Detection of Periodic Radars March 2002 May 2002 Detection of Periodic Radars Peter Larsson Ericsson Research peter.larsson@era.ericsson.se Peter Larsson, Ericsson Research

May 2002 Problem Detection of periodic radars by means of periodic quiet times is ”unsuitable”. Peter Larsson, Ericsson Research

Periodic Radars & Periodic Quiet Times May 2002 Periodic Radars & Periodic Quiet Times Example Radar pulse TQuiet_Duration Quiet time Quiet time Quiet time Quiet time Beacon Beacon Beacon Beacon Beacon Time TQuiet_Offset TQuiet_Offset TQuiet_Offset TQuiet_Offset Does this occur in reality? The draft states ”All STAs in an IBSS shall continue to schedule the quiet periods by including a Quiet element in any Beacons or Probe Responses, with the same Quiet Period, Quiet Duration and Quiet Offset, if any previous Beacons or Probe Responses in the IBSS contained a Quiet element” Peter Larsson, Ericsson Research

Periodic Radars & Periodic Quiet Times May 2002 Periodic Radars & Periodic Quiet Times Zero probability of detecting periodic radars when n/PRF=Quiet_period_field·Beacon_interval, where n is an integer. Long detection time of periodic radars, when Quiet times and Radar Pulses are (slowly) sliding relative to each other. It is a real problem in an IBSS. It is also a real problem in a BSS when fixed quiet parameters are deployed. Peter Larsson, Ericsson Research

The remedy - part 1 Randomization of the Quiet Offset! Radar pulse May 2002 The remedy - part 1 Randomization of the Quiet Offset! Beacon TQuiet_Offset Quiet time Radar pulse Time Peter Larsson, Ericsson Research

Randomization partly enabled by current BSS Solution May 2002 Randomization partly enabled by current BSS Solution The AP may change the value of the Quiet Offset field, provided scheduling of Quiet times is stopped first. STA may miss Beacon and then accidently TX during the Quiet time STA TXing Quiet time Quiet time Quiet time Quiet time Beacon Beacon Beacon Beacon Beacon Sleep time Time Quiet Offset1 Quiet Offset2 Quiet Offset3 Peter Larsson, Ericsson Research

Problems with current BSS solution May 2002 Problems with current BSS solution The ”existing” randomization operation: Only applies to BSS, but not to IBSS Does not provide desired robustness Is not ”sleep mode compatible” Peter Larsson, Ericsson Research

The remedy - part 2 Introduce appropriate redundancy. May 2002 Peter Larsson, Ericsson Research

Main Solution Multiple Quiet Offsets indicated in each Beacon Time May 2002 Main Solution Multiple Quiet Offsets indicated in each Beacon Quiet time Quiet time Quiet time Quiet time Quiet time Beacon Beacon Beacon Beacon Beacon Quiet Offsetn+1 Offsetn+2 Offsetn+N Quiet Offsetn+2 Offsetn+3 Offsetn+1+N Time TQuiet_Offset TQuiet_Offset TQuiet_Offset TQuiet_Offset TQuiet_Offset Peter Larsson, Ericsson Research

New Quiet Element Format May 2002 New Quiet Element Format Element ID (40) 1 Length (6) 1 Octets: Quiet Count 1 Quiet Period 1 Quiet Duration 2 Quiet Offset n Quiet Offset n+1 Quiet Offset n+2 2 2 2 Octets: Peter Larsson, Ericsson Research

Alternative Solution Quiet time Quiet time May 2002 Alternative Solution Indicate a state for a pseudo-random generator in the Beacon Derive the Quiet Offset from the state Update the state for every Beacon Delayed Beacon Quiet time Quiet time Beacon Beacon Time TQuiet_Offset TQuiet_Duration TBTT TQuiet_Offset TQuiet_Duration TBTT TBeacon_Interval Quiet Offset state N.B. Beacon interval used for illustration here. DCF Interval may be used instead Peter Larsson, Ericsson Research

New Quiet Element Format May 2002 New Quiet Element Format Element ID (40) 1 Length (6) 1 Octets: Quiet Count 1 Quiet Period 1 Quiet Duration 2 Quiet Offset state 2 Octets: Peter Larsson, Ericsson Research

May 2002 Conclusions Current Quiet time method unsuitable for detection of periodic radars Introduce quiet time randomization and appropriate redundancy Advantages Statistically guaranteed detection* Robustness Sleep mode compatibility Operation in both BSS and IBSS The Main Solution is more flexible than the alternative solution, but the latter is more compact and suits IBSS well *N.B. Not entirely true for the IBSS case using the main solution, but yet sufficiently good Peter Larsson, Ericsson Research