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.