Multi-hop communication system, radio control station, radio station and multi-hop communication method

The multi-hop communication system according to the present invention is configured by a radio control station 1 connected to a core network, and radio stations 21, 22, 31-33. The radio control station 1 has a control signal TX/RX unit 12 which transmits/receives the control signal, an information signal TX/RX unit 13 which transmits/receives the information signal, and a communication channel controller 15 which transmits a “usage notification” indicating usage of communication channels managed by the radio control station 1 to the radio stations 21, 22, 31-33 using the control signal. The radio station has a control signal TX/RX unit 42 which transmits/receives the control signal, and an information signal TX/RX unit 43 which transmits/receives the information signal.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. P2003-044945, filed on Feb. 21, 2003; the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multi-hop communication system and a multi-hop communication method, which are configured by a radio control station and radio stations. The present invention also relates to a radio control station and a radio station used in the multi-hop communication system. Specifically, the present invention relates to a cellular phone system, in which a radio base station acts as the radio control station, and a wireless LAN, in which an access point acts as the radio control station.

2. Description of the Related Art

In an ad hoc network, communications between two stations, which cannot directly communicate with each other, are achieved using multiple stations. The multiple stations relay signals transmitted by the origin to the destination (which is called “multi-hop connection”). As for the ad hoc network, various kinds of commutation route discovery protocols have been proposed, i.e., DSR (Direct Source Routing) and AODV (Ad hoc On demand Distance Vector).

In such ad hoc networks, a centralized control station like the base station in the cellular phone system does not exist. Therefore, normally the radio station itself discovers a communication route autonomously.

In this regard, the multi-hop cellular, in which a communication route from the radio station to the base station is acquired by a multi-hop connection, has also been proposed. In the multi-hop cellar, since the radio station can access to a core network via the base station, communications with the destination becomes possible if a communication route up to the base station is acquired. Therefore, probability of connection with the distant destination is higher than the ad hoc network.

However, in the multi-hop cellular, methods of communication between the radio stations and acquisition of a communication route between the radio station and the base station have still been considered. It is highly desirable that such processes are performed within a short period of time with a small amount of controlling signals.

Further, communications cannot be performed between the radio stations if communication channels handled by the base station are fully occupied. In this case, since packets, which are not received by any radio station/base station, are transmitted, resources of the multi-hop cellular are wasted.

Moreover, it is known that in a radio communication system, higher transmission power is required as transmission speed (bit rate) increases. Therefore, power consumption at the radio station/base station may become excessive if the bit rates for transmitting both control signal and information signal are increased simultaneously.

BRIEF SUMMARY OF THE INVENTION

The present invention has been made in view of the above perspectives, and thus has an object of providing a multi-hop communication system, a radio control station, a radio station and a multi-hop communication method, which enables acquisition of an appropriate communication routes between the radio stations or between the radio station and the radio control station with small amount of control signals.

In order to achieve the above object, a multi-hop communication system according to the present invention, which is configured by a radio control station that is connected to a core network, and radio stations that relay an information signal transmitted by other radio stations, determines a communication route for a control signal and a communication route for an information signal by different independent processes. The control signal and the information signal are transmitted by the determined communication routes.

DETAILED DESCRIPTION OF THE INVENTION

FIRST EMBODIMENT

A first embodiment of the present invention will now be described with reference to the drawings.FIG. 1Ais a schematic diagram showing overall configuration of the multi-hop communication system.

As shown inFIG. 1A, the multi-hop communication system is configured by a radio control station1connected to a core network, e.g., a wideband wired network, and radio stations21,22,31-33located either inside or outside of an area A that is covered by the radio control station1. Incidentally, area A means an area, in which the radio control station1can directly transmits/receives an information signal with the radio stations21,22,31-33. The information signal contains user information, e.g., data packets and voice packets.

As shown inFIG. 1B, a plurality of the area A is deployed. Further, an area B is located outside of each area A. Each area B has a shape of a ring and is overlapped with the other areas B.

As shown inFIG. 1A, the radio stations21,22are located in the area A of the radio control station1. On the other hand, the radio stations31-33are located outside of the area A. Therefore, the radio stations31-33cannot directly transmit the information signal to the radio control station1. However, the radio stations31-33are located in the area B of the radio control station1. The radio stations31-33can transmit/receive only the control signal, i.e., a pilot signal, to the radio control station1.

