One of the things that is just as important as capacity on the UMTS or LTE air interface is the ability to shuffle the data back and forth between a base station and the rest of the network. This link is called the "backhaul" and for data rates beyond just a couple of megabits, fiber connections or microwave Ethernet links are required. Recently, Ericsson has released an interesting paper that describes the state of the art and the future of microwave backhaul.
So here are some technical details I found quite interesting: Like everywhere in telecoms, it seems there is an ETSI version (European Technical Standards Institute) of the equipment used globally and an ANSI version (American National Standards Institute) for the Americas. The current state of backhauling uses up to 56 MHz wide channels, and 256 QAM modulation and the maximum transmission power is given as around 2 watts. Traditional (virtual) E1 connections can be mixed with Ethernet connectivity and the Ethernet line rate on such a link is 345 MBit/s, equaling 80 traditional E1 lines. That is good enough for a combined GSM, UMTS and LTE base station today with three sectors, as 100 MBit/s per sector is unlikely to be achieved simultaneously in all three sectors. For more technical details see their data sheet.
The smallest channel offered by Ericsson's current product is a 3.5 MHz Channel and QPK modulation, resulting in a line rate of 2 E1s, equaling 4.1 MBit/s. The spec sheet that can be found here also reveals that dish antennas can be used with a diameter from 0.2m and single polarization up to dishes with 3.7m diameter and dual polarized antennas inside.
Used frequencies in ETSI land are anywhere between 6 and 38 GHz today, so far above the current frequency bands used between the base station and actual user devices, which transmit and receive anywhere between 700 MHz and 2600 MHz today. As high frequencies and high modulation are sensitive to rain a bandwidth adaptation feature ensures that during bad weather the line rate is reduced to ensure the link stays up.
I also had a quick look a competing microwave solution from Dragonwave to see where they are at the moment. They claim that their current microwave compact product is capable of speeds in the order of 800 MBit/s. That's about twice that of the Ericsson product and is likely because they use XPIC (cross polarization interference cancelation), which allows two data streams to be sent over the same channel simultaneously. Some but unfortunately not very detailed information can be found here.
So what does the future hold? Ericsson says that the 80 GHz band looks promising for microwave in the future, 112 MHz channels should be introduced shortly and 4x4 MIMO via two separate antennas and XPIC in each antenna will push microwave backhaul datarates beyond 1 Gbit/s.