Abstract:
The present invention relates to a method of synchronizing a plurality of base stations in a wireless telecommunications system ( 100 ) comprising base stations (BS 1  to BS 5 ) interconnected by an IP architecture local area network ( 10 ), the method including a step of hierarchical organization carried out in each group of base stations (BS 1  to BS 4 ) covering interconnected picocells (P 1  to P 4 ) by assigning positive integer numbers (No. 1  to No. 4 ) in increasing order and a fine synchronization step involving listening to a lower numbered base station of its group.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to a telecommunications system comprising base stations, for example of the Digital European Cordless Telecommunications (DECT) type, managing picocells and interconnected by a local area network, in particular an Internet Protocol (IP) local area network. 
   A local area network using the IP as its transmission protocol cannot synchronize DECT base stations with each other and thereby provide handover, i.e. continuity of call transmission when a mobile terminal such as a DECT mobile telephone moves from one cell to another. This is known in the art. 
   The document WO-01-78246 describes a telecommunications system comprising a plurality of base stations using the BLUETOOTH technology for short-range radio communication, the base stations each covering a picocell and being interconnected by a local area network. 
   The above document discloses a method of synchronizing base stations including the following steps:
         a step of coarse synchronization of each base station adjoining an active base station, i.e. one communicating with a mobile terminal, the active base station sending a coarse synchronization signal to the adjoining base stations via the local area network,   a step of sending a reference clock, the active base station periodically sending the reference clock at a high power level in the context of the frequency hopping aspect of the BLUETOOTH technology, and   a step of fine synchronization of an adjoining base station when the mobile is detected in the intersection area between the active picocell and the adjoining picocell, by listening either to the active base station or to the moving mobile station.       