In the multi-hop communication system in the embodiment, the control signal, i.e., a pilot signal, is transmitted with a lower bit rate in comparison with the information signal.

FIG. 2Ais a block diagram of the radio control station1, andFIG. 2Bis a block diagram of the radio stations21,22,31-33.

As shown inFIG. 2A, the radio control station1has a radio communication unit11which transmits/receives the information signal and the control signal using a radio communication scheme (e.g., CDMA), a control signal TX/RX unit12which transmits/receives the control signal, a control signal processor14which generates/processes the control signal.

Further, the radio control station1has a communication channel controller15which transmits a “usage notification” indicating usage of communication channels managed by the radio control station1to the radio stations21,22,31-33using the control signal.

Moreover, the radio control station1also has a communication route determiner17which determines a communication route for the information signal and the control signal, an information signal TX/RX unit13which transmits/receives the information signal, and an information signal processor16which generates/processes the information signal.

The communication route determiner17determines a communication route for transmission/reception of the control signal by a different independent process from the process for determination of a communication route for transmission/reception of the information signal.

Specifically, the communication route determiner17determines if the radio control station1can directly transmit/receive the information signal with the radio station based on a reception level of the control signal (e.g., a pilot signal) received at the control signal TX/RX unit12.

At this point in time, transmission/reception of the control signal is performed directly with the radio station as a different independent process using the control signal TX/RX unit12. The process has no relations to the processes for determination of a communication route for the information signal. It means that two different communication routes may be set between the radio control station1and the radio station. Such as a communication route for the control signal, of which bit rate is lower than the information signal, and a communication route for the information signal.

The control signal, of which bit rate is lower than the information signal, can cover a larger transmission area in comparison with the information signal. Therefore, the radio control station1and the radio station (21,22,31-33) can transmit/receive the control signal even if the radio control station1and the radio station (21,22,31-33) is distant, as they cannot directly transmit/receive the information signal.

Further, the communication route determiner17can estimate the distance between the radio control station1and the radio station according to the reception level of the control signal (e.g., the pilot signal) transmitted by the radio station. The communication route determiner17can also determine whether transmission/reception of the information signal directly performed with the radio station.

Incidentally, the control signal TX/RX unit12can directly transmit/receive the information signal between the radio control station1and the radio station. Further, the control signal TX/RX unit12can perform the multi-hop connection for transmission/reception of the information signal via a plurality of the radio stations.

Moreover, in this embodiment, although the communication route determiner17is located in the radio control station1, the present invention is not limited to such embodiment. The communication route determiner having above described function may be located in the radio station21,22,31-33in order for the radio stations to determine the communication routes for the information signal and the control signal by a different independent processes.

As shown inFIG. 2B, the radio stations21,22,31-33have a radio communication unit41which transmits/receives the information signal and the control signal using a radio communication scheme (e.g., CDMA), a control signal TX/RX unit42which transmits/receives the control signal, a control signal processor44which generates/processes the control signal.

Further, the radio stations21,22,31-33also have an information signal TX/RX unit43which transmits/receives the information signal and an information signal processor46which generates/processes the information signal.

Incidentally, the control signal processor44can transmit a usage inquiry for inquiring usage of the communication channel handled by the radio control station1through the control signal. The control signal processor44can also transmit/receive the information signal according to a usage notification that is a response to the usage inquiry.

(Operation of the Multi-hop Communication System)

Hereinafter, an operation of the multi-hop communication system according to the embodiment will be described.FIG. 3is a flowchart showing an operation of the multi-hop communication system.

As shown inFIG. 3, in step S101, the radio station (either one of radio stations21,22,31-33) transmits the usage inquiry for inquiring usage of the communication channel to the radio control station1without a relay by the other radio station, i.e., directly, when commencing communications via the radio control station1.

In step S102, the radio control station1checks whether or not a vacant communication channel is available according to the usage inquiry transmitted in the step S101.

