This specification describes a method comprising responding to a detection of a variation in at least one measured characteristic of a reference signal transmitted from an access point to a portable device by triggering participation by the portable device in vehicle-to-portable device communications. In some examples, the triggering of the participation in the vehicle-to-portable device communications in response to the detection of the variation in the at least one measured characteristic of the reference signal may be performed only if it has been determined that a location of the portable device satisfies a predetermined condition.