Abstract:
In a system handling communication between base transceiver stations (BTS 1 -BTS 12 ) and base station controllers (BSC) of a cellular radio network the transmission capacity is composed of a predetermined number of capacity units (T 0 -T 31 ). At least one capacity unit (TCH) can be allocated to a given base station group which comprises at least two base transceiver stations. Said capacity unit is allocated to a certain base transceiver station in the base station group when the instantaneous volume of traffic handled by said base transceiver station requires temporary allocation of additional capacity, and said capacity unit is again released so as to be allocatable to the base station group when the instantaneous volume of traffic handled by said base transceiver station no longer requires temporary allocation of additional capacity.

Description:
This is a national stage of PCT application no. PCT/FI98/00821, filed on Oct. 22, 1998. Priority is claimed on that application, and on patent application no. FI 974034, filed on Oct. 23, 1997. 
    
    
     The invention relates to a method defined in the preamble of claim  1  for distributing the capacity of a transmission system in a base station network, which method makes it possible to increase the number of base stations using a given transmission system and/or improve the utilization rate of the transmission system. The invention also relates to a transmission system applying such a method. 
     Communication between a base station controller (BSC) and the base transceiver stations (BTS) controlled by it in a GSM (Global System for Mobile telecommunications) network, for example, are usually arranged as follows: Transmission is realized using bi-directional time-division-based 2-Mbps systems. A system includes thirty-two 64-kbps time slots each of which can be divided into four 16-kbps partial time slots. One (point-to-point) or several base transceiver stations may be connected to such a system in a chain, multidrop, loop or star configuration. Base transceiver stations have one or more transmitter/receiver (TRX) units which comprise eight 16-kbps bi-directional traffic channels (TCH). For each TRX unit the transmission system allocates in a fixed manner two time slots for traffic channels and one 16-kbps partial time slot for TRX signalling (TRXSIG). In addition, the system reserves in a fixed manner for each base transceiver station one 16-kbps partial time slot for operation and maintenance unit signalling (OMUSIG). Thus, one transmission system suffices for 12 TRX units. In this maximum case only a few partial time slots are left unused; the exact quantity depends on how many base transceiver stations the TRX units are divided into. There are also arrangements in which the traffic of 10 TRX units is placed in the 2-Mbps system. Furthermore, there are arrangements in which some of the transmission system time slots contain GSM traffic and some contain NMT (Nordisk MobilTelefon) traffic or the traffic or paging traffic of some other cellular radio system. 
     The method according to the prior art and a system embodying it are disclosed e.g. in a Nokia Telecommunications document “TRUA Base Station Transmission Unit, Product Description”, NTC C33315002SE_B0, Nokia Telecomnunications Oy 1995-1996. FIG. 1 a  shows a possible base station network using a 2-Mbps transmission system. The base transceiver stations in it are chained through a cable originating from a base station controller  101 . Base transceiver station BTS 1  ( 102 ) has six TRX units sectored e.g. in such a manner that each of the three sectors has two TRX units. Base transceiver stations BTS 2 , BTS 3  and BTS 4  each have two omnidirectional TRX units. The interface unit  103  in base transceiver station BTS 1  connects in bi-directional manner time slots T 1 -T 12  to the radio channels of the TRX units. Placement of traffic channels TCH in the time slots is shown in more detail in FIG.  2 . Rows in the table correspond to time slots T 0 -T 31  and X indicates that the partial time slot in question is unused. Similarly, base transceiver station BTS 2  reserves time slots T 13 -T 16 , BTS 3  time slots T 17 -T 20 , and BTS 4  time slots T 21 -T 24 . 
     Separate time slots must be allocated for signalling (TRXSIG) and operation and maintenance (OMUSIG). In the exemplary case, base transceiver station BTS 1  uses for these purposes time slots T 25 , T 26  and T 27 , BTS 2  uses time slot T 28 , BTS 3  time slot T 29  and BTS 4  time slot T 30  in accordance with FIG.  2 . 
