Rotated Walsh-Hadamard Spreading with Robust Channel Estimation for a Coded MC-CDMA System

EURASIP Journal on Wireless Communications and Networking 2004:1, 74–83 c 2004 Hindawi Publishing Corporation
Rotated Walsh-Hadamard Spreading with Robust Channel Estimation for a Coded MC-CDMA System Ronald Raulefs Research Group for Mobile Radio Transmission, Institute of Communications and Navigation, German Aerospace Center (DLR), Oberpfaffenhofen, 82234 Wessling, Germany Email: Armin Dammann Research Group for Mobile Radio Transmission, Institute of Communications and Navigation, German Aerospace Center (DLR), Oberpfaffenhofen, 82234 Wessling, Germany Email: Stephan Sand Research Group for Mobile Radio Transmission, Institute of Communications and Navigation, German Aerospace Center (DLR), Oberpfaffenhofen, 82234 Wessling, Germany Email: Stefan Kaiser Research Group for Mobile Radio Transmission, Institute of Communications and Navigation, German Aerospace Center (DLR), Oberpfaffenhofen, 82234 Wessling, Germany Email: Gunther Auer NTT DoCoMo Euro-Labs, 80687 Munich, Germany Email: Received 1 November 2003; Revised 2 April 2004 We investigate rotated Walsh-Hadamard spreading matrices for a broadband MC-CDMA system with robust channel estimation in the synchronous downlink. The similarities between rotated spreading and signal space diversity are outlined. In a multiuser MC-CDMA system, possible performance improvements are based on the chosen detector, the channel code, and its Hamming distance. By applying rotated spreading in comparison to a standard Walsh-Hadamard spreading code, a higher throughput can be achieved. As combining the channel code and the spreading code forms a concatenated code, the overall minimum Hamming distance of the concatenated code increases. This asymptotically results in an improvement of the bit error rate for high signalto-noise ratio. Higher convolutional channel code rates are mostly generated by puncturing good low-rate channel codes. The overall Hamming distance decreases significantly for the punctured channel codes. Higher channel code rates are favorable for MC-CDMA, as MC-CDMA utilizes diversity more efficiently compared to pure OFDMA. The application of rotated spreading in an MC-CDMA system allows exploiting diversity even further. We demonstrate that the rotated spreading gain is still present for a robust pilot-aided channel estimator. In a well-designed system, rotated spreading extends the performance by using a maximum likelihood detector with robust channel estimation at the receiver by about 1 dB. Keywords and phrases: code division multiaccess, Walsh-Hadamard spreading sequences, multicarrier, fading channels, concatenated channel coding. 1. INTRODUCTION Multicarrier code-division-multiple access (MC-CDMA) is a promising candidate for the downlink for the fourth generation of mobile radio systems. MC-CDMA systems of- fer the spectral efficiency of orthogonal frequency division multiplexing (OFDM) combined with CDMA to combat multiuser interference losses and to offer a flexible multiuser access scheme. However, in a fully loaded system, MC-CDMA experiences a significant loss of performance Rotated Walsh-Hadamard Spreading for a Coded MC-CDMA System due to multiple-access interference. Mobile system operators, for example, in Germany, have paid tremendous amounts of money for licenses of the third mobile radio generation. Therefore, it is essential to identify solutions for fully loaded systems with high spectral efficiency in the presence of multiple-access interference. One solution is to perform complex interference cancellation. These schemes are proposed to enhance the system performance of multiuser systems [1, 2]. The complexity of multiuser detectors (MUDs) depends on the maximum number of users to detect. In addition, a cellular system offers more flexibility with shorter spreading codes. As the cells of a mobile radio system reduce further to increase data rates, especially in hotspots, intercellular interference is more likely. Combining the users in several small user groups allow by explicit use of frequency bandwidths for a distinct user group at the radio cell boundaries to avoid intercellular interference. The spectral efficiency in a cell would be reduced, but the system would be flexible as long as the number of users in a user group is small. On the other hand, performance gains through exploitation of diversity by the spreading code increase with the length of the spreading code. A reasonable spreading length is based on the overall system parameters. 1.1. Signal space diversity In this paper, we focus on an MC-CDMA system with a high-rate convolutional channel code and rotated spreading sequences. Both combined allow to exploit diversity in Rayleigh faded channels more efficiently. Rotated spreading sequences are derived from signal space or modulation diversity approaches. In the following, we briefly recap some references that focus on signal space diversity. Signal space or modulation diversity defines a multidimensional signal constellation. The signal constellation is transmitted over different, ideally independently faded channels. The latter could be realized, for example, by interleaving. A high spectral efficiency without the reduction through redundancy, for example, through channel coding, could be accomplished by these diversity schemes. Boullé and Belfiore [3] presented an N-dimensional modulation scheme to exploit time diversity with a lattice decoder. Kerpez presented a coordinated modulation diversity scheme for several channels of a digital subscriber line [4]. The performance improves significantly for Rayleigh faded channels by taking into account a small performance loss for the AWGN channel. DaSilva and Sousa [5] introduced a fading-resistant modulation scheme by transmitting the distinct signal points on different, uncorrelated transmitter antennas. Boutros and Viterbo [6] presented a rotated approach of the modulation alphabet that comes close to the AWGN channel bound for a Rayleigh fading channel. It does not perform worse without rotated modulation in case of an AWGN channel. Latter is identified as an important fact that no degradation occurs in AWGN channels. In [7], Lamy and Boutros compared Walsh-Hadamard sequences and rotated lattice structures (random and algebraic rotations). They investigated the different schemes for a 75 16-QAM modulation alphabet. The authors could show that the possible diversity gains for rotated lattices in a Rayleigh fading channel are especially significant in comparison to short lengths of the Walsh-Hadamard sequences. They further demonstrated that for a dimension of 512, the diversity gains for the rotated lattice structures and the pure WalshHadamard approach differ only marginally. In [8], Brunel applied a lattice decoder for an MC-CDMA system. The author construes the possible dimensions generated by the Walsh-Hadamard sequences as an N-dimensional sphere. He used this approach to detect and decode the generated chips by a lattice de (...truncated)