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When military radar causes fixed broadband wireless interference

Military radar may cause fixed broadband wireless interference. This expert tip explains tests and ideas to check if this is the case and how to address this interference.

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We established a RF link (using an Infinet router) to connect two locations about 40 KM apart with fixed broadband wireless. It was working fine for 2 weeks and suddenly it started going out for 2 to 3 hours and coming back on automatically. The ISP is telling us this is due to military radar operating in that area. If it is due to a radar issue what are the possible solutions?

It is quite possible that your ISP has landed on the root cause of your problem.  Military radar and commercial Wi-Fi systems have been known to cause co-interference with each other, as the military C-band occupies frequencies in the range of 5290 to 5925 MHz, overlapping with the IEEE 802.11a wireless local area network standard allocations of 5150 to 5725 MHz. Likewise, the Military E, F, and G Bands occupy frequencies from 2 to 6 GHz, which overlap with the radio frequencies allocated to many Fixed Broadband Wireless Access (FBWA) solutions such as the InfiNet Wireless InfiLINK products.  The InfiLINK R5000-Omx, in particular, operates in the 2.3-2.6, 4.9-6.4 GHz ranges, leaving the possibility of co-interference with military systems that occupy frequencies in this range.

The only way to be certain that interference with military radar is causing the problems is to scan the radio environment over an extended period of time to determine working frequencies so that collisions can be avoided.  This can be accomplished with a dedicated spectrum analyzer, or by using a built-in spectrum analyzer that is included with most FBWA products such as the InfiNet Wireless R5000-Omz.

To address the concerns of co-interference, the Dynamic Frequency Selection (DFS) specification was created to define a set of procedures to detect and avoid interference with Radar systems operating in the 5 GHz range. If radar is detected the Wi-Fi device must alter the channel it is operating on, and (ideally) notify associated stations what channel they will be moving to.  One possibility is that one of your devices has detected radar interference with DFS and has switched frequencies without notifying its peer device so that it can make the associated change. Another is that DFS is not enabled through configuration on both devices. This may explain the intermittent outages that you are occurring.

The following note from the technical discussion forum on http://forum.infinetwireless.com/ suggests that DFS must be enabled through configuration.  I would suggest verifying that this has been done on all devices as a first step.

Radar detection is available if:

#DFS configuration
 dfs rf5.0 dfsradar
 dfs rf5.0 freq auto
 dfs rf5.0 cot 00:00

 

Tom Kunath, CCIE no 1679, is a Solutions Architect in Cisco’s Advanced Services Performance and Validation Test group. With nearly 20 years in the networking industry, Tom has helped architect, deploy, and operate many of Cisco’s largest Enterprise and Financial customer networks. Before joining Cisco, Tom worked at Juniper Networks’ Professional Services Group as a Resident Engineer supporting several Service Provider IP and MPLS backbones, and prior to that as a Principal Consultant at International Network Services (INS). In addition to his CCIE, Tom holds several industry certifications including a Juniper JNCIS, and Nortel Networks Router Expert. Tom lives in Raleigh, NC with his wife and two children.

This was first published in September 2011

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