RSSI is Under-Appreciated

transcript

RSSI is Under-AppreciatedRSSI is Under-Appreciated

Kannan Srinivasan & Philip LevisStanford Information Networking Group (SING)

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What is RSSI?What is RSSI?

• Before going anywhere: what is RSSI?– RSSI is Received Signal Strength Indicator– It’s an estimate of average received signal

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The Buzz about RSSIThe Buzz about RSSI

• RSSI is a bad indicator of link quality

• Why is it believed so?– Many studies on wireless platforms– Zhao et al.– Ganesan et al.– Son et al.– Aguayo et al. (802.11 Roofnet nodes)

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Plot of Aguayo et al.Plot of Aguayo et al.

SNR averaged over 1 sec

(Aguayo et al. SIGCOMM 2005)

Little correlation with PRR

SNR (dB)

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A New Parameter By Newer RadiosA New Parameter By Newer Radios

• CC2420 provides a new parameter, LQI for every successful packet– LQI from a single (previous) packet is

believed to be a good indicator – Many protocols have adopted single packet

LQI– No extensive evaluation to support this claim

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Why Evaluate CC2420?Why Evaluate CC2420?• Several motes are based on it: Micaz,

Telos and Intel2

• Based on IEEE 802.15.4 standard

• Operates in 2.4 GHz ISM band

• High data rate ~ 250 Kbps

• Different modulation – OQPSK– Older WSN radios used OOK (mica1) and

FSK (mica2)

• Uses Direct Sequence Spread Spectrum

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OOK, FSK & OQPSKOOK, FSK & OQPSK

1 1 10

OOK/ASK

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DSSS in CC2420DSSS in CC2420

Several chip errors can still result in a successful decoding of a symbol

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Frame Control

Seq No. Addressing Fields Frame Payload FCS

2 1 0-20 variable 2

Preamble SFD Frame Length/Rsrvd

PHY Payload

4 1 1 variable

LQI (~ CER) between 50 & 110 (8 Symbols)

RSSI (8 symbols)

CC2420 RSSI & LQICC2420 RSSI & LQI

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Experimental MethodologyExperimental Methodology1 4

14 5 6 7 8 9 1

All nodes sent packets to all other nodes.Transmit power level was changed.

Repeated experiment.

Mirage, Intel Research, Berkeley

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Results: RSSIResults: RSSI

Transmit Power Level: 0 dBm

OutliersNarrow cliff => Difference in noise floor

Distribution of RSSI for a link

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Noise Floor at NodesNoise Floor at Nodes

-98 -97 -96 -95 -94 -93 -92

# of Nodes

5 8 4 3 2 3 1

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Results: LQIResults: LQI

Large variation over time

Single LQI could mean many things

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Results: Average LQIResults: Average LQI

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What Window Size To Use?What Window Size To Use?

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LQI > 100: Single value is fine

Single LQI is not fine

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Smaller Mean PRR Error due to averaging

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Going back to Aguayo et alGoing back to Aguayo et al

(Aguayo et al Data)

Look at the SNR variation

If SNR varies so does the PRR

What matters is how signal and noise look like when you receive a packet: not before not after.

Averaging SNR maps to different PRR

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ConclusionConclusion

• RSSI is NOT a bad indicator– Above -87 dBm and stable => good link– Below -87 dBm or unstable => don’t know

• Single packet LQI is NOT a good indicator of intermediate links

• LQI, when averaged, has potential

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Some Open QuestionsSome Open Questions

• What is going on with the outliers in RSSI and LQI plots?

• Can combined knowledge of RSSI, average LQI and noise estimate PRR accurately?

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Kannan Srinivasan (srikank@stanford.edu)

Philip Levis (pal@cs.stanford.edu)

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• RSSI is a bad link quality indicator

• Why is it believed so?– After many evaluations on older radios– Zhao et al

SS > 550

PLR ~ 80%

PLR < 5% had SS > 550 but converse not true

(Zhao et al ENSS 2003)

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• RSSI is a bad link quality indicator• Why is it believed so?

– After many evaluations on older radios– Zhao et al

• (PLR < 5%) => (SS > 550) but converse not true

– Son et al (evaluated concurrent transmissions)

SINR threshold: SINR with PRR > 0.9

(Son et al ISI-TR-2005)

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Transmit Power Level: -7 dBm

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Outliers

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Narrow cliff => Difference in noise floor

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Wide cliff due to miscalibration

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Results: LQIResults: LQI

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Nicer curve fit may be due to difference in time

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CC2420 RSSI and LQICC2420 RSSI and LQI

• RSSI calculated over 8 symbols– From analog signal

• LQI (Link Quality Indicator):– 2 ways to calculate (Chipcon)

• From RSSI• Chip correlation in a byte (can be looked at as

Chip Error Rate) – provided by CC2420– Statistical in nature

» Remember: many chip errors can still lead to a correct symbol decoding

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Plot for Son et alPlot for Son et al

Difference in distortion of signal from different interfering nodes

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Average Window Size = 5

LQI > 100: Not hard to find

Still a mess

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Results from another evaluationResults from another evaluation

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LQI > 100: Single value is fine

Due to outliers or change in “quality”??

Single LQI is not fine

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RSSI is Under-Appreciated

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