Specifically, the communication channel controller15that manages usage of the communication channel(s) handled by the radio control station1checks the usage of the communication channel and decides whether or not the vacant communication channel is available.

If the vacant channel is available (“Y” in the step S102) in step S103, the radio control station1discovers a communication route for transmission/reception of the information signal.

In step S104, the radio control station1commences transmission/reception of the information signal, i.e., communications with the radio station using the communication route discovered in the step S103.

On the other hand, if no vacant channel is available (“N” in the step S102), in step S105, the radio control station1determines that communication is not possible with the radio station and then discontinues setting of the communication route to the radio station.

According to the first embodiment heretofore described, the radio station (the radio stations21,22,31-33) inquiries usage of the communication channel handled by the radio control station1before transmission/reception of the information signal. Further, the radio station determines setting of a communication route based on the availability of the vacant channel.

Therefore, performing the process of the communication channel setting by the radio station with the radio control station1can be reduced even if no vacant communication channel is available.

Specifically, since packets for communication route discovery is not transmitted when the communication channels are fully occupied, a waste of resources in the multi-hop communication system can be prevented.

SECOND EMBODIMENT

Hereinafter, a second embodiment of the present invention will be described.FIG. 4is a schematic diagram showing overall configuration of the multi-hop communication system according to the embodiment. In the embodiment, whether or not communication is directly conducted between the radio station and the radio control station is determined based on a reception level of the control signal (e.g., a pilot signal).

As shown inFIG. 4A, it is similar to the first embodiment, the radio control station1has a radio communication unit11which transmits/receives the information signal and the control signal using a radio communication scheme (e.g., CDMA), a control signal TX/RX unit12which transmits/receives the control signal, a control signal processor14which generates/processes the control signal.

Further, the radio control station1has a communication channel controller15which transmits a “usage notification” indicating usage of communication channels managed by the radio control station1to the radio stations21,22,31-33using the control signal.

Moreover, the radio control station1also has a communication route determiner17which determines a communication route for the information signal and the control signal, an information signal TX/RX unit13which transmits/receives the information signal, and an information signal processor16which generates/processes the information signal.

The communication route determiner17determines a communication route for transmission/reception of the control signal by a different independent process from the process for determination of a communication route for transmission/reception of the information signal.

Specifically, the communication route determiner17determines if the radio control station1can directly transmit/receive the information signal with the radio station based on a reception level of the control signal (e.g., a pilot signal) received at the control signal TX/RX unit12.

At this point in time, transmission/reception of the control signal is performed directly with the radio station as a different independent process using the control signal TX/RX unit12. The process has no relations to the processes for determination of a communication route for the information signal. It means that two different communication routes may be set between the radio control station1and the radio station. Such as a communication route for the control signal, of which bit rate is lower than the information signal, and a communication route for the information signal.

The control signal, of which bit rate is lower than the information signal, can cover a larger transmission area in comparison with the information signal. Therefore, the radio control station1and the radio station (21,22,31-33) can transmit/receive the control signal even if the radio control station1and the radio station (21,22,31-33) is distant, as they cannot directly transmit/receive the information signal.

Further, the communication route determiner17can estimate the distance between the radio control station1and the radio station according to the reception level of the control signal (e.g., the pilot signal) transmitted by the radio station. The communication route determiner17can also determine whether transmission/reception of the information signal directly performed with the radio station.

Incidentally, the control signal TX/RX unit12can directly transmit/receive the information signal between the radio control station1and the radio station. Further, the control signal TX/RX unit12can perform the multi-hop connection for transmission/reception of the information signal via a plurality of the radio stations.

Moreover, in this embodiment, although the communication route determiner17is located in the radio control station1, the present invention is not limited to such embodiment. The communication route determiner having above described function may be located in the radio station21,22,31-33in order for the radio stations to determine the communication routes for the information signal and the control signal by a different independent process.

As shown inFIG. 4B, the radio stations21,22,31-33have a radio communication unit41which transmits/receives the information signal and the control signal using a radio communication scheme (e.g., CDMA), a control signal TX/RX unit42which transmits/receives the control signal, a control signal processor44which generates/processes the control signal.

Further, the radio stations21,22,31-33have an information signal TX/RX unit43which transmits/receives the information signal and an information signal processor46which generates/processes the information signal.

