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1、INTERLEAVINGInterleaving is used to obtain time diversity in a digital communications system without adding any second generation digital cellular system, due to the rapid proliferation of digital speech coders which transform analog voices into efficient digital messages that are transmitted over w
2、ireless links (speech coders are presented in Chapter 7).Because speech coders attempt to represent a wide range of voices in a uniform and efficient digital format, the encoded data bits (called source bits)carry a great deal of information, and as explained in Chapter 7 and 10, some source bits ar
3、e more important than others and must be protected from error .It is typical for many speech coders to produce several “important” bits in succession, and it is the function of the interleaver to spread these bits out in time so that if there is a fade or noise burst , the important bits form a bloc
4、k of source data are not corrupted at the same time . By spreading the source bits over time, it becomes possible to make use of error control coding(called channel coding)which protects the source data from corruption by the channel. Since error control codes are designed to protect against channel
5、 errors that may occur randomly or in a bursty manner , interleavers scramble the time order of source bits before they are channel code.An interleaver can be one of two forms-a block structure or a convolutional structure. A block interleaver formats the encoded data into a rectangular array of m r
6、ows and n columns, and interleaves nm bits at a time. Usually, each row contains a word of source data having n bits. An interleaver of degree m (or depth m) consists of m rows. The structure of block interleaver is shown in Figure 6.17. As seen, source bits are placed into the interleaver by sequen
7、tially increasing the row number for each successive bit, and filling the columns. The interleaved source data is then read out row-wise and transmitted over the channel. This has the effect of separating the original source bits by m bit periods.At the receiver, the de-interleaver stores the receiv
8、ed data by sequentially increasing the row number of each successive bit, and then clocks out the data row-wise, one word (row) at a time.Convolutional interleavers can be used in place of block interleavers in much the same fashion. Convolutional interleavers are ideally suited for use with convolu
9、tional codes.There is an inherent delay associated with an interleaver since the received message block cannot be fully decoded until all of the nm bits arrive at the receiver and de-interleaved. In practice, human speech is tolerable to listen to until delays of greater than 40 ms occur. t is for t
10、his reason that al of the wireless data interleavers have delays which do not exceed 40ms.The interleaver word size and depth are closely related to the type of speech coder used, the source coding rate and the maximum tolerable delay.FUNDAMENTAL OF CHANNEL CODINGChannel coding protects digital data
11、 form errors by selectively introducing redundancies in the transmitted data. Channel codes that are used to detect errors are called error detection codes, while codes that can detect and correct errors are called error correction codes.In 1948,Shannon demonstrated that by proper encoding of the in
12、formation, errors induced by a noisy channel can be reduced to any desired level without sacrificing the rate of information transfer. Shannons channel capacity formula is applicable to the AWGN channel and is given by C=BB)=B(1+S/N)Where C is the channel capacity (bits per second), B is the transmi
13、ssion band-width (Hz), P is the received signal power (watts), and N0 is the single-sided noise power density (watts/Hz). The receiver is given as P=Where is the average bit energy, and is the transmission bit rate. Equation can be normalized by the transmission bandwidth and is given byWhere C/B de
14、notes bandwidth efficiency.The basic purpose of error detection and error correction techniques is to introduce redundancies in the data to improve wireless link performance. The introduction of redundant bits increases the raw data rate used in the link , hence increases the bandwidth requirement f
15、or a fixed source data rate. This reduces the bandwidth efficiency of the link in high SNR conditions, but provides excellent BER performance at low SNR values.It is well know that the use of orthogonal signaling allows the probability of error to become arbitrarily small by expanding the signal set
16、, i.e., by making the number of waveforms M, provided that the SNR per bit exceeds the Shannon limit of SN=-1.6dBVit79.In the limit, Shannons result indicates that extremely wideband signals could be used to achieve error free communications, as long as sufficient SNR exists. Error control coding waveforms, on the other hand, have bandwidth expansion factors that grow only linearly with the code block length. Error correction coding thus offers advantages
