GB2328841A - Means of increasing capacity in cellular radio (mobile and fixed) systems - Google Patents

Abstract

A method of arranging a plurality of directional beams in a cellular radio system having a plurality of antennas 65 each communicating over a corresponding respective cell area. Interference between geographically close cells is reduced by the method resulting in an improvement in carrier to interference ratio performance. Carrier frequencies of inner two beams 62A and 63A transmitted by antenna 65A exchange for inner two beams 62B and 63B which are transmitted in substantially the same direction by antenna 65B. This results in an improved carrier to interference performance for all four beams transmitted by the antennas. The techniques disclosed are applicable to center-excited or corner excited (tri-cellular) systems.

Description

MEANS OF INCREASING CAPACITY IN CELLULAR RADIO (MOBILE AND FIXED) SYSTEMS Field of the Invention The present invention relates to a method of operating an antenna arrangement in a cellular communications system and more particularly to a method of assigning frequencies to multi-beam directional antennas.

Background to the Invention In conventional cellular radio systems, geographical areas are divided up into a plurality of adjoining cells, in which mobile stations within a cell communicate with a base transceiver station. In general, each mobile (or set of mobiles sharing a multiplexed channel) communicating with a base station in a cell uses a different carrier frequency to other mobiles in the cell, to avoid interfering with the other mobiles. Thus the number of mobiles which can be served in a cell is limited by the number of available carrier frequencies. There is increased capacity demand for use of cellular radio systems, however the frequency band within which cellular radio systems operate is limited in width, and so to provide increased capacity in the system available carrier frequencies are re-used from cell to cell.

The re-use of frequencies in a locality is restricted by co-frequency interference between different cells which re-use the same or close frequencies and which are geographically close to each other. To obtain maximum capacity in a system comprising a plurality of cell areas, cellular radio system designers aim to re-use as many different carrier frequencies of the set of available carrier frequencies as possible in each cell. However there are limits on the re-usage of carrier frequencies in a cell due to other potentially interfering signals, particularly: interference between a carrier frequency in a first cell and an identical frequency re-used in neighboring cells interference between a carrier frequency used in a first cell and adjacent carrier frequencies used in neighboring cells.

The minimum physical distance between geographic cells which re-use a same canier frequency or an adjacent carrier frequency is limited by the required quality of signals received at the carrier frequency. One metric used to describe the quality of the signal is referred to in the art as the carrier to interference ratio (C/l ratio). The C/I ratio is a ratio of signal strength of a received desired carrier frequency to a signal strength of received interfering carrier frequencies and noise. A number of physical factors can affect the C/I performance in cellular systems, including reflections from buildings, geography, antenna radiation pattems, mobile station transmitting power, and mobile station locations within a cell. In general, calculating the distances between cells which re-use an interfering carrier frequency is a complex problem, however a general approach to the calculations may be found in Mobile Cellular Telecommunications Systems by William Chien-Yeh Lee published by McGraw Hill Book Company, New York 1989.

Taking as an example a Digital Amps TDMA (time division and multiple access) system having available 12.5 MHz of frequency spectrum, for example in the 850 MHz band, individual carrier frequencies are spaced apart from each other centered at spacings of every 30 KHz, giving a total of 416 carrier frequencies available across the network as a whole. The 416 carrier frequencies are partitioned so that individual carrier frequencies are re-used from cell to cell.

Taking as an example a base station re-use factor n of 7 (n=7), for centerexcited cells each cell is allocated 416+7=59 carrier frequencies per cell.

However, with a base station re-use factor of n=4, this gives 416+4=104 carrier frequencies per cell, resulting in a higher capacity than for an n=7 re-use factor. At a base station re-use factor of n=4 cells which re-use a same carrier frequency (the frequency re-use cells) are closer to each other than at a base station re-use factor n=7, resulting in more interference, and a lower C/I ratio in the base station re-use factor n=4 case that in the base station re-use factor n=7 case. To implement the lower base station re-use factor (n=4) frequency re-use cells must be closer together than with a higher base station re-use n=7.

However, the distance between the re-use cells must be great enough that the carrier to interference ratio is high enough to allow the cellular radio telecommunications apparatus to distinguish signals at each re-used carrier frequency in one cell from the interfering frequencies present in other cells across the network. The C/I performance is a limiting factor in implementation of a lower base station re-use factor.

