1. Field of the Invention
The present invention relates to a radio communication system, a radio communication device and a radio communication method for mutual communication among a plurality of radio stations such as a wireless LAN (Local Area Network), and a computer program for implementing the method, and in particular to a radio communication system, a radio communication device and a radio communication method for a radio network in which terminals asynchronously and directly communicate with one another, and a computer program for implementing the method.
More particularly, the present invention relates to a radio communication system, a radio communication device, and a radio communication method in which a radio network is established by ad hoc (Ad-hoc) communication without particularly arranging a device as a control station, and a computer program for implementing the method, especially to a radio communication system, a radio communication device, and a radio communication method in which communication terminals performing random access may not interfere one another among networks under a communication environment where a plurality of ad hoc networks adjoin one another, so as to form a radio network, and a computer program for implementing the method.
2. Description of the Related Art
As a system which releases a user from LAN wiring by means of cables, the wireless LAN has attracted attention. According to the wireless LAN, most wiring cables may be omitted in working spaces such as an office, so that the communication terminals such as a personal computer (PC) can be moved comparatively easily.
In recent years, as wireless LAN systems have improved in speed and cost, their demands have remarkably increased. In particular, in order to establish a small-scale radio network among a plurality of electronic devices existing in the environments and to perform information communications, an introduction of a personal area network (PAN) has been considered. Different radio communication systems are defined by means of frequency bands which do not need any license from the controlling authorities, such as for example, 2.4 GHz band, and 5 GHz band.
In order to constitute a local area network using wireless technologies, it is usual to employ a method in which one device as a control station called an “access point” is provided in the area, so that the network is established under the general control of the control station.
In the radio network having an access point, when information is transmitted from a certain communication device, an access control method based on bandwidth reservation is widely employed in which a bandwidth required for the information transmission is first reserved to an access point so that a transmission channel may be used to avoid a collision with information transmission of another communication device. In other words, by arranging the access point, synchronized radio communications may be carried out in which the communication devices in the radio network synchronize with one another.
However, when asynchronous communication between the communication devices on a transmission side and a reception side are carried out in the radio communication system in which the access point is detected, the wireless communication through the access point is always required, so that there is a problem that performance of the transmission channel is considerably reduced.
In contrast, as another method of constituting the radio network, a so-called “ad hoc (Ad-hoc) communication”, in which the terminals communicate with one another directly and asynchronously by radio has been proposed. Amongst these, in a small-scale radio network constituted by a comparatively small number of clients located in proximity, it is considered that the ad hoc communication is appropriate in which arbitrary terminals can mutually perform the direct and asynchronous wireless communication without using a specific access point.
On the other hand, nowadays, a method called “ultra wideband (UWB) communication” which puts information on a very weak impulse train and performs wireless communication has received attention as a radio communication system which allows short-distance ultra high-speed transmission, and such method is expected to be realized (see “Ultra Wideband, Delivering Child of Revolution in Radio Communications,” Nikkei Electronics 11 Mar. 2002, pp. 55-66, for example).
There are two types of UWB transmission systems. One is a DS-UWB system in which a spread rate of an information signal of DS (direct sequence) is increased to the maximum. The other is an impulse-UWB system in which an information signal is constituted by using an impulse signal train of very short period, approximately a few hundred pico seconds so that the signal train may be transmitted and received. Both systems spread the signal train over an ultra wide frequency band of 3 GHz through 10 GHz, for example, so as to transmit and receive the signal trains and realize high-speed data transmission. The occupied bandwidth is on the order of GHz such that a quotient of the occupied bandwidth divided by its center frequency (for example, 1 GHz-10 GHz) is substantially 1 (one), and therefore the bandwidth is wider than those which are usually used in the so-called W-CDMA system, the so-called CDMA2000 system, as well as a wireless LAN using the SS (Spread Spectrum) system or the OFDM (Orthogonal Frequency Division Multiplexing) system.
For example, in the standardization process of IEEE 802.15.3, a method of communicating by forming a piconet among radio communication devices which perform ultra wideband wireless communication, for example, is being standardized.
Now, considering that information devices, such as a personal computer (PC), become popular and there is a communication environment where many devices are mixed in an office and the devices are connected to one another by means of the radio network, two or more radio networks may be crushed within a small operation space so that their communication service areas may overlap with one another.
