OPTIMIZATION OF CDMA SYSTEM BY USING HYBRID DS TH SPREAD SPECTRUM TECHNIQUE

Abstract

The main motivation of this thesis is to optimize the non-coherent detector in code division multiple access (CDMA) communication system. The choice to use non-coherent detection is made because of the fact that non-coherent detectors does not utilize the phase information in receiver side, resulting in lowering detector complexity. In the ultra wideband (UWB) technique both direct-sequence spread spectrum (DS-SS) and time hopping SS (TH-SS) can be combine, since UWB system has ability of combining the advantages of direct-sequence (DS)-CDMA and time-hopping (TH)-CDMA systems and avoiding their disadvantages. This hybrid DS-TH CDMA system can mitigate the channel effect in communication system, naming inter-symbol interference (ISI), multiuser interference (MUI) and multipath fading effects. The correlation detector does not have ability to mitigate these channel effect while linear detector minimum mean-square error (MMSE) has ability to mitigate these channel effect. In single user environment where there is no obstacle between transmitter and receiver and are in line of sight with each other, the correlation detector performance is near optimum and low complexity. However, correlation detector can also be used for multiuser environment if channel effect is ignored.Furthermore, MMSE detector which has ability to tackle channel effect can use hybrid DS-TH UWB system to mitigate channel effect. This novel has three phases, in first phase both correlation and MMSE detector have been simulate in multiuser environment ignoring channel effect, we find out that correlation detector perform well with lower complexity and near ideal BER performance. In second phase channel effect have been introduced and simulate the MMSE for hybrid DSTH UWB system, result shows MMSE produced good result by varying DS iii and TH spreading factor of DS-TH CDMA system. However, the complexity for DS-TH CDMA increases for MMSE detector. In third phase, the adaptive detector least-mean square (LMS) is investigated for DS-TH CDMA system in term of their convergence speed, robustness, computational complexity and BER performance. LMS has lower complexity then correlation detector and has BER performance better than MMSE, since it does not require channel estimation. However, LMS result low BER performance in low SNR region.