     The base station network shown in FIG. 1 a  has a chain topology. In FIG. 1 b , the base station network has a star topology as a connection is branched from base transceiver station BTS 3  to two other base transceiver stations BTS 5  and BTS 6 . TRX units are distributed between the base transceiver stations slightly differently from FIG. 1 a  in order to keep their total number the same. In FIG. 1 c  the base station network has a loop topology as a direct communications connection is provided between base transceiver station BTS 4  and the base station controller BSC. The loop topology is used in prior-art base station networks mainly for securing communications as in this configuration all base transceiver stations in the base station network have two alternative communications connections with the base station controller (the alternative connections are in the opposite directions of the loop formed by the base transceiver stations). FIG. 3 schematically illustrates a base transceiver station  300  in such a loop-configured base station network. Communication between the base transceiver station  300  and other apparatus in the same base station network takes place through a transmission unit  301  (TRU). The transmission unit  301  is a cross-connect in which a certain branching table (not shown) determines how the various time slots are connected straight through the transmission unit  301  from left to right (or from right to left) and which time slots are connected via the lower part of the transmission unit  301  to the base transceiver station control functions (BCF) part. Through the latter, the transmission capacity represented by the time slots is distributed between the TRX units  302  and  303  of the base transceiver station. In accordance with the usual practice, FIG. 3 shows the various time slots as separate signal lines although in reality they are transferred via the same physical connection. This example assumes that six time slots are connected straight through the transmission unit  301  (lines  304 ) and two time slots are connected to the base transceiver station&#39;s TRX units  302  and  303  (lines  305  and  306 ). 
     In FIG. 3 the transmission unit  301  comprises two so-called Y-type protection switches  307  and  308  by means of which the system utilizes the loop topology of the base station network. The transmission unit monitors the so-called pilot information accompanying the signals coming from the different transmission directions and determines whether the time slots used by the base transceiver station&#39;s TRX units  302  and  303  should be routed via the left-hand-side path or via the right-hand-side path between the base transceiver station and base station controller. In FIG. 3, the transmission unit has detected that the time slots represented by lines  305  and  306  should be transmitted via the left route, so the protection switches  307  and  308  have been set so as to connect the base transceiver station&#39;s TRX units  302  and  303  to the left branches of lines  305  and  306 . In case of a different measurement result one or both of the protection switches  307  and  308  could be set into the other position indicated by the broken line, in which case the traffic in the time slot in question would be routed via the right-hand-side path in the base transceiver station  300 . Setting of the protection switches  307  and  308  is realized such that a change is made in the current branching table in the transmission unit  301 . It is obvious that in this context the directional terms “left”, “right” and “down” only refer to the orientation shown FIG.  3  and bear no relation to the actual situation. 
     A disadvantage of the present method is that the transmission system reserves capacity for the base transceiver stations&#39; TRX units according to the maximum traffic, regardless of the actual traffic situation. Thus, at times, the network operator has to pay for unnecessary transmission capacity. A further disadvantage of the present method is that if additional mobile communications capacity has to be built in a given area to such an extent that the number of TRX units exceeds 12, the operator has to provide a new, even more underutilized transmission system. 
     The object of the invention is to reduce the disadvantages mentioned above. The method according to the invention is characterized by what is expressed in the independent claims. 
     The basic idea of the method is as follows: At least part of the time slots in the transmission system are shared by the base transceiver stations and their TRX units. A given time slot or partial time slot can at different moments be allocated to different TRX units according to the traffic situation. Some of the traffic time slots are allocated to the TRX units in a fixed manner and the rest are shared, or all time slots are shared. In the latter case, too, it is preferable to allocate fixed partial time slots for TRX signalling (TRXSIG) and operation and maintenance signalling (OMUSIG). 
