Wireless base station and mobile station

A wireless base station in a wireless communication system includes a memory that stores a plurality of timing patterns that are set such that timings of using the resource are distributed for each of the mobile stations when the mobile stations change a timing of using the resource in accordance with a traveling speed and control unit that allocates one of the timing pattern to the mobile stations.

FIELD

The embodiments discussed herein are related to a wireless base station and a mobile station.

BACKGROUND

Recently, development of ITSs has been underway aiming at reduction in traffic accidents. One type of the ITSs includes wireless base stations (roadside devices) installed in or on traffic lights for roadside-vehicle communication wherein information, such as traffic light information at an intersection (e.g., whether the light is red or green) or whether there is any vehicle turning to the right is sent to a wireless terminal (hereinafter referred to as a “on-vehicle device” or “mobile station”) mounted on a vehicle that is a few hundred meters from that intersection. In addition, there is another type of inter-vehicle communication in which on-vehicle devices that are separated with a smaller distance inform each other of vehicle information, such as the respective travel direction and traveling speed.

For such inter-vehicle communication, application of narrow-area ad-hoc communication based on the common wireless Local Area Network (LAN) techniques (IEEE802.11 series standards), as disclosed in Non-Patent Document 1 listed below is considered. The narrow-area ad-hoc communication in accordance with Non-Patent Document 1 is a type of communication in which mobile stations can directly communicate with each other without requiring any central control apparatus, such as a base station. Since no base stations are involved with allocating communication (wireless) resources, each mobile station manages communication resources that are employed, and carry out communication while contending each other for communication resources (hereinafter, referred to as shared resources) shared among mobile stations in a decentralized autonomous manner.

That is, each wireless terminal sends signals while monitoring a state of usage of the shared resources (frequency) by using the Carrier Sense Multiple Access (CSMA) technique so that the sending signals do not collide with each other. Furthermore, each wireless terminal waits for receiving data while it is not sending, and searches for beginnings (preambles) of signals being sent by other wireless terminals. This means that wireless terminals are not synchronized with each other. Note that IEEE 802.11 series standards indicate that cells having radii within several hundred meters are constructed as communication areas and the Orthogonal Frequency Division Multiplexing (OFDM) is employed as a modulation scheme.

Note that well-known techniques related to allocating communication resources include techniques described in Patent Documents 1 and 2 listed below.

The technique of Patent Document 1 is related to a packet relaying scheme in a wireless transmission. It is directed to providing a scheme in which reliability as a system is ensured for failure of an apparatus during relay while requiring only a single carrier for conveying wireless signals, in terms of the relaying scheme of wireless signals applied when a distance between locations of a number of sites distributed and a center for collecting information is beyond the limit of the propagation distance of wireless signals.

For this purpose, Patent Document 1 discloses that a plurality of portable terminals that transmit sensor information and monitor control information in a packet to a main terminal are linked via a linear transmission path. A time slot related to an installation location is allocated to each portable terminal, thereby permitting wireless transmission in this time span for relaying packets among portable terminals in a time division multiplexing manner as bucket brigade while relaying by bypassing an adjacent portable terminal when this terminal fails.

In addition, the technique of Patent Document 2 relates to a method for allocating wireless channels in a wireless communication system and a corresponding wireless communication system. It is directed to providing an improved method and wireless communication system for allocating wireless channels.

For this purpose, the Patent Document 2 discloses that a wireless channel in the downlink direction (tsx) is allocated to a mobile station (MS) by a wireless station (BS) depending on a channel allocation pattern (CAP) that indicates a transmission output (txp) for a time slot, and corrects the allocation of the wireless channel (tsx) by mobile station (MS) under the control of the mobile station depending on an interference pattern (IP) that indicates an interference output (inp) of the time slot.

As described above, in the technique of Patent Document 2, by carrying out allocating depending on the occupy rate of the time slots according to channel allocation patterns, it is possible to distribute the transmission output from a wireless station in the downlink direction as evenly as possible among existing time slots. By combining this feature and the fact that correction of such allocated time slots under the control of mobile stations is available (that is, correction to a time slot having a lower interference output based on the interference pattern), the shortcoming of soft handover may be avoided in the case in which the frequency repetition interval (cluster reuse) is equal to one and interferences may be problematic.

Patent Document 1: Japanese Patent Publication No. H09-36916

Patent Document 2: Japanese Translation of PCT International Application No. 2002-510916

However, none of Patent Documents teaches control on communications between mobile stations.

The related techniques described above may be problematic when the number of mobile stations is increased and thus there is not sufficient resources used for communications between mobile stations.

