MXPA97008291A - Allocation of frequency in a celu radio telecommunications network - Google Patents

Abstract

The present invention relates to a cellular telecommunications network including separate base stations. Each base station has three directional antennas for transmitting and receiving signals to subscriber units. Eight frequency groups are assigned among all the antennas, so that each group of frequencies is reused in an average of three out of eight stations

Description

ALLOCATION OF FREQUENCY IN A CELLULAR RADIO TELECOMMUNICATIONS NETWORK DESCRIPTION OF THE INVENTION The present invention relates to a method for assigning carrier frequencies to base stations in a cellular radio telecommunications network, and to a radio telecommunications network including said frequency assignments. In known cellular radio systems, such as mobile phones, a network of base stations is provided, each having antennas. Mobile subscriber units have antennas, which are necessarily omni-directional since subscribers usually move, both within cells and from one cell to another. Accordingly, a seven-cell frequency reuse pattern is common, as described in, for example, Cellular Radio Systems, Balston DM Macari RCV Editors, Artech House Inc., 1993, pages 9-13. The invention is defined in the claims, to which reference must now be made. Preferred aspects are presented in the dependent claims. The present invention preferably provides a cellular telecommunications network that includes separate base stations, each base station comprising three directional antennas for transmitting and receiving signals to subscriber units, eight frequency groups being allocated between the antennas, so that each Frequency group is reused on an average of three out of eight base stations. The azimuth sight directions of the three antennas of a base station are preferably substantially 120 ° from one another. The base stations are preferably separated in order to be equidistant from their six surrounding stations, and lie in alternating first and second parallel rows, so that the antennas in the first rows point to azimuth angles of 0o, 120o and 240o , and the antennas in the second rows point to azimuthal angles of 60 °, 180 ° and 300 °. The azimuthal angles preferably have a clockwise direction and the north being 0o. A preferred base station has an antenna with a look direction that uses the same frequency group as an oppositely directed aiming direction of an antenna of a surrounding base station in an adjacent row. The frequency groups are preferably assigned to any of the following rules: (a) a frequency group is not assigned to more than one antenna in a base station, (b) the same frequency groups are assigned to corresponding antennas every sixth base along a row, (c) the same frequency groups are assigned to antennas corresponding to each 24th. base station along a column, (d) the antennas in a row having the same direction of view from an azimuth angle of 0o or 180 °, are assigned alternating frequency groups along the row, and the antennas in the row that have other directions of sight are assigned to other frequency groups, so that the frequency groups are not reused in areas of coverage of adjacent antennas. In particular, the frequency groups fi, f3, f, are used in base stations in column i, row n; the frequency groups f2, f5, f3, are used in the column i + 1, row n, the frequency groups fi, f4, fs, are used in column i + 2, row n, (e) column i + 2, row n + 2 has frequency groups corresponding to those for row i, row n, where the frequency groups are selected as: fi + 2, n + 2 = fi, n + 3 + where f is the frequency group in a sector of the base station in column i, row n, fi + 2, n + 2 is the frequency group in the corresponding sector of the base station in column i, row n , + indicates a circular addition of up to 8. + The antennas in the base stations in the network are preferably directional antennas of beam width of 120 °, but can be of a lower beam width, such as from 60 to 85 °. The network may comprise a selection of configured base stations and having frequency groups assigned in order to avoid selected poor communication paths, specifically those having a high co-channel interference. To avoid the poor communication paths selected, the alternating rows of base stations in the network can substantially use alternating signal polarizations. The selected base stations may have polarizations different from other members of their row in order to avoid selected poor communication paths. The present invention is preferably for use with subscriber units having directional antennas. Preferably, the addresses and locations of the or each of the antennas of a subscriber unit are fixed. The present invention also relates to corresponding methods of assigning frequency groups. A frequency group can be a frequency, a frequency scale or a frequency selection. A preferred embodiment of the present invention will now be described by way of example and with reference to the drawings, in which: Figure 1 illustrates an individual base station with three antennas; Figure 2 illustrates the location of frequency bands within the base station network, Figure 3 illustrates some selected signal paths for which noise ratios were determined, and Figure 4 illustrates the modified base station network, so that the alternating rows of the base stations transmit and receive a different polarization.

