MMSE TURBO EQUALIZATION FOR MULTICODE DS-CDMA Kazuaki TAKEDA + and Fumiyuki ADACHI ++ Dept. of Electrical and Communication Engineering, Graduate School of Engineering, Tohoku University 6-6-05 Aza-Aoba, Aramaki, Aoba-ku, Sendai, 980-8579 Japan E-mail: + takeda@mobile.ecei.tohoku.ac.jp, ++ adachi@ecei.tohoku.ac.jp ABSTRACT Frequency-domain equalization (FDE) based on the minimum mean square error (MMSE) criterion can replace the conventional rake combining with significantly improved bit error rate (BER) performance of multicode DS-CDMA in a frequency-selective fading channel. However, the presence of residual inter-chip- inference (ICI) after MMSE-FDE produces the orthogonality distortion among the spreading codes and the BER performance degrades as the number of multiplex order increases. Recently, we have proposed a frequency-domain ICI cancellation to improve the uncoded performance. To further improve the BER performance, in this paper, we propose MMSE turbo equalization. In the proposed scheme, MMSE-FDE and ICI cancellation are incorporated into iterative maximum a posteriori (MAP) decoding, resulting in an MMSE turbo equalization. MMSE weight taking into account the residual ICI is updated in each iteration. The effect of MMSE turbo equalization is confirmed by computer simulation. Keywords-component; DS-CDMA, frequency-domain equalization, ICI cancellation, MMSE turbo equalization 1. INTRODUCTION In the third generation mobile communication systems, direct sequence code division multiple access (DS-CDMA) is successfully used [1]. Wireless channel is composed of many distinct propagation paths having different time delays, resulting in a frequency-selective fading channel [2]. In the present cellular systems adopting DS-CDMA, rake combining is applied to combat with the frequency-selective fading channel for the data transmissions of up to around a few Mbps [1], [3]. Recently, a lot of research attention has been paid to the next generation mobile communication systems that will support data services higher than several tens of Mbps. The wireless channel for high speed data transmission is severely frequency-selective and the BER performance with the rake combining degrades due to a strong inter-path interference. Hence, an advanced equalization technique is indispensable. Recently, it has been shown [4]-[7] that FDE based on the minimum mean square error (MMSE) criterion can replace the rake combining and improve the BER performance for the DS-CDMA signal reception over a severe frequency-selective channel. Although FDE can significantly improve the downlink BER performance, the presence of residual inter-chip interference (ICI) after FDE distorts the orthogonality among the spreading codes and the BER performance of orthogonal multicode DS-CDMA degrades as the code multiplexing order increases. The joint use of FDE and multi-access interference (MAI) cancellation for DS-CDMA uplink is considered in [8]. Recently, we have proposed a joint MMSE-FDE and frequency-domain ICI cancellation to improve the uncoded BER performance of the DS- CDMA downlink signal transmission [9]. For the data transmissions higher than 100Mbps, the use of strong error correction technique (e.g., turbo coding) is inevitable [10]. Recently, the turbo equalization technique has been drawing much attention since it can suppress the interference while achieving high coding gain by iteratively performing channel equalization and channel decoding [11]-[13]. In this paper, we propose MMSE turbo equalization to suppress the residual ICI present after FDE and to improve the decoded BER performance of multicode DS-CDMA. In the proposed scheme, MMSE-FDE, ICI cancellation and maximum a posteriori (MAP) decoding are repeated to successively suppress the residual ICI and to obtain the higher coding gain. MMSE weight taking into account the residual ICI is updated in each iteration. 2. TRANSMISSION SYSTEM MODEL 2.1. Overall transmission system The transmission system model for DS-CDMA with turbo equalization is illustrated in Fig.1. At the transmitter, after turbo coding and channel interleaving, a binary data sequence of Klog 2 M bits is transformed into a data modulated symbol sequence {d(n); n=0~K−1}, where M is the modulation level. The resulting data modulated symbol sequence is transformed, by serial-to- parallel (S/P) converter, into U parallel symbol sequences { ) (m d u ; m=0~K/U−1}, u=0~U−1, and then each sequence is divided into a sequence of blocks of N c /SF chips. Here, N c is the FFT window size and SF is the spreading factor. In this paper, one block transmission of { ) (m d u ; m=0~N c /SF−1} is considered for simplicity (i.e., K/U=N c /SF). One block of data symbols { ) (m d u ; m=0~N c /SF−1} is spread by multiplying an orthogonal spreading sequence c u (t) of spreading factor SF. The resultant U chip sequences are multiplexed and further multiplied by a common scramble sequence c scr (t) to make the resultant multicode DS-CDMA signal like white-noise. Then, the 1-4244-0411-8/06/$20.00 ©2006 IEEE.