Paper presentation Ultra-Portable Devices Paper: Christian C. Enz, - PowerPoint PPT Presentation

Paper presentation – Ultra-Portable Devices Paper: Christian C. Enz, Nicola Scolari, et al. Ultra Low-Power Radio Design for Wireless Sensor Networks, International Workshop on RF Integration Technology Nov. 30 – Dec 02, 2005. Presented by: Dejan Radjen 2009-02-09 Paper Presentation - Ultra Portable Devices 1

Outline • Introduction • The Power Consumption Challenge in WSN • Wireless Sensor Network Architectures • Transceiver Design Considerations • Low-Power Transceiver Architectures • Analog-to-Digital Converter • Summary and Conclusions 2009-02-09 Paper Presentation - Ultra Portable Devices 2

Introduction • A Wireless Sensor Network consists of a large number of sensor nodes • Each node locally processes and stores data so it can be used by other nodes • The nodes might be placed in regions difficult to access and must therefore be energetically autonomous • A targeted node lifetime ranges typically between 2 – 5 years 2010-02-09 Paper Presentation - Ultra Portable Devices 3

The Power Consumption Challenge in WSN Lifetime of a sensor node powered by a 1.5 V AA battery Capacity: 2.6 Ah, Assumed leakage current: 30 μA • Lifetime 2-7 years requires an average powers of 10 – 100 μW • Can be reached only by duty cycling and by minimizing the sleep mode current 2010-02-09 Paper Presentation - Ultra Portable Devices 4

The Power Consumption Challenge in WSN • Severe constraints on sensor node design – Limited processing speed and storage capacity – Keep communication bandwidth at minimum • Several ways of reducing the power consumption of a sensor node – Use of ad-hoc networks and multi-hop communication – Trade-off between communication and local computing – More efficient radio design (low duty cycle) – More energy efficient protocols and routing algorithms 2010-02-09 Paper Presentation - Ultra Portable Devices 5

Wireless Sensor Network Architectures • Infrastructure networks → the sensor nodes communicate via a base station • Ad hoc networks → multi hop communication between sensor nodes – Takes advantage of the exponential decrease in radiated power – The following is a quite optimistic estimate d = distance d in a single hop α = radiated power exponent 2010-02-09 Paper Presentation - Ultra Portable Devices 6

Transceiver Design Considerations • The end-of-life voltage of a 1.5 V AA alkaline battery is 0.9 V • Low supply voltages force the transistors to operate in moderate or even weak inversion • Moderate inversion offers a good trade of between current generation efficiency and speed • Small currents lead to small g m and significantly higher impedances than 50Ω are required for RF -design 2010-02-09 Paper Presentation - Ultra Portable Devices 7

Transceiver Design Considerations Duty Cycle and Optimum Data Rate • Smaller data rate → smaller signal and noise bandwidth • Higher system noise figure is acceptable assuming a given receiver sensitivity • The above statements are true if the duty cycle is 100 % • The radio has to be duty cycled to reach desired power consumption • The power consumption is roughly divided as – P 0 = Fixed power mainly due to the frequency synthesizer – P dem = Demodulation chain power (LNA, mixers, filters, etc…) 2010-02-09 Paper Presentation - Ultra Portable Devices 8

Transceiver Design Considerations Duty Cycle and Optimum Data Rate 2010-02-09 Paper Presentation - Ultra Portable Devices 9

Transceiver Design Considerations Modulation • Binary modulation schemes for simple transceiver architectures – OOK, FSK and BPSK OOK and FSK Modulation OQPSK Modulation 2010-02-09 Paper Presentation - Ultra Portable Devices 10

Transceiver Design Considerations Modulation • OOK is the most basic modulation – Special encoding used to avoid long series of identical bits – Requires automatic gain control → might be power consuming and slow – Peak output power twice as large as for FSK or BPSK • FSK suitable for direct conversion architectures – Wide band FSK (WBFSK) further simplifies the receiver – Δf > BW, very little signal power around the carrier – Poor spectral efficiency • BPSK is the most efficient binary modulation – Requires less SNR than OOK and FSK – Requires an ADC which becomes power hungry 2010-02-09 Paper Presentation - Ultra Portable Devices 11

