MXPA99003325A - Method for improving co-channel interference in a cellular system - Google Patents

Abstract

A fractional loading scheme is used to improve the spectral efficiency of a cellular system, and therefore increase the number of users that the system can support. The fractional loading scheme allows only a fraction of the total number of available communication channels within each cell to be used simultaneously. Thus, each cell is deliberately underloaded to operate at less than its full capacity. The underloading of the individual cells reduces the spectral efficiency within each cell. However, the underloading of each cell means that there will be fewer interfering users at any given time so that the co-channel interference is reduced. This reduction in co-channel interference allows the reuse distance between co-channel cells to be reduced thereby increasing the reuse of frequencies throughout the system resulting in an increase in spectral efficiency in the system as a whole.

Description

METHOD FOR IMPROVING CO-CHANNEL INTERFERENCE IN A CELLULAR SYSTEM Field of the Invention The present invention relates to cellular radio systems and more particularly to cellular structures and frequency plans for reducing co-channel interference in a cellular system. BACKGROUND OF THE INVENTION Traditionally, satellite systems use a single beam to cover a large geographic area. Within the coverage area, each carrier frequency is used only once. In 1995, the American Mobile Satellite Communications System was operational. This system uses a few spot beams to cover the Continental United States, Alaska and Hawaii. However, two carrier frequencies are not used simultaneously in the system. Since the available bandwidth limits the number of available channels, traditional satellite systems can not support a large number of users. In cellular communications systems, frequency re-use plans allow the same frequency to be used more than once within the system. Instead of using a single high-power transmitter to cover a large geographic area, cellular systems employ a large number of low-power transmitters that broadcast a signal in relatively small geographic areas referred to as cells. Each cell can be only a few kilometers across and theoretically can be as small as a few city blocks. By reducing the coverage area of the transmitter and creating a large number of cells, it is possible to reuse the same frequency in different cells. In this way, a single frequency can be used multiple times throughout the cellular system, to increase the capacity of the calling party. For example, considering that a particular geographic region is served by a single high-power transmitter that has ten frequency channels. The system would be able to handle only ten simultaneous calls. The eleventh calling party will be blocked because other channels are not available. If the same geographical region is divided into one hundred cells and the same frequencies can be used in all the cells, then one thousand simultaneous calls can be supported. This cellular approach can be used in satellite systems to increase the capacity of the system. Unfortunately, the immediate re-use of all frequencies in adjacent cells is not practical due to co-channel interference. Current cell boundaries in the real world are poorly defined and subject to constant changes due to signal fluctuations. In this way, the coverage area in adjacent cells overlaps and intermixes. A vehicle operating near the boundary of a cell will be in an ambiguous zone where the signal strength of two adjacent cells using the same frequency is approximately equal. This balanced zone or zone of interference makes communications difficult. The mobile unit will interlock first in one transmitter then in another, as the signal strength of the transmitters in the adjacent cells fluctuates. This constant jump between transmissions would make communication impossible. To avoid the problem of co-channel interference, cells operating on the same frequency are spatially separated such that the mobile unit operating within a cell receives the desired signal at a higher level than any potential interference signal from a cell. co-channel cells. Cells that operate at different frequencies are placed between any two co-channel cells. In this way, the mobile unit will change frequencies during transfer as it approaches a cell boundary before entering the zone of interference between any two co-channel cells. In general, the power of any interference signal decreases as the distance between users that interfere increases. A carrier frequency can be re-used if the level of interference is sufficiently reduced by separating between co-channel calls. The level of interference is measured by the ratio of carrier power to interference power, C / I. The C / I ratio is the primary criterion used in designing frequency re-use plans. From the foregoing, it should be apparent that the number of times a given frequency can be reused in a system is related to the separation distance or re-use distance between any two co-channel cells. Developing new frequency allocation plans that reduce co-channel interference by allowing greater re-use of frequencies without sacrificing signal quality will result in increased system capacity. COMPENDIUM OF THE INVENTION The present invention provides a frequency assignment plan to improve the spectral efficiency of a cellular system and therefore increase the number of users that the system can support. The method to assign frequencies is based on the concept of fractional load of cells within the system. The fractional loading technique is used to reduce co-channel interference, thus allowing the re-use distances between co-channel cells to be reduced. By reducing the re-use distance, the same carrier frequency can be used more often to increase the spectral efficiency of the system as a whole. In accordance with the present invention, each cell within a cellular communication system is assigned a group of carrier frequencies. A multiple access scheme is used to divide the carrier frequencies available in each cell, in a plurality of different communication channels. In the preferred embodiment of the invention, multiple access with time division is used. Each carrier frequency is divided into a number of time slots, with each slot representing a different communication channel within the cell. Within each cell, a fractional load scheme is used to assign the communication channels available to individual users in the call configuration. The fractional load scheme allows only a portion of the total number of communication channels available within each cell to be used simultaneously. In this way, each cell is deliberately sub-charged to operate unless the capacity integrates. The underload of the individual cells reduces the spectral efficiency within each cell. However, the underload of each cell means that there will be fewer users of interference at any given time, so that the co-channel interference is reduced. This reduction in co-channel interference allows the re-use distance between co-channel cells to be reduced, thus increasing the re-use of frequencies through the system, resulting in an increase in spectral efficiency in the system like an everything. A variety of fractional load schemes can be used to assign frequencies to individual users within each cell. The simplest fractional charge scheme is to establish a maximum number of simultaneous users per cell or carrier frequency. Once the threshold is reached, any additional users are blocked. Alternatively, a signal quality test can be used to determine if additional frequencies are assigned once the threshold is reached. A test signal is transmitted on an available channel before a channel is assigned. After transmitting the test signal, the quality of the test signal and / or its effect on the communication channels already in use in co-channel cells, can be measured. If the test signal does not degrade the signal quality, then the channel can be assigned. In another aspect of the present invention, the communication channel available within each cell can be assigned randomly. Alternatively, communication channels can be assigned to minimize, as much as possible, the use of each communication channel at any given time between all the co-channel cells. For each available communication channel the total number of users in all co-channel cells for that particular channel is determined. The channel is assigned with the minimum number of users considering the co-channel cells. Other objects and advantages of the present invention will be apparent and apparent from a study of the following description in the accompanying drawings that are merely illustrative of said invention. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an illustration of a system of Multiple beam satellite communication. Figure 2 is a diagram of the distribution of cells for a cellular communication system having a plurality of cells hexagonally packed. Figure 3 is a diagram representing one simple carrier frequency in a TDMA system.

