There are some very important distinctions between analog & digital trunked radio systems, all of which use FDD (frequency division duplexing) and LTE networks, which use FDD or TDD (time division duplexing).
For TDD LTE systems, surprising cases of long distance interference are possible due to super-refraction caused by tropospheric ducting. Other sources of anomalous propagation are important and exhibit frequency dependence as well. The shortest path may not be the path traveled–implications for optimistic terrain-shielded frequency reuse. Dual-frequency 240MHz + 2GHz 50km & 140km oversea paths sometimes 240 MHz does better and sometimes 2.015 GHz propagates better! 240 km LTE interference between Korea and Japan 50 Watt base stations contend with 0.2 Watt devices. Base station to base station power was nearly -90 dBm over 240 km!
FM and TV broadcasts have long been known to be subject to tropo ducting. A main motivation to move FM from low-band VHF (42-50 MHz) to 88-108 MHz was to get away from sporadic-E ionospheric skip, which could lead to far more interference from hundreds to thousands of kilometers away along with tropo ducting. Of course, other transmission modes like aircraft AM are also subject to tropo ducting. The rise of FM translators has made the general public more aware of tropo ducting, particularly as over the air TV audiences have dwindled, and HDTV limits visibility of interference.
VHF tropo ducting has been known for decades from Hawaii to California–a 4000 km path. However the antennas required are large and the signals are weak. In summers repeaters 150 km away can be heard almost every day at certain times of day. On some days the 150 km distant repeaters were so strong you could hear them on an indoor desktop scanner with internal antenna! Whether trunked or conventional, the impact of ducting on your VHF system depends on the signal ratio desired/undesired, which should be > 21 dB in a 12.5 kHz analog system. If two stations wishing communications are more than 50 km (base-base) you may have some issues with ducting interference. This gets trickier in places like New England with a long highly-populated irregularly shaped coast. The interference on the repeater input can be the issue, especially with high-power base stations accessing repeaters. These distant base stations can “jam” portable units into your repeater. Possible resolve: check other user license. Are they keeping to legal EIRP/antenna height for their control stations? DMR: MotoTRBO will change to a new rest channel if interference is detected on the current rest channel.
Tropo ducting of UHF (450MHz) including GMRS is more rare than VHF, but can equally interfere with commercial users when it happens. We have experienced 300 km distant repeaters on an omni base antenna. Tropo ducting was a plague when UHF trunking came into vogue on 12.5 kHz “split” channels because for years before, low power mobile operation had been permitted on the repeater input (455-460,465-470) MHz range. Of course, pretty soon people put base stations on those frequencies and we had to threaten FCC action to get them to stop. Repeater panels would mark those channels as “last-used”. Be careful when planning a new trunked system so that the “home” or “collect” frequencies are not plagued by these legacy operators. LTR trunking on any band has only a single bit “area code” to distinguish co-channel systems – and of course the repeater number and group code. Perhaps once every year or two where due to tropo ducting a customer would hear someone else on their area code, repeater and talkgroup on LTR. LTR systems are being rapidly replaced with DMR systems, so these problems hopefully won’t be relevant too much longer. Passport system color code has 2 bits (?) and with other unique ID measures gave much better resiliency against directly hearing “skip”. But as an analog system 21 dB D/U was required for good audio quality in the presence of tropo ducting. These systems are disappearing so I won’t go into further detail. Ditto for LTR-Net and MultiNet II.
800 MHz two-way radio especially suffered from tropo-ducting for co-channel Nextel and two-way radio systems. A customer was 40 km from the repeater with an omni base antenna would experience several hours per year when they could not access the SMR LTR system on 800 MHz due to tropo skip. A MultiNet II 800 MHz trunking repeater system would sometimes hear tropo skip on the input frequencies. That system had other coverage problems such that the skip may have just gotten indistinguishable from the other problems.
For Motorola analog Privacy Plus trunking and AMPS cellular phones, the very limited number of connect / SAT tones meant that after a call was established, an undesired signal could bleed through on Privacy Plus trunking systems or AMPS analog cellular. Privacy Plus control channels rotate every day (and could be manually encouraged to do so earlier).
900 MHz two-way radio systems are still operating (including 33cm ham radio) despite the relatively frozen license structure. The nature of 900 MHz systems made them less prone to interference from tropo skip since the receive sensitivity (due to misconfiguration) was often not great already, and the systems tended to have users close to the repeater. If the systems had been constructed correctly, 900 MHz would have been slightly more prone to tropo skip due to 12.5 kHz bandwidth (21dB D/U instead of 17dB D/U required for good audio 12.5 kHz vs. 25 kHz). The coverage is not as good on 900 MHz as 800 MHz because 12.5 kHz analog FM suffers a 4 dB impairment relative to 25 kHz bandwidth analog FM. The radio technology available at the time of early 900 MHz systems was a bit rudimentary by even year 2000 standards. The mobiles had trouble holding frequency accuracy, microphonics were more of an issue, and every radio model had different compandoring. System implementers were often as not careless with tower-top amplifier and distribution amplifier configuration, leading to IMD/spectral regrowth problems (harming repeater receive sensitivity). These issues existed at 800 MHz but given adjacent spectrum uses became aggravated at 900 MHz. Anterix got FCC Report and Order 17-200 authorizing broadband LTE on 900 MHz, which adds a more dynamic RF environment.