Summary of the Invention An object of the present invention is to provide improved carrier to interference ratio for a plurality of beams which re-use frequencies from beam to beam, and to provide an acceptably low level of interference overall, thereby allowing greater re-use of frequencies and providing a capacity gain for a cellular radio communication system.

According to one aspect of the present invention there is provided in a cellular radio system having a plurality of base stations each capable of communicating over at least one corresponding respective cell area using a plurality of directional beams, a method of arranging said plurality of directional beams, said method comprising the steps of: at a first base station, forming a first set of beams in a first cell area; at a second base station, forming a second set of beams in a second cell area; wherein at least one beam of said first set is directed in a substantially same direction to and re-uses a first same frequency as at least one beam of said second set; and at least one remaining beam of said first set re-uses a second same frequency as at least one remaining beam of said second set, said remaining beam of said first set being directed in a direction pointing away from said remaining beam of said second set.

Said first set of beams use a set of carrier frequencies, each beam of said first set being assigned a corresponding respective carrier frequency from said set of carrier frequencies, said carrier frequencies being different to each other, and said second set of beams re-use said set of carrier frequencies.

Preferably each said set of beams comprises at least one inner beam and at least one outer beam; wherein a said outer beam of said second set re-uses a frequency of an outer beam of said first set, said outer beam of said second set directed in a substantially same direction as said outer beam of said first set; and an inner beam of said second set re-uses a frequency of an inner beam of said first set, said inner beam of said second set being directed away from said inner beam of said first set.

According to a second aspect of the present invention there is provided in a cellular radio system having a plurality of base stations each communicating over at least one corresponding respective cell area, by means of a plurality of directional beams, a method of arranging said plurality of directional beams, comprising the steps of: assigning a first set of said beams to a first said base station of a first said cell area; assigning a second set of beams to a second said base station of a second said cell area; wherein each beam of said first set corresponds with a respective beam of said second set: wherein a beam of said first set has a common carrier frequency and common direction with a corresponding beam of said second set; and a beam of said first set has a common carrier frequency with but a different direction to a corresponding beam of said second set.

Preferably each set of beams comprises an outer beam and an inner beam; and an outer beam of said first set has a common carrier frequency and common direction with an outer beam of said second set; and an inner beam of said first set has a common carrier frequency but a different direction to an inner beam of said second set.

In one embodiment, said base stations each serve to communicate over three corresponding respective adjoining hexagonal edge excited cells each of substantially equal area in a tri-cellular arrangement.

The use of a plurality of directional beams in a single cell of a tri-cellular arrangement may enable improved carrier to interference ratio, and allow tighter frequency re-use in a cellular radio system.

Preferably each said set of beams comprises four directional beams.

Preferably each said set of directional beams comprises two inner beams and two outer beams.

The invention includes a cellular radio system having a plurality of base stations and an overall network control frequency configuration table which defines which carrier frequencies are assigned to which beams transmitted by said antennas for operating a method according to the second aspect herein.

According to a third aspect of the present invention there is provided a cellular radio apparatus comprising a plurality of base stations capable of communicating with a plurality of cellular areas, each said base station capable of forming a plurality of directional beams covering at least one said cellular area, wherein: a first said base station and a second said base station are spaced apart from each other; a first set of said beams extend from said first base station in a first pattern; a second set of said beams extend from said second base station in a second pattem, said second pattem substantially replicating said first pattem; a plurality of carrier frequencies are assigned to said first set of beams in a first order; said plurality of carrier frequencies are re-used by second set of beams, said set of canier frequencies being assigned to said second set of beams in a second order, different to said first order.

Preferably said beams of said first set extend along directions diverging within an angle of 60O from a main direction of said first cell; said beams of said second set extend along directions diverging within an angle of 60O from a main direction of said second cell; and said main directions of said first and second cells being substantially coincident with each other.

According to a fourth aspect of the present invention there is provided in a cellular radio system having a plurality of base stations each capable of communicating over at least one corresponding respective cell area, each of said plurality of base stations operating a common set of carrier frequencies re-used as between said base stations, a method of allocating said carrier frequencies comprising the steps of: at a first said base station forming a first set of directional beams, each said beam directed in a respective one of a plurality of directions; at a second said base station, forming a second set of directional beams each beam of said second set also directed in a respective one of said plurality of directions; allocating said set of carrier frequencies to said first set of beams in a first order; and allocating said same set of carrier frequencies to said second set of beams in a second order, said second order being different from said first order.

