METHOD OF ALLOCATING FREQUENCY BANDS TO DIFFERENT CELLS, AND TDMA CELLULAR RADIO SYSTEM

Abstract: The invention relates to a TDMA cellular radio system, and a method of allocating available frequency bands to different cells in a TDMA cellular radio system wherein at least some of the receivers within the system employ interference cancellation methods for cancelling the effects of co-channel interference in a desired signal. To enable an efficient utilization of interference cancellation methods and the resulting capacity increase in the system, the frequency bands available for the cellular radio system are, according to the method of the invention, so allocated to different cells that groups (50-55) of adjacent service areas are formed in the system, each group employing the same frequency band within the service areas, whereby at least one co-channel signal interfering with the desired signal is significantly stronger than other co-channel interfering signals.

Description: Method of allocating frequency bands to different cells, and TDMA cellular radio system The present invention relates to a method of allocating available frequency bands to different cells in a TDMA cellular radio system wherein at least some of the receivers within the system employ interference cancellation methods for cancelling the effects of co- channel interference in a desired signal. As far as narrow-band cellular radio systems are concerned, co-channel interference is one of the most significant factors limiting the system capacity. The term co-channel interference refers to interference in the desired signal caused by signals transmitted in near-by cells on the same frequency. For this reason, cellular radio systems have conventionally been so designed that the same frequency is only used in cells located at a sufficient distance from each other, whereby the interfering signals remain within acceptable limits as a result of propagation attenuation of the signals. This leads to a so-called re-use pattern in the cellular structure. For example, the re-use pattern seven means that one seventh of all the available frequency bands is allocated to every cell, and the same frequencies are re- used at a relative re-use distance of 21 from each other. If it is desired that the system capacity be increased, the need arises to shorten the re-use distance without, however, decreasing transmission quality. Conventionally, an increase in the system capacity has been achieved by decreasing cell sizes and reducing transmission power levels. Another method for solving the problems concerning capacity is to keep up with the fast development of digital signal processing, and utilize the interference cancellation algorithms developed. The cancelling of co-channel interference in the receiver enables a more efficient frequency re-use within the networ . In the present-day cellular radio systems, co- channel signals are approximated in the receiver as a random additive white Gaussian noise. This approximation is sufficient as long as the interfering co-channel signals are not too strong. It is an aim of the conventional cell planning to ensure this matter by a sufficient re-use distance. In interference limited cellular radio systems, however, the co-channel interference is typically of a deterministic nature, suggesting that it should be possible to cancel, at least partly, its effects on the desired signal. Previously, interference cancellation methods have been developed for use in code division multiple access (CDMA) systems, which are of interference limited type. It is, however, much more difficult to apply interference cancellation techniques to TDMA systems. The Finnish patent publication 944736, which is herein incorporated as reference, discloses a method for utilizing interference cancellation methods in TDMA systems. Thus, interference cancellation methods are based on detecting some interfering signals and cancelling their effect in the desired signal. The interference cancellation algorithms function the better the more accurately the interfering signals can be detected. Hence, the most advantageous situation for the implementation of interference cancellation algorithms is such where the receiver is receiving, along with the desired signal, a number of interfering signals which are clearly stronger than the other sources of interference, and which can be cancelled by means of the algorithms. In frequency and cell planning of conventional cellular radio systems, however, the aim is that all the interfering signals would be as weak as possible from the point of view of the desired signal. As a result, the interference cancellation methods do not function in an optimum manner in systems designed by conventional methods . It is an object of the present invention to provide a cellular radio system in which the use of interference cancellation methods is taken into account already at the planning stage of the system, and in which an efficient application of said methods, as a result of network planning carried out according to the invention, enables a higher capacity than what has been possible heretofore. This object is achieved by a method set forth in the introduction, characterized in that the available frequency bands of the cellular radio systems are so allocated to different cells that in cell at least one co- channel signal interfering with the desired signal is significantly stronger than other co-channel interfering signals. The invention further relates to a TDMA cellular radio system which comprises in each cell at least one base station communicating