Mixed modulation signals for CDMA capacity improvement

It is well known that the capacity of the code division multiple access (CDMA) system is limited by the amount of self-interference, which is also called CDMA noise, and is generated because of multiple signals coexisting in the same band. Use of sectors, power control, voice activity factor, directional antennas, diversity combining, Rake receivers, and multiple user detection schemes are some of the techniques used to improve the user capacity. In most of the systems a single modulation scheme such as binary phase shift keying (BPSK), Gaussian minimum shift keying (GMSK), or some form of quadrature phase shift keying (QPSK) is used. All these signals have a power spectral density (PSD) with a centered peak. One class of split spectrum modulation signals, such as binary offset carriers (BOC), have their PSD peak away from the center. In certain scenarios it is possible to use a combination of different modulation schemes (mixed modulation signals) with their correspondingly minimally overlapping spectral shapes, to achieve up to 45% improvement in the CDMA capacity. In this paper a brief description of all the modulation schemes used currently in a wireless communication system is described. Examples of spectrally mixed modulation schemes are described, followed by a discussion of the capacity improvement expected for each case.