CA2345855A1 - Method for assigning frequencies in cellular communications systems - Google Patents

Abstract

A wireless cellular frequency assignment method optimized for both frequency division duplex and time division duplex communications, consisting of a number of identical clusters enabling coverage of a geographical area. The clusters are divided into arrays of 16 cells.
Each cell is further subdivided into sectors, with each such sector being capable of operating on either vertical or horizontal electromagnetic polarizations. Within each cluster, a frequency reuse of two is employed to obtain maximum reuse of the available frequencies with minimum co-channel and adjacent channel interference. This method permits an initial installation containing a minimum number of frequencies, and permits expansion to a system utilizing all available frequencies.

Description

METHOD FOR .ASSIGNING FREQUENCIES IN
CELLULAR COMMUNICATIONS SYSTEMS
FIELD
This invention relates to cellular communications systems and, more specif=ically, to a method of assigning frequencies within a cellular communications system.
BACKGROUND
There is a growing demand for wireless communication services, such as cellular mobile telephone, mobile digital data cellular networks, mobile personal communications services, as well as fi~:ed wireless networks. In such systems, it is essential_ to maximize the use of the available radio spectrum over a geographic territory.
In order to utilized the available frequencies in an optimal manner, a large geographic territory is typically divided into a number of sub-areas called cells. Each cell typically includes a radio base station and an antenna for effecting communication; between the base station and the remote stations, which may be either fixed or mobile.
The available frequencies are allocated to the individual cells, in order to minimize both adjacent-channel interference andl co-channel interference. In order to further increase the utilization of the available frequencies, a cell may be subdivided into a number of sectors, with each sector serviced by an associated directional antenna. Each sector typically consists of an integral fraction of 360 degrees, although, in practice, sectors are not limited to such a precise integer.
In order to provide for extended coverage in large geographic areas, arrays of cells, known as clusters, are employed. A cluster consists of a basic cell group.
Clusters are typically aligned such that a symmetrical pattern is established in two dimensions. Such alignment is used to ensure that lboth co-channel and adjacent-channel interference is minimized between clusters.
In any frequency plan, frequency reuse is normally employed to achieve a system capacity significantly greater than the total number o:E available channel frequencies. In frequency reuse, a frequency will be assigned to cells that are separated sufficiently from each other to achieve relatively low interference between radio channels in different cells having l~he same frequency, i.e., low co-channel interference.

2 Existing cellular frequency planning methods and techniques have been developed for frequency division duplex, the communications technique most commonly employed. Newer systems utilize time division duplex, which has different frequency assignment considerations as well as different inters°erence mechanisms. It would be desirable if frequency plans could be developed that would not only accommodate time division duplex, but would also be adaptable to frequency division duplex in a cellular system.
In a fixed wireless cellular system, the subscriber units are permanently mounted, and may be shared by multiple subscribers. 'rhi.s technique is used for providing a communications link to a large building, which might have a plurality of individual subscribers. Such fixed cellular systems can also be used in developing countries and in rural areas where conventional wireline service is either non-existent or uneconomical.
Frequency reuse patterns have been extensively studied in the cellular industry. Frequency reuse patterns minimizing adjacent-channel interference for frequency division duplex have been proposed for cell clusters of

3 greater than nine cells in size. However, claims have been made in existing studies that adjacent channel interference may be unavoidable when the number of cells in a cluster is less than nine. Existing frequency reuse patterns of a modulo of less than nine cells all suffer from the problem of adjacent-channel interference.
In U.S. Pat No. 5,549,292 issued to Doner, entitled "Obtaining Improved Frequency Reuse in wireless Communication Systems", the cells are sub-divided into six radial sectors. The frequencies are assigned to the sectors in such a manner as to enable reuse of each available frequency in every third cell (i.e. N=3).
However, in such a scheme the start-up cost is high, since at least two complete sets of transceiver equipment have to be located in each cell, even in initial low-density systems.
While an N=3 system can be shown to be highly efficient, it is not easily adapted to existing low-density plans. Low-density reuo~e patterns are typically implemented using a reuse factor of seven, and thus the reuse patterns do not f:it well into a reuse grid of three because seven is not divisible by three. Even if the low-