SUMMARY

(1) According to an aspect of the embodiments, an apparatus includes a wireless base station in a wireless communication system including the wireless base station and a plurality of mobile stations which carry out communication between mobile stations with other mobile stations using a resource allocated by the wireless base station, the wireless base station including: a memory that stores a plurality of timing patterns that are set such that timings of using the resource are distributed for each of the mobile stations when the mobile stations change a timing of using the resource in accordance with a traveling speed; and control unit that allocates one of the timing patterns to the mobile stations.

(2) In addition, according to an aspect of the embodiments, an apparatus includes a mobile station in a wireless communication system in which a communication between mobile stations is carried out using a resource allocated in advance, the mobile station including: a memory that stores a plurality of timing patterns that are set such that timings of using the resource are distributed for each of the mobile stations when the mobile stations change a timing of using the resource in accordance with a traveling speed; timing pattern selection unit that autonomously selects one of the timing patterns; and communication between mobile stations control unit that carries out the communication between mobile stations in accordance with the selected timing pattern.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments will be described with reference drawings. It is not intended that various modifications and applications of techniques that are not explicitly described are excluded. That is, each embodiment may be varied (such as combining embodiments, for example).

(A) Description of One Embodiment

(A1) System Configuration and Fundamental Operation

FIG. 1is a block diagram illustrating fundamental configuration and operations of an ITS as one example of a wireless communication system according to one embodiment.

The ITS depicted inFIG. 1is configured to include a wireless base station (BS)1as a road-side device for forming a communication area1-2, mobile stations (MS)2-1and2-2as on-vehicle devices located outside the communication area of the BS1(hereinafter, referred to as wide-area communication area)1-2, and mobile stations (MS)2-3and2-4as on-vehicle devices located within the communication area1-2of the BS1. Note that the number of BSs and the number of MSs within/outside the communication area1-2are not limited to those depicted inFIG. 1. In addition, in the following description, the MSs2-1,2-2,2-3, and2-4may be collectively referred to as “MS2” when distinction among them is not necessary.

In the above ITS, the MSs2-1and2-2located outside the communication area1-2of the BS1carry out inter-MS communications1-31and1-41while autonomously selecting within a range of a resource (wireless resource (hereinafter, referred to as “resources”)) that are allocated between the MS2-1and2-2in advance as shared resource. Note that the term “resource” includes time slots in the time division multiplexing scheme, frequencies in the frequency division multiplexing scheme, codes in the code division multiplexing scheme, bursts in the orthogonal frequency division multiplexing scheme (this applies throughout the entire specification).

On the other hand, the MSs2-3and2-4located within the communication area1-2of the BS1(or, the MS2-1and2-2that are migrated within the communication area1-2) carryout communication (1-52and1-62) with the BS1(for example, transmit a resource allocation request) using a resource allocated in advance for the resource allocation request, and receives, from the BS1, information on a resource to be used for communications (roadside-vehicle communications) with the BS1and information on a resource to be used for communications (inter-vehicle communications) between the MSs2-3and2-4. Alternatively, among a resource to be used for roadside-vehicle communications and a resource to be used for inter-vehicle communications, the resource to be used for inter-vehicle communications may be requested, information on an allocated resource may be received from the BS1, and inter-vehicle communications may be carried out.

Note that the MS2-3and the MS2-4respectively may receive allocation of transmission resources, carry out respective communications using the allocated resources, and communicate with each other by carrying out reception from other MSs using other times or the like.

When allocations are made for both roadside-vehicle communications and inter-vehicle communications, the MSs2-3and2-4can carry out roadside-vehicle communications (1-52and1-62) with the BS1using the resource allocated by the BS1while carrying out inter-vehicle communications (1-51and1-61) each other (and other MSs that are not depicted). Allocations of resources may be carried out by notifying of a resource to be allocated in a DL_MAP of MAP information (DL/UL_MAP) as long as a frame is any frame compliant with the Orthogonal Frequency Domain Multiple (OFDM) scheme (including the OFDMA) compliant with the WIMAX standards, for example. Alternatively, other wireless communication schemes other than the WIMAX may be used, and allocations of resources may be carried out by notifying of information on allocated resources or otherwise.

Note that in the following description, a communication (roadside-vehicle communication) between the BS1and an MS2may be referred to as a “wide-area infrastructure communication” or “wide-area communication,” and a wireless resource for that communication may be referred to as a “wide-area communication resource.” In addition, a communication (inter-vehicle communication) between MSs2may be referred to as a “narrow-area ad-hoc communication” or “narrow-area communication,” and a wireless resource for that communication may be referred to as a “narrow-area communication resource.”

Incidentally, when the number of MSs located within the communication area1-2is increased compared to the amount of narrow-area communication resources available for the communication area1-2of the BS1, the BS1allocates a single narrow-area communication resource to a plurality of MSs2such that it is shared among them. Thus, when there are MSs2that try to carry out a narrow-area ad-hoc communication in the same cycle (timing) within the communication area1-2, contention of the narrow-area communication resource, that is, a collision (interference) of a communication occurs.