Figure 4 is a graph of the carrier-to-interference ratio as a function of the rate or code rate. Figure 5 is a graph of an antenna discrimination pattern is typical for a satellite antenna. Figure 6 is a graph of the relationship between the carrier-to-interference ratio and the fractional charge effect in a seven cell cell system with six interferers; Figure 7 is a graph of the relationship between the carrier-to-interference ratio and the fractional charge effect in a seven-cell cellular system with eighteen interferers; Figure 8 is a flow diagram illustrating how the available communication channels are allocated in a fractional load scheme. DETAILED DESCRIPTION OF THE INVENTION Now with reference to the drawings and particularly to Figure 1, a satellite communications system is illustrated and indicated generally by the number 100. The satellite communication system 100 includes one or more satellites 110 having antennas of multiple beams 120 projecting a plurality of point beams on the surface of the earth covering a designated geographical area. Preferably, the system includes 100 or more point beams providing a means of communication, for communications between stations on the ground. The ground station may be a fixed station 130 or a mobile station 140. The fixed station 130 for example may be a mobile services switching center (MSC = Mobile Services Switching Center) which provides interphase between the satellite communications system 100 and the public switched telephone network (PSTN = Public Switched Telephone Network). Calls between two ground stations, either fixed or mobile, are transmitted via satellite 110. Switching circuits on board satellite 110 allow calls originating from a point beam to be transmitted to one station on the ground at another point beam. In systems using more than one satellite, the connection can be made by a cross-link between two or more satellites 110. The satellite communication system 100 resembles a land-based mobile cellular communications system. The point beams projected on the surface of the earth are analogous to the cells of the earth-based cellular system. The satellite 110 is analogous to a base station serving as the mobile units are interconnected with the communication network. However, unlike land-based cellular systems, a single satellite can serve as a base station for multiple cells or point beams. Similar to land-based cellular systems, the satellite communications system 100 employs the concept of frequency re-use to increase the spectral efficiency of the system. Frequency reuse means that the same frequency can be used simultaneously in different cells or spot beams within the communications network. Quite obviously, the ability to re-use the same frequencies, often within a communications system, has enormous potential to increase the spectral efficiency of the system. An increase in spectral efficiency means that a greater number of simultaneous users can be supported. Spectral efficiency is measured in terms of users per Mhz per square kilometer or users per Mhz per cell. Now with reference to Figure 2, a seven-cell re-utilization plan is illustrated. As in a conventional cellular system, the available frequencies used for communication are sub-divided into groups of frequencies that are then assigned in a way to reduce co-channel interference. Frequency groups are assigned to cells in such a way that adjacent cells do not use the same frequency group. The frequency assignment scheme is called a reuse plan. Cells to which the same frequency has been assigned, they are called co-channel cells. Co-channel cells are spatially separated from each other to reduce co-channel interference. The cells are grouped into swarms that include a cell of each frequency group. Within each cell or point, a multiple access or multiple access scheme is used to assign communication channels to individual user stations. For example, multiple access with frequency division (FDMA = Multiple Access Frequency-Division) multiple access with time division (TDMA = Multiple Access Time-Division), or multiple access with division of code (CDMA = Code-Division Multiple Access) can be used In the present invention, TDMA is employed. The TDMA scheme has been standardized by the electronic industry association (IEA = Electronics Industry Association) and the telecommunications industry association (TIA = Telecommunication Industry Association) as IS-54, which is incorporated herein by reference. Figure 3 is an illustration of a TDMA carrier. In TDMA, each carrier frequency is divided into a series of frames that are also sub-divided into time slots. The frames are repeated at a fixed interval in time. In the preferred embodiment of the invention, each frame is divided into sixteen time slots. Each time slot represents a different communication channel that can support a single user station. When a call is established, each user station is assigned a different time slot during which the user station transmits and receives voice or data in short bursts. The burst transmissions must be carried out in the assigned time slot so as not to interfere with the transmission of other user stations using a different time slot on the same carrier frequency. In this way, there may be sixteen users assigned to each carrier frequency. The selected reuse pattern for a cellular system (either terrestrial or satellite based) affects both the amount of co-channel interference experienced, and the capacity of the