Frequency Assignment in the Base Station Network As shown schematically in Figure 1, a base station has three directional antennas, each directed 120 ° apart in the horizontal plane, such that the base cell or site of each base station is divided into three sectors. A network of these base stations is shown in Figure 2. In this network, the base stations are distributed so that each base station is equidistant from its six closest surrounding stations. Considering that the base stations are in rows, which are shown horizontally in Figure 2, each base station has a close on either side in the same row and two close on either side of the adjacent rows. It will be seen from Figure 2 that the antennas in alternating rows are aligned differently. For example, antenna lookouts for antennas at the base stations in rows n, n + 2, n + 4, ... n + 2i point to North (0 °), 120 ° and 240 °. The antennas in the base stations in rows n + 1, n + 3 ... n + (2i-1) signal at 60 °, 180 ° and 300 °. The base stations of rows n + 1, n + 3, are offset in an east-west direction halfway between the base stations in a row. Eight frequency groups are used and these are designated as follows: (i) On each row, the same frequency groups are assigned on the antennas corresponding to each sixth base station (ib8 = 1, 7, 1 3, 19, etc. .). (¡I) On each column, running from north to south, the same frequency groups are assigned to the corresponding antennas at each 24th. base station, (ibs = 1, 25, 49, 73, etc.). (iii) The confrontation factors north of the base stations in row n, are alternatingly assigned frequency groups denoted by the Frequency group numbers, 1 and 2, are marks for the frequency groups, surrounding numbers, for example 1 and 2, but they do not need to be, frequency adjacent on a frequency scale. The other sectors of the base stations in this row use frequency groups denoted by 3, 4 and 5, assigned such that a particular frequency group is not used for more than one antenna in a base station or in two of the sectors of the surrounding base station. As shown in Figure 2, the base station in column i (row n) uses frequency groups 1, 3, 4; the station in column i + 1 (row n) uses frequency groups 2, 5, 3; the base station in column i + 2 (row n) uses frequencies 1, 4, 5; and so on. As shown in Figure 2, each base station and a surrounding base station in the adjacent row have antennas with opposite directions of sight directed using the same frequency group. The base stations along row n + 1 have frequency groups 1 and 2 alternately assigned to the antennas facing south, and two frequency groups selected from the groups 6, 7 and 8 on their other antennas, so that a particular frequency group is not used for more than one antenna in the base station or in any of the two surrounding base station sectors. The base stations in column i + 2, row n + 2 are configured to correspond to those distributed for column i, row n, where the frequency groups are selected as, f? + 2, n + 2 = fi, n + 3 + where f is the frequency group in a sector of the base station in column i, row n, fi + 2, n + 2 is the frequency group in the corresponding sector of the base station in column i, row n, + indicates a circular addition of up to 8, that is, 8 + 1 = 1 + + 7 + 3 = 2 + 2 + 3 = 5. + In this way, if a frequency group f is distributed to an antenna that has a 60 ° orientation in a base station in row n + 1, column i + 0.5, frequency group f is also designated to the antenna with an orientation of 240 ° in a base station in row n + 2, column i + 1. Also, if a frequency group f is distributed to an antenna having an orientation of 300 ° at a base station in row n + 1, column i + 0.5, the frequency group f is also assigned to the antenna with an orientation of 120 ° in a base station in row n + 2, column i + 1.

Signal for the Preferred Network Interference Analysis To consider the possible signal to interference ratios in the base station network, a simple analysis has been taken to determine the signal to interference ratios in "worst case" sites. Figure 3 shows a subgroup of the base station network, which is shown in Figure 2, where base stations from A to O are identified. The points with circles, which are denoted by P to Z, indicate where there is a poor reception, that is, low signal-to-noise ratios are expected. The frequency group used in the various sectors is shown in Figure 3. The dotted lines, shown in Figure 3, indicate signal transmission paths selected for the determination of signal-to-noise ratios. The calculations of the signal to interference ratio were made assuming real signal resistance values and that the signal strength is reduced as a function of the cube distance, ie, d3, where d is the distance. The antennas in the base stations were taken as having a beam width of 120 ° and a front-to-back ratio, that is, the maximum attenuation due to the directionality of the antenna of 18dB. The antennas of the fixed subscriber unit were assumed to be highly directional, having a beam width of 10 ° and a front-to-back ratio of 20dB. Points P to Z were established to lie at the edge of the sector, where they are located and at a distance r from the nearest antenna, where the separation of antennas is at least 2r. The base station that serves the sector where a point P to Z is located is denoted as the server. An interference base station is an "interfering station".

TABLE 1 Server Losses at Sample Points The server and the frequency group for each of the points P to Z are shown in Table 1. The azimuth deviation angle between the direction to point P to Z and the direction of sight of your antenna in service is also indicated together with the corresponding azimuthal loss on the service antenna. To determine the signal to interference ratios, the effect of the interference bases on the signals are determined, assuming that all the transmitters are of the same energy. The trajectories to points P to Z and, in each case, the surrounding base, which uses the same frequency groups and thus interfere, are shown as the first two columns of Table 2. The frequency groups are shown in the third column, and the path distances whose interference signals must travel relative to the corresponding path distances for the desired signals are shown in the fourth column of Table 2.

