GB2366962A

GB2366962A - Adaptive frequency response plan

Info

Publication number: GB2366962A
Application number: GB0127919A
Authority: GB
Inventors: Steven Ray Henson
Original assignee: Ericsson Inc
Current assignee: Ericsson Inc
Priority date: 1997-02-10
Filing date: 1998-02-06
Publication date: 2002-03-20
Anticipated expiration: 2018-02-06
Also published as: GB2366962B; GB0127919D0

Abstract

A cellular telecommunications network comprises N cells associated to form a cluster and a plurality of clusters are associated to form a modified cluster. The network has T frequency channels available which are distributed over the modified cluster according to a first frequency reuse plan. The plan involves, assigning a first group of frequencies to a first cell, assigning a different group of frequencies to a second cell and when an increase in call capacity requires further channels, reusing frequencies from the second cell within the first cell. In practice a seven cell cluster may be used A7-G7. The modified cluster may contain seven clusters, wherein the cells are labelled in the same pattern for each cluster, e.g. A3-G3. Since all of the T frequencies are used only once over the modified cluster, the frequency borrowed to accommodate extra traffic is chosen from a cell in the modified cluster which is in the same position in the basic cluster e.g. G7 uses G3 as its source of extra channels. The whole scheme thus ensures sufficient channel separation of the new frequency and an acceptable spatial separation to reduce co-channel interference.

Description

<Desc/Clms Page number 1> ADAPTIVE FREQUENCY REUSE PLAN BACKGROUND OF THE INVENTION Technical Field of the Invention The present invention relates to a cellular telecommunications network and, in particular, to a cell pattern within such a network using an adaptive frequency reuse plan.

Description of Related Art Frequency reuse patterns are cell-based schemes for assigning the frequency channels available within a particular cellular telecommunications system. The most basic unit of any frequency reuse pattern is a cell. Each cell within a frequency reuse pattern is assigned a number of frequency channels. A plurality of cells are then associated together to as a cluster and utilizeg all of the frequency channels available to a particular cellular telecommunications system. Groups of clusters are then used to provide a cellular coverage area within the cellular telecommunications system and the frequency channels allocated .for one cluster are reused in other clusters. The scheme for recycling or reassigning the frequency channels throughout the serving coverage area is referred to as a reuse plan. The distance between a first cell using a particular frequency channel within a first cluster and a second cell using the same frequency channel within a second cluster is further known as a reuse distance.

The reuse of the same frequency channels by a number of different cells implies that cells may suffer from cochannel interferences. It is therefore desirable for the received strength of the serving carrier (C) within each cell to be higher than the total co-channel interference level (I). As a result, the higher the carrier to interference (C/I) value, the better the speech quality.

A higher C/I value is obtained partly by controlling the channel reuse distance. The larger the reuse distance between adjacent cells utilizing the same frequency channels, the lesser the co-channel interferences created between those cells.

The C/I ratio is further related to a frequency reuse plan (N/F) where N indicates the number of cells included within a single cluster and F indicates the number of frequency groups. For example, the C/I ratio is directly related to the following equation: DR- (3*F) iiz*R Where: DR is the reuse distance; F is the number of frequency groups; R is the radius of a cell.

Accordingly, the larger the F value, the greater the reuse distance. However, it is not always desirable to use a larger F value to increase the C/I ratio. Since the total number of available =frequency channels (T) is fixed within a'particular mobile network, if there are F groups, then each group will contain T/F channels. As a result, a higher number of frequency group (F) would result in a fewer channels per cell and lesser call capacity.

For most cellular systems, capacity is not a major issue when the system initially goes into operation. Therefore, in order to achieve a high C/I value and to improve the quality of speech connection, a high frequency reuse plan (N/F), such as 9/27, is initially used. However, as the capacity increases, the cellular telecommunications network has to resort to a lower frequency reuse plan, such as a 7/21 or 4/12, to allocate more frequency channels per cell. Consequently, the whole cellular telecommunications network and its associated clusters and cells need to be reconfigured with a new frequency reuse plan. Such reconfiguration and reallocation requires an investment of considerable time and resource. on the other hand, due to poorer speech connection quality, it is undesirable to use a low

frequency reuse plan from the beginning when there is no need for high capacity.

