EP1131977A1

EP1131977A1 - Channel allocation method and cellular radio system

Info

Publication number: EP1131977A1
Application number: EP99972415A
Authority: EP
Inventors: Petteri Hakalin; Eija Saario
Original assignee: Nokia Networks Oy; Nokia Oyj
Current assignee: Nokia Oyj
Priority date: 1998-11-16
Filing date: 1999-11-15
Publication date: 2001-09-12
Also published as: AU1388600A; WO2000030388A1; FI982473A0; NO20012385D0; NO20012385L; FI982473A; JP2002530958A; CN1326657A

Abstract

The invention relates to a cellular radio system and a channel allocation method. In the method, allocation of cellular radio network channels to subscribers is controlled on the basis of the mutual interference between the given cell (200) and the neighbouring cells (220 to 250). In the solution of the invention, neighbouring cell connections receiving interference from the given cell (200), which neighbouring cells (220 to 250) use at least partly the same frequency set as the given cell (200), are defined on the basis of the measurements the subscriber terminals located in the neighbouring cells (220 to 250) report on their strongest neighbours and on the basis of the common frequencies used by the neighbouring cells (220 to 250) together with the given cell (200). Thus the decision on allocating a channel to the subscriber terminal can be made on the basis of optimal data.

Description

CHANNEL ALLOCATION METHOD AND CELLULAR RADIO SYSTEM

FIELD OF THE INVENTION

The invention relates to a method of allocating channels in a cellular radio system to terminals and a cellular radio system.

BACKGROUND OF THE INVENTION

One of the key problems in constructing and maintaining cellular radio networks is the limited scope of the available radio spectrum. Careful planning of the use of radio frequencies aims to minimize co-channel interference and adjacent channel interference. By means of various complex models, the frequencies are divided into different cells with the intention to minimize the interference occurring in radio connections and thus maximize the network capacity. In a cell repeat pattern, the same or adjacent frequencies must not be too close to one another, because this causes excessive interference in the system. As the use of mobile telephones and other subscriber terminals be- comes more common, the capacity of networks must constantly be increased. This incurs high costs in frequency planning and various measurements.

In known mobile systems, the allocation of a traffic channel to a given subscriber terminal generally depends on whether a radio cell has an available traffic channel at that instant or not. In other words, if said radio cell has a free traffic channel, it is allocated to the subscriber terminal when needed. The GSM system (Global System for Mobile Telecommunications) is one example of a system where the traffic channel allocation is carried out in the above-described manner. Since the GSM system is time-divisional, to the subscriber terminal is allocated a traffic channel which comprises a time slot on a given frequency channel. Alternatively, if frequency hopping is used, the frequency channel of the traffic channel changes from one time slot to another according to a predetermined frequency hopping sequence. Thus the subscriber terminal transmits alternately on all frequency channels included in frequency hopping sequence. Consequently, when the reuse coefficient of a channel in a network using frequency hopping is less than 9, the capacity is no longer limited by available traffic channels (so-called 'hard blocking') but by interference (so-called 'soft blocking'). The network can thus be considered interference limited. When loading in the interference limited network increases, the quality decreases correspondingly. The basic idea of a so-called Dynamic HotSpot feature (DHS) is to control the traffic load of a cellular radio network employing frequency hopping on the basis of interference and hence to achieve higher network capacity. The Dynamic Hotspot feature is thus used for controlling the load in an interference limited cellular radio network. When the load of a given cell exceeds a predetermined value and hence the interfer- ence in the rest of the cellular radio network increases, so-called soft blocking can be used in channel allocation. A blocking criterion is checked in each new call allocation and inter-cell handover. The decision to allow a new call to enter the network is based on the connection qualities of the neighbouring cells. If the number of poor connections in the interfered neighbouring cells is large, the call is blocked even though there were free channels available in the cell. In prior art dynamic channel allocation methods, the channel allocation is based, for instance, on the average quality of the calls connected at said instant in the interfered cells, i.e. the quality of each cell equals the quality average of all calls in said cell. The quality class of each cell is determined according to the average of the qualities of calls already connected, and this quality class of the cell determines the probabilities of getting the call through. The final probability of reserving a channel is the product of the probabilities of the neighbouring cells.