Moreover, the radio stations21,22,31-33of the embodiment also have a relay controller45, a reception signal level measurer47, a decision unit48and a communication route selector49.

The relay controller45transmits a “relay control signal” for requesting a relay of the information signal to the other station and sets a communication route to the radio control station via the other station according to a “response relay control signal” that is a response to the relay control signal. Incidentally, in the embodiment, the relay controller45configures a first relay controller.

Further, the relay controller45receives a relay control signal requesting a relay of the information signal from the other station. The relay controller45then transmits a response relay control signal that is a response to the relay control signal and sets a communication route from the other radio station to the radio control station1. Incidentally, in the embodiment, the relay controller45configures a second relay controller.

The reception signal level measurer47measures a reception level of a pilot signal transmitted by the radio control station1and sends a result of the measurement to the decision unit48.

The decision unit48decides whether or not communication is directly conducted with the radio control station1based on a reception level of the pilot signal received by the control signal TX/RX unit42.

Specifically, the decision unit48stores the threshold(s) for the reception level of the pilot signal. Then whether or not information signal is directly transmitted to the radio control station1is decided by the decision unit48based on the result of comparison of the reception level and the threshold.

The communication route selector49selects a radio station satisfying a prescribed condition regarding a communication state if a plurality of the other radio stations transmitted the response relay control signal.

The communication route selector49can select a communication route according to the methods described below.

(1) Selection Based on SIR and the Number of Hops

In this method, the communication route selector49selects a communication route to the radio control station1based on an SIR (signal to interference ratio) and the number of hops.

As shownFIG. 5A, the communication route selector49is configured by a decoder49afor decoding a received control signal, an interference level measurer49dfor measuring a level of interference imposed on the pilot signal, a selector49cfor selecting the other radio station which relays the information signal, a memory49bfor memorizing the number of hops, and a relay control signal generator49efor adding information of the radio station itself to the received relay control signal (or the response relay control signal) and generating a new response relay control signal.

Further, as shown inFIG. 5B, a station ID, source station ID, an interference level and the number of hops up to the radio control station1are included in the response relay control signal. The radio station adds the station ID, the interference level and the number of hops to the response relay control signal received from the other radio station and then transmits the response relay control signal.

Each radio station received the relay control signal (or the response relay control signal) selects a radio station to be located on a next hop based on the SIR and the number of hops.

In other words, each radio station selects a radio station, of which the number of hops to the radio control station1is smallest based on the response relay control signal. Further, if two or more communication routes having the same number of hops are available, each radio station selects a ratio station, of which SIR is highest.

Specifically, as shown inFIG. 6, in step S501, the control signal TX/RX unit42receives the relay control signals (or the response relay control signals) from the other radio station.

In step S502, the communication route selector49initializes the counter i for counting the number of hops.

In step S503, the communication route selector49selects the response relay control signal, of which number of hops (Nhop) is equal to “i”, and generates a family “S” configured by the response relay control signal, of which number of hops (Nhop) is equal to “i”.

In step S504, the communication route selector49checks whether or not the family “S” is null.

If the family “S” is null (“Y” in the step S504), in step S505, the communication route selector49checks whether or not the value of the counter “i” is below the maximum number of hops determined in advance.

If the value of the counter “i” is not below the maximum number of hops (“N” in the step S505), in step S506, the communication route selector49adds “1” to the counter “i”

The communication route selector49again performs processes starting from the step S503based on the new value of the counter “i”.

On the other hand, if the family “S” is not null (“N” in the step S504), in step S507, the communication route selector49computes a relative transmission power of the response relay control signal (the pilot signal) included in the family “S”.

In step S508, the communication route selector49selects a radio station, of which relative transmission power is smallest based on the relative transmission power.

In step S509, the communication route selector49adds “1” to the counter “i” and then concludes the process.

Further, if the value of the counter “i” is equal to the maximum number of hops (“Y” in the step S505), in step S506, the communication route selector49reduces “1” from the counter “i” and then concludes the process.