Some existing systems have used other approaches to increase capacity and reduce co-channel interference. Rappaport (WO 94/18804), for example, generally describes designating a portion of the frequency channels assigned to each face of a cell for lending to adjacent cells. When a11 the channels assigned to an adjacent cell are utilized, a frequency channel designated for lending is temporarily assigned to the adjacent cell in order to temporarily increase the capacity of the adjacent cell. Benveniste (EP 0 684 744 A2) generally describes another approach where channels are borrowed from neighboring cells in a specified order, and a specified number of channels are borrowed from each neighboring cell before returning to a particular cell to borrow additional channels. Kallin (WO. 95/07013), on the other hand, attempts to reduce co-channel interference by assigning different highest priorities- for frequency channels in each co-channel cell (e.g., cells which are assigned the same frequencies) and by selecting for communication the frequency channel having the highest priority. This approach reduces the likelihood of two co-channel cells selecting the same frequency channel at the same time.

Other systems have taken a different approach than those-described above. For example, Faruque (WO 95/02308) generally describes an N=3 frequency plan where channels are assigned to 60 degree sectors in accordance with an odd-even cyclic distribution thereby providing a three channel separation between sectors and an eight channel separation between cells. Hamabe (EP 0 616 481) generally describes assigning a channel group to a sector of each cell in such a manner that the same channel groups are assigned to sectors which have substantially the same directions determined by antenna directivity. Channels within each sector are allocated for communication if the carrier- to= interference ratio is above a predetermined

level. Finally Naeini (JP 0536864 A2) generally describes a cellular time division multiplexed wavefcrm which allows time slots to be reused across a service area by allowing all similarly numbered cells or clusters to transmit simultaneously.

Although these approaches alleviate some of the problems experienced by service operators, these approaches fail to adequately address the problem of adapting frequency plans. Accordingly, there is a need for a mechanism to enable service operators to adapt their frequency plan according to their capacity and C/I without reconfiguring the channel allocation.

SUMIARY OF THE INVENTION According to the invention, there is provided a method for adaptively changing a first reuse plan to a second reuse plan within a cellular telecommunications network, said method characterized by the steps of: associating N cells as a cluster; associating a plurality of clusters as a modified cluster; distributing T number of frequency channels within said modified cluster according to said first reuse plan by: assigning a first group of frequency channels to a first cell; and assigning a second group of frequency channels to a second cell, wherein said first group is different from said second group; and reusing, in order to accommodate an increase in call capacity within said first cell, frequency channels from said second group of frequency channels within said first cell to adaptively change to said second reuse plan.

The present invention overcomes the foregoing and other problems with a modified cells cluster and an adaptive frequency reuse plan. The plan supports a gradual change from a high reuse plan to a low reuse plan to adapt to an increase in call capacity without requiring a reconfiguration of the channel allocation throughout the network.

BRIEF DESCRIPTION OF THE DRAWINGS A more complete understanding of the method and apparatus of the present invention may be had by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein: FIGURE 1 is a diagram of a seven cell per cluster pattern using an omni-directional antenna to provide radio coverage over a particular area; FIGURE 2 is a diagram of a modified forty-nine cells per cluster pattern using,: -uni-directional antennas in accordance with the teachings of the present invention; FIGURE 3 is a diagram of a cell plan illustrating different reuse distances; FIGURE 4 is an illustration of a center-excited sectorized antenna configuration within an seven cells per cluster pattern; FIGURE 5 illustrates the assignment of frequency channels to each sector within each cell of FIGS. 2 and 4; FIGURE 6 is a diagram of a 49/147 cell plan of the present 'invention illustrating the assignment of frequency channels to each sector within each cell; and FIGURE 7 is a diagram of a 49/147 plan adapted to a 7/21 cell plan in accordance with the teachings of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS Reference is now made to FIG. 1 illustrating a pattern with seven (7) cells per cluster 5. An omni-

directional antenna is used in each cell to provide radio coverage over a particular area. The pattern is schematically represented by a hexagonal grid with a single cell in the middle and six (6) surrounding additional cells. (This pattern and the frequency assignment scheme associated therewith, which will be more fully discussed later, provide all of the basic properties of a conventional reuse pattern.

