A fractionally spaced decision feedback joint detection proposal for multi carrier CDMA

Significant attention has been paid lately on Multiple Access (MA) schemes based on Code Division Multiple Access (CDMA) as well as Orthogonal Frequency Division Multiplexing (OFDM), especially in the context of the UMTS Terrestrial Radio Access (UTRA) selection process. The advantages and weaknesses of CDMA schemes are relatively well known as they provide high immunity against multipath, high flexibility and they are well suited for variable rate links. On the other hand, their performance and capacity is strongly dependent and limited by Multiple Access- and Self- Interference (MAI and SI respectively). The advances in DSP technology resulting in compact and relatively easy to implement digital modulators combined with the favourable characteristics of OFDM such as immunity to Inter Symbol Interference (ISI), bandwidth efficiency and frequency diversity have resulted over the last years in a number of different radio access scheme proposals for mobile channels that combine OFDM and CDMA. In the context of this paper, we focus in the system implementation known as Multi Carrier CDMA (MC-CDMA). The use of equalising receiver structures versus the use of the Guard Time technique in MC transmission schemes is discussed and a Fractionally Spaced Decision Feedback Equaliser (FS/DFE) receiver structure for an MC-CDMA system is proposed. Its performance is evaluated by means of computer simulations and compared to the use of the Guard Time technique. A FS Multiple Input Multiple Output (MIMO) DF equalising structure performing Joint Detection (JD) further improves the performance of the proposed equalising scheme in terms of MAI.