RSSI is Under-Appreciated
Kannan Srinivasan & Philip Levis
Stanford Information Networking Group (SING)
What is RSSI?
• Before going anywhere: what is RSSI?
– RSSI is Received Signal Strength Indicator
– It’s an estimate of average received signal
power
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The 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.
PRR
Little correlation
with PRR
SNR averaged
over 1 sec
(Aguayo et al. SIGCOMM 2005)
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SNR (dB)
4
A 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?
• 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 & OQPSK
1
0
1
1
Data
mica1
OOK/ASK
mica2
FSK
telos
QPSK
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DSSS in CC2420
Several chip errors can still result in a successful decoding of a symbol
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CC2420 RSSI & LQI
LQI (~ CER) between 50 & 110
(8 Symbols)
Preamble
SFD
4
1
Frame Length/
Rsrvd
1
PHY Payload
variable
RSSI
(8 symbols)
Frame
Control
Seq No.
Addressing Fields
2
1
0-20
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Frame Payload
variable
FCS
2
9
Experimental Methodology
1 2 3 4 5 6 7 8 9 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3 3 3 3 3 3 3 3 3 4 4
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
A
12
25
19
B
C
14
4
D
E
7
3
6
2
5
15
11
20
18
23
24
28
F
G
H
I
8
10
21
13
J
26
17
27
29
22
All nodes sent packets to all other nodes.
Transmit power level was changed.
Repeated experiment.
K
L
M
N
O
P
1
9
16
30
Mirage, Intel Research, Berkeley
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Results: RSSI
Distribution of
RSSI for a link
Transmit Power
Level: 0 dBm
Outliers
Narrow cliff => Difference
in noise floor
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Noise Floor at Nodes
Noise
(dBm)
-98
-97
-96
-95
-94
-93
-92
# of
Nodes
5
8
4
3
2
3
1
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Results: LQI
Large variation over
time
Transmit Power
Level: 0 dBm
Single LQI could
mean many things
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Results: Average LQI
Transmit Power
Level: 0 dBm
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What Window Size To Use?
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Results: Average LQI
Single LQI is not fine
LQI > 100: Single
value is fine
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Results: Average LQI
Smaller Mean
PRR Error due to
averaging
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Going back to Aguayo et al
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
(Aguayo et al Data)
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Conclusion
• 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 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 ([email protected])
Philip Levis ([email protected])
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The Buzz about RSSI
• RSSI is a bad link quality indicator
• Why is it believed so?
SS > 550
– After many evaluations on
older radios
PLR ~ 80%
– Zhao et al
PLR < 5% had SS > 550
but converse not true
(Zhao et al ENSS 2003)
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The Buzz about RSSI
• RSSI is a bad link quality indicator
SINR threshold: SINR
• Why is
believed
so?
withitPRR
> 0.9
– After many evaluations on older radios
– Zhao et al
• (PLR < 5%) => (SS > 550) but converse not true
– Son et al (evaluated concurrent transmissions)
(Son et al ISI-TR-2005)
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Results: RSSI
Transmit Power
Level: 0 dBm
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Results: RSSI
Transmit Power
Level: -7 dBm
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Results: RSSI
Transmit Power
Level: 0 dBm
Outliers
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Results: RSSI
Transmit Power
Level: 0 dBm
Narrow cliff => Difference
in noise floor
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Results: RSSI
Wide cliff due to
miscalibration
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Results: LQI
Transmit Power
Level: -7 dBm
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Results: Average LQI
Nicer curve fit
may be due to
difference in time
Transmit Power
Level: -7 dBm
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CC2420 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 al
Difference in distortion
of signal from different
interfering nodes
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Maximum Absolute
Results: Average LQI
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Results: Average LQI
Average Window Size = 5
Still a mess
LQI > 100: Not
hard to find
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Results: Average LQI
Average Window Size = 10
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Results from another evaluation
LQI
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Results: Average LQI
Average Window Size = 1
Single LQI is not fine
Due to outliers or
change in “quality”??
LQI > 100: Single
value is fine
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Results: Average LQI
Average Window Size = 20
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