The proposed N=7 frequency plan for an omni- directional antenna site as shown in FIG. 1 is based on dividing all of the available frequency channels (T) in the spectral band available to a particular cellular telecommunications network into seven or multiples of seven frequency groups with approximately T/7 channels per frequency group. Table 1 shows the channel assignments for such an omni-directional antenna system.

Table 1 Frequency Channel Group A B C D E F G Channel 1 2 3 4 5 6 7 Number 8 9 10 11 12 13 14 15 16 17 18 19 20 21 As can be seen from Table 1, the frequency channels are assigned sequentially to each frequency channel group. Therefore, the difference in frequency channel numbers between frequency channels assigned to any channel group is seven. A frequency channel group is then associated with each cell in a manner that eliminates adjacent frequency channels within the cluster and with respect to adjacent clusters. These same frequencies, after being assigned to a first cluster, may then be reused by other clusters according to the same assignment configuration in order to provide cellular coverage over a specific area.

The seven cells within each cluster are typically alphabetically labeled. For example, a G-cell is in the middle surrounded by six A-F cells. Cells with the same label are then associated as a cell group. Each frequency channel group described above is then allocated to each corresponding cell within a cluster.

As an illustration, a11 frequency channels associated with the A frequency group are allocated to the A cells A1-A7. Similarly, frequency channels associated with the rest of the frequency groups B, C, D, E, F, and G, are allocated to the remaining cells B1-B7, C1-C7, D1-D7, E1- E7, F1-F7, and G1-G7, respectively. The same frequency channels are utilized by corresponding cells in each cluster 5 creating a potential for co-channel interference. For example, the G7 and G3 cells reuse the same frequencies. The distance between two cells utilizing the same frequency channels is known as a reuse distance 30. The greater-the-_reuse distance, the lesser the chance of co-channel interference. However, in order to allocate more frequency channels per cell to increase call capacity, the number of frequency groups is decreased resulting in a lesser reuse distance. By reducing the reuse distance 30, a potentially higher co-channel interference arises. As a result, with an increase in call capacity, a decrease in speech connection quality may follow.

Reference is now made to FIG. 2 illustrating a modified forty-nine cells per cluster pattern using unidirectional antennas in accordance with the teachings of the present invention. An initial determination is made as to which frequency reuse (N/F) plan is ultimately going to be used in the system for maximum capacity. Hereinafter, this is referred to as the "target" reuse plan. For example, the 7/21 plan as illustrated in FIG. 1 is determined. Thereafter, seven contiguous clusters are associated together as a modified cluster 40 creating a modified (N*7) / (F*7) plan. Accordingly, the modified

cluster 40 includes seven times N (forty-nine for FIG. 2) number of cells associated within seven clusters. The number of frequency groups is further increased to F*7. As disclosed above, an increase in the number of frequency f' groups (F) increases the reuse distance (DR). The (N*7)/(F*7) plan then takes the allotted frequencies available to the serving cellular telecommunications network and distributes them over (N*7) cell sites. As an illustration of such a distribution:

Table 2 Cell Numbers A1 B1 C1 D1 E1 F1 G1 A2 B2 ..... G7 Channel 1 2 3 4 5 6 7 8 9 ...... 49 Number 50 51 52 53 54 55 56 57 58 ...... 98 99 100 101 102 103 104 105 106 107 ..... 147 Reference is now made to FIG. 3 illustrating a reuse distance between two modified clusters 40 within the modified forty-nine cells per.cluster pattern. Assuming that the width of each cell is 0.60 measurement units and the height is 0.52 measurement units, the reuse distance 30 between the two cells G7 and G3 using the same frequency group within a conventional frequency reuse plan (e.g., 7/21) as shown in FIG. 2 is 1.38 measurement units. On the other.hand, a reuse distance 50 between two cells G7 using the same frequency group within the modified reuse plan (e.g., 49/147) is 3.64 measurement units. As a result, the use of a cluster and its six surrounding clusters to distribute the T number of frequency channels, rather than distributing the channels all within one cluster, creates an improvement of up to 2.6 times in the reuse distance FIGURE 4 is an illustration of a center-excited sectorized antenna configuration within a seven-cell cluster. Each site contains a single antenna site 60 with

three sectors 70 having antenna pointing azimuth separate. by 120 . Tt should be understood that while FIG. 4 is described with respect to a three sector configuration, other multi-sector configurations may be used. Each sector 70 is approxifated by the shape of a rhombi. Each sector can use, for example, a 60 , 90 , or 120 transmit antenna and two corresponding diversity receiver antennas with the same pointing azimuth. The center-excited three sector pattern splits the hexagon representing a cell into three rhombi. The frequency group assigned to that cell is accordingly split into three subgroups.

