tion Authority (see http:// standards.ieee.org/regauth/index.html for more information). 4.1 IEEE 802. 16m mobile station state diagrams Sleep mode Sleep Listening Interval Interval From Access State To Idle State Active Mode To Initialization State/Power ON/ Scanning Mode Failed Operation Path Normal Operation Path FIGURE 4-5 Connected state procedures [3] Active Mode An MS in Active Mode can be scheduled for downlink or uplink transmissions at appropriate intervals by the serving BS. The MS may conduct channel quality measurements based on downlink reference signals and report the measurements to the BS to assist the BS scheduler in properly adapting to the link characteristics and selecting the most appropriate transmission formats for data and control signals sent to (or received from) the MS. The MS may transit from Active Mode to Sleep Mode during sleep intervals (with intermittent DL/UL traffic) or to Idle State (when it has no DL/UL traffic and following deregistration with the BS) in order to conserve power. Sleep Mode Sleep is a mode in which an MS becomes unavailable to the serving BS during pre-negotiated intervals while connected to the BS. The Sleep Mode can only be activated when an MS is in the Connected State. When Sleep Mode is activated, the MS is provided with a series of sleep cycles that typically consists of a listening window followed by a sleep window (as shown in Figure 4-6). A sleep cycle the sum of the sleep and listening intervals. During the sleep window, the BS does not transmit any downlink unicast MAC PDU to the MS; therefore, the MS may power down its transmission chain or perform other activities that do not require communication with the BS. During the listening window, the MS may receive downlink transmissions in the same way as in normal operation. The MS must ensure that it has the most up-to-date system information. The downlink synchronization and system information acquisition may be performed by the MS, if it wakes up at superframe intervals (i.e., every CHAPTER 4 IEEE 8