According to a fifth aspect of the present invention there is provided in a cellular radio system having a plurality of base stations each operating a common set of carrier frequencies, said plurality of base stations each operating a set of directional beams in a common pattern, a method of frequency allocation comprising the steps of: for each of a first set of said base stations, allocating said common set of carrier frequencies to said set of beams in the first order; and for each of a second set of said base stations, allocating said common set of frequencies to said set of beams in a second order, wherein said second order is different from said first order.

Preferably said first set of base stations are arranged substantially along a first line and said second set of base stations are arranged substantially along a second line. Preferably said first line is substantially parallel to said second line.

Brief Description of the Drawings For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which: Fig. 1 illustrates an arrangement of center excited tri-sectorized hexagonal cells in which each sector is served by a plurality of directional beams; Fig. 2 illustrates a prior art tri-cellular arrangement wherein each of three cells of a tri-cellular area are served by a separate beam; Fig. 3 illustrates beam coverage of one cell of a tri-cellular area of the prior art arrangement of Fig. 2, showing a -3dB contour and a -10dB contour, illustrating coverage of the cell from a beam originating at a corner of the cell; Fig. 4 illustrates a directional beam layout having frequency re-use between cells for edge excited cells with four beams per cell in a tri-cellular arrangement; Fig. 5 illustrates a canier to noise and interference ratio graph corresponding to the layout in Fig. 4; Fig. 6 illustrates a directional beam layout for edge excited cells having frequency re-use between cells with four beams per cell in a tri-cellular arrangement according to a specific method of the invention herein; and Fig. 7 illustrates a carrier to noise and interference ratio graph corresponding to the layout in Fig. 6.

Detailed Description of the Best Mode for Carrying Out the Invention There will now be described by way of example the best mode contemplated by the inventors for carrying out the invention. In the following description numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent however, to one skilled in the art, that the present invention may be practiced without using these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the present invention.

A capacity increase over a prior art center-excited tri-sectorized cellular arrangement having three 1200 sectors each served by a separate 1200 azimuth beam can be achieved by the use of a plurality of directional beams in each sector, as shown in Fig. 1 herein.

Referring to Fig. 1 there is shown an example of a geographical area served by a cellular radio network covering a plurality of nominally hexagonal center excited cells each cell divided into three 1200 sectors, each sector served by four directional beams. First, second and third hexagonal cells 100, 101, 102 respectively re-use a common set of frequencies, and are spaced apart from each other by intermediate cells 103 - 105 which use different frequencies to the first to third cells and which are non-interfering with the common frequencies used in the first to third cells. Each of the first second and third cells is trisectorized into three 1200 sectors, wherein four directional beams per sector are provided.

Using a plurality of directional beams it is possible to increase the capacity of a cellular radio system by reducing interference from neighboring beams which re-use common frequencies. Using the example of a base station re-use factor n=7, where each cell is allocated 416i7=59 carrier frequencies per cell, where the cell is tri-sectored, each sector having four directional beams, 19 carrier frequencies can be allocated to each set of four beams 106-109 in a sector as shown in Fig. 1. Most of these frequencies can be allocated to traffic, but some are reserved for control purposes.

A base station re-use factor of n=4, gives 416+4=104 carrier frequencies per cell. In a tri-sectored hexagonal cell, each sector having a multibeam arrangement of four beams per sector, 34 canier frequencies can be allocated per sector, ie across four directional beams, in a 1200 sector. Improvement in the C/I ratio obtained by the use of directional beams within a sector enables the implementation of a lower base station re-use factor n than would otherwise be available with a tri-sectorized center excited cell arrangement.

Considering now a conventional tri-cellular arrangement (otherwise known as edge excited or corner excited cells) as shown in Fig. 2 herein, in which a base station serves three nominally hexagonal cells comprising a tri-cellular region, from a position at a center of the tri-cellular region where the three cells meet each other, the tri-cellular arrangement has an inherent advantage in terms of C/I performance compared to an equivalent conventional center excited trisectorized hexagonal cell of comparable area, due to the narrower beam which can be used in the tri-cellular arrangement as compared to the known center excited tri-sectorized center excited arrangement.

