Method for assigning telecommunications channels in a cellular telephone system.

<u>FIELD OF THE INVENTION</u>

This invention relates to a method for cell splitting and frequency assignment in a cellular telephone system, and to a distribution of frequency assignments in a cellular telecommunications system.

<u>DESCRIPTION OF THE PRIOR ART</u>

Although the concept of mobile telephones has been known for nearly as long as telephones have been known, it is only recently that mobile telephone systems have been used significantly by the general public. The recent increase in the use of mobile telephones has given rise to the present invention, which is used to accommodate a large number of customers (also known as subscribers or users) without an undue increase in the expense of providing the service. The difficulties that arise with an increase in the number of customers are as follows.

Mobile telephone systems use a separate frequency channel for each call from a mobile customer. The use of the same channel by another nearby customer may lead to interference on the channel and a reduction in service quality. This interference is known as co-channel interference.

It is therefore desirable to use as many channels as possible. Since there are only a set number of frequencies available, the frequencies must be assigned in groups to particular service areas. Each service area is known as a cell, from which the term "cellular system" is derived. Cells having the same group of frequencies are separated as much as possible to reduce co-channel interference.

Each cell is served by a cell site, at which the transmission and control equipment is located. Calls are routed to and from the cell site to a mobile telecommunications switching office and from there to the telephone network.

The equipment at each cell site includes an antenna, which may be either directional or omnidirectional. An omni-directional antenna will typically be located at the center of a cell and radiate the entire cell. A directional antenna will typically be located at the corner of a cell and radiate the entire cell, known as end radiating the cell.

Cell shapes are determined by the radiation patterns of the antenna and the local conditions at the cell site. However, they may be idealized for analysis as a hexagonal pattern, since such a pattern closely approximates the ideal distribution of cell sites.

A useful introduction to this cellular system may be found in <u>Mobile Cellular Telecommunications Systems</u> by William C. Y. Lee, McGraw-Hill Book Company, New York, 1989, particularly chapters 1, 2 and 6, and in <u>The Bell System Technical Journal</u>, January 1979, No.1, Part 3, particularly pages 15 - 42.

In the AMPS system used in North America and several other parts of the world, the available 333 frequencies are divided among the cells so that there will be K different groups of frequencies divided among a repeated pattern of K cells. For the common case where K = 7 there are about 45 frequencies for each cell.

The normal development of a cellular system in North America is to start with the 7 cell layout with up to 45 channels per cell. Each cell typically includes an omni-directional antenna radiating all of the cell from the mid-point of the cell. With large cells (12 Km radii) it is possible to assign the frequencies to the kth cell so that it includes all the frequencies defined by: {333 + k + 7j + 21i} for i = 0, 1, 2...14 and j = 0, 1 and 2.

As the number of subscribers in a given area increases, frequency re-use is required in smaller and smaller areas. To accommodate this, cell splitting is required. Normally the first split is a 2:1 division of the cell radius (i.e. 4 times the number of cells). When this occurs, the interference from cells using the same frequency becomes a problem. To provide additional co-channel interference protection, the cells are sectorized or partitioned with sectors A, B and C. Each sector will now have up to 14 channels (Normally the 15th voice channel is dropped to accommodate the control and locate channels). Sectorization is accomplished by using directional antennas at about the mid-point of each cell and separate control channels per sector. Radiation patterns from the antennas would be as close as possible to provide end radiation of each sector.

The major problems with this method are:

1. Decreased trunking efficiency (refers to the statistical improvement of decreased blockage to phone calls as the number of channels are increased). The telephone traffic tables (see appendix 1.1 in Mobile Cellular Telecommunications Systems cited above) indicate approximately a 30% reduction per channel of traffic handling capability when the trunk group is changed from 44 to 14 channels. (Normally only 14 voice, control and locate channels are used per sector).

2. The high cost of cell division. This results from: (a) the necessity of obtaining many new sites; (b) additional signalling and paging channels; and (c) the increased number of voice channels resulting from decreased trunking efficiency.

In order to provide radio coverage to mobile cellular car phones and to allow frequency re-use, the cellular systems have been laid out with a frequency re-use based on K sub-groups (where K = 4, 7, 9 or 12, see Figure 2.3 of <u>Mobile Cellular Telecommunications Systems</u> cited above). The 666 frequencies of the primary frequency allocations have been allocated equally to the A &amp; B bands. Normally the B Band has been assigned by the communication commission to the wireline companies and the A Band has been assigned to the Non-wireline companies.

