Low Power Design for Wireless Sensor Networks Aki Happonen

Table of Contents Introduction Digital Design RF Design Conclusions

Introduction Many cases wireless sensors are deployed to remote location without capability to replace battery. One of the key elements for distributed sensors is long lifetime covering both reliability and energy efficiency. This presentation covers low power design methods – focus on digital design source:

Digital Design - Efficiency One key element in digital design is energy-flexibility trade-off The most efficient solution is Application Specific IC while the worst energy efficiency is in synthesizable processors [J. M. Rabaey 2000]

Digital Design – Asynchronous Operating voltage is the key element in energy consumption – lower the voltage lower the power consumption Asynchronous design is one method to adopt lower operating voltage and lower clock frequency Two methods for data path: single rail and dual rail

Digital Design – Single Rail In single-rail design controller generates initial request signal. Request signal goes through delay that is matched to processing delay and it is also latching signal for output. Acknowledge is generated by delaying request signal by the time latching the output takes An advantage of single rail design is that synchronous designs’ blocks could be used so data paths have good area constraints.

Digital Design – Dual Rail In dual-rail design data is encoded using two wires for each bit. Codes “01” and “10” present “1” and “0” data values and code “00” represents spacer or idle state. This is robust method that guarantees correct operation with arbitrary delays in the circuit or logic. A disadvantage of dual- rail logic is larger area due the two wires per bit encoding

Dynamic Voltage Scaling (DVS) DVS allows the processor operating voltage, and frequency to be reduced when workload levels are low Challenge: How to get the optimum voltage? Huge power saving potential: 53% savings [Sinha A. et al] Current consumption is not dependent on code – only voltage and clock frequency matters

Processing Load One important issue in power management is to minimize processing activity variation. Battery lifetime decreases as a function of increase in workload variance even if the average workload remains fixed.

RF Design Power amplifier dominates total power budged if the transmission range is more than 10 meters But short-range communication having gigahertz carrier frequency the radio the frequency synthesizer dominates power rather than the actual transmit power In packet based communication the start- up time of radios start to dominate in energy consumption Table shows low power design examples for BT

Conclusions The most attractive technology to reduce power consumption is dynamic voltage scaling. In digital design energy per operation scales with technology but communication energy per transmitted bit does not scale at the same rate. Compute do not communicate. In the end implementation selections will be minor issue if the system level constraints are wrong.