Multi-Carrier CDMA Systems Using Bridge Functions

Multi-Carrier CDMA systems appear to be a good candidate for high speed wireless data transmission. These systems avoid time domain equalization and have the capability of achieving high diversity gains over frequency selective fading channels. Orthogonal Walsh-Hadamard spreading waveforms have been commonly considered for these systems. However, reliable system performance over frequency selective fading channels requires complicated detector structures with a complexity that increases with the length of the spreading code. In this paper a multi-carrier CDMA system using the Bridge functions as spreading waveforms is considered and its performance over frequency selective fading channels is evaluated. The Bridge functions are three valued orthogonal functions which take the values +1, 0, and −1. Such a structure adds flexibility to the multi-carrier CDMA where the number of zeros in the spreading code can be used to decide on the frequency separation between the modulated subcarriers and thus avoiding the need for frequency interleaving. Another benefit is the possibility to use more powerful detectors at the receiver. Infact, this combined system can be seen as a generalization to multi-carrier CDMA where the Bridge functions play the role of bridging between the regular OFDM with no spreading and multi-carrier CDMA with WalshHadamard functions.