Performance analysis of M-ary orthogonal DS system for infrared wireless communications

The performance of a direct-sequence spread-spectrum system using M-ary orthogonal modulation under the effect of multipath dispersion is evaluated through theoretical analysis and computer simulations. The performance of the system employing (i) random orthogonal codes and (ii) Walsh codes are investigated in a simulated non-directed line-of-sight infrared channel. An approximation to the bit error probability is given by using a Gaussian approximation for the intersymbol interference due to multipath propagation. The average probability of error as a function of the signal-to-noise ratio is used as the performance criteria. Results show that an M-ary orthogonal DS system is able to combat the effect of multipath dispersion since only small power penalties are incurred while increasing the system spectral efficiency. It is observed that Walsh codes have slightly worse performance than random orthogonal codes in a dispersive infrared channel, and the Gaussian approximation becomes more accurate as the length of the code sequence increases.