     It is thus an advantage of the invention that the capacity of the transmission system can be utilized more efficiently, because it can always be directed to those TRX units, base transceiver stations and areas which have the most traffic. Compared to the current practice, more TRX units can be attached to the transmission system. This is significant, especially in the case where the network operator has to lease the transmission connections. Consequentially, it is a further advantage of the invention that as the amount of traffic increases in a given area, the introduction of a new transmission system can be postponed, as compared to the current practice. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in detail. In the description, reference will be made to the accompanying drawings wherein 
     FIGS. 1 a - 1   c  are examples of known base station networks, 
     FIG. 2 illustrates the current usage of transmission system time slots in a network according to FIG. 1 a,    
     FIG. 3 shows a known base transceiver station in a loop-configured base station network, 
     FIG. 4 shows the network of FIG. 1 a  expanded such that part of the transmission system capacity is shared by the TRX units in accordance with the invention, 
     FIG. 5 illustrates the usage of transmission system time slots in accordance with the invention in a network according to FIG. 4, 
     FIG. 6 shows a network expanded such that the whole traffic capacity of the transmission system is shared by the TRX units in accordance with the invention, 
     FIG. 7 illustrates the usage of transmission system time slots in the network of FIG. 6, 
     FIG. 8 illustrates the principle of allocating a time slot or partial time slot, 
     FIGS. 9 a  and  9   b  show a base transceiver station applying the principle according to the invention, 
     FIG. 10 shows a base station network applying the principle according to the invention, and 
     FIG. 11 shows a second base station network applying the principle according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 4 and 5 illustrate an example of the method according to the invention. One base transceiver station BTS 5  with two TRX units and one base transceiver station BTS 6  with one TRX unit employing the transmission system have been added to the network. Similar to the prior art, some of the transmission system time slots are allocated in a fixed manner to TRX units, but certain time slots are always shared by two TRX units. Time slots T 1 -T 8  ( 401 ) are allocated in a fixed manner to four TRX units in base transceiver station BTS 1 , and time slots T 9  and T 10  are allocated to one TRX unit in BTS 2 . Likewise, time slots T 11 -T 16  are allocated to one TRX unit in base transceiver stations BTS 3 , BTS 4  and BTS 5 , and time slots T 17  and T 18  are allocated to the only TRX unit in BTS 6 , as shown in FIG.  5 . Time slots T 19  and T 20  are initially allocated to TRX  4  ( 402 ) in base transceiver station BTS 1  and TRX  10  in BTS 3 . Similarly, time slots T 21  and T 22  are initially allocated to TRX  6  in BTS 1  and the other TRX unit ( 403 ) in BTS 4 . Further, time slots T 23  and T 24  are initially allocated to the other TRX units in base transceiver stations BTS 2  and BTS 5  in accordance with FIG.  5 . 
     Let units TRX  1  and TRX  2  in base transceiver station BTS 1  represent sector  1 , TRX  3  and TRX  4  sector  2 , and TRX  5  and TRX  6  sector  3 . If, for example,  8  traffic channels are not enough at a given moment in sector  2 , base transceiver station BTS 1  reserves time slot T 19  if it is free. In this situation, base transceiver station BTS 3  can use at most 12 traffic channels (8+4). If 12 channels are not enough in sector  2  of base transceiver station BTS 1 , the base transceiver station also reserves time slot T 20  if it is free. This would leave only eight channels for base transceiver station BTS 3 . Likewise, BTS 3  may reserve time slots T 19  and T 20  when they are free. So, time slots T 19  and T 20 , as well as time slots T 21 -T 24 , are used according to the traffic situation. Since the traffic peaks in the various cells and sectors of cells usually do not coincide, the method described above does not significantly increase network congestion. Network design must allow for the temporal distribution of traffic peaks; in an area containing office buildings the traffic peaks occur at office hours, and in residential areas they occur outside office hours. A base station group, in which base stations share initially allocated time slots, should include base stations in which the traffic peaks are not likely to coincide. 
     FIG. 5 also shows a possible way of locating in the transmission system time slots the signalling and operation and maintenance signals, marked TRXSIG and OMUSIG. 
     FIGS. 6 and 7 illustrate a second example of the method according to the invention. The number of TRX units is 36, or threefold compared to the case depicted by FIG.  1 . Dynamic allocation of the transmission system capacity is now taken further than in the previous example. Let us assume that the rush-hour traffic peak value E h  is the same in each TRX unit pair area and that E h =16. Traffic volume is measured in traffic channels in use. Since instantaneous traffic volumes in different areas are independent of each other, the peak traffic values of the areas are summed squared. There are 18 TRX pairs, so the peak traffic E hk  of the whole base station network is calculated from the formula 
     
       
           E   hk =16{square root over (18)}=67.9≈17.0·4 
       
     
     Since one time slot can have four traffic channels, a 17-time slot capacity is enough for the traffic of the whole base station network if the traffic channels in all base stations are freely selectable. The example of FIGS. 6 and 7 assumes so. In accordance with FIG. 7, the number of traffic time slots is 18. All 72 traffic channels are shared by all 36 TRX units and their 288 radio channels, so it can be said that the time slots are initially allocated to all base stations. 
