The technologies used and underlying principles have changed enormously from 2G/GSM to 3G/WCDMA to 4G/LTE/OFDMA but a lot of it comes down to questions of how m

The technologies used and underlying principles have changed enormously from 2G/GSM to 3G/WCDMA to 4G/LTE/OFDMA but a lot of it comes down to questions of how many individual connections each tower can handle and the minimum amount of airtime communicating with each device requires. At one point (still?) there are also questions of how strongly each device and tower must be broadcasting to keep the received signal at each end within acceptable levels. Edit: Expanding on this a little while I still have time to edit, and all of this is from a non-telecom person so it's likely I have errors in here. Still, this should provide a starting point for the history and what to look for to get more current/accurate info. Most 2G was GSM which used TDMA (Time Division) - basically, each device connecting to the tower was assigned a "slice" of the available time and had to broadcast within that time slice. When the tower ran out of slices, some devices simply couldn't connect to it. Sprint and Verizon were the exceptions, they used CDMA which allowed many devices to broadcast at the same time and was able to decode data for/from each device based on the Code Division used to encode it. CDMA towers "breathed" which meant that their coverage area could vary based on how many devices were connected because the devices and towers had to keep broadcasts within particular power levels at the endpoints. CDMA allowed more devices, TDMA could allow larger coverage from each tower by reducing the number of time slices (which also reduced the number of devices that could connect). Notably, CDMA allowed devices to talk to multiple towers at once with reconciliation being handled by a smarter backend, while TDMA required more explicit handoffs between towers. Way back when this was why on GSM your call would simply drop, but on CDMA you'd simply develop a severe case of robot voice. 3G almost all moved to WCDMA, which I believe had many of the same advantages in density as CDMA. 4G moved to somethind different (OFDMA?), which I haven't read enough about to really understand but which is technically quite different from WCDMA. Most LTE uses yet another protocol. On T-Mobile, this was noticeable when T-Mo "refarmed" its network, converting a bunch of frequency bands from 4G to the newer higher-capacity but incompatible LTE. I know a few people who had 4G phones on T-Mobile and were offered free replacement phones because T-Mobile had basically yanked the actual 4G network out from under them. Basically, there's a lot out there, it has changed a LOT over the last 10-15 years, and many of the changes were actually complete replacements rather than incremental upgrades.