BACKGROUND TO THE INVENTION Within a wireless communication networks it may be necessary to have both directional transmission 101, from a node 102 to another node 103, and broadcast transmission from the node 101 to any receiving equipment within the coverage area 104, as shown in figure 1.

To provide directional transmission, a cylindrical multibeam antenna has been developed, as shown in figure 2, and this antenna is disclosed in the following co-pending US Patent Applications: Nortel reference 158971D : Damian Bevan, Steve Baines and Simon Gale entitled'Wireless Communication"US Patent Application No. 10/683, 300; Filed : October 10,2003 Nortel reference 159071D : Martin Smith and Andrew Urquhart entitled "Multibeam Planar Antenna Structure and Method of Fabrication"US Patent Application No. 10/683,301 ; Filed : October 10,2003.

Nortel reference 159121D : Martin Smith, Chris Ward, Damian Bevan et al entitled"Wireless Antennas, Networks, Methods, Software and Services"US Patent Application No. 10/683,408 ; Filed : October 10,2003.

Figure 2 shows a 6 beam antenna 201 comprising individual elements arranged in columns 203.

Figure 3 shows a schematic diagram of a beam pattern of such a multibeam antenna having 8 overlapping beams 11-18. To achieve this beam pattern, 8 antenna elements 202 are arranged at equal angular spacing of 45° on the circumference of a circle-402, as shown in figure 4. The antenna elements are

typically separated by a distance of 1-1.5 times the wavelength (.) used in order to allow a minimum cost construction with the elements and the column distribution networks side by side.

For broadcast transmission, an omni azimuth beam pattern is preferred. This can be formed from a circular array through use of"phase mode excitation". In order to provide an omni directional beam pattern, the elements 202 of the array must be arranged close together, of the order of half wavelength spacing, as shown in figure 5. The reason for the selection of this spacing is shown in figure 6.

Figure 6 shows a graph of the angular power of the omni directional beam for three difference elements with three different spacings. Line 601 shows that for a spacing of 0.5 X, the ripple on the angular power is very small, however, if the spacing is increased to 1 X (line 602) or 2 X (line 603) the ripple becomes unacceptably large. Ideally for an omni directional beam pattern, the angular power should remain constant (i. e. there should be no ripple).

In addition to careful control of the spacing, for phase mode excitation it is necessary to carefully control the phasing to the elements and the graph in figure 6 assumed that the phasing was carefully controlled. If the phase is not carefully controlled, the angular power ripple becomes very large and uncontrolled, as shown in figure 7, line 701.

Referring to figures 4 and 5 it can be seen that the design constraints mean that a single antenna cannot be used to provide both a directional beam pattern and an omni directional beam pattern.

OBJECT TO THE INVENTION The invention seeks to provide an omni directional antenna which mitigates at least one of the problems of known methods.

SUMMARY OF THE INVENTION According to a first aspect of the present invention there is provided an antenna arrangement for forming an omni directional beam comprising a plurality of antenna elements arranged around a structure, wherein each said element or collection of elements forms a directional beam having a polarisation which is orthogonal to the polarisation of adjacent beams.

In one embodiment each said element is arranged at substantially equal angular spacing around said structure.

The antenna arrangement maybe capable of forming a polarisation diverse omni directional beam, further comprising a switch element capable of changing the polarisation of each directional beam between two orthogonal polarisations.

According to a further aspect of the present invention there is provided an antenna arrangement comprising: a plurality of antenna elements arranged around a structure; and a switching element for switching between a first and a second beam arrangement, wherein said first beam arrangement is a directional multiple beam pattern and said second beam arrangement is an omni directional beam pattern.

According to a further aspect of the present invention there is provided an omni directional beam pattern comprising: a plurality of beams formed by a plurality of antenna elements arranged around a structure, and wherein adjacent beams have orthogonal polarisation.

According to a further aspect of the present invention there is provided a complex switch for switching between a first and a second input and a sum of said first and second inputs wherein the switch includes only four switching elements and a combining element, arranged with no cross over portions.

The invention also provides for a system for the purposes of communications which comprises one or more instances of apparatus embodying the present invention, together with other additional apparatus.

The invention is also directed to methods by which the described apparatus operates and including method steps for carrying out every function of the apparatus.

The invention also provides for computer software in a machine-readable form and arranged, in operation, to carry out every function of the apparatus and/or methods.

According to a further aspect of the present invention there is provided a method of forming an omni directional polarisation diverse beam pattern comprising the steps of: forming a plurality of beams from a plurality of antenna elements arranged around a structure.

The preferred features may be combined as appropriate, as would be apparent to a skilled person, and may be combined with any of the aspects of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS An embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows a schematic diagram of antenna coverage; Figure 2 shows a schematic diagram of a cylindrical multibeam antenna; Figure 3 shows a schematic diagram of a beam pattern of a multibeam antenna; Figure 4 shows a schematic diagram of the angular spacing of elements of a multibeam antenna; Figure 5 shows a schematic diagram of the angular spacing of elements of an omnidirectional antenna; Figure 6 shows a schematic diagram of the angular power of an omnidirectional antenna; Figure 7 shows a schematic diagram of the angular power of an omnidirectional antenna without phase control; Figure 8 shows a schematic diagram of an omnidirectional antenna according to the present invention; Figure 9 shows a schematic diagram of angular spacing of elements of an omnidirectional antenna according to the present invention; Figure 10 shows a schematic diagram of a beam pattern of an omnidirectional antenna according to the present invention; Figure 11 shows a schematic diagram of polarisation diversity in an omnidirectional antenna according to the present invention; Figure 12 shows a schematic diagram of angular power of an omnidirectional antenna according to the present invention;

Figure 13 shows a schematic diagram of angular power of an omnidirectional antenna according to the present invention; Figure 14 shows a schematic diagram of angular power of an omnidirectional antenna without phase control according to the present invention; Figure 15 shows a schematic diagram of angular power of an omnidirectional antenna without phase control according to the present invention ; Figure 16 shows a schematic diagram of switch architecture for multibeam antennas; Figure 17 shows a schematic diagram of switch architecture for multibeam antennas; Figure 18 shows a schematic diagram of a switching unit according to the present invention; Figure 19 shows a schematic diagram of switch architecture for multibeam antennas according to the present invention; Figure 20 shows a schematic diagram of switch architecture for multibeam antennas according to the present invention.

