GPS signal is very week.. how to solve it? add gps antenna?

i think, this mi note lte have problem about GPS antenna, cause if use GPS on field without high building, the smartphone get only 3 GPS signal (american satelite). if use on car, the smartphone dont get any gps sattelite.

i try the gps software tools, downgrade / upgrade MIUI version, but still not work..

How to solve it, where is gps antena? i want to add gps external antena