Communication method between at least one subscriber station and at least two base stations

In order to prevent breaks in a communication network comprising at least one subscriber station and at least two base stations synchronised and connected to each other by a communication link, a method is provided comprising a time resource allocation step for the transmission and/or reception of packets by the at least one subscriber station, the time resource being specific to each subscriber station and being able to be used with all the base stations.

This application claims the benefit, under 35 U.S.C. §119, of European Patent Application No. 0655928 on 22 Dec. 2006.

1. SCOPE OF THE INVENTION

The present invention relates to the wireless telecommunications field and more specifically to the passing of a communication associated with a mobile station from one relay station to another (known as handover).

2. TECHNOLOGICAL BACKGROUND

According to the prior art, several wireless network architectures are known. Some of them are based on a centralised architecture. Hence, the Wi-fi system (based on the IEEE 802.11a standard) has a non-centralised architecture with a contention channel access. Such an architecture does not enable Quality of Service (or QoS) to be managed effectively enough for some applications. The Wimax system (based on the IEEE 802.16 standard) has a centralised architecture that allows a more suitable quality of service to be implemented for some applications.

Nevertheless, the techniques implemented in the Wimax networks do not enable a quality of service to be guaranteed for all the applications, for example for video type communications, data being transmitted by wireless cameras moving around in noisy radio-frequency environments, subject to interference or disturbed by obstacles creating signal losses or echoes. Hence, a communication with a wireless station can be cut off suddenly (for example, when the mobile station is moving around).

3. SUMMARY OF THE INVENTION

The purpose of the invention is to overcome the disadvantages of the prior art.

More particularly, the purpose of the invention is to enable the transmission and/or reception of data by at least one wireless station intended for or coming from relay stations, with a guaranteed quality of service and more specifically with an absence of cutting off of the communication (namely, with no loss of packets sent or having to be received by the wireless station or stations) under normal conditions of use.

For this purpose, the invention proposes a communication method implementing at least one subscriber station and at least two base stations synchronised and connected to each other by a communication link. In order to enable a transmission over the communication channel without any cut off, the method comprises a time resource allocation step for the transmission and/or reception of packets by at least one subscriber station, the said time resource being specific to each subscriber station and being able to be used with all the base stations.

According to one advantageous characteristic, for each of the stations in a set comprising at least one part of the subscriber station or stations, the method comprises the following steps:connection of the subscriber station to a first base station, the base station emitting and/or receiving data intended for and/or coming from the subscriber station,handover decision of the connection of the first base station to a second base station, the decision being made by the subscriber station,connection of the subscriber station to a second base station, the base station emitting and/or receiving data intended for and/or coming from the subscriber station.

The method comprises the steps above for a set that comprises a part of the subscriber stations (for example, a single station or a few subscriber stations) and all the subscriber stations. The handover decision being made by the subscriber station concerned, the handover is made advantageously according to criteria specific to the subscriber station (for example, reception quality of the signals sent by each base station, this quality being able to comprise the reception level and/or the error rate after decoding). The connection of a subscriber station to a base station means that the subscriber station can transmit data to this base station, receive data from this base station or both send and receive data intended for and coming from this base station.

Advantageously, during a handover phase, the first and second base stations share the same time resource to send and/or receive data intended for and/or coming from the said subscriber station.

The time resource corresponds to a time slot in a given frequency band. The frequency band can be use both for the upward and downward direction, or, on the contrary, two separate frequency bands are used for the upward direction and the downward direction. The time resource is assigned either to one of the directions or to both directions.

According to a particular characteristic, the first base station transmits data intended for the subscriber station and recorded before the start of the handover phase.

According to another particular characteristic, the second base station transmits data intended for the subscriber station and recorded after the start of the handover phase.

Advantageously, the method comprises a transmission step of data representative of the start of the handover phase.

According to an advantageous characteristic, the method comprises a radio packet transmission step indicating the time slot used for the time resource.

According to a particular characteristic, the method comprises an assignment step of a connection identifier associated with each subscriber station.

The connection identifier is advantageously representative of the base station with which the subscriber station communicates and/or the direction of communication with the subscriber station.

5. DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a communication network comprising a wireless network1and a wired Ethernet network.

The wireless network1comprises one or more wireless stations, fixed or, advantageously mobile. The wireless stations are for example two mobile stations (MS) or subscriber stations SS110and SS211.

The Ethernet comprises a hub14and two nodes15and16connected to each other by an Ethernet link.

Stations10and11can transmit or receive data intended for or coming from the node15via relay stations or base stations (BS) BS112and BS213. The base stations12and13enable the interface between the wireless network1and the Internet network to be provided. Hence, the base station12(respectively13) is connected via a bidirectional wireless link101(respectively102) to the station MS110and111(respectively112) to the link MS211. The network architecture1is such that the network1comprises enough base stations to cover the entire zone in which the mobile stations are likely to be found. Hence, at any time, each mobile station of the network1is connected to at least one base station of the network1by a wireless link enabling a wireless communication to be provided. The base stations12and13are connected to the hub14via an Ethernet link respectively121and131. Hence, for example, if the station SS1is connected to the station BS1, the station SS1can transmit data to the node15via the links101,121and141. One of the nodes15and16can be dedicated to the synchronisation of the base stations and regularly broadcast a synchronisation signal intended for the base stations.

The stations SS1and SS2are for example mobile cameras, equipped with wireless communications means and the node15an image processing system (for example, a video recorder, a studio entry point, etc.). Hence, the network ofFIG. 1enables a continuous transmission (namely uninterrupted) of the videos transmitted by the cameras to the processing system).

Advantageously, the base stations share a same radio frequency channel, the radio spectrum being a resource to economise. The base stations can possibly listen mutually to each other on the radio channel. According to one embodiment, the base stations cannot listen mutually to each other on the radio channel.

Advantageously, the communications used between the nodes of the network of theFIG. 1are of the IP type (Internet Protocol), the SS, BS and the node15each having an IP address. IP is used to transport the flow in streaming mode, for example for transporting video and/or audio in unidirectional or bidirectional mode.

The mobile station2comprises, connected to each other by an address and data bus24, also transporting a clock signal:a microprocessor21(or CPU),a non-volatile memory of the ROM type (Read Only Memory)22,a Random Access Memory or RAM23,a transmission module25of a signal on the wireless link,a reception module26of a signal on the wireless link, andan interface27to an application.

Moreover, each of the elements21to26is well known by a person skilled in the art. These common elements are not described here.

It is noted that the word “register” used in the description of the memories22and23designates in each of the memories mentioned, a memory zone of low capacity (some binary data) as well as a memory zone of large capacity (enabling a whole programme to be stored or all or part of the data representing an audio/video service received).

The application is, for example, of the video type and constitutes respectively the source and destination of the data respectively transmitted and received by the mobile station2(the mobile station2is for example a camera).

The ROM memory22notably comprises a “prog” programme220.

The algorithms implementing the steps of the method specific to the invention and described below are stored in the memory ROM22associated with the mobile station2implementing these steps. When powered up, the microprocessor21loads and runs the instructions of these algorithms.

The random access memory23notably comprises:in a register230, the operating programme of the microprocessor21responsible for switching on the mobile station2,data or PDUs containing this data in a register231,an IP address of the mobile station2in a register232,connection identifiers or CID in a register233,BS identifiers of the network1, an identifier comprising the IP or Ethernet MAC address in a register234, andinformation relating to the quality of reception of the frame and/or packet header transmitted by each BS in a register235.

The base station3comprises, connected to each other by an address and data bus34, also transporting a clock signal:a microprocessor31(or CPU),a non-volatile memory of the ROM type (Read Only Memory)32,a Random Access Memory or RAM33,a transmission module35of a signal on the wireless link,a reception module36of a signal on the wireless link, andan interface37to an Ethernet network.

Moreover, each of the elements21to26is well known by a person skilled in the art. These common elements are not described here.

It is noted that the word “register” used in the description of the memories32and33designates in each of the memories mentioned, a memory zone of low capacity (some binary data) as well as a memory zone of large capacity (enabling a whole programme to be stored or all or part of the data representing an audio/video service received).

