GB2380611A

GB2380611A - Antennas for cellular networks

Info

Publication number: GB2380611A
Application number: GB0113192A
Authority: GB
Inventors: John Thornton; David Grace; Myles Capstick; Timothy Conrad Tozer
Original assignee: University of York
Current assignee: University of York
Priority date: 2001-05-31
Filing date: 2001-05-31
Publication date: 2003-04-09
Anticipated expiration: 2021-05-31
Also published as: GB0113192D0; GB2380611B

Abstract

A cellular communications system includes an antenna disposed above the earth's surface and arranged to illuminate a cell on the earth's surface, wherein the directivity of the antenna is maximised at the edge of the cell, thereby to optimise the power distribution across the cell. Each cell may have an equal circular footprint. An array of such antennas may be carries on a platform, mast or satellite.

Description

ANTENNAS FOR CELLULAR NETWORKS This invention relates to antennas, and is concerned particularly with antennas for cellular networks.

With an ever-increasing demand for capacity for future generation multimedia applications, service providers are looking toward the millimetnc wave bands to provide a solution. Such bands, e. g. those specified for Local Multi-Point Distribution Systems (LMDS), offer relatively uncluttered spectrum and wide frequency allocations, and are capable of delivering very high data rate services.

However, propagation is essentially line-of-sight, and for a terrestrial network this imposes limitations of coverage and heavy demand for base-stations. One possible solution is to use High Altitude Platforms (HAPs), which operate in the stratosphere, at an altitude of (for example) 17-22km. HAPs have the potential capability to serve a large number of users, situated over a large geographical area, using considerably less communications infrastructure than that required if delivered by a terrestrial network. Such systems will employ a frequency re-use cellular architecture in order to provide overall system capacity, with cells defined by means of a number of antenna spot beams from the HAP.

The performance of terrestrial cellular architectures has been described by

Lee * 1989/W. C Y Lee,'S Lee in vg iii Cellmlar--'IEEE Travra-liovr o. 1 , pee, liwm E I Vehic & lar Te.-17, vology, p riV'/ar K/'7% pp. < -Z/ ! /. Terrestrial (non-sectored) schemes normally involve a base station in each cell, using omni-directional antennas. To provide wide coverage the cells are tessellated, with different channels assigned to neighbouring cells in order to manage co-channel interference ; assignments can take the form of frequency channels, time slots or codes. Conventionally, cells are clustered into groups of 3,4, 7 or 9, dividing the overall frequency allocation between them. The larger the number of cells in the cluster, the greater the reuse

distance and the higher the carrier to interference ratio (CIR), but the fewer the number of channels per cell ; this trade-off is fundamental to most cellular systems. The exact type of re-use plan depends on the system and terrain, and in terrestrial networks considerable time may be spent on frequency planning. Fixed channel assignment has been shown to produce the highest capacities in non-shadowed environments, but when the traffic load per cell varies, dynamic channel assignment has been shown to provide higher capacity. Dynamic channel assignment is also particularly useful when the environment and/or traffic load is hard to predict.

A HAP cellular system has both similarities and differences with a terrestrial system. Providing the elevation angle remains above a certain limit, propagation is by line-of sight and hence terrain and buildings will not be an important factor. Reuse plans are still applicable (either fixed or dynamic), but an important difference is the way the interference arises and how it decays with distance. In a HAP system, interference is caused by antennas serving cells on the same channel, and arises from an overlapping main lobe or side-lobes.

An ideal antenna beam illuminates its corresponding cell with uniform power across the cell and with zero power falling outside the cell-in this respect the antenna is acting as a spatial filter. In practice, spot beams that are realisable fall short of this ideal, particularly at mm-wave frequencies where array beam synthesis techniques are difficult. The most practicable antennas for this application are likely to be aperture types, whose radiation characteristics are well established. To minimise interference, beams with very low sidelobes and a steep roll-off in the main lobe are highly advantageous. While sidelobe suppression may be achieved with corrugated horn designs, the rate of roll off is primarily influenced by the main lobe width and hence directivity. If too high a directivity is chosen, the cell will suffer excessive power roll-off at its edge; if too low a directivity is chosen, excessive power will fall outside the cell. At the frequencies

allocated for HAPs, such as 48 GHz, limited available transmit power combined with rain fade gives rise to marginal link budgets particularly at cell edges.