Therefore, by setting the cycle of the narrow-area ad-hoc communication according to a traveling speed of an MS2(shorter cycle as the speed increases), such collision of communication may be reduced to a certain degree. However, in the case there are MSs2within the communication area1-2which are traveling at the same traveling speed, the cycle is not changed and collision of communications may be experienced unless the traveling speed is changed.

In order to avoid such a phenomenon above, the BS1of this example allocates different phases (transmission timings) such that communications are carried out at different times (timings) even when MSs2that use a narrow-area communication resource have the same status (traveling speed, for example) and carry out narrow-area ad-hoc communications at the same cycle, on the basis of setting the cycle of a narrow-area ad-hoc communication according to traveling speeds of the MSs2.

(A2) Configuration of BS

The BS1of this example includes, as depicted inFIG. 2, for example, a communication processing unit11, a narrow-area communication resource management unit12, a wide-area communication resource management unit13, and an antenna14.

Here, the communication processing unit11includes a transmission and reception function that carries out reception processing on signals received from an MS2via the antenna14and transmission processing on signals bound for the MS2. The reception processing includes processing on signals received by the antenna14, such as low noise amplification, a frequency conversion (downconversion), an AD conversion, demodulation, decoding, and the transmission processing includes processing on transmission information, such as coding, modulation, a DA conversion, a frequency conversion (upconversion), and high power amplification.

In addition, the transmission processing includes processing for transmitting synchronization signal8-1and a synchronization signal number that is incremented every time the synchronization signal8-1is transmitted which will be described later with reference toFIG. 5, and processing for generating and transmitting a wireless frame (base frame) at a timing synchronized with that synchronization signal8-1such that an MS2within the communication area1-2can communicate with the BS1and other MSs2.

The narrow-area communication resource management unit12is equipped with a function that manages availabilities of narrow-area communication resources and manages allocation status of phases allocated to MSs2for each narrow-area communication resource, and carries out allocation processing and deallocation processing of narrow-area communication resources and phases in response to requests from the communication processing unit11.

For this purpose, the narrow-area communication resource management unit12includes a transmission cycle table12-1and a phase table12-2which will be descried later with reference toFIG. 4, a memory121for storing a narrow-area communication resource/a transmission pattern management table12-3which will be descried later with reference toFIG. 7, and a control unit122that carries out the allocation processing and the deallocation processing narrow-area communication resources and phases based on the contents stored on the memory121.

The wide-area communication resource management unit13includes a function that manages availabilities of wide-area communication resources, and carries out allocation processing and deallocation processing of wide-area communication resources in response to requests from the communication processing unit11.

The antenna14includes a transmission and reception function of wireless signals with MSs2within the communication area1-2. Note that the communication processing unit11may be connected to a communication processing unit11of another BS1or a server apparatus or the like that contains various information, although not depicted inFIG. 2.

(A3) Configuration of MS

On the other hand, MS2includes a wide-area communication processing unit21, a wide-area communication antenna22, a narrow-area communication processing unit23, a narrow-area communication antenna24, and a status management unit25, as depicted in, for example,FIG. 3.

Here, the wide-area communication processing unit21includes a function that carries out a wide-area infrastructure communication with the BS1via the wide-area communication antenna22in synchronized with a synchronization signal from the BS1in the communication area1-2of the BS1, and notifies the narrow-area communication processing unit23of the allocation information on narrow-area communication resource from the BS1.

The narrow-area communication processing unit23is adapted to carry out a narrow-area ad-hoc communication with other MSs2via the narrow-area communication antenna24. The narrow-area communication processing unit23includes a function that carries out a narrow-area ad-hoc communication with one other MS2while autonomously selecting the narrow-area communication resource that has been allocated in advance when the local MS2is outside the communication area1-2of the BS1, and carries out a narrow-area ad-hoc communication (transmission) according to allocation information of a narrow-area communication resource and a phase specified by the wide-area communication processing unit21when the local MS2is located within the communication area1-2of the BS1.

The narrow-area communication processing unit23includes a memory231that stores a phase table23-1that is similar to the phase table12-2owned by the BS1side, a transmission pattern selection unit232that selects one of transmission patterns (timing patterns) in the phase table12-2according to an instruction from the BS1, and a control unit233that carries out a narrow-area ad-hoc communication (transmission) and a narrow-area ad-hoc communication (inter-MS communication) according to the selected transmission pattern. Note that, as for reception, the MS2is preferably ready at anytime to receive a transmission signal using a narrow-area communication resource from one other MS2.

The status management unit25includes a function to manage status information of the local MS2, such as the traveling speed (vehicle speed) of the local MS2, and notify the narrow-area communication processing unit232of the status information in response to a request from the narrow-area communication processing unit23or in a certain cycle. The traveling speed may be detectable using a vehicle speed sensor installed in a vehicle, for example.

