On Iterative Detection, Demodulation and Decoding for OFDM-CDM Armin Dammann 1 , Serkan Ayaz 2 , Stephan Sand 1 , Ronald Raulefs 1 1 Institute of Communications and Navigation, German Aerospace Center (DLR), Oberpfaffenhofen, D-82234 Wessling, Germany, Email: {Armin.Dammann, Stephan.Sand, Ronald.Raulefs}@DLR.de 2 Chair of Mobile Communications, University of Erlangen-N¨ urnberg, Cauerstrasse 7, D-91058 Erlangen, Germany, Email: sayazim@yahoo.com Abstract In this paper, we address iterative receiver processing for OFDM code division multiplexing (OFDM-CDM). The receiver algorithm we focus on is soft parallel interference cancellation in combination with a-priori demodulation. We investigate the convergence behavior of OFDM-CDM as a serial code concatenation, consisting of CDM as inner code and rate-1/2 convolutional codes as outer codes, by means of EXIT chart analysis. The EXIT charts will induce further adaptive modifications for iterative receivers in order to improve the bit error performance. As we will see, performance improvements can be mainly achieved at low signal-to-noise ratios resp. for medium bit error ranges. 1 Introduction OFDM-CDM and in particular its multiple access vari- ant multicarrier code division multiple access (MC- CDMA) are candidate techniques for a 4 th genera- tion mobile radio system (4G). Both schemes use typically orthogonal Walsh-Hadamard (WH) spreading sequences for their CDM resp. CDMA parts. One focus of ongoing research in OFDM-CDM are advanced receiver algorithms with the cost of an increased com- putational complexity. Optimum detection/decoding of the entire signal, however, is far to complex for the proposed 4G air interface designs. Iterative detection and decoding algorithms are known to be good subopti- mum alternatives. Soft parallel intererence cancellation (S-PIC) is one representative [1] of iterative schemes, which is proposed for receivers of OFDM-CDM based systems. For the analysis of the convergence behavior of such concatenated coding schemes with iterative re- ceiver structures, extrinsic information transfer (EXIT) charts have become a popular tool [2]. Throughout this paper, we focus on iterative re- ceivers for OFDM-CDM based on the S-PIC receiver principle. Compared to [1] and [3], we additionally include demodulation with a-priori information and use extrinsic rather than a-posteriori information, provided by the outer channel decoder, as a-priori input for the S-PIC. Based on the analysis of the EXIT charts of the OFDM-CDM systems under investigation, we propose approaches for improving the system perfor- mance, which are mainly based on adaptive detection and hybrid usage of modulation. Both modifications affect the CDM resp. S-PIC part of the receiver. In Section 2, we describe the transmitter and receiver structures, we focus on throughout this paper. In Sec- tion 3, the principles of adaptive detection and hybrid modulation are introduced together with simulation results in terms of EXIT charts and bit error rate investigations. 2 System Description The System, we focus on, is a coded OFDM-CDM system in general. In our approach, we use orthog- onal Walsh-Hadamard (WH) spreading, where all the L available spreading sequences are assigned to one user [3]. Hence, we consider WH-spreading as inner code of a serial concatenated coding scheme rather than as a multiple access component as is typically done in multi-carrier code division multiple access (MC-CDMA). The user separation, i.e. the multiple access, in OFDM-CDM is applied by either FDMA, TDMA or a combination of both. For the channel, we consider an uncorrelated Rayleigh fading channel in frequency domain, i.e. fading is flat per subcarrier with fading coefficients being independently and identically distributed complex valued Gaussian random variables. With this assumption and the respective simulation implementation in frequency domain, we implicitly neglect both inter-symbol interference (ISI) and inter- carrier interference (ICI). 2.1 Transmitter The Transmitter is shown in Fig. 1. We exemplarily consider one user, denoted as user 1. The information bits of that user are encoded using rate-1/2 convo- lutional codes with different constraint lengths. The codebits are then interleaved by a random interleaver and mapped to complex valued data symbols out of a M -QAM modulation alphabet. For our investigations, we use different mappings S : GF(2) m →S (1)