AY4434Occupancy Analysis of the

2.4GHz ISM Band

Mike Biggs, mbiggs@mass.co.uk

Simon Day , swd@phasor-design.uk.com

February – May 2003

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

Spectrum Analyser

GPIB

Laptop with GPIB card

& CD-writer

Antenna

Tripod

Powersource

• 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%

Sub-bandLower band edge Upper band edge

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

– 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

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

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

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

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