A flexible multicarrier CDMA system with time-varying data rates and services

We consider an enhanced multicarrier code-division multiple access (CDMA) system that performs the spreading in the time and frequency domains simultaneously, where time-domain spreading codes are used as a main code and frequency-domain spreading codes (FDSCs) are used as a subcode. By allocating the FDSCs to users, rates and services can be dynamically matched to users' needs. The resulting system can support different time-varying source rates and, therefore, enable multimedia applications. In addition, since the proposed scheme uses multiple carriers, it has some advantages such as the mitigated multipath fading, frequency/path diversity and narrowband interference suppression capability, in contrast to a single carrier (SC) system such as the multicode CDMA scheme. Finally, the closed-form expression for probability of bit error is derived for a frequency-selective Rayleigh fading channel. It has been also shown that the proposed system has outperformed the conventional SC RAKE system if the available bandwidth is fixed.