Preferred embodiments of the present invention aim to provide optimised antenna arrays which are particularly suitable for use with HAPs.

According to one aspect of the present invention, there is provided a cellular communications system including an antenna disposed above the earth's surface and arranged to illuminate a cell on the earth's surface, wherein the directivity of the antenna is maximised at the edge of the cell, thereby to optimise the power distribution across the cell.

Such a system preferably includes a plurality of antennas as aforesaid, each arranged to illuminate a respective cell on the earth's surface.

Preferably, each of said cells has a substantially circular footprint.

Preferably, all of said cells are of substantially equal size.

Preferably, said cells are tessellated.

Preferably, the directivity of the or each antenna is maximised in each of two orthogonal directions, at the azimuth edge and at the elevation edge of the cell, thereby to optimise the power distribution across the cell in said two directions.

Preferably, the boresight of the or each antenna is orientated towards the centre of its respective cell.

Preferably, the or each antenna is carried on a High Altitude Platform.

The or each antenna may be carried on a tethered platform, high terrestrial mast or LEO satellite.

According to another aspect of the present invention, there is provided a method of designing an antenna for use in a cellular communications system, including the step of selecting the required beamwidth of the antenna for illuminating a cell of given dimensions on the earth's surface, such that the directivity of the antenna is maximised at the edge of the cell, thereby to optimise the power distribution across the cell.

The invention extends to an antenna designed by such a method.

According to another aspect of the present invention, there is provided an array of antennas arranged to be disposed above the earth's surface and each to illuminate a respective cell on the earth's surface, wherein the directivity of each antenna is maximised at the edge of its respective cell, thereby to optimise the power distribution across the cell.

According to another aspect of the present invention, there is provided an array of antennas arranged to be disposed above the earth's surface and each to illuminate a respective cell on the earth's surface, wherein each of said cells has a substantially circular footprint and all of said cells are of substantially equal size.

For a better understanding of the invention, and to show how embodiments of the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which : Figure 1 shows a typical corrugated horn radiation pattern for an aperture antenna ;

Figure 2 shows directivity at the edge of a cell; Figure 3 shows a comparison of directivity for three beams with differing edge powers; Figure 4 shows a co-ordinate system for a hexagonal cell layout, with a 121 cell layout shown as an inset; Figure 5 shows elevation and azimuth angles subtended by a cell beam from a HAP ; Figure 6 illustrates derivation of antenna directivity at a point on the ground; Figure 7 shows as contour plots a comparison of OR coverage for circular beams and elliptic beams; Figure 8 shows a comparison of geographical coverage of the circular and elliptic beams, for one of four channels ; Figure 9 shows (a) coverage for four channels at a OR threshold of 18 dB and (b) geographical overlap between channels 1,2 and 3 at a OR threshold of 10 dB ; Figure 10 shows OR channel overlap for combinations of four channels; Figure 11 shows OR contours for 1 of 7 channels with (a) sidelobes at- 40 dB and (b) sidelobes at-50 dB; Figure 12 illustrates the effect of sidelobe level on coverage, for one of four or seven channels ;

Figure 13 shows geographical channel overlap with seven channels, as multi-channel coverage; and Figure 14 shows a comparison of OR and power for (a) a central row of cells and (b) an adjacent row of cells.

High Altitude Platforms (HAPs) are increasingly being cited as a potential means of supporting high capacity communications networks. As with terrestrial cellular networks, the capacity is directly dependent on the number of times a channel may be re-used, which is in turn limited by co-channel interference. The interference mechanism in a HAP based architecture is a function of the antenna beamwidth, angular separation and sidelobe level. At the millimetre wave frequencies proposed for HAPs, an array of aperture type antennas on the platform is a practicable solution for serving the cells.

