Brough Turner brings up an interesting point how most backhaul from mobile cell sites are only 1 or 2 E1 or T1's and that just doesn't do the trick for mobile broadband. Nyquist Capital made a quick remark that this would be booming times for mobile backhaul. My opinion is that in many countries where broadband in the rural areas hasn't arrived yet will also not benefit from mobile broadband as it takes broadband to the cell site just to be able to make mobile broadband.
Broadband btw is not defined as 256kbit or 2mbit/s but at least 10mbit to the cell site in order for it to maintain adequate capacity for modern usage. WiMax sounds interesting for backhaul until you realize that the advertised 40mbit is half duplex and only reached within short range of the antenna. So for Wimax or LTE to fly in Rural areas at advertised speeds you need a minimum of VDSL/SDSL to the cell site, but a strong preference for VDSL2 or Docsis 3.0 as a minimum. However VDSL2 requires being closer than a 500 metres from the cell site. At which point you could say... why bother and just dig all the way. Docsis is tricky for different reasons most of them being that Cableco's don't do SLA's as a general rule. And there we have it... mobile broadband over any technology requires a finely distributed fixed network. Gentleman start digging!