system. For example, a four-cell re-use pattern will improve spectral efficiency compared to a seven-cell re-use pattern. In a four-cell re-use pattern, the same frequency can be used in one in four cells. In a seven-cell re-use pattern, the same frequency is used in one out of seven cells. Considering that the number and size of cells is fixed, a re-use pattern of four cells will increase the spectral efficiency by approximately 75%. However, the co-channel cells in a four-cell pattern are closer together than the co-channel cells in a seven-cell reuse pattern. In this way, a four-cell reuse pattern increases the amount of co-channel interference that will be experienced. The carrier to interference ratio C / l is the fundamental parameter in calculations of reuse factors. In system design, co-channel interference must be addressed from two respective ones. First, the required C / l required, such that the degradation in EB / N0 is less than a specified value, must be determined. The ratio EB / N0 represents the energy per bit of information about the interference spectral density. Second, the system must be designed in such a way that the current C / l experienced due to co-channel interference, is above the minimum C / l required by a predetermined percentage of time. The C / l that is required in such a way that the EB / N0 ratio is not substantially degraded, can be derived as follows. The compound EB / (N0 + I0) can be written as: Since EB = C / RB, I0 = I / BW, and BW = Rb / (wR), equation 1 can be rewritten as: where m is the modulation order and i? is the effective code rate. For QPSK, the modulation order m = 2. Solving equation 2 for C / l produces the following expression: The minimum required C / l that results in maximum degradation in EB / N0 is given by the following expression (in dB): -rlOlogR +3 (4) In Figure 4, equation 4 is plotted for probable losses of 0.5, 1, 2 and 3 decibels, as a function of the channel code rate. The EB / N0 required for BER = 10 ~ 3 for different code rates is obtained from Clark and Cain for a Rice factor K = 6. The values for EB / N0 are 6.7 dB for a code rate of 1, 3.9 dB for a code rate of 3/4, 3.5 dB for a code rate of 2/3, 3.0 dB for a code rate of 1/2, 2.6 dB for a code rate of 1/3, and 2.3 dB for a code rate of code rate of 1/4. A reasonable operating point therefore is a C / L of 10.5 to 12.5 dB, which allows the use of either a 1/2 speed or 1/3 speed code with a? loss to co-channel interference no greater than 0.5 dB. Once the minimum required C / l is determined, the distribution of cells and the reuse pattern is designed, so that the current C / l experienced will exceed the minimum C / l required. In conventional mobile cellular systems (both ground based and satellite based) cell distribution and frequency assignment are designed to obtain the minimum C / l required under fully loaded conditions. That is, the calculation of re-utilization distances considers that all available channels will be used simultaneously. The present invention departs from this prior art practice. A fractional loading technique is used to reduce co-channel interference thereby allowing the re-use distances between co-channel cells to be reduced. By reducing the reuse distance, the same carrier frequency can be used more often to increase the spectral efficiency of the system as a whole. A computer simulation of a satellite communications system has been used to demonstrate the fractional loading technique. The current C / l for a satellite communication system can be described mathematically by the following equation: where N is the total number of co-channel interferers, Antdis is the antenna discrimination between the user and the i-th interferent, and dP ± is the difference of the EbNQ between the user and the i-th interferente. Antenna discrimination is a measure of the ability of the antenna to reject signals that are received off the axis to the antenna sight. A typical antenna discrimination pattern is illustrated in Figure 5. As seen in Figure 5, the relative gain as the distance or angle from the antenna sight increases, declines non-monotonically and includes a number of side lobes. The sidelobes may result in the reception of interference signals in an undesired direction. The expression in Equation 5 is used to model a satellite communication system that has 61 cells in a hexagonally packed beam pattern. Each beam is considered to result from a uniformly illuminated circular aperture. All the beams were charged evenly. Depending on the user's location within a beam, the power for that user is modulated in such a way that all users within a beam have equal ground power for the forward link and, on the contrary, equal received signal strengths in the satellite for the return link. Users were randomly placed within 25% of the crossing distance from the center of a beam. Table 1 below is a summary of the simulation showing the relationship between C / l and the crossing distance. For this simulation, the co-channel interference of 60 interference users is considered. The crossing distances between adjacent beams are varied between 2 dB and 10 dB. The results indicate that a crossing distance of approximately 4.5 dB produces the highest C / l for the 98th percentile case and is almost optimal for the 90th and 95th percentile cases. It also indicates that without DTX, speed coding of a third channel or more is required to meet a 0.5 dB loss requirement due to co-channel interference.

