ay4434 occupancy analysis of the 2.4ghz ism band mike biggs, [email protected] simon day,...
TRANSCRIPT
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AY4434Occupancy Analysis of the
2.4GHz ISM Band
Mike Biggs, [email protected]
Simon Day , [email protected]
February – May 2003
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Presentation Contents
• Introduction to MASS• 2.4GHz Monitoring Exercise
– Scenario– Objectives– Equipment– Measurement and Analysis Techniques– Surveys– Results– Conclusions
• Questions
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Mass Consultants – Who Are We?
• Systems House
• 3 Divisions– Systems Engineering
– Real-Time Systems
– Managed Services
• Vital Statistics– 100 people strong
– Turnover ~ £12M
– >90% defence
– Based in St Neots
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Eurofighter in the Electronic Warfare Test Facility
• Experts in the measurement and analysis of jamming signals
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MASS and RA
• RA concerned with the effects of radio interference
• Jamming is intentional RF interference
• Applied EW expertise to commercial radiocomms
• 3 projects for RA– AY4119 - Man-Made Noise Measurement Programme– AY4364 - EMC Implications of Software Radio– AY4434 - 2.4GHz Monitoring Exercise
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2.4GHz Monitoring Exercise- Scenario
• 2,400MHz to 2,483.5MHz ISM band• Many uncoordinated services
– Microwave oven leakage– Broadcasting services– RF identification devices– Licensed FWA systems– BlueTooth– WLAN / WiFi / IEEE802.11b
• Danger of band congestion
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Objectives
• Assess band occupancy– Time-varying nature of usage– Spatial distribution of users
• Investigate anecdotal evidence of WLAN congestion– Many instances reported to RA– Is there any evidence of congestion?– If so, why is it occurring?
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Equipment• Omni antenna• Spectrum analyser• Laptop PC• Bespoke data
logging software
Spectrum Analyser
GPIB
Laptop with GPIB card
& CD-writer
Antenna
Tripod
Powersource
• Reasonably portable• Capable of running for 7 days continuously
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Measurement Process
• Continuously scan band of interest– Include 30MHz ‘guard bands’ to check for out-of-band
activity– Use 1 MHz bandwidth
• Data collected in 10-minute blocks– > 10,000 samples– Good statistical set– Tangible period of time
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Activity Percentile Plot• Percentiles
– Blue, 50%– Red, 10%– Green, 1%– Magenta, 0.1%– Black, 0.01%
Sub-bandLower band edge Upper band edge
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Reference Study (1)
Microwave Oven BlueTooth (carrying speech)
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Reference Study (2)
Video Transmitter WLAN (Internet browsing)
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Surveys
• Private Sector Duration– Internet café 1½ hrs– Hub of broadband wireless provider 24 hrs– Roving exercise around Cambridge 1 day
• Public Sector– Primary school school hrs– Secondary school school hrs– Cambridge hospital 24 hrs– Heathrow airport 2 hrs
• City centre– London 7 days– Glasgow 7 days
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Cambridge Internet Café
– Proportion of time with any activity only 5%
– Clear WLAN activity• 3 channels
– Strong continuous signal• In sub-band
– Underlying µwave oven activity• catering
• Provides wireless internet service to customers– Limited take up of service
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Secondary School
– Proportion of time with activity up to 30%
• Highest of sites surveyed
– Clear WLAN activity• 4 channels
– Reputedly very effective– Recorded activity fluctuates
according to school timetable
• 50 WLAN access points supporting > 1,000 laptops– Internet access (capped at 2MB/s)– File storage
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Secondary School – WLAN Interference
– Strong sensor signal in central sub-band
– WLAN and burglar alarm installed correctly by professionals
– Radiation limits not exceeded– But Interference experienced– Work-around is to avoid channels
close to sub-band
• Report of WLAN interference– Initial period of problem-free operation– Then interference experienced on some channels
• Microwave burglar alarm sensors suspected
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Central London (1)
– Energy evenly distributed across band
– Band limits adhered to– Large number of emitters– Difficult to identify individual
sources– Several continuous signals
• Located on roof of IEE building
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Central London (2)
• 7-day activity– Repeated daily pattern– Peak activity at lunchtime
(microwave ovens)– Occupancy level 5-10%
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Conclusions• Recorded levels of activity low in terms of
– Signal strength– Proportion of time on
• Microwave oven leakage appears to be main source• Microwave movement detectors commonplace
– Generally low signal strenths– 1 case where microwave movement detectors were found to
be causing interference to WLAN
• BlueTooth emissions not observed– But not surprising, as low power, low duty cycle devices
• Evidence of widespread usage of WiFi observed– No cases of congestion (due to traffic density) observed
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Summary
• Introduction to MASS
• 2.4GHz Monitoring Exercise– Scenario– Objectives– Equipment– Measurement and Analysis Techniques– Surveys– Results– Conclusions
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Further Reading
• Study Report on RA website– http://www.radio.gov.uk/topics/research/topics.htm
• Article in LPRA News– Published September 2003
• Article in Electronics Weekly– Published 12 November 2003
• Article submitted to IEE Proceedings, Communications, special issue on WLANs– Due for publication in 2004