Actually it is pretty tough to improve the efficiency of the antenna. The improvement usually comes from trading isotropic radiator proporties for directionality, in effect concentrating the direction of transmission. Works well for space missions, and the Mars probes because they can maintain their spatial orientation. Generally doesn't work well for cell phones because people don't stand still, so their spatial orientation changes, as a result most cell phone antennae at the mobil end are close to isotopic radiators. The situation at the Base is a bit different, you have an antenna to cover each 120 degree area, and it is a safe bet that no one is going to be doing much transmitting either below the ground, or above the top floor of the highest building, so those antennae can be designed to provide substantial gain over an isotropic radiatior in part because there is more than one of them, and in part because they know where the customer has to be relative to receiver antenna. The phone is not so lucky.
bottom line is that 'passive' cell phone boosters are an excellent way of performing wallet surgery (surgircal removal of funds from the wallet of the buyer).