A method of generating preamble sequence is disclosed. A channel used by a wireless device may be divided into four sub-channels, and the method includes forming a preamble sequence of a first sub-channel, making three replicas of the preamble sequence of the first sub-channel, each replica with a phase rotation of a first angle, a second angle, and a third angle respectively, for forming each preamble sequence of a second sub-channel, a third sub-channel, and a fourth sub-channel, and arranging the preamble sequences of the first, the second, the third, and the fourth sub-channels to form a preamble sequence of the channel.