METHOD FOR ALLOCATING A FREQUENCY TO A CELL IN A CELLULAR RADIO SYSTEM, AND A CELLULAR RADIO SYSTEM

Abstract: The invention relates to a method for allocating a frequency to a cell (200) in a cellular radio system, and a cellular radio system comprising a plurality of neighbouring cells (100) surrounding a cell (200), and means (204) for transmitting a measuring signal from the cell (200) to a neighbouring cell (100). The cells comprise a plurality of subscriber terminals (102, 202) and at least one base station (101, 201) serving the subscriber terminals (102, 202) within the area of the cell (100, 200) through channels. The cellular radio system comprises first field strength measuring means (301) for measuring disturbance field strength, second field strength measuring means (302) for measuring the field strength of the channel serving the subscriber terminal (102, 202), and means (305) for forming ratios of field strength to disturbance field strength, the ratios being utilized for forming disturbance probability distributions utilized for allocating a suitable frequency to the cell (100, 200).

Description: METHOD FOR ALLOCATING A FREQUENCY TO A CELL IN A CELLULAR RADIO SYSTEM, AND A CELLULAR RADIO SYSTEM The invention relates to a method for allocating a frequency to a cell in a cellular radio system, said cell transmitting a measuring signal, and said cellular radio system comprising a plurality of neighbouring cells adjacent to said cell, said cells comprising a plurality of subscriber terminals and at least one base station serving the subscriber terminals within the area of the cells through channels. The invention further relates to a method for allocating a frequency to a cell in a cellular radio system, said cell transmitting a measuring signal, and said cellular radio system comprising a plurality of neighbouring cells adjacent to said cell, said cells comprising a plurality of subscriber terminals and at least one base station serving the subscriber terminals within the area of the cells through channels. The present invention is suitable for allocating a frequency to a cell to be added to a cellular radio system, particularly in the GSM and DCS cellular radio networks (GSM = Global System for Mobile Communications, DCS = Digital Cellular System). The solution of the invention is used for selecting the frequencies that will be used in the cell and that serve the traffic in the cellular radio system in the best possible manner. In a solution of the prior art, the frequency of a base station within the area of a new cell is allocated by means of a special tool. The tool is used for calculating the interference probabilities, or l-matrices, between the base stations of the cellular radio system. The predicted cell coverage areas are utilized in the calculation of the l-matrix. A separation matrix is deduced from the calculated l-matrix. On the basis of the separation matrix, it is possible to find out how frequencies are used in a base station of the cellular radio system. The frequency of the base station is allocated by a heuristic algorithm utilizing the results of the separation matrix. The prior art solution has, however, drawbacks. The calculation of the l-matrix is slow. In addition, it takes a long time to allocate the frequency. The solution requires detailed information on the environment of the base station to be added to the cellular radio system, particularly of the terrain. Data on the environment are fed into the tool mentioned above in the form of a digital map. The tool calculates different prediction models. However, the prediction models obtained are not sufficiently reliable and accurate. Since the prior art is based on the use of prediction models, the frequency allocation achieved with the prior art is not satisfactory. An object of the present invention is to allow a frequency to be rapidly allocated to a cell to be added to a cellular radio system. A further object of the invention is to ensure that the frequency to be used in the cell does not disturb other cells of the system. This is achieved with a method of the type described in the introductory portion. The method of the invention is characterized in that the neighbouring cell measures the disturbance field strength caused by the measuring signal and the field strength of the channel serving a subscriber terminal of the neighbouring cell, from which ratios of field strength to disturbance field strength are formed, said ratios being utilized in the formation of a probability distribution for the disturbance caused by the measuring signal in the neighbouring cell, said probability distribution being utilized in the allocation of a suitable frequency to said cell. The method of the invention is further characterized in that said cell measures the disturbance field strength caused by the channel serving a subscriber terminal of the neighbouring cell and the field strength of the channel serving a subscriber terminal of said cell, from which ratios of field strength to disturbance field strength are formed, said ratios being utilized in the formation of a probability distribution for the disturbance caused by the base station of the neighbouring cell in said cell, said probability distribution being utilized for allocating a suitable frequency to said cell. The invention further relates to a cellular radio system comprising a plurality of neighbouring cells surrounding a cell, and means for transmitting a