with the subscriber terminal equipments within its service area, which system is allocated a group of frequency bands, and in which system at least some of the receivers of the system employ interference cancellation methods for cancelling the effects of co-channel interference in the desired signal . The TDMA cellular radio system in accordance with the invention is characterized in that the available frequency bands in each cell of the cellular radio system have been so selected that at least one co-channel signal interfering with the desired signal is significantly stronger than the other co-channel interfering signals. According to the basic idea of the method of the invention, the available frequency bands of the cellular radio system are so allocated to different cells that several groups of adjacent service areas are formed in the system, each group employing the same frequency band within the service areas. Thus, for each group, such a situation is intentionally created in which one co-channel signal interfering with the desired signal is significantly stronger than co-channel interfering signals originating in other signal sources. This enables an efficient application of interference cancellation algorithms, which leads to the terminal equipments and the base stations being capable of operating at a lower signal to noise ratio, but nevertheless at the same transmission quality as before. As a result of the method according to the invention, a better sensitivity of the receiver is achieved. It is possible to utilize the improved sensitivity in a multitude of ways. The increase in the capacity of the cellular network was already mentioned above. In addition, it is possible to improve the quality of the connections as a result of the receivers tolerating a higher level of interference. Furthermore, cell sizes and data transmission rates may be increased. In the following, the invention will be described in greater detail by means of the preferred embodiments with reference to the examples of the attached drawings, in which figure 1 shows a conventional frequency allocation of a cellular radio system, figure 2 shows a cell divided into three sectors, figure 3 shows a frequency allocation with a re-use factor of three, figure 4 illustrates an example of overlapping microcells and macrocells, and figures 5a - 5b illustrate the frequency allocation of a cellular radio system implemented by means of cell pairs, with re-use factors of three and five. The method of the present invention for allocating frequencies to different cells may be applied to any digital TDMA cellular radio system. The method of the present invention does not set any limits to the interference cancellation algorithm used, either. The performance of all prior art interference cancellation methods suitable for a cellular radio environment will be enhanced, as long as the network planning is carried out according to the present invention. Figure 1 illustrates the conventional way of allocating frequencies to different cells, with a re-use factor of seven. This means that one seventh of all the frequency bands of the system is in use in each cell, and the same frequency bands are re-used as illustrated by figure 1, in which the frequencies used in cell 10 are also used in cells 11 - 16. It is assumed that the desired connection is located in cell 10, and that the same channel (the same time slot on the same frequency) is also used in cells 11 - 16. In such as case, as illustrated by figure 1, the interfering signals 17a - 17f arrive in the cell 10 from approximately an equal distance. Naturally, each signal undergoes an individual multipath propagation and fading, which means that the signal levels vary to some extent, but the attenuation caused by distance is equal. In the solution according to the invention, the aim, already at the stage of determining the structure and operation of the network, is to take into account that at least some of the receivers of the network could utilize interference cancellation. In practice, this may be taken into account by aiming at such a network solution where one or several interfering co-channel signals are in all probability significantly stronger than the overall interference level. As far as frequency planning is concerned, this leads to irregular re-use patterns. Figure 2 illustrates a cell which has been divided into three sectors 20 - 22 by means of directional antennas. Conventionally, each sector has employed a dedicated frequency different to the one used in the other sectors, and the same group of three frequency bands has been used with the re-use factor of three. The re-use is illustrated in figure 3. Figure 3 shows a group of cells by means of base stations located in the midpoint, the base stations being indicated by small circles. Figure 3 shows by bold circles those base stations that employ the same specific frequency band. According to the method of the invention, all the three sectors 20 - 22 employ the same frequency, and the other two frequencies are left for use by other cells. In this situation, it is essential that all the receivers be able to utilize interference cancellation. As far as interference is concerned, the worst situation emerges at the border of two adjacent sectors. An example of such a situation is illustrated in figure 3, where the terminal equipment 23 communicates with the receiver of sector 21, and the terminal equipment 24 communicates with the receiver of sector 22, and both of them are close to each other at the border of the sectors. If both the terminal equipments, which