According to the method described above, the radio station can select a radio station, of which number of hops up to the radio control station1is smallest as well as the SIR is largest as a radio station to relay the information signal. The radio station then transmits the response relay control signal containing information of the radio station itself (i.e., the station ID, the interference level and the number of hops) to the selected radio station.

(2) Selection Based on Relative Transmission Power

As a second method, the communication route selector49can select a radio station, of which relative transmission power is smallest. The relative transmission power is computed base on an SIR of the response relay control signal (the pilot signal) and an interference level imposed to the response relay control signal.

In this method, the radio station computes a relative transmission power of each response relay control signal received from the other stations. The radio station then sorts the values of the relative transmission power in an ascending order.

Further, the radio station checks whether or not the number of hops is below a prescribed value about the sorted response relay control signal. If the number of hops is below the prescribed value, the radio station selects the radio station that transmitted such response relay control signal as a radio station to relay the information signal.

Incidentally, the relative transmission power is computed based on the difference between the SIR of the response relay control signals and the interference level imposed to the response relay control signal.

As shown inFIG. 7, in step S601, the control signal TX/RX unit42receives the response relay control signals (or the relay control signals) from the other radio station.

In step S602, the communication route selector49computes the relative transmission power of all the response relay control signals.

In step S603, the communication route selector49sorts the values of the relative transmission power in an ascending order.

In step S604, the communication route selector49initializes the counter i for counting the position in the sorted values of the relative transmission power.

In step S605, the communication route selector49checks whether or not value positioned at “S” is null.

If value positioned at “S” is not null (“N” in the step S605), in step S606, the communication route selector49checks that the number of hops (Nhop) up to the radio control station1is equal to or less than the prescribed value.

If the number of hops is larger than the prescribed value (“N” in the step S606), in step S607, the communication route selector49adds “1” to the counter i and again performs processes starting from the step S605based on the value of the response relay control signal positioned at a new position “S”.

Further, if value is not located at the position “S” (“N” in the step S605), in step S610, the communication route selector49reduces “1” for the number of hops (Nhop) and then concludes the process.

If the number of hops is equal to or smaller than the prescribed value (“Y” in the step S606), in step S608, the communication route selector49selects the radio station that transmits the response relay control signal positioned at “S”.

In step S609, the communication route selector49adds “1” for the number of hops (Nhop) and then concludes the process.

According to the method described above, the radio station can select a next hop radio station, of which the number of hops up to the radio control station1is equal to or below the prescribed value, and of which relative transmission power is smallest, as a radio station to relay the information signal.

Incidentally, large transmission power is normally required for transmission of the information signal, which is transmitted with a high bit rate. In view of this, the radio station (decision unit48) may change threshold value for the reception level according to a transmission speed of the information signal.

Specifically, the radio station (decision unit48) changes the threshold value for the reception level of the response relay control signal according to the bit rate (transmission speed) used in the information signal TX/RX unit43. For example, the radio station (decision unit48) increases the threshold value if the bit rate used in the information signal TX/RX unit43increases.

(3) Selection Based on the Total Transmission Power

As a third method, the communication route selector49can select a communication route so as to minimize the total transmission power of the radio stations relaying the information signal.

In this method, as shown inFIG. 8A, the communication route selector49is configured by a decoder49a, a memory49b, a selector49c, an interference level measurer49d, a relay control signal generator49eand a total transmission power computation unit49f.

Further, as shown inFIG. 8B, in a response relay control signal, a transmitting station ID indicating a radio station that transmits a relay control signal, or a relay station ID indicating a radio station that relays the relay control signal (or the response relay control signal), and required transmission power are included.

The radio station to relay the relay control signal (or the response relay control signal) adds the identifier of the radio station itself and value of the required transmission power. Therefore, such information for all the radio stations located on the communication route can be gathered.

Specifically, as shown inFIG. 9, instep S701, the control signal TX/RX unit42receives the relay control signals (or the response relay control signals) from the other radio station.

Instep S702, the communication route selector49measures an SIR of the response relay control signal and retrieves the value of the required transmission power included in the response relay control signal.

In step S703, the communication route selector49computes the required transmission power at the radio station based on the measured SIR in the step S702, the desired SIR and predetermined value of the transmission power at the radio station that transmitted the relay control signals (or the response relay control signals).