measuring signal from said cell to a neighbouring cell, said cells comprising a plurality of subscriber terminals, and at least one base station serving the subscriber terminals within the area of the cell through channels. The cellular radio system of the invention is characterized by comprising first field strength measuring means for measuring disturbance field strength, second field strength measuring means for measuring the field strength of the channel serving the subscriber terminal, and means for forming ratios of field strength to disturbance field strength, said ratios being utilized for forming disturbance probability distributions utilized for allocating a suitable frequency to the cell. In the following, the invention will be described in greater detail with reference to the examples illustrated in the accompanying drawings, in which Figure 1 shows a subscriber terminal of the invention, Figure 2 shows a first embodiment of the cellular radio system of the invention, and Figure 3 shows a second embodiment of the cellular radio system of the invention. Figure 1 illustrates the essential parts of a subscriber terminal 102 of the invention. The subscriber terminal 102 comprises a receiver part A and a transmitter part B. The receiver part A comprises an antenna 11 , radio frequency parts 12, a demodulator 13, a decoder 14, and an earpiece 18. The transmitter part B comprises a microphone 21 , a coder 22, a modulator 23, radio frequency parts 24, and an antenna 25. The subscriber terminal 102 further comprises control means 31 , which are common to the receiver part A and the transmitter part B. The subscriber terminal 102 further comprises first field strength measuring means 301 , second field strength measuring means 302, channel measuring means 303, and power measuring means 304. Figure 2 illustrates a cellular radio system of the invention, comprising two cells 100, 200. Each cell 100, 200 comprises at least one base station 101 , 201 within its area. In addition, the cellular radio system comprises a plurality of subscriber terminals 102, 202, positioned in the area of the cells 100, 200. In the cellular radio system of Figure 2, cell 200 is a new cell added to the cellular radio system. The new cell 200 comprises transmission means 204, which transmit a measuring signal. Figure 3 also illustrates a cellular radio system of the invention, comprising two cells 100, 200. Each cell 100, 200 comprises at least one base station 101 , 201 within its area. In addition, the cellular radio system comprises a plurality of subscriber terminals 102, 202, positioned in the area of the cells 100, 200. A new cell 200 is added to the cellular radio system of Figure 3. The receiver part A of Figure 1 operates as follows. A radio- frequency analogue signal received by the antenna 11 is switched to an intermediate frequency and filtered by the radio frequency parts 12. The demodulator 13 restores a broadband signal to a narrow-band data signal. The data signal is decoded in a suitable manner by the decoder 14. The decoder 14 typically decodes a convolution-coded signal, and the operation of the decoder 14 is based, for example, on the Viterbi algorithm. The decoder 14 usually also decrypts and demultiplexes a pre-treated signal. From the decoder 14, the signal is supplied to the earpiece 15. The transmitter part B operates as follows. The microphone 21 receives an audio signal and transmits an electric equivalent of the signal to the coder 22. The coder 22 convolution-codes and typically encrypts the signal. In addition, the coder 22 multiplexes the bits or bit groups of the signal. The convolution-coded narrow-band signal is pseudo-noise-coded to a broadband spread-spectrum signal by the modulator 23. Thereafter the spread-spectrum signal is converted into a radio-frequency signal according to the prior art in the radio frequency parts 24, and transmitted through the antenna 25 to the radio path. Control means 31 control the operation of both the receiver part A and the transmitter part B. The antennas 11 and 25 are transmitter and receiver antennas of radio systems of the prior art. In practice, the functions of the transmitter and receiver antennas 11 and 25 are combined into one antenna. The microphone 21 , earpiece 15, and radio frequency parts 12 and 24 are components of the prior art, used in known radio systems. The first field strength measuring means 301 measure the field strength of the channel which serves the subscriber terminal 102, 202. The second field strength measuring means 302 measure the disturbance field strength. The field strength data obtained from the first 301 and the second field strength measuring means 302 are supplied to means 305 for forming field strength probability distributions from the field strength data. The means 305 are connected to means 307 for forming interference and separation matrices from the probability distributions of field strength. The channel measuring means 303 and power measuring means 304 are connected to means 307 through calculation means 306. In the cellular radio system of Figure 2, the new cell 200 belongs to the list of neighbouring cells of sufficiently many adjacent cells 100. The transmission means 204 of the new cell 200 transmit a measuring signal, which is different from other signals of the cellular radio system. In practice, the transmission means 204 are positioned in the base station 201. The transmission means 204 may also be