Andrea Conti

Barbara Masini

Flavio Zabini

English

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)

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

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Archivio istituzionale della ricerca - Alma Mater Studiorum Università di Bologna

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Archivio istituzionale della ricerca - Università di Ferrara