Under such a wireless communication environment, one radio network uses a certain frequency channel and other radio networks use different frequency channels, so that they can coexist even if they overlap with one another in space.
However, in a wireless ad hoc network system, since a network is (i.e. not via the control station) established simply between a radio communication device and another radio communication device within its own service area (distance) in the same frequency channel, it is difficult for them to coexist by means of different frequency channels. For example, when expanding ad hoc radio networks within a predetermined space domain, a radio communication device installed in an area which overlaps an adjoining ad hoc radio network can communicate with a radio communication device included in another ad hoc network. Therefore, the radio communication device might repeat an unnecessary authentication process with a radio communication device outside its network.
The UWB transmission system (as mentioned above) expected as a radio communication system for realizing short-distance ultra high-speed transmission performs high-speed data transmission by spreading a transmission data in a very wide frequency band, so that it is quite likely to overlap an adjoining wireless communication network and it is difficult to realize the coexistence by means of the different frequency channels.
Alternatively, a plurality of radio networks may coexist by performing the time division multiplexing with respect to one frequency channel to be utilized, even if the networks spatially overlap one another.
For example, a communication system is envisaged in which a plurality of radio communication devices are grouped, a representative wireless communication terminal is provided for each group, the representative wireless communication terminal specifies time to be used in the group, and a beacon signal is transmitted so as to notify another terminal in the group and an adjoining representative wireless communication terminal in another group (see Japanese Laid Open Patent 2000-165930, for example).
However, when the method in which the representative wireless communication terminal thus specifies the time to be used within the group is employed, the representative wireless communication terminal in the group is specified in advance, which can be a problem that it should be notified to all the wireless communication terminals in the group. Therefore, it may not be applied to the ad hoc network in which the wireless communication terminals operate on an equal basis, and the terminals in the service area are asynchronously and directly connected to one another.
When another group approaches an adjoining area, the surrounding groups need a process for synchronizing one another, which can be a problem that operations become complicated in an environment where many groups exist.
On the other hand, a network system is proposed in which a transmission frame is divided into a plurality of slots, at least one of which is assigned to a control slot so that information is transmitted suitable as a network status or content of information to be transmitted (see Japanese Laid Open Patent 2000-299670, for example). Each terminal station informs the control station of the transmission slot which is being interfered, so that the control station avoids the transmission slot and uses another transmission slot.
However, in this case, when a specific control station is established in each network, if the synchronization is not achieved among networks via the control station, the slot can not be avoided so as to use another transmission slot continuously. In other words, the network system can not be applied to the ad hoc network system in which any specific control station is provided.
In the case of the UWB transmission system as described above, since the impulse signal train to be used does not have a specific frequency carrier, it is difficult to perform career sensing. Therefore, when a UWB radio communication system is applied as a PHY layer of IEEE802.15.3, as a specific carrier signal does not exist, the access control of the section may not be performed by using the standardized carrier sensing and there is no other way but to depend on access control by means of the time division multiplexing system.
According to the standard IEEE802.15.3, a mechanism which makes neighboring piconets coexist is proposed by devising the time division multiplexing process. As the standard IEEE802.15.3, a structure is envisaged in which a plurality of networks are synchronized so as to perform the time division multiplexing, for example (see Japanese Laid Open Patent 2003-143644, for example).
In this case, as to each radio network, the control station (coordinator) assigns a band to each predetermined transmission frame period. Then, on receiving beacon information from another network, the network control station decodes the beacon information, sets up its band allocation area (time domain) as an unused domain in its network, based on the bandwidth allocation information written there. Further, the band allocation domain (time domain) used in another network is excluded, so that the band allocation area (time domain) used in its network can be set up again. As a result, while performing a network operation on a mutually equal basis, each of the radio networks may avoid a collision with other networks, to thereby realize the coexistence on the same frequency channel.
However, also in this case, networks coexist in the same frequency channel, so that it is necessary to achieve the time synchronization among the networks. In other words, since the system cannot be managed if the synchronization is not achieved among networks via the control station, so that it may not be applied to the ad hoc network system which does not prepare any specific control station.