A method of waking up a small cell base station from a sleep mode, the method includes: by the mobile station: determining, based on stored location information indicating a location of the small cell base station and on a monitored current location of the mobile station, that the mobile station is in a coverage area of the small cell base station, determining, from serving cell information and a neighbor cell list, that the small cell base station is in the sleep mode, changing a transmission characteristic of the standard access signal so as to increase its transmission energy, in response to the determinations, and transmitting the changed access signal; and by the small cell base station: harvesting energy from the changed access signal that is transmitted by the mobile station, using a passive energy harvesting sensor, the harvested energy triggering a wake-up procedure in the small cell base station.