DETAILED DESCRIPTION OF INVENTION Embodiments of the present invention are described below by way of example only. These examples represent the best ways of putting the invention into practice that are currently known to the Applicant although they are not the only ways in which this could be achieved.

As described earlier, within a wireless communication networks it may be necessary to have both directional transmission, from node A to node B, and broadcast transmission, from node A to any receiving equipment within the coverage area. Ideally both of these functions would be performed by the same antenna (but not concurrently), but currently this is not possible for the reasons described above.

Referring to figures 8-15, there is shown a first aspect of the present invention.

According to a first aspect of the present invention, an omni directional antenna 801 is disclosed which can be wrapped around a structure 802, as shown in

figure 8. The antenna comprises a number of antenna elements 803. As shown in figure 9, the elements 803 are arranged at substantially equal angular spacing 902 around a virtual point 904. The elements may lie on the circumference of a circle 906 if all the elements are substantially equidistant from the virtual point, however the elements do not need to be equidistant from the virtual point. The elements 803 may comprise columns of individual antenna elements, as shown in figure 8.

The antenna produces a number of overlapping beams 1002 as shown in figure 10. Each beam produced by the antenna has at least one of two orthogonal polarisations (P1 and P2 as shown in figure 11). Using this polarisation diversity, an omni directional beam pattern can be selected by using alternate polarisations on adjacent beams, as shown in figure 11. Figure 11 shows the two possible omni directional beams 1101,1102.

If polarisation diversity is required within the omni directional beam pattern, this can be achieved using the two different omni directional beams 1101,1102 shown in figure 11. To achieve polarisation diversity, it is necessary that each of the overlapping beams 1002 can be switched between its two orthogonal polarisations referred to herein as P1 and P2.

Figure 8 shows and antenna having 6 elements and figures 9-11 show multi beam antennas each having 8 elements, these are by way of example only and other numbers of elements may be used.

The performance of an antenna as shown in figures 9-11 is shown in figures 12- 15. Figure 12 shows a graph of the angular power of the omni directional beam for elements spaced by one wavelength, using the alternating polarisations as shown in figure 1. 1. It can be seen that the ripple in the power is small, at only about 2dB. Figure 13 shows a similar graph for elements spaced by two wavelengths and again the ripple in the power is only about 2dB.

According to this invention it is not necessary to control the phasing of the elements within the antenna (figures 12 and 13 assumed good control of phasing of elements) and this is clearly shown by figures 14 and 15. Figure 14 shows the effect of random phase where the elements are spaced by one wavelength (corresponding to figure 12) and figure 15 shows the same effect where the elements are spaced by two wavelengths (corresponding to figure 13). In both cases it can be seen that the ripple in the power is not significantly affected by the lack of phasing control.

According to a second aspect of the present invention, a single antenna is disclosed which can be switched so as to provide either a directional transmission or a broadcast transmission. The antenna comprises the antenna described above in reference to figures 8-15, combined with a switching architecture described below.

Referring to figures 16-20, there is shown a third aspect of the present invention. Where appropriate the same reference numerals have been used throughout.

As described above, it is possible to use a single multibeam antenna to provide either directional beams or an omni directional beam pattern. According to a third aspect of the present invention, a switch architecture is described to allow the antenna to be switched between operation in one configuration (directional beams) and operation in the other configuration (omni directional beam pattern).

Figure 16 shows a switch architecture suitable for use with a multibeam antenna for switched directional beams only. The architecture includes a plurality of switches 1601 joined by electrical connections 1602 to the antenna elements 1603.

Figure 17 shows a switch architecture suitable for use with a multibeam antenna for an omni directional beam pattern only. The architecture includes a plurality of combiners 1701 joined by electrical connections 1602, such as wires or tracks on a printed circuit, to the antenna elements 1603.

The design of figure 17 shows a hierarchical combination structure so that each beam has the same losses. The design also shows +/-45° polarisation inversion for each omni-combination.

Figure 18 shows a switching unit 1801 suitable for use with a multibeam antenna in a hierarchical structure to permit switching between the two different modes of operation; the first mode of operation being the switched directional beams and the second mode of operation being the omni directional beam pattern. The switching unit includes no track crossovers and keeps the number of elements (switches 1601 and combiners 1701) to a minimum. It is beneficial to avoid wire crossovers as it enables the antenna to be made using a single metal layer process which reduces cost. Keeping the number of switching or combining elements to a minimum also assists in minimising costs but more importantly reduces the electrical losses within the circuit. This is a repeatable unit that can be cascaded in a hierarchical switch arrangement or daisy-chain.

Figures 19 and 20 show two examples of architectures using the switching unit 1801 to connect antenna elements 1603 using electrical connections 1602, to permit switching between the two different modes of operation (omni-directional mode and directional mode).

It will be understood that the above description of a preferred embodiment is given by way of example only and that various modifications may be made by those skilled in the art without departing from the spirit and scope of the invention.