Calculating error probabilities for DS-CDMA systems: when not to use the Gaussian approximation

Error probabilities for various unbalanced DS-CDMA systems are calculated using the standard Gaussian approximation, improved Gaussian approximation and the Fourier series based schemes. A scarcely populated system, a system with a dominant interferer, a system in a fading channel and a multimedia system with a single high processing gain user are considered. It is seen that the standard Gaussian approximation in all of these cases gives inaccurate results, especially when the number of users active in the system is low. The improved Gaussian approximation on the other hand, gives more accurate results for a scarcely populated system and a system with a dominant interferer. For systems in fading channels and multimedia systems, neither of the Gaussian approximations are accurate. For all of these cases, the Fourier series based scheme gives very accurate results without computational complexity.