Further, the communication route selector49computes the total sum of the values of the required transmission power included in the response relay control signal.

In step S704, the communication route selector49compares the total sum of the required transmission power computed in the step S703and the total sum of the required transmission power, which has already been memorized in the memory49b.

If the value of computed total sum is smaller than the memorized total sum (“Y” in the step S704), in step S705, the communication route selector49replaces the memorized total sum by the computed total sum so as to update the information in the in the memory49b.

In step S706, the communication route selector49checks whether or not the other response relay control signal has been received at the control signal TX/RX unit42.

If the other response relay control signal has been received (“Y” in the step S706), the communication route selector49again performs the processes of the steps S701through S705for the other response relay control signal.

On the other hand, if no response relay control signal has been received (“N” in the step S706), in step S707, the communication route selector49starts a timer.

In step S708, the communication route selector49checks elapsed time.

If a prescribed time elapsed (“Y” in the step S708), in step S709, the communication route selector49selects a radio station of a next hop, which relays the information signal, according to the information included in the response relay control signal. Further, the communication route selector49adds the identifier of the radio station itself and the value of the computed transmission power to the response relay control signal.

In step S710, the communication route selector49transmits the response relay control signal, which the identifier and the value have been added, to the other radio stations of the next hop.

(Operation of the Multi-hop Communication System)

Hereinafter, an operation of the multi-hop communication system according to the embodiment will be described.FIG. 10is a flowchart showing an operation of the multi-hop communication system.

As shown inFIG. 10, in step S201, the radio station (either one of radio stations21,22,31-33) transmits the usage inquiry for inquiring usage of the communication channel to the radio control station1without a relay by the other radio station, i.e., directly, when commencing communications via the radio control station1.

In step S202, the radio control station1checks whether or not a vacant communication channel is available according to the usage inquiry transmitted in the step S201.

Specifically, the communication channel controller15that manages usage of the communication channel(s) handled by the radio control station1checks the usage of the communication channel and decides whether or not the vacant communication channel is available.

If no vacant channel is available (“N” in the step S202) in step S208, the radio control station1determines that communication is not possible with the radio station, and then the radio control station1discontinues setting of the communication route to the radio station.

On the other hand, if the vacant channel is available (“Y” in the step s202), in step S203, the radio station measures a reception level of the pilot signal transmitted by the radio control station1.

In step S204, whether or not the information signal is directly transmitted to the radio control station1is decided by the radio station based on the result of comparison of the reception level and the threshold being set.

If the reception level exceeds the threshold (“Y” in the step S204), in step S209, the radio station directly transmits the information signal to the radio control station1(i.e., single hop connection).

On the other hand, if the reception level does not exceed the threshold (“N” in the step S204), in step S205, the radio station transmits a relay control signal in order to request the other radio stations to relay the information signal.

In step S206, the radio station determines whether or not a relay of the information signal by the other radio station is possible based on the response relay control signal.

If the relay of the information signal by the other radio station is possible (“Y” in the step S206), in step S207, the radio station transmits the information signal to the radio control station1through the other radio station (i.e., multi-hop connection).

Incidentally, the radio control station1can assign a communication channel to be used in the radio station located on the determined communication route.

Hereinafter, the multi-hop communication method shown inFIG. 10will be further described.FIG. 11is a sequence chart showing a communication sequence between the radio stations21,31,32and the radio control station1.

As shown inFIG. 11, in step S301, the radio stations21,31,32receive the pilot signal from the radio control station1periodically.

In step S302, the radio station32transmits a usage inquiry for inquiring usage of the communication channel handled by the radio control station1.

In step S303, the radio control station1checks whether or not a vacant communication channel is available based on the usage inquiry. The radio control station1transmits a usage notification that notifies usage of the communication channel to the radio station32.

In step S304, whether or not the information signal is directly transmitted to the radio control station1is determined by the radio station32based on a reception level of the pilot signal received most recently.

Incidentally, in this communication sequence, although the radio station32determines the communication route when commencing communication, the radio station32may also determine the communication route when it receives the pilot signal every time. This alternative allows discovering an appropriate communication route more rapidly.

In step S305, the radio station32transmits a relay control signal for requesting a relay of the information signal to the radio stations21and31.