For identification purposes, the seven clusters within a modified cluster are numbered one through seven (1-7). Each cell associated with a particular cluster is then further identified by its alphabetical label plus the numerical label assigned to the parent cluster. The three sectors within a cell are further identified by retaining the label from its parent-and further adding a sector subscript (e.g., 1-3). As an illustration, the cell A1 is sectored into three sectors labeled A11, Al., and A13. Similarly, the A2 cell within the next cluster is sectored into A21, A22, and A23. The available frequency channels are then assigned on a one-by-one basis starting with A11 where all sectors with the same subscript are sequentially assigned a frequency channel before assigning the next subscript sector. When a11 of the sectors within a first cluster are each assigned a frequency channel, the sectors within the rest of the clusters are assigned in a similar manner. This sectorization and labeling may be applied to the pattern illustrated in FIG. 2.

FIGURE 5 illustrates the frequency channel allocation for the modified 49/147 plan (of Figs. 2 and 4) in accordance with the teachings of the present invention. As illustrated by row 100, A11 is assigned first frequency channel number two (2). A sector from each of the cells within the same cluster with the same subscript (1) label is then sequentially numbered as shown. After all of the

sectors with the first subscript label have been assigned a frequency channel, sectors with the second subscript label are then similarly assigned a frequency channel as shown in row 110. As a result, the difference between assigned channel numbers for two sectors within the same cell is in the magnitude of seven (7). For example, A11 is assigned channel number two (2) and A12 within the same cell is assigned channel number nine (9). When a11 of the sectors within the first cluster are assigned a frequency channel, the sectors within a second cluster are similarly assigned a frequency channel as shown in row 120. Accordingly, the difference between assigned channel numbers for two sectors within the same cell group with the same subscript label is in the magnitude of twenty-one (21). For example, A11 for the first cluster is assigned channel number two (2), and A21 with the same subscript for the A cell group associated with the second cluster is assigned channel number f.wetty-three (23).

When all sectors associated with seven clusters within a modified cluster are assigned a frequency channel, the remaining frequency channels are reassigned repeatedly for the same sectors in a similar manner. There are one hundred forty seven (7*7*3) sectors within each modified cluster. Therefore, the last sector G73 is assigned channel number one -hundred-forty-eight (148). Assignment of remaining channels starts over again at sector All with channel number one- hundred-forty-nine (149) as illustrated in column 140. This process continuous until all T available channels have been assigned. As a result, the difference between multiple channel numbers assigned to the same sector is in the magnitude of one -hundred-forty-seven (147). As described above, there are forty-nine (49) cells within a modified cluster. Accordingly, the 49/147 plan is introduced.

FIGURE 6- is a diagram of the 49/147 cell plan (of Figs. 2, 4, and 5) illustrating the assignment of frequency channels to each sector within each cell. As

fully described in FIG. 5, the difference between assigned channel numbers to a particular sector is in the magnitude of one-hundred-forty-seven (147). Accordingly, since no same frequency channel is reused within the seven clusters, the reuse distance with the neighboring modified cluster is much greater. As a result, a higher C/I ratio and improved speech quality is introduced.

In response to an increase in demand for capacity at a particular sector (i.e., A11), the prior art teaches reallocating all of the frequency channels using a lower reuse plan. In accordance with the teachings of the present invention, however, a frequency channel from a different sector within the same cell group having the same subscript label is advantageously reused within that particular sector. As an illustration, in case sector A11 needs to be assigned more frequency channels for additional call capacity, a frequency channel previously assigned to sector A21 (belonging to the same cell group A and having the same subscript label one) is reused within sector A11. Similarly, A1 lmay reuse frequency channels previously assigned to A31, A41, A51, A61, and A7,. Since, sector All was initially assigned frequency channels numbers two (2) and one-hundred-forty-nine (149), reusing frequency channels twenty-three (23) and one- hundred-seventy (170), for example from sector A21, decreases the difference in channels numbers to the magnitude of twenty-one (21). Accordingly, as far as those two sectors are concerned, they are using the 7/21 reuse plan as in FIG. 1.