Note that the wide-area communication antenna22and the narrow-area communication antenna23are depicted as being discrete elements inFIG. 3. However, if it is possible to integrate the functions of the two antennas22and23, for some reasons, such as the same radio frequency band is used for a wide-area communication and a narrow-area communication, the wide-area communication processing unit21and the narrow-area communication processing unit23may be configured to share one antenna.

(A4) Information on Transmission Cycle and Phase

Next, the relationship between the status of an MS2and the cycle of a narrow-area ad-hoc communication (transmission cycle), and the relationship of phase information derived from that relationship is illustrated inFIG. 4. InFIG. 4, the transmission cycle table12-1indicates that the relationship between the traveling speed (that is exemplified by the traveling speed of an MS2as a factor to determine the cycle of narrow-area ad-hoc communications) and the transmission cycle of narrow-area ad-hoc communications. It is indicated that the transmission cycle is reduced as the traveling speed of the MS2increases.

That is, in the transmission cycle table12-1, the transmission cycle is 1000 ms (i.e., 1 second) when the traveling speed V of the MS2detected by a sensor or the like is 0 km/h (stopped status); the transmission cycle is 700 ms when 0 km/h<V≦10 km/h; the transmission cycle is 500 ms when 10 km/h<V≦20 km/h; the transmission cycle is 400 ms when 20 km/h<V≦30 km/h; the transmission cycle is 300 ms when 30 km/h<V≦40 km/h; the transmission cycle is 200 ms when 40 km/h<V≦50 km/h; and the transmission cycle is 100 ms when 50 km/h<V.

The phase table12-2is generated based on the transmission cycle table12-1. The specific procedure for the generation will be described. Firstly, since it is known from the transmission cycle table12-1that the minimum value of the transmission cycle is 100 ms and the maximum value is 1000 ms, a narrow-area communication resource is configured from a base frame8-2of 100 ms and a super frame of 1000 ms is configured10base frames8-2(#i), as depicted inFIG. 5, for example. Note that “i” indicates a base frame number (identifier), and “i” is one of integers from 1 to 10. InFIG. 5, a non-transmitting period, such as a guard interval, is provided in the base frame8-2.

That is, synchronization signals8-1transmitted by the BS1includes synchronization signal numbers, the beginning of each base frame8-2having a length of 100 ms is defined at the timing in synchronized with that synchronization signal. Furthermore, a super frame having a length of 1000 ms is defined by repetition of 10 base frames8-2. Note that the synchronization signal8-1and the base frame8-2is generated at the communication processing unit11of the BS1.

The synchronization signal number employed is determined based on the transmission cycles described in the transmission cycle table12-1. In the case of the transmission cycle table12-1, an integer obtained by dividing each by the length of base frame time (100 ms) which ranges from 1 to 420, the maximum value of 420 being the least common multiple of 1, 2, 3, 4, 5, 7, and 10. Each MS2determines which base frame8-2is to be transmitted (transmission timing phase) on the basis of the base frame number1(F1), and carries out transmission using the determined transmission timing.

In addition, each base frame #i are further divided into finer time slots8-3(TSs #1-#n), and one time slot TS #j defined within one base frame #i is allocated to each MS2from the BS1as a narrow-area communication resource. Note that “j” indicates time slot number (identifier), and j is one of integer from 1 to n. For example, when an MS2travels at V>50 km/h and carries out a narrow-area ad-hoc communication at 100 ms, the MS2will carry out transmission using the allocated time slot TS #j at every base frame #1to #10.

Note that, althoughFIG. 5illustrates an example in which a narrow-area communication resource is time division multiplexed, frequency division multiplexing, code division multiplexing, or the like may be applied. In addition, a base frame #i may have a frame length shorter than 100 ms. Furthermore, although the synchronization signal8-1is transmitted by the BS1at every base frame #i, the BS1may transmit synchronization signals at every base frames #i, or other signal sent from other systems, such as a global positioning system (GPS) may be used as synchronization signals.

Here, according to the frame configuration of the narrow-area communication resource depicted inFIG. 5, a certain time slot number j appears ten times within a super frame of 1000 ms (this means that there are ten opportunities of transmission). Here, assuming the case in which the transmission cycle is set to the longest value of 1000 ms, it is possible to avoid collision of narrow-area ad-hoc communications by allocating a different base frame number Fi to each MS2as a phase (transmission timing phase) if the number of MSs2sharing a single time slot number j is ten or less since only one of ten slots is required for a transmission.

As described above, the number of MSs for which any collision can be completely avoided by displacing phase (the base frame number Fi) is (the transmission cycle/the length of base frame time) when the same time slot number j is used. The phase table12-2includes various combinations of phases in each transmission cycle as transmission patterns when the number of MSs using the same time slot number j is equalized.