In step S306, the radio stations21and31measure a reception level of the pilot signal transmitted by the radio control station1in response to the relay control signal transmitted by the radio station32.

In step S307, the radio station21determines that the information signal to be transmitted by the radio station32can be relayed to the radio control station1.

In step S308, the radio station21transmits a response relay control signal to the radio station32to notify that information signal to be transmitted by the radio station32can be relayed.

In step S309, the radio stations21and32determine the communication route for transmission/reception of the information signal based on the response relay control signal.

Incidentally, if the radio station21cannot directly transmit the information signal to the radio control station1, the radio station21further transmits the response relay control signal, which the information of the radio station21is added, to the other radio station (e.g., the radio station22).

Then, whether or not the information signal to be transmitted by the radio station32can be directly relayed to the radio control station1is determined by the radio station22. If the radio station22can relay the information signal to be transmitted by the radio station32, the radio station22transmits a response relay control signal to notify the ability of the relay.

Further, the radio station21relays the response relay control signal transmitted by the radio station22to the radio station32so as to determine a communication route between the radio station32and the radio control station1.

Moreover, the radio station may also add a value of a reception level of the pilot signal to the response relay control signal (or the relay control signal).

According to this arrangement, the radio station relaying the information signal can be selected based on the reception level in addition to the SIR and the number of hops. This allows that the radio station can select only the radio station, of which reception level of the pilot signal is higher. It is therefore prevented that the radio station located in more distant position from the radio control station1is selected to set a communication route via other stations.

THIRD EMBODIMENT

Hereinafter, a third embodiment of the present invention will be described.FIG. 12is a sequence chart showing a communication sequence between the radio stations21,31,32and the radio control station1.

In the embodiment, if the radio station determines that the information signal is directly relayed to the radio control station1, the radio station transmits a communication route acquisition request to the radio control station1.

Further, the radio control station1sets a communication route according to the communication route acquisition request, and transmits a communication route acquisition notification to each radio station. Incidentally, the radio control station1may transmits the communication route acquisition request and the radio station may transmit the communication route acquisition notification.

As shown inFIG. 12, in step S401, the radio stations21,31,32receive the pilot signal from the radio control station1periodically.

In step S402, the radio station32transmits a usage inquiry for inquiring usage of the communication channel handled by the radio control station1.

In step S403, the radio control station1checks whether or not a vacant communication channel is available based on the usage inquiry. The radio control station1transmits a usage notification that notifies usage of the communication channel to the radio station32.

In step S404, whether or not the information signal is directly transmitted to the radio control station1is determined by the radio station32based on a reception level of the pilot signal received most recently.

Incidentally, in this communication sequence, although the radio station32determines the communication route when commencing communication, the radio station32may also determine the communication route when it receives the pilot signal every time. This alternative allows discovering an appropriate communication route more rapidly.

In step S405, the radio station32transmits a relay control signal for requesting a relay of the information signal by a multi-hop connection to the radio stations21and31.

In step S406, the radio stations21and31measure a reception level of the pilot signal transmitted by the radio control station1in response to the relay control signal transmitted by the radio station32.

In step S407, the radio station21determines that the information signal to be transmitted by the radio station32can be relayed to the radio control station1.

In step S408, the radio station21transmits a communication route acquisition request for requesting acquisition of a communication route between the radio station32and the radio control station1to the radio control station1.

In step S409, the radio control station1determines a communication route to the radio station32according to the communication route acquisition request transmitted by the radio station21. The radio control station1then transmits a communication route acquisition notification for notifying information of the communication route (e.g., a station ID) to the radio stations21and32.

The both radio stations21and31can recognize the communication route between the radio station32and the radio control station1by the communication route acquisition notification.

Further, in the multi-hop communication method according to the embodiment, if a plurality of the communication routes is available, a communication route, which has the smallest number of hops between the radio station and the radio control station1is selected.

FIG. 13is a diagram explaining an operation of the multi-hop communication system to select a communication route, which has the smallest number of hops.

InFIG. 13, the case where the radio station31commences communication with the radio control station1(i.e., the core network) is illustrated.

