Low Power Wireless DesignLow Power Wireless Design

Dr. Ahmad Bahai

National Semiconductor

New paradigm in WirelessNew paradigm in Wireless

Bits/s/Hz J/Bits/s/HzJ/Bits/s/Hz

Design for worst case

Configurable Configurable DesignDesign

Power Efficiency

Configurability

Centralized HybridHybrid

Architecture

Power efficiencyPower efficiency

Cellular

WLAN

UWB

Distance to IP Network

Miles

Yards

Feet

Pervasive IP

TX Power

100 mW

10 mW

mW

Tx power ~ Circuit power(1nJ/bit transmission energy- 10 m distance)

Power = Tx power + Circuit power

Data Rate

100s kbps

10s mbps

100s mbps

Comm Theory, Asym ValuesComm Theory, Asym Values

dBWCN

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WC

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Absolute minimum energy for reliable transmission Absolute minimum energy for reliable transmission of 1 bit of informationof 1 bit of information

JEb4.171069

Min switching energy for digital gate (1 electron @100mV): 1.6X10-20

Transmission vs. Circuit EnergyTransmission vs. Circuit Energy

Communication Theory usually Communication Theory usually considers Transmission energy considers Transmission energy

only!only!

Transmission EnergyTransmission Energy Spectral EfficiencySpectral Efficiency

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Optimal Bandwidth-time pair? Optimal Bandwidth-time pair?

Total Energy (MQAM) Total Energy (MQAM)

PlatformPlatform

MAC layer MAC layer including including ARM and PCIARM and PCI

PCI PCI interfaceinterface

Phy Phy Tx/RXTx/RX

RF/Analog RF/Analog supporting up supporting up to 4 radiosto 4 radios

Power profile in WLAN (TX)Power profile in WLAN (TX)

261, 22%

40, 3%

36, 3%

47, 4%

288, 24%

530, 44%

DSP+MAC DAC BBFE LO RF FE PA

Power profile in WLAN (RX)Power profile in WLAN (RX)

176, 24%

200, 28%72, 10%

47, 6%

60, 8%

174, 24%

DSP+MAC ADC BBFE LO RF FE FEC

Channel EffectChannel Effect

IMEC IMEC CollaborationCollaboration

Comm Theory Approach Comm Theory Approach

Bandwidth

Power Mask

Interference

Data rate

BER

Channel

Modulation

Coding

Synchronization

SiNR

Dynamic Range

Margin

Noise figure

EnergyEnergy

QoSQoS

Statistical Statistical performanceperformance

MAC State machinesMAC State machines

Adaptive designAdaptive design

Energy and ThroughputEnergy and Throughput

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GPR

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Common Approach:

Define SINR and capacity as

Assume BPSK with BER target of 10e-q, bandwidth W and target data rate of R>C; then we can show that minimal power vector supporting network topology for low SIR can be derived as:

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Design StrategyDesign Strategy

System level approach to low power communication design

Case study: ZigBee

•Profile the power consumption

•Study effect of multi-layer optimization

•A new design strategy

IEEE 802.15.4 PHYIEEE 802.15.4 PHY

Direct Sequence Spread Spectrum (DSSS) radio 2Mchip/s OQPSK modulation 1 symbol = 32-chip PN sequence 1 symbol = 4 bits PHY data rate: 250kbps Transmit power up to 0 dBm

BAND COVERAGE DATA RATE CHANNEL(S)

2.4 GHz ISM Worldwide 250 kbps 16

868 MHz Europe 20 kbps 1

915 MHz ISM Americas 40 kbps 10

802.15.4 spec. summary802.15.4 spec. summary

Symbol rate and Tx RF accuracy: +/- 40 ppm

Packet Error Rate (PER) Defined for PSDU of 20x8 bits

Sensitivity: -85 dBm (PER < 1%)

RSSI: sens. level +10 dB, 40 dB range (+/- 6dB)

Max input level: -20 dBm

Jamming resistance (interference performance) 0 dB for adjacent channels (ref: -82 dBm) 30 dB for alternate channels (ref: -82 dBm) Interferer is 802.15.4 compliant interferer

Tx Error Vector Magnitude : < 35% for 1000 chips

Tx PSD: -20 dB or –30 dBm |f-Fc| > 3.5 MHz (rbw 100kHz)

Output power: > -3 dBm (@ max power setting)

Rx-Tx turnaround time: 12 Symbols (192 ms)

ZBIC, one-chip solution ZBIC, one-chip solution

ZBIC

4-state/Transition Energy Profile4-state/Transition Energy Profile

Shutdown80 nA

Idle396 uA

RX19.6 mA

TX-25 dBm: 8.42 mA-15 dBm: 9.71 mA-10 dBm: 10.9 mA-7 dBm: 12.17 mA-5 dBm: 12.27 mA-3 dBm: 14.63 mA

-1 dBm: 15.785 mA0 dBm: 17.04 mA

VDD = 1.8V

970 us691pJ

194 us6.63 uJ

194 us6.63 uJ

Transition Energy

T(transition) x I(target state) x VDD IMEC/MITIMEC/MIT

Power ProfilePower Profile

ObservationsObservations

Efficiency (energy/bit) changes with:

Larger packet size

Transmit power control

Network Load

Link layer performance

Contention

Channel Coding

Power BreakdownPower Breakdown

Breakdown between the states

In high load, the node spends more time in RX than in TX mode!

IMEC/MITIMEC/MIT

More comprehensive Energy modelMore comprehensive Energy model

Energy efficiency metric:

New model for total energy was used to optimize back off strategy in an ad-hoc network.

TX, RX, Collision, sensing, Transitions, ramp up

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PayloadE

EnergyE total

Energy Efficient BackoffEnergy Efficient Backoff

Proposed backoff

Standard backoff

Resetting back-off is more energy efficient than DCF backoff due to carrier sensing overhead.

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SummarySummary

Statistical Performance Analysis: New design paradigm in communication

Mixed signal processing and cross layer optimization

Configurable and low power design: Key Design objectives

Multimode/Multi-layer Optimization

Analog/mixed signal: critical in power consumption