The ROM memory32notably comprises a “prog” programme320.

The algorithms implementing the steps of the method specific to the invention and described below are stored in the memory ROM32associated with the base station3implementing these steps. When powered up, the microprocessor31loads and runs the instructions of these algorithms.

The random access memory33notably comprises:in a register330, the operating programme of the microprocessor31responsible for switching on the base station3,data or PDUs containing this data in a register331,data associated with the synchronisation of the base station3in a register332,an IP address of the base station2in a register333,IP addresses of mobile stations of the network1in a register334,connection identifiers or CID in a register335,mobile station statuses (for example, associated or connected to the station3or to another station in a register336, andidentifiers of the other BS or other BSs connected to the BS3(via for example an Ethernet link), an identifier comprising the IP or Ethernet MAC address in a register337.

FIGS. 8 and 9illustrate an example of communication between the mobile station10, the base stations12and13and the node15(these elements are represented by vertical lines; actions, events and/or successive transmissions are chronologically illustrated).

During step10, the node15transmits synchronisation signals700and701to each of the base stations12and13. To illustrate this, it is the same node15that enables the synchronisation of the base stations and that receives or transmits the data coming from or intended for the mobile station10. According to one variant, it is the node16that enables the synchronisation. Moreover, the signals700and701are advantageously combined into a single signal (broadcast signal).

The different base stations are synchronised with a precision slot Δ compatible with the size of the frames. Δ is advantageously less than 500 μs and even more advantageously less than 50 μs.

Several synchronisation mean can be considered:a synchronisation via the Ethernet network (as illustrated inFIG. 8) according, for example, to an IEEE1588 protocol PTP (Precision time protocol) that enables a precision in the order of 1 μs,a synchronisation via a radio transmission controlled by an atomic clock, a clock being locked onto a radio transmission over a wide geographic coverage (for example a country). It may be the radio clock transmitter of Anthorn in the United Kingdom at 60 kHz, the radio clock of Mainflingen in Germany at 77 kHz or Fort Collins in the USA at 60 kHz,a synchronization via an Ethernet/IP network according to an IP based protocol based on NTP which can supply a time base with a precision in the order of a few hundred microseconds.

For the synchronization with the other base stations, each base station receives a signalling signal to generate a start pulse for each radio frame carrying a same number and transiting on the wireless network1. Hence, each base station can emit its own frame in a slot allocated in a synchronized manner with the others. A single base station emits at a given moment, the transactions not intersecting.

This is illustrated byFIG. 4that chronologically describes the emission and reception of a frame4numbered N. The frame4is divided into two slots40and41corresponding respectively to the downward direction, base station to mobile station and to the upward direction, mobile station to base station.

The slot40comprises:one part reserved for frame headers in the time slots assigned to each BS,one part reserved for the transmission of data to the mobile station or stations connected in the time slots assigned to each BS, anda part (not shown) enabling exchanges in contention mode (notably to enable mobile stations not associated or not connected to do it).

In the first part of the slot40, the station BS1first emits a frame header or FH400. Next, the station BS2emits a frame header or FH401. The start of the header emission400(respectively401) corresponds to the start of a slot allotted to BS1(respectively BS2) with the precision A. Each FH comprises the time mapping of resource allocations by connection in the current frame. The assignment of slots for the FHs is unremarkable (for example determined according to the MAC address of the BSs or to the order of declaration in the network). When an MS is associated with the BS, the BS receiving the association request (or by another BS (for example a BS according to a variant)) allocates in a non-equivocal manner time resources for the transmission and/or reception of data packets. A time resource is assigned to a single MS (for example, if a single BS allocates the resources or if an allocation mechanism shared between several BSs prevents two separate MSs from being allocated a same time resource).