There is no previously known, advanced method to define actual, interfered cells. A problem with the above-described arrangement is, for instance, that each interfered cell must have been defined case-specifically, i.e. in order to define the interfered neighbouring cells, it has been necessary to collect information for each individual serving cell, for instance manually from the network configuration and to send said information to the channel alloca- tion process. The interfered cells are not necessarily the same as the cells that have the strongest signal level in the cell where a connection will be established based on the subscriber terminal measurements. The reason for this is that method can only be utilized in measuring uplink interference which is lower in relation to the downlink interference, and it has not been possible to use the method for assessing the interfered cells with the desired accuracy.

BRIEF DESCRIPTION OF THE INVENTION

The object of the invention is thus to provide a method and a cellular radio system such that the above problems can be solved. This is achieved with a solution of the invention. The invention relates to a channel allocation method in a cellular radio network, which cellular radio network comprises at least one base station of a given cell, at least one subscriber terminal communicating with the base station, and neighbouring cells of the given cell and subscriber terminals communicating therewith, in which cellular radio system a request is received for allocating a traffic channel in a given cell to a sub- scriber terminal, in which cellular radio system the subscriber terminal reports the measurement results of the broadcast control channel transmitted by base stations detected by the terminal, in which method allocation of cellular radio network channels to the terminals is controlled on the basis of the mutual interference between the given cell and its neighbouring cells. In the method, neighbouring cell connections receiving interference from the given cell, which neighbouring cells use at least partly the same frequency set as the given cell, are defined on the basis of the measurements the subscriber terminals located in the neighbouring cells report on their strongest neighbours and on the basis of the common frequencies used by the neighbouring cells together with the given cell.

The invention also relates to a cellular radio system which comprises a base station of at least one given cell, at least one subscriber terminal communicating with the base station, and neighbouring cells of the given cell and subscriber terminals communicating therewith, which cellular radio system is arranged to receive a request for allocating a traffic channel in the given cell to a subscriber terminal, in which cellular radio system the subscriber terminals communicating with the base station are arranged to report neighbouring cell measurement results of the broadcast control channel transmitted by the base stations, and which cellular radio system is arranged to control allocation of cellular radio network channels to terminals on the basis of the mutual interference between the given cell and its neighbouring cells. The cellular radio system is arranged to define the neighbouring cell connections receiving interference from the given cell, which neighbouring cells use at least partly the same frequency set as the given cell, on the basis of the measurements the subscriber terminals located in the neighbouring cells report on their strongest neighbours and on the basis of the common frequencies used by the neighbouring cells together with the given cell.

The preferred embodiments of the invention are set forth in the dependent claims. In accordance with the prior art, interference caused to a cell using the same frequency set is estimated, for instance, by calculating an average quality of all ongoing calls at that instant. In that case, it is assumed that all poor quality results from interference. This is not necessarily so, but poor quality may also result from the fact that a subscriber terminal is in a weak field, for instance, indoors. The method of the invention in turn is based on performing the interference estimation by employing only the connections that are likely to be interfered. This is concluded by whether the subscriber terminal can hear the broadcast control channel of the interfering cell, if the answer is yes, a conclusion is drawn that also the traffic channel frequencies of said cell having co-channel or adjacent channel interference interfere with one another. By means of the invention, the functionality of the Dynamic HotSpot method can be improved in the cellular radio network. The method of the invention provides a better method over the prior art for selecting interfered neighbouring cells. In the present application, the neighbouring cell refers to a cell which is so close to the cell whereto a traffic channel is going to be allo- cated that the new traffic channel can be assumed to affect the ongoing connections in the neighbouring cell. In other words, the neighbouring cell need not necessarily be located next to the cell whereto the channel will be allocated but there can be one or more other radio cells between said cells.