The formula cited above gives the standard frequency layout (a chart is given at page 246 of <u>Mobile Cellular Telecommunications Systems</u> cited above). The first 21 channels are used for control channels and the remaining channels are used for voice channels. This allocation provides for 15 voice channels (the last 3 groups have only 14 voice channels) in each sub-group or 45 channels in the larger group (i.e. Group 1 consists of 15 voice channels in each sub-group 1A, 1B and 1C). The A sub-group corresponds to the j = 0 frequencies in the formula cited above, the B sub-group to the j = 1 frequencies and the C sub-group to the j = 2 frequencies. Hence, all of the frequencies assigned to the sectors or parts of the cell were originally in the cell, and have simply been divided between the sectors or the cell.

Referring to Figure 1A, there is shown a plurality of cells identified by a numeral from 1 to 7 in a conventional 7 cell pattern. Each cell is center radiated with an omni-directional antenna located about at its center. Channel assignment for each cell is made according to the table in Figure 2, in which N = 333 channels are divided into 21 groups, each with 3 sub-groups, or equivalently according to the formula: {333 + k + 7j + 21i} for i = 0, 1, 2,...14 and j = 0, 1 and 2, where k is the cell number, and j defines the sub-group number (0 = A, 1 = B, 2 = C). In the distribution shown, cells having the same k identification, hence having the same assigned frequencies, will be separated by a minimum two intervening cells, thus providing a measure of protection against co-channel interference, but in the cluster of cells adjacent to the K = 1 cell, for example, all of the available frequencies are used.

Figure 1B also shows a first stage of cell splitting of the conventional K = 7 cell distribution. Thus, most of the cells have been shown with sectors A, B and C, to which are assigned the frequency subgroups as shown in Figure 2. By this distribution, greater trunking efficiency is obtained, which results in a net re-use of frequency, and thus a greater degree of protection against co-channel interference may be obtained, than without sectoring.

This method of cell splitting requires that a directional antenna be used for each sector at each cell site. Normally the sectors have the same orientation, i.e. 1A is oriented in the same direction as 2A, 3A, 4A, etc.

A second stage of cell division is also shown in Figure 1C in which the same frequency distribution is re-used on a smaller scale.

<u>SUMMARY OF THE INVENTION</u>

The invention described in this patent disclosure has been created by a new frequency allocation and by end radiating 3 cells from one cell site.

This procedure overcomes the major objections of the 7 cell sectorization. That is, the capacity of the system can be increased beyond the 7 cell sectored plan with 1/3 the number of cell sites. No loss in trunking efficiency occurs when directional antennas are required as the omni pattern is evolved into 3 directional cells as traffic dictates.

The co-channel interference is further minimized with the use of highly directional antennas and high gain antennas. Other methods that are known in the art may also be used. The adjacent channel interference from co-located cells is prevented by avoiding adjacent channels at a site.

Thus in one broad aspect of the invention there is provided a method of available frequencies to a plurality of cells in a cellular telecommunications system, the method comprising: <ul id="ul0001" list-style="none"><li>(a) assigning a first group of frequencies taken from the available frequencies to a first cell;</li><li>(b) for each cell adjacent to the first cell, assigning to it a distinct group of frequencies from the first group and from any group assigned to any other cell adjacent to the first cell, such that at least a second group and a third group of distinct frequencies remains unassigned after each cell adjacent to the first cell has been assigned frequencies;</li><li>(c) assigning frequencies from the second group to each of three second cells that are non-adjacent to the first cell and that are adjacent to a cell adjacent to the first cell, the three second cells being spaced substantially equally from each other about the first cell;</li><li>(d) assigning frequencies from the third group to each of three third cells that are non-adjacent to the first cell and that are adjacent to a cell adjacent to the first cell, the three third cells being spaced substantially equally from each other about the first cell and the second cells.</li></ul>

By repeating this process, the entire frequency assignment for the cellular system may be made. The steps of the process are not necessarily applied sequentially in the order given.

In a further aspect the method includes r e - assigning the first group of frequencies to a first part of the first cell;

assigning the second group of frequencies to a second part of the first cell; and

assigning the third group of frequencies to a third part of the cell, such that the first, second and third parts of the cell substantially cover the cell.