a separate test transmitter. The new cell 200 belongs, for example, to three or four cell bands surrounding it. The first field strength measuring means 301 of the subscriber terminals 102 of the neighbouring cell 100 measure the disturbance field strength caused by the measuring signal of the transmission means 204 of the new cell 200 added to the cellular radio system. The disturbance field strength measured by the first field strength measuring means 301 is described by an l-value. The second field strength measuring means 302 of the subscriber terminal 102 of the neighbouring cell 100 measure the field strength of the BCCH (Broadcast Control Channel) of the subscriber terminal 102. The BCCH measured by the second field strength measuring means 302 gives a C-value, which is proportional to the field strength of the BCCH. The BCCH is used for transmitting control data between the base station 101 , 201 and the subscriber terminal 102, 202. The BCCH carries, for example, identification data of the cells 100, 200. The channel measuring means 303 of the channel of the subscriber terminal 102 measure the quality of the traffic channel of the subscriber terminal 102. The term 'traffic channel' refers herein to the channel formed between the subscriber terminal 102 and the base station 101. The measuring means 303 measure, for example, the signal-to-noise ratio of the traffic channel. The signal-to-noise ratio measured by the channel measuring means 303 gives a Q-value, which is proportional to the quality of the channel. In addition, the transmission power measuring means 304 of the subscriber terminal 102 of the neighbouring cell 100 measure the transmission power of the subscriber terminal 102. The effect of the power control function on the magnitude of transmission power is taken into account in the measurement of the transmission power. The power control allows the current consumption of the subscriber terminal 102, 202 to be reduced, for example. If power control, frequency hopping or discontinuous transmission are not used in the subscriber terminals 102, 202 of the cell 100, 200, the C-value can be calculated from the traffic channel instead of the BCCH. In discontinuous transmission DTX, the transmission part B of the subscriber terminal 102, 202 is switched off when the subscriber terminal 102, 202 has nothing to transmit. The C- and l-field strength values obtained in the field strength measurement are transmitted to means 305 which, from these values, form C/l pairs, which in practice are ratios between field strengths. The means 305 also form the probability distribution of the disturbance caused by the measuring signal transmitted by the transmission means 204 in the downlink direction of the neighbouring cells 100. In addition, the calculation means 306 included in the cellular radio system are arranged to calculate the magnitude of the disturbance caused by the subscriber terminals of the neighbouring cell 100 in the uplink direction of the new cell 200. The power measurement means 304 of the neighbouring cell 100 measure the transmission power of the subscriber terminals 102, and the channel measuring means 303 measure the quality of the channel. The measurement values obtained are utilized for calculating the magnitude of the disturbance caused by the subscriber terminals 102 of the neighbouring cell 100 in the uplink direction of the new cell 200. From the measurement of the first field strength measurement means 301 and that of the second field strength measurement means 302, the means 305 included in the subscriber terminal 102 form the probability distribution of the disturbance caused by the measuring signal in the neighbouring cell 100. If an instantaneous measured Q-value is low, but the C- value is high, the C-value has been raised, for instance, by an interfering signal. In the above-mentioned situation, the means 305 have been arranged to ignore the probability value in question, or the means 305 weight the probability value obtained in such a manner that the significance of the probability value is reduced. The probability distribution of the disturbance caused by the base station 201 of the new cell 200 in the downlink direction of the neighbouring cells is obtained by means of the field strength measurements carried out. In addition to field strength, the instantaneous transmission powers of the subscriber terminals 102, 202 within the area of the new cell 200 and its neighbouring cells 100 are also measured. The magnitude of the disturbance caused by the neighbouring cells 100 in the new cell 200 is calculated from the instantaneous transmission powers. The probability distribution by which the subscriber terminals 102 of the neighbouring cells 100 disturb the traffic of the new cell 200 in the uplink direction is calculated from the transmission powers. The means 305 included in the base station 201 of the new cell 200 are arranged to measure the strength of the signal received by the station 201 from the subscriber terminals 202. The probability distribution of the measured field strength is formed from the measured signal strength. The probability distribution of the disturbance caused by the subscriber terminals 102 of the neighbouring cell 100 in the uplink direction of the new cell 200 is obtained from the probability distribution of the measured field strength and the probability distribution of field strength caused by the disturbance. Means 307 included in