By means of the method of the invention, it is also possible to cal- culate the ratio of poor quality in the interfered cells. The method of the invention advantageously utilizes a new kind of a weighting factor which can also be used for further improving the original DHS algorithm.

Several advantages are achieved with the method and system of the invention. The method provides an automatic method for defining inter- fered cells for a given cell, and a ratio of poor quality to overall quality of the interfered cells. Hence the method gives a better overall picture of the interference, and so the capacity of the cellular radio network can be increased. The DHS method enables very tight frequency reuse by transceivers preferably using frequency hopping without that the quality deteriorates, because a base station controller can limit the traffic load to areas where the value of the interference variable is within acceptable limits. In addition, the traffic can be controlled dynamically. This characteristic is very useful, particularly when the traffic amounts increase occasionally and temporarily. Local peaks in the traffic load can thus be allowed if the quality of the interfered cells still remain on an acceptable level. In accordance with the prior art, poor quality data is collected from the cells that are interfered by the cell to which a new call will be connected. However, the cellular radio network does not update the list on these interfered cells. The method of the invention can be used for finding out interfer- ence effects on each cell under one base station controller. In prior art methods, poor quality data was collected from all connections of the interfered cells. In the method of the invention, the connection quality of only the actually interfered connections is measured. In addition, the method of the invention employs a new frequency weighting factor which takes into account the situa- tion where the frequencies of the interfered cells are only partially overlapping, whereas in prior art solutions, the interference in different cells was assumed to be the same and independent of the number of common frequencies.

Advantageously, the solution of the invention can be applied to a GSM-type cellular radio network, for instance. The solution does not require any changes in the current GSM specification. All necessary information is available in the base station controller. The method can be adopted by amending the software alone, i.e. by updating network device software. The actual equipment need not be changed.

The system of the invention provides the same advantages as the above-described method. It is obvious that preferred embodiments and details thereof can be combined into various combinations in order to achieve the desired technical effect.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will be described in connection with the preferred embodiments with reference to the accompanying drawings, wherein

Figure 1 shows a cellular radio network in general;

Figure 2 shows a cellular radio network of the invention; and

Figure 3 illustrates a method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 shows an example of the structure of a common cellular radio network. The figure shows coverage areas, i.e. cells, of a base station

100, 102, 130 as hexagons. The base stations 100, 102, 130 communicate via a connecting line 112 with a base station controller 114. The task of the base station controller 114 is to control the operation of its subordinate base sta- tions. Normally, the base station controller 114 has a connection to a mobile services switching centre 116, and therefrom further to a public switched telephone network 118. In office systems, the functions of the base station 100, the base station controller 114 and even the mobile services switching centre 116 can be combined into one device, from which there is a connection to the public switched telephone network 118, for instance, to a switching centre of the public switched telephone network 118. Subscriber terminals 104, 106 in a cell 200 have bidirectional radio connections 108, 110 to the base station 100 of the cell. Subscriber terminals 122, 124 in a cell 220 have bidirectional radio connections 126, 128 to a base station 130 of the cell. In addition, a network part, i.e. a fixed part of the cellular radio network, can comprise additional base stations, base station controllers, transmission systems and network management systems of various levels. It is obvious to a person skilled in the art that the cellular radio network also comprises a variety of other structures that need not be explained in greater detail herein.

Figure 2 shows a more extensive example of a cellular radio network, to which the Dynamic HotSpot algorithm of the invention can be applied in order to control the traffic load. The radio network comprises a set of cells to each of which is allocated a set of frequency bands for subscriber terminal connections. The cells 200, 220, 230, 240, 250 of Figure 2 use at least partly the same frequency group, i.e. the same frequencies. In this example, let us assume that the frequencies used are cell-specifically as follows:

Table 1

In the cellular radio networks, the base stations generally send a so-called broadcast control channel which comprises base-station-related general information, by means of which the subscriber terminals can contact the base stations. The broadcast control channel is also used when assessing the subscriber terminals' need for handover. For instance in the GSM system, the broadcast control channel is referred to as BCCH. Advantageously, the broadcast control channel also comprises an identifier on the basis of which the subscriber terminal knows from which base station the signal is coming. In the GSM system, the identifier data is a so-called identity code BSIC (Base Station Identity Code).