4 density reuse pattern is selected to be a multiple of three, mixing the N=3 cell patterns with such patterns creates unacceptable interference between homologous cells at the periphery of the cell patterns of a particular reuse factor. This is especially true where one attempts to locate a sectorized cell adjacent to an un-sectorized cell.
A further development in U.S. patent 5,974,323 issued to Doner, entitled "Frequency Plan for wireless Communication System that Accommodates Demand Growth to High Efficiency Reuse Factors" attempts to overcome this problem by starting with a low-density reuse pattern of 12 cells, with growth to a.n N=3 reuse factor. Such a scheme overcomes the aforementioned difficulty of attempting to integrate an N=3 schemes into a reuse pattern which is not divisible by 3. However, it requires the radio spectrum to be divided into three groups, which is not always desirable in some smaller frequency bands. Also, it is specifically optimized for frequency division duplex systems, and does not address the interference mechanisms, which arise with time division duplex systems.
In some existing frequency reuse patterns, adjacent channel frequencies may be deployed in the same cell. This

5 deployment can create a problem with the power amplifiers used in the cell. Power amplifiers operate with the greatest efficiency close to their saturation region.
However, as the saturation region is approached, a form of distortion known as third order intermodulation distortion can negatively affect t:he performance of adjacent channels operating in the cell using the power amplifier.
Therefore, frequency reuse methods should be designed to forbid adjacent channels from occupying the same cell.
The object of this invention is to provide a method of assigning frequencies, polarizations, and sectorization in both time division duplex and frequency division duplex wireless cellular communications systems.
It is a further object of this invention to optimize the frequency planning process for both frequency division duplex and time division duplex.
It is still a furtl:~er object of this invention to provide a method of assigning frequencies that prohibits adjacent frequency channels from occupying the same cell or adjacent sectors on adjacent cells, to prevent third order intermodulation distortion from the power amplifiers. This

6 should also allow the power amplifiers to be operated closer to the saturation region for improved efficiency.
STJMMARY
The method of assigning frequencies starts by dividing the geographic region for wireless coverage into a number of clusters. Each cluster is then divided into 16 cells.
The cells are arranged :into 4 columns of 4 cells, with the cells preferably arranged in a triangular pattern such that the cluster shape is a :rhomboid.
The available frequencies are divided into two groups, with adjacent frequencies assigned to separate groups. One group of frequencies is allocated to the first and third columns of cells in the cluster, and the second group of frequencies to the second and fourth columns of cells.
Each cell is then divided into n equal sectors of 360/n degrees, with n being an even integer greater than or equal to four. Basic examples include 4 sectors of 90 degrees or 8 sectors of 45 degrees. The number of sectors is restricted to twice the number of frequencies available in one group or the total number of frequencies available.

7 Each sector is then assigned a frequency. The frequencies available a:re assigned to the sectors in each cell such that no two adjacent sectors in a cell are assigned the same frequency. Finally, each frequency assigned either a horizontal or vertical polarization such that no two cells in a cluster have an identical combination of polarizat.ions and frequencies assigned.
Generally, the number of sectors per cell is equal to the total number of available frequencies. The greater the number of frequencies and polarizations assigned to a cell, the greater the traffic capacity that can be handled by that cell and the corresponding cluster and network. The number of sectors n is E=_qual to S, where S is equal to the number of available frequencies. Alternatively, the number of sectors per cell may be equal to S/2 or S/4, with the result that only one-half or one-quarter of the available frequencies are assigned. In this manner, future growth is accommodated by allowing for the division of the cell into additional sectors, as opposed to requiring the addition of new frequencies. For e:~cample, if eight (S=8) total frequencies are available, the cells may be originally divided in four (S/2) sectors each, resulting in the

