GB2303490A - An omnidirectional antenna scheme - Google Patents

Description

AN OMNIDIRECTIONAL ANTENNA SCHEME This invention relates to cellular radio communication systems and in particular relates to a base station arrangement comprising means to improve reception quality for an omnidirectional antenna arrangement.

Cellular radio systems are currently in widespread use throughout the world providing telecommunications to mobile users. In order to meet the capacity demand, within the available frequency band allocation, cellular radio systems divide a geographic area to be covered into cells. At the centre of each cell is a base station through which the mobile stations communicate, typically being equipped with directional antenna arrays arranged in three or six sectored sub-cells where the higher gain of the narrow beamwidth antennas improve the uplink from the lower power mobiles. The distance between the cells is determined such that cochannel interference is maintained at a tolerable level.

Obstacles in a signal path, such as buildings in built-up areas and hills in rural areas, act as signal scatterers and can cause signalling problems.

These scattered signals interact and their resultant signal at a receiving antenna is subject to deep and rapid fading and the signal envelope often follows a Rayleigh distribution over short distances, especially in heavily cluttered regions. A receiver moving through this spatially varying field experiences a fading rate which is proportional to its speed and the frequency of the transmission. Since the various components arrive from different directions, there is also a doppler spread in the received spectrum. All these effects combine so that, in all practical systems, the antenna arrangements must be capable of overcoming at least some of these effects.

A prime consideration in all systems is the cost of the apparatus. A significant cost of any base station is determined by the type of antenna used. Omnidirectional antennas are not dedicated to a particular sector and transmit a 360" azimuthal beam. Such a beam is, typically, narrow in elevation. Omnidirectional antenna installations are therefore simple and cheap to install. An omnidirectional antenna, by its very nature, needs no beam steering and thus with this type of antenna there is no requirement for beam control electronics, further reducing costs. Other types of antennas, for instance, the flat plate antennas, especially of the adaptive variety, have beam steering electronics whereby a beam formed by an array of antenna elements is steered towards, for example, a mobile.

Thus sectored antennas are more expensive, not only because the greater number of antennas employed, but also because they require more transceivers per sitre at initial deployment. In particular, a simple omnidirectional site requires only one transceiver whereas a trisectored site will require three transceivers.

When a new cellular radio system is initially deployed, operators are often interested in maximising the range in order to minimise start up costs. Any increase in range means that fewer cells are required to cover a given geographic area, hence reducing the number of base stations and associated infrastructure costs.

The range of the link, either the uplink or the down link, can be controlled principally in two different ways: by adjusting either the power of the transmitter or the sensitivity of the receiver. On the downlink the most obvious way of increasing the range is to increase the power of the base station transmitter. The output power of a transmitter, however, is constrained to quite a low level to meet national regulations. National regulations, which vary on a country to country basis set a maximum limit on the effective isotropic radiation power (EIRP) which may be emitted.

Accordingly other methods of improving the transmitted gain must be implemented.

One method of improving the receiver sensitivity and to reduce the effect of fading is to include some form of diversity gain. The object of a diverse system is to provide the receiver with more than one path, with the paths being differentiated from each other by some means, e.g. space, angle, frequency or polarisation. The use of these additional paths by the receiver provides the diversity gain. The amount of gain achieved depends upon the type of diversity, number of paths used, and the method of combining the various signals from the several signal paths.

The known primary methods of improving gain for omnidirectional antenna include: (i) The provision of two antennas spaced from each other by typically, 20 wavelengths; which is known as spatial diversity, and (ii) The provision of a linear array of radiating elements vertically stacked one above the other.

The use of two antennas in a spatially diverse system is typically used for repeater stations alongside highways and the like. The two antennas are placed along a line perpendicular to the highway. Spatial diversity is employed for reception in the far-field, whilst no measures are specifically required for reception in the near-field. The vertical stacking of omnidirectional antennas on the other hand improves the gain generally and provides a stronger beam, which is pencil-like in elevation, but does not provide any diversity effects.

The present invention seeks to provide an improved omnidirectional antenna scheme whereby the simple architecture of omnidirectional antennas can be utilised more effectively.