TABLE 1 C / I 90% 95% 98% DTX PC Err. Dist.

Prom. Slow PC Cruz. 0. 1 -0.5 -0.7 -1.0 NO YES NO 2.0 dB 2. 4 1.7 1.4 1.2 NO YES NO 2.5 dB 6. 9 5.6 5.2 5.0 NO YES NO 3.O dB 11. 2 9.2 8.8 8.4 NO YES NO 3.5 dB . 7 10.3 10.1 10.0 NO YES NO 4.0 dB . 5 10.0 9.8 9.5 NO YES NO 4.5 dB 1 100..11 9 9..44 9 9..33 9 9..00 N NOO YES NO 5.0 dB 9. 7 9.0 8.9 8.6 NO YES NO 6.0 dB 9. 5 8.8 8.7 8.5 NO YES NO 6.5 dB 9. 4 9.9 8.9 8.7 NO YES NO 7.5 dB 9. 7 9.3 9.2 9.0 NO YES NO 10.0 dB Figure 6 is a graph showing the effect of fractional load in C / l for a seven-cell pattern with 18 interfering swarms, based on a computer simulation. Figure 7 is a graph showing the effect of fractional charge in C / l for a seven-cell pattern with 6 interfer swarms, based on a computer simulation. These graphs show a peak in C / l at a normalized re-utilization distance of approximately 2. There are circumstances where this peak in C / I can be advantageously used to increase the spectral effici of the system by fractional charge of the cell. For example, with refer to Figure 6, considering that a C / I of at least 12 is required to maintain an acceptable signal quality. If the system is fully charged at 100% capacity, then a normalized re-utilization distance of approximately 3.3 is required. However, a minimum C / l of 12 can also be obtained at a standardized reuse of 2.0 when loading the cell to 62.5% of its full capacity. That is, if it is considered that no more than 62.5% of the available channels in any given cell will be used simultaneously, then the re-use distance can be reduced from 3.3 to 2.0. Fractional charge in a cell results in a loss of spectral effici in that cell. However, that loss is displaced by a gain in the spectral effici of the system as a whole resulting from a reduction in the re-utilization distance between co-channel cells. In many cases, the gain attributable to a reduction in the distance of use will exceed the loss attributable to the fractional load of the cell. In these cases, the fractional charge of the cells can result in much greater total spectral effici.

