WO1998052373A2

WO1998052373A2 - A three-cell reuse pattern in a cellular radio network

Info

Publication number: WO1998052373A2
Application number: PCT/FI1998/000388
Authority: WO
Inventors: Harri Halonen; Risto Uola
Original assignee: Nokia Networks Oy
Priority date: 1997-05-09
Filing date: 1998-05-06
Publication date: 1998-11-19
Also published as: AU7337298A; WO1998052373A3

Abstract

The invention relates to a three-cell reuse pattern in a cellular radio network. The cellular radio network comprises base stations. Each base station comprises a decentralized antenna system. The decentralized antenna system comprises antennas. Each antenna has its own separate coverage area, and the separate coverage areas are interposed with respect to each other. The cell antenna system operates like one antenna. Between an antenna of a first cell and an antenna of a second cell operating at the same frequency, there are at least two of the following things: an antenna of a third cell operating at a different frequency, an area having the size of an antenna coverage area. The antenna system comprises at least six antennas. When the arrangement according to the invention is combined with known techniques, a desired coverage area and capacity can be provided cost-effectively for a cellular radio network.

Description

A THREE-CELL REUSE PATTERN IN A CELLULAR RADIO NETWORK

FIELD OF THE INVENTION

The present invention relates to a three-cell reuse pattern in a cellular radio network. DESCRIPTION OF THE PRIOR ART

A cellular radio network uses a certain part of the radio spectrum. The network operator tries to utilize this limited part of the radio spectrum as efficiently as possible as regards both costs and capacity. The spectrum is divided into carriers; for example in the GSM system the interval between the medium frequencies of a carrier is 200 kHz. Modulation takes place at a slightly higher frequency, so that a carrier utilizes effectively the entire frequency band of 200 kHz.

The operator plans the distribution of the frequency range it has been given by dividing the intended geographical coverage of the cellular radio network into coverage areas or cells. The cells may be of different sizes and their diameter may vary from a few dozens of meters to a few dozens of kilometers. The cells can be divided into different types: a macrocell, a normal cell, a microcell and a picocell. The names are not defined very accurately. The diameter of a macrocell is dozens of kilometers, that of a normal cell is a few kilometers, the diameter of a microcell is a few hundreds of meters, and that of a picocell a few dozens of meters. The same carrier cannot be used in adjacent cells, since otherwise two separate but adjacent transceivers operating at the same frequency would interfere with one another. The operator plans the implementation of the frequency reuse. This means that it is designed by means of different models how the frequencies between the cells are divided so that they do not interfere with one another. These different models are called cell reuse patterns. During modelling a cell is considered to be a hexagon. Hexagons that are interposed with respect to each other describe the mutual placement of coverage areas. A classic model is the seven-cell reuse pattern. Examine Figure 1.

The hexagons are cells. The numbers from 1 to 7 denote carriers. The carriers can be arranged for example in such a manner that carrier 1 is used in the cell situated in the middle. Carriers 2, 3, 4, 5, 6 and 7 are situated around it clockwise beginning from one o'clock. Such groups of seven cells are interposed with respect to one another. By means of this arrangement the same carrier is not used in adjacent cells, and the co-channel interference caused by transmitters operating at the same carrier is thus avoided.

The reuse factor of the model described above can be raised if the capacity is to be increased. It is possible to use, for example, sectorized base stations, which means that a base station cell is divided into three sectors each of which has its own transceiver and each of which operates at its own frequency. A 21-cell reuse pattern is formed in this manner. The drawback is naturally that the pattern requires twenty-one carriers instead of seven.

