展频通讯原理
S ST TU UT T E EE E L Li in n 1展頻通訊原理 (技職 92 版) Principles of Spread Spectrum Communication 林福林 南台科技大學電子工程學系 中華民國九十二年五月 S ST TU UT T E EE E L Li in n 2S ST TU UT T E EE E L Li in n 3主要參考書籍 1. J. K. Holmes, “Coherent Spread Spectrum Systems”, 1982, John Wiley & Sons, Inc. 其他參考書籍 1. S. Haykin, “Communication Systems”, 2000, John Wiley & Sons, Inc 2. J. Viterbi, “CDMA - Principles of Spread Spectrum Communication”, 1995, Addison-Wesley Publishing Company. 3. R. C. Dixon, “Spread Spectrum Systems with Commercial Applications”, 1994, John Wiley & Sons, Inc. 4. D. J. Torrieri, “Principles of Military Communication Systems”, 1981, Artech House, Inc. 5. M. K. Simon, J. K. Omura, R. A. Scholtz and B. K. Levitt, “Spread Spectrum Communications”, 1985, 6. R. E. Ziemer, R. L. Peterson, “Digital Communications and Spread Spectrum Systems”, 1985, 7. J. C. Liberti and T. S. Rappaport, “Smart Antannas for Wireless Communications: IS-95 and Third Generation CDMA Applications” 1999, Prentice Hall, Inc. 8. W. W. Peterson and E. J. Weldon, “Error-Correcting Code”, 1972. S ST TU UT T E EE E L Li in n 4目錄： 第一章 展頻通訊原理簡介 p1-32 第二章 虛擬隨機序列和金氏碼 第三章 展頻碼之同步(獲取與追蹤) 第四章 展頻通訊之性能分析 第五章 展頻通訊的例子 S ST TU UT T E EE E L Li in n 1第一章 展頻通訊原理簡介 1.1 Definition and benefits: 1. Spread spectrum is a means of transmission in which the data sequence occupies a bandwidth in excess of the minimum bandwidth necessary to send it. 2. The spectrum spreading is accomplished before transmission through the use of a code that is independent of the data sequence. The same code is used in the receiver to de- spread the received signal so that the original data sequence may be recovered. * FHSS: Bluetooth, IEEE 802.11, DSSS: IS-95, cdma-2000, W-CDMA, IEEE 802.11b, GPS, TRC-4, THSS: UWB, S ST TU UT T E EE E L Li in n 2展頻通訊的優點包括： 1. 對人為的刻意干擾 (Ja m m i n g ) 訊號有良好的抵禦能力。 2. 通訊過程被截收的可能性較低。 3. 具有簡單的保密通訊能力。 4. 利用此技術可完成分碼多工通訊。 5. 可以改善多路徑干擾所造成通訊品質惡化的現象。 S ST TU UT T E EE E L Li in n 31.2 虛擬隨機序列 Pseudo-random sequences A pseudo- noise (PN) sequence is a periodic binary sequence with a noise- like waveform that is usually generated by means of a feedback shift register, a general block diagram of which is shown in Figure 1.1. Fig. 1.1 Feedback shift register. S ST TU UT T E EE E L Li in n 4A feedback shift register consists of an ordinary shift register made up of m flip- flops and a logic circuit that are interconnected to form a multiloop feedback circuit. The flip- flops in the shift register are regulated by a single timing clock. The PN sequence so generated is determined by the length m of the shift register, its initial state, and the feedback logic. With a total number of m flip- flops, the number of possible states is at most m2. It follows that the PN sequence must eventually become periodic with a period of at most m2. A feedback shift register is said to be linear when the feedback logic consists entirely of modulo-2 adders. In such a case, the zero state is not permitted. Consequently, the period of a PN sequence cannot exceed 12m. When the period is exactly 12m, the PN sequence is called a maximal-length-sequence or simply m-sequence. S ST TU UT T E EE E L Li in n 5EX1.1: Assume that the initial state of the shifter register is 100. Then, the succession of states will be as follows: 100, 110, 111, 011, 101, 010, 001, 100, The output sequence (the last position of each state of the shift register) is therefore: 0011101001110100 . 123Cl o c kS 0S 1S 2S 3Fl i p - Fl o pO u t p u t S e q u e n c e)1)(1)(1 (13327XXXXXX+=+ S ST TU UT T E EE E L Li in n 6Fig. 1.2 Maximum- length sequence generator for m=3. S ST TU UT T E EE E L Li in n 7虛擬隨機序列具有一些良好的特性，包括： (1) 假設移位暫存器的個數為 m，則其重複週期為 12=mn 個位元。 如果每次取出m個位元 ， 則除了m個 0 位元以外的狀況會各出現一次 。 (2) 在每一個序列週期內，0 與 1 的個數相差一個。(balance property) (3) 如果我們定義位元串 (Runs) 為相同的位元串接在一起的集合，則 在每一個序列週期內，位元串長度 (Run Lengths) 出現為 1 的出現機率 是 1/2 ， 位元串長度出現為 2 的出現機率是 1/4 ， 依此類推 。 (run property) S ST TU UT T E EE E L Li in n 8(4) 自相關函數 (Auto- Correlation Function) 如式 1.1 所示，所以當 m 很 大時，此序列看起來像是白色的 (White)。(correlation property) += periodtheofremaindertheforNTNTN Rc c /1|11)(1.1) Figure 1.3 展示了一個m=3的例子，其中圖(b)為自相關函數，圖(c)為功率頻譜密度。 S ST TU UT T E EE E L Li in n 9Fig. 1.3 (a) Waveform of maximum- length sequence for length m=3. (b) Autocorrelation function. (c)Power spectral density. S ST TU UT T E EE E L Li in n 10底下是另一個 5=m 的例子，其週期 31=n，取個週期得到結果如下： 1 0 1 1 1 0 1 1 0 0 0 1 1 1 1 1 0 0 1 1 0 1 0 0 1 0 0 0 0 1 0 其自相關函數的運算結果如圖所示。 0102030405060708090100-2002040Auto-correlation function for m-sequence baswav2.m0102030405060708090100-2-1012PN sequence time waveformFig. 1.4 準隨機序列 5=m，週期 31=n 之自相關函數的運算結果示意圖 S ST TU UT T E EE E L Li in n 11(未除N) 1.3 直接序列展頻 Direct-sequence Spread Spectrum )(tb: the information- bearing (data) signal )(tc: the
