RLANs and weather radars in the 5 GHz band rev 3

© 2006 Cisco Systems, Inc. All rights reserved. Cisco ConfidentialPresentation_ID 1

RLANs and weather radars in the 5 GHz bandrev 3

Jan Kruys

Dec 7, 2006


Purpose

Collect and document that technical side of the issues – at a high level

Get all concerned on the same line


Intro The ITU-R in 2003 defined sharing criteria for WLANs operating

in the 5GHz range (5.25 – 5.725 GHz)These criteria include detection requirements:

1 usec pulse length

>> 1 pulses per sweep (depending on radar sweep rate)

Canada has always insisted on special protection for its weather radars (5.6 – 5.65 GHz)

They are economically important

They are hard to detect due to their pulse type and scan patterns

Other countries, notably Australia and Japan, are adopting Canada’s stance

This could lead to blocking of the 5.6 GHz subband by the ITU-R

The loss of capacity is significant

Airborne Wi-fi is not considered here due to its very different nature


The technical problem at first sight

Weather radars use short pulses (necessary to get resolution): typically .5 usec

Detection requires high sensitivity which potentially leads to many false positive detections

Detection during PACKET RECEIVE is also a problem

Weather radars use complex and sometimes fast scan patterns while analyzing cloud systems

Means we see few pulses per burst and therefore we cannot separate false positive detections from real detections

This leads to a high false alarm rate and therefore significant service disruption due to the 30 minute re-entry delay

Means long intervals in which we see nothing: as much as 10 minutes

Radar operators worry that we will be transmitting in their band while they are “looking the other way” and cause interference when they do look our way


Looking more closely

The service interruption issue arises from the requirement to do in-service monitoring of the channel:

Traffic interferes with detection and therefore we may not see a short burst of pulses

New trends in weather radar design point towards less but longer pulses (with pulse compression)

The industry does not want to vacate this band for no good reason

A (false) detection means vacating the channel for at least 30 minutes

Service interruption is an issue but only close to radar sites and on- channel

Locally determined channel selection in the “footprint” of the radar can avoid interference without closing the band nation/world-wide


Short pulse detection Off-line detection of weather radars should not be an issue

The RLAN receiver has nothing to do but listen – see also below.

The channels that need extra care are known/can be known locally

At the edge of the radar (horizon) footprint, the radar signal is still very strong:

Approx -20 dBm for a 20KW radar with a 40 dBi antenna

This is easily detected - even if the pulses are shorter than 1 usec but not while the channel is being used for WLAN traffic

We need many pulses to assure no false detection

Weather radars have variable scan patterns – in sweep rate and elevation.

We will only see them at low to medium scan rates that deliver enough pulses to compensate for the shorter pulse widths


Weather radar activity and footprint

These radars operate 24/365 – if you see them once you will see them always

Allows RLANs to employ off-line detection to establish being in view of a weather radar or not

If a radar is seen, it will always be seen

If no radar is seen in the weather radar band, it will never be seen

Radar footprints are limited geographically

Inside that footprint, the above applies

Outside that footprint, the radar is not seen or affected

This example was provided by Environment Canada


Analysis1. Some radars / radar scan types cannot be detected while the

RLAN is using the channel (ISM)Because of short pulses or short burst lengthThere is no issue during Channel Availability Checking (CAC)

2. All of these radars are fixed and in more or less continuous service

3. All of these radars are stationary and have fixed footprintShort pulse detection is required only within the foor print

4. A 10 minute CAC + channel blocking would meet the needs of the (fixed) radar community and the RLAN community

Once per 24 hours would be enoughMark the channel as available or not available

Requires that operators maintain consistent operational schedules

5. ISM assures protection of mobile radars that appear out of the “blue”

Due to ISM, no RLAN will interfere with a weather radar for more than 5 minutes –at any time of day: one sweep at the right scan rate will silence all RLANs within range


Conclusion

The “weather radar issue” stems mostly from imprecise regulatory language

Does not distinguish between CAC and ISM as means to achieve the applicable protection requirements

The “issue” can be removed by:Restricting the requirement to detect sub micro second pulses to CAC in 5600-5650 MHz

Allowing a 24 hr “validity period” for a CAC for fixed (weather) radars

Detection means the channel must be blocked for 24 hrs

No detection means use of the channel is allowed – with ISM and with normal 60 second re-entry CAC

Regulatory Impact:Rule change as per above (partially already implemented

in EU)


Another perspective Knowledge = power. By ignoring the cognitive side of the story we

limit our capability to solve these problemsKnowledge of channels, location, operational patterns, etc

The FCC is committed to Cognitive Radio techniques to facilitate spectrum sharing

802.11 TGY is “riding” that commitment – it develops means to share presence information between spectrum users

We should leverage this FCC policy and propose off-air means to facilitate spectrum sharing with, in this case, weather radars

Cognitive spectrum sharing can use geographical data to allow systems outside a given radar’s horizon to rely exclusively on the current DFS profile

Government, the radar operators or a third party can maintain the necessary data base on the web

System installers would have to check that data base – in fact it can be automated


Summary The “weather radar issue” can be solved by resorting to

off-line detection and a validity period for fixed radarsAssures that the radar channel is not used - but only within the radar horizon

Removes the motivation to block the 5600-5650 MHz band (Australia)

Data base access to check proximity of weather radars would add resilience

Regulators and radar operators have to be engagedFirst talk with key regulators, then involve WFA formally.

Eventually we want the regulatory language to be changed to broaden the means of radar detection and avoidance


Questions/Comments?

RLANs and weather radars in the 5 GHz band rev 3

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