RLANs and weather radars in the 5 GHz band rev 3

1. RLANs and weather radars in the 5 GHz bandrev 3 Jan Kruys Dec 7, 2006

2. Purpose • Collect and document that technical side of the issues – at a high level • Get all concerned on the same line

3. 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

4. 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

5. 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

6. 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

7. 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

8. Analysis • Some radars / radar scan types cannot be detected while the RLAN is using the channel (ISM) Because of short pulses or short burst length There is no issue during Channel Availability Checking (CAC) • All of these radars are fixed and in more or less continuous service • All of these radars are stationary and have fixed footprint Short pulse detection is required only within the foor print • 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 enough Mark the channel as available or not available Requires that operators maintain consistent operational schedules • 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

9. 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)

10. Another perspective • Knowledge = power. By ignoring the cognitive side of the story we limit our capability to solve these problems • Knowledge 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

11. Summary • The “weather radar issue” can be solved by resorting to off-line detection and a validity period for fixed radars Assures 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 engaged First 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

12. Questions/Comments?