TABLE 2 Interference and C / l in Poor Locations The fifth column shows the angles of deviation between the viewing directions of the antennas that are intended to communicate with points P to Z and the direction from which the interference signal of the same frequency group originates. The sixth column shows the angular deviation between the interference signal and a subscriber unit at the respective point. Each subscriber unit has a directional antenna pointing directly to its service antenna. Losses due to angular deviations are shown in columns seven and eight. These predicted attenuations are more severe than what is likely to actually occur during practice. The interference signal values evaluated, are shown in the last column of Table 2, from which it can be seen that the location with the worst signal-to-noise ratio is signaled at 5, which has the signal-to-interference ratio of 23 dB. Other poor locations are the U, Y, W, V and Z points. These points are indicated by asterisks in Table 2.

Ways to Improve Communications in Preferred Networks The signal to interference ratios can be improved by selecting different antenna beamwidths at the base stations and / or using signal polarization. These are considered in turn, later.

Antenna Beam Widths Instead of using antennas with a beam width of 120 °, antennas with a beam width of 60 ° can be used. Taking the attenuation at an angular deviation of 60 ° from the direction of the antenna's sight as 12 dB, the signal-to-noise ratio in the worst case could be only 32 dB. Since the use of three 60 ° antennas at each base station could leave gaps in the covered area, antennas having a sensitivity to an angular deviation of 60 ° can be used, which is approximately equal to that of an omnidirectional antenna. The worst signal-to-interference ratio can be improved by about 3 dB, using antennas at 85 ° without affecting the coverage.

Polarization An alternative method to increase the signal to interference ratio is to use cross polarization. As shown in Figure 4, alternating rows are given alternating horizontal and vertical polarizations. This reduces the interference along paths to points corresponding to S V Z, as shown in Figure 3, at approximately 10 dB, since the worst interference uses polarized signals in cross-over to those of the server. The interference to points W and Y is not changed since the server and the interfering party use the same polarization. By setting base station networks in restricted areas, poor, particular signal-to-interference paths can be avoided. For example, if a long narrow area is to be covered, two rows of base stations can be used, the polarity being alternated along each row. The poor signal to interference ratio along the paths corresponding to points V and Z may not increase, since they are caused by 3 distant rows. S, W and Y scenarios could be avoided due to the attenuation polarization. Another example is where the area that is covered is relatively small, so that very few base stations are required. By selecting up to eight base stations, as shown within the example planning area shown in Figure 4, the worst signal-to-noise ratios are avoided through the appropriate polarization selection and since the selected planning area avoids interference of the U and Y scenarios. As shown in Figure 4, within the example planning area, two base stations use the polarization alternative to the row of stations where they lie. These two base stations are shown with a diagonal line shaded in Figure 4. Using crossover polarization, signal enhancements are made to interference. The worst relationship is about 26 dB (path to the Y point), and many unacceptable paths are avoided, which have such signal to interference ratios of less than 30 dB. Many other planning areas within a larger national network, in addition to the example shown in Figure 4, can be selected and used on the ground in order to avoid poor communications.

Claims (8)

1 .- A cellular telecommunications network that includes separate base stations, each of said base stations comprises three directional antennas for the transmission and reception of signals to subscriber units, eight frequency groups being assigned between the antennas, so that each frequency group is reused on an average of three of each of the eight base stations, the base stations being separated in order to be in alternating first and second parallel rows, so that the antennas in the first rows they point at azimuthal angles of 0 °, 120 ° and 240 °, and the antennas in the second rows point to azimuthal angles of 60 °, 180 ° and 300 °.

2. - A cellular telecommunications network according to claim 1, wherein the azimuthal sight directions of the three antennas of each base station are substantially at 120 ° to each other.

3. A cellular telecommunications network according to claim 1 or claim 2, wherein the base stations are separated in order to be equidistant from their six closest surrounding stations.

4. A cellular telecommunications network according to claim 3, wherein the azimuthal angles are dextrorotatory and with a north being 0 °.

5. - A cellular telecommunication network according to any of the preceding claims, wherein a first station has an antenna with a targeting direction using the same frequency group as an oppositely directed aiming direction of an antenna of a surrounding base station in an adjacent row.