8 assignment of four of the available frequencies, or half the total available in t=he cluster. The cells can later be divided into eight (S) ;hectors each, allowing for the assignment of all eight available frequencies and doubling the traffic-handling abilities of the system. This allows the system to grow and handle more wireless traffic without demanding additional frequencies beyond those initially available.
This invention also permits the power amplifiers used at cell hubs to be operated more efficiently by restricting adjacent channels from occupying the same cell.
BRIEF DESCRIPTION OF THF~ DRAWINGS
The invention itse7_f both as to organization and method of operation, as well as additional objects and advantages thereof, will_ become readily apparent from the following detailed description when read in connection with the accompanying drawings:
Figure 1 illustrates a typical wireless communication frequency spectrum, con"isting in this case of eight remote to hub frequency channels and eight hub to remote frequency channels;

9 Figure 2 illustratE~s the assignments of frequency and polarizations within the basic 16-cell cluster for an initial deployment;
Figure 3 illustrates the assignments of frequency and polarizations within the basic 16-cell cluster for a growth strategy, in which all 8 available frequencies have been allocated within the ba:~ic 16-cell cluster; and Figure 4 illustrates a rectangular and a rhomboidal arrangement of cells.
DETAILED DESCRIPTION
A representative frequency spectrum ranging from 38.6 GHz to 40.0 GHz is depicted in Figure 1. This frequency spectrum is further subdivided according to licensed frequency usage. In the following description, reference is made to "hub to remote", and "remote to hub"
transmission. This is used to illustrate the well-known technique of frequency division duplex. The terminology and the techniques of this invention apply equally to time division duplex, in which both "hub to remote" and "remote to hub" transmissions utilize the same frequency, but at different time periods.

The overall frequency spectrum consists of two major blocks, a remote to hub block 1 and a hub to remote block 2. Further, the remote to hub block 1 is sub-divided into shared portions 3, and an exclusive portion 4. Similarly, the hub to remote block 2 is sub-divided into shared portions 5, and exclusive portion 6. This invention is concerned with, but not restricted to, the exclusive portions 4 and 6.
Exclusive portions 4 and 6 are divided into, for purposes of illustration, two sets of eight frequency channels each. The frequency channels are numbered from fl to f8 for the exclusive portion 4 of remote to hub block 1, and from fly to f8~ for the exclusive portion 6 of hub to remote block 2. Only the frequency channels fl to f8 in the exclusive portion 4 of remote to hub block 1 will be described. The frequency planning methods described apply equally to the frequency channels fl' to f8' contained in the exclusive portion 6 of hub to remote block 2.
The frequency plan of this invention is based on an n=16 repeat pattern and a frequency reuse of N=2. In addition, for the N=2 frequency reuse pattern, all frequencies must be able to be operated on either the vertical or the horizoni~al polarization.
This frequency plan can be used in an initial low-density application, as shown in Figure 2, and can be subsequently expanded to use all allocated frequencies indicated in the exclus_Lve portion 4 of Figure 1. The frequency plan utilizin<i all frequency channels fl to f8 of exclusive portion 4 is depicted in Figure 3.
Referring to Figure: 2, an arbitrary area has been mapped into a number of r_ells. A specific area of such cells has been designated as a cluster 11, which consists of 16 cells, numbered 1 to 16, corresponding to an n=16 repeat pattern. The cluster 11 is designed to be repeated in two dimensions, by placing adjacent clusters contiguous to the cluster 11, such that cluster symmetry is maintained. Lines extending from cluster 11 in Figure 2 show the contiguous boundaries of these adjacent clusters.
The cluster 11 is rhomboidal-shaped arising from triangular cell spacing as opposed to a more conventional rectangular or hexagonal shape. The basic unit of each pattern is shown in more detail in Figure 4, wherein it can be seen that closer cell spacing is achieved by triangular cell spacing, as well as a smaller area of reduced coverage.
In an initial deployment, four of the available frequencies are used. These are chosen from the eight available in exclusive portion 4 in Figure l, to be as widely separated in frequency as possible, to reduce adjacent channel interference. Each of the cells 1 to 16 is then split into four 90-degree sectors. For purposes of description, the cells 1. to 16 are assigned to columns and rows, with column 1 consisting of cells 1 through 4, column 2 consisting of cells 5 through 8, column 3 consisting of cells 9 through 12, and column 4 consisting of cells 13 through 16. Likewise, row 1 consists of cells 1, 5, 9, 13, row 2 consists of cells 2, 6, 10, 14, row 3 consists of cells 3, 7, 11, 15, and row 4 consists of cells 4, 8, 12, 16.
With reference to Figure 2, there are two frequencies, fl and f5, deployed in column 1. The frequencies fl and f5 are always displaced 90 degrees with respect to one another. Furthermore, in column 1 the sector locations of fl and f5 repeat every second cell. In order to further reduce the co-channel interference caused by sectors that carry the same frequency, polarization separation of the frequency into vertical polarization (V) and horizontal polarization (H) is intraduced. Each frequency is identified as fxy, where x is the frequency number and y is the polarization. This i.s shown in column 1, where horizontal polarization i.s introduced as f1H in cell 3 and f5H in cell 2. By using two separate polarizations for the same frequency, the bore-sight frequency repetition distance is increased over that found in low N frequency reuse methods without frequency polarization. All sectors within a given cell using the same frequency channel are given the same polarization, to prevent potential problems from depolarization caused by external effects, such as rain.
Again with reference to Figure 2, the other two chosen frequencies f2 and f6 in the four-frequency plan are introduced in column 2. The sectors and polarizations used for f2 and f6 in column 2 are the same as those used for fl and f5 in column 1, respectively i.e. where f1H was used in column 1, f2H is used is column 2, similarly, where flv was used in column 1, f2V is used in column 2, etc.

