## 4 Original PS-Request Protocol  
Figure 3 shows the format of a general medium access control (MAC) frame of the WiFi system based on the IEEE802.11g specification. It comprises a set of fields that occur in a fixed order in all frames, as shown in Figure 3. The conventional WiFi system has three frame types: control, data, and management. Each of the frame types has several defined subtypes. The control frame consists of sixteen subtypes. Eight subtypes are being used, and eight subtypes are reserved. Detailed explanations of the types and subtypes are given in the IEEE802.11g specification [16]. Figure 3: Format of general MAC frame of WiFi system. Figure 2: Desired TDO of WiFi and WiMAX systems. The original PS-Request protocol is based on the use of the vestigial power management (PwrMgt) bit within the frames transmitted by a WiFi AP. In the case of an infrastructure configuration, the conventional WiFi system based on IEEE 802.11g specification can only support the power management mode for WiFi STAs. A WiFi STA can be in one of the two power management modes: an active mode (AM) or a power-saving mode (PSM). A WiFi STA in the AM is fully activated. It can send or receive frames at any time. A WiFi STA in the PSM can be in one of two states: a sleep state or an awake state. The WiFi STA in the PSM is usually in the sleep state, and periodically moves to the awake state to receive frames from a WiFi AP, such as beacon frames. The WiFi STA consumes much smaller energy than it does in the AM, because the WiFi STA in the PSM does not send any frames. In the legacy WiFi system of an infrastructure configuration, a WiFi STA that wishes to move into the PSM shall inform its WiFi AP of an indication by setting the PwrMgt bit within the Frame Control field of the frame including the indication to '1'. The WiFi STA should be acknowledged by its WiFi AP, and move into the PSM. However, a WiFi AP cannot be in the PSM, because none of its WiFi STA can communicate for the PSM duration of the WiFi AP. Therefore, the PwrMgt bit within the Frame Control field of all frames transmitted by the WiFi AP is always set to '0'. This indicates that the WiFi AP is always in the AM. If all WiFi STAs within the coverage of a WiFi AP are in the PSM, they do not transmit any signals. Thus, they do not generate interference to other systems during the PSM period. That is, if a WiFi system, including a WiFi AP and all of its WiFi STAs, do not transmit a signal during a specified period, the other wireless communication systems, such as a WiMAX system, can communicate without interference during the period. However, this can be achieved, only if the WiFi AP and all of its WiFi STAs move into the PSM. As mentioned before, the PSM of the conventional WiFi system can be controlled only by the WiFi STAs. That is, the WiFi AP cannot control the signal transmission of its WiFi STAs. We previously proposed to use the vestigial PwrMgt bit within the Frame Control field of frames transmitted by a WiFi AP for the WiFi AP to control the signal transmission of all of its WiFi STAs. We named it the PS-Request protocol. The PS-Request frame transmitted by the WiFi AP commands all its WiFi STAs to move into the PSM to stop them transmitting a signal. Figure 4 shows a format of  
Figure 4: Format of original PS-Request frame. the proposed PS-Request frame. One of the eight reserved subtypes, such as 0000, can be allocated, and the PwrMgt bit is set to '1' in the Frame Control field. The Duration/ID field represents the value of a network allocation vector (NAV). This means that the WiFi AP will not transmit a signal during the PSM period of the NAV, and the WiFi AP stops its WiFi STAs from transmitting a signal during the period. Figure 5 shows an example of the original PS-Request protocol. There are three cases after the start time of WiFi Tx ON period equal to the end time of WiMAX Tx ON period, as shown in Figure 5.  
Case (a):  
A WiFi AP collaborating with a WiMAX BS senses a wireless medium during a distributed inter-frame space (DIFS) time.