Sorry, I have been imprecise and my knowledge somewhat outdated. The basic issue is above TDMA/OFDMA; unlicensed bands require that you have some mechanism to d

Sorry, I have been imprecise and my knowledge somewhat outdated. The basic issue is above TDMA/OFDMA; unlicensed bands require that you have some mechanism to detect if the channel is in use before you begin transmission, to ensure you do not hog the entire spectrum and can coexist with incompatible/foreign technologies. This is the best summary I could find: „Supporting LTE over unlicensed bands is not trivial. The key challenge is how to achieve har- monious coexistence between LTE and other sys- tems that are already operating in these bands. Conventional LTE cannot operate in unlicensed spectrum as it has no concern for cross-technol- ogy coexistence. For example, transmissions in an LTE radio access network (RAN) are continuous in time, and subject to centralized scheduling at the eNodeB (eNB). Even in the absence of data traffic, control and reference signals are transmit- ted over the air (at the OFDM symbol level) and are ubiquitous over time and its channel band- width.“ From Huang, Yan, et al. "Recent advances of LTE/WiFi coexistence in unlicensed spectrum.", use https://doi.org/10.1109/MNET.2017.1700124 https://doi.org/10.1109/MNET.2017.1700124 in sci-hub.