Last week a friend and radio hobbyist obtained a report that a radio systems integrator prepared for a rural county on their options to for upgrading their public safety radio system. The report recommends a simulcast 800MHz system. They want to use 800MHz so they can use the P25 radio standard. The desired coverage area is big enough to benefit from multiple antenna sites. Since the county only has access to a few 800MHz frequencies, they need to simulcast on the same frequency from all antennas. 
Simulcast systems have precisely controlled timing so that signals arrive to the area in range of more than one antenna at the same time. Timing is not an issue near any one transmitter because its signal dominates. In the zone where the strength of signals from multiple transmitters are within 10dB of each other, the timing has to be exactly the same. Radio waves propagate at 5 microseconds per mile. So if the distance from one transmitter to the zone of equal strength is 2 miles different from the distance to the other, engineers need to add a 10us delay so the signals arrive at the zone at the same time.
Before the county adopted a P25 system, it used a VHF system. My friend tells me from his monitoring the system worked well. The VHF band is a hodgepodge of single-system input and output frequencies. The UHF band is more organized, with 5 MHz offsets between output and input frequencies, but it’s still mostly one system per frequency. The trunked systems, which share a handful of frequencies among many users, are mostly located in the 800MHz region. That’s a much more efficient use of spectrum, but paradoxically this rural county finds scarcity of frequencies to be a problem as it transitions to 800MHz. This is a failure of the system of licensing frequencies, not a real shortage of spectrum in rural America.
This same issue came up in a completely unrelated paper in the latest issue of IEEE Wireless Communications about future scarcity of spectrum space in the unused TV channel frequencies, TV white space (TVWS). These frequencies are being opened for unlicensed use. The media sometimes refer to it as “Wi-Fi on steroids” because the free-space loss is lower at these frequencies, which results in more range at given output power and receiver sensitivity. The researchers calculated the amount of interference expected if these frequencies are widely used in densely populated cities. They conclude that in some areas congestion will be a huge problem, and they recommend the use of a dynamic frequency selection (DFS) scheme. 
DFS is something that Wi-Fi users in the 5GHz band could benefit from right now. Some of the channels require users to monitor for radar and vacate the channel if radar is detected. There is no standard for a client who has to vacate the channel to ask an access point, which may not be in range of the radar, if it would be willing to change to a channel the client is able to use. As a result, 2.4GHz Wi-Fi is crowded and 5GHz is under-used.
As more devices get wireless capability, the need for systems to share the spectrum increases. It seems a shame that a rural county, where most of the RF spctrum is vacant, must deal with a lack of licensed frequency space. We are on a slow march for the hodgepodge of VHF systems, 800MHz trunked systems, to cognitive radios that share the spectrum intelligently and give priority to emergency communications. Much of that innovation comes from unlicensed bands. Municipalities, factories, and mining operations often use the unlicensed bands, including the Wi-Fi bands, over distances of several miles because it’s easier to share the band than to deal with getting a license. More spectrum will move toward an unlicensed model simply because it works. How much RF Leviathan is needed to make users play nice remains to be seen.
Images are from a Monitoring Times article on Simulcasting and the article on dynamic frequency selection in IEEE Wireless Communications.