Calculation of Bit-Error Probability for Direct-Sequence Spread-Spectrum Communications with Multiple-Access Interference of Rayleigh Distribution

Even with a small user base, recent cellular mobile systems showed capacity saturation in major urban areas. Further generations of wireless systems will offer higher data rates and flexibility. This demand required a large capacity increase. Multiple digital methods were used to solve the cellular mobile system capacity problem. Different users can share a fixed-spectrum resource using two digital strategies. One uses different frequencies (FDMA), and the other uses different time slots (TDMA). FDMA, TDMA, and hybrid capacities are well-defined. When RF channels or time slots are unavailable, no more customers can be served. Military applications of spread spectrum (SS) have been successful for decades. This spread spectrum uses Code Division Multiple Access, a new multiple access method. CDMA's higher capacity and multipath resistance make it an attractive scheme. The biggest factor limiting CDMA capacity is Multiple Access Interference (MAI). In this paper, we examine how MAI affects DS-CDMA system Bit Error Probability. Many methods have been reported for calculating the DS-CDMA bit error probability. They include three methods: Standard Gaussian Approximation (SGA), Improved Gaussian Approximation (IGA) and simplified IGA. These methods use the Central Limit Theorem (CLT), which approximates theMAI distribution as a Gaussian with a zeromean. We model MAI as a Rayleigh distributed random variable. This model estimates the average BEP in an asynchronous DS-CDMA system well. SGA with a nonzero mean is used to compare our methods to previous work.