Analysis of Direct Sequence and Chirp Spread Spectrum over Flat Fading Channel for Multiple Access

This paper compares the performance of phased chirp spread spectrum and direct sequence spread spectrum technique for multiple access over different fading channels. The direct sequence spread spectrum technique is motivated by the inherent interference rejection capability of such spread-spectrum type system, especially in circumstances where immunity against Doppler shift and fading due to multipath propagation are important. Linear chirp of different chirp rates and phases spreads the modulated signal that creates a pseudo-orthogonal set of spreading codes. An efficient system for chirp spread spectrum, comparable to direct sequence spread spectrum can be made by combining linear chirp modulation with polar signaling, which reduces the multiple access interference. Simulation models for both the systems have been outlined in MATLAB and bit error rates over fading channels in different Doppler environments, have been calculated when the coherence time is of the order of bit duration. Simulation results show that under proper selection of parameters; the phased chirp spread spectrum technique performs approximately same to direct sequence spread spectrum in all flat fading channels in terms of bit error rate.