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MAI-Free MC-CDMA Systems Based on Hadamard–Walsh Codes
Tsai, S.-H.; Lin, Y.-P.; Kuo, C.-C.J.;
Signal Processing, IEEE Transactions on [see also Acoustics, Speech, and Signal Processing, IEEE Transactions on]
Volume 54,
Issue 8,
Aug. 2006
Page(s):3166
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3179
Abstract:
It is known that multicarrier code-division multiple-access (MC-CDMA) systems suffer from multiaccess interference (MAI) when the channel is frequency-selective fading. In this paper, we propose a Hadamard–Walsh code-based MC-CDMA system that achieves zero MAI over a frequency-selective fading channel. In particular, we will use appropriately chosen subsets of Hadamard–Walsh code as codewords. For a multipath channel of length$L$, we partition a Hadamard–Walsh code of size$N$into$G$subsets, where$G$is a power of two with$Ggeq L$. We will show that the$N/G$codewords in any of the$G$subsets yields an MAI-free system. That is, the number of MAI-free users for each codeword subset is$N/G$. Furthermore, the system has the additional advantage that it is robust to carrier frequency offset (CFO) in a multipath environment. It is also shown that the MAI-free property allows us to estimate the channel of each user separately and the system can perform channel estimation much more easily. Owing to the MAI-free property, every user can enjoy a channel diversity gain of order$L$to improve the bit error performance. Finally, we discuss a code priority scheme for a heavily loaded system. Simulation results are given to demonstrate the advantages of the proposed code and code priority schemes.
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