In the radio system of the invention, the subscriber terminals measure the broadcast control channel from the neighbouring cells. Each subscriber terminal having a connection to its own base station measures continuously the BCCH signals of the strongest neighbouring cells to evaluate possible handover candidates. The measurement information advantageously includes the received signal level and the base station identity code. Since the measuring capacity and measuring time of the subscriber terminals are finite, the number of base stations to be measured is restricted. One advantageous maximum number of base stations to be measured is six, which is used in the GSM system. For instance, the subscriber terminal 122 in the cell 220 measures the strength of the BCCH signals of the six strongest received neighbouring cells and reports the measurement results to its base station 130 which forwards them to the base station controller 114.

Let us assume that the subscriber terminal 120 in the cell 200 at- tempts to establish a connection to the base station 100. According to the DHS principle, if the number of free traffic channels in a cell is lower than a given threshold, the decision on the acceptance of the set-up request of the subscriber terminal 120 depends on the interference the cell 200 causes to nearby cells using the same frequencies. The basic idea of the invention is that the BCCH measurings executed in the neighbouring cells are utilized in estimating the effects of interference. If a call is connected to the cell 200, it may affect the quality of other calls using the same frequencies in the neighbouring cells 220, 230, 240, 250.

Let us examine this example in more detail by means of Figures 1 and 2. Each subscriber terminal having a connection to its base station measures on a continuous basis the BCCH signals of the strongest neighbouring cells. Thus the subscriber terminals located in the cells surrounding the cell 200 also execute measurings. Substantial in this context are the cells where the same frequencies are used as in the cell 200 where a decision must be made on a new connection between a subscriber terminal and the base station. In principle, the method of the invention has two steps. First, interference information is collected from the neighbouring cells 220, 230, 240 and 250 of the cell 200 in the following manner. Let us start with the cell 220, for instance. The base station controller collects BCCH measurement results from the subscriber terminals communicating with said cell 220. Each subscriber terminal performs independent measurings preferably on six strongest BCCH signals. For the terminals located on different sides of the cell said six signals can be different. The number of common frequencies between the cell 220 and the reported neighbouring cells is calculated. Then it is examined whether the number of the common frequencies exceeds zero. If not, no considerable interference occurs between these cells. For instance, the adjacent cell 210 employs different frequencies from those of the cell 220. For this reason, the reports on the BCCH signal of this base station will be omitted in this context, even though a plurality of terminals located on that side of the cell probably measures that signal. On the basis of the BCCH measurements performed by the subscriber terminals, the neighbouring cells that have at least one common frequency with the serving cell 220 are tabulated of the strongest ones. In this example, the cells using the same frequencies are thus 200, 230, 240 and 250. In other words, if the number of common frequencies exceeds zero, the table of the neighbouring cells 200, 230, 240 and 250 interfered by the cell 220 is updated, the table showing, for instance, the identifiers of the neighbouring cells 200, 230, 240 and 250 using the same frequencies, the ratio of the common frequencies of the cell 220 and its neighbouring cells, i.e. the same frequencies, in relation to all available frequencies (added up from the cell 220 and each neighbouring cell at a time) in said neighbouring cells, and the poor quality samples in proportion to all samples. The table for the cell 200 may appear, for instance, as follows:

Table 2

In the following, the poor quality samples in proportion to all samples are denoted with an abbreviation PQ%. Similar tables are drawn up for each neighbouring cell 220, 230, 240 and 250 of the cell 200.

