resource assignment. In synchronous HARQ, resource allocation for the re-transmissions in the uplink can be fixed or adaptive according to control signaling. The default operation mode of HARQ in the uplink is non-adaptive, i.e., the parameters and the resource for the re-transmission are known a priori. The BS can signal an adaptive uplink HARQ mode. Similar to the HARQ operation in the downlink, on receiving a UL Basic Assignment A-MAP IE, the MS transmits a HARQ sub-packet in the assigned resource. The BS attempts to decode the data packet. If the decoding is successful, the BS will send a positive acknowledgement to the MS; otherwise, the BS will send a negative acknowledgement to the MS. If re-transmission becomes necessary, and if the MS does not receive a UL Basic Assignment A-MAP IE for the failed HARQ sub- packet, the MS transmits the next sub-packet through the resources assigned in the previous sub-packet transmission with the same ACID. A UL Basic Assignment A-MAP IE may be sent to signal the re-transmission with corresponding ACID and unchanged AI_SN. On receiving the UL Basic Assignment A-MAP IE, the MS performs the HARQ re-transmission. The maximum number of HARQ channels per MS is 16 in the uplink. The maximum number of re-transmissions of the same CHAPTER 7 The IEEE 802. 16m MAC CPS (Part II) Frame N Frame N+1 Subframe Assignment Assignment + DL Burst + DL Burst Feedback Feedback Example FDD DL HARQ Timing Frame N Frame N+1 Subframe Assignment + DL Burst Assignment + DL Burst Feedback Feedback Example TDD DL HARQ Timing FIGURE 7-7 An example of FDD and TDD downlink HARQ timing for 5, 10, and 20 MHz channel bandwidths [2] data packet is [N_MAX_ReTx = 4]. An example HARQ operation in the uplink for TDD (DL/UL ratio of 5:3) and FDD is shown in Figure 7-8. Note that the processing time at the BS and MS is assumed to be equal to three sub-frames. Synchronous HARQ re-transmissions are supported when persistent allocation is utilized. With group resource allocation, the HARQ re-transmissions are allocat