As an example, it is considered that a re-use pattern of seven cells in a fully loaded system due to co-channel interfer is required. The subloading of carrier freques would reduce the spectral effici within a carrier but will result in fewer interfers and consequently a higher total C / l when all the interferers are considered. Whereas the reduction in C / l resulting from a load of 75% would allow a reuse pattern of four cells. The net result would be only 31% in spectral effici through the system. There are a number of different ways to implement the fractional loading technique in the satellite communications system of the present invention. The simplest technique is to randomly assign the time slots available on a TDMA carrier frequ, until a pre-determined maximum charge level is reached. For example, if a maximum load level of 75% is set and there are 16 available time slots on each carrier frequ, then only 12 of the available time slots will be allocated at any given time. In this way, at any given moment in time, there will be at least four unused time slots.

A slightly more complex technique involves the use of a signal quality test to determine if additional freques can be assigned after the predetermined threshold has been reached. The satellite can determine if additional channels are assigned by transmitting a test signal on one of the available channels and the quality of the test signal is measured. If the quality of the test signal is at an acceptable level, then the channel can be assigned. Instead of measuring the quality of the test signal, the effect of the test signal on the channels that are already in use can also be measured. If the signal quality in the channels already in use in the co-channel cells remains at an acceptable level during transmission of the test signal, then the channel can be assigned. This process allows some flexibility in assigning communications channels over the predetermined threshold and can increase the spectral efficiency. Figure 8 is a flow chart of the decision process to determine if a new channel is assigned. After a request is received for a communication channel (block 200), it is determined whether the number of channels already in use meets a predetermined threshold (block 202). If the number of users is less than the threshold, then a new channel is assigned (block 204). Otherwise, a signal quality test is conducted (block 206). If the result of the signal quality meets certain predetermined criteria, ie the test is passed (block 208), then a new channel is assigned to the user (block 204). If the signal quality test is not passed (block 208) then access is denied (block 210). Instead of randomly assigning communication channels within a cell, it can be made to minimize an adaptive assignment, as much as possible, the disproportionate use of a particular channel in a group of two channel cells, to minimize co-channel interference . An example of adaptive assignment would be to assign channels in a cell based on the speed of use of that channel in all co-channel cells. For example, when a communication channel is requested by a user, the available channels in that cell are determined. The next stage is to determine which of the available channels is the least used considering all the co-channel cells. The channel that is used the least when all the co-channel cells are considered is assigned. Other adaptive application schemes in which channel allocation is made according to predetermined criteria will occur to those with skill in the specialty.

The present invention can of course be carried out in other ways specific to those established herein, without departing from the spirit and essential characteristics of the invention. For example, the particular dimensions used to describe the prototype constructed in accordance with the present invention are not intended to limit the scope of the claims but are provided only as examples. The present modalities will therefore have to be considered in all aspects as illustrative and not restrictive and all changes that fall within the meaning and range of equivalence of the appended claims, are intended to be encompassed here.

Claims (33)

1. - Method for reducing co-channel interference in a cellular radio communication having a plurality of cells, characterized in that it comprises: a) allocating the carrier frequencies available within the system to the cell within each carrier frequency, providing one or more channels of different communications such that a plurality of communication channels are available in each cell; and b) establishing a fractional load scheme for selected cells such that the maximum number of channels used at any given time in select cells is less than the total number of available channels in the selected cells.

2. - The method according to claim 1, characterized in that it also includes the step of spacing co-channel cells in the cellular communication system, to obtain a carrier power to interference power (C / I) ratio, with base in the fractional load scheme for the selected cells.

3. - The method according to claim 1, characterized in that the communication channels available in the fractionally loaded cells are assigned randomly.

4. - The method according to claim 1, characterized in that the allocation of communication channels in the fractionally loaded cells is made to minimize the simultaneous use of the communication channel in all the co-channel cells.

5. - The method according to claim 1, characterized in that it also includes the step of establishing a threshold for the total number of simultaneous users in each cell.

6. The method according to claim 5, characterized in that it includes the step of denying access when the threshold is reached.

7. - The method according to claim 5, characterized in that it also includes the step of transmitting a test signal on a channel not used when the threshold is reached and perform a signal quality test to determine if additional channels should be assigned .

8. - The method according to claim 7, characterized in that the signal quality test comprises testing the signal quality in the test channel.

