Copyright (c) 2011 IEEE. Personal use is permitted. For any other purposes, permission must be obtained from the IEEE by emailing pubs-permissions@ieee.org. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. 1 Superimposed Training Based Channel Estimation and Data Detection for OFDM Amplify-and-Forward Cooperative Systems under High Mobility Lanlan He, Yik-Chung Wu, Shaodan Ma, Tung-Sang Ng and H. Vincent Poor Abstract In this paper, joint channel estimation and data detection in orthogonal frequency division mul- tiplexing (OFDM) amplify-and-forward (AF) cooperative systems under high mobility is investigated. Unlike previous works on cooperative systems in which a number of subcarriers are solely occupied by pilots, partial data-dependent superimposed training (PDDST) [8] is considered here, thus preserving the spectral efficiency. Firstly, a closed-form channel estimator is developed based on the least squares (LS) method with Tikhonov regularization and a corresponding data detection algorithm is proposed using the linear minimum mean square error (LMMSE) criterion. In the derived channel estimator, the unknown data is treated as part of the noise and the resulting data detection may not meet the required performance. To address this issue, an iterative method based on the variational inference approach is Lanlan He was with the Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong. She is now with the Huawei Tech. Investment Company (email: llhe@eee.hku.hk). Yik-Chung Wu and Tung-Sang Ng are with the Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong (email: {ycwu, tsng}@eee.hku.hk). Shaodan Ma was with the Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong. She is now with the Department of Electrical and Computer Engineering, University of Macau, Macau (email: shaodanma@umac.mo). H. Vincent Poor is with the Department of Electrical Engineering, Princeton University, Princeton, NJ 08544 USA (email: poor@princeton.edu). The work was supported in part by the General Research Fund (GRF) from Hong Kong Research Grant Council (Project No.: HKU 7154/08E), in part by the GRF (Project No. HKU 7191/11E), and in part by the U.S. National Science Foundation under Grant CNS-09-05398. ∗ The corresponding author is Lanlan He. September 10, 2011 DRAFT