Aperture antennas of medium and high directivity (D) have main lobe patterns which are often approximated by

For example, Figure 1 shows a typical corrugated horn radiation pattern, which is circularly symmetric, and a curve fit of the above type. The fit is very good in the main lobe until the directivity falls to-26 dB of it peak value. Beyond this, while the curve fit does not reproduce the sidelobe structure, a flat sidelobe floor may be fixed for modelling purposes (such a floor is usually specified by manufacturers).

When sidelobe levels are very low, such as in the above example, peak directivity is often approximated by :

where #3dB and #3dB are the 3dB beamwidths in two orthogonal planes.

These values are equal for a circularly symmetric beam, where (1) can be re-written:

D = ( < )- (3) 2 20 3dB

hence the directivity may be expressed as a function of 0 and 1/only :

We may now set D at the cell edge by fixing (edge and varying n. For example, Figure 2 shows that for a given angle subtended at the cell edge, the directivity is maximised for a single value of n. As the angle subtended increases, the edge of cell (EOC) directivity is maximised for lower values of) Y. The value of n may be derived by setting the derivative D to zero where D is the partial derivative with respect to/ :

Thus the curve fit method used is a convenient way of choosing the optimum edge of cell directivity as a function of a single parameter 11.

For example, from Figure 2 we can see that to maximise the power at the edge of a cell subtending 100 at the HAP, we choose a value of/1 = 65. The antenna may then be chosen by deriving the peak directivity using (2).

To compare the beam pattern for the maximum EOC condition with suboptimum beams, Figure 3 shows three cases, where the cell edge subtends 90 at the HAP.

In Figure 3 we see that fitting the 3dB antenna beamwidth to the cell edge produces a directivity at the cell edge very similar to the maximum case, but the roll-off is worse. For the lOdB fit, the improved roll-off would result in potentially less interference in neighbouring cells (depending on their angular separation) but at the expense of a reduced link budget at the cell edge. Any curve other than the optimum curve has at least a portion between the centre and the edge of the cell, where the power is less than the optimum curve.

In a more general formulation, the cell will subtend different angles in elevation and azimuth, and two orthogonal beamwidths may be derived to yield an elliptic beam which optimises EOC power in both planes. Techniques for producing elliptic beam antennas for optimising geographical coverage are also

discussed in TV , j5. r < .,'2//'/7/z < w in'polaited ellolieal heam a) vletiiia for falellile IEEE"N/I) vlemalioval S . 1 ~ymposi & m Dgesl 4) vte) v) var a) vd Propaga, li, v. (7Vew Yorh) iol2. p is Pro ?/./'A. / ?/ ''. < '-/. While the effect on AR is presented below, it should be reiterated that beamwidths other than those derived using the above method will yield a worse link budget at the edge of the cell.

Attention is now directed to the prediction of Co-Channel Interference.

The HeliNet programme IHel., Alel Prie,-I, w, 4vw. heli, el , bol,, Io. it 2000J proposes a variety of communications applications for a solar powered

stratospheric aircraft, including broadband services at millimetre wave frequencies.

Power and payload considerations are likely to limit the number of antennas, and hence cells, to between 100 and 200.

For the cells sharing a given channel, the antenna pointing angles are first calculated and the azimuth and elevation angles subtended by each cell may be used to derive optimum elliptic beams using the above method. The angles must be derived as a function of the HAP height. To this end, the cells'co-ordinates are expressed as {nr, nc} where nr specifies the number of a concentric hexagonal ring and lit the number of the cell within the ring. The convention is illustrated in Figure 4, which also shows cells along the first side of the 4th ring by way of example.

