182,D259Linear binary Block Code or (n,k) code:The ratio of the code = k/n (50/53)Minimum Distance d0 = n-k+1The error bits can be corrected = (d0-1)/2April 9, 2022信息产业部电信传输研究所7+ Mobile Communications + Bluetooth + Radio &am

8、p; Satellite Communications + Wireless Technologies + Copyright ritt7layers2) 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,D2593) 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 fro

9、m:D(e)=1+T3+T4 The odd output bits are derived from:D(o)=1+T+T3+T4 Where + means EXOR,T means delay of one clock period.91 Even bits91 odd bits78 class II bitsApril 9, 2022信息产业部电信传输研究所8+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt

10、7layersTTTTTIa bits in189b/20ms+Fig. 2 GSM Convolutional Encoder for Speech SignalD(e) )=1+T3+T4D(o) = 1+T+T3+T4April 9, 2022信息产业部电信传输研究所9+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layersBlock Interleaving and Diagonal Interlea

11、ving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。 April 9, 2022信息产业部电信传输研究所10+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layers57 bits8 bits8 blocks of 57

12、bitsD0 D1 D2 D3 D4 D5 D6 D7D8 D9 D10 D11 D12 D13 D14 D15D438 D439 D450 D451 D452 D453 D454 D455 April 9, 2022信息产业部电信传输研究所11+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layers而后进行所谓对角线交织交织，相邻20ms帧的（82）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。

13、这样， （82）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。April 9, 2022信息产业部电信传输研究所12+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright Yao Bin0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19448 44945557bits57bits57bits57bits57bits57bi

14、ts57bits57bits57bits57bits57bits57bits57bits57bits0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19448 449455116 bits116 bits2 stealing flags added per burst114 bits burstsTo even bits positionsTo odd bits positionsTo even bits positionsApril 9, 2022信息产业部电信传输研究所133B57A57260033B57A57260033B57A5726003

15、3B57A57260033A5757260033A5757260033A57572600335726003A573C57B57260033C57B57260033C57B57260033C57B5726003April 9, 2022信息产业部电信传输研究所14+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layers184bBlock Encoder（n/k=224/184）Tail bitConvoluti

16、onal Encoder 1/2224b228b456bChannel 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。April 9, 2022信息产业部电

17、信传输研究所15+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layersChannel Encoding for data traffic（1）9.6、4.8和2.4kb/s的数据输入通常需要加入一些控制信息，使数据速率变为12kb/s，即240比特/20ms。（2）加入4个尾比特“0000”，使卷积编码器清零。（3）244比特加入与语音相同的卷积编码器，输出为488比特/20ms。（4）通过删缩（ punc

18、turing ）使488比特减少到 456比特，这意味着删去一些比特，即比特11、26、41以及其后的每个第15比特，Integer (488-11)/15+1= 32 。总共删除32比特，输出为456比特/20ms。Insert Control bitsAdd Tail bit240b244b488bPuncturing456b/ 20ms192b/ 20msConvolutional Encoder9.6kb/s(192b/20ms)April 9, 2022信息产业部电信传输研究所16+ Mobile Communications + Bluetooth + Radio & Sa

19、tellite Communications + Wireless Technologies + Copyright ritt7layers（5）数据业务采用比语音业务更复杂的交织方式，114比特扩散在19个突发脉冲，6比特/突发脉冲。这是因为数据业务误码要求比语音业务高，而延时要求比语音业务低。语音业务必须在交织深度和延时之间权衡。April 9, 2022信息产业部电信传输研究所17 Copyright ritt7layersGSM Loop Type and BERsReference and absolute sensitivity The reference sensitivity:

20、When a signal with defined level (MS= 102 dBm, BTS= 104 dBm for GSM900) is applied to the receiver,if the measured BER is below the specified limit, the receiver is passed. The absolute receiver sensitivity:The level of the test signal is varied until a defined BER is obtained.Obviously, absolute re

21、ceiver sensitivity takes more time to measure than reference sensitivity. April 9, 2022信息产业部电信传输研究所18+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layersGSM Loop Type and BERsMeasurements: Acoustic Interface - DAI Interface (e.g.

