Physical Layer Energy Consumption in IEEE 802.15.4 Lecture

1. MAC, Physical Layer, Energy Consumpion and IEEE 802.15.4Lecture 8 September 28, 2004EENG 460a / CPSC 436 / ENAS 960 Networked Embedded Systems &Sensor Networks Andreas Savvides andreas.savvides@yale.edu Office: AKW 212 Tel 432-1275 Course Website http://www.eng.yale.edu/enalab/courses/eeng460a

2. Announcements • Appointment schedule for projects • Student presenter for Oct 12 – Diffusion routing • Project proposal • 1 page description of your project (including references) • Should include: • What is the problem your solving and what is the new feature that you are adding to the problem • Narrow down the problem you will be working on, be very precise with what you are going to do • Give an initial list of paper references on which your paper will be based • A list of resources that you will need for the project (any additional HW, SW and sensors) • Do not exceed 1-page!!!! • Email to andreas.savvides@yale.edu • Filename: name1_and_name2_proposal • Email Subject: EENG460 Project Proposal

3. Frequency Bands and Data Rates • In 2.4GHz band 62.5 ksymbols/second • 1 symbol is 4 bits • 1 symbol is encoded into a 32-bit pseudorandom sequence the chip • chip rate = 62.5 x 32 = 2000 kchips/s • Raw data rate = Symbol rate * chips per symbol • = 62.5 * 4 = 250kb/s • In 868/915 MHz bands • 1 bit symbol (0 or 1) is represented by a 15-chip sequence

4. Physical Layer Transmission Process Binary Data from PPDU Bit to Symbol Conversion Symbol to Chip Conversion O-QPSK Modulator RF Signal

5. Radio Characteristics • Power output • The standard does not specify a power output limit. • Devices should be able to transmit -3dBm • In US 1Watt limit in Europe 10mW for 2.4GHz band • Receiver should be able to decode a packet with receive power of • -85dBm in 2.4GHz and -92dBm in the lower frequency bands • What does that mean in terms of range?

6. Going from Watts to dBm

7. Friss Free Space Propagation Model Same formula in dB path loss form (with Gain constants filled in): How much is the range for a 0dBm transmitter 2.4 GHz band transmitter and pathloss of 92dBm?

8. Friss Free Space Propagation Model • Highly idealized model. It assumes: • Free space, Isotropic antennas • Perfect power match & no interference • Represent the theoretical max transmission range Same formula in dB path loss form: How much is the range for a 0dBm transmitter 2.4 GHz band transmitter and pathloss of 92dBm?

9. Propagation Mechanisms in Space with Objects • Reflection • Radio wave impinges on an object >> λ (30 cm @1 GHz) • Earth surface, walls, buildings, atmospheric layers • Diffraction • Radio path is obstructed by an impenetrable surface with sharp irregularities (edges) • Secondary waves “bend” arounf the obstacle • Explains how RF energy can travel without LOS • Scattering • When medium has large number of objects < λ (30cm @1 GHz) • Similar principles as diffraction, energy reradiated in many directions • Rough surfaces, small objects (e.g foliage, lamp posts, street signs) • Other: Fading and multipath

10. A more realistic model: Log-Normal Shadowing Model • Model typically derived from measurements • Statistically describes random shadowing effects • values of n and σ are computed from measured data using linear regression • Log normal model found to be valid in indoor environments!!!

11. Transmit Power Levels in Chipcon CC2420 Radio supply voltage= 2.5V And Power = I*V = 1mW = 43.5mW

12. Budgeting Battery Power • Assuming power drain is the same for Transmitting and Receiving = 43.5mW • We need to power the device from a 750mAh battery for 1 year • What is the duty cycle we need to operate at?

13. Budgeting Battery Power • Assuming power drain is the same for Transmitting and Receiving = 43.5mW • We need to power the device from a 750mAh battery for 1 year • What is the duty cycle we need to operate at? 1 year has 365 x 24 = 8760 hours The average current drain from the battery should be Average power drain

14. Computing Duty Cycle

15. Energy Implication • Active transceiver power consumption more related to symbol rate rather than raw data rate • To minimize power consumption: • Minimize Ton - maximize data rate • Also minimize Ion by minimizing symbol rate • Conclusion: Multilevel or M-ary signalling should be employed in the physical layer of sensor networks • i.e need to send more than 1-bit per symbol

