The OFDMA PHY supports two mandatory contention-based Bandwidth Request mechanisms:
The SS sends the bandwidth request header as specified for the OFDM Layer. The OFDM Layer focused contention and subchannelisation considerations no longer apply to OFDMA where subchannels (subcarriers) are distributed instead of OFDM symbols.
Conversely, use the CDMA-based mechanism described below.
As specified in Chapter 9, the OFDMA PHY specifies a ranging subchannel. The BS provides in the UCD message a subset of ranging codes that are used for contention-based Bandwidth Requests and initial ranging. The BS can determine the purpose of the received code by the subset to which the code belongs. The requesting bandwidth SS selects, with equal probability, a ranging code from the code subset allocated to Bandwidth Requests. This ranging code modulates the ranging subchannel and is then transmitted during the appropriate uplink allocation. Upon detection, the BS provides an implementation-dependent uplink allocation for the SS, but instead of indicating a Basic CID, the broadcast CID is sent in combination with a CDMA_Allocation_IE, which specifies the transmit region and ranging code that were used by the SS. This allows an SS to determine whether it has been given an allocation by matching these parameters with the parameters it used. The SS then uses the allocation to transmit a Bandwidth Request MAC PDU and/or data.
If the Bandwidth Request does not result in a subsequent allocation of bandwidth, the SS assumes that the ranging code transmission resulted in a collision and then follows the contention resolution mechanism.