the cellular radio system are arranged to form interference and separation matrices from the probability distributions. The interference and separation matrices are utilized in the allocation of a frequency to the new cell 200. Figure 3 illustrates another way of allocating a frequency to the new cell 200. In the method, the first field strength measuring means 301 of the subscriber terminal 202 in the new cell 200 measure the field strength of the BCCH between the subscriber terminals 202 and the base station 201. In addition, the second field strength measuring means 302 of the subscriber terminal 202 measure the disturbance field strength caused by the BCCHs of the neighbouring cells 100 in the new cell 200. The power measurement means 304 are arranged to measure the actual transmission power of the subscriber terminals 202 in the new cell 200. The measured field strength of the BCCH of the new cell 200 gives a C-value, which is proportional to the field strength, and the disturbance field strength caused by the BCCH of the neighbouring cells 100 gives an l-value, which is proportional to the disturbance field strength. The effect of power control on transmission power is taken into account in the measurement of the actual transmission power of the subscriber terminals 202. From the C- and l-values, means 305 form the probability distribution of the disturbance caused by the base stations 101 of the neighbouring cell in the downlink direction of the new cell 200. In addition, the calculation means 306 included in the cellular radio system are arranged to calculate the magnitude of the disturbance caused by the subscriber terminals 202 of the new cell 200 in the uplink direction of the neighbouring cells 100. The measured field strength values are further utilized in the formation of various probability distributions. Means 305 included in the base station 101 located within the area of the neighbouring cells 100 in Figure 3 are arranged to measure the strength of the signal transmitted by the subscriber terminals 102 within the area of the base station. Means 305 are arranged to form cell-specific probability distributions of field strength from the measured signal strength. Furthermore, means 305 are arranged to form the probability distribution of the disturbance caused by the subscriber terminals 202 of the new cell 200 in the uplink direction of the neighbouring cell 100.This probability distribution is formed from the cell-specific probability distributions of field strength and from the probability distributions of disturbance field strength. Means 307 included in the cellular radio system form the interference and separation matrices. The probability distributions are utilized in the formation of the matrices. The interference and separation matrices are used for allocating a frequency to the new cell 200. It is possible to make a probability distribution with a tool of the prior art. Another alternative is to use measurements of the invention and to replace the BCCH of each cell 100, 200 with a separate measurement frequency. If the measurement range of the subscriber terminal 102, 202 is not sufficient for accurate determination of the field strength ratios, the transmission power of the base station 201 transmitting the test frequency can be increased, whereby the level of the disturbing signal is raised. The transmission power of the base station 201 transmitting the test frequency can be increased, for example, by 10 dB from the normal transmission power. The solution of the invention allows the base station 201 to be replaced with a test transmitter, which transmits a transmission frame of the cellular radio system in question. The building of a base station 201 can thus be avoided by the use of a test transmitter. It is also possible to make measurements when the subscriber terminal 102 of the neighbouring cell 100 has no call in progress. In this case, idle state measurements of the subscriber terminal 102 are used. If, according to the measurements made by the subscriber terminal 202, the field strength of the cell of the subscriber terminal 202 is, for example, -50 dBm, and the field strength of the neighbouring cell 100 is, for example, -60 dBm, the value of the field strength ratio is 10 dBm. The average interference matrix between the cells 100, 200 is calculated from the measurement results obtained. When the disturbance tolerance limits of the cellular radio system are taken into account, the separation matrix is obtained. The separation matrix gives the frequencies to be used in the new cell 200. One or more cells 100, 200 of the cellular radio system form predetermined sites. The separation matrix is calculated by means of various parameters obtained from different sites of the cellular radio network. The separation matrix values of cells 100, 200 located on the same site are not calculated, but the values are obtained from the parameters of the cellular radio system. If the cells 100, 200 are located in different sites, the value of the separation matrix is calculated on the basis of the measurement results obtained from the field strength measurements made in the cellular radio system. Although the invention has been described above with reference to the example illustrated in the accompanying drawings, it will be clear that the invention is not limited to this example but can be modified in many ways within the scope of the invention disclosed in the appended claims.