     The procedure of allocating time slots or portions of time slots is described below, referring to FIG.  8 . In step  80  a base transceiver station finds that it needs more capacity for the communication between itself and the base station controller. This find may be based on the fact that all time slots available to the base transceiver station are already full or that a portion (say, 80%) of the available time slots are fill, so that in the latter case the base transceiver station in a way anticipates that the capacity is about to come to an end. Step  81  in FIG. 8 represents an allocation request for a time slot or part of it, sent by the base transceiver station to the base station controller in the form of a standard message in a time slot available to the base transceiver station, preferably in a control information time slot like the OMUSIG or TRXSIG channel. In step  82  the base station controller checks the base station network&#39;s time slot allocation situation from a table that it maintains. If the base transceiver station&#39;s allocation request is acceptable (a suitable time slot or partial time slot is free) the base station controller indicates to the base transceiver station the identification of the time slot or partial time slot which is allocated to the base transceiver station, step  83 . This indication is sent preferably in a control information time slot like the OMUSIG or TRXSIG channel. In response to said indication the base transceiver station activates a control, step  84 , which sets up a connection between the allocated time slot (or partial time slot) and the downlink radio channel in the base transceiver station&#39;s cross-connect, or transmission unit. A more detailed discussion on the various connection types in the transmission unit follows later on. The base transceiver station may also inform the base station controller, in accordance with step  85 , that a connection was made so that in response to that information the base station controller updates the allocation table such that the time slot or partial time slot is marked allocated to the base transceiver station in question, step  86 . If the acknowledgment procedure according to steps  85  and  86  is not used, the allocation table is updated in conjunction with the checking of the allocation situation in step  82 . 
     Allocation of additional capacity to a base transceiver station may also be initiated by the base station controller. In step  87  in FIG. 8 the base station controller detects that a mobile station located in the area of a given base transceiver station is about to receive a paging message, i.e. a request to establish a new connection. The base station controller also detects that the time slots allocated to the base transceiver station are already full of other traffic. So the base station controller starts the procedure according to step  82  in order to allocate a new time slot to the base transceiver station in the manner described above. 
     If the base station controller, when checking the allocation situation in accordance with step  82 , finds that there are no suitable time slots or partial time slots free, it will not indicate allocation of a new time slot or partial time slot to the base transceiver station in accordance with step  83 . FIG. 8 does not show the release of the additional capacity when the base transceiver station no longer needs it. The release may be based on a notice sent by the base transceiver station, indicating that additional capacity allocated earlier is no more needed. Alternatively, the base station controller may measure the traffic in each time slot and/or partial time slot and thus detect time slots or partial time slots that have no active traffic. If the allocation table shows that such a time slot or partial time slot belongs to the initially allocated “shared” capacity of a base station group and is now temporarily allocated to a given base transceiver station, the base station controller may send to that base transceiver station a deallocation notice and update the allocation table such that the time slot or partial time slot is again allocatable to any base transceiver station in that base station group. 
     FIGS. 9 a  to  11  depict in more detail technical implementations used in the transmission units of base transceiver stations to make the base station network operate in accordance with the invention. FIGS. 9 a  and  9   b  show a base transceiver station  900  which has a transmission unit (TRU)  901  and a control functions part (BCF)  902  which distributes communications capacity to the TRX units (not shown). The base transceiver station  900  is allocated one time slot in a fixed manner. In addition, the base transceiver station  900  belongs to a base station group in which the base transceiver stations are initially allocated a time slot represented by signal line  904 . In FIG. 9 a  the base transceiver station  900  has not reserved said time slot, so its transmission unit  901  connects signal line  904  representing the time slot straight through. In FIG. 9 b  the base transceiver station  900  has reserved, with permission from the base station controller (not shown), the time slot represented by signal line  904 . In FIG. 9 b  the allocation has been made by adding to the branching table (not shown), which controls the operation of the transmission unit  901 , a Y-type protection switch  905  which connects the left branch of signal line  904  to the base station control functions part  902 . Instead of the protection switch one could have a similar straight connection from the left branch of signal line  904  to the base station control functions part as with signal line  903 . If the base transceiver station depicted in FIGS. 9 a  and  9   b  belonged to a loop-configured base station network, it would be possible to disclose branching tables the first of which (in the situation depicted in FIG. 9 a ) would have one Y-type protection switch for signal line  903  and a straight connection through the transmission unit  901  for signal line  904 . The second branching table (in the situation depicted in FIG. 9 b ) of the transmission unit  901  would include two Y-type protection switches both of which would be used in order to produce the best possible connection with the base station controller in the same manner as described above in conjunction with the description of the prior art, referring to FIG.  3 . 
