4.5G

short for "fourth and a half generation", is the generation of mobile telecommunications technology between 4G and 5G (aka LTEe), succeeding 3G

4G

short for "fourth generation", is the fourth generation of mobile telecommunications technology (aka LTE), succeeding 3G

5G

short for "fifth generation", is the fifth generation of mobile telecommunications technology (aka "New Radio"), succeeding 4G

FDD

Frequency Division Duplex

FMC

FPGA Mezzanine Card (FMC) is an ANSI/VITA standard that defines I/O mezzanine modules with connection to an FPGA or other device with re-configurable I/O capability. It specifies a low profile connector and compact board size for compatibility with several industry standard slot card, blade, low profile motherboard, and mezzanine form factors. (Wikipedia, FPGA Mezzanine Card)

FRU

Field Replaceable Unit