Since reusing other frequency channels is only required for a particular .sector with a need for additional.capacity, as frequency channels are reused by neighboring sectors within the same modified cluster, the overall frequency reuse layout can be different throughout the system and can continually be updated without affecting the frequency assignment already in place.

As the system grows, additional capacity issues can be addressed by only drawing from one sector until al? frequencies from that sector have been reused. Upon utilizing all of the frequencies within a particular sector, frequency channels previously assigned to a next sector within the same cell group having the same subscript label can be reused. For example, in order to address an increase in. the call capacity for sector A11, frequency channels previously assigned to sector A21 are reused. Upon exhausting a11 frequency channels associated with that sector, other frequency channels from sector A31, for example, are reused for sector A11: As the same frequency channels within the same modified cluster are being used within more than one cluster, a corresponding reuse distance decreases causing the C/I to also decrease.

Reference is now made to FIG. 7 illustrating a 49/147 plan adapted to a 7/21 plan:--As sectors utilize a11 of the frequency channels assigned to other sectors within the same cell group with the same subscript label, each cluster will be utilizing the same frequency channels transforming the modified 49/147 plan into the target 7/21 plan. As an illustration, in order to handle maximum capacity, sector All uses all frequency channels assigned to sector A21 as well as frequency channels from a11 other sectors within the same cell group with the same subscript label. The rest of the sectors similarly reuse frequency channels previously assigned to other sectors. Since, the frequency channels being used by the two sectors are the same within a particular modified cluster, the reuse distance is accordingly reduced and an increase in co- channel interference is effectuated. As a result, the overall reuse plan is ultimately changed into the originally targeted 7/21 reuse plan.

In accordance with the teachings of the present invention, a service operator can initially deploy a cellular system with an attractive high reuse plan and

selectively decrease the reuse plan to the targeted reuse plan to accommodate an increase in call capacity throughout the network.

Even though the present invention has been described using the 7/21 target reuse plan with the 49/147 modified reuse plan, it is to be understood that the present invention is applicable for other reuse plans, including but not limited to, 3/9, and 4/12 with the modified plan being 21/63, and 28/84, respectively. Other reuse plans and modified plans will be apparent to those skilled in the art.

Although a preferred embodiment of the method and apparatus of the present invention has been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it will be understood that the invention is not limited to the embodiment disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope of the invention as set forth and defined by the following claims.

Claims

CLAIMS 1. A method for adaptively changing a first reuse plan to a second reuse plan within a cellular telecommunications network, said method characterized by the steps of: associating N cells as a cluster (5); associating a plurality of clusters as a modified cluster (40) ; distributing T number of frequency channels within said modified cluster (40) according to said first reuse plan by: assigning a first group of frequency channels to a first cell; and assigning a second group of frequency channels. to a second cell, wherein said first group is different from said second group; and reusing, in order to accommodate an increase in call capacity within said first cell, frequency channels from said second group of frequency channels within said first cell to adaptively change to said second reuse plan.
2. The method of claim 1, wherein said step of reusing- further includes reusing, within said first cell associated with a first cluster within said modified cluster (40), a first frequency channel assigned to a second cell associated with a second cluster within the same modified cluster (40) wherein the difference in channel numbers between a second frequency channel assigned to said first cell and said first frequency channel reused from said second cell is at least seven.

<Desc/Clms Page number 16>
3. The method of claim 1 further characterized by the steps of: partitioning each cell within said modified cluster (40) into D number of sectors (70); and assigning said D number of sectors (70) one or more of said T number of frequency channels according to said first reuse plan.
4. The method of claim 3 further characterized by the step of reusing, within a first sector within said first cell of a first cluster, a first frequency channel assigned to a second sector within said second cell of a second cluster within the same modified cluster (40), wherein the difference in channel numbers between a second frequency channel assigned to said first sector and said first frequency channel being reused from said second sector is at least twenty-one.
5. ' The method of claim 4, wherein said step of reusing further includes the step of reusing a third frequency channel from a third sector within a third cluster only after all frequency channels from said second sector have been reused.
6.- The method of claim 1 wherein said N includes a numerical value of seven (7).