Patent Publication Number: US-2007123274-A1

Title: Wireless communications network and method of operation thereof

Description:
FIELD OF THE INVENTION  
      The present invention relates to wireless communications networks, in general, and in particular, to a communications network and a method for assigning a mobile station to a traffic channel.  
     BACKGROUND OF THE INVENTION  
      In a cellular communication system (like for example the Global System for Mobile communication (GSM) and TETRA (TErrestrial Trunked RAdio)), a geographical region is divided into a number of cells each of which is served by a base station. One base station, also termed as Base Transceiver Station (BTS) with a switching infrastructure is referred as a node of the network. The switching infrastructure supporting BTS may be shared with other BTSs. The system infrastructure in a TETRA system is generally referred to as a switching and management infrastructure (SwMI), which substantially contains all of the communication elements apart from the MSs. This includes base transceiver stations (BTSs) connected to a conventional public-switched telephone network (PSTN) or internet through base station controllers (BSCs) and mobile switching centres (MSCs). In geographical terms reference to a node is a reference to an area served by one BTS, and in network terms reference to a node is a reference to one BTS and supporting it infrastructure. A remote unit referred also as subscriber or mobile station (MS) or communication unit is served via a radio communication link by the base station of the cell within which the remote unit is operating.  
      One of the basic advantages of the cellular communications systems is that subscriber may move from one geographical location to another one while receiving services from the network. To provide seamless service there are regions of overlapping coverage of two base stations. As the subscriber moves from area served by a first base station towards area served by a second base station it enters the region of overlapping coverage. Within the region of overlapping coverage the subscriber changes the serving base station. This is known as cell reselection or handover.  
      The communication link from a BTS to a subscriber is generally referred to as a downlink communication channel. Conversely, the communication link from a subscriber to a BTS is generally referred to as an up-link communication channel.  
      To enable communication between two handsets operating in two different cells a fixed network interconnects the base stations. The fixed network is operable to route data between any two base stations and this way allows for communication between these two remote handsets. In addition, the fixed network may comprise gateway functions for interconnecting to external networks such as the Public Switched Telephone Network (PSTN) or the internet. This allows subscribers to communicate with landline telephones and other communication terminals connected by a landline. Additionally the fixed network is adapted to perform functions required for managing a conventional cellular communication network including routing data, admission control, resource allocation, subscriber billing, mobile station authentication etc.  
      The TETRA communications system may be used as a public cellular communication system. However its main target group are organizations or groups such as emergency services. Special functions and services implemented in the TETRA system make this system especially suitable for services like police, emergency, fire rescue or others. One of such features provided by TETRA system, which is especially useful for the emergency services is controlling group calls as well as managing the membership of these groups. Other features and services provided by TETRA include, push-to-talk channel allocation, broadcast calls etc. In addition to trunked mode operation wherein remote units communicate via a base station, TETRA provides for the possibility of communication directly between remote units without participation of the infrastructure. This is known as Direct Mode Operation (DMO).  
      One of the important issues related to providing communications in cellular networks is mobility management. Main goal of the mobility management is providing and keeping up-to-date the infrastructure with information on location of the subscribers (i.e. in which cell particular subscriber is operating).  
      It is known that the range of any digital radio system greatly depends on the bit rate. This is because radio transmitters are usually power limited, so the more bits to send, the less energy can be devoted to each bit. As the receiver depends on the received energy per bit, this naturally requires less path loss for higher bit rate, and hence shorter range. When adding a high speed data overlay to an existing voice or low speed data network, the range of the overlay will therefore typically be less than the existing network. This means that “filler” cells must be added to the network in order to provide carpet coverage—increasing the number of cells by a factor of 2-3. With an air interface such as the TETRA TEDS (TETRA Enhanced Data Service) high speed data, this means adding 2-3 times more 25 kHz pi/4 Differential Quaternary Phase Shift Keying (DQPSK) main carriers as well. This latter point will have severe impact on the existing network planning, or will require significant air interface resources without providing much benefit.  
     SUMMARY OF THE INVENTION  
      According to a first aspect of the present invention there is provided a wireless communications network as claimed in claim  1 .  
      According to a second aspect of the present invention there is provided a method of assigning a mobile station to a traffic channel in a wireless communications network as claimed in claim  10 .  
      The present invention beneficially allows for increasing data speed in a communication network while minimizing resources necessary to provide the service in the network (no additional control channels are required). 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:  
       FIG. 1  is a simplified diagram illustrating a wireless communication network in one embodiment of the present invention,  
       FIG. 2  is a flow chart illustrating a method of operating a wireless communication network in one embodiment of the present invention. 
    