As described above, the efficiency of resource utilization of communications between MSs2can be improved by allocating, by the BS1, phases such that frequencies of usage of respective phases in a predetermined phase group is equalized when allocating phases to group of MSs2that use a part of the same narrow-area communication resource. Note that definition that gives biases in usage may be made, other than the equalization as described above.

In other words, the phase table12-2defines a plurality of timing patterns (patterns of timing phases) that are defined such that each timing for each MS2is dispersed upon the MS2when the timing for using a narrow-area communication resource is changed according to the traveling speed thereof, more specifically, defines patterns of a plurality of timing phases having different phases of usage cycles for a plurality of MSs2having their usage cycles of narrow-area communication resources matched according to the traveling speeds of the MSs2.

The BS1allocates one of the phases (phase combination) indicated by a transmission pattern ID (PT-ID) in the phase table12-2, and transmits the allocated PT-ID to an MS2. Since the MS2which received the allocated PT-ID has the phase table23-1that is the same as the phase table12-2on the BS1side in the memory231(the MS2may appropriately receive one from a road-side device side, or the PT-ID may include information on a transmission timing phase for each transmission cycle where necessary), the MS2determines (selects), in a transmission cycle that is decided according to the detection result of a sensor or the like in the local the traveling speed of the MS2, which phase (base frame number Fi indicating the initiation timing of the transmission cycle) is to be used for narrow-area ad-hoc communication by looking for the received PT-ID based on the phase table23-1.

As depicted specifically inFIG. 6, for example, when three MSs2having a time slot identifier (TS-ID)=j and transmission pattern identifiers (PT-ID)=1, 2, and 3 are present within the communication area1-2of the BS1, transmission timing phases are dispersed and no collision of narrow-area ad-hoc communications occurs even if the transmission cycles depending on the traveling speed of the MSs2become all 300 ms.

That is, since the MS2having its allocated PT-ID=1 carries out transmission at a cycle of 300 ms as an initial phase using the time slot TS #j of TS-ID=j of the base frame number=F1, the MS2having its allocated PT-ID=2 carries out transmission at a cycle of 300 ms as an initial phase using the time slot TS #j of TS-ID=j of the base frame number=F2. Similarly, since the MS2having its allocated PT-ID=3 carries out transmission at a cycle of 300 ms as an initial phase using the time slot TS #j of TS-ID=j of the base frame number=F3, no collision of narrow-area ad-hoc communications occurs.

Note that the BS1maintains the above-described transmission cycle table12-1and the phase table12-2in the memory121of the narrow-area communication resource management unit12depicted inFIG. 2as tabular form data, and an MS2maintains the phase table23-1that is the same as the phase table12-2stored by the BS1the memory231of the narrow-area communication processing unit23depicted inFIG. 3as tabular form data.

(A5) Allocation Processing of Narrow-Area Communication Resource and Transmission Pattern

The narrow-area communication resource management unit12of the BS1maintains the narrow-area communication resource/the transmission pattern management table12-3as depicted inFIG. 7in the above-described memory or the like, and carries out allocation processing of narrow-area communication resources (time slots) and transmission patterns based on the management table12-3.

Here, the vertical axis of the management table12-3represents time slot identifiers (TS-ID=TS#1-TS#n) defined in base frames #i, the horizontal axis represents transmission pattern identifiers (PT-ID=PT#1-PT#10), and the intersections contain numbers of MSs using the time slot TS #j identified by the corresponding TS-ID and the transmission pattern by the PT-ID.

That is, when the narrow-area communication resource management unit12in the BS1receives a narrow-area communication resource allocation request from an MS2, time slots to be allocated and transmission patterns are searched from the top left of the management table12-3inFIG. 7to the direction indicated by the dotted direction. The logic of the search is as follows:

(1) If there is an unallocated time slot TS #j, the time slot TS #j (and PT#1) are allocated.

(2) If there is no unallocated time slot TS #j, an unallocated transmission pattern PT#i among allocated time slots TS #j is allocated.

(3) If there is no unallocated transmission pattern PT#i, the transmission pattern PT#i that has been allocated to the smallest number of MSs among allocated transmission patterns PT#i is allocated.

(4) Once the combination of time slot TS #j to be allocated and the transmission pattern PT#i is determined, the value in the field of that combination is incremented by one.

When an MS2leaves the communication area1-2of the BS1and the combination of the time slot TS #j and the transmission pattern PT#i is not used, the value in the field of that combination in the management table12-3is decremented by one.

Note that further information may be added to the information in the management table12-3depicted inFIG. 7in order to speed up such search processing.

When the narrow-area communication resource management unit12recognizes that there are multiple MSs2that use a certain one time slot TS#2in the course of the allocation processing of a time slot TS #j and a transmission pattern PT#i described above, the narrow-area communication resource management unit12instructs the MSs2that use the time slot TS#2to initiate a cycle control of the narrow-area ad-hoc communication according to the traveling speeds.

