Airborne RLAN and Weather Radar Interference Studies

at C Band

Paul Joe1, Frank Whetten2, John Scott1 and Dennis Whetten2

1Environment Canada2The Boeing Company

Background

• International Telecommunication Union (ITU) approves C Band for Radio Local Area Network use on non-interfering basis with weather radar (2003) • eleven 18 MHz bands between 5470-5725MHz

• RLAN to implement Dynamic Frequency Selection (DFS) to avoid interference

• Previous report on stationary, ground tests, bench version of a DFS detector and Access Point (AP)

• Commercial units now available• Mobile versus stationary Access Points• Performance of DFS “algorithm” to mitigate

interference on weather radar• Performance of weather radar on Access Point

Three RLAN Tests

• Vancouver tests• Boeing 777 with AP’s fly in vicinity of Mt

Sicker radar• Coincident data collection by weather

radar, DFS detector in operational mode• Ground test

• Focus is on the DFS detector performance• Calgary tests

• Repeat Vancouver, external antenna, special scans

C Band Channels and RLAN’s

Note: Weather radars use only 1 MHz bandwidth in 5600-5650MHz.

How are the AP’s implemented?

>> 500 Mbps

Power Spectrum

Power spectrum

Time series of the AP

18 Mhz

The PlayersTwo weeks from concept to flight!

Want to know if they can operate 5 GHz AP’sProvided airplane and RLANs

Want to protect CBand10 min cycle

Performance of 5GHz networksProvided special hardware

Protect C Band through regulation

Vancouver/Mt Sicker TestsNetwork radar in standard 10 min cycleOperate “normally”

Dots indicate detectionsBright dots are > thresholdBoxes are peaks

Flight Legs Around Radar

No interference identified by weather radar!

King City Radar Ground TestsStudy the DFS detection

Transmitter Room Results!

DFS Detector

Antenna Raised from 0o to 35o

DFS detects radar at 48 km range(> 25 km Line of Sight range)

AP DFS Radar Detections

-90

-80

-70

-60

-50

-40

-30

-20

-10

0

16:0

7:30

16:0

7:45

16:0

8:00

16:0

8:15

16:0

8:30

16:0

8:45

16:0

9:00

16:0

9:15

16:0

9:30

16:0

9:45

16:1

0:00

Time (H:M:S)

Rad

ar P

ow

er D

etec

ted

(d

Bm

)

Radar Tx Off Radar Tx Off

09:00 ~ 09:3008:00 ~ 08:30

Interference into radar (16.8 km)

AP Interference Level into Radar in dBm

-120

-100

-80

-60

-40

-20

0

0 50 100 150 200 250

Slant Range (km)

Rec

eive

d P

ow

er (

dB

m)

AP Signal Level

Radar Noise Floor

16.8 km

DFS Measures Radar Power vs Range

Highly non-linear!Automatic Gain Control

Calgary Flight Test Pattern10 sec cycle rate/external antenna

50 nmi

25 nmi

Special PPI’s•Disable filters•No range averaging•0.5 azimuth•6 rpm•10 s update

DFS has no problem seeing Radar (time between hits)

Radar sees the External Antenna (40 dBm)

Aircraft is seen asa “hot” spot

RLAN signal

Aircraft is seen asa “hot” spot

RLAN signal

1. Could only see the RLAN with external antenna at powers greater than normal operations!

2. Aircraft was RF hardened (different windows), DFS still detects the weather radar!

Link Budget

Thermal Noise Floor

FCC Spurious Emissions Limit

-20 dBm

-40 dBm

-60 dBm

-80 dBm

-100 dBm

-120 dBm

-140 dBm

1 m 10 m 100 m 1,000 m 10,000 m 100,000 m

684 m

FuselageAttenuation

~30 m

Distance from Transmitter

Scenario:100 mW 802.11a transmitter17 dB Fuselage Attenuation

SignalStrength

593 m

Thermal Noise Floor

FCC Spurious Emissions Limit

-20 dBm

-40 dBm

-60 dBm

-80 dBm

-100 dBm

-120 dBm

-140 dBm

1 m 10 m 100 m 1,000 m 10,000 m 100,000 m

684 m

FuselageAttenuation

~30 m

Distance from Transmitter

Scenario:100 mW 802.11a transmitter17 dB Fuselage Attenuation

SignalStrength

593 m20 dBm

Isotropic

Link Budget

Appears that we have a ~8dBm discrepency however AP does not transmit a constant amount of power!

~-110 dBm

Access Point Response to DFS “hit”The “DFS Algorithm”

• DFS detects radar by pulse counting not by “detection of leading edge of pulse” !!! (μs)

• AP sends signal to clients to cease transmission (ms)

• AP ceases transmission (seconds)• So, AP may create interference into radar

• Seconds response is slow, will create interference

• Vacates channel 30 min• New channel(s) randomly selected and

monitored before use

DFS Summary Comments

• Commercial DFS systems can be manufactured to easily detect the weather radar

• Weather radar will impact RLAN operations particularly for streaming applications (maybe transparent for TCP/IP type apps)

• Colubris systems reports any detections within the equipment sensitivity which was below the -63 dBm requirement, this may not be true for all manufacturers.

Weather Radar Definition

• DRAFT ITU specification for weather radar are deficient!!!!

• ETSI and US definitions• low surveillance PRF’s (250 vs 700 vs 300 Hz)• small pulsewidths (1 μs vs 0.8 vs 0.5 μs)• staggered PRT’s, phase coding

• Colubris AP’s were able to detect small pulse widths and low PRFs• may not be true in general !!!

Standard DFS Algorithm

• Time from detection to vacating channel is of the order on seconds!

• Impact• Initial 30 minute frequency channel check• If DFS detects weather radar, vacate

channel – but radar may experience interference since the sweep rate is high

• 30 minutes later can use channel again, repeated interference possible, no checking required

• Strong multiplier effect of many AP’s!!!

Canadian Channel Availability Check

• 10 minute check before re-use of channel • match cycle time of scan strategy• If not in maintenance mode, DFS will

detect the radar and never use the channel

• Without this check, potential for continuous interference from multiple Access Points

Summary

• Interference tests with real-world scenario and one particular AP by Colubris

• DFS can see weather radar very well and experience interference for streaming applications

• Moving platform requires additional criteria for operations

• Annoyance is link budget doesn’t balance• Colubris DFS is highly non-linear detection system

(not quantitative, not unexpected)• Power output is variable, AGC for detection

• AP can not use the channels ! • Weather radar did not see the airborne RLAN • Colubris system is designed for high POD. Do other

manufacturer’s AP’s work similarly?