Power Management in SDR

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