Overview

In Chapters 2 5 our attempts at dealing with the multipath channel effect have focused on the receiver end. Our point of view is that broadband DS-CDMA signals, when transmitted through frequency-selective fading channels, are inevitably distorted and thus can only be recovered through receiver side processing.

Instead of relying on sophisticated reception algorithms, the technique of "multicarrier transmission" copes with hostile radio channels with low-speed, parallel operations. By transmitting high-speed data through low-rate streams, the symbol duration in each substream increases, leading to higher immunity against multipath dispersion. The advantages and success of multicarrier modulation (MCM) and CDMA techniques motivated many researchers to investigate the suitability of combining MCM with CDMA for wideband multiple-access communications [1 9]. This combination, known as MC-CDMA allows one to benefit both the advantages of both schemes.

Several schemes have been proposed to date. An overview of the three most popular proposa1s, multitone CDMA (MT-CDMA) [10], multicarrier direct-sequence CDMA (MC-DS-CDMA) [11, 12, 13], and multicarrier CDMA (MC-CDMA) [14, 15], is given in [16]. In all three schemes, users are allowed to transmit on many available subchannels, thus obtaining the maximum benefit from multicarrier transmission. Each user is assigned a CDMA code, which is used to differentiate between signals belonging to different users at the receiver.

In this chapter, we will first study the modulation and detection strategies of MC-CDMA based on a common framework that encompasses several existing MC-CDMA schemes. The sensitivity of MC-CDMA in the presence of carrier offsets will then be investigated.