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Analysis of techniques to enhance the performance of direct sequence spread spectrum signaling for wireless data communications

University of British Columbia

The processing gain of a spread-spectrum (SS) system is generally defined as the ratio between the transmission bandwidth and data bandwidth, and is a measure of its interference rejection capability. Due to limited spectrum, most licence-free commercial ISM-band direct sequence (DS) SS modems suffer from insufficient processing gain if they are to support a reasonably high data rate as required for wireless local area network applications. This thesis presents two DS-SS signaling systems, Code-Phase Multiplexing (CPM) and Code-Phase-Shift Keying (CPSK), both designed to increase the processing gain without reducing the aggregate data rate. In CPM several data bits are modulated with different phase shifts of the same pseudonoise (PN) code sequence and multiplexed for parallel transmissions, whereas CPSK is an M-ary DS-SS modulation method which employs a signaling set consisting of different phase shifts of a single PN code sequence. The performances of both systems against a single tone jammer, over multipath fading channel and against a single tone jammer in the presence of multipath fading are analyzed. Theoretical and simulation results show that the proposed CPM scheme is able to increase immunity against single tone jamming by increasing the number of parallel signaling streams. Simulation results also show that the performance of CPM DS-SS signaling in the mobile fading channel is very similar to that of conventional DS-SS signaling, and could be improved by the use of a RAKE receiver. However, in the presence of tone jamming in the fading channel, CPM DS-SS signaling gives better performance than conventional DS-SS signaling. While CPM, like conventional DS-SS modulation, gives no advantage over BPSK with respect to thermal noise, CPSK offers increasing thermal noise immunity as M increases. With M=64, CPSK offers a 3.5 dB reduction in E b / N 0 over BPSK. Furthermore, the effect of single-tone interference at the carrier frequency is totally mitigated. It is also shown that CPSK maintains good performance in a multipath fading channel with a dominant direct path, a situation common to wireless local communications. A RAKE receiver could also be used to improve performance in a multipath channel without a dominant direct path. At the expense of increased receiver complexity, CPSK is effective in increasing both the power and bandwidth efficiency of DS-SS signaling.

Thesis/Dissertation

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2009-03-06

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http://hdl.handle.net/2429/5665

Electrical and Computer Engineering

University of British Columbia

UBCV

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