xing of the legacy and new protocols in the downlink and uplink legacy/new zones. More specifically, the legacy and new zones are time division multiplexed in the downlink and are frequency division multiplexed in the uplink. Figure 1 illustrates an example where the legacy system is supported in an IEEE 802.16m system. The overhead channels corre- sponding to each system (i.e., synchronization, control, and broadcast channels) are duplicated due to incompatibility of the physical structures and transmission formats of these overhead channels. Although IEEE 802.16m specifies handover mechanisms to and from the legacy systems, the handover protocols, MAC messages, and triggers are different, requiring a separate protocol/software stack for dual-mode implementation of the two systems. Table 1 compares the physical layer and lower MAC features of the legacy mobile WiMAX and IEEE 802. 16m. It can be seen that many important features and functions such as HARQ, subchannelization, control channels, and MIMO modes have changed in the IEEE 802.16m, making migration from legacy systems to the IEEE 802.16m systems not straightforward and also expensive. The complexity of later upgrades is similar to that of migration of UMTS/HSPA systems to 3GPP LTE systems given the non-backward compatible nature of 3GPP LTE enhancements relative to UMTS. The features and functions listed in this table will be described in Chapters 9 and 10. As a result of extensive changes and enhancements in the IEEE 16m standard relative to legacy mobile WiMAX, it will not be surprising to realize that the throughput and performance of the IEEE A-MAP Region A-MAP Region A-MAP Region A-MAP Region A-MAP Region Legacy Legacy Legacy Uplink Uplink Legacy Uplink Legacy Uplink Uplink New Uplink New Uplink New Uplink Control Control Control Channels Channels Channels New Uplink New Uplink Control Control Channels Channels New Uplink New Uplink New Uplink New Uplink New Uplink DL Subframe UL Subframe DL Subframe UL Subframe DL Subframe UL Subframe DL Subframe U