The invention in another aspect also provides a cell layout for a cellular telecommunications system in which there are K groups of frequencies in order from k = 1, 2,...K, each of the frequencies of the kth group differing from the respective frequencies of the kth + 1 group by one channel, the layout comprising:    a first cell having assigned to it a first group of frequencies;    six adjacent cells distributed in a hexagonal pattern about the first cell having assigned to them a plurality of groups of frequencies;    three second cells distributed regularly about the six adjacent cells so that their centers form an equilateral triangle with the first cell in the center of the equilateral triangle, the three second cells having assigned to them a second group of frequencies that are different from the plurality of groups of frequencies and the first group of frequencies;    three third cells distributed regularly about the six adjacent cells so that their centers form an equilateral triangle with the first cell in the center of the equilateral triangle, the three third cells having assigned to them a third group of frequencies that are different from the plurality of groups of frequencies and the first and second groups of frequencies, and being located between respective pairs of the second cells; and    the frequencies of the first, second and third groups of frequencies differing from each other respectively by at least three channels.

In another aspect the invention provides a cell layout for a cellular telecommunications system having a hexagonal pattern of cells and having available a set of at least K = 9 groups of distinct frequencies, in order from k = 1, 2,...K, each of the frequencies of the kth group differing from the respective frequencies of the kth + 1 group by one channel, the cell layout comprising:    a first cell having assigned to it the K = 1 group of frequencies;    adjacent cells to the first cell having assigned to them respectively in order around the first cell the K = 2, 3, 8, 6, 5 and 9 groups of frequencies;    a first set of three equally spaced cells adjacent to and located around the adjacent cells each of the first set of three equally spaced cells having assigned to them the K = 4 group of frequencies; and    a second set of three equally spaced cells adjacent to and located around the adjacent cells and equally spaced between respective ones of the first set of three equally spaced cells, each of the second set of three equally spaced cells having assigned to them the K = 7 group of frequencies.

<u>BRIEF DESCRIPTION OF THE FIGURES</u>

There will now be described preferred embodiments of the invention with reference to the accompanying drawings by way of illustration, in which like numerals denote like elements and in which: <ul id="ul0002" list-style="none"><li>Figure 1A is a diagram showing the standard prior art cell layout of K = 7;</li><li>Figure 1B is a diagram showing 7 co-located sectored cells from the prior art before cell division;</li><li>Figure 1C is a diagram showing the cells of Figure 1B after cell division;</li><li>Figure 2 is a table showing the frequency allocation for the prior art K = 7 cell, 3 sector, plan;</li><li>Figure 3 shows a cellular 9 cell pattern according to the invention;</li><li>Figure 4 is a diagram showing an application of an embodiment of a method according to the invention showing the conversion of omni-directional cells to end-radiated cells;</li><li>Figure 5 is a diagram of a second application of an embodiment of a method according to the invention showing the splitting of end-radiated cells;</li><li>Figure 6 is a table showing the frequency allocation for a cellular pattern according to the invention;</li><li>Figure 7 is a diagram of a cellular system showing interleaving of cells; and</li><li>Figure 8 is a diagram showing frequency assignment for both K = 9 and K = 12.</li></ul>

<u>DESCRIPTION OF PREFERRED EMBODIMENTS</u>

This description begins with a short set of definitions, then deals with (a) the layout of frequency groups according to the invention, (b) a first stage of cell splitting and (c) a second stage of cell splitting, together with a discussion of interleaving of cells. Finally, a discussion of advantages of the invention is given, with suggestions for the implementation of the invention.

In this patent disclosure, a cell is an area on the surface of the earth that is served by a given antenna. Each cell will be assigned a group of distinct frequencies, and thus the antenna (whether omni-directional or directional) will have its associated filters tuned to those frequencies. Distinct frequencies means frequencies that do not interfere with each other at an unacceptable level, which in the case of frequencies assigned to a specific cell means at least that the frequencies are separated by at least one channel and preferably more, and in the case of frequencies assigned to adjacent cells means the frequencies are at least not the same.