   The base stations are synchronized by tricking a moving mobile terminal into believing that it still in the same picocell so that it retains both the synchronization and the frequencies of the original active base station with the associated frequency hopping law. 
   This synchronization method is complex and must be executed “dynamically”, i.e. immediately handover is required. 
   Furthermore, to effect this synchronization, it is necessary to provide a set of supplementary radio means in each base station adapted to be configured when a mobile terminal is moving. These means must be available for the moving mobile terminal and cannot be used for other tasks. This impacts on the cost of the configurations to be implemented. In particular, this type of technique is not suitable for high traffic levels. 
   Users of a transmission system, for example a professional transmission system, are committed to maximizing the return on the investment in their infrastructures. 
   U.S. Pat. No. 5,519,759 describes a method of synchronizing a plurality of base stations in a DECT wireless telecommunications system comprising base stations each covering a picocell and interconnected by a local area network comprising a plurality of switches. The method includes a system initialization phase including a step, carried out within each group of base stations covering adjoining picocells, of organizing the base stations hierarchically. This hierarchical organization step consists in a first base station of the group concerned broadcasting a synchronization signal, the other stations of the group that are capable of receiving the signal receiving it and measuring its power, transmitting the results of the power measurements to a central server, comparing the results to determine the station that received the highest power, instructing that adjoining station to synchronize to the first station and then to send in its turn a synchronization signal, and repeating the above process until all the stations constituting a group have been synchronized in succession. 
   BRIEF SUMMARY OF THE INVENTION 
   An object of the present invention is to provide a simpler method that is less costly to implement for synchronizing base stations, in particular DECT base stations, interconnected by a local area network, in particular an IP local area network. 
   To this end, the present invention proposes a method of synchronizing a plurality of base stations in a wireless telecommunications system comprising base stations each covering a picocell and interconnected by a local area network, the method including a system initialization phase including a step of hierarchically organizing the base stations carried out in each group of base stations covering adjoining picocells. The method is characterized in that the step of hierarchically organizing the base stations includes assigning positive integer numbers in increasing order to each of the cells in a group of base stations covering adjoining picocells as a function of the distribution of said base stations. 
   The method of the invention, based on synchronizing base stations when initializing the network, relies on the time division multiple access (TDMA) modulation characteristic conventionally used for DECT calls and enabling a plurality of calls to be set up simultaneously on the same frequency. 
   The invention also provides a telecommunications system comprising base stations each covering a picocell and interconnected by a local area network and a synchronization server including means for executing a system initialization phase including a step of hierarchically organizing the base stations carried out in each group of base stations covering adjoining picocells. The system is characterized in that said synchronization server includes means for assigning positive integer numbers in increasing order to each of the cells in a group of base stations covering adjoining picocells as a function of the distribution of said base stations. 
   The system of the invention has the advantage of being transparent for the mobile terminals, necessitating no addition of costly equipment to each base station. The radio transmitters and the radio receivers already present in each base station are respectively used to send the reference clock or the fine clock and to listen to one of the base stations with a lower number. 
   The system preferably includes means for dynamically managing the system initialization phase connected to means for storing the topology of the system and in particular the distribution of the picocells. 
   Each base station includes means for receiving IP messages such as the coarse synchronization signal in the form of packets of data. 
   Each base station can advantageously include IP voice and/or data packet timing means ahead of TDMA radio transmission of the packets to the mobile terminals of the system. The timing means eliminate the effect of the jitter generated by transmission via the local area network, i.e. the uncertainty as to the arrival time of the data packets at the base stations. 
   Of course, the foregoing description has been given by way of illustrative example only. Any means can be replaced by equivalent means without departing from the scope of the invention. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features and objects of the present invention will emerge from the detailed description given hereinafter with reference to the accompanying drawing, which is offered by way of illustrative and non-limiting example. 
     The single FIGURE is a diagrammatic representation of a transmission system  100  of the invention comprising five DECT base stations BS 1  to BS 5  covering respective picocells P 1  to P 5  and interconnected by an IP architecture local area network  10 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   In this preferred embodiment of the invention, four of the base stations BS 1  to BS 4  have picocells connected to each other, each picocell of a base station overlapping at least one other picocell of an adjoining base station. DECT mobile terminals of the system  100 , such as mobile telephones T 1  to T 7 , are able to move around in the picocells P 1  to P 4  covered by that group of base stations. Thus the invention can execute a handover as soon as the system  100  is initialized. 
   Conversely, the fifth base station BS 5  covers an isolated picocell P 5  that functions independently of the others. 
   The system  100  includes a synchronization server  1  including:
         means  2  for storing the topology of the system  100 , and   a coarse synchronization module  3  including:
           means  3   a  for generating IP messages to be transmitted via the local area network to the base stations in particular,   means  3   b  for hierarchically organizing the grouped base stations BS 1  to BS 4 , and   means  4  for dynamically managing the initialization phase of the system  100 .   
               

   The hierarchical organization is defined by assigning positive integer numbers No. 1 , No. 2 , No. 3 , No. 4  in increasing order to the respective base stations BS 1 , BS 2 , BS 3 , BS 4 , the assignment being fixed as a function of the order of the picocells P 1  to P 4 , which is communicated by the means  2 . The hierarchical organization is conceived so that each grouped base station BS 1  to BS 4  can have an adjoining base station with a lower number. By way of example, table 1 below sets out three other arrangements of the numbers, with the same priority base station BS 1 . 
   
     
       
             
             
             
             
             
           
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
               Base 
               Base 
               Base 
               Base 
             
             
                 
               station BS1 
               station BS2 
               station BS3 
               station BS4 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               Configuration 1 
               No. 1 
               No. 2 
               No. 4 
               No. 3 
             
             
               Configuration 2 
               No. 1 
               No. 3 
               No. 2 
               No. 4 
             
             
               Configuration 3 
               No. 1 
               No. 3 
               No. 4 
               No. 2 
             
             
                 
             
           
        
       
     
   
   The system  100  is reinitialized as soon as necessary (reorganization or extension of the system, failure of a priority base station, etc.) so that a slave or isolated base station can become the master station and/or the distribution of the picocells can be changed. It is therefore preferable for all base stations to be equipped with the same means. 
   Each base station BS 1  to BS 5  therefore includes:
         means  11  to  51  for receiving IP messages transmitted via the local area network by voice and/or data packets and coupled to means  11 ′ to  51 ′ for timing the packets to be transmitted by radio,   means  12  to  52  for sending time division multiple access (TDMA) digital radio signals, and   means  13  to  53  for receiving modulated radio signals that are used by the grouped base stations BS 1  to BS 4  to listen to one of the base stations with a lower number.       