9. - The method according to claim 7, characterized in that the signal quality test comprises testing the effect of the test signal on a channel that is already in use.

10. A cellular radio communications system, characterized in that it comprises: a) a plurality of cells, each cell is assigned one or more carrier frequencies that provide a plurality of different communication channels in each cell; b) the plurality of cells includes at least one fractional load cell where the maximum number of channels in use at any given time is less than the total number of channels available in the cells.

11. The cellular radio communication system according to claim 10, characterized in that the assignment of communication channels in the fractionally loaded cells is done randomly.

12. - The cellular radio communications system according to claim 10, characterized in that the allocation of communication channels in the fractionally loaded cells is that it minimizes the simultaneous use of communication channels in all the co-channel cells.

13. - The cellular radio communication system according to claim 10, characterized in that the pre-determined threshold is established for the total number of users in the fractional load cell that is less than the total number of available channels.

14. - The cellular radio communications system according to claim 13, characterized in that the allocation of communication channels to users is blocked when the predetermined threshold is reached.

15. - The cellular radio communication system according to claim 13, characterized in that the allocation of communication channels to users once the predetermined threshold is reached, is based on a signal quality test.

16. The cellular radio communication system according to claim 15, characterized in that the signal quality test comprises transmitting a test signal in an available channel and measuring the signal quality of the test signal.

17. The cellular radio communications system according to claim 15, characterized in that the signal quality test comprises transmitting a test signal in an available channel and measuring the effect of the test signal in other users assigned to it. channel in other co-channel cells.

18. A method for increasing the number of users in a multi-beam satellite communication system having a plurality of spot beams, characterized in that it comprises the steps of: a) assigning the available carrier frequencies within the system to the spot beams , with each carrier frequency providing one or more different communication channels, such that a plurality of communication channels are available in each cell; and b) establish a fractional charge scheme for selected point beams, in such a way that the maximum number of channels used at any given time in the selected point beams is less than the total number of channels available in the selected beams and c) space the Co-channel data to obtain the minimum acceptable proportion of code to interference C / I based on the fractional load scheme.

19. The method according to claim 18, characterized in that the communication channels available in the fractionally charged point beams are assigned randomly.

20. The method according to claim 18, characterized in that the allocation of communication channels in the fractionally charged point beams is made to minimize the simultaneous use of the communication channel in all the co-channel point beams.

21. The method according to claim 18, characterized in that it also includes the step of establishing a threshold for the total number of simultaneous users in each point beam.

22. - The method according to claim 21, characterized in that it also includes the step of denying access when the threshold is reached.

23. - The method according to claim 21, characterized in that it also includes the step of transmitting a test signal on a channel not used when the threshold is reached and performing a signal quality test to determine if additional channels should be assigned .

24. - The method according to claim 23, characterized in that the signal quality test comprises testing the signal quality in the test channel.

25. The method according to claim 23, characterized in that the signal quality test comprises testing the effect of the test signal on a channel that is already in use. 26.- Method for reducing co-channel interference in a cellular radio communication having a plurality of cells, characterized in that it comprises: a) assigning the available carrier frequencies within the system to the cell, with each carrier frequency providing one or more different communication channels, such that a plurality of communication channels are available in each cell; b) establish a fractional load scheme for selected cells in such a way that the maximum number of channels used at any given time in the selected cells is less than the total number of available channels in the selected cells and c) assign channels within each cell to users, according to a pre-determined adaptive allocation scheme. 27. The method according to claim 26, characterized in that the adaptive allocation scheme includes selecting a channel within a cell based on the use ratio of that channel in all co-channels. 28. The method according to claim 26, characterized in that the allocation of communication channels in the fractionally loaded cells is carried out to minimize the simultaneous use of the communication channel in all the co-channel cells. 29. The method according to claim 26, characterized in that it also includes the step of establishing a threshold for the total number of simultaneous users in each cell. The method according to claim 29, characterized in that it also includes the step of denying access when the threshold is reached. The method according to claim 29, characterized in that it also includes the step of transmitting a test signal on a channel not used when the threshold is reached and performing a signal quality test to determine whether additional channels should be assigned. . 32. The method according to claim 31, characterized in that the signal quality test comprises testing the signal quality in the test channel. 33. - The method according to claim 32, characterized in that the signal quality test comprises testing the effect of the test signal in a channel already in use.