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Unformatted text preview: Kevin Buckley - 2007 1 ECE 8770 Topics in Digital Communications - Sp. 2007 Lecture 13 6 Spread Spectrum & Multiuser Communications There are several reasons for using a spread spectrum scheme for digital communications. Two historical reasons, motivated in large part by military applications, are: interference mitigation – a narrow band interference signal will corrupt only a fraction of the communi- cations bandwidth; and security – signal transmission power spread over a large bandwidth will be difficult to detect. A third reason, which has become particularly important with the advent of mobile cellular systems, is added multiuser capacity. Some of the more important communication theory and signal processing issues associated with spread spectrum systems are: 1. modulation (e.g. FSK, PSK, QPSK), and spectral spreading and multiuser multiplex- ing (e.g. TDMA, frequency hopping, direct sequence CDMA (DS-CDMA)) schemes; 2. spreading code; 3. capacity, efficiency and bandwidth; 4. synchronization (e.g. carrier and bit); 5. matched filtering and fractional sampling; 6. ISI, signal fading, interference and multiple users; 7. symbol error rate and mean squared error; 8. detection, sequence estimation, equalization and interference reduction; 9. training and decision directed modes; 10. power control, hand off, and antenna diversity; and 11. multichannel (i.e. multiple antenna) processing. After a brief general introduction, our focus will be on DS-CDMA, consideration of basic multiuser issues, and an overview of sequence estimation. Kevin Buckley - 2007 2 6.1 Overview of Spread Spectrum Methods Consider a bit rate of R bits/sec. (that is, R bps). More generally, R could be the symbol rate. The bit interval is T b = 1 R . (1) R , or T b , dictate the transmission bandwidth if R bps are transmitted directly. Three methods which effectively increase the transmission bandwidth (spread the spec- trum) are: 1. Frequency Hopping (FH) : Over a bit interval T b , the carrier frequency is switched a number of times, according to some pseudo-random sequencing scheme dictated by a PN (pseudo noise) generated code. 2. TDMA (time-division multiple access) : Strictly speaking, this is not a spread spectrum method. However, if only a fraction of the time is allocated to a user to transmit, to achieve an average rate of R bps, shorter duration symbols must be crammed into a smaller allocated time slot, effectively increasing transmission...
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This document was uploaded on 10/12/2009.
- Spring '09