Combined GSM/UMTS/LTE base stations require a fat backhaul pipe today so traditional means of connecting them via one or more 2 Mbit/s E-1 links is no longer an option. A UMTS base station can see peaks of over 30 Mbit/s in a sector in the downlink direction and for LTE, the peak for a 20 MHz carrier is well over 80 Mbit/s. Combined and averaged over the three technologies and three sectors (GSM requires only little compared to those numbers) there are only two real options for connecting base stations in the age of high speed wireless Internet access: Fiber or microwave Ethernet.
Fiber sounds great but I guess it must be a challenge to connect base station sites via fiber in practice. But it seems this problem is being worked on, especially by those network operators also owning fixed line assets. A1, the former state carrier in Austria is now reported to have have connected over two thirds of their base station sites via fiber. Quite a significant number that should prevent backhaul bottlenecks well into the future.