<u>FREQUENCY LAYOUT ACCORDING TO THE INVENTION</u>

A preferred embodiment of the initial distribution of frequencies according to the invention is shown in Figure 3, with a K = 9 cell distribution, each cell being center radiated by a single omni-directional antenna. With the 9 cell layout the same frequency sub-groups can be used as with the K = 7 layout, only they are distributed differently. Also, this frequency division uses the same control channels as the K = 7 pattern. However, the larger groups consist of combinations of different sub-groups. That is, group 1 of the K = 7 layout (See Figures 1 and 2) consists of 1A, 1B and 1C which corresponds to subgroups 1, 8 and 15 as shown in Figures 3 and 6, whereas group 1 of the layout of the present invention would consist of sub-groups 1 and 10. The channel distribution of the N channels between 9 cells in the K = 9 pattern for the distribution of channels in the B band (frequencies from 333 to 666) is therefore {333 + k + 9j + 21i} for i = 0, 1, 2,...14 and j = 0 and 1, where k is the cell number.

The signalling channels are numbered 334 to 354 inclusive. The base transmit frequency is (870 + n x 0.030)MHz, and the mobile transmit frequency is (825 + n x 0.030)MHz, where n is the channel number, for an allocation of 20MHz per system.

The remaining frequencies are wildcard frequencies that can be used where required, such as in inner city areas with dense mobile telephone traffic. The wildcard groups are groups 19, 20 and 21 of Figure 6. The channel assignment pattern allows for 30 voice channels per cell. In addition the wildcard groups can be used to increase this number up to 44 channels in selected cells.

The use of the same subgroups and control channels has advantages when K = 9 systems need to interface with the conventional systems (K = 7). For example, the control channels are compatible and the frequencies are the same in the sub-groups for both systems (that is, where 15 or fewer channels are used).

In the establishment of the system using omni-directional cells, the available frequencies (presently 333) are assigned to the cells by choosing a first cell, assigning to it a first group of frequencies (for example, those belonging to group K = 1), and then assigning the surrounding adjacent cells distinct groups of frequencies but leaving out the groups of frequencies (re-use frequencies) that are to be used later during cell splitting. This seeming inefficient use of frequencies results in benefits during cell splitting. As shown in Figure 3, frequencies of the 1st group are assigned to the cell labelled 1, and then one of the adjacent surrounding cells is assigned frequencies from another group (cell 2). This process continues for each surrounding cell, but leaving out the frequencies that are to be used later for cell splitting, which are preferably those that minimize co-channel interference. Thus, in Figure 3, groups 4 and 7 are not assigned to cells adjacent cell 1, groups 5 and 8 are not assigned to cells adjacent cell 2 and groups 6 and 9 are not assigned to cells adjacent cell 3.

This layout is made with clusters of 3 colocated cells, and with K = 9 distinct groups of frequencies, the cluster re-use frequency is 9/3 = 3. The system may also work with a cluster re-use frequency of 4, hence K = 12. In this case, the cells surrounding the first cell may be assigned frequency groups 2, 3, 4, 6, 7 and 8, leaving out groups 5 and 9 for later re-use. The frequencies assigned to each of these groups is made on the same principles as used to establish the frequency groups shown in Figure 6.

The method of frequency allocation is applied to all of the cells in the system, resulting, in the case of K = 9, in the assignments shown in Figure 3 and, in the case of K = 12, in the assignments shown in Figure 8.

If the re-use frequencies for a particular cell k are designated by letters A, B and C, and the groups then identified by the letters kA, kB and kC, for k = 1, 2 and 3 (Figure 3 and 8) or k = 1, 2, 3 and 4 (Figure 8 for the case m = 4), then the frequencies are distributed such that any group of cells kA, kB and kC forms an equilateral triangle, with a cell having a distinct group of frequencies between any pair of cells kA and kB, kB and kC, and kA and kC. The complete cell distribution is thus formed by leaving out the re-use frequencies (for cell kA these are kB and kC) in the cells adjacent kA, and then assigning the re-use frequencies to non-adjacent cells. When the cells are later sub-divided, and directional antennas used, the equilateral distribution of the cells with re-use frequencies permits the antennas in cells with the same frequencies to be separated by the maximum uniform distance. This may be seen in Figures 3, 4 and 8. As seen in Figure 4, the orientation of the directional antennas in the smaller cells results in there being 7 hexagonal cell radii between co-channel cells in the direction of orientation of the antenna. That is, a random pattern may result in greater distance between some co-channel cells, but others will be closer together.

In this manner, the cells adjacent to any particular cell are assigned frequencies that are separated from the frequencies assigned to that particular cell by at least one channel. At least two of the cells that are separated by one cell from any particular cell are assigned frequencies that differ from the frequencies assigned to the particular cell by at least three channels.