The elevation and azimuth pointing angles oo and, po from the HAP to the centre of any cell may then be derived from:

and

where h is the HAP height, d is the cell width and the ground distance g from the cell centre to the sub platform point is derived from the cosine rule:

Since the results are repeated for each side of the hexagonal ring, the counter c'is used to identify the cell's location with respect to the first cell along the side:

where I1r is an integer between 1 and 6 identifying the side of the hexagon:

and Floor is an operator which rounds down to an integer. The elevation and azimuth angles subtended by the circle of radius r which encloses the cell are illustrated in Figure 5 and given by :

and

p ~h * calculated for each The power at each point on the ground {y, j} is calculated for each antenna beam by deriving the elevation and azimuth angles Sa and #2 relative to boresight in antenna polar co-ordinates. A rotation through cell pointing azimuth is first applied:

and

then,

where Xu is the displacement xo transformed to a plane normal to and centred on antenna boresight, as shown in Figure 6:

The pointing angles and subtended angles for each cell are thus a function of cell coordinates {nr, nc} and dimensions b and d only, and may hence be rapidly generated on changing HAP height h or cell width d Let n# and n# be the indices for the curve fits of the form in (1) for an elliptic beam, fitted to optimise

directivity at the cell edges QfWh and respectively. Then the directivity seen at {x,y} is:

where

Thus, for each beam, a data array is generated of the form {. ; j < MM ?/} where power is derived from the antenna directivity seen at a point oxyl on the ground, minus the excess free space loss with respect to the sub-platform point. Depending on the required spatial resolution, data array sizes are typically 104 for each beam. Having derived the data arrays for power, carrier-to-interference ratio (QR) may be derived for the group of co-channel cells from:

where v,, is the number of co-channel cells. At each point (xy), all arrays

ximu {j '/} are tested to find the maximum RMx (. ), which is the maximum power from an individual beam and effectively therefore the carrier. The denominator in (21) is the sum of powers in all the other beams, which is the interference. Thus for each cell group a further array may be generated of the form {x, ~y, CIR). Further processing may be applied to illustrate and quantify the geographical coverage at various threshold values of CIR. Some representative results are presented, which illustrate the effects of antenna beamwidths discussed above.

While OR is not a function of free space loss, it is useful to include the term in the derivation of power, so that the variation in power across the service area is properly scaled, as shown in Figure 14.

One of the cellular layouts that we have studied in some detail is a conventional hexagonal cell layout, where 121 cells of 6.3 km diameter service a 60

km diameter coverage area. Choosing a channel re-use number of 4 yields 3 groups of 30 cells and a further group of 31 cells. OR patterns for the latter case are illustrated in Figure 7, where sidelobes have been modelled as a flat floor at-40 dB.

In Figure 7 a comparison of OR coverage for circular beams and elliptic beams are shown as contour plots where spacing is 3 dB and the labels are for the "top"contours. In the former case, the azimuth angle subtended by each cell from the HAP is used to derive the antenna 3dB beamwidth. Since, with the exception of the central cell, each cell subtends a lesser angle in elevation, the beams are excessively wide in this plane, while narrower circular beams would yield a degraded link budget in some parts of the cells. The resulting OR pattern shows considerable distortion of the cells, which tend to be pushed radially away from their intended location. In contrast, when optimised elliptic beams are adopted, the regions of high OR are both better geographically defined and exhibit higher or The geographical coverage for both cases is quantified in Figure 8 as the fractional area of the co-channel cell group served at a given OR threshold. The optimised elliptic beams offer a clear advantage in terms of OR offered. The same trend is followed very closely for the other 3 cell groups in this re-use plan.

By further processing of the data arrays, the geographical relationships between the coverage areas of the 4 cell groups may be illustrated. Gaps in the coverage may be shown at a chosen OR threshold, and regions of overlap may also be studied. To illustrate these effects, Figure 9 shows the coverage at a OR threshold of 18 dB and also the geographical overlap between channels 1,2 and 3 at a threshold of 10 dB. (The dark shaded regions are above the threshold OR and the cells are labelled with their channel numbers.)

