Zones are time zones/ frequency regions assigned to the legacy system operation. The Mzones and LZones are time-mul- tiplexed in the downlink. For uplink transmissions both TDM and FDM approaches are supported for multiplexing of legacy and new mobile stations. Note that downlink or uplink traffic/control for new mobile stations can be scheduled in either MZone or Lzone, depending on the operational mode with which the new MS is connected to the BS, but it cannot be scheduled in both zones at the same time, whereas the DL or UL traffic/control for the legacy mobile stations can only be scheduled in the LZones. Frequency Region 1 Time Zone 1 Time Zone 2 Frequency Region 2 Radio Frame DL/UL Subframes Switching Gap FIGURE 9-19 An illustration of the concept of time zones and frequency regions in TDD mode CHAPTER 9 The IEEE 802. 16m Physical Layer (Part I) In the absence of any legacy system, the LZones will disappear and the entire radio frame will be allocated to the new systems. In a mixed mode deployment of legacy and new systems, the allocation of time zones in the TDD mode is as shown in Figure 9-19. The duration of the zones may vary. Unlike the legacy standard, where the minimum size of zone was two OFDM symbols, in IEEE 802.16m the minimum size of a zone is one subframe, thus, there is less granularity compared to the legacy standard. Note that each legacy frame starts with a legacy preamble and the DL/UL MAPs, followed by a DL zone that is expected by the legacy mobile and relay stations. Similarly, in a mixed mode deployment, the UL portion of the frame in TDD mode starts with the legacy UL control channels as expected by the legacy mobile and relay stations. The coexistence is defined as a deployment where the new and legacy base stations coexist in the same frequency band and in the same or neighboring geographical areas. In a green-field deployment, the LZones can be removed. The DL to UL and UL to DL switching points must be synchronized across the network to reduce inter-cell interference. The switchi