1. Field of the Invention
The present invention relates to a communication system and a communication apparatus allowing information apparatuses to perform large-volume data communication. Particularly, the present invention relates to a communication system and a communication apparatus allowing information apparatuses to perform data communication by using an electrostatic field or an inductive electrical field without causing interference with another communication system. Also, the present invention relates to a communication system and a communication apparatus allowing information apparatuses to perform data communication by using an inductive magnetic field without causing interference with another communication system.
More specifically, the present invention relates to a communication system and a communication apparatus allowing information apparatuses placed in a short range to transmit radio frequency (RF) signals by using an electrostatic field or an inductive electrical field. Also, the present invention relates to a communication system and a communication apparatus allowing information apparatuses placed in a short range to transmit RF signals by using an inductive magnetic field. Particularly, the present invention relates to a communication system and a communication apparatus allowing couplers mounted on respective information apparatuses to efficiently transmit RF signals so as to enable large-volume transmission in a short range using electrical field coupling or magnetic field coupling.
2. Description of the Related Art
Recently, a radio interface has been used more instead of a multi-purpose cable such as an AV (audio visual) cable or a USB (universal serial bus) cable and a medium such as a memory card in order to transfer data from a compact information apparatus to another, for example, to exchange image data or music data between personal computers. Using the radio interface eliminates the need for connecting a cable to a connector at every data transmission, which is convenient for a user. Also, many information apparatuses provided with various cableless communication functions have emerged. As a method for performing cableless data transmission between compact apparatuses, a radio wave communication method for transmitting/receiving radio signals by using an antenna, such as communication using a radio LAN (local area network) represented by IEEE802.11 or Bluetooth®, has been developed.
A communication method called “ultrawideband (UWB)”, which has been receiving attention in recent years, is a radio communication technique that uses a very wide frequency band of 3.1 GHz to 10.6 GHz and that realizes large-volume data radio transmission of about 100 Mbps in a short range. Thus, the UWB communication method is capable of transferring large-volume data, for example, moving pictures and music data of a CD (compact disc), at high speed and in short time.
The UWB communication, of which communication distance is about 10 m due to transmission power, is used for radio communication in a short range, such as a PAN (personal area network). For example, a method for transmitting data having a packet structure including a preamble has been devised as an access control method of UWB communication in IEEE2802.15.3 and the like. Also, Intel Corporation in the United States has been considering a wireless version of the USB, widespread as a multi-purpose interface for a personal computer, as an application of the UWB.
Also, a transmission system using a UWB low-band of 3.1 to 4.9 GHz has been actively developed under consideration that the UWB communication enables data transmission of over 100 Mbps without occupying a transmission band of 3.1 GHz to 10.6 GHz and that an RF circuit can be easily made.
Under the Radio Law in Japan, weak radio waves having an electrical field intensity (radio wave intensity) of a predetermined level or lower at a distance of three meters from radio facilities, that is, weak radio waves of a noise level for a neighboring radio system, do not require a license of a radio station, so that development and manufacturing costs of radio systems can be reduced. By applying the above-described UWB communication, a short-range radio communication system can be constituted at a relatively low electrical field level based on its transmission power. However, if a UWB communication system is constituted by using a radio wave communication method of transmitting/receiving radio signals by using an antenna, it is difficult to suppress a generated electrical field to a weak level.
Many of radio communication systems according to related arts adopt the radio wave communication method, in which signals are propagated by using a radiated electrical field that is generated when a current is flown to an aerial (antenna). In this case, a transmitter side emits radio waves regardless of presence/absence of a receiver side, and thus may become a source of disturbing radio waves to a neighboring communication system disadvantageously. Also, an antenna on a receiver side receives not only desired waves from a transmitter but also radio waves coming from a remote site, and is thus subject to external disturbing radio waves, which results in a decrease in reception sensitivity. If there are a plurality of other ends of communication, complicated setting is performed to select desired one from among the other ends. For example, when a plurality of pairs of radio apparatuses perform radio communication in a narrow range, division multiplexing including selection of frequencies is performed to avoid mutual interference. Furthermore, polarized waves orthogonal to each other prevent communication from being performed, and thus the directions of polarized waves of antennas should be matched with each other between a transmitter and a receiver.
