MC-CDMA Systems: a General Framework for Performance Evaluation with Linear Equalization

Abstract

The adaptation of wireless technologies to the users rapidly changing demands is one of the main drivers of the wireless access systems development. New high-performance physical layer and multiple access technologies are needed to provide high speed data rates with flexible bandwidth allocation, hence high spectral efficiency as well as high adaptability. Multi carrier-code division multiple access (MC-CDMA) technique is candidate to fulfil these requirements, answering to the rising demand of radio access technologies for providing mobile as well as nomadic applications for voice, video, and data. MC-CDMA systems, in fact, harness the combination of orthogonal frequency division multiplexing (OFDM) and code division multiple access (CDMA), taking advantage of both the techniques: OFDM multi-carrier transmission counteracts frequency selective fading channels and reduces signal processing complexity by enabling equalization in the frequency domain, whereas CDMA spread spectrum technique allows the multiple access using an assigned spreading code for each user, thus minimizing the multiple access interference (MAI) (K. Fazel, 2003; Hanzo & Keller, 2006). The advantages of multi-carrier modulation on one hand and the flexibility offered by the spread spectrum technique on the other hand, let MC-CDMA be a candidate technique for next generation mobile wireless systems where spectral efficiency and flexibility are considered as the most important criteria for the choice of the air interface. Two different spreading techniques exist, referred to as MC-CDMA (or OFDM-CDMA) with spreading performed in the frequency domain, and MC-DS-CDMA, where DS stands for direct sequence and the spreading is intended in the time domain. We consider MC-CDMA systems where the data of different users are spread in the frequency-domain using orthogonal code sequences, as shown in Fig. 1: each data symbol is copied on the overall sub-carriers or on a subset of them and multiplied by a chip of the spreading code assigned to the specific user. The spreading in the frequency domain allows simple methods of signal detection; in fact, since the fading on each sub-carriers can be considered flat, simple equalization with one complex-valued multiplication per sub-carrier can be realized. Furthermore, since the spreading code length does not have to be necessarily chosen equal to the number of subcarriers, MC-CDMA structure allows flexibility in the system design (K. Fazel, 2003)