Post on 02-Jan-2016
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?