Claims: CLAIMS 1. A method for allocating a frequency to a cell (200) in a cellular radio system, said cell (200) transmitting a measuring signal, and said cellular radio system comprising a plurality of neighbouring cells (100) adjacent to said cell (200), said cells (100, 200) comprising a plurality of subscriber terminals (102, 202) and at least one base station (101 , 201) serving the subscriber terminals (102, 202) within the area of the cells (100, 200) through channels, c h a r a c t e r i z e d in that the neighbouring cell (100) measures the disturbance field strength caused by the measuring signal and the field strength of the channel serving a subscriber terminal (102) of the neighbouring cell (100), from which ratios of field strength to disturbance field strength are formed, said ratios being utilized in the formation of a probability distribution for the disturbance caused by the measuring signal in the neighbouring cell (100), said probability distribution being utilized in the allocation of a suitable frequency to said cell (200). 2. A method for allocating a frequency to a cell (200) in a cellular radio system, said cell (200) transmitting a measuring signal, and said cellular radio system comprising a plurality of neighbouring cells (100) adjacent to said cell (200), said cells (100, 200) comprising a plurality of subscriber terminals (102, 202) and at least one base station (101 , 201) serving the subscriber terminals (102, 202) within the area of the cells (100, 200) through channels, c h a r a c t e r i z e d in that said cell (200) measures the disturbance field strength caused by the channel serving a subscriber terminal (102) of the neighbouring cell (100) and the field strength of the channel serving a subscriber terminal (202) of said cell (200), from which ratios of field strength to disturbance field strength are formed, said ratios being utilized in the formation of a probability distribution for the disturbance caused by the base station (101) of the neighbouring cell (100) in said cell (200), said probability distribution being utilized for allocating a suitable frequency to said cell (200). 3. A method according to claim ^ c h a r a c te r i z e d in that said method comprises forming a probability distribution of the disturbance caused by the measuring signal in the downlink direction of the neighbouring cell (100). 4. A method according to claim ^ c h a r a c t e r i z e d in that said method comprises calculating the magnitude of the disturbance caused by the subscriber terminals (102) of the neighbouring cell (100) in the uplink direction of said cell (200). 5. A method according to claim 4, characterized in that said method comprises measuring instantaneous transmission power of the subscriber terminal (202), and utilizing the measurement result obtained for calculating the magnitude of the disturbance caused by the subscriber terminal (102) of the neighbouring cell (100) in the uplink direction of said cell (200). 6. A method according to claim 4, characterized in that said method comprises measuring the quality of the channel serving the subscriber terminal (102), and utilizing the measurement result obtained for calculating the magnitude of the disturbance caused by a subscriber terminal (102) of the neighbouring cell (100) in the uplink direction of said cell (200). 7. A method according to claim 6, characterized in that the ratio of field strength to disturbance field strength is ignored or underrated, if the quality of the serving channel is poor as compared with the measured field strength of the serving channel. 8. A method according to claim 1, characterized in that an interference probability matrix and a separation matrix are formed from the probability distribution of the disturbance caused by the measuring signal in the neighbouring cell (100), said matrices being used for allocating a frequency to said cell (200). 9. A method according to claim 1, characterized in that a base station (201) located within the area of said cell (200) measures the strength of the signal received from the subscriber terminals (202) within its area, said signal strength being used for forming the probability distribution of the measured field strength. 10. A method according to claim 9, characterized in that the probability distribution of the disturbance caused by the subscriber terminals (102) of the neighbouring cell (100) in the uplink direction of said cell (200) is formed from the probability distribution of the measured field strength and the probability distribution of the disturbance caused by the measuring signal in the neighbouring cell (100). 11. A method according to claim 1 or 2, characterized in that the channel serving the subscriber terminal (102, 202) is a BCCH or a traffic channel. 12. A method according to claim 2, characterized in that the method comprises forming the probability distribution of the disturbance caused by the base station (101) of the neighbouring cell (100) in the downlink direction of said cell (200). 13. A method according to claim 2, characterized in that the method comprises calculating the magnitude of the disturbance caused by the subscriber terminals (202) of the cell (200) in the uplink direction of the neighbouring cell (100). 