Can one communicate with smartphones only in a directional cone?
Directional antennae are used to obtain directionality as well as increase signal strength. Just like a flashlight concentrates the beam from a light bulb into a smaller area, a directional antenna increases signal strength as it narrows the coverage area.   

Bluetooth should be ideal because smartphones already have it, the frequency is high enough to make a compact directional antenna, and range is short so it should be less affected by signals from outside the 'cone'. To minimize stray signal pickup you should use the lowest transmitting power that can still be received inside the distance you want to cover. 

Your antenna doesn't need to be very efficient, so its dimensions are not particularly critical. You can make one out of an old tin can, a metal box open at one end, or a curved plate acting as a reflector.

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