Airborne RLAN and Weather Radar Interference Studies at C Band Paul Joe 1, Frank Whetten 2, John Scott 1 and Dennis Whetten 2 1 Environment Canada 2 The 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 MHz 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 MHz. How are the AP’s implemented? >> 500 Mbps

Power Spectrum Power spectrum Time series of the AP 18 Mhz

The Players Two weeks from concept to flight! Want to know if they can operate 5 GHz AP’s Provided airplane and RLANs Want to protect CBand 10 min cycle Performance of 5GHz networks Provided special hardware Protect C Band through regulation

Vancouver/Mt Sicker Tests Network radar in standard 10 min cycle Operate “normally” Dots indicate detections Bright dots are > threshold Boxes are peaks

Flight Legs Around Radar No interference identified by weather radar!

King City Radar Ground Tests Study the DFS detection

Transmitter Room Results!

DFS Detector

Antenna Raised from 0 o to 35 o

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

Interference into radar (16.8 km) 16.8 km

Radar Detects Access Point

DFS Measures Radar Power vs Range Highly non-linear! Automatic Gain Control

Calgary Flight Test Pattern 10 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) 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 20 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?