While Figure 9 (a) shows an arbitrary 18 dB threshold level, the graphic shows the tendency for the CIR coverage (and hence quality of service) to be worse in the central region, while coverage exists beyond the intended 60 km diameter circle which is shown. The result is instructive in illustrating the difficulty of controlling the geographical coverage with a fixed re-use plan. Also, Figure 6 (b) shows, by way of example, the overlap regions for channels 1,2 and 3. The dark areas receive coverage with a OR of at least 10 dB by all 3 of these channels. To quantify such effects Figure 10 shows the fractional coverage for various channel overlap combinations. In this case the overlap is shown as the fraction of the total service area (of 60 km diameter) which is served at a given OR threshold exceedance. For example, at a C [R of at least 10 dB, 80 % of the coverage area is served by at least two channels, 40 % by at least 3, and 2 % by all 4 channels.

An intuitive conclusion is that the cell edges tend to receive multi-channel coverage. This may be useful to boost the capacity at the cell edges where the QR and power budget due to the primary channel is weakest. It is also worth observing that coverage everywhere is at least 14 dB. This is consistent with Figure 8 where the coverage on channel 1 become less than unity at about 14 dB.

Coverage for different re-use schemes will now be considered.

Having established a formulation for optimised antenna beamwidths, it is interesting to compare different re-use plans and also the effect of sidelobe levels.

For an increase in the channel re-use number, while the bandwidth allocated to each channel is linearly reduced, an improvement in OR is expected due to the decrease in the number of interfering beams. On increasing the number of channels to 7 and retaining the optimised elliptic beams, Figure 11 shows contour plots of the QR for one of the channels, where there are 17 cells. This cellular pattern is repeated 6 times, while the 7th channel (which also includes the central cell) has a

total of 19 cells. The contours spacing is again at 3 dB intervals, and two cases are shown for sidelobe levels at-40 dB and-50 dB.

The geographical coverage for the two cases is shown in Figure 12 below, which also shows results for the 4 channel scheme. For 7 channels and sidelobes at - 50 dB, the maximum OR is increased by 10 dB as expected and the roll-off occurs at a higher OR threshold. For the 4 channel case the roll-off is less clearly improved on adopting-50 dB sidelobes because there is less angular separation between cells and the cell edges suffer interference from neighbouring antenna main lobes. Therefore, on reducing the sidelobe power, the cell centres experience reduced interference but the cell edges retain interference from neighbouring lobes.

When 7 channels are used, the increased angular separation obviates main lobe overlap, and the predominant interference mechanism is from sidelobes.

Therefore, on reducing sidelobe levels, the majority of the coverage area experiences reduced interference in the 7 channel case.

Channel Overlap will now be considered.

Returning to OR coverage using 7 channels and sidelobes at-40 dB, the overlap between channels is presented in Figure 13 which shows the fractional simultaneous coverage by a number of channels. For example, there is 100 % coverage by at least 1 channel up to a OR of 19 dB, and by any 2 channels up to a OR of 13 dB and so on. The trend that the coverage reduces for an increasing number of overlapping channels is an expected one.

When traffic loads are uneven the overlap can be exploited to provide additional capacity when one cell is full, provided the received power is sufficiently high. The high degree of overlap is not usually seen in conventional terrestrial situations because it implies :

m Base station redundancy, as power falls off typically as distance-4 ; . Higher transmit power, which is required to serve longer link lengths; and z More channels to complete the reuse plan, e. g. to achieve full overlap base stations can be arranged in multiple overlapping layers, each with its own unique conventional reuse plan.

However, despite these disadvantages it is used in indoor systems where the propagation environment is harsh and base station redundancy is needed to maintain coverage. In the case of the HAP architecture, the interference mechanism is different (power falls off less sharply at first, followed by a steeper roll-off beyond the edge of cell). A high degree of overlap is achieved without the need for additional channels and base station redundancy, with the added advantage that the link lengths do not change significantly.

Finally, a comparison of the 4 and 7 channel schemes is shown in Figure 14 as plots of OR and power for cross-sections through the centre of rows of cells.

In both schemes, the antennas and hence power budgets are identical, and both cases of OR are shown. The power is scaled as the maximum antenna directivity as seen on the ground, minus the free space loss relative to the sub-platform point.