14. A method according to claim 2, characterized in that the method comprises measuring instantaneous transmission power of the subscriber terminal (102), said instantaneous transmission power being utilized in the calculation of the disturbance caused by a subscriber terminal (202) of the cell (200) in the uplink direction of the neighbouring cell (100). 15. A method according to claim 2, characterized in that an interference probability matrix and a separation matrix are formed from the probability distribution of the disturbance caused by the base station (101) of the neighbouring cell (100) in said cell (200), said matrices being used for allocating a frequency to said cell (200). 16. A method according to claim 2, characterized in that the base station (101) of the neighbouring cell (100) measures the strength of the signal received from the subscriber terminals (102) within its area, said signal strength being used for forming a probability distribution of the measured field strength. 17. A method according to claim 16, characterized in that the probability distribution of the disturbance caused by the subscriber terminals (202) of said cell (200) in the uplink direction of the neighbouring cell (100) is formed from the probability distribution of the measured field strength and the probability distribution of the disturbance caused by the base station (101) of the neighbouring cell (100) in said cell (200). 18. A cellular radio system comprising a plurality of neighbouring cells (100) surrounding a cell (200), and means (204) for transmitting a measuring signal from said cell (200) to a neighbouring cell (100), said cells comprising a plurality of subscriber terminals (102, 202), and at least one base station (101, 201) serving the subscriber terminals (102, 202) within the area of the cell (100, 200) through channels, characterized by comprising first field strength measuring means (301) for measuring disturbance field strength, second field strength measuring means (302) for measuring the field strength of the channel serving the subscriber terminal (102, 202), and means (305) for forming ratios of field strength to disturbance field strength, said ratios being utilized for forming disturbance probability distributions utilized for allocating a suitable frequency to the cell (100, 200). 19. A cellular radio system according to claim 18, characterized in that the first field strength measuring means (301) are arranged to measure the disturbance field strength caused by the measuring signal transmitted from said cell (200) to the neighbouring cells (100). 20. A cellular radio system according to claim 18, characterized in that the first field strength measuring means (301) are arranged to measure the disturbance field strength caused by the channel serving the subscriber terminal (102) of the neighbouring cell (100). 21. A cellular radio system according to claim 18, characterized in that the means (305) are arranged to form a probability distribution of the disturbance caused by the measuring signal in the downlink direction of the neighbouring cell (100) from the ratios of field strength to disturbance field strength. 22. A cellular radio system according to claim 18, characterized in that the means (305) are arranged to form the probability distribution of the disturbance caused by the base station of the neighbouring cell (100) in the downlink direction of said cell (200) from the ratios of field strength to disturbance field strength. 23. A cellular radio system according to claim 18, characterized in that it comprises calculation means (306) for calculating the magnitude of the disturbance. 24. A cellular radio system according to claim 23, character- i z e d in that the calculation means (306) are arranged to calculate the magnitude of the disturbance caused by the subscriber terminals (102) of the neighbouring cell (100) in the uplink direction of said cell (200). 25. A cellular radio system according to claim 23, characterized in that the calculation means (306) are arranged to calculate the magnitude of the disturbance caused by the subscriber terminals (202) of said cell (200) in the uplink direction of the neighbouring cell (100). 26. A cellular radio system according to claim 18, characterized in that it comprises means (307) for forming an interference probability matrix and a separation matrix from the disturbance probability distributions, said matrices being used for allocating a frequency to the cell (100, 200). 27. A cellular radio system according to claim 18, characterized in that the first field strength measuring means (301), the second field strength measuring means (302), and the means (303) are provided in the subscriber terminal (102, 202). 28. A cellular radio system according to claim 18, character- ized in that the channel serving the subscriber terminal (102, 202) in the cellular radio system is a BCCH or a traffic channel.

Inventors: JOLMA PETRI, LEPPAENEN RISTO, JOLMA, PETRI, LEPPAENEN, RISTO
Applicants: JOLMA PETRI, LEPPAENEN RISTO, NOKIA TELECOMMUNICATIONS OY, JOLMA, PETRI, LEPPAENEN, RISTO
Application year: 1997
Main classification: H04Q7
IPC codes: H04W16