What are Coexistence Filters?

With the introduction of new technologies and standards every year, the global RF spectrum is getting more and more congested. This has resulted in a significant decrease in the bandwidth or elimination of the frequency guard bands which are allocated between two adjacent technologies to avoid interference.

As a result of this, the filter requirements have changed and now require a very steep transition between the passband and the stopband. This is required to avoid interference due to co-existence of different technologies in adjacent frequency bands.

Filters that are designed to have very sharp transitions between its passband and stopband are called Coexistence Filters. 

For example, as seen in the figure above, bands 40 (2300 to 2400 MHz) and 41 are used for providing TDD-LTE service in China and Wi-Fi operates from 2400 to 2482 MHz. The upper edge of band 40 (2400 MHz) has a common boundary with the lower edge of Wi-Fi and thus there is no transition band. In this case, what we need are RF filters that provide a steep transition from the passband and stopband, thereby rejecting adjacent frequencies. This is when we need coexistence filters.

Using traditional filters in these cases will result in considerable interference and thus we would need coexistence filters.