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Study of Actual State of Wireless Technology in Residential Environments
Mo ShaPeng LiJing Xia
Link study TelosB
802.15.4 compliant Chipcon CC2420 radio
250 kbps maximum bit rate
Can be programmed to work on 16 channels
Received Signal Strength Indicator (RSSI) to measure signal strength
Spectrum Analyzer Use Wi-Spy spectrum
analyzer Can track all radio
activity from 2.4 GHz Connected to PC
though USB
2
15 Days
5+ GB Data Trace
Link Snooping + 2.4GHz Spectrum Scanning
Questions? 1. Is every 802.15.4 channel covered by exclusive wireless signal?
2. How long does every 802.15.4 channel maintain consecutive clean?
3. Is channel diversify beneficial?4. Is the interference narrow-band or wide-band?5. Does the interference change over time?6. Is there cyclic behavior (time correlation)?7. Is there correlation between adjacent channels (frequency
correlation)?8. Does the interference mainly cause by Wifi?9. Is every 802.15.4 channel clean if we remove modeled Wifi signal?10. How many 802.15.4 channel become clean if we remove modeled Wifi signal?
Is every 802.15.4 channel covered by exclusive wireless signal?
RSSI Threshold=30dB+noise floor
Is every 802.15.4 channel covered by exclusive wireless signal?
RSSI Threshold=40dB+noise floor
Is every 802.15.4 channel covered by exclusive wireless signal?
Insight: Each 802.15.4 channel covered by exclusive wireless signal, but the impact on each channel is different.
Impact
How long does every 802.15.4 channel maintain consecutive clean?
RSSI Threshold=30dB+noise floor
How long does every 802.15.4 channel maintain consecutive clean?
RSSI Threshold=40dB+noise floor
How long does every 802.15.4 channel maintain consecutive clean?
Insight: Channel with long-time consecutive clean condition is rare.
Impact
Is channel diversify beneficial?
Insight: channel diversify is beneficial.
With an ideal channel hopping approach
Is the interference narrow-band or wide-band?
Insight: The interference is mainly narrow-band, hence channel diversify is helpful.
Does the interference change over time?
Insight: The interference on each channel changes over time.
Hamming Distance• Purpose
• Evaluating the difference between two equal length strings
• Definition• Number of positions at which the corresponding symbols are
different
• Normalized Hamming Distance (NHD)• Hamming distance divided by the length of string• NHD described the difference between two strings• Digitalized wireless signals are strings of 0’s and 1’s• We use 1-NHD to quantify the similarity (correlation) between two
signal sets
Frequency Correlations - 1
Window size = 3 min; Threshold = 30 dBm + Noise floor
Frequency Correlations - 2
Window size = 3 min; Threshold = 40 dBm + Noise floor
Time Correlations - 1
Window size = 3 min; Threshold = 30 dBm + Noise floor
Time Correlations - 2
Window size = 3 min; Threshold = 40 dBm + Noise floor
Signal dependency between channels
Window size = 3 min; Threshold = 30 dBm + Noise floor
Signal dependency between channels
Window size = 3 min; Threshold = 40 dBm + Noise floor
Wireless Sensors interfered by WiFi Questions
8. Does the interference mainly cause by Wifi?
21
15 Days Link Snooping + 2.4GHz Spectrum Scanning
802.11(WiFi channels) and 802.15.4( TelosB channels)
Does the interference mainly cause by Wifi?Packet Receive Ratio (PRR)
TelosB : 16 channels range from 2.405GHz to 2.485GHz
PRR of TelosB 16 channels
Percentage of strong signal per second• TelosB reciever sensitivity -90dBm (min) -94dBm
www.xbow.com • Threshold = 30dBm + nosie floor• Percentage of signal >threshold per second
Correlation of consecutive days of TelosB 16 channels
16 TelosB channels14 consecutive days
16 TelosB channels14 consecutive days
Correlation of consecutive days of WiFi 13 channels
1 2 3 4 5 6 7 8 9 10 11 12 13
0
5
10
15-0.4
-0.2
0
0.2
0.4
0.6
13 WiFi channels14 consecutive days
13 WiFi channels
Conclusion
Random occurrence of WiFi signals (no correlation among time) High frequency of occurrence of WiFi signals in residential
apartments Interfere with wireless sensors