     The Y-type protection switch is not the only connection type that can be used in the transmission units of base transceiver stations in order to realize the invention. FIG. 10 shows a base station network comprising three base transceiver stations  1001 ,  1002  and  1003  where each base transceiver station is allocated in a fixed manner three of twelve possible time slots (tripled signal lines  1004 ). In addition, the time slots represented by signal lines  1005 ,  1006  and  1007  are initially allocated to the base station group comprised of the base transceiver stations in question. The transmission unit of each base transceiver station includes three branching switches  1008 ,  1009  and  1010  the positions of which determine the base station to which each of the initially allocated time slots is connected. In FIG. 10, these time slots are not allocated to any given base transceiver station, so all the branching switches  1008 ,  1009  and  1010  are in the upper position. If, for example, the time slot represented by signal line  1007  is allocated to base transceiver station  1003 , the corresponding switches  1010  in base transceiver stations  1001  and  1002  are kept in the upper position, and switch  1010  in base transceiver station  1003  is turned to the lower position depicted by a broken line. So, in this case there is no need to introduce a totally new branching table in any of the base transceiver stations since certain two-position switches are already defined in the branching tables of the transmission units of all base transceiver stations. 
     The branching switch shown in FIG. 10, which connects the inbound signal line either through the transmission unit or from the transmission unit to the control functions part, could be applied to the case depicted by FIGS. 9 a  and  9   b . The branching table of the base station transmission unit of FIGS. 9 a  and  9   b  would in that case include a branching switch with which signal line  904  would be directed either through the transmission unit  901  or via it to the control functions part. 
     FIG. 10 assumes that each base transceiver station can take additional capacity regardless of which initially allocated time slot is addressed to it. So, each base transceiver station can use any one of the time slots represented by signal lines  1005 ,  1006  and  1007 . However, base station architecture may cause that a given base transceiver station can only use a particular time slot as additional capacity. FIG. 11 shows a base station network in which the invention is applied in such a case. Base transceiver stations BTS 1  to BTS 9  form a chain-configured base station network where each base transceiver station is allocated one time slot (signal lines  1101 ) in a fixed manner. For simplicity, base transceiver stations BTS 4  to BTS 7  are left out of the drawing, but their position and connections can be easily deduced from the rest of the drawing. Three time slots (signal lines  1102 ,  1103  and  1104 ) are initially allocated to a base station group which includes all the base transceiver stations. Each base transceiver station has three branching switches  1105 ,  1106  and  1107  with one common line to the control functions part  1108 . This case requires the use of so-called conditional branching tables, i.e. if a connection is modified in the transmission unit of a base transceiver station, the branching table in that transmission unit has to be changed. The base station control functions part need not know which time slot was allocated as additional capacity, because the transmission unit, controlled by the new branching table, directs the additional capacity represented by the allocated time slot to the base transceiver station always in the same way as seen from the control functions part. 
     Above it was discussed mainly 2-Mbps connections between base stations and base station controllers, in which traffic is divided into 32 time slots which can be further divided into four partial time slots. The invention is in no way limited to systems based on these figures but the inventional idea can be applied to all systems in which the communication between base stations and base station controllers is based on the allocation of time slots or similar capacity units. For example, other widely used data transmission rates apart from 2 Mbps are 1.5 Mbps, 1 Mbps and 0.5 Mbps. If the communication between the base stations and a base station controller takes place on multiple parallel frequency bands, the dynamic allocation method according to the invention can be used on all frequency bands or on some of the frequency bands. 
     Above it was-assumed that as regards time slot allocation the base stations are equal i.e. a free, allocatable time slot is allotted to the base station which first reserves it. Base stations may also be given priorities so that a given time slot may be primarily reserved to a certain base station and other base stations may reserve that time slot only if the primary base station does not need it.