<u>FIRST STAGE OF CELL SPLITTING</u>

The first stage of cell splitting using an embodiment of the method of the present invention is shown in Figure 4, where the same cell group as outlined in Figure 3 is shown in heavy outline. The initial large cells with circles marking omni-directional antennas are shown numbered as cells 1 - 9, having effective hexagonal radii R approximately equal to 12 Km. This is similar to K = 7 start except for frequency groupings (see Figure 6, Sub-groups 1 and 10, 2 and 11, etc.)

As capacity needs to be increased, additional directional cells are provided where needed as shown in Figure 4. In Figure 4, the cells without hatching, identified by the numerals in circles, are the larger (omni-directional) cells before splitting, while the smaller cells with hatching are the cells after splitting. Each of the smaller cells is preferably end radiated using a set of three co-located directional antennas. The direction of the maximum radiation from the antenna (referred to as the orientation of the antenna) is indicated for each antenna by the apex of the triangle.

Each larger cell is split by assigning to a part of it frequencies of a non-adjacent cell, so that after the split some of the cell is assigned the original group of frequencies and at least one part of the cell is assigned frequencies from a non-adjacent cell to form a new cell, while maintaining coverage of the cell from the same, or essentially the same cell site. Preferably, the non-adjacent cells from which the frequency assignments are derived are separated from the cell being split by at least two intervening cells of the size of cell after splitting, and where K = 9, the cell is preferably split into 3 parts.

In the example shown, cell 2 is divided into parts or cells having assigned to them frequencies from the 2, 5 and 8 cells. These cells (5 and 8) are the only two cells that are non-adjacent the 2 cell before splitting, hence providing for a minimum cell separation of two intervening cells of the size of cell after cell splitting.

For a hexagonal grid system, with 6 cells surrounding any one cell, it is believed that optimal frequency assignment may be obtained by using a frequency repeat pattern based on K = 9.

With increasing telephone traffic, typically in the inner city, further cell splitting may become necessary. In that case, new cells may be provided at the corners of the inner cells, as described in more detail in the next section.

<u>SECOND STAGE OF CELL SPLITTING</u>

If additional cell divisions are necessary, this can be accomplished with an embodiment of the method of the present invention by adding the new site about midway between 2 existing sites and using the cell number not in use at either of the existing sites.

That is, if a cell site is to be added between a first cell site with cells 1, 4, and 7, and a second cell site with cells 2, 5, and 8 then the new site will contain cells 3, 6, and 9, as shown in Figure 5.

The groups of cells 1, 4 and 7 and 2, 5 and 8 are the first and second sets of three co-located end radiated cells, each radiated by directional antennas at first and second cell sites respectively. The cell splitting method is then to locate a third cell site at about the mid-way point between the first and second cell sites; and to radiate at least a third cell from the third cell site.

According to the K = 9 cell distribution, the first and second sets of cells are assigned distinct groups of frequencies (1, 4, 7, 2, 5 and 8) and in the method of cell division frequencies are assigned to the third cell that are distinct from the frequencies assigned to the first and second sets of cells (3, 6 and 9).

In this method, it is preferable that directional antennas are used at the third cell site to end radiate a third set of three co-located cells.

It is also preferable that the assigning of frequencies is done by assigning to each of the cells in the third set of cells frequencies that are distinct from the frequencies assigned to the first and second sets of cells.

In a cellular system based on the hexagonal cell distribution, each cell will be surrounded by 6 cells, so that the K = 9 distribution will always yield 2 cells that are non-adjacent the center cell and which have distinct frequency assignments. In the general case, if the cell is sub-divided into m different parts then each cell must be surrounded by no more than K - m cells. This yields at least m - 1 non-adjacent cells from which frequency subsets can be taken for cell splitting.

<u>INTERLEAVING OF CELLS</u>

In the first stage of cell division best illustrated in Figure 4, when the directional antennas are introduced, the smaller cell extends the boundaries of the old omni-directional cell. This extension of the cell, or equivalently, extension of the footprint of the directional antenna, requires the directional antenna to have a narrower footprint than the directional antennas used in the conventional cell distribution illustrated in Figure 1. A narrower footprint, as is well known in the art, may be obtained by increasing the horizontal directivity of the antenna.

The edge of a footprint is normally defined in relation to adjacent cells since the footprint of an antenna is the area where the signal of that antenna is the predominant signal. Because handoff algorithms assign a mobile customer the dominant frequency receivable by that customer, the footprint of an antenna is defined as the area where that antenna will be the one used by a mobile customer. Otherwise, the edge of the footprint may be defined in terms of the signal level, for example, where the signal level coverage is at the 39 dB microvolt contour.