The capacity of a typical GSM system will be examined below. Assume that the operator has been granted a frequency band of 5 MHz. This frequency band can hold in principle 5 MHz / 200 kHz = 25 carriers. A guard band or a protective area at the edge of the frequency range must be subtracted from the aforementioned number, i.e. in this example 3 carriers. In practice, there are thus 25 - 3 = 22 carriers. Each carrier comprises 8 channels (bidirectional), except for the first carrier which comprises 7 channels. Assume that the system uses the seven-cell reuse pattern, in which case there may be 22 / 7 = 3 carriers in one cell. The cell capacity is therefore the capacity of the three carriers, i.e. 1 * 7 + (3 - 1) ^* 8 = 23 channels. There may therefore be 23 simultaneously ongoing calls in a cell. Frequency reuse in a cellular radio network depends on the C/l requirement of the system. C/I refers to the Carrier/Interference ratio and describes the effect of interference on the reception of a signal. In order that a signal can be identified from a received carrier, the amount of interference at the frequency must not exceed a certain limit. For example in the GSM system, this limit is about 12 dB. Therefore, in different reuse patterns the same channel is not used in adjacent cells, since otherwise the co-channel interference would become too great. There must often be even two cells between cells operating on the same channel, as in the example shown in Figure 1. This is due to the fact that different geographical obstacles affect the propagation of radiowaves. In such a case, the cell boundary does not form an ideal hexagon, but in practice it can be a very curved line. The shape and outline of a cell cannot be defined very accurately, and therefore a sufficient safety margin must remain between cells utilizing the same frequency. Examine Figure 2. Cells 200 and 202 operate on the same channel. The actual boundary of cell 200 is a curved line 210 and the actual boundary of cell 202 is a curved line 212. There is a sufficient margin between them, i.e. two cells 204, 206 operating on a different channel, and therefore the required C/l ratio is at least 12 dB in both cells 200, 202.

The effect of a bad C/l ratio on the reception can be improved by providing a better receiver, i.e. the receiver must be able to receive a signal having a smaller C/l ratio than what could be received normally. In such a case, the frequency reuse of the system can be increased. The drawback of this arrangement is that the manufacturing costs of the receiver increase due to the more complicated technology.

SUMMARY OF THE INVENTION The purpose of the present invention is to provide a cellular radio network wherein a considerably higher capacity is achieved compared to conventional cellular radio networks.

The present invention provides a three-cell reuse pattern in a cellular radio network comprising base stations, said base station comprising a decentralized antenna system, which comprises antennas each of which has its own separate coverage area, said separate coverage areas being interposed with respect to each other, and the cell antenna system operating like one antenna.

The use of the invention provides several advantages. The greatest advantage results from the fact that the frequency reuse of the system can be increased. In such a case, the radio frequency range granted to the operator is used considerably more effectively and the system capacity increases to more than a double. In the invention, the C/l ratio of the system remains at a sufficiently good level for all the receivers of the system. The network planning and implementation are facilitated when the boundaries between the cells are clearer.

The field of application of the present invention will be described in greater detail below. However, it should be understood that even though the detailed description and the specific examples indicate the preferred embodiments of the invention, they are only intended to illustrate the invention by way of an example, since various changes and modifications of the invention will be evident to those skilled in the art without departing from the spirit and scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, the invention will be described in greater detail with reference to the accompanying drawings, in which Figure 1 shows a current seven-cell reuse pattern,

Figure 2 shows the actual boundaries of cells,

Figure 3 shows an imaginary three-cell reuse pattern,

Figure 4 shows a three-cell reuse pattern according to the invention,

Figure 5 shows an example of the equipment required by the invention,

Figure 6 shows a motorway model with a three-cell reuse pattern. DETAILED DESCRIPTION OF THE INVENTION According to the invention, a three-cell reuse pattern is used.

Considerably more capacity is obtained for the system in this manner. Examine the capacity of a GSM system according to the invention. Assume again that the operator has been given a frequency band of 5 MHz. This frequency band can hold, in principle, 5 MHz / 200 kHz = 25 carriers. The guard band or the protective area at the edge of the frequency band, i.e. in this example 3 carriers, must be subtracted from the aforementioned number. There are thus 25 - 3 = 22 carriers in use. Each carrier comprises 8 channels (bidirectional), except for the first carrier which comprises 7 channels. Assume that the system utilizes a three-cell reuse pattern, wherein 22 / 3 = 7 carriers may be used in one cell. The cell capacity is therefore the capacity of these seven carriers, i.e. 1 ^* 7 + (7 - l) ^* 8 = 55 channels. There may thus be 55 simultaneously ongoing calls in a cell. If this number is compared to the capacity of 23 channels set forth above for a seven-cell reuse pattern, the result is 55 / 23 = 2.4. The system capacity can thus be increased to a 2.4-fold value.

Examine Figure 3. The hexagons are cells. The numbers from one to three denote channels. If carriers are arranged in such a manner, the C/l ratio of the system deteriorates too much. This is due to the aforementioned reasons: the boundaries of macrocells described above are not clear, and therefore it is not sufficient that there is only one cell operating at another frequency between two cells operating at the same frequency.

