Comparison of the Total Inter-Carrier Interference Caused by the Doppler Effect in OFDMA and a Proposed Hybrid CDMA-OFDMA System

Luis LOYOLA  Tetsuya MIKI  

IEICE TRANSACTIONS on Communications   Vol.E86-B   No.4   pp.1256-1264
Publication Date: 2003/04/01
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: PAPER
Category: Wireless Communication Technology
CDMA,  Doppler spread,  Gold Codes,  Inter-Carrier Interference,  OFDMA,  

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Time variations of the wireless channel cause Inter-Carrier Interference (ICI) between different subcarriers in an OFDM system. In a highly mobile environment this interference may become so high that it degrades up to unacceptable levels the communication channel. In this paper, firstly we obtain a simplified expression for the total ICI experienced by every subcarrier in an OFDMA system. Unlike other previous works, the result establishes an explicit and useful relation between the ICI on each subcarrier and the speed of the rest of the terminals in the system. Then, by means of a mathematical analysis we extend the scope of that expression to a hybrid system in which OFDMA and CDMA are combined. Using the good autocorrelation and cross-correlation properties provided by Gold-sequences we propose a Gold-Code-based CDMA-OFDMA transmission technique for the asynchronous uplink channel. We show that the proposed method can reduce the total ICI and potentially increase the capacity of the system in comparison to a conventional OFDMA system.