The extension of the footprints formed by adjacent first and second antennas at a first cell site results in a gap, and the extension of the footprint of an antenna of an adjacent cell site into the gap results in the interleaving of the footprints. Thus, for example, the footprint of the smaller 2 cell in the center of Figure 4, extends between and interleaves with the footprints of the small adjacent 4 and 7 cells. With a 60 degree antenna that is 10 dB down at the side of the footprint, this provides for a more distinct handoff than is available from the sectorization system shown in Figure 1.

In addition, in the case where a customer is moving around the edge of a group of co-located cells, for example, small cells 2 and 8 in the center of Figure 4, handoff to the adjacent cell 6 will occur. Although this results in an increase in the number of handoffs, the handoff is more distinct. Also the number of handoffs is compensated for by a decrease in the total number of handoffs required in the system.

In the preferred implementation of the invention, handoff (which requires extensive control equipment, all readily commercially available) may be controlled by the mobile telecommunications switching office rather than, as commonly used, the cell site.

In Figure 7, the interleaving is made more apparent by the replacement of the idealized hexagonal cells by approximations of the actual footprints. As shown in Figure 7, interleaving of cells may also be introduced before the second stage of cell splitting where appropriate as a method of filling in frequency assignments. In Figure 7, the interleaved cells are the densely hatched cells surrounding the 2 cell site with the three end radiated 2, 5 and 8 cells. This assignment is facilitated by making available frequencies to be assigned to the new interleaved cells.

The use of highly directional antennas with narrow footprints diminishes interference from laterally spaced cells but increases it from cells in the direction of orientation of the antenna. Thus, it is desirable, as with the methods described here to have similar channels significantly separated in the antenna orientation direction.

Various types of antennas may be used to implement the present invention. A KATHREIN antenna model no. A 633061 with a vertical beamwidth of 19 degrees may be used, or a DECIBEL DB 874 antenna with vertical beamwidth of 14 degrees. These antennas are readily commercially available with various horizontal beamwidths, and mechanical tilting is available on the Decibel products.

<u>ADVANTAGES OF THE INVENTION AND PRACTICAL CONSIDERATIONS OF THE IMPLEMENTATION OF THE INVENTION</u>

Some practical considerations arising from the implementation of the method of the invention are as follows. As shown in Figure 4, 3 cells are end radiated from one cell site. Consider cells designated by k = 2, 5 and 8. These cells would be served with directional antennas oriented at 30 degrees, 150 degrees and 270 degrees. As one desires to cover the cell, the gain of the antenna should be high (i.e. +16 dB at 0 degrees, + 13 dB at plus and minus 30 degrees, and + 6 dB at plus and minus 60 degrees).

For maximum cell site utilization 3 cells are co-located, however due to topography or building blockage, cells can be separated into stand alone directional cells, without violating the cellular pattern. The back lobe radiation will usually allow sufficient radiation behind the antenna (fill in) for separations up to 1/4 R, where R is the radius of the hexagonal cell (also equal to the length of one side of the hexagon).

The 9 cell re-use pattern allows for 29 voice channels per cell with up to an additional 14 more voice channels per cell for congested areas. With this pattern, using the wildcards, and once in every 9 cell re-use, higher traffic capacity from 7 sites is obtainable than for 21 sites using the sectored K = 7 pattern.

To maintain minimum co-channel interference, the base station antennas and power should be selected to provide the required decrease in signal level in areas where interference could be expected. Greater uniformity of signal strength within a cell may be obtained with antennas at high heights as the point of reflection from the ground (or other obstacles) increases directly with antenna height.

The use of high sites is also facilitated with the invention. If, for example, an omni-directional antenna is located at a cell site on one side of a hill and it is desired to replace it with a set of three co-located directional antennas, then two of the directional antennas may be located at the cell site, and a third located nearer the top of the hill, using the back lobe of the antenna radiation pattern to fill in the intervening area.

In the initial stages of cellular development, high gain omni-directional antennas are used. In Figure 3, the apex of the cell is vertical, and boundaries between adjacent cells may be considered to define axes. However, when directionally radiated cells are introduced the apex becomes horizontal, equivalent to a rotation of the axes through 30°. This change allows for 3 times the number of cells using the same site and no change in frequency allocations. This change allows the system to change from N cells to 3N cells, with the corresponding 3 fold increase in capacity. The next stage would be by cell division as shown in Figure 5. This creates a four fold increase in cells to 12N cells.

