A proposal of simultaneous spread of PPM in frequency and time axes for adaptive CDMA

We have two goals in this paper. One is the comparison of Spread Spectrum ( SS) CDMA and Spread Time (ST) CDMA. The other is to propose a new SS-ST CDM A system, which is an adaptive CDMA with both merits of SS and ST CDMA. SS and ST CDMA are compared from the view point of two dimensional space "freq uency (B Hz)-time (T Sec)" together with their communication capacity. A pr imary modulation is assumed to be PPM in ST CDMA, and FSK in SS CDMA which is regarded as PPM in frequency axis. Both SS and ST CDMA are combined to g ive the proposed SS-ST CDMA, where transmitted signals are spread both in t ime and frequency domain. In order to realize the proposed system, a transm itter model is presented, and two receiver structures are discussed. The di screte Fourier transform (DFT) is employed for the system flexibility Altho ugh SS. ST and SS-ST CDMA are shown to have the same capacity of 0.7213, th e combined SS-ST CDMA has a merit of adaptability to adjust spreading gain of ST and SS according to property of channels, an impulsive noise dominate d or a CW interference dominated channel. Numerical results of DFT are also shown to illustrate the waveform and spectrum of the proposed SS-ST CDMA s ystem. Finally the symbol error probability performance of ST PPM, SS FSK a nd combined SS-ST systems in CW and impulsive environment is presented.