   The base stations BS 2 , BS 3 , respectively No. 2  and No. 3 , can listen to the priority base station BS 1  independently. Also, the base station BS 3 , which has a higher number, can be synchronized before the base station BS 2 . 
   The means  4  manage the initialization phase of the system  100  dynamically, by registering on the fly the base stations and their synchronized terminals, managing waiting base stations not yet synchronized, and updating the means  3  and in particular the location tables. 
   To initialize the system  100 , the means  2  establish the optimum shape and dimensions of each picocell and the distribution of the picocells to maximize listening between adjoining base stations. The means  2  then store the meshing of the picocells of the system  100  and construct location tables for the terminals T 1  to T 7  to be synchronized. This information on the topology of the system  100  is accessible to the means  3  and  4 . 
   Consider the execution of a phase for initializing the system  100 . The fine synchronization of the non-priority base stations is symbolized by thick double-headed arrows and that of the mobile terminals by thin double-headed arrows. 
   First of all, each base station BS 1  to BS 5  receives a coarse synchronization signal s g  sent in the form of an IP message by the means  3   a  and broadcast via the local area network  10 . 
   After communicating with the hierarchical organization means  3   b,  the base station BS 1  is identified as the priority base station, and thus station No. 1 , and decides to send via the sending means  12  a reference clock h, in the form of a TDMA radio signal containing synchronization time information. The terminals T 1 , T 2 , T 3  in the picocell P 1  of the priority base BS 1  at this time are also synchronized. 
   In parallel with or consecutively to the priority base station BS 1 , the base station BS 2 , after communicating with the hierarchical organization means  3   b,  is identified as non-priority base station No. 2 . Also, using its means  23  and the information contained in the signal s g , the base station BS 2  opens a receive window that is sufficiently wide to find and capture the reference clock h 1  for its fine synchronization. The sending means  32  then send a fine clock h 2  containing said time information: the terminal T 4  in its picocell P 2  is synchronized. 
   In a variant, the base station BS 2  can also be synchronized by listening to a signaling message updating the priority base station BS 1  and the mobile terminal T 3  in the area of intersection of the picocells P 1  and P 2 , in which case the time information is contained in the signaling frame. In fact, the base station BS 2  is then synchronized indirectly to the priority base station BS 1 . 
   In parallel with or consecutively to the other base stations, the base station BS 3  is identified as non-priority base station No. 3 . The base station BS 3  waits for fine synchronization and listens to the priority base station BS 1 . After receiving the reference clock for its synchronization, the base station BS 3  sends a fine clock h 3  containing said time information: the terminals T 5 , T 6  in its picocell P 3  are synchronized. 
   Finally, in an analogous fashion, the base station BS 4  is identified as non-priority base station No. 4 . Also, it captures the fine clock h 3  sent by the adjoining base station BS 2  of lower number for its synchronization and sends a fine clock h 4  enabling the terminal T 7  in its picocell P 4  to be synchronized. 
   Also, the fifth base station BS 5  sends its own reference clock h 5  to synchronize with the terminals T 8 , T 9  of its isolated picocell P 5 . 
   Of course, the invention is not limited to the embodiment that has just been described. 
   The invention also applies to a set of base stations interconnected by means of the Internet Protocol and using TDMA modulation with a protocol other than that for the DECT. 
   Finally, any means can be replaced by equivalent means without departing from the scope of the invention.