onfiguration in the IEEE 802.1 16m systems that support the legacy mobile and base stations are indexed in a different manner. The DL subframe index ranges from 0 to D - 1, where D is the number of DL subframes dedicated to IEEE 802.16m compliant systems. Similarly, the UL subframe index ranges from 0 to U - 1, where U is the number of UL subframes dedicated to IEEE 802.16m compliant systems. Figure 7-11 shows an example of subframe indexing where the ratio of the total DL to UL subframes is 5 to 3. The Frame Offset parameter is 2. While operating the legacy and the new systems simultaneously in TDD mode, the legacy and new uplinks are frequency-division multiplexed to mitigate the uplink link budget issues of the legacy terminals. Thus, the offset parameters in Equations (7-1) to (7-5) are adjusted to accommodate the effect of Frame Offset in IEEE 802.16m systems with legacy support. Legacy Frame Legacy Radio Frame from the Point of View of Legacy BS/MS Frame Offset New Frame New Radio Frame from the Point of View of New BS/MS FIGURE 7-11 Relative positions of the legacy and new radio frames in TDD mode with DL/UL ratio 5:3 [3] 7.3 MAC PDU formation The Group Resource Allocation (GRA) mechanism allocates resources to multiple users as a group in order to reduce the control overhead. The mechanism takes advantage of common traffic charac- teristics and grouping is done based on some common parameters, such as modulation and coding scheme, MIMO mode and resource size. On receiving a DL Group Resource Allocation A-MAP IE, the scheduled MS attempts to decode the received data packet. If the decoding is successful, the MS will send a positive acknowledgement to the serving BS; otherwise, the MS will send a negative acknowl- edgement to the BS. With DL Group Resource Allocation, the HARQ re-transmissions are allocated individually and performed using a DL Basic Assignment A-MAP IE that carries the same ACID as the one used by the DL Group Resource Allocation A-MAP IE for the first HARQ sub-packet transmission. On rece