Further, the numbers in the rectangle is correlated with a reception level of the pilot signal transmitted by the radio control station1. The number means that the reception level is higher as the number decreases. For example, “1” shown in the rectangle of the radio station22means the highest reception level in the radio stations21,22,31-33.

Since the information signal cannot directly be transmitted between the radio station31and the radio control station1, the radio station31transmits a relay control signal for requesting a relay of the information signal to the other radio stations.

Here, it is assumed that the radio stations32and33received the relay control signal transmitted by the radio station31. It is further assumed that the information signal cannot directly be transmitted between the radio station32,33and the radio control station1.

The radio stations32and33add the station ID, etc. (e.g. reception level of the pilot signal) to the relay control signal and transmits the relay control signal as a response relay control signal that the station ID, etc. is added.

In this case, the radio station32receives the response relay control signal from the radio station33. Similarly, the radio station33receives the response relay control signal from the radio station32. Here, since the reception level of the pilot signal is added to the response relay control signal, the response relay control signal transmitted by the radio station33is discarded in the radio station32based on the reception level.

In other words, since the number correlated to the reception level is “4” in the radio station33, and it is “5” in the radio station32, the radio station32discards the response relay control signal transmitted by the radio station33.

Similarly, the response relay control signals transmitted by the radio station32and the radio station33are also discarded in the radio station31.

As heretofore described, the response relay control signal is only relayed to the radio stations, which has a higher reception level. Therefore, the response relay control signal reaches the radio stations21and22, which can directly relay the information signal to the radio control station1.

The radio stations21and22transmit a response relay control signal for notifying that a relay of the information signal is possible to the radio station31.

Under the configuration shown inFIG. 13, the following communication routes can be set (the number below means either radio station or radio control station).

The radio station31selects the communication route (6) which has the smallest number of hops (i.e., 3 hops).

InFIG. 13, four communication routes ((3)-(6)) exist between the radio station32and the radio control station1. The radio station32notifies only the communication route (6), which has the smallest number of hops, to the radio station31.

Accordingly, only three communication routes (i.e., (1) (2) and (6)) are notified to the radio station31. This alleviates a load of selection of the communication route in the radio station31.

Further, as shown inFIG. 12, the radio control station1may notify the communication route acquisition notification of all the radio stations located on the communication route. In this method, the response relay control signal including the information of the radio station relayed the response relay control signal is also received by the radio control station1.

The radio control station1selects the communication route (6) as a communication route to the radio station31based on the response relay control signal received.

In the embodiment heretofore described, the communication route is determined according to the relay control signal transmitted by the radio station commencing communication. However, in the certain situation, e.g., an incoming call to a cellular phone (or mobile terminal) and delivery of e-mail to a cellular phone, the radio control station1has to transmit the relay control signal to the radio station.

In this case, the radio control station1can transmit a communication route acquisition request for requesting acquisition of a communication route, and the radio station can discover a communication route between the destination radio station and the radio control station1according to the communication route acquisition request.

Further, the radio control station1determines the communication route discovered by the radio station as a communication route between the between the destination radio station and the radio control station1.

As heretofore described, according to the multi-hop communication system, the radio control station1, the radio stations (radio stations21,22,31-33) and a multi-hop communication method, it is feasible that acquisition of an appropriate communication routes between the radio stations or between the radio station and the radio control station with small amount of control signals.

Further, according to the embodiments described above, since the radio station transmits the usage inquiry to check whether or not a vacant communication channel is available before it commences communication, transmission of the information signal while no vacant communication channel is available is prevented. Therefore, communication can be conducted more surely in the multi-hop communication system even large amount of traffic occurs in the multi-hop communication system.

Moreover, according to the embodiments described above, since the radio station, of which distance to the radio control station1is shorter, is selected based on a reception level of the pilot signal, the communication route between the radio station and the radio control station1, which has the minimum number of hops, can be set.

The invention has been described in detail by referring to the embodiments. It is obvious to those skilled in the art that the invention is not restricted to the embodiments described above. The invention may be carried out as a corrected or modified embodiment not departing from the gist and scope specified by the scope of the claims of a patent. Therefore, the description of this specification aims at the representation of examples but does not have any limitation on the present invention.