Column 3 utilizes t:he same sector assignment for frequencies as used in column 1, however, all polarizations used in column 3 are them opposite of those used in the same row in column 1. For e~s:ample, where column 1 uses fxV, column 3 uses fxH, and v~rhere column 1 uses fxH, column 3 uses fxV. The opposite polarization of the same frequency in different columns further reduces co-channel interference.
Similarly, column 4 utilizes the same sector and frequency assignments as. used in column 2 and all polarizations used in column 4 are the opposite of those used in the same row column 2.
Referring to Figure: 2, each cell uses a unique combination of palarizations and frequencies such that no two cells are identical.
The basic plan described in the preceding can be expanded to permit progressive growth to enable maximum use to be made of the available frequency spectrum. The polarized frequencies have been arranged within the sectors of the cells such that no two cells have an identical combination of polarizations and frequencies within their sectors. Figure 2 is based on using the four frequencies f1, f2, f5 and f6. Howcwer, there are eight total frequencies available, iEl to f8. Further subdividing the existing four 90-degree sectors into eight 45-degree sectors allows use of all eight frequencies. Figure 3 illustrates the method of this invention wherein all eight frequencies are utilized.
Figure 3 retains the basic symmetry of Figure 2, l0 except that where originally one sector and one polarized frequency were used in 1?figure 2, now two sectors and two polarized frequencies occupy the same location in Figure 3.
This is most readily de:~cribed in the following manner.
Each sector originally occupied by fl in Figure 2 is now sub-divided into two sub-sectors in Figure 3, one occupied by fl and the other by f3. The polarization of the sub-sector frequencies is the same as those of the sector frequencies in the original sector. For example, the lower-left sector oi= cell 1 in Figure 2 is occupied by flV. In Figure 3, the .Lower-left sector of cell 1 has been sub-divided in two sub-sectors, one occupied by flV and the other occupied by f3V. The sub-division method described above is applied to each sector of each cell, with f2 repl aced by f 2 and f 4 , iE 5 by f 5 and f 7 , and f 6 by f 6 and f8.
The expanded frequency plan described in Figure 3 may be applied to each cluster 11 as needed to accommodate increased wireless traff=ic. As the frequencies added to the sector assignments are not present in the original low-traffic frequency plan in Figure 2, there is no additional co-channel or adjacent channel interference along the cluster boundaries between a low-traffic cluster and a high-traffic cluster.
The embodiments described in the preceding use a specific sectorization a.nd available frequency group to illustrate the process used in this invention. In this description, a maximum of eight frequency channels and eight sectors per cell were employed. This technique can be extended to any number of frequencies and any degree of sectorization. One such. extension would be to 16 frequencies and 16 sectors. However, it should be noted that the number of frequencies used in a cluster is always equal to the number of sectors per cell.

