MAI-Free MC-CDMA Systems Based on Hadamard-Walsh Codes

• Published in: IEEE transactions on signal processing 2006-08, Vol.54 (8), p.3166-3179
• Main Authors: Shang-Ho Tsai, Yuan-Pei Lin, Kuo, C.-C.J.
• Format: Article
• Language: English
• Subjects: Applied sciences; Carrier frequencies; Carrier frequency offset (CFO); Channel estimation; Channels; Coding, codes; Detection, estimation, filtering, equalization, prediction; Error correction; Error correction codes; Estimates; Exact sciences and technology; Fading; Frequency division multiaccess; Frequency-selective fading channels; Gain; Hadamard-Walsh code; Information, signal and communications theory; interference free; large-area synchronized (LAS) code; Miscellaneous; Multiaccess communication; multiaccess interference (MAI)-free; Multicarrier code division multiple access; multicarrier code-division multiple-access (MC-CDMA); Multipath channels; Multiple access interference; multiuser detection (MUD); Permissible error; Priorities; Robustness; Signal and communications theory; Signal processing; Signal, noise; Simulation; Studies; Telecommunications and information theory
• ISSN: 1053-587X; 1941-0476
• DOI: 10.1109/TSP.2006.875387

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 GgesL. 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