A CID is associated with each upward or downward connection between an MS and a BS. Hence, each CID identifies precisely a connection between an MS and a BS as well as the downward or upward direction (two separate connections therefore have a different CID). According to one variant, the CID is the same for the upward and downward direction. When a mobile station is associated with a base station, it is automatically associated with the other base stations. Advantageously, the other base stations deduce from the first CID allocated for an association operation, according to a predefined law, the CIDs associated with the connection with themselves of the mobile station concerned and for each communication direction. According to one variant, the first base station attributing a CID also attributes the CIDs for the connections of the relevant mobile station with the other base stations. According to another variant, each base station and mobile station can deduce the CIDs from the first CID attributed. The predefined law is based for example on parts of the CID identifying respectively the base station, the mobile station, the direction and, possibly, a particular association (if particularly several connections concern for a given direction a same mobile station and a same base station). As an illustration, for SS1, a CID is equal to:14 for a BS1connection to SS1,15 for an SS1connection to BS1,24 for a BS2connection to SS1, and25 for an SS1connection to BS2,
Hence, if CID is noted in hexadecimal mode, the first quartet is associated with a BS (here 1 for BS1and 2 for BS2) and the first three bits of the second quartet are associated with the SS (here 4) and the last bit is associated with the direction (1 for SS to BS and 0 for BS to SS). Advantageously, all the base stations and all the mobile stations know the CID assignment law according to a first CID and can therefore effectively manage (for the listening and the transmission of messages with specific CIDs as well as for creation) the CIDs (this is the role of the classifier).

It is assumed that during frame4, SS1(respectively SS2) exchanges data with BS1(respectively BS2). Hence, in the part reserved for the transmission of data from BSs to SSs, BS1first transmits to SS1in a slot402assigned to the connection with SS1, data with the CID14. Then, BS2transmits to SS2in a slot403assigned to the connection with SS2, data with the CID26.

The slot41comprises a part reserved for the transmission of data by the mobile station or stations connected in polling mode, time slots being assigned to each MS. Hence, in slot41, SS1first transmits to BS1in a slot410assigned to the connection with SS1, data with the CID15. Then, SS2transmits to BS2in a slot411assigned to the connection with SS2, data with the CID27.

When each BS is synchronised, it is synchronised again regularly (for example every second) or each time that it is necessary.

After the synchronisation phase70,FIG. 8illustrates an association and connection phase71.

As an illustration, this phase begins with the transmission of an association request710by SS1to BS1. This message is transmitted during the contention period in the slot40. During one of the frames that follow, BS1sends a frame header comprising a message711of a response type for the association. The format of the messages710and711is for example such as defined according to the standard IEEE 802.16.

Then, the SS1having chosen BS1as base station to which it wants to connect, it transmits a connection request711to BS1during the contention period. According to one variant, BS1consults SS1in polling mode to enable SS1to emit, for example, a connection request711.

Then, BS1determines a CID during a step713: a CID worth14is associated with the connection of BS1to SS1. BS1then transmits an information message714to BS2, this message containing the parameters of the connection, notably with the CID. BS2thus determines a CID during a step716according to the message714: a CID worth24is associated with the connection of BS2to SS1. After the step713, BS1also transmits a connection message715to SS1with the identifier corresponding to CID worth14. SS1can then record this value and from it deduce the value of the CID corresponding to the BS2connection to SS1or to the connections SS1to BS1and SS1to BS2. According to a variant of the invention, the CIDs corresponding to the different connections are explicitly requested from the BSs via similar messages to the message712.

After the phase, SS1is linked to BS1and can receive or transmit data. Hence, SS1transmits during the slot15a message720associated with the CID worth24and containing data with a destination address corresponding to the IP address of the node15. BS1then transmits to node15the content of the message720to the node15via the link121. The node15transmits via the link141a message722received by BS1and BS2and whose destination address is the IP address of SS1. BS1being connected to SS1, BS1transmits the content of the message722to SS1during one or more slots402with a CID worth14.

After the phase72, SS1requests a connection handover from BS1to BS2during a phase73.

Phase73is illustrated byFIG. 5that chronologically describes the emission and reception of a frame5numbered N+1. The frame5is divided into two slots50and51corresponding respectively to the downward direction, base station to mobile station and to the upward direction, mobile station to base station.

The slot50comprises parts similar to the parts reserved for the slot40.

In the first part of the slot50, the station BS1first emits a frame header500. Next, the station BS2emits a frame header501.