Thus at the sub-platform point-the centre of the central cell row in Figure 11 (a)the antenna directivity is 22 dB. An interesting result is that as the ground distance increases, the directivity of the elliptic beams increases more rapidly than the free space loss and hence the power budget improves slightly. The tendency for the dR to improve at the edge of the coverage region compared to the centre, which has been illustrated in Figures. 4,6 (a) and 8, is again clearly shown. One mechanism contributing to this effect is that the central region experiences worse sidelobe interference, compared to the outer region, due to the lesser free space loss.

Antenna array pointing angles, directivities and sizes will now be considered.

The properties of each cell's antenna are repeated for each of 6 sides of a hexagonal"ring". Because of this symmetry, there is an inherent sub-set of 20 antennas, 6 copies of which are used, plus a single antenna for the central cell. Thus for each repetition of the subset, the azimuth pointing angles are incremented by multiples of 600.

The properties of the antenna sub-set are tabulated in Table 1.

0 1 2 3 4 5 6 Ring 0 17. 79 32. 7 43. 9 52. 1 58. 1 60. 8 29. 7 40. 3 49. 2 55. 8 59. 5 40. 3 48. 0 54. 4 59. 0 49.2 54.4 59.5 55.8 60.8 Pointing Azimuth (deg) 0 0 0 0 0 0 9.0 30.0 19.1 13.9 10.9 19.1 40.9 30.0 23.4 30.0 46.1 36.6 40.9 49.1 51.1 Elevation 17. 6 16.0 12.6 9.3 6.8 5.0 4.3 3dB Beamwidth (deg) 13. 6 10. 4 7.6 5. 7 4.6 10. 4 8. 0 6. 0 47 7.6 6.0 4.6 5. 7 4.3 Azimuth 17. 6 16.8 14.8 12. 7 10.9 9. 4 7 3dB Beamwidth (deg) 15. 4 13.5 11.6 10.0 9.0 13. 5 11. 8 10. 3 9. 1 11. 6 10. 3 9. 0 10.0 8. 7 Directivity (dBi) 20. 7 21.3 22.8 24. 7 26.5 28.1 28.9 22.4 24.0 25.8 27.5 28.5 24.0 25.5 27. 1 28. 4 25.8 27.1 28.5 27. 5 28.9

Approx. 27.1 29.0 34.6 42.7 52.6 63.4 69.7 Aperture Length (nun) 32.8 39. 6 48.5 58.8 66.6 at 48 GHz for aperture 39.6 47.1 56.4 65.5 efficiency = 0. 5 48.5 56.4 66.6 58.8 69.7 Approx. 46.4 49.8 59.3 73.3 90.1 108.6 119.6 Aperture Length (mm) 56.2 67.9 83.2 100.7 114.2 at 28 GHz for apetture 67.9 80.8 96.6 112.4 efficiency = 0. 5 83.2 96.6 114.2 100. 7 119. 6

Table 1 Antenna beamwidth, pointing angles and aperture lengths The approximate aperture length is denved from the square root of the . is aperture area for each antenna. The sum of the areas of the above apertures is 0. 35 m2 at 48 GHz and 1. 02 m2 at 28 GHz.

In the above, a number of issues related to cellular planning for broadband services delivered from HAPs have been explored. A key factor is the shape of antenna beams and their effect on obtainable QR patterns. While the work is not specific to any RF frequency, the emphasis is on bands between 28 GHz and 48 GHz and aperture antenna radiation patterns have been approximated using curve-fit methods. Elliptic beams have been shown to offer advantages in terms of optimised power at cell edges, which is of most importance where RF link budgets are marginal. By tailoring each antenna's beamwidths to its corresponding cell's subtended angles an array of antennas is suggested to serve a chosen coverage area. While the physical demonstration of these beams has not been presented, the form of the modelled beams is both realistic and tractable for the purposes of cellular service characterisation which has been presented in detail.

