PERIODIC RANGING IN SLEEP MODE | Power Management in Mobile WiMAX

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.

1. During the listening window, a BS may allocate an uplink transmission opportunity for periodic ranging.
   
   
2. 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.
   
   
3. 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.

HYBRID CASE AND IMPORTANT ISSUES | Power Management in Mobile WiMAX

As described in the standard, Figure 1 shows two PSCs: One for type I, in which the sleeping window is doubled as long as no traffic is addressed to it, and one for type II where the sleeping intervals are of constant length. Finally, availability and unavailability intervals are shown too. During unavailability interval the MS is said to be “sleeping.”

 

Figure 1: Hybrid case for IEEE 802.16e.

Usually most of NRT-VR and BE connections are handled by PSC type I, whereas each UGS connection requires a single type II PSC. This is because bandwidth allocation for the first two scheduling classes is fully under control by the BS and does not necessarily strictly follow the traffic arrival pattern. On the other hand, for the case of UGS, each traffic source might have an individual, periodic interarrival time (ON/OFF pattern). Assume two VoIP calls, for example, one with 20 ms and another one with 30 ms interarrival time. If the larger interarrival time is used as a sleep window then the first one would suffer a multiple of 10 ms additional delays for each transmission (without adding any other network delay). But if a sleeping window less than 30 ms would be used, then the ON interval for the second call could not fit into, which also means extra delay. If, however, both applications had the same interarrival time, then the sleep window is specified as the OFF periods between these calls. Nevertheless, management reasons of these two flows dictates that a seperate intance of PSC type II is used.

Finally, it has to be mentioned that as long as the number of UGS connections increases, the MS’s total ON time does increase as well. This relationship would be directly proportional if the MS would only hold PSCs of type II. In WiMAX networks, however, ranging and other management operations require the use of PSC type III, and follow a decoupled operation. Deriving the final unavailability periods becomes more complex in this real hybrid case. The above analysis yields some deductions for the PSC type (e.g., VoIP calls) and the sleeping time:

• Deduction 1. Each UGS and RTPS-CR/VR connection needs to be handled by a separate power saving class type II with individual parameters.
  
  
• Deduction 2. The energy efficiency is inversely proportional to incoming load.
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  Deduction 3. The access delay is improved when load is high, but not in a straightforward manner.

MESSAGES AND TRANSACTIONS | POWER MANAGEMENT

The 802.16 standards can support 255 different management type of messages. According to IEEE 802.16e, 66 messages are defined from which 50–66 are used explicitly for handover and sleep mode operation. 

In the following, we describe the most important messages for sleep mode operation. Each message has three parts. The first part is casually MOB (implying Mobility) while the second part carries the scope of the message (SLP, TRF, HO, BSHO, MSHO, PAG, etc.). For the case of sleep mode operation, we are mainly concerned with SLP (implying Sleep) and TRF (implying Traffic). The final part of each message is usually REQ (implying Request), RSP (implying Response), or IND (implying Indication).

MOB_SLP-REQ: Sleep Request Message, type: 50, Connection: Basic.

A MS sends this message to request definition/activation of several PSCs. The actual definition occurs when a MS suggests a PSC for an incoming connection. The message contains a Power Saving Class ID, which is a unique identifier for a group of PSCs associated with a MS. This ID is also used for the exchange of many similar message types and response messages. Other parameters are also defined like the Initial Sleep Window, Final Sleep Window Base, and Exponent quantities measured in IEEE 802.16e frames. Moreover if the MS handles more than one class, the Number_of_CIDs field carries this number. 

MOB_SLP-RSP: Sleep Response Message, type: 51, Connection: Basic.

This message is sent in response to a request for the definition/activation of a PSC from the BS to a MS using a broadcast CID (Connection ID) or the MS’s basic CID. If a new definition was requested, then a new PSC is defined and the assigned ID is returned. When the MS receives this message, it activates the defined PSC. The MOB_SLP-RSP message contains fields such as the Length of Data, for the number of bytes per PSC, the start frame number for the first sleep window, and the relative intervals. Additionally, CIDs and windows (base and exponent) are described in MOB_SLP-REQ.

MOB_TRF-IND: Traffic Indication Message, type 52, Connection: Broadcast.

By the time the MS wakes up, it starts listening for a possible MOB_TRF-IND message, sent from the BS on broadcast CID or sleep mode multicast CID. This message is sent from a BS to a MS in the listening interval to indicate if there has been traffic addressed to the MS while it remained in sleep mode and functions only when there are one or more PSC IDs defined for PSC type I (as described below). Any other MS ignores this message. An explicit occasion in which the BS may arbitrarily include a positive indication for a MS is if the MS’s periodic ranging operation is scheduled to start in the next sleep window.