Referring to Fig. 3 herein, one cell of a three cell tri-cellular arrangement is illustrated in which one hexagonal cell 300 is covered by a beam having a coverage pattem having a 60 angular beamwidth 301 at -3 dB gain. Such a beamwidth may give adequate performance for coverage of a 1200 angle azimuth 302 at the comer of the cell where the base station is situated, as the coverage pattem of the beam falls away in power in such a way that at a -10dB contour 303 of the beam, nearest comers 304, 305 of the nominally hexagonal cell which are adjacent to the comer occupied by the base station are relatively close to the base station compared to oppositely facing comers 306, 307 and are within the -10 dB contour of the beam with the result that acceptable power levels are available for communicating with mobile stations at the nearest comers 304, 305.

Thus, the known tri-cellular arrangement has an inherent advantage over the known center excited tri-sectorized hexagonal cell arrangement in terms of the narrower beam which can be used to cover an equivalent area. The reduced power received on the downlink by mobile stations located at the nearest comers to the base station comer is compensated by the fact that these comers are closer to the base station comer than other parts of the tri-cellular cell. Typically a -3 dB azimuth beamwidth of 60 to 700 is acceptable for coverage of the tricellular cell.

Referring to Fig. 4 of the accompanying drawings there is illustrated a cellular radio system serving a geographical area divided into a plurality of adjoining hexagonal edge-excited cells 40 of substantially equal area to each other in a tri-cellular configuration in which a plurality of base stations B are each surrounded by a corresponding respective set of three hexagonal cells, which they serve. Each base station has one or more directional beam antennas 45.

Each base station supports coverage of its three surrounding cells comprising a tri-cellular region. Tri-cellular regions are shown enclosed by a thickened line in Fig. 4.

A plurality of frequency re-use base stations B which use a common set of frequencies are arranged in a plurality of substantially straight lines which are approximately parallel to each other, the base stations of a line being spaced approximately equidistantly from each other along the line. Base stations of one line are positioned off-set to base stations of a neighboring line. Each tri-cellular area comprises three nominally hexagonal cell areas. Each cell area is served by a plurality of substantially radially extending beams extending outwardly from the base station and covering the area of the cell. The plurality of beams extend either side of a main length of the cell, the main length extending between a corner of the hexagonal cell at which the base station is situated, and a furthermost comer of the cell opposite the comer at which the base station is located. Each beam is of relatively narrow beamwidth, typically of the order 45" to 500 azimuth at the -3dB gain contour.

For ease of description, hereinafter a method corresponding to one cell of a tri-cellular region, the tri-cellular regions supported by two base stations which are spaced apart from each other and re-use a common set of carrier frequencies will be described. It will be understood that coverage of all cells in the cellular radio system requires duplication of the method described hereinafter. In Fig. 4 a first set of directional beams has been labeled 41A, 42A, 43A and 44A for one of the cells covered by first frequency re-use base station 45A and a second set of directional beams has been labeled 41B, 42B, 43B and 44B for one of the cells covered by second frequency re-use base station 45B. When referring to Fig. 4 herein, a beam referred to by a number 41 shall represent beam 41 A, 41 B or any other beam of equivalent re-used canier frequency and substantially similar direction transmitted by any other frequency re-use base station 45 in Fig. 4.

Likewise beams referred to by a number 42, 43 or 44 shall represent beams of identical re-used carrier frequency and substantially similar direction of any frequency re-use base station 45 in Fig. 4. All other cells in Fig. 4 have a corresponding pattem of four beams 41 to 44 which use other frequencies but these are not illustrated for clarity.

In the arrangement of beams shown in Fig. 4, outer beam 41A supported by first base station 45A re-uses the same carrier frequency as outer beam 41 B supported by second base station 45B. Likewise all inner beams 42, have the same carrier frequency as each other, and similarly inner beams 43 re-use another same carrier frequency, and outer beams 44 re-use a further same carrier frequency, as between the first and second base stations 45A, 45B in Fig.

4.

