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This paper examines the detection problem of the preamble sequence index in the orthogonal frequency division multiplexing (OFDM) system. The receiver knows all the possible preamble sequences and should estimate which preamble sequence has been transmitted. Preamble sequence detection is straightforward with the knowledge of channel conditions. However, since the preamble OFDM symbol is usually the first symbol that is received, the channel is unknown to the receiver, which makes this problem complicated. In this paper, the joint maximum likelihood (ML) estimator of the preamble sequence and the channel is derived and compared with a simple decoupled estimator. The estimators are applied to IEEE 802.16e systems, and it is shown by simulation that the joint ML estimator detects the preamble sequence index very well in the absence of the channel knowledge.