 received. After decoding the Group Resource Allocation MAC control message, if an MS realizes that it has been deleted from the group, it will not expect allocations in that group starting in the same subframe in which deletion information was sent. The MS sends an ACK to the BS to acknowledge receipt of the deletion information. The user bitmap may be rearranged via Group Resource Allocation MAC control message in order to indicate open user bitmap positions in the current user bitmap. The open positions indicated in the CHAPTER 6 The IEEE 802. 16m MAC CPS (Part I) User Bitmap De-allocated Mobile Stations De-allocated De-allocated De-allocated De-allocated De-allocated MS Index MS Index MS Index MS Index MS Index FIGURE 6-24 An example of de-allocation with a de-allocated MS index rearranged bitmap implicitly identify the available positions for switching user bitmap. For HARQ- enabled transmissions, an ACK/NACK is transmitted to acknowledge the successful/unsuccessful decoding of a data burst. The starting HARQ feedback channel for the group is signaled in the DL Group Resource Allocation A-MAP IE and scheduled mobile stations belonging to the group will determine their individual HARQ feedback channels. The GRA uses bitmaps to signal resource allocation information for mobile stations within a group. These bitmaps are sent in the Group Resource Allocation MAC control message. The first bitmap is the User Bitmap which uses one bit per user, to signal which users are scheduled in that frame. The second bitmap is MIMO Bitmap which is used to indicate the assigned MIMO mode, when multiple MIMO modes and their associated parameters are supported in the group. When the MIMO Mode Set of the group includes MU-MIMO, PSI Bitmap and Pairing Bitmap appear to determine the two mobile stations that share the same resource. The Pairing Bitmap is used to indicate a pair of mobile stations using different number of streams. The number of bits per pair in the Pairing Bitmap depends on the total number of pairs in the group. If