According to the invention, the base station of each cell comprises a decentralized antenna system. The decentralized antenna system comprises antennas. Each antenna has its own separate coverage area. These separate coverage areas are modelled in the same way as normal cells, i.e. into areas having a hexagonal shape. The separate coverage areas are interposed with respect to each other. Since a separate coverage area is smaller than a normal cell, the transmit power can be decreased. The theoretical and actual boundaries between the cells also correspond to each other better. This is due to the fact that when a normal macrocell is divided into smaller separate coverage areas as described above, the obstacles caused by geographical barriers to radiowaves can be taken into account better by means of the correct placement of the antennas. The cell antenna system operates like one antenna. This means that the antenna system is visible to the base station as one antenna. Therefore all the antennas in the antenna system transmit the same information. Examine Figure 4. In this figure, numbers 1 to 3 denote again carriers. The hexagonal areas are separate coverage areas. The six similar carriers grouped together form one cell. As the figure shows, there are at least two coverage areas having a different frequency between two different cells operating at the same frequency. Therefore the C/l ratio remains at a sufficiently good level. This is due to the fact that even though the distances of the different cells have not necessarily increased compared to Figure 3 (since the cells of Figure 3 are macrocells and the cell of Figure 4 consists of several coverage areas having the size of a microcell), the transmit powers are lower, however, and the actual boundaries of the cells follow the theoretical boundaries more closely.

It can be said in general that between an antenna of a first cell and an antenna of a second cell operating at the same frequency there are at least two of the following things:

- an antenna of a third cell operating at a different frequency, - an area having the size of an antenna coverage area.

The antenna system preferably comprises at least six antennas. There may also be more antennas.

Examine Figure 5 which shows how a part of the system shown in Figure 4 is implemented. The cellular radio network comprises base stations 500 that communicate with other parts of the network, for example the base station controller or the mobile services switching centre. The structure of the base station 500 is known from the prior art. It comprises an antenna input and output 502. The antenna input and output 502 is connected via an adapter 510 to a bidirectional transmission network 520, which may be implemented for example in connection with a cable television network. The adapter 510 adapts, if required, the frequency, phase and power of the signal to be transmitted from a form suitable for the transmission network 520 to a form suitable for the base station 500, and vice versa. The transmission network 520 comprises amplifiers for the signal to be transmitted, if the transmission path is so long that the signal starts to deteriorate. The base station is connected to the antenna system via the transmission network 520. The antenna system consists of separate antennas 530A, 532A, 534A, 536A, 538A, 540A. The antennas are known from the prior art. Each antenna has its own separate coverage area 530B, 532B, 534B, 536B, 538B, 540B. An adapter 530C, 532C, 534C, 536C, 538C, 540C adapts, if required, the frequency, phase and power of the signal to be transmitted from a form suitable for the transmission network 520 to a form suitable for the antenna 530A, 532A, 534A, 536A, 538A, 540A, and vice versa.

In practice, the structure of the networks and their coverage areas is not always as regular as described above. Examine Figure 6, which shows how a cellular radio network has been implemented for a part of a motorway. Separate coverage areas according to the invention have been positioned in parallel with the motorway 600. There is also at least one umbrella cell 610 which operates at a fourth frequency. The function of the umbrella cell 610 is to cover the places shown in Figure 6 where the required capacity is so small that it is not advantageous to implement it with separate small coverage areas according to the invention. When the three-cell reuse pattern according to the invention is combined with known techniques, such as the use of umbrella cells, a desired network structure can be provided cost-effectively as regards both the coverage and the capacity. Different variations and modifications of the invention will be evident for those skilled in the art without departing from the spirit and scope of the invention disclosed in the appended claims.

Claims

1. A three-cell reuse pattern in a cellular radio network comprising base stations, said base station comprising a decentralized antenna system, which comprises antennas each of which has its own separate coverage area, said separate coverage areas being interposed with respect to each other, and the cell antenna system operating like one antenna.

2. A system according to claim 1 , wherein there are at least two of the following things between an antenna of a first cell and an antenna of a second cell operating at the same frequency:

- an antenna of a third cell operating at a different frequency,

- an area having the size of an antenna coverage area.

3. A system according to claim 1 , wherein the antenna system comprises at least six antennas.

4. A system according to claim 2, wherein the antenna system comprises at least six antennas.

5. A system according to claim 1 , 2, 3 or 4, which system also comprises at least one umbrella cell operating at a fourth frequency.