ified by an SPID. In order to specify the start of a new transmission, a single-bit HARQ Identifier Sequence Number (AI_SN) is toggled on every new HARQ transmission attempt on the same ACID. If the AI_SN changes, the receiver will treat the corresponding HARQ transmission as a new encoder packet and discard previous HARQ transmissions associated with the same ACID. 7.2 Hybrid automatic repeat request functions 7.2.3 HARQ Timing and Signaling Protocols The IEEE 802.16m uses adaptive asynchronous HARQ in the downlink. In adaptive asynchronous HARQ, the resource allocation and transmission format for the HARQ re-transmissions may be different from the initial transmission. In the case of re-transmission, appropriate signaling required to indicate the resource allocation and transmission format, along with other HARQ parameters [2]. On receiving a DL Basic Assignment Advanced Medium Access Protocol Information Element (A-MAP IE), the MS attempts to receive and decode the data burst. If the decoding is successful, the MS sends a positive acknowledgement to the BS; otherwise, the MS will send a negative acknowl- edgement to the BS. The process of re-transmissions is controlled by the BS using the ACID and AI_SN fields in the DL Basic Assignment A-MAP IE. If the AI_SN field for the ACID remains the same between two HARQ burst allocations, it indicates re-transmission. The BS may allocate different resource and transmission formats that are signaled through the DL Basic Assignment A-MAP IE for each re-transmission. If the AI_SN field for the ACID is toggled, it indicates the transmission of a new HARQ packet. The maximum number of HARQ channels per MS in the downlink is 16. The delay between two consecutive HARQ transmissions of the same data burst does not exceed the maximum [T_ReTx_Interval = 1, 2, 8]. The number of re-transmissions of the same data packet does not exceed the maximum [N_MAX_ReTx = 4]. An example of HARQ operation in the downlink for TDD (DL/UL ratio of 5:3) and FDD is shown in Figure 7-7. Note that th