IB-DFE Receiver Techniques for CP-Assisted Block Transmission within DS-CDMA and MC-CDMA Systems Rui Dinis, Paulo Silva and Ant´ onio Gusm˜ ao CAPS-IST, Tech. Univ. of Lisbon, Av. Rovisco Pais, 1049-001 Lisboa, Portugal, Phone: +351 218419358; Fax: +351 218465303; email: rdinis@ist.utl.pt Abstract - MC-CDMA (MultiCarrier Code Division Multiple Access), currently regarded as a promissing multiple access scheme for broadband communications, is known to combine the advantages of an OFDM-based (Or- thogonal Frequency Division Multiplexing), CP-assisted (Cyclic Prefix) block transmission with those of CDMA systems. Recently, it was recognised that DS-CDMA (Di- rect Sequence) implementations can also take advantage of the beneficts of the CP-assisted block transmission approach, therefore enabling an efficient use of FFT-based (Fast Fourier Transform), chip level FDE (Frequency- Domain Equalisation) techniques. In this paper we consider the use of IB-DFE (Iter- ative Block Decision Feedback Equalisation) FDE tech- niques within both CP-assisted MC-CDMA systems with frequency-domain spreading and DS-CDMA systems. Our simulation results show that an IB-DFE receiver with moderate complexity is suitable in both cases, with ex- cellent performances that can be close to the single-code matched filter bound (especially for the CP-assisted DS- CDMA alternative), even with full code usage. I. Introduction It is widely known that a CP-assisted block transmission approach, allowing low-complexity FDE receiver techniques, is suitable for high data rate transmission over severely time- dispersive channels. This approach can be employed with ei- ther MC (MultiCarrier) or SC (Single-Carrier) modulations [1], [2]. When adopted in CDMA systems, it leads to MC-CDMA implementations [3], [4], [5], and also, as recently recognized, quite efficient DS-CDMA implementations [6]. These CP- assisted schemes are especially interesting for multicode and/or downlink transmission, since all codes are synchronised, which simplifies the receiver implementation. Conventional, linear FDE techniques are known to lead to a significant noise enhancement when a ZF (Zero Forcing) criterion is adopted in channels with deep in-band notches, which can lead to significant performance degradation. For this reason, an MMSE (Minimum Mean-Squared Error) FDE equaliser is usually preferable [7]. However, an MMSE FDE does not perform an ideal channel inversion. Therefore, when this type of equaliser is employed within CP-assisted CDMA systems, we are not able to fully orthogonalise the different spreading codes. This means severe interference levels, espe- cially when different powers are assigned to different codes. It is well-known that nonlinear equalisers can significantly outperform linear equalisers. For this reason, a promising IB- DFE (Iterative Block Decision Feedback Equalisation) ap- proach proposed for CP-assisted SC schemes [8], with both the feedforward and the feedback parts implemented in the frequency domain (a similar concept was also proposed in [9]). Since the feedback loop takes into account not just the hard-decisions for each block, but also the overall block reliability, the error propagation problem is significantly re- duced. Consequently, the IB-DFE receivers offer much better performances than the linear, non-iterative FDE receivers [8], [10]; moreover, their implementation is much less complex than that of frequency-domain turbo-equalisation [11]. In this paper, we consider the use of IB-DFE techniques for CP-assisted block transmission within MC-CDMA systems with frequency-domain spreading and DS-CDMA systems. This paper is organized as follows: Sec. II describes the CP- assisted CDMA schemes. The IB-DFE receiver is described in sec. III. A set of performance results is presented in sec. IV, and sec. V is concerned with the conclusions and complementary remarks of this paper. II. CP-Assisted CDMA Schemes In this section we describe the CP-assisted DS-CDMA and MC-CDMA schemes to be considered, involving a multicode transmission with constant spreading factor (the extension to VSF schemes (Variable Spreading Factor) is straightforward). In both cases, the receiver can be based on a linear FDE, as depicted in fig. 1A. As with other CP-assisted techniques, after removing the cyclic extension, the received time-domain block fy n ; n =0; 1;:::;N ¡ 1g is passed to the frequency domain, leading to the block fY k ; k =0; 1;:::;N ¡ 1g. When the cyclic extension is longer than the overall channel impulse response, the samples Y k can be written as Y k = H k S k + N k ; (1) where H k and N k denote the channel frequency response and the noise term for the kth frequency, respectively, and fS k ; k =0; 1;:::;N ¡ 1g = DFT fs n ; n =0; 1;:::;N ¡ 1g,