User guide

45 Mbps Backhaul User Guide
Issue 3, December 2004 Page 64 of 113
have a very large impact on the operation of the wireless link so it is important that the user understands how
the UK regulations effect the wireless link when impulsive radar interference is detected.
The UK DFS requirements are specified in ETSI EN 301 893.
Their primary aims are:
Detect interference from other systems and to avoid co-channel operation with these systems, notably radar
systems;
Provide on aggregate a uniform loading of the spectrum across all devices.
Prevent transmission in the RTT (Road Traffic Tolling) part of the spectrum 5794 to 5818 MHz.
6.3.7.1 Radar Detection
To address the primary aims the DFS algorithm implements a radar detection function which looks for
impulsive interference on the active channel only. If impulsive interference is detect DFS will mark the current
active channel as having detected a radar and initiate a channel hop to an available channel. The previous active
channel will remain in the radar detected state for thirty minutes after the last impulsive interference pulse was
detected. After the thirty minutes have expired the channel will be returned to the available channel pool.
The radar detection algorithm will always scan the active channel for 60 seconds for and radar interference
before using the channel. This compulsory channel scan will mean that there is a 60 seconds service outage
every time a radar is detected. This scan is mandated by the UK regulator.
6.3.7.2 Aggregate Uniform Loading
To address the “provide aggregate uniform loading of the spectrum across all devices” requirement, the channel
selection algorithm will choose a channel at random from a list of available channels. The channel selection
algorithm is initiated at link initialization and when radar interference is detected.
6.3.7.3 UK DFS Control
The only controls available to the master are the Statistics Window and Interference Threshold attributes. They
will have no effect of the operation of the wireless link and will only affect the generation of the channel
spectrum graphics.
A channel marked with a no entry symbol with their associated statistics colored black are the RTT prohibited
channels. These channels are never used to host the wireless link, but CAC measurements are still taken so that
adjacent channel biases can be calculated correctly and so the user can see if RTT equipment is in use.