For example, in a noncontact data communication system in an extremely short range of several millimeters to several centimeters, it is preferred that a transmitter and a receiver strongly couple to each other in a short range while that signals do not reach a remote site so that interference with another system can be avoided. Also, it is desired that apparatuses performing data communication couple with each other while being independent from each other's attitude (orientation), that is, without directivity, when the apparatuses are close to each other. Also, it is desired that wideband communication is possible when large-volume data communication is performed.
Other than the above-described radio wave communication using a radiated electrical field, communication methods using an electrostatic field or an inductive electrical field are used in radio communication. For example, in an existing noncontact communication system mainly used in RFID (radio frequency identification), electrical field coupling or electromagnetic induction is applied. An electrostatic field and an inductive electrical field are inversely proportional to the third power and the square of the distance from a source, respectively. Thus, the electrostatic field and the inductive electrical field can realize weak radio waves having an electrical field intensity (radio wave intensity) of a predetermined level or lower at a distance of three meters from radio facilities, and a license of a radio station is not required. In this type of noncontact communication system, transmitted signals steeply attenuate in accordance with a distance, and thus no coupling occurs when no other end for communication exists in the neighborhood. Accordingly, any communication system is not disturbed. Furthermore, even if radio waves come from a remote site, a coupler does not receive the radio waves, and thus interference from another communication system can be avoided. That is, ultrashort range noncontact communication through electrical field coupling using an inductive electrical field or an electrostatic field is suitable for realizing weak radio waves.
The ultrashort range communication system in a noncontact manner has some advantages compared to an ordinary radio communication system. For example, when a radio signal is transmitted/received between apparatuses that are relatively separated from each other, the quality of the signal in a radio zone degrades in accordance with the existence of a surrounding reflective object or extension of a communication distance. However, in the short range communication, there is no dependency on a surrounding environment and a high-quality signal of low error rate can be transmitted at a high transmission rate. Furthermore, in the ultrashort range communication system, an improper apparatus that intercepts transmitted data does not intervene and thus there is no need to consider prevention of hacking on a transmission path and securement of confidentiality.
In the radio wave communication, an antenna needs to have a length of about a half or a quarter of a used wavelength λ, and thus the size of apparatus becomes large inevitably. Such constraints do not exist in the ultrashort range communication system using an inductive electrical field or an electrostatic field.
For example, Patent Document 1 (Japanese Unexamined Patent Application Publication No. 2006-60283) suggests an RFID tag system. This RFID tag system is capable of stably reading and writing information even if RFID tags attached to a plurality of items are overlapped each other, by forming a set of communication auxiliaries between which the RFID tags are placed.
Patent Document 2 (Japanese Unexamined Patent Application Publication No. 2004-214879) suggests a data communication apparatus using an inductive magnetic field. This data communication apparatus includes a main body, an attaching unit used to attach the main body to a user's body, an antenna coil, and a data communication unit to perform data communication with an external communication apparatus in a noncontact manner via the antenna coil. The antenna coil and the data communication unit are placed in an outer case provided at an upper part of the main body.
Patent Document 3 (Japanese Unexamined Patent Application Publication No. 2005-18671) suggests a mobile phone apparatus with an RFID ensuring a communication distance without losing portability, as a configuration in which an antenna coil to perform data communication with an external apparatus is mounted on a memory card inserted into a mobile information apparatus and an RFID antenna coil is placed outside a memory card slot of the mobile information apparatus.
The RFID system according to the related art using an electrostatic field or an inductive electrical field uses low-frequency signals and thus communication speed thereof is low, which is not suitable for large-volume data transmission. In the communication method using an inductive magnetic field generated by an antenna coil, problems about mounting arise. For example, it may be impossible to perform communication when a metal plate exists on the back of the coil, and a large area is required on the plane for placing the coil. Furthermore, loss in a transmission path is high and signal transmission efficiency is poor.
Under these circumstances, the inventors of the present invention believe that high-speed data transmission ensuring confidentiality can be realized by a weak electrical field that do not require a license of a radio station by using an ultrashort range communication system to transmit RF signals through electrical field coupling, that is, to transmit UWB communication signals by using electrical field coupling in an electrostatic field or an inductive electrical field or using magnetic field coupling in an inductive magnetic field. Also, the inventors of the present invention believe that large-volume data, such as moving pictures or music data of a CD, can be transferred at high speed and in short time in the UWB communication system using an electrostatic field or an inductive electrical field.