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This presentation covers the analysis of occupancy in the 2.4GHz ISM band from February to May 2003. It includes the scenario, objectives, equipment used, measurement and analysis techniques, surveys conducted, results obtained, and conclusions drawn. The study focused on assessing band occupancy, spatial distribution of users, and investigating WLAN congestion. The equipment used included an omni antenna, spectrum analyzer, and laptop PC with bespoke data logging software. Surveys were conducted in both the private and public sectors to gather data on wireless activity. Findings indicated low levels of activity, with microwave oven leakage being a main source. Despite widespread WiFi usage, congestion was not observed. For further reading, refer to the Study Report on the RA website and articles in LPRA News and Electronics Weekly.
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AY4434Occupancy Analysis of the 2.4GHz ISM Band February – May 2003 Mike Biggs, mbiggs@mass.co.uk Simon Day , swd@phasor-design.uk.com
Presentation Contents • Introduction to MASS • 2.4GHz Monitoring Exercise • Scenario • Objectives • Equipment • Measurement and Analysis Techniques • Surveys • Results • Conclusions • Questions
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
Eurofighter in the Electronic Warfare Test Facility • Experts in the measurement and analysis of jamming signals
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
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
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?
Equipment • Omni antenna • Spectrum analyser • Laptop PC • Bespoke data logging software • Reasonably portable • Capable of running for 7 days continuously
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
Activity Percentile Plot • Percentiles • Blue, 50% • Red, 10% • Green, 1% • Magenta, 0.1% • Black, 0.01% Lower band edge Upper band edge Sub-band
Reference Study (1) Microwave Oven BlueTooth (carrying speech)
Reference Study (2) Video Transmitter WLAN (Internet browsing)
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
Cambridge Internet Café • Provides wireless internet service to customers • Limited take up of service • Proportion of time with any activity only 5% • Clear WLAN activity • 3 channels • Strong continuous signal • In sub-band • Underlying µwave oven activity • catering
Secondary School • 50 WLAN access points supporting > 1,000 laptops • Internet access (capped at 2MB/s) • File storage • 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
Secondary School – WLAN Interference • Report of WLAN interference • Initial period of problem-free operation • Then interference experienced on some channels • Microwave burglar alarm sensors suspected • 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
Central London (1) • Located on roof of IEE building • Energy evenly distributed across band • Band limits adhered to • Large number of emitters • Difficult to identify individual sources • Several continuous signals
Central London (2) • 7-day activity • Repeated daily pattern • Peak activity at lunchtime (microwave ovens) • Occupancy level 5-10%
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
Summary • Introduction to MASS • 2.4GHz Monitoring Exercise • Scenario • Objectives • Equipment • Measurement and Analysis Techniques • Surveys • Results • Conclusions
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