The cell served by first base station 45A containing first set of directional beams 41A44A uses a same set of frequencies as second set of beams 41B44B of second base station 45B serving the second tri-cellular area. Similarly, other surrounding frequency re-use base stations 45C, 45D, 45E, 45F, 45G, each serving a corresponding respective tri-cellular area, re-use the same frequencies as first base station 45A, allocating those re-use frequencies to corresponding respective third to seventh beam sets 41 CAl G, 42C-42G, 43C43G, 44C44G as shown in Fig. 4. Each frequency re-use cell contains a set of directional beams 4144. In each case, the directional beams extend radially about the corresponding respective base station, and either side of a main length of the corresponding respective cell served by the beam set. Each cell containing a beam set re-using a same set of frequencies has a main length extending in a same direction to each other cell re-using the same frequency set.

Each beam of first beam set 41A44A extends in a respective general direction which is the same as a corresponding respective beam 41 B A4B of a corresponding cell comprising second tri-cellular area supported by second first tier re-use base station 45B.

The plurality of frequency re-use base stations 45 are arranged in such a way that for each cell of the tri-cellular area supported by the corresponding respective re-use base station 45, beams 41, 44 at an outer edge of each individual cell of the tri-cellular area extend along a line of sight pointing midway between corresponding respective outermost frequency beams 41, 44 of neighboring first tier re-use base stations. For example, outer beam 41A extends along the line of sight pointing to an area midway between corresponding respective outer beams 41B, 41C re-using a same frequency as 41A. Because beams 41A-C are directional, the likelihood of interference between these frequency re-use beams is reduced.

Referring to Fig. 5 herein, there is illustrated carrier to interference ratio graphs corresponding to four beams of one cell of the layout shown in Fig. 4.

Graph line 51 shows a plot of carrier to interference level in decibels on a vertical axis, against beam width on a horizontal axis for outer beam 41 in Fig. 4 over beamwidths in the range 20 to 500. Likewise graph lines 52, 53 and 54 in Fig. 5 correspond to inner beams 42, 43 and outer beam 44 in Fig. 4 respectively.

As can be seen from graph lines 51 and 54 in Fig. 5 the outer two beams 41 and 44 of a cell in Fig. 4 have a relatively higher carrier to interference performance compared to inner beams 42, 43. Innermost beams 42A, 43A of the first base station 45A extend in a direction which points towards the corresponding respective inner beams 42B, 43B of adjacent first tier re-use cell of first tier re-use base station, second base station 45B. Areas covered by inner beams 42B, 43B receive interference from corresponding inner beams of adjacent first tier frequency re-use base station 42A, 43B respectively. The beams 42B and 43B in Fig. 4 experience reduced canier to interference performance due to the interference which results from beams 42A and 43A transmitted by antenna 45A having the same carrier frequencies and being directed in substantially the same direction.

Fig. 6 herein illustrates a directional beam layout in a cell of a tri-cellular radio system with identical apparatus components to those shown in Fig. 4 but employing a specific method of arranging frequency re-use beams which is subject of the present invention. For ease of description hereinafter a method corresponding to one cell of a tri-cellular region supported by a base station will be described. It will be understood that coverage of all three cells supported by a base station requires duplication of the method described hereinafter. For this section of the description a beam referred to by a number 61 shall represent first outer beam 61A, 61B or any other beam of substantially similar direction supported by any base station in Fig. 6 which re-uses a common set of carrier frequencies. Likewise beams referred to by a number 62, 63 shall represent inner beams of substantially similar direction supported by any frequency re-use base station 65 in Fig. 6 and beams referred to by number 64 shall represent second outer beams of any frequency re-use base station 65. First outer beam 61 has a same carrier frequency for all base stations 65 in Fig. 6. Second outer beam 64 also has a same carrier frequency for all base stations 65 in Fig. 6.

However the canier frequencies of inner two beams 62 and 63 have been exchanged for each other as between first and second base stations 65A and 65B so that inner beam 62A of first frequency re-use cell served by first base station 65A cell has the same carrier frequency as opposite inner beam 63B of second frequency re-use cell of the second first tier frequency re-use base station 65B, and inner beam 63A of the first frequency re-use cell has the same carrier frequency as opposite inner beam 62B of the second frequency re-use cell. The pattem of altemating the carrier frequencies of the two inner beams transmitted by base stations 65A and 65B is repeated throughout the layout of frequency reuse base stations so that the inner two beams of all adjacent base stations have altemated carrier frequencies in order to minimize overall interference.

