e, in IEEE 802. 16m, the initial ranging channel can be scheduled as frequently as 5 ms. The handover ranging channel can also be allocated though A-MAP in any subframe, except the subframe that has already been used for a regular resource allocation. Padded Zadoff-Chu sequences with cyclic shifts are used for the periodic ranging preamble codes as follows: ,k=0,1,...,Nrp-1;n = 0,1,2 (10-18) where NTCS denotes the time-domain cyclic shift per OFDM symbol. The start root index of the Zadoff-Chu sequence and the periodic ranging preamble code information are broadcast via the AAI_SCD MAC control message for non-femto-cell deployment scenarios. The number of periodic ranging preamble codes can be 8, 16, 24, or 32, depending on the system configuration. The infor- mation regarding periodic ranging channel configuration includes subframe offset (OSF) for ranging channel allocation in the time-domain, where OSF is the same as that for the initial ranging, as well as the sub-band index for ranging channel allocation in the frequency domain. For FDM-based UL-PUSC legacy system support, the same pseudo noise codes specified in the IEEE 802.16-2009 standard are used by the new mobile stations for the initial, periodic, and handover ranging. 10.3.5 Bandwidth Request Channel A contention-based random access mechanism is used by the mobile stations to transmit bandwidth request information to the base station. Prioritized bandwidth requests are supported over the band- width request channel. In the legacy uplink zone with PUSC subchannelization, a BW-REQ tile is formed by four contiguous sub-carriers over six OFDM symbols. There are three BW-REQ tiles per BW-REQ channel, where each BW-REQ tile carries one BW-REQ preamble. In the new uplink zones, a BW-REQ tile is formed by six contiguous sub-carriers over six OFDM symbols, where each BW- REQ channel consists of three distributed BW-REQ tiles, and each BW-REQ tile carries one BW-REQ preamble and one quick access message. The MS may only transmit the BW-REQ sequence and leave t