communicate with different sectors as mentioned above, use the same frequency and time slot for communicating with the base station, the terminal equipments heavily interfere with each other, and the signal to noise ratio is 0 dB at its worst. It is not possible for the ratio to decrease from that, because at that stage handover to another sector is carried out . From the point of view of the interference cancellation algorithms, it is clear that this solution provides a desired situation where one or two co-channel signals are significantly stronger than interference from any other source. The potential benefit in the capacity increase can be estimated to exceed 30%. The method of the invention is by no means restricted to dividing the cell into three sectors as in the exemplary case above. It is possible to divide each cell into two or more sectors; in principle, the number of sectors is unlimited. Figure 4 illustrates a network structure in which the network is implemented by means of a group of small microcells 41 - 45 and a large macrocell 40 covering the same geographical area. The macrocell corresponds to a present-day conventional cell, which handles mobile communication. The microcells may be cells inside buildings, for example, or cells providing extra capacity to traffic nodes. In the latter case, the macrocell is a so-called umbrella cell, by means of which an easy handover from one microcell to another can be achieved. In the frequency planning solution according to the invention, both the macrocell 40 and the microcells 41 - 45 use the same frequency band. There is no need for the macro and microcells to be synchronized with each other. Compared to the macrocells, transmission power levels used by the microcells are low, which means that co-channel interference caused by the microcells to the macrocell is negligible, and, consequently, no interference cancellation is required in the macrocell. The co-channel interference in the communication links from a macrocell to a microcell may be eliminated by means of interference cancellation. Interference cancellation should be utilized by all the microcell receivers. Co-channel interference between the microcells is negligible due to the low transmission power levels used in the cells. The arrangement described above enables an advantageous implementation of office microcells, for example, in an area where there already exists a macrocell in the same frequency range. Office microcells located in other buildings do not interfere with each other due to a severe propagation attenuation and the low transmitting power mentioned above. Figure 5a illustrates a third preferred embodiment of the invention, in which the frequency bands available are so allocated that the cellular network consists of adjacent cell groups 50 - 55 within which the same frequency band is used in the cells. The cell groups may comprise two or more cells, and the number of cells per cell group may vary on a group-to-group basis. In the example of figure 5a, each cell group comprises two cells, i.e. a cell pair. As in figure 3 above, figure 5a illustrates a group of cells by means of base stations in their midpoint so that small circles represent the base stations. The bold circles in the figure represent those base stations using the same, specific frequency band. In the example according to figure 5a, in both cells of the cell pair, a co-channel signal originating in the other cell of the cell pair and interfering with the desired signal is significantly stronger than the co-channel interfering signals originating in the other cells. It is assumed that the cells of the cell pairs are synchronized with regard to each other. In figure 5a, the frequencies of the cell pairs are re-used with a re-use factor of three. Correspondingly, in figure 5b, the frequencies of the cell pairs are re-used with a re-use factor of five. At its best, the capacity increase achieved by the solution is 150%. In the cell group solution, the capacity may be increased even further by using directional antennas in the cells, enabling a further decrease in the interference originating in the other cell groups that use the same frequency range. The idea according to the invention may also be implemented by decreasing, in comparison with conventional frequency planning, the re-use factor of the frequency bands of the cellular radio network, which re-use factor determines the distance at which the frequency band may again be used. In such a case, co-channel interference is commonly increased compared with the normal situation. By an appropriate positioning of the base stations, a situation is created where in all probability a few co- channel signals interfering with the desired signal are in each cell significantly stronger than the other co-channel interfering signals. This is based on each signal having an individual propagation and attenuation on the radio path. The idea of the invention may also be carried out by using the same time slot of the same frequency band within the service area of one base station of the cellular radio system on two separate connections between the base station and the terminal equipment. This means that the strongest co-channel interference (cancellable by interference cancellation algorithms) originates in the area of the same cell. Although the invention is described above with reference to the examples in the accompanying drawings, it is obvious that the invention is not restricted thereto, but it may be modified in a number of ways within the scope of the inventive idea disclosed in the attached claims .