16. Radio Energy Model: the Deeper Story…. Tx: Sender Rx: Receiver • Wireless communication subsystem consists of three components with substantially different characteristics • Their relative importance depends on the transmission range of the radio Incoming information Outgoing information Channel Power amplifier Transmit electronics Receive electronics

17. Radio Energy Cost for Transmitting 1-bit of Information in a Packet The choice of modulation scheme is important for energy vs. fidelity and energy tradeoff

19. Examples Medusa Sensor Node (UCLA) Nokia C021 Wireless LAN GSM nJ/bit nJ/bit nJ/bit ~ 50 m ~ 10 m ~ 1 km • The RF energy increases with transmission range • The electronics energy for transmit and receive are typically comparable

20. Power Breakdowns and Trends Radiated power 63 mW (18 dBm) Intersil PRISM II (Nokia C021 wireless LAN) Power amplifier 600 mW (~11% efficiency) Analog electronics 240 mW Digital electronics 170 mW • Trends: • Move functionality from the analog to the digital electronics • Digital electronics benefit most from technology improvements • Borderline between ‘long’ and ‘short’-range moves towards shorter transmit distances

21. What is wrong with this model? • Does not include many parameters • DC-DC converter inefficiencies • Overhead for transitioning from on to standby modes • Different power consumptions for receiver and transmitter • Battery discharge properties • Does not include the processor power and any additional peripherals

22. Where does the Power Go? Processing Programmable Ps & DSPs (apps, protocols etc.) ASICs Memory Communication RF Transceiver Radio Modem Peripherals Disk Display Power Supply DC-DC Converter Battery

23. DC-DC Converter Inefficiency Current drawn from the battery Current delivered to the node

24. Battery Capacity from [Powers95] • Current in “C” rating: load current normalized to battery’s capacity • e.g. a discharge current of 1C for a capacity of 500 mA-hrs is 500 mA

25. Microprocessor Power Consumption CMOS Circuits (Used in most microprocessors) Static Component Bias and leakage currents O(1mW) Dynamic Component Digital circuit switching inside the processor Dynamic Static

26. Power Consumption in Digital CMOS Circuits - current constantly drawn from the power supply - determined by fabrication technology • short circuit current due to the DC path between the • supply rails during output transitions - load capacitance at the output node - clock frequency - power supply voltage

27. DVS on Low Power Processor Number of gates Maximum gain when voltage is lowered BUT lower voltage increases circuit delay Dynamic Power Component Load capacitance of gate k Propagation delay Transistor gain factor CMOS transistor threshold voltage

28. Now Back to IEEE 802.15.4 MAC • MAC supports 2 topology setups: star and peer-to-peer • Star topology supports beacon and no-beacon structure • All communication done through PAN coordinator

29. Star: Optional Beacon Structure Generic Superframe Structure Beacon packet transmitted by PAN Coordinator to help Synchronization of network devices. It includes: Network identifier, beacon periodicity and superframe structure GTS: Guaranteed time Slots assigned by PAN coordinator

30. Star Network: Communicating with a Coordinator

31. Star Network: Communicating from a Coordinator Beacon packet indicates that there is data pending for a network device Device sends request on a data slot Network device has to ask coordinator if there is data pending. If there is no data pending the Coordinator will respond with a zero Length data packet

32. Peer-to-Peer Data Transfer • Peer-to-peer data transfer governed by the network layer – not specified by the standard • Four types of frames the standard can use • Beacon frame – only needed by a coordinator • Data frame – used for all data transfers • ACK frame – confirm successful frame reception • A MAC Command Frame – MAC peer entity controltransfers

33. Beacon Frame

34. ACK & Data Frames ACK Frame Data Frame

35. MAC Command Frame

36. Wrap-up Low Power MAC • You now have enough information to do a more detailed power consumption analysis for IEEE 802.15.4 • Need to factor in different packet structures header and MAC overheads • What are the issues related with low power MAC protocols? • Design of low power schemes for peer-to-peer networking…

37. Concept of Primitives Request: To initiate a service Indication: Indicate an N-layer event that is significant to the used Response: to complete a procedure previously invoked by an indication primitive Confirm: conveys the results of one or more associated previous service requests

38. Next Lecture • Time Synchronization • Read the paper [Elson02] Fine-Grained Network Time Synchronization using Reference Broadcasts, Jeremy Elson, Lewis Girod and Deborah Estrin, Proceedings of the Fifth Symposium on Operating Systems Design and Implementation (OSDI 2002), Boston, MA. December 2002. UCLA Technical Report 020008.