In recent years, systems have been used in which communications are established with radio communication devices, such as radio frequency IDentification (RFID) tags, non-contact integrated circuit (IC) cards, and portable terminal devices (mobile phones or personal digital assistants (PDA)) via a reader/writer and an antenna at, for example, an access point. For example, a goods identification system and a management system that use RFID tags are commonly used in various fields. RFID is a technology in which a tag and a reader/writer communicate with each other using radio communications using radio waves, i.e., electromagnetic waves, to identify and manage persons or goods.
For communications with an RFID tag, a non-directional antenna having a wide radio wave radiation angle, for example, is generally used. FIG. 12 is an explanatory view of a radio wave control system using a non-directional antenna. In the radio wave control system represented in FIG. 12, a no-communication area in which the non-directional antenna cannot communicate with the RFID tag (for example, a “null point” is included) may be caused in the communication area due to something called multipath interference, in which a direct wave BW from the non-directional antenna and a reflected wave aW, which is reflected on the floor surface, interfere with each other.
To reduce the effect of multipath interference, radio wave control systems using a variable beam antenna that has high directionality are used. FIG. 13 is an explanatory view of a radio wave control system using a variable beam antenna. In the radio wave control system represented in FIG. 13, a communication area in one radiation direction is narrow in which the variable beam antenna radiates beams. Therefore, selecting a radiation direction in consideration of wave reflections from the floor surface and the ceiling leads to a tendency that no no-communication area is caused in the communication area and the effect of multipath interference is prevented.
Technologies have been also disclosed in which such a variable beam antenna radiates radio waves such that the radio waves intersect with the floor surface, which is the reflection surface from which the strongest reflected waves are generated, thereby to reduce the occurrence of no-communication areas (see, for example, Japanese Laid-open Patent Publication No. 2006-20083).
However, a beam antenna having high directionality cannot efficiently communicate with an RFID tag even under the circumstances where no no-communication areas are caused in the area in which the beam antenna communicates with the RFID tag.
Specifically, the beam antenna having high directionality sequentially changes at different timing the direction in which the beam antenna radiates radio waves. Thus, in order to radiate radio waves over the entire area in which the beam antenna communicates with the RFID tag, the time that meets the number of radiation directions is required. When an RFID tag is moving in the communication area, data is sometimes inadequately read and thus efficient communications with the RFID tag cannot be established.
When the beam antenna having high directionality communicates with the RFID tag, radio waves may be radiated at maximum power all the time in order to extend the communication area. However, it is preferable that the power with which radio waves are radiated be as small as possible in consideration of the effect of interference on other neighboring RF ID systems. This problem similarly arises in systems, other than the RFID system, in which communications with radio communication devices are established.