So-called frequency weighting factors TS can also be calculated in the table, so that they could be utilized as interference weighting factors. The utilization will be described later on in the text. The weighting factors are calculated by utilizing the information on the number of common frequencies between the interfered and interfering cells. One way to form the weighting factors is to determine the ratios of common frequencies to all available frequencies as described in the above table. Another advantageous way to form weighting factors is to calculate them with the formula

_{τc =} f_a TRXLKM_i nterferer f 'interferer *f 'interfered

where f_a„ = the number of common frequencies (interfering and interfered cell),

TRXLKM_interferer = the number of transceivers in the interfering cell, fi_nterferer ^{= tne} number of frequencies in the interfering cell, fi_nter_fered ^{= tne} number of frequencies in the interfered cell. Thus in tabulation the interfering cell is the cell whose broadcast control channel is detected among the strongest ones, and correspondingly the interfered cell is the cell whose table is being updated. Table 2 is intended for indicating the percentages of poor quality PQ% that are periodically updated. Thus, when the subscriber terminal reports the cell which has the common frequencies with the serving cell 220 and which is included in the group of the strongest, e.g. the six strongest, neigh- bouring cells, the ratio of poor quality to overall quality is updated until said cell is no longer among the strongest (in this example, the six strongest) ones. If a plural number of the neighbouring cells 200, 230, 240, 250 reported on the list operate on the same frequencies with the six strongest ones, the poor quality percentage PQ% is updated for each neighbouring cell in the table. The method of the invention then measures only the poor qualities that result from the interference caused by the cells of the same frequency group. The poor quality resulting from the low signal level of the serving cell 220, for instance, can then be ignored.

In the second step of the method of the invention, the procedure is as follows. Figure 3 shows a flow chart of an advanced Dynamic HotSpot algorithm, in which the method is used for collecting poor quality information and for calculating a frequency weighting factor for the cell 200. The frequency weighting factor and the poor quality percentage PQ% are collected from each interfered neighbouring cell 220, 230, 240, 250 by using the above method. What is novel and inventive is how, in particular by what method, the poor quality information of the interfered cells 220, 230, 240, 250 is collected according to the above tabulation principle and also the use of the frequency weighting factor.

According to the DHS algorithm of Figure 3, the subscriber terminal 120 sends a service request to the cell 200. It is checked next, whether the number of traffic channels (TCH) in use exceeds a desired preset limit value. If said number does not exceed the desired limit value, the subscriber terminal 120 is connected to the cell 200 that received the service request. However, if the number of traffic channels already in use exceeds the desired limit value, the interfered neighbouring cells 220 to 250 are looked for. It is checked for each cell, whether the table of said cell 220 to 250 contains the cell 200 that received the service request. Next is determined a connection ratio for each interfered neighbouring cell 220 to 250 by means of the poor quality percentage PQ% calculated in Table 2. This ratio indicates the intensity of interfer- ence. The ratio is intended for scaling the quality value within a given range. Determination of the connection ratio on the basis of the PQ% value can be performed, for instance, by means of the following table:

Table 3

The values of the connection ratio are determined as follows: if the calculated, so-called excessively poor quality percentage PQ% exceeds the preset, standard poor quality limit value, the connection ratio in the table is zero. If the measured poor quality percentage PQ% is lower than or equal to the preset poor quality limit value and simultaneously exceeds the preset sig- nal quality limit value 1 , the ratio in the table is a constant 'Prob 1'. If the measured poor quality percentage PQ% is lower than or equal to the preset signal quality limit value 1 and exceeds the preset signal quality limit value 2, the ratio in the table is 'Prob 2'. Likewise, if the measured poor quality percentage PQ% is lower than or equal to the preset signal quality limit value 2 and exceeds the preset good quality limit value, the ratio in the table is 'Prob 3'. If the good quality limit value exceeds the above-mentioned poor quality percentage PQ%, the ratio is 1. When necessary, it is possible to scale the table to be more detailed or less detailed, i.e. the ratio may vary with desired steps between 0 and 1. The obtained ratios are weighted next with the frequency weighting factors TS, the calculation of which was described earlier, and the total probability PROB of allocation is calculated, for instance, by multiplying the ratios of each neighbouring cell 220, 230, 240, 250 between 0 and 1 in the following manner: PROB = TS_200/220 ^*Prob(Cell 220) * TS_200/230*Prob(Cell 230)*

TS_200/240 ^*Prob(Cell 240) * TS_200/250*Prob(Cell 250), where each term Prob(Cell) is the connection ratio of said cell and the term TS_200/N is the frequency weighting factor between the cell 200 and a cell N. By means of the frequency weighting factor it is possible to enhance the impor- tance of each cell in proportion to the number of common frequencies.

