Factors to Consider for Intersystem EMC ITU-R Radar Seminar Geneva 24 September 2005 Frank Sanders

1. Factors to Consider for Intersystem EMC ITU-R Radar Seminar Geneva 24 September 2005 Frank Sanders Chief, ITS Telecommunications Theory Division, U.S. Department of Commerce, NTIA 325 Broadway, Boulder, CO fsanders@its.bldrdoc.gov 303.497.7600 Institute for Telecommunication Sciences – Boulder, Colorado

2. Introduction -Proposals have been made to operate communication systems in radar bands. -These proposals represent a change from the historical approach of separate band allocations due to perceived electromagnetic compatibility issues between radar and communication services. -This talk addresses the technical challenges analyzing intersystem EMC. Institute for Telecommunication Sciences – Boulder, Colorado

3. Some justifications offeredfor sharing 1) Radar systems make little use of existing spectrum allocations. 2) Radar receivers are inherently robust against interference. 3) Perhaps interference to radars might be accommodated on a statistical basis. 4) Spectrum might be shared by communication devices that sense radar signals and avoid local radar frequencies. Institute for Telecommunication Sciences – Boulder, Colorado

4. Evaluation of these ideas… …may create opportunities for new patterns of spectrum use. But careful technical consideration of EMC is needed to test these concepts. We will consider the four ideas from the previous slide in this presentation… Institute for Telecommunication Sciences – Boulder, Colorado

5. Measured spectrum utilization of radars Radars usually consist of high power transmitters operating in concert with very sensitive receivers. EIRP transmit levels are often more than 1 GW, while receiver noise figures are often only a few decibels above the theoretical thermal noise limit. Representative radar emission parameters as a function of mission. Institute for Telecommunication Sciences – Boulder, Colorado

6. Measured spectrum utilization of radars, continued -Some spectrum surveys have not observed radar emissions. Is this evidence that radars don’t utilize spectrum allocations very much? -But published NTIA spectrum surveys show significant spectrum utilization by radars on an ongoing basis at every location in the US where such measurements have been done (for example, in Denver, San Diego, Los Angeles, and San Francisco). Why might this be? Institute for Telecommunication Sciences – Boulder, Colorado

7. Measured spectrum utilization of radars, continued San Diego, California measured radar utilization of L-band (2 weeks) Source: NTIA spectrum survey report Maximum Mean Minimum Institute for Telecommunication Sciences – Boulder, Colorado

8. Measured spectrum utilization of radars, continued San Diego, California measured radar utilization of C band (2 weeks) Source: NTIA spectrum survey report Maximum Mean Minimum Institute for Telecommunication Sciences – Boulder, Colorado

9. Measured spectrum utilization of radars, continued Why don’t some spectrum surveys show radar signals whereas the NTIA results do show substantial usage? Radar emissions don’t show up with conventional survey techniques. Instead, radars need to be observed with specialized, computer-controlled techniques. NTIA publications (e.g., a recent NTIA Report on RSEC measurement procedures) explain how. See also ITU-R New Recommendation M.1177. Specialized RF front ends are also needed… Institute for Telecommunication Sciences – Boulder, Colorado

10. M.1177 radar meas. front-end

11. Radar receiver performance in the presence of interference In some EMC studies that have been performed by NTIA, radars have been intentionally subjected to low levels of interference from various waveforms. Protocols: Radar receiver is disconnected from its antenna. In place of live targets, test targets are injected into the radar receiver. Target power level is adjusted so that the probability of detection of these targets is as close as possible to 90% (Pd=0.9). Interference is injected with a selected modulation. Target losses are counted as a function of interference power level relative to radar inherent receiver noise level (I/N). Institute for Telecommunication Sciences – Boulder, Colorado

12. Radar receiver performance in the presence of interference Example radar plan position indicator (ppi) display during interference testing. The bright wedge is a time-lapse effect of the radar ppi scan line. Institute for Telecommunication Sciences – Boulder, Colorado

13. Radar receiver performance in the presence of interference Equipment room of a radar station Examination of a radar receiver circuit card during interference testing Institute for Telecommunication Sciences – Boulder, Colorado

14. Radar receiver EMC performance in the presence of interference Example data showing target losses as a function of I/N ratio for a search radar.

15. Radar receiver EMC performance in the presence of interference Example data showing target losses as a function of I/N ratio for a maritime search radar. Pulsed signals representative of other Radar emissions. Communication-type signals

16. Radar receiver EMC performance in the presence of interference Typical I/N ratios at which measurable target losses begin to occur. Based on results of measurements to date and grouped by radar mission. Institute for Telecommunication Sciences – Boulder, Colorado

17. Radar receiver EMC performance in the presence of interference Do target losses only occur at the edge of radar coverage? Interference causes loss of targets within some number of decibels (call it X dB) of radar inherent noise... …therefore target losses don’t depend upon range, per se. To the extent that distant targets are weaker than closer targets, losses would correspond somewhat to range. But target cross sections vary… …so some targets are weak and close. Since X dB is X dB, target losses don’t strictly translate into range reduction. Weak targets are not necessarily unimportant targets. Institute for Telecommunication Sciences – Boulder, Colorado

18. Radar receiver EMC performance in the presence of interference Target losses at low I/N levels of interference are insidious. Low-level interference doesn’t make any weird features appear. Targets simply fade away, so there is no way to tell that targets have been lost. Low-level interference has caused all targets to disappear. But there is no overt indication of Interference. 10 targets with no interference present Institute for Telecommunication Sciences – Boulder, Colorado

19. Radar receiver performance in the presence of interference Example of ppi screen display effects of high-level interference (such as dramatized in movie portrayals). Institute for Telecommunication Sciences – Boulder, Colorado

20. Technical consideration of a statistical basis for radar interference The statistical approach raises several questions: 1) Which targets is it acceptable to lose? 2) How can communication systems coordinate activities with radars to assess target losses? 3) How can target losses be correlated with communication system interference? Institute for Telecommunication Sciences – Boulder, Colorado

21. Mitigation of interference by avoiding radar frequencies -In ‘dynamic frequency selection’ (DFS), communication units sense radar signals on the communication systems’ frequency, and then move operations to another frequency. -DFS depends upon communication systems to sense radar signals and then avoid the conflicted frequencies. -No theoretical reason has been shown why this approach cannot work; it is possible in principle. But it needs to be tested. -In the USA, NTIA, other Government agencies, and industry are currently working together to test this technology in the 5 GHz part of the spectrum. Institute for Telecommunication Sciences – Boulder, Colorado

22. Summary -Radar activity fills allocated spectrum bands at surveyed locations in the US. But proper monitoring techniques are necessary for successful surveys. -In tests performed to date, radars typically begin losing targets when interference is at about -10 dB I/N. (That is, at 10 dB average power below radar receiver inherent noise.) -Effects of low-level interference are insidious; targets fade away without overt indications of interference. -Proposals to allow interference on a statistical basis are in early stages; the technical feasibility of this approach is being studied. -An alternative approach, dynamic frequency selection, may be feasible. It typically requires substantial amounts of listening time. The concept is undergoing tests at this time. Institute for Telecommunication Sciences – Boulder, Colorado