Claims: Claims 1. A method of allocating available frequency bands to different cells in a TDMA cellular radio system wherein at least some of the receivers within the system employ interference cancellation methods for cancelling the effects of co-channel interference in a desired signal, c h a r a c t e r i z e d in that the available frequency bands of the cellular radio systems are so allocated to different cells that in cell at least one co-channel signal interfering with the desired signal is significantly stronger than other co-channel interfering signals . 2. A method as claimed in claim 1, c h a r a c t e r i z e d in that the available frequency bands of the cellular radio system are so allocated to different cells that several groups of adjacent service areas are formed in the system, each group employing the same frequency band within the service areas . 3. A method as claimed in claim 1, c h a r a c t e r i z e d in that the available frequency bands are so allocated that each cell is divided into two or more sectors (20 - 22) by means of directional antennas, and that the same frequency band is used in each sector, whereby in each sector at least one co-channel signal originating in an adjacent sector and interfering with the desired signal is significantly stronger than the co-channel interfering signals from the other cells. 4. A method as claimed in claim 1, c h a r a c t e r i z e d in that the available frequency bands are so allocated that the system comprises large macrocells (40) , each employing a specific frequency band, and that within the service area of each macrocell there are a group of microcells (41 - 45) which are significantly smaller than the macrocells, and each of the microcells using the same frequency band as the corresponding macrocell, whereby in each microcell one macrocell co-channel signal interfering with the desired signal is significantly stronger than the co-channel interfering signals originating in the other cells. 5. A method as claimed in claim 4, c h a r a c t e r i z e d in that receivers of the microcells (41 - 45) employ interference cancellation methods. 6. A method as claimed in claim 1, c h a r a c t e r i z e d in that the available frequency bands are so allocated that the system consists of adjacent cell groups (50 - 55) , which groups comprise two or more cells, which cells employ the same frequency band, whereby in each cell of the cell group at least one co- channel signal interfering with the desired signal is significantly stronger than the co-channel interfering signals originating in the other cells. 7. A method as claimed in claim 6, c h a r a c t e r i z e d in that the number of cells in each cell group (50 - 55) varies cell group specifically. 8. A method as claimed in claim 1, c h a r a c t e r i z e d in that the re-use factor of the frequency bands of the cellular radio system is chosen to be low enough, whereby in each cell one or more co-channel signals interfering with the desired signal are significantly stronger than the other co-channel interfering signals. 9. A method as claimed in claim 1, c h a r a c t e r i z e d in that the same time slot of a given frequency band is used on two separate connections within the service area of one base station. 10. A TDMA cellular radio system which comprises in each cell at least one base station communicating with the subscriber terminal equipments within its service area, which system is allocated a group of frequency bands, and in which system at least some of the receivers of the system employ interference cancellation methods for cancelling the effects of co-channel interference in the desired signal, c h a r a c t e r i z e d in that the available frequency bands in each cell of the cellular radio system have been so selected that at least one co- channel signal interfering with the desired signal is significantly stronger than the other co-channel interfering signals.

Inventors: GANDINI GIULIO, HONKASALO ZHICHUN, JOLMA PETRI, RANTA PEKKA, GANDINI, GIULIO, HONKASALO, ZHICHUN, JOLMA, PETRI, RANTA, PEKKA
Applicants: GANDINI GIULIO, HONKASALO ZHICHUN, JOLMA PETRI, NOKIA TELECOMMUNICATIONS OY, RANTA PEKKA, GANDINI, GIULIO, HONKASALO, ZHICHUN, JOLMA, PETRI, RANTA, PEKKA
Application year: 1995
Main classification: H04Q7
IPC codes: H04B7, H04W16