Thursday, March 31, 2011

Power Management in Mobile WiMAX

Mobile devices tend to incorporate more and more processing units and functionalities which has a negative effect on battery lifetime. For this reason IEEE 802.16e workgroup has standardized those mechanisms that would augment battery lifetime in a worldwide interoperability for microwave access (WiMAX) network without affecting the quality-of-service (QoS) performance. This chapter gives an overview of those mechanisms, includes the methods to analyze them, and identifies those elements that could decrease the waiting time when in sleep mode.


By definition, designed for mobile devices, whose utility is inherently constrained by limited energy supply, efficient power saving mechanisms are decisive criteria for IEEE 802.16e to become a compelling alternative for mobile broadband wireless access (BWA). This applies to any mobile worldwide interoperability for microwave access (WiMAX) device, regardless of its size and application and therefore ranging from a tiny personal digital assistant (PDA) to mobile phones to table PCs and notebooks.

Power saving becomes important as all these devices continue to incorporate more and more functionalities per integration unit, a development that has been predicted already years ago and which is known as Moore’s law. By doing so, manufacturers serve customer’s demand for consolidation; the ideal device is small, light, but feature rich. Hence, while early mobile phones were solely designed for voice services, today’s generation integrate MP3 player, camera, positioning service and many other energy-hungry features. The combined energy demand of such peripheral features, for instance, make up for almost 30 percent of the total for a Nokia 6630. Clearly, this also entails a whole set of new thermal requirements on mobile device design.

This development poses a considerable challenge to battery performance just as with computing power increasing energy accumulation is expected. Yet advances in battery capacity lacks considerably behind, like which claims that battery capacity has only improved by 80 percent in the last ten years while computing power doubles every 18 months according to Moore’s law. Nevertheless and on top of that, a recent Taylor Nelson Sofres (TNS) research project reveals that “Two-thirds of mobile phone and PDA users rate ‘two-days of battery life during active use’ as the most important feature of the ideal converged device of the future”. Similar to this finding, it is generally agreed that the operational time for a notebook shall be in the range of 2–5 hours. As of today, the only efficient means to cater to these user expectations is to counter this development by ever more efficient power saving mechanisms.

Motivating research on power consumption to overcome usability and performance constraints can be considered as the standard argumentation. But in fact, there is a much more important, frequently neglected reason: Saving energy means safeguarding our environment. In 2008, 70 million notebooks will be sold, roughly double the number than back in 2001. That means 70 million new power consumers and equal number of new batteries, yet one of the most expensive energy source in terms of production, which usually involves many valuable resources and hazardous chemicals, but also in terms of deployment. Disposing unusable batteries is a complicated business and the implicative environmental consequences do not require any further elucidation. Hence, saving power—and amid the total lifetime of a battery—can only be of utmost importance.

In conclusion, effective power saving mechanisms for IEEE 802.16e, which is supposed to seize the lead in BWA technology, are important to improve the usability of mobile devices and to accomplish this in the most environmentally friendly manner. The chapter is divided into two main parts. In the first part, we analyze how the power management mechanism functions in both point-to-point and relay architecture, and set some important deductions for the sleeping time. In the second part, we identify the tools for the performance analysis of power saving mechanisms and set the initial pace for some major, open research issues.
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