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

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Author(s): Raulefs Ronald | Dammann Armin | Sand Stephan | Kaiser Stefan | Auer Gunther

Journal: EURASIP Journal on Wireless Communications and Networking
ISSN 1687-1472

Volume: 2004;
Issue: 1;
Start page: 713902;
Date: 2004;
Original page

Keywords: code division multiaccess | Walsh-Hadamard spreading sequences | multicarrier | fading channels | concatenated channel coding

ABSTRACT
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 signal-to-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.
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