Wilson has solutions along with $$$. I have a older 3G unit with outside antenna on rear roof and inside panel amplifier that points forward and gives cell access inside and outside perhaps 50'. The stated separation between the two antennas is 20' with the amplifier in between. In my initial testing 15' worked fine. My outside antenna is omni directional and higher gain directional antennas are available.
Mine is not a mobile setup meaning that when I'm close to a cell tower with the MH it will overload, enter a error state and requires manual intervention. But it has served me well for 12+ years.