    
     DESCRIPTION OF AN EMBODIMENT OF THE INVENTION  
      The following description focuses on an embodiment of the invention applicable to a TETRA cellular communication system. However, it will be appreciated that the invention is not limited to this application but may be applied to many other communication systems (e.g. ASTRO/APCO 25).  
      Referring to  FIG. 1  one embodiment of the communications network  100  is shown.  
      The nodes  116 - 128  of the network operate with an increased bit rate of traffic channels. This increase is achieved by reducing energy-per-bit factor. When increasing bit rate of an existing voice or low speed data network, the range of the traffic channels affected by said increase will therefore typically be less than before said increase. This range reduction is a result of a power limit of a transmitter (more bits to be sent using the same energy). By increasing data speed in a network the changes affect the traffic channels and not the control channels. In result the range RCC of control channel remains on the same level whereas range RTC 1  of traffic channels will be reduced. To compensate the reduced range additional sites, in form of overlay, must be added.  
      The network  100  comprises a plurality of sites  102 - 114  each of which is served by a node  116 - 128 . Said nodes  116 - 128 , are parts of the network  100  infrastructure and form with other elements of the network infrastructure (not shown on for the sake of clarity) a switching and management infrastructure (SwMI). The network also comprises a plurality of mobile stations  130 ,  134  that operate in different sites of the network  100 . In one embodiment, a first site  102  has a first control channel  140 , whereas, remaining sites  104 - 114  do not have control channels. In this embodiment by increasing data speed in a network the changes affect the traffic channels and in result a range of the traffic channels RTC 1 -RTC 3  is reduced and less than a range RCC of the first control channel  140 , which is not affected by said data speed increase.  
      Said SwMI is adapted, using said first control channel  140 , to direct a mobile station  130 ,  134  located in a site without or with a control channel to a traffic channel depending on a geographical location of said mobile station  130 ,  134 .  
      To direct the mobile station  130 ,  134  to a traffic channel the SwMI must be informed in a range of which node the mobile stations  130 ,  134  operate.  
      In one embodiment, if the mobile station  130  is equipped with a Global Positioning System (GPS) unit, the SwMI obtains said GPS location data and uses it for directing the mobile station  130  to a traffic channel.  
      In alternative embodiment said SwMI obtains said geographical location of said mobile station  134  by analyzing signals received from said mobile station  134 . Said SwMI calculates the position of the mobile station  134  based on an angle of arrival or RSSI or triangulation or any combination thereof. It is clear for the specialist in the art how to apply the mentioned methods.  
      It is clear that in alternative embodiments the number of sites with control channel can be higher than one.  
      Referring to  FIG. 2  one embodiment of a method of assigning a mobile station to a traffic channel in a wireless communications network is shown.  
      If a mobile station  130  is within a site, which does not have a control channel, but within a range RCC of a first control channel  140  said first control channel  140  can be used to direct the mobile station  130  to a traffic channel, in most situations it is a traffic channel provided by a node which is closest to the mobile station  130 . However in specific situations caused, for example, by topography of the area in which the mobile station operates, the traffic channel need not be provided by the closest node, but by a node providing a traffic channel of the highest signal strength.  
      When the mobile station  130  using the first control channel  140  requests connection to a traffic channel the SwMI determines  204  geographic location of the mobile station  130  to identify which node should provide said traffic channel to said mobile station  130 .  
      In one embodiment, if the mobile station  130  is equipped with a GPS unit  206 , the mobile station  130  transmits to the SwMI, using the first control channel  140 , its location obtained via GPS channel  144 . When said location of said mobile station  130  is determined  210  the SwMI identifies the node which should provide said traffic channel to said mobile station. Finally a traffic channel  142  is assigned  212  to provide communication between said mobile station  130  and said node  128  (and in consequence with remaining part of the network and also with other communications networks).  
      In alternative embodiment, when GPS signal is not available, or when the mobile station does not have GPS unit the SwMI calculates  208  location of the mobile station  134  based on some parameters of a radio signal received from said mobile station  134 . It may be an angle of arrival of the signal from said mobile station  134  or Received Signal Strength Indication (RSSI), or triangulation, or any combination thereof.  
      In various embodiments the calculations of location of the mobile stations, and other functions of the SwMI, may be carried out by a Base Transceiver Station, a Base Station Controller or by a Mobile Switching Centre or by a stand-alone computing device operably connected to a fixed network that interconnects the infrastructure of the network  100 .