Study of bandwidth efficient DS-CDMA systems.

This thesis studies the modulation and coding aspects of bandwidth efficient direct-sequence CDMA systems. The novel concepts proposed and investigated are: DS-CDMA employing combined modulation schemes; MFSK/DS-CDMA employing orthogonal codes; and DS-CDMA employing permutation modulation. Under each concept, one or more schemes are developed and examined. The first concept suggests that two or more modulation schemes can be considered together as a combined modulation in a DS-CDMA system. By combining two modulation schemes, the signal energy per symbol for a given bit energy is increased. This in turn increases the system power efficiency and also the bandwidth efficiency. By combining MFSK and MPSK, we proposed the MFSK-MPSK/DS-CDMA scheme. By combining MFSK and differential MPSK, we proposed the MFSK-DMPSK/DS-CDMA scheme. The second concept can be thought of as an application of the first. By combining MFSK and the orthogonal PN codes, we proposed the MFSK-OC/DS-CDMA scheme. It is found in this thesis that the orthogonal code concept has many significant applications in both performance improvement and complexity reduction in DS-CDMA systems. An analysis also shows that the ideal orthogonality of the codes can be relaxed with very little performance degradation. Thus, quasi-orthogonal codes can be used in practical applications. In the third concept, permutation modulation is applied to the DS-CDMA systems and it is proved to be a very powerful scheme in fading channels. By using the multi-tone FSK scheme, we proposed the MT-FSK/DS-CDMA system. In a fading channel, this system introduces an implicit diversity, and thus is robust in mobile environments. The concept of orthogonal codes are used here to reduce the implementation complexity. By using a simple frequency tone selection rule, we further proposed a novel scheme, namely OMT-FSK/DS-CDMA in which all symbols are orthogonal each other. For the proposed schemes, performance analyses are made over AWGN and fading channels with and without FEC coding. Computer simulation is done for the MFSK-OC/DS-CDMA and MT-FSK/DS-CDMA systems.