6. A cellular telecommunications network according to any of the preceding claims, wherein the frequency groups are assigned according to the rule that a frequency group is not assigned to more than one antenna in a base station. 7 - A cellular telecommunications network according to any of the preceding claims, wherein the frequency groups are assigned according to the rule that the same frequency groups are assigned to corresponding antennas in each sixth base throughout a row . 8. A cellular telecommunications network according to any of the preceding claims, wherein the frequency groups are assigned according to the rule that the same frequency groups are assigned to corresponding antennas at each 24th. base station along a column. 9 - A cellular telecommunications network according to any of the preceding claims, wherein the frequency groups are assigned according to the rule that the antennas in a row having the same direction of azimuth angle of 0 ° or 180 ° they are assigned alternating frequency groups along the row, and the antennas in the row that have other directions of sight are assigned other frequency groups, so that the frequency groups are not reused in areas of antenna coverage adjacent. 10. A cellular telecommunications network according to claim 9, wherein the frequency groups fi, f3, f, are used in a base station in column i, row n; frequency groups f2, fs, f3 are used in a base station in column i + 1, row n, frequency groups Í1 F f4 f5 are used in a base station in column i + 2, row n. 1 - A cellular telecommunications network according to any of the preceding claims, wherein the frequency groups are assigned according to the rule that a base station in column i + 2, row n + 2, has frequency groups that correspond to those for a base station in column i, row n, where the frequency groups are selected as, fi + 2, n + 2 = fi, n + 3 + where f is the group of frequency in a sector of the base station in column i, row n, fi + 2, n + 2 is the frequency group in the corresponding sector of the base station in column i, row n, + indicates a circular addition of up to 8. + 12 - A cellular telecommunications network according to any of the preceding claims, wherein the antennas at the base stations in the network are directional antennas of 120 ° beamwidth. 13. A cellular telecommunications network according to any of claims 1 to 11, wherein the antennas in the base stations in the network have a beam width of less than 120 °. 14 - A cellular telecommunications network according to claim 13, wherein the antennas have a beam width in the range of 60 ° to 85 °. 15 - A cellular telecommunications network according to any of the preceding claims, comprising a selection of base stations having frequency groups assigned in order to avoid poor, selected communication paths. 16. A cellular telecommunications network according to claim 15, wherein to avoid poor, selected communication paths, the alternating rows of base stations in the network substantially use alternating signal polarizations. 17 - A cellular telecommunications network according to claim 15 or claim 16, wherein the selected base stations have polarizations different from the other members of their row, in order to avoid poor, selected communication paths. 18. A cellular telecommunications network according to any of the preceding claims, wherein the subscriber units have directional antennas. 19. A cellular telecommunications network according to claim 18, wherein the addresses and locations of the or each antenna of a subscriber unit are fixed. 20. A cellular telecommunications network according to any of the preceding claims, wherein a frequency group is a frequency, a frequency scale or a frequency selection. 21 - A method of assigning frequency groups in a cellular telecommunications network that includes separate base stations, each base station comprises three directional antennas for the transmission and reception of signals to subscriber units, the base stations being separated in order to lie in the first and second alternating parallel rows, so that the antennas in the first rows point to azimuth angles of 0 °, 120 ° and 240 °, and the antennas in the second rows point to azimuth angles of 60 °, 180 ° and 300 °, whereby eight frequency groups are assigned between the antennas, so that each frequency group is reused in an average of eight out of eight base stations. 22.- A frequency group assignment method according to claim 21, wherein the frequency groups are assigned according to the rule that a frequency group is not assigned to more than one antenna in a base station . 23.- A frequency group assignment method according to claim 21 or claim 22, wherein the frequency groups are assigned according to the rule that the same frequency groups are assigned to corresponding antennas in each sixth base along a row. 24. A method of assigning frequency groups according to any of claims 21 to 23, wherein the frequency groups are assigned according to the rule that the same frequency groups are assigned to antennas corresponding to each 24ava . base station along a column. 25. A method of assigning frequency groups according to any of claims 21 to 24, wherein the frequency groups are assigned according to the rule that the antennas in a row having the same azimuthal angle direction of 0 ° or 180 ° are assigned alternating frequency groups along the line, and the antennas in the row that have other viewing directions are assigned to other frequency groups, so that the frequency groups are not reused in adjacent antenna coverage areas. 26. A method of assigning frequency groups according to claim 25, wherein the frequency groups fi, f3, f4, are used in a base station in column i, row n; frequency groups f2, fs, f3 are used in a base station in column i + 1, row n, frequency groups f ^ f4 f5 are used in a base station in column i + 2, row n. 2

7. A method of assigning frequency groups according to any of claims 21 to 26, wherein the frequency groups are assigned according to the rule that a base station in column i + 2, row n + 2, has frequency groups that correspond to those for a base station in column i, row n, where the frequency groups are selected as, fi + 2, n + 2 = fi, n + 3 + in where f is the frequency group in a sector of the base station in column i, row n, fi + 2, n + 2 is the frequency group in the corresponding sector of the base station in column i, row n, + indicates a circular addition of up to

8. +