22、SLR, RLR)Acoustic Interface - RF Interface (e.g. echo loss)DAI Interface - RF Interface (speech Codec test - not for DCS 1800) A/DD/A ConverterSpeech codecChannel codec FECMouthEarAcoustic InterfaceDigital Audio interfaceRF InterfaceRF UnitCA&B(FR) D,E,F(HR)April 9, 2022信息产业部电信传输研究所19+ Mobile Co

23、mmunications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layersBER test loop type: A,B, C,D,E,F AND NEW ONES Types A, B, D, E and F the tester generates a pseudo-random bit stream, which is channel-coded and applied via the RF interface to the receiver

24、 of the mobile. There the data stream is channel decoded and then is sent back to the tester via channel coder, RF interface and channel decoder. What precisely the mobile sends back depends on the type of loop: Loop B:MS returns exactly what it has received. Loop A:MS does not returns the received

25、voice frames with non-correctable class 1a errors but marked as erased frames(EF). In this case the mobile sends back a voice frame consisting entirely of zeroes. On receiving such a voice frame, the tester increments the FER (frame error rate) counter. April 9, 2022信息产业部电信传输研究所20 Copyright ritt7lay

26、ersLoop types: D, E and F:They are used for half-rate connections and are of minor significance in production. Like loop type A, certain frames are not tested in the BER measurement (unreliable frames, erased SID frames and erased valid SID frames). Such frames are marked by zeroes.Loop types C:The

27、larger of numbers of mobile phones produced, the more important it becomes to cut testing times. This was the reason for introducing loop type C. The mobile sends back the received data stream without channel decoded. The advantage here is that, for the same transmission period, about five times as

28、many bits are available for determining bit errors. This type of loop is not supported by all mobile phones however. New loop types and test modes have defined for the new and upcoming mobiles that support HSCSD (high-speed circuit-switched data) and GPRS (general packet radio services).April 9, 202

29、2信息产业部电信传输研究所21+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright ritt7layers1)Bit Error Ratio (BER): The bit error rate is defined as the ratio of the bits wrongly received to all data bits sent. All voice and data bits are regarded, but n

30、o signalling frames. Within the voice frame no difference is made between class Ia, Ib and II bits. 2)Residual Bit Error Ratio (RBER):The residual bit error ratio is defined as the bit error rate (BER) in frames which have not been declared as erased. A voice frame is declared as invalid, when the p

31、arity bits (CRC) can not be calculated from the Ia bits. The RBER is determined by bit error rate of the class Ib and class II bits. 3)Frame erasure Ratio (FER):April 9, 2022信息产业部电信传输研究所22+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright r

32、itt7layersA frame is defined as erased if the error detection function in the receiver indicates an error. For full rate speech this is the result of the 3 bit cyclic redundancy check (CRC) as well as other processing functions that cause a Bad Frame Indication (BFI).For signalling channels it is th

33、e result of the FIRE code or any other block code used. For data traffic, FER is not defined.The System Simulator commands the MS to create the loop back facility from either the decryption unit output or the receivers channel decoder output to the transmitters encoder input. Erased frames are signa

34、lled to the System Simulator. The loop back facility is integrated into the HW of the MS.April 9, 2022信息产业部电信传输研究所23+ Mobile Communications + Bluetooth + Radio & Satellite Communications + Wireless Technologies + Copyright Yao BinStealing Flags:There are 2 stealing flags per incoming burst of 11

35、4bits which are used to indicate if control data(FACCH) is sent.During interleaving The traffic channel(TCH) dada is broken down into blocks of 114 bits.these are combined with two further bits called Stealing Flags to indicate if the information sent in this bust is traffic(speech,external data,etc

36、) or control data.When the flags are set to 1,it implies that control data is being sent.This means the space of traffic data is stolen by higher priority control data.Setting to 0 means it is traffic data.3575726113April 9, 2022信息产业部电信传输研究所24+ Mobile Communications + Bluetooth + Radio & Satelli

37、te Communications + Wireless Technologies + Copyright Yao BinBurst AssemblyInterleaved DataControl DataStealing FlagsTraining SequenceTail BitsModulationMEMORYDEVICEInput 22.8kb/scontinuallyOutput270.83kb/sIntermittentlyApril 9, 2022信息产业部电信传输研究所25+ Mobile Communications + Bluetooth + Radio & Sat

38、ellite Communications + Wireless Technologies + Copyright Yao BinIt is important to remember that during speech codec, channel codec and interleaving process, the data of 456 bits per 20ms is delivered, hence the bit rate is 22.8kb/s (43s per bit).When transmitting each bit occupies 3.69 s.so the bit rate is about 270.83kb/s.To achieve this conversion of bit rate or time compression ,it is