In the absence of any synchronization, users can still perform ‘sense and avoid’. In this scheme, before a device begins transmission, it senses the spectrum. This is achieved by keeping the receivers active and the transmitter switched off.

If there have been no transmissions by other radars, the spectrum is silent. The ADC data should only show the thermal noise floor and the noise figure. If, on the other hand, there are transmissions from another radar device, expect spikes in the ADC data corresponding to the points where the crossing occurs.

XWR devices can generate fast chirps of the order of 250 megahertz per microsecond, allowing for fast scans.

A max hold of ADC data cross chirps shows the interferer clearly. In Figure 3-3, the interferer is chirping between frequencies f₁ and f₂. Thus, the ADC output shows the energies between f₁ and f₂. If the period of scanning is long enough to cover multiple frames, estimate the number of aggressor radars using the number of discrete bands of frequencies used. The user can also estimate frame periodicity bandwidth occupied by chirps. Most importantly, the user can find free spectra, or time slots, where interference-free transmission is possible.

Figure 3-3 Sense and Avoid

When the interferers have been identified, the radar can start transmission in regions where the interferer is not active.