That instruction includes that TS-ID (#2), and is notified to the MSs2by means of broadcast-type communication within the communication area1-2, for example. Alternatively, it is possible to notify the individual targeted MSs2of the TS-ID (#2) by maintaining identification information of the MSs2that carryout a narrow-area ad-hoc communication within the communication area1-2.

In addition, when the number of MSs2that uses a certain one time slot TS #j is reduced to one for some reasons, such as a certain MS2having moved outside the communication area1-2, the MS2is instructed to terminate the cycle control of the narrow-area ad-hoc communication according to the traveling speed. That instruction may be carried out in the same manner as the initiation instruction of the above-described cycle control.

For the cycle control according to the statuses of MSs2of narrow-area ad-hoc communication, a cycle control may be carried out constantly irrespective of the usage situation of time slots TS #j, rather than the BS1instructing dynamically to initiate/terminate a cycle control according to the usage situation of time slots TS #j as in the manner described above.

With the configuration descried above, the BS1can notify MSs2in advance of information indicating a pattern of transmission timing phase in a wireless frame that is used when transmission cycle is changed. The MSs2can make a change for increasing a length of the transmission cycle when the traveling speed is decreased based on the information notified in advance, identify the transmission timing phase corresponding to the transmission cycle after the change, and carry out transmission using that transmission timing phase.

(A6) Description of Operation of BS

Next, on the basis of the above description, operations of the BS1will be described with reference to the flowcharts depicted inFIG. 8andFIG. 9.FIG. 8illustrates a processing sequence when the BS1receives a connection request from an MS2, andFIG. 9illustrates a processing sequence when the BS1receives a disconnection request from an MS2.

(Upon Reception of Connection Request)

As depicted inFIG. 8, when the communication processing unit11of the BS1receives a connection request from an MS2(Step4-1), the communication processing unit11requests the wide-area communication resource management unit13to allocate a wide-area communication resource (Step4-2) and requests the narrow-area communication resource management unit12to allocate a narrow-area communication resource and a phase.

When the wide-area communication resource management unit13receives the above-described request from the communication processing unit11, the wide-area communication resource management unit13determines the wide-area communication resource to be allocated to the MS2that made the connection request. On the other hand, when the narrow-area communication resource management unit12(control unit122) receives the above-described request from the communication processing unit11, as described above with reference toFIG. 7on the basis of the management table12-3, the narrow-area communication resource management unit12carries out allocation processing of a narrow-area communication resource (time slot) and a phase (transmission pattern, pattern of transmission timing phase) (Step4-3).

Once the allocation processing above is completed, the communication processing unit11transmits a connection response to the MS2that made the connection request (Step4-4). Inclusion of a PT-ID in the connection response enables the MS2to identify a particular transmission pattern in the phase table23-1to be used for carrying out a narrow-area ad-hoc communication when transmission cycle is modified.

Thereafter, the narrow-area communication resource management unit12determines whether or not there are multiple MSs2that use the same narrow-area communication resource (time slot number j) as the narrow-area communication resource (time slot number j) allocated in the above-described allocation processing based on the management table12-3(Step4-5).

When it is determined that there are multiple MSs2as a result of the determination, the narrow-area communication resource management unit12requests the MSs2using the same time slot number j to initiate a transmission cycle control based on the statuses of the MSs2and the phase allocated by the BS1via the communication processing unit11(Yes route from Step4-5to Step4-6). Note that the above TS-ID may be included in the initiation request. On the other hand, when there are not multiple MSs2, the processing is terminated and reception of another connection request is waited (No route of Steps S4-5).

On the other hands, the communication processing unit11of the BS1receives a disconnection request from an MS2(Step5-1), the communication processing unit11requests the wide-area communication resource management unit13to deallocate a wide-area communication resource (Step5-2) and requests the narrow-area communication resource management unit12to deallocate a narrow-area communication resource and a phase (Step5-3). Inclusion of a phase (PT-ID) allocated from the BS1in this deallocation request enables the BS1to identify the narrow-area communication resource (time slot) and the phase (transmission pattern) to be deallocated.

When the wide-area communication resource management unit13receives the deallocation request from the communication processing unit11, the wide-area communication resource management unit13carries out deallocation processing of the corresponding wide-area communication resource. In addition, the narrow-area communication resource management unit12carries out deallocation processing of the corresponding narrow-area communication resource (time slot number j) and the phase using the search logic for the management table12-3that has been described above with reference toFIG. 7based on the PT-ID received from an MS2(Step5-3).

Once that deallocation processing is completed, the communication processing unit11transmits a disconnection response to the MS2that made the disconnection request (Step5-4).

