Modeling and simulation of mixed modulation formats for improved CDMA bandwidth efficiency

It is well known that CDMA systems are capacity limited because of the code division multiple access noise. Most of the CDMA systems today use single modulation format such as non-return-to-zero with modulation schemes such as Gaussian minimum shift keying (GMSK) some form of quadrature phase shift keying (QPSK) or quadrature amplitude modulation (QAM). Because of the centered peak in the power spectral density (PSD) of these signals, the bandwidth is not efficiently utilized. There is another formatting scheme based on a generalization of Manchester coding which leads to signals with their PSD peak away from the center, and the PSD of these signals around the center is very small. These are generally known as split spectrum modulation signals, and the binary offset carrier (BOC) is one such example. Bandwidth efficiency can be improved by combining these two modulation formats beyond what is achievable using a single format. The capacity in this case is set by both the CDMA noise of each differently formatted signal and by the mutual interference between the NRZ and BOC signals. Mutual interference is reduced by the separation of the PSD peaks of the two formats there by resulting in the overall improved capacity. This concept of mixing the formats on the same link also will potentially provide the means to share the same frequency between two categories of users by assigning each group different formats possibly at different performance levels. In this paper a brief description of a scheme using this concept of mixing the modulation formats to achieve bandwidth efficiency is discussed. Simulation performed using SystemView^@ software is described and the results obtained in support of the viability of this concept are also presented.