It should be noted herein that the above-described method for calculating the total probability PROB is only one option. The value corresponding the total probability can also be calculated by other known methods.

Thereafter, the calculated value of the total probability PROB is compared with a connection threshold value formed in a predetermined manner. This threshold value can be, for instance, a random number within a given range, or more preferably, a preset value (for instance 0.5). If PROB is lower than said value, the subscriber terminal 120 is connected to the cell 200 that received the service request. If PROB exceeds said value, the call is blocked and it is not connected to said cell 200.

In the cellular radio network, the functions in accordance with the invention can preferably be implemented in the base station controller 114 or in a corresponding unit that is responsible for allocating channels to the terminals. The method can be adopted by amending the programs alone, i.e. by updating the network device software such that they carry out the method steps required by the invention. Equipment changes are not necessarily needed.

Even though the invention is described in the above with reference to the example of the accompanying drawings, it is obvious that the invention is not restricted thereto but is can be modified in a variety of ways within the scope of the inventive idea disclosed in the attached claims.

Claims

1. A channel allocation method in a cellular radio network, which cellular radio network comprises at least one base station (100) of a given cell (200), at least one subscriber terminal (104, 106) communicating with the base station (100), and neighbouring cells (220 to 250) of the given cell (200) and subscriber terminals (122, 124) communicating therewith, in which cellular radio system a request is received for allocating a traffic channel in a given cell (200) to a subscriber terminal (120), in which cellular radio system the subscriber terminal (104, 106) re- ports the measurement results of the broadcast control channel transmitted by base stations detected by the terminal, in which method cellular allocation of radio network channels to the terminals is controlled on the basis of the mutual interference between a given cell (200) and its neighbouring cells (220 to 250), c h a r a c t e r i z e d in that neighbouring cell connections receiving interference from the given cell (200), which neighbouring cells (220 to 250) use at least partly the same frequency set as the given cell (200), are defined on the basis of the measurements the subscriber terminals located in the neighbouring cells (220 to 250) report on their strongest neighbours and on the basis of the common frequencies used by the neighbouring cells (220 to 250) together with the given cell (200).

2. A method as claimed in claim 1 , c h a r a c te r i z e d in that in the method a base station controller (114) collects measurement results of a broadcast control channel from the subscriber terminals communicating with the cell (220), the number of common frequencies is calculated between the cell (200) and adjacent cells transmitting the broadcast control channels measured by the terminals, and of the cell's (220) strongest neighbours, the cells (200, 230, 240,

250) having at least one or more common frequencies with the cell (220) are tabulated.

3. A method as claimed in claim 2, c h a r a c t e r i z e d in that in the method the base station controller (114) receives a connection setup request relating to a subscriber terminal (120) and a given cell (200), it is checked whether the number of traffic channels in use exceeds a preset limit value, whereby interfered neighbouring cells (220, 230, 240, 250) are looked for by checking if the cell (200) appears in the tables of the neighbouring cells (220, 230, 240, 250), a connection ratio is calculated for the interfered neighbouring cells (220, 230, 240, 250) by means of a poor quality percentage, a total probability is calculated, the calculated total probability is compared with a connection threshold value formed in a predefined manner and a decision is made on call connection or rejection on the basis of the comparison.

4. A method as claimed in claim 3, characterized in that in calculating the total probability the connection ratios are weighted with frequency weighting factors that are calculated by means of the number of the common frequencies between the interfered and the interfering cells.