Thereafter, the narrow-area communication resource management unit12determines whether or not there are multiple MSs2that use the deallocated narrow-area communication resource (time slot number j) (Step5-5). When there are not multiple MSs2, the narrow-area communication resource management unit12requests the MS2that uses the time slot number j to terminate the transmission cycle control based on the status of the MS2and the phase allocated by the BS1(from No route of Step5-5to Step5-6), and terminates the processing and waits for another disconnection request when there are multiple MSs2(Yes route of Step5-5).

Note that, when no MS2uses deallocated narrow-area communication resource in the above-described Step5-6, Step5-6(termination request of transmission cycle control) may not be carried out. In addition, following this deallocation procedure, if there are multiple MSs2that use a certain one narrow-area communication resource that is different from the one that has just been deallocated, the narrow-area communication resource deallocated in the above procedure may be allocated to such multiple MSs2. Note that, following this procedure, the BS may reallocate narrow-area communication resources and PT-IDs that have been allocated to the MSs, thereby equalizing the frequency of use of the narrow-area communication resources and the PT-IDs.

(A7) Description of Operation of MS

On the other hand, as depicted inFIG. 10, when an MS2is turned on and the wide-area communication processing unit21does not receive a signal from the BS1, the narrow-area communication processing unit23autonomously selects a narrow-area communication resource and a phase and initiates a narrow-area ad-hoc communication based on the status of the local MS2notified by the status management unit25(Step6-1). In Step6-1, the MS2selects a narrow-area communication resource and a phase at random from a known group of narrow-area communication resources and a phase group that have been allocated in advance.

When the MS2moves and the wide-area communication processing unit21receives a signal from the BS1, the wide-area communication processing unit21transmits a connection request to the BS1(Step6-2). When the MS2receives a connection response to that connection request at the wide-area communication processing unit21, the MS2notifies the narrow-area communication processing unit23of allocation information of the narrow-area communication resource allocated by the BS1(TS-ID) by means of that connection response (Step6-3).

The narrow-area communication processing unit23initiates a narrow-area ad-hoc communication based on the notified allocation information using the narrow-area communication resource allocated by the BS1(Step6-4). In this case, the transmission cycle can be regarded as 100 ms, for example, irrespective of the speed.

Thereafter, when the wide-area communication processing unit21receives an initiation request for a transmission cycle control from the BS1(Step6-5), the wide-area communication processing unit21notifies the narrow-area communication processing unit23of that initiation request. In the case in which the connection response does not include any PT-ID, the narrow-area communication processing unit23autonomously selects a PT-ID and looks up (searches) the management table23-1based on the autonomously selected PT-ID and the status (speed) the local MS2notified by the status management unit25. Thereby, the narrow-area communication processing unit23identifies the narrow-area communication resource and the phase for a narrow-area ad-hoc communication and carries out the narrow-area ad-hoc communication using the narrow-area communication resource and the phase that are identified.

On the other hand, in the case in which a PT-ID is included in the connection response, the narrow-area communication processing unit23will carry out the narrow-area ad-hoc communication the transmission cycle and phase according to the traveling speed using that transmission pattern (Step6-6).

Note that the control described in the description of Step6-6may be carried out without waiting for an initiation request.

Thereafter, when the wide-area communication processing unit21receives a termination request for a transmission cycle control from the BS1, the wide-area communication processing unit21notifies the narrow-area communication processing unit23of that termination request, and the narrow-area communication processing unit23terminates the transmission cycle control based on the status of the local MS2(Step6-7). In addition, the wide-area communication processing unit21determines whether or not the local MS2is leaving out of the communication area1-2of the BS1by receiving the termination request, from the status of radio waves from the BS1, the location information or the traveling direction of the local MS2measured by a GPS, or the like (Step6-8).

When it is determined that the local MS2is leaving out of the communication area1-2as a result of the determination, the wide-area communication processing unit21transmits a disconnection request to the BS1(Yes route from Step6-8to Step6-9), and carries out processing following Step6-1after receiving a response thereto (Step6-10). On the other hand, when the local MS2is not leaving out of the communication area1-2of the BS1(when No in Step6-8), the wide-area communication processing unit21determines whether or not an initiation request for a transmission cycle control has been received and the narrow-area communication processing unit23has initiated a transmission cycle control (Step6-11).

The narrow-area communication processing unit23has initiated a transmission cycle control (when Yes in Step6-11), the processing following Step6-6is carried out. Otherwise (when No in Step6-11), the processing following Step6-4is carried out.

Note that the above processing sequence is terminated when the MS2is turned off, although this is not depicted in the processing sequence inFIG. 10.

In addition, in this example, although an MS2determines whether or not the MS2is leaving out of the communication area1-2of the BS1, such a determination may be difficult to be made depending on the status of radio waves from the BS1and the MS2may leave from the communication area1-2of the BS1without being able to send a disconnection request to the BS1.