In the arrangement of Fig. 6 herein, first base station 65A communicates with first cell area served by first set of beams 61A-64A and second frequency reuse base station 65B communicates with second cell area served by second set of frequency re-use beams 61B-64B. Outer beams 61A, 64A of the first beam set are directed in a same direction as corresponding respective outer beams 61, 64 of the plurality of other beam sets (second to seventh beam sets 61-64 corresponding to second to seventh frequency re-use base stations 65B-65G).

Because of the layout of the base stations, arranged substantially along straight lines parallel to each other where frequency re-use base stations are spaced substantially equidistantly from each other along each line, the outer beams 61, 64 of a cell of a tri-cellular area extend along a line of sight which points towards an area between nearest adjacent corresponding respective frequency re-use beams 61, 64 of adjacent frequency re-use base stations, and interference between outer frequency re-use beams 61, 64 of adjacent frequency re-use cells is relatively low.

First frequency re-use inner beams 62, of each frequency re-use cell along a line of base stations, for example a first line comprising fourth base station 65D, first base station 65A and seventh base station

In other words, examining the relationship between frequency re-use at first base station 65A and second base station 65B, first base station 65A communicates with a first cell area of a tri-cellular area using a first set of beams, second frequency re-use base station 65B communicates with second cell of second tri-cellular area using a second set of beams, at least one beam of the first set being directed in a substantially same direction as a corresponding beam of the second set, and at least one remaining beam of the first set which re-uses a second same frequency as a beam of the second set, being directed away from that beam. Outer beams 61A, 64A of the first set of beams have a same direction as corresponding respective outer beams 61 B, 64B of the second set of beams, corresponding respective beams of each set pointing in the same direction as each other and using the same frequency as each other. Inner beams 62A, 63A of first beam set and inner beams 62B, 63B of second beam set re-use the same two frequencies as each other, however first inner beam 62A of the first set having a common re-used carrier frequency with second, opposite inner beam 63B of the second set are directed in different directions to each other, and second, opposite inner beam 63A of the first set having a same common carrier frequency as first inner beam 62B of the second set also are directed in different directions to each other.

The first set of beams 61A - 64A extending from the first base station 65A are arranged in a first pattern, extending radially from the first base station, whereas the second set of beams 61 B-64B extend in a second pattem substantially radially outwardly from the second base station 64B, the first and second sets of beams re-using a common set of carrier frequencies, the carrier frequencies being assigned to the first set of beams 61A, 64A in a different order as compared with their assignment to the second set of beams 61 B-64B.

Fig. 7 herein illustrates carrier to interference ratio graphs corresponding to four beams transmitted by a base station 65 in the beam layout shown in Fig. 6.

Graph line 71 shows a carrier to interference level in decibels on a vertical axis plotted against beam width for beam 61 in Fig. 6 over beamwidths in the range 20 to 500. Likewise graph lines 72, 73 and 74 correspond to beams 62, 63 and 64 in Fig. 6 respectively.

As can be seen from graph lines 71 and 72 in Fig. 7 outer two beams 61 and 64 in Fig. 6 achieve a relatively higher carrier to interference performance for beamwidths in the range 20 to 500. An improvement in carrier frequency to interference performance resulting from altemating the re-used carrier frequencies between inner beams 62 and 63 in Fig. 4 is seen for both inner beams represented by graph lines 72 and 73, as compared to the arrangement of Fig. 4 herein. For graph line 72 (representing beam 62 in Fig. 6) the carrier to interference performance is improved significantly. For graph line 73 (representing beam 63 in Fig. 6) the carrier to interference performance is also improved.

References [1] Mobile Cellular Telecommunications Systems by William Chien-Yeh Lee published by McGraw Hill Book Company, New York 1989

Claims (14)

Claims:

1. In a cellular radio system having a plurality of base stations each capable of communicating over at least one corresponding respective cell area using a plurality of directional beams, a method of arranging said plurality of directional beams, said method comprising the steps of: at a first base station, forming a first set of beams in a first cell area; at a second base station, forming a second set of beams in a second cell area; wherein at least one beam of said first set is directed in a substantially same direction to and re-uses a first same frequency as at least one beam of said second set; and at least one remaining beam of said first set re-uses a second same frequency as at least one remaining beam of said second set, said remaining beam of said first set being directed in a direction pointing away from said remaining beam of said second set.

