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Power Management in SDR

This article provides an overview of power management in SDR technology, including power fundamentals, approaches, current state of technology, and the challenges of power management for SDR. It discusses software-controlled power, general power management techniques, and application and hardware-based strategies. It also explores the state-of-the-art in power management, including ACPI and OSPM, and explores the specific challenges and needs of power management for SDR.

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Power Management in SDR

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  1. Power Management in SDR Max Robert, Jeffrey H. Reed Mobile and Portable Radio Research Group (MPRG) Virginia Tech September 14, 2004

  2. Overview • Power fundamentals • Overview of approaches • Current state of technology • Power management for SDR • Operation states • Interface descriptions • Conclusion

  3. Power Basics • Terms: • α: switching activity • C: capacitance • V: voltage • f: operating frequency • Fixed attributes • Switching activity (algorithm-specific) • C is fixed • Attributes open to modification • V, f

  4. Software-controlled power • Some attributes are determined at design time and cannot be changed at run-time • Compiler optimizations • Waveform design • Attributes that can change at runtime • Operating voltage • Operating frequency • Timing control • Thread management in the case of processors • Active components

  5. General Power Management • Power management split into three principal categories • Previous work on each section varies in depth

  6. Software-Controlled Attributes • Timing management • Thread priority in the case of a GPP or (sometimes) DSP • Bus/message management in system • Algorithm may optimize wait times to cluster work for component • Voltage and Frequency selection are related • Higher voltage will allow higher frequencies • Optimal voltage for frequency not necessarily the best choice • Voltage switching may be slower than frequency switching • May desire to maintain operating range for quick response • Active component selection is a subset • Set voltage or frequency to zero for that component • Flexible RF • Still unclear what attributes of the RF will be software-controlled • Mixer bias, filter BW, others • Framework- and application-based strategies need to be sufficiently flexible to allow smooth integration of flexible RF control

  7. Application & Hardware • Significant previous research • Adaptive management algorithms • Advanced power hardware-level power management techniques • Application- and HW-based strategies well suited for static applications • Current way of developing power-saving strategies • Fixed waveform • Can be optimized to specific platform

  8. Operating System/Environment • Software structure necessary to support power management functionality • Standard interface • Switch between different states • i.e.: sleep (several levels), active • States not necessarily limited to sleep modes • Standard management structure • Maintain state of all devices in system • State machine for describing system • Unified structure for handling associated devices

  9. State-of-the-Art • Development limited to PC needs • Power management for laptops • Sleep mode management • BIOS-based management • BPM (BIOS power management) • Has no awareness of the user’s (or application’s) needs • Operating-system based management • OSPM (OS power management) • Current de-facto standard • Most publications today are algorithms for the efficient switching between states using OSPM

  10. ACPI • Advanced Configuration and Power Interface • State machine used to describe machine configuration • States associated with different parts of the system • Common interface provided to enact changes in the state of the system

  11. ACPI States • Basic set of states • Cx • CPU states • Dx • States for peripheral device • Modem • Network card • Screen • Hard drive

  12. Interface Descriptions • Multiple standardized interfaces provided • Example • AcpiEnterSleepStatePrep • AcpiEnterSleepState • AcpiLeaveSleepState • Provides common interface for the change of states for the system

  13. OSPM • Operating System Power Management • Model describing partitioning of power consumption management • Operating system determines when to trigger power management features • BIOS determines how to perform power management features

  14. OSPM/ACPI • OSPM and ACPI integrated • OSPM provides mechanism for selection of mode • Kernel initiates action • ACPI provides common interface to hardware

  15. Power Management For SDR • SDR places challenges different from classic communications system • Can support application swapping • Needs to support wide set of devices • Variety of needs and states • Difficult to narrow to small, well-defined set of states • Requires sophisticated power control structures • Applications can be more predicable than PC • Possible to determine “fast enough” speed • Blind throttle for the application may not be enough

  16. State Support • ACPI supports mesh state machine • Assumes basic device states can be throttled • Linear transitions (throttle) are a subset of the mesh state machine

  17. Problems with Mesh SM • Assumes that all transitions are fundamentally “equal” • Does not take into account QoS issues related with state change • Example: • Voltage and frequency are fundamentally linked • Increased voltage will allow a higher set of frequency settings to be supported • Throttle transitions based on the assumption that lowest possible voltage is supported for the desired frequency • If a change in voltage incurs a higher time delay than a change in frequency, could lead to unplanned additional latencies

  18. Rate-Change Support in Communications • Example (802.11b): • Support alternate processing speeds for different sections of received frame • Benefits • Minimizes required computing power • Provides ability to discard frame before high-speed processing is necessary

  19. Rate Change and SDR • Waveform takes place of “user” in SDR • Latencies associated with change of state need to be taken into account • State switching needs to be in order of microseconds • Millisecond-level switches may be too slow for some waveforms • Ideally, should cluster state changes into transition state • Example: • Crusoe TM5400 automatically controls voltage and frequency settings • Slow ramp in voltage for up-frequency changes followed by fast frequency change • Fast down frequency change followed by slow voltage change • Changes performed automatically • Possible for some equipment to leave change requests up to the application • Voltage regulator can have a significant impact on the transition speeds in core operating voltage • May be too slow (ms+) for some waveforms

  20. State Machine Description • Break down state machine into slow-change states and related fast-change states • Provides application with ability to change states quickly during waveform operation • Also supports sleep or standby operation

  21. Sample Operation • Fast operation • Can cycle between 500 and 700 MHz • 500 MHz may be more efficient at 1.5V • May choose not to transition, since change to 600 or 700 MHz expected soon • Can still transition to lower powers • Support significantly lower power consumption levels • Same concept can apply to other devices • FPGAs, ASICs, CCMs, DSPs

  22. Common Interface • Design of common interface will have to wait until conceptual framework is finalized • Will rely on ACPI to determine appropriate interfaces • Will also rely heavily on SCA 3.0 interface specifications • SCA 3.0 concentrates on non-CORBA interface descriptions • Challenging task • Generic nature of hardware makes static definition of interfaces unlikely • Will most likely require a generic structure • May be able to leverage AML

  23. Application-Level Power Management • Algorithm development • Field of research currently has large number of contributions • Primarily concentrating on PC-based systems • ACPI/OSPM • Clear from OEPM that SDR will have some unique characteristics • Optimization strategies will be based on the permutations possible by conceptual framework • This research venue cannot proceed until conceptual framework is complete

  24. Conclusion • Some concepts in power management are fairly mature • PC power management • Voltage and frequency scaling • Policies and algorithms • Current state-of-the-art does not cover all needs of SDR • Unique issues related to nature of SDR • Actively developing techniques to resolve these issues

  25. Acknowledgement • This work is funded by the DCI Postdoctoral Research Fellowship and the MPRG Affiliates Program

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