Hybrid Multi-carrier CDMA Modulation based on VLSI

Abstract

CDMA is a multiple access technique, which is based on spread spectrum modulation scheme. It allows a multiple users to access the channel simultaneously, with each user's data being multiplied by a unique and orthogonal pseudo-random code sequence. Spread spectrum, as the name suggests, modulates the narrow band data and spreads it over a much wider bandwidth, thus transmitted data has a very low power spectral density. Moreover the inherent interference rejection capability of CDMA makes it suited to radio frequency environment. CDMA is interference limited, due to the multiple access interference of multiple users. Although orthogonal codes could result in no multiple access interference (MAI), in flat fading channels, the orthogonality will not be guaranteed in frequency selective fading channels because of interchip interference, which will cause MAI and degrade the system performance. One approach to suppress the effect of inter-chip interference in frequency selective fading channels is the combination of CDMA and multi-carrier modulation, such as OFDM that can achieve high spectral efficiency because the spectrum of successive sub-carriers is allowed. The combined CDMA system with OFDM is mainly categorized into MC/DS-CDMA and MCCDMA.