In accordance with the present invention there is provided an antenna arrangement comprising at least 4 vertically oriented omnidirectional antenna, wherein the omnidirectional antenna are spaced apart in a regular fashion.

By providing at least 4 omnidirectional antenna in a regular spaced apart arrangement, spaced diversity is provided by at least 3 antennas at any given time. Thus the minimum gain, for a four antenna arrangement, is > 0.75 of the maximum gain when taking relative position of a mobile within the cellular space into account. The use of four elements provides an optimal arrangement taking cost, complexity and space requirements into account.

In accordance with a further aspect of the invention, there is provided a method of operating an omnidirectional antenna scheme comprising at least four vertically arranged omnidirectional antennas, the method comprising the steps of feeding each antenna with a signal to be transmitted whereby a receive station may receive the signals with a receive gain determined by spatial diversity.

In order to enable a full understanding of the invention, reference will now be made to the figures as shown in the accompanying drawing sheets, wherein: Figure 1 shows a space station arrangement including four omnidirectional antennas in accordance with the invention; Figure 2 shows a mobile within a cell site in conditions of maximum gain; and Figure 3 shows a mobile in a cell site under conditions of minimum gain.

Referring now to Figure 1 there is shown an antenna structure 10 comprising a support structure 12 and four antenna elements 14,16,18,20 vertically oriented and arranged in a spaced apart relation to each other.

Transmission lines 22 feed signals from base station controller 24. The base station controller comprises a transmitter 26 and receiver 28 which communicate with the transmission lines via diplexer 30, the transmitter preferably includes a power amplifier 32 which can be situated prior to signals being fed into the diplexer. Incoming signals from the antennas are directed by the transmission line 22 through the diplexer 30 through an amplifier 34 towards a diversity combiner and beam former 36. The purpose of a diversity combiner is to ensure that the signals received from each of the antenna elements 14,16,18,20 are combined in phase, with only the best signals being combined or otherwise, depending upon the type of combiner, from where they are eventually sent to the receiver.

With the arrangement shown, at least 3 antennas receive differing power in the presence of scatterers (multipath reflections). With reference to Figure 2 the mobile (M) is in direct line of sight communication with all four antenna elements. In contrast, the mobile of Figure 3 is only in direct line of sight communication with 3 of the antenna elements, and thus is operating at 75% efficiency (ignoring fading losses etc.). It is to be noted that an antenna not in line of sight view of the mobile may in fact receive reflected signals which, depending upon type of combiner, can be used constructively to add to the signal quality.

The availability of digital signal processing (DSP) has made possible the practical use of high performance diversity schemes such as maximum ratio combination and the switched co-phasal combiner (GB 9421538.1) With the advent of advanced cellular standards such as GSM, it is no longer necessary to site large numbers of transceivers per base station site. For example one GSM transceiver provides 8 full rate voice channels.

GSM is also considerably more robust against the effects of interference than earlier analogue standards and so the use of omnidirectional, high capacity base sites can be considered and thus overcome the trunking inefficiency inherent in sectored arrangements. With the use of an omnidirectional base station in accordance with the invention, reuse patterns as tight as five can be viably employed. Using spatial combination on transmit with diplexers, high electronic isotropic radiation powers can be achieved by avoiding the losses usually incurred in either hybrid or cavity combiners. In order to balance a high power down link, a four branch diversity scheme can be employed. In the case where the signal format provides some form of identification for the wanted signal (for example, the GSM training sequence), robust beamforming and/or null steering techniques can be employed even in high interference scenarios.

Claims (4)

CLAIMS:

1. An antenna arrangement comprising at least 4 vertically oriented omnidirectional antenna, wherein the omnidirectional antenna are spaced apart in a regular fashion.

2. A method of operating an omnidirectional antenna scheme comprising at least four vertically arranged omnidirectional antennas, the method comprising the steps of feeding each antenna with a signal to be transmitted whereby a receive station may receive the signals with a receive gain determined by spatial diversity, irrespective of the psition of the receive station with respect to the antenna scheme.

3. An antenna arrangement substantially as described herein, with reference to any one or more of the accompanying figures as shown in the drawing sheets.

4. A method of operating an omnidirectional antenna arrangement substantially as described herein, with reference to any one or more of the accompanying figures as shown in the drawing sheets.