Low-Power Transceiver architectures Receiver Architectures Super heterodyne Super regenerative 2010-02-09 Paper Presentation - Ultra Portable Devices 12

Low-Power Transceiver architectures Receiver Architectures Low-IF Zero IF or direct conversion 2010-02-09 Paper Presentation - Ultra Portable Devices 13

Low-Power Transceiver architectures Transmitter Architectures • Direct modulation of the VCO-signal – Changing the reference frequency directly – Alteration of the frequency divider ratio – Direct modulation of the control voltage of the LC-tank VCO varactors • Upconversion architecture 2010-02-09 Paper Presentation - Ultra Portable Devices 14

Low-Power Transceiver architectures The 1st Generation of WiseNet Transceivers 2010-02-09 Paper Presentation - Ultra Portable Devices 15

Low-Power Transceiver architectures Main Measured Results of the 1st generation WiseNet 2010-02-09 Paper Presentation - Ultra Portable Devices 16

Low-Power Transceiver architectures The 2nd Generation of WiseNet Transceivers 2010-02-09 Paper Presentation - Ultra Portable Devices 17

Low-Power Transceiver architectures Main Measured Results of the 2nd Generation WiseNet 2010-02-09 Paper Presentation - Ultra Portable Devices 18

Analog to Digital Converter • For higher data rates and better spectral efficiency phase modulation can be used • An ADC is required in the receiver chain for successful demodulation • Two approaches for analog to digital conversion are proposed – Quadrature ∑Δ converters – Phase Analog-to-Digital Converters 2010-02-09 Paper Presentation - Ultra Portable Devices 19

Analog to Digital Converter Quadrature ∑Δ converters • A direct conversion receiver suffers from flicker noise and self coupling of the LO • In addition to low pass filtering, high pass filtering is required → a band pass filter is needed in front of low pass ∑Δ - converter • Low corner frequency is hard to achieve → solution: a very low IF receiver • With a non zero IF, the quantization noise becomes an issue → solution: shift the low pass NTF to IF 2010-02-09 Paper Presentation - Ultra Portable Devices 20

Analog to Digital Converter Quadrature ∑Δ converters NTF-shift Band pass STF 2010-02-09 Paper Presentation - Ultra Portable Devices 21

Analog to Digital Converter Quadrature ∑Δ converters Low pass ∑Δ -modulator Quadrature ∑Δ -modulator 2010-02-09 Paper Presentation - Ultra Portable Devices 22

Analog to Digital Converter Quadrature ∑Δ converters GmC implementation of a complex resonator 2010-02-09 Paper Presentation - Ultra Portable Devices 23

Analog to Digital Converter Phase Analog-to-Digital Converters • After direct conversion the two normalized signal components are • For QPSK it is ideally sufficient to keep track in which quadrant in the constellation diagram the signal vector is • Due to intersymbol interference the vector might not always cross from one quadrant to another 2010-02-09 Paper Presentation - Ultra Portable Devices 24

Analog to Digital Converter Phase Analog-to-Digital Converters • Quantization of quadrants • Linear combination yields 2010-02-09 Paper Presentation - Ultra Portable Devices 25

Analog to Digital Converter Phase Analog-to-Digital Converters 2N – 1 Signals Detection of the signs of the signals Thermometer code 2010-02-09 Paper Presentation - Ultra Portable Devices 26

Analog to Digital Converter Phase Analog-to-Digital Converters Generation of I k -signals Resistive bridge Resistive current divider Pseudo resistance implementation implementation implementation Voltage outputs Current outputs Current outputs 2010-02-09 Paper Presentation - Ultra Portable Devices 27

Summary and Conclusions • The paper addresses different issues faced when designing ultra low power WSN transceivers • Keep things as simple as possible • The type of modulation used has a strong impact on the complexity of the radio • Simple binary modulation formats are preferred due to simple demodulation • Direct conversion and low IF receiver architectures are preferred due to their potential for higher integration 2010-02-09 Paper Presentation - Ultra Portable Devices 28