Airborne RLAN and weather radar interference at C-Band

At the World Radio Conference, the 5GHz band, was opened up to unlicensed Radio Local Area Network (RLAN) use. The RLAN were granted use on a non-interfering basis with incumbent weather radar operations. The RLAN's operate in 15 channels of 18 MHZ bandwidth wide in the frequency range 5250-5710. The technology to mitigate the interference is Dynamic Frequency Selection (DFS) at the Access Points (AP). There are a variety of applications on airborne platforms where interference by weather radar can not be tolerated such as streaming video. So the efficacy of DFS is crucial not only to weather radars but also for airborne use. Note that, in some countries, such as Australia, use of bands in the weather radar frequencies (5600-5650 MHz) is forbidden. The current DFS regulations nor algorithms were not specified nor designed with mobile RLAN applications in mind.

To assess the effectiveness of the DFS algorithm in high-speed airborne mobile platforms, flight testing was conducted where weather radars were monitored from the aircraft as well as operating airborne 5GHz RLAN's to maximize the potential for weather radar interference. The tests were conducted were Environment Canada operational radars was scanned in volume scan and PPI modes to detect the aircraft and where the transmitter was turned on and off to create a definitive pattern in the DFS data. In additional to operating the AP's inside the fuselage, an external antenna for the AP was mounted on the Boeing 777 aircraft fuselage so that a definitive RLAN signal could be detected by the weather radar.

The results show that the onboard RLAN's reliably detect terrestrial radars in all the conditions tested which included a RF hardened aircraft body. Under the RLAN operations and weather radar scan conditions, the radars products are not compromised due to fuselage attenuation of the RLAN signal.

Ground based testing was also conducted to understand the performance of a commercially manufactured AP. Previous studies used home grown DFS detectors or RLAN packets from a signal generator. . At near ranges (~3km), the DFS was able to detect the weather radar even when the radar was not incident on the AP. The manufacturers have great leeway in developing algorithms used to detect weather radar and vacate the channelThe AP tested exceeded the specification outlined by the International Telecommunication Union.