In the part reserved for the transmission of data from the BSs to the SSs, BS1first transmits to SS1in a slot502assigned to the connection with SS1, data present in its buffer memories with the CID14(this data corresponding, for example, to packets or parts awaiting positive acknowledgment, a negative acknowledgement having previously been received or no positive acknowledgement having previously been received). Then, BS2transmits to SS1in a slot504assigned to the connection with SS1, with the CID24of data coming from the node15. The slot402encompasses the slots502and504. The slots502and504do not intersect. The slot502is advantageously smaller than504(the duration of502is, for example, less than 1/10thof the time of504, or even less, the buffer memories of BS1generally containing little amounts of data). The sharing of the slot402in slots502and504follows a determinist diagram (for example a fixed ratio for the connection with the old BS during a predetermined number of frames or even according to an exchange protocol between the BS concerned or with an arbitrating BS This sharing can also take into account the quantity of data in the buffer memories to empty. Next, BS2transmits to SS2in a slot503assigned to the connection with SS2, data with the CID26coming from the node15.

The interval51comprises a part reserved for the transmission of data by the mobile station or stations connected in polling mode, time slots being assigned to each MS. Hence, in the slot51, SS1first transmits to BS1in a slot512, the data present in its buffer memories with the CID15(for example data awaiting acknowledgement) then in a slot510of new data destined for node15to BS2. Then, SS2transmits to BS2in a slot51assigned to the connection with SS2, data with the CID27. The slot412encompasses the slots512and510. The slots510and512do not intersect. The slot512is advantageously smaller than510(the duration of512is, for example, less than 1/10thof the time of510, or even less, the buffer memories of SS1generally containing little amounts of data destined for BS1after the start of the handover). The sharing of the slot412in slots512and510follows a determinist diagram (for example a fixed ratio for the connection with the old BS during a predetermined number of frames or even according to an exchange protocol between the BS concerned or with an arbitrating BS This sharing can also take into account the quantity of data in the buffer memories to empty.

Phase73is initiated by the transmission of a specific message730from SS1to BS1and BS2, this message indicating the frame number from which the handover is effective (here the number N+1, the specific message730being transmitted for example 10 frames beforehand).

BS1thus transmits a message731to BS2to inform it of the number (for example number of an IP frame) of the last packet that BS1is responsible for transmitting to SS1. BS2will transmit the following packets to SS1. The message731marks the start of the handover phase from which the data reaching BS1and BS2, are no longer recorded by BS1but by BS2. During the handover phase, BS1empties its buffer memory by transmitting the data to SS1, whereas BS2begins to send the data to SS1. BS1and BS2share the time resource allocated to SS1for each direction.

The node15transmits via the link141a message732received by BS1and BS2and whose destination address is the IP address of SS1. When the handover is made, BS1empties its buffer memory by transmitting to SS1the data corresponding to the messages733(associated with the slot502) corresponding to CID worth14whereas BS2transmits the data transmitted by the node15in messages734corresponding to CID worth24(in the slot504).

SS1transmits during the slot512a message735associated with the CID worth15and containing data present in its buffer memories before the handover and with a destination address corresponding to the IP address of the node15. Then, SS1transmits during the slot510a message736associated with the CID worth25and containing data generated after the handover and with a destination address corresponding to the IP address of the node15. BS1and BS2thus transmit the data received from SS1to the node15in the messages respectively737and738.

After the handover phase73, SS1is linked to BS2and can receive or transmit data via BS2in a phase74illustrated inFIG. 9.

FIG. 6chronologically described the emission and reception of a frame4numbered N+x (where x is worth for example 2 to 5). Frame4is divided into two slots60and61corresponding respectively to the downward and upward direction.

The slot60comprises the headers600and601similar to the headers respectively400and401. The slot402is replaced by a slot602assigned to the connection between BS2and SS1with a CID worth24. The slot600also comprises a slot603similar to the slot403.

In the slot61, SS1first transmits to BS2in a slot610assigned to the connection with SS1, data with the CID25. Then, SS2transmits to BS2in a slot611assigned to the connection with SS2, data with the CID27.