Further, by reference to Figures 2 and 3, it can be seen that adjacent channel. frequencies never occupy the same cell, thus permitting power amplifiers to be operated closer to saturation, resulting in greater efficiency.
Power amplifiers operate most efficiently when operated close to the saturation region. However, as the saturation region is approached, a form of distortion known as third order intermodulation d_Lstortion can affect the performance of adjacent channels. =Cn the prior art, it was common to deploy adjacent channel; in the same cell. The method described herein permits more efficient operation of power amplifiers by prohibiting adjacent channels from occupying either the same cell or adjacent sectors in adjacent cells.
This method also prohibits identical frequencies with opposite polarizations f=rom occupying the same cell, or adjacent sectors within a cell. This advancement over the prior art eliminates the potential deterioration in performance that occurs when depolarization, caused by rain, reduces the co-cha.nnel separation.
Accordingly, while this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to this descr_Lption. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the scope of the invention.

Claims (12)

WHAT IS CLAIMED IS:

1. A method of assigning channels within a wireless communications system, comprising:
(a) dividing a geographic region covered by said wireless communications system into a plurality of clusters;
(b) dividing each said cluster into a 4x4 array of cells;
(c) partitioning each said cell into n sectors, where each said sector covers an angle of 360/n degrees, and where n is an even integer greater than or equal to four;
(d) splitting frequencies within a frequency band assigned to said channels equally into a first group of frequencies and a second group of frequencies wherein no two frequencies in a group are adjacent within the frequency spectrum;
(e) allocating said first group of frequencies to a first group of channels and said second group of frequencies to a second group of channels;
(f) allocating said first group of channels to the cells in the first and third columns of said 4x4 array of cells and said second group of channels to the cells in the second and fourth columns of said 4x4 array of cells;
(g) assigning a channel from each said allocated group of channels to two sectors of said each cell such than said each cell contains a number of different channels equal to the number of sectors in that cell divided by two and such that no two adjacent sectors in the same cell are assigned the same channels; and (h) assigning a polarization to each said frequency in said channels in said each sector of said each cell into one of vertical or horizontal polarization such that no two cells in said cluster are assigned an identical set of polarized frequencies.

2. The method according to claim 1, wherein said wireless communications system is a frequency division duplex wireless communications system.

3. The method according to claim 1, wherein said wireless communications system is a time division duplex wireless communications system.

4. The method according to claim 1, wherein the number of sectors used in said each cell is equal to the number of available channels.

5. The method according to claim 1, wherein the number of sectors used in said each cell is equal to half the number of available channels.

6. The method according to claim 1, wherein the shape of said cluster is a rhomboid.

7. The method according to claim 1, wherein each cell is divided into a number of sectors equal to one half of the number of channels available in said first and said second group of channels and one half of the channels available in said first and said second group of channels are allocated to said sectors of said cells.

8. The method according to claim 7, wherein each sector can be sub-divided into two sub-sectors such that the total number of sub-sectors thus created is equal to the number of channels available in said first and said second group of channels and all of the channels available in said first and said second group of channels are then allocated to said sub-sectors of said cells.

9. A method of assigning channels within a wireless communications system, comprising:
(a) providing a first group of channels having frequencies f1, f3, f5, . . . . . , f j and a second group of channels having frequencies f2, f4, f6, . . . . . , f k, where each frequency f j and f k includes horizontal and vertical polarizations of said frequency and where no two frequency channels in a group are adjacent in a corresponding frequency spectrum of said group of channels;
(b) dividing a geographic region into a plurality of clusters and each cluster into arrays of cells;
(c) partitioning each cell of each array of cells into n segments, where n is an even number;
(d) allocating said channels in said first group of channels to segments of cells in odd columns of each array of cells, so that no two adjacent segments in the same cell are assigned the same channel; and (e) allocating said channels in said second group of channels to segments of cells in even columns of each array of cells, so that no two adjacent segments in the same cell are assigned the same channel.

10. The method of claim 9, wherein no two adjacent segments of adjacent cells are assigned the same channel.

11. The method of claim 9, wherein each of said arrays of cells is a 4x4 array of cells.

12. The method of claim 9, wherein the cells are circular or polygonal and said segments are sectors.