Figure 8: Subchannelization procedure for bidirectional communication using the IEEE 802.16e protocol. In uplink, each user is allocated slots spanning subchannels in frequency over slot-durations. Each slot-duration comprises 3 consecutive OFDM symbols, whereas each subchannel contains 6 tiles distributed throughout of the useful frequency band. A tile contains 4 subcarries over a slot-duration with the 4 pilots at the corners of the tile and the remaining 8 tones used for data. In the above figure, there are tiles with , , and . The circles filled with violet and red colors indicate the pilot tones of users 1 and 2, respectively, whereas the circles filled with black and blue colors indicate the data tones of users 1 and 2, respectively.