As shown inFIG. 9, SS1transmits during the slot610a message742associated with the CID worth25and containing data with a destination address corresponding to the IP address of the node15. BS1then transmits to the node15the content of the message742to the node15via the link121. The node15transmits via the link141a message740received by BS1and BS2and whose destination address is the IP address of SS1. BS1being connected to SS1, BS2transmits the content of the message740to SS1during one or more slots602with a CID worth24.

FIG. 7summarises the handover processing method. This method begins with an initialisation phase90during which the different parameters of the network are updated.

Then during a step91, each SS of the network1is associated with a BS.

Next, during a step92, each SS of the network1is connected to a BS.

Next, during step93, each SS manages its connections to the BSs by choosing the BS to which it wants to be connected and by handing over to another BS than the one to which it is connected when a change is required.

FIG. 10illustrates the management of the communication between the BSs, as implemented in SS1.

This management begins with an initialisation phase1000during which the SS2initialises the different parameters and useful variables.

Then, during a step1001, SS1measures the level or the quality of reception of the FHs sent by each of the BSs. Each BS indeed emits FHs even if no SS is connected.

Then, during a step1002, SS1selects the BS that corresponds to the best reception.

Next, during a step1003, SS1is associated and connects to a BS as illustrated inFIG. 8.

Then, during a step1004, SS1receives data (transmitted by a BS) that the CPU sends to the application and transmits data generated by the application to the BS to which it is connected.

Next, during a step1005similar to step1001, SS1assesses the level or the quality of reception of the FHs sent by each of the BSs.

Then, during a test1006, SS1checks whether or not a handover to another BS is necessary.

In the affirmative case, during a step1007, SS1carries out a handover operation to the BS determined during the test1006.

After the step1007or if the result of the test1006is negative, the step1004is reiterated.

FIG. 11illustrates the management of the connections with the SSs, as implemented in each BS.

This management begins with an initialisation phase1100during which the BS initialises the different parameters and useful variables.

Then, during a step1101, the BS waits then receives association and connection requests.

Next, during a test1102, the BS checks whether the association and connection requests are derived directly from a SS.

In the affirmative, during a step1103, after receiving a connection request from the SS having requested the association, the BS assigns a CID to the accepted connection. Then, during a step1105, the BS transmits via an Ethernet link the CID corresponding to the connection.

If the result of the test1102is negative, an information sent by another BS and indicating a connection between a SS and this other BS is received. The BS thus assigns, during a step1104a CID according to the CID associated with the connection with the other BS.

Next, during a test1106, the BS checks whether a handover announcement is received. In the affirmative case, during a step1107, the BS processes this handover by determining the packet from which the handover will be effective and the BS to which the handover is made is informed of this.

After step1107or if the result of the test1106is negative, during a test1108, the BS checks whether a connection announcement transmitted by another BS is received. In the affirmative case, during a step1109, the BS assigns a CID according to the CID associated with the connection with the other BS.

After the step1107or if the result of the test1108is negative, the BS verifies whether association and connection requests coming from an SS are received.

In the negative case, the test1106is repeated. In the affirmative case, the step1103is repeated.

Naturally, the invention is not limited to the embodiments previously described.

In particular, the architecture of the mobile stations and base stations can be different from the architectures illustrated inFIGS. 2 and 3, in the respective function and/or form of the elements (the functions of the electronic elements can notably be grouped into a restricted number of components or, on the contrary, expanded into several components) and their layout.

The invention is not limited to an architecture as described with respect toFIG. 1but involves any architecture implementing a wireless network with local (for example a few tens of meters) or remote (for example a few kilometers according notably to a standard IEEE 802.16) coverage with one or more SS, each SS being connected at any time to at least one BS. According to one variant, the link between the BSs and/or between the BSs and the destination and/or source node is a wireless link (local or remote link).

The invention can also be applied with different communication protocols than those described above. Hence, the control data can be transmitted according to any protocol (for example with a contention access or in polling mode). The communication channels between the SS and the BS can use the same frequency channels for the upward and downward directions (mode known as “half duplex”) or different frequency channels (mode known as “full duplex”).