The standard defines periodic ranging in sleep mode functions in great detail. Ranging is a very important mechanism in IEEE 802.16 networks since the MS is using it to adjust the power and sychronize with the OFDM symbol. When the MS is in sleep mode, the ranging uplink transmission can be allocated in three different ways.
- During the listening window, a BS may allocate an uplink transmission opportunity for periodic ranging.
- A BS can activate a PSC type III to keep the MS in active state until the assignment of an uplink transmission opportunity for periodic ranging.
- As mentioned above, the RNG-RSP (or MOB_SLP-RSP) can include the next periodic ranging TLV. From this the MS may know when the next periodic ranging opportunity shall occur. From the next periodic ranging TLV a MS may decode all consequent UL-MAP (uplink MAP) messages waiting for an uplink unicast transmission opportunity. When its own opportunity takes place the ranging procedure can be executed as normal.
It is important to mention that successful ranging does not deactivate the PSCs. After successful ranging, the BS announces the next ranging time in an RNG-RSP message. Additionally, if there is downlink traffic in the BS’s queue then a DL traffic indication is addressed to the MS. This way, the MS can exit the sleep mode algorithm earlier and reduce the imminent response delay.
