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Summary

Summary. Direct-sequence MPSK and Frequency-hop MFSK represent two principle categories of spread-spectrum communications. Another technique is time-hope PPM, may be popular in the future. They rely on the use of a pseudo-noise (PN) sequence, which is applied differently in these categories.

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Summary

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  1. Summary Direct-sequence MPSK and Frequency-hop MFSK represent two principle categories of spread-spectrum communications. Another technique is time-hope PPM, may be popular in the future. They rely on the use of a pseudo-noise (PN) sequence, which is applied differently in these categories. The PN sequence is a periodic binary sequence with a noiselike waveform that is usually generated by means of a feedback shift register. There are two components of the synchronization in SS system. The first component, called acquisition, is the determination of the initial code phase; the second component, called tracking, is the problem of maintaining code synchronization after initial acquisition. 2.4.9

  2. Summary Many kinds of jamming waveforms occur in practice. In any event, there is no single jamming waveform that is worst for all SS systems, and there is no single SS system that is best against all possible jamming waveforms. Smart antenna can enhance the BER performance of CDMA systems or increase the number of simultaneous subscribers that can be supported in each cell. 2.4.9

  3. References [1] Simon Haykin, Communication Systems, 3th ed., John Wiley & Sons, 1994. [2] J. K. Holmes, Coherent Spread Spectrum Systems, John Wiley & Sons, Inc. 1982. [3] R. C. Dixon, Spread Spectrum Systems with Commercial Applications, 1994, John Wiley & Sons, Inc. [4] R. E. Ziemer, R. L. Peterson, Digital Communications and Spread Spectrum Systems, 1985. [5] T. S. Rappaport, Wireless Communications: Principles & Practice, 1996, Prentice Hall, Inc. [6] W. W. Peterson and E. J. Weldon, Error-Correcting Codes, (MIT Press), 1972. [7] S. Haykin and M. Moher, Modern WirelessCommunications, Pearson Prentice Hall, 2005. [8] I. Oppermann, M. Hamalainen and J. Iinatti, UWB Theory and Applications, John Wiley & Sons, 2004. 2.4

  4. References [9] R. A. Scholtz, “Multiple access with time-hopping impulse modulation,” in Proc. MILCOM, Oct. 1993. [10] J. C. Liberti and T. S. Rappaport, Smart Antannas for Wireless Communications: IS-95 and Third Generation CDMA Applications, Prentice Hall, Inc., 1999. [11] R. E. Ziemer, R. L. Peterson, Introduction toDigital Communication, Prentice Hall, Inc., 2001. 2.4

  5. Problems Fig. P1 shows a four-stage feedback shift register. The initial state of the register is 0001. Find the states of shift register and the output sequence of the shift register. Fig. P1 A PN sequence is generated using a feedback shift register of length m = 7. The chip rate is chips per second. Find the following parameters: (a) PN sequence length. (b) Chip duration of the PN sequence. (c) PN sequence period. In a DS/BPSK system, the feedback shift register used to generate the PN sequence has length m = 7. The system is required to have an average probability of symbol error due to externally generated interfering signals that does not exceed . Calculate the following system parameters in decibels: (a) Processing gain. (b) Antijam margin. 2.4

  6. Problems Using the SSRG shown in Fig. P4, (a) find the characteristic polynomial. (b) By directly computing the states using the recursion relation, determine the output sequence when the register is initially loaded with and . How many maximal linear code sequences are available from an 11-stage sequence generator? How many runs of three ones would be expected in a chip m sequence? How many runs of three zeros would be expected in the same sequence? What are the autocorrelation values for a chip Gold sequence? Fig. P4 2.4

  7. Problems Assume a code has N PN symbols and the dwell time (integration time) is second. (a) If and , find the mean acquisition time in half chip increments (2N cells). (b) Find the mean acquisition time with , Mchips/sec, and N =2047. (a) Plot a typical PN code acquisition system for DSSS system. (b) Plot a typical diagram of -dither code tracking loop. When and , the variance of the acquisition time can be given by find the approximate variance as and . Consider a DS/BPSK system, suppose that the average interference power, ；the signal energy per bit, ; the chip time, ; and the bit rate, , determine the average probability of error. 2.4

  8. Problems A FH/MFSK system has the following parameters: Number of MFSK tone : M=4 Bit rate : Hop rate : (a) Find the minimal tone spacing for noncoherent orthogonal signaling; (b) determine the chip rate. Is this system a slow FH system? A slow FH/MFSK system has the following parameters: Period of the PN sequence : 15 Number of MFSK tone : M=4 Length of PN segment per hop : k=3 Total number of frequency hops : Calculate the processing gain of the system in decibels. 2.4

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