2. The method as claimed in claim 1, wherein said first set of beams use a set of carrier frequencies, each beam of said first set being assigned a corresponding respective carrier frequency from said set of canier frequencies, said carrier frequencies being different to each other, and said second set of beams re-use said set of carrier frequencies.

3. The method as claimed in claim 1, wherein each said set of beams comprises at least one inner beam and at least one outer beam; wherein a said outer beam of said second set re-uses a frequency of an outer beam of said first set, said outer beam of said second set directed in a substantially same direction as said outer beam of said first set; and an inner beam of said second set re-uses a frequency of an inner beam of said first set, said inner beam of said second set being directed away from said inner beam of said first set.

4. In a cellular radio system having a plurality of base stations each communicating over at least one corresponding respective cell area, by means of a plurality of directional beams, a method of arranging said plurality of directional beams, comprising the steps of: assigning a first set of said beams to a first said base station of a first said cell area; assigning a second set of beams to a second said base station of a second said cell area; wherein each beam of said first set corresponds with a respective beam of said second set: wherein a beam of said first set has a common carrier frequency and common direction with a corresponding beam of said second set; and a beam of said first set has a common carrier frequency with but a different direction to a corresponding beam of said second set.

5. A method as claimed in claim 4, wherein each set of beams comprises an outer beam and an inner beam; and an outer beam of said first set has a common carrier frequency and common direction with an outer beam of said second set; and an inner beam of said first set has a common canier frequency but a different direction to an inner beam of said second set.

6. The method as claimed in claim 4 or 5, wherein said base stations each serve to communicate over three corresponding respective adjoining hexagonal edge excited cells each of substantially equal area in a tri-cellular arrangement.

7. The method as claimed in claim 4, 5 or 6, wherein each said set of beams comprises four directional beams.

8. The method as claimed in any one of claims 4 to 7, wherein each said set of directional beams comprises two inner beams and two outer beams.

9. A cellular radio apparatus comprising a plurality of base stations capable of communicating with a plurality of cellular areas, each said base station capable of forming a plurality of directional beams covering at least one said cellular area, wherein: a first said base station and a second said base station are spaced apart from each other; a first set of said beams extend from said first base station in a first pattern; a second set of said beams extend from said second base station in a second pattern, said second pattem substantially replicating said first pattern; a plurality of carrier frequencies are assigned to said first set of beams in a first order; said plurality of carrier frequencies are re-used by second set of beams, said set of carrier frequencies being assigned to said second set of beams in a second order, different to said first order.

10. The cellular radio apparatus as claimed in claim 9, wherein said beams of said first set extend along directions diverging within an angle of 60O from a main direction of said first cell; said beams of said second set extend along directions diverging within an angle of 60O from a main direction of said second cell; and said main directions of said first and second cells being substantially coincident with each other.

11. In a cellular radio system having a plurality of base stations each capable of communicating over at least one corresponding respective cell area, each of said plurality of base stations operating a common set of carrier frequencies re-used as between said base stations, a method of allocating said carrier frequencies comprising the steps of: at a first said base station forming a first set of directional beams, each said beam directed in a respective one of a plurality of directions; at a second said base station, forming a second set of directional beams each beam of said second set also directed in a respective one of said plurality of directions; allocating said set of carrier frequencies to said first set of beams in a first order; and allocating said same set of carrier frequencies to said second set of beams in a second order, said second order being different from said first order.

12. In a cellular radio system having a plurality of base stations each operating a common set of carrier frequencies, said plurality of base stations each operating a set of directional beams in a common pattem, a method of frequency allocation comprising the steps of: for each of a first set of said base stations, allocating said common set of carrier frequencies to said set of beams in the first order; and for each of a second set of said base stations, allocating said common set of frequencies to said set of beams in a second order, wherein said second order is different from said first order.

13. The method as claimed in claim 12, wherein said first set of base stations are arranged substantially along a first line; and said second set of base stations are arranged substantially along a second line.

14. The method as claimed in claim 13, wherein said first line is substantially parallel to said second line.