5. A method as claimed in claim 2 or 4, characterized in that the subscriber terminals report the desired number of only the strongest interfered neighbouring cells.

6. A method as claimed in claim 1 or 2, characterized in that the measurement results are compiled in a table or the like cell- specifically.

7. A method as claimed in claim 1 or 2, characterized in that the ratios of the numbers of common frequencies to all frequencies between the cell (220) and the neighbouring cells, as well as poor quality percentages, are compiled in the table or the like.

8. A method as claimed in claim 3, characterized in that the connection threshold value is a random number.

9. A method as claimed in claim 3, characterized in that the connection threshold value is a preset limit value.

10. A method as claimed in claim 1, characterized in that the method is applied to a so-called Dynamic HotSpot algorithm.

11. A cellular radio system which comprises a base station (100) of at least one given cell (200), at least one subscriber terminal (104, 106) communicating with the base station (100), and neighbouring cells (220 to 250) of the given cell (200) and subscriber terminals (122, 124) communicating therewith, which cellular radio system is arranged to receive a request for allocating a traffic channel in a given cell (200) to a subscriber terminal (120), in which cellular radio system the subscriber terminals(104, 106) communicating with the base station are arranged to report neighbouring cell (220, 230, 240, 250) measurement results of the broadcast control channel transmitted by the base stations, and which cellular radio system is arranged to control allocation of cellular radio network channels to terminals on the basis of the mutual interference between the given cell (200) and its neighbouring cells (220, 230, 240, 250), c h a r a c t e r i z e d in that the cellular radio system is arranged to define the neighbouring cell connections receiving interference from the given cell (200), which neighbouring cells (220 to 250) use at least partly the same frequency set as the given cell, on the basis of the measurements the subscriber terminals(122, 124) located in the neighbouring cells (220 to 250) report on their strongest neighbours and on the basis of the common frequencies used by the neighbouring cells (220 to 250) together with the given cell (200).

12. A cellular radio system as claimed in claim 11 , c h a r a c t e r i z e d in that in the cellular radio system the base station controller (114) is arranged to collect measurement results of the broadcast control channel from the subscriber terminals (122, 124) communicating with the cell (220), the cellular radio system is arranged to calculate the number of the common frequencies between the cell (200) and the neighbouring cells, and of the strongest neighbours of the cell (220), the cellular radio system is arranged to tabulate the cells (200, 230 to 250) having at least one or more common frequencies with the cell (220).

13. A cellular radio system as claimed in claim 12, c h a r a c t e r i z e d in that in the cellular radio system the base station controller (114) receives a connection setup request relating to the subscriber terminal (120) and the given cell (200), the cellular radio system is arranged to check whether the number of traffic channels in use exceeds a preset limit value, whereby the cellular radio system is arranged to look for the interfered neighbouring cells (220 to

250) by checking if the cell (200) appears in the tables of the neighbouring cells (220 to 250), the cellular radio system is arranged to calculate a connection ratio for the interfered neighbouring cells (220 to 250) by means of the poor quality percentage, the cellular radio system is arranged to calculate a total probability, the cellular radio system is arranged to compare the connection probability value with the calculated total probability and to make a decision on call connection or rejection.

14. A cellular radio system as claimed in claim 11 or 13, characterized in that the subscriber terminals are arranged to report the desired number of only the most strongly interfered neighbouring cells.

15. A cellular radio system as claimed in claim 12 or 13, c h a r- acterized by being arranged to compile the measurement results in a table or the like cell-specifically.

16. A cellular radio system as claimed in claim 12 or 13, c h a r- acterized by being arranged to compile in a table or the like data on the ratios of the numbers of common frequencies to all frequencies between the given cell (220) and the neighbouring cells (200, 230 to 250), as well as poor quality percentages.

17. A cellular radio system as claimed in claim 13, characterized in that the connection probability value is a random number.

18. A cellular radio system as claimed in claim 13, charac- t e r i z e d in that the connection probability value is a preset limit value.