The cell distribution of the present invention (Figure 4) is compared with the conventional K = 7 pattern (Figure 1) as follows. In each case, there are 21 equal sized cells. The wildcard groups are distributed over cells 2, 5 and 9. The wildcard groups are sub-groups 19, 20 and 21 from Figure 6, indicated in the figure by the notation WC. <tables id="tabl0001" num="0001"><img file="EP0470831A2_D0001.tif" /></tables>

Based on the above the cost is expected to be more than 50% less for the present invention over the conventional system because of fewer sites, higher trunking efficiencies and fewer control channels. (Note 1: Taken from Traffic Tables given on P 34 &amp; P 35 of the book <u>Mobile Cellular Telecommunications Systems</u> for 2% blocked calls during busy hour.)

Handoff occurs when a mobile traverses from 1 cell to another, or from 1 cell sector to another. The total number of handoffs is expected to be more than twice as high on the K = 7 pattern. This excessive handoff makes handoff between sectors of the same site necessary for the conventional system, but not for the present invention.

The K = 7 Pattern requires 120 degree antennas whereas the present invention requires 60 degree antennas. The 60 degree antenna will provide additional co-channel interference protection. Both systems require careful selection of sites and the use of antenna tilt to ensure maximum co-channel interference protection. Co-channel interference is expected to be similar between the two systems.

Since both systems do not use adjacent channels from the same site, the adjacent channel interference will be negligible.

The present invention allows for more flexibility as each cell can stand alone. This would allow all three co-located cells to be separated when necessary. The transition from omni-directional large cells to small directional cells can evolve smoothly as there is not the jump down in trunking efficiency when directional cells are introduced. Contrasted to the present invention, the K = 7 conventional pattern cell division is required because sectorization results in approximately 30% reduction in trunking efficiency when changing from a 44 voice channel (vc) omni-directional cell to a 14 vc sectored cell.

Having 3 groups of frequency available for wildcard usage is beneficial, since additional channels are required in selected cells to accommodate the large fluctuations in traffic density from the city core to urban areas. The ability of being able to provide up to an additional 28 channels at a site would result in significant savings when providing a cellular system.

The following are advantages that arise when the present invention is adopted over the conventional K = 7 system.

Trunking Efficiency: the trunking efficiency does not decrease when directional cells are introduced, as the cells are changed from a number of R = 12 Km omni cells to 3 times the number of R = 6 Km directional cells without adding sites.

Fewer cell sites: as shown above the conventional pattern (K = 7) has up to 3 times the number of cell sites as the present invention. Few handoffs: each directional cell covers 3 times the area of a 7 x 3 sector and thus less than one third of the handoffs would be expected on the present invention.

Lower facility cost: in addition to fewer voice and control channels (approx. 30% less), if serviced with private microwave or fibre services, the cost of going to fewer sites would be up to 70% less, than going to sectored cells. (Note: this does not apply to rural sites as traffic does not dictate 3 directional cells per site).

Flexibility is increased by the availability of non-assigned wildcard sub-groups. Normally the highest traffic density would be in the city core and therefore additional channels would be assigned to these cells, however as long as co-channel and adjacent channel protection is maintained, these wildcard frequencies can be assigned to other congested areas.

Co-located cells: as handoff is significantly less with the present approach, the need for intersite handoff (from control at site) is unnecessary. Without this handoff limitation the 3 co-located cells could be separated. This type of separation would occur when the topography prevents serving three areas from one site.

Evolution from centre to end radiated cells: This is effected by observing that the centre radiated cell (see Figure 3) can provide 3 end radiated cells from each site if we change from a vertical to a horizontal pattern. (A vertical pattern refers to the position of the apex of the cell). If we start with the cell radius equal to 12 Km in Figure 3 then the cell radius of these new cells will be 6 Km. The next cell division will give the small 3 Km cells. It should be noted that this rotation of patterns would also be valid for other cell orientations (i.e. A horizontal pattern for omni cells would change to a vertical pattern on the end radiated cells).

Preferred examples of the invention have been described, but the invention includes all immaterial variations on the embodiments disclosed. The invention has been described with reference to the existing systems in use in North America, but is believed to have applicability to any cellular system in which frequency re-use is required, including digital implementations of the system.