In order to address such a case, a configuration is possible in which the BS1and MSs2periodically carry out communications with each other for ensuring that the BS1and the MSs2can communicate with each other. When it is detected that communication is no more available with one MS2, the BS1may terminate communication with the MS2and deallocate the narrow-area communication resource and the phase that are allocated to the MS2. The MS2may terminate the communication with the BS1, and autonomously select a narrow-area communication resource for carrying out a narrow-area ad-hoc communication.

As described above, according to this embodiment, when an MS2changes the cycle of a narrow-area ad-hoc communication that is carried out using a narrow-area communication resource allocated by the BS1according to the status of each MS2(traveling speed), the BS1instructs each MS2to shift the phase (frame number) of the communication cycle of each MS2with respect to each other by allocating a PT-ID (that is, transmission pattern (pattern of transmission timing phase)) to the MS2to prevent a narrow-area ad-hoc communication from being carried out by multiple MSs2to which a part of the same narrow-area communication resource (the same time slot number j) has been allocated at the same timing.

Accordingly, as long as a communication is carried out according to a different phase, any collision (interference) of narrow-area ad-hoc communication can be avoided. That is, even when there are multiple MSs2that use the resource at the same cycle while each MS2changing the cycle of a narrow-area ad-hoc communication according to the traveling speed of the local MS2, it is possible to avoid collision of narrow-area ad-hoc communications since the initiation timings of the cycle are shifted.

In addition, the BS1and the MSs2maintain the same phase table (transmission pattern group)12-2, and a PT-ID that is identification information of a transmission pattern is notified (selection instruction) rather than transmitting the transmission pattern from the BS1to the MSs2. Thus, since it is possible to allocate the transmission pattern, that is, to make the MSs2identify (select) transmission pattern, the amount of communications between the BS1and the MSs2may be minimized, thereby reducing the amount of communications in the entire system. In other words, it is possible to improve the efficiency of resource utilization between the BS1and the MSs2.

Especially, since it is sufficient to carry out allocation of a transmission pattern that the BS1only once when an MS2enters the communication area1-2of the BS1, the amount of communications between the BS1and the MSs2may be minimized.

In addition, when there are multiple MSs2to which a certain time slot number j are allocated, the BS1notifies the MSs2that use the time slot of the time slot number j of the TS-ID (TS #j). When the communication cycle for carrying out the narrow-area ad-hoc communication is changed, the BS1instructs to initiate a modification control of the transmission timing phase according to the PT-ID that has been allocated. Thus, even if there is an MS2that does not change the transmission cycle according to the traveling speed (for example, MS2that carries out a narrow-area ad-hoc communication with a shortest transmission cycle as possible considering safety issues), it is possible to force the MS2to follow the above-described phase control and thus it is ensured that collision of narrow-area ad-hoc communications is avoided.

Furthermore, the BS1instructs termination of a phase control of a communication cycle for carrying out a narrow-area ad-hoc communication when there is only one MS2that uses that time slot number j after instructing the initiation of the phase control. Thus, the MS2may carry out a narrow-area ad-hoc communication at any timing irrespective of transmission patterns. Accordingly, it is possible to carry out a narrow-area ad-hoc communication with a shortest transmission cycle possible considering safety issues.

Although the above-described embodiment assumes that the BS1allocates narrow-area communication resources, narrow-area communication resources may be allocated to each MS2in advance, as in the case in which MSs2that are located outside the communication area1-2of the BS2carry out communication with each other. In such a case, it is possible to reduce the occurrence of collisions of narrow-area ad-hoc communications when each MS2autonomously (for example, at random, or according to a certain condition) selects a PT-ID in the phase table23-1, instead of using PT-IDs notified by the BS1.

In addition, the above described embodiment has been described focusing on the function and operation of the transmission on narrow-area ad-hoc communications (transmission by the MSs2), a phase control similar to the above-described embodiment may be applied for reception (allocation of reception patterns).

The present invention provides at least one of effects or advantages listed below:

(1) Since each timing for each MS2is distributed when each MS2changes the timing of inter-MSs communications (resource usage timing) according to its traveling speed, it is possible to avoid collisions of inter-MS communications.

(2) In addition, the efficiency of resource utilization of the inter-MSs communication may be improved.

(3) In addition, since it is sufficient for the BS1to allocate the timing patterns only once without receiving reports on traveling speeds from MSs2, the amount of communications between the BS1and the MSs2may be minimized, thereby reducing the amount of communications in the entire system.

(4) In other words, it is possible to improve the efficiency of resource utilization between the BS1and the MSs2.

According to the embodiments, efficient usage of a wireless resource employed in a communication between mobile stations can be achieved.

As described in the above in detail, since it is possible to reduce the occurrence of collisions of narrow-area ad-hoc communications (communications between mobile stations), the embodiments are considered quite useful in the technical field of wireless communication, such as the technical field of the ITS.