Conventional re-use patterns of 4 and 7 channels, using hexagonal cells, have been studied. In each case, elliptic antenna beams which provide maximum power at cell edges have been utilised. Where 4 channels are used, cell edges suffer interference from neighbouring beams and a reduction in sidelobe level produces a

corresponding improvement in OR at cell centres only. When 7 channels are used, the benefit of reducing sidelobe levels is enjoyed across the majority of the coverage area. A significant amount of overlap between channels has been shown, which suggests a useful mechanism for adaptive resource allocation and handover between cells.

In previous proposals, cell sizes in an array can vary considerably for a given type of antenna, particularly because of the relatively low height at which a HAP operates. Preferred embodiments of the invention provide cell arrays in which the cells are substantially the same size (e. g. within 1%, 2% or 5%), and preferably of circular shape. This has the advantage of allowing the use cf conventional terrestrial re-use plans which have already been calculated A further advantage is that a user can use the same type of receiving antenna over the overall footprint or coverage area. More generally, embodiments of the invention can be used with cells that are of differing sizes and/or shapes, which need not be circular.

This may be of relevance where it is desired to illuminate a geographical area of a particular shape.

The above-described and illustrated embodiments of the invention are particularly for use with HAPs, which may typically operate at a height of around 20 km and, more generally, within a height range of about 10 to 50 km. The antenna design aspects are particularly relevant to the geometry of platforms at such heights. However, the invention may be adapted to other platforms-for example, tall masts up to a height of about 3km above sea level, tethered platforms up to a height of about 10 km, any platform at a height in the range 50 to 700 km, and Low Earth Orbit (LEO) satellites typically at a height of, for example, 1,350

km.

In this specification, the verb"comprise"has its normal dictionary meaning, to denote non-exclusive inclusion. That is, use of the word"comprise" (or any of its derivatives) to include one feature or more, does not exclude the possibility of also including further features.

The reader's attention is directed to all and any priority documents identified in connection with this application and to all and any papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment (s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims

CLAIMS : 1. A cellular communications system including an antenna disposed above the earth's surface and arranged to illuminate a cell on the earth's surface, wherein the directivity of the antenna is maximised at the edge of the cell, thereby to optimise the power distribution across the cell.
2. A cellular communications system according to claim 1, including a plurality of antennas as aforesaid, each arranged to illuminate a respective cell on the earth's surface.
3. A cellular communications system according to claim 2, wherein each of said cells has a substantially circular footprint.
4. A cellular communications system according to claim 2 or 3, wherein all of said cells are of substantially equal size.
5. A cellular communications system according to claim 2,3 or 4, wherein said cells are tessellated.
6. A cellular communications system according to any of the preceding claims, wherein the directivity of the or each antenna is maximised in each of two orthogonal directions, at the azimuth edge and at the elevation edge of the cell, thereby to optimise the power distribution across the cell in said two directions.
7. A cellular communications system according to any of the preceding claims, wherein the boresight of the or each antenna is orientated towards the centre of its respective cell.

<Desc/Clms Page number 22>
8. A cellular communications system according to any of the preceding claims, wherein the or each antenna is carried on a High Altitude Platform.
9. A cellular communications system according to any of the preceding claims, wherein the or each antenna is carried on a tethered platform, high terrestrial mast or LEO satellite.
10. A method of designing an antenna for use in a cellular communications system, including the step of selecting the required beamwidth of the antenna for illuminating a cell of given dimensions on the earth's surface, such that the directivity of the antenna is maximised at the edge of the cell, thereby to optimise the power distribution across the cell.
11. An antenna designed by a method according to claim 10.
12. An array of antennas arranged to be disposed above the earth's surface and each to illuminate a respective cell on the earth's surface, wherein the directivity of each antenna is maximised at the edge of its respective cell, thereby to optimise the power distribution across the cell.
13. An array of antennas arranged to be disposed above the earth's surface and each to illuminate a respective cell on the earth's surface, wherein each of said cells has a substantially circular footprint and all of said cells are of substantially equal size.