0 Slide 0 Interference Temperature: Objections from the Real World National Spectrum Managers Association May 18, 2004 Mitchell Lazarus | |

1 Slide 1 Interference Temperature Concept (1)  Mechanism to provide unlicensed “underlay” in licensed spectrum:  estimate ambient noise at licensed receivers  determine maximum noise licensed receivers can tolerate  let unlicensed devices exploit any headroom in between  Goal: extract maximum use from available spectrum.

2 Slide 2 Interference Temperature Concept (2) Distance from licensed transmitting antenna Licensed Signal New Opportunities for Spectrum Access Original Noise Floor Power at Receiver Figure 1 Interference Temperature Limit Diagram from Establishment of an Interference Temperature Metric, 18 FCC Rcd (2003).

3 Slide 3 Interference Temperature Concept (3)  “Interference temperature” (IT) is power spectral density expressed in temperature units: IT (deg. K) = P (W) / bandwidth (Hz) / k k = 1.38 x watt-sec/K (Boltzman’s constant).

4 Slide 4 Consequence of Temperature Model  Conversion from power spectral density to temperature units:  mathematically -- just division by a constant  but... conversion to temperature implicitly changes interference model from vector fields to a scalar field loses information on directions of origin.

5 Slide 5 FCC View of Interference Temperature  FCC regards IT as:  scalar or isotropic  changing gradually with location  independent of azimuth... like a fog resting over the landscape.  Model is misleading because:  most receivers are directional (some more than others)  most receivers are sensitive to direction of interfering sources  IT model ignores this information.

6 Slide 6 Interference Temperature Monitoring  IT monitoring is basic to preventing interference  FCC-proposed monitoring sites: at licensed receiver (best; hardest to do) grid of monitoring stations (expensive) at unlicensed transmitter (least effective but easiest)  Unlicensed transmitters receive data on:  interference temperature  maximum permissible noise in licensed band  Unlicensed transmitters dynamically control power, frequency, antenna pattern  goal: stay above noise floor, below receiver limit.

7 Slide 7 Requirements to Prevent Interference  System must measure IT:  at licensed receiving antenna  in direction of antenna sensitivity  continuously, relative to desired signal (which may be weak and may fade abruptly)  take terrain into account  System must communicate available margin at each instant to unlicensed transmitters in the field  Unlicensed transmitters must shut down instantly if desired signal fades.

8 Slide 8 Conclusions 1.Monitoring IT without reference to direction of interference will not protect licensed receivers (especially if highly directional). 2.The FCC has received no practical proposals for estimating IT at victim receivers and communicating the available margin to unlicensed transmitters. 3.Use of IT to prevent interference to licensed receivers is not practical at this time.

9 Slide 9 Thank you! Mitchell Lazarus | |