GB2299488A

GB2299488A - Radio antenna arrangement

Info

Publication number: GB2299488A
Application number: GB9505307A
Authority: GB
Inventors: Andrew William Drewry Watson
Original assignee: Motorola Ltd
Current assignee: Motorola Solutions UK Ltd
Priority date: 1995-03-16
Filing date: 1995-03-16
Publication date: 1996-10-02
Also published as: GB9505307D0

Abstract

A radio receiver is provided comprising at least two antennas 42-44 each having different directional antenna patterns 34-36, a variable delay 21-23 for delaying signals received at the antennas with respect to signals received at the other antennas, a combiner 48 for combining the signals received at each antenna and outputting a received signal, and an equaliser for combining components of the received signal. The combination of antennas behaves like a single antenna having an adjustable coverage area, 37. A transmitter operating on the same principle is also disclosed.

Description

RADIO ANTENNA ARRANGEMENT Field of the Invention This invention concerns radio receiver and transmitter antenna arrangements providing combined and irregular coverage area. It is, specifically, applicable to radio transmission systems which use digital modulation and which incorporate equalisers for the reduction of multi-path propagation effects.

Background to the Invention Radio transmitting and receiving stations provide coverage of their signals in their intended area of service. An example is the cell sites used to provide service coverage for cellular mobile radio telephone systems.

The desired area of coverage is determined by the radiated power of the transmitting system, the sensitivity of the receiving system, the shape of the antenna radiation patterns, the direction and height of the installed antennas, as well as intervening terrain between the cell site and the subscriber to the radio service. (e.g. the mobile or portable station for the subscriber of a cellular mobile telephone system). Cellular mobile telephone systems usually operate under interference conditions and therefore the effective boundary of the cell (best server) is determined in a complex way by the coverage of the interfering cells and the neighbour cells.

Cell site antennas can employ omni-directional antennas or directional antennas. Over flat, even terrain and in the absence of interference, the shape of the coverage area will largely follow the radiation pattern of the antenna. This will normally be a regular symmetrical shape.

It is sometimes desirable to achieve irregular coverage, for example to follow a particular terrain or boundary feature. This would require a cell site antenna with an irregular pattern.

Conversely however, very often, terrain features will modify the coverage of a cell site antenna, compared to the coverage that it would have over regular terrain. The result of such modified coverage may be poor service in some areas and interference to subscribers using a different cell in other areas. Thus, it is desirable to compensate for the effect of modified coverage by means of an antenna with a compensatory irregular radiation pattern.

It is very difficult to design a single antenna having a complex irregular radiation pattern. Furthermore, it would be prohibitively costly to tailor the design of antennas to particular cell sites.

Antennas with complex and irregular radiation patterns can be made by means of the phased array antenna technique. However, such antennas are costly, difficult to set up to the desired pattern, sensitive, delicate, require regular maintenance and are usually large.

Simple combination of multiple antennas with different radiation patters in order to produce a combined pattern of the desired shape have not been successful. Simple combination (i.e. additions) of the signals from multiple antennas causes severe interference lobes, particularly at the boundary between the patterns of the individual antennas.

A method of combination is desired that overcomes the problem and provides reliable irregular shaped coverage areas.

Summarv of the Invention In accordance with a first aspect of the invention, a radio receiver is provided comprising first and second antennas each having different directional antenna patterns; a variable delay means for delaying signals received at the antennas with respect to signals received at the other antennas; a combiner for combining the signals received at each antenna and outputting a received signal; and an equaliser for combining components of the received signal.

In accordance with a second aspect of the invention, a corresponding transmitter is provided as defined in the claims.

The invention in all its aspects provides a inexpensive and simple arrangement in which irregular coverage can be provided.

Brief Description of the Drawings FIGs. 1, 2 and 3 show radiation patterns of antennas.

FIG. 4 shows a receiver in accordance with an embodiment of the present invention.

FIG. 5 shows a transmitter in accordance with an embodiment of the present invention.

FIG. 6 shows a microcellular environment.

FIG. 7 shows a cell cite transmitter arrangement.

Detailed description of the preferred embodiments A first aspect of the present invention provides the means by which two or more antennas may be combined to produce a complex radiation pattern which is the superposition of the simpler radiation patterns of the individual antennas. Thus, irregular coverage area may be achieved in order to follow a particular terrain or boundary feature. This is done according to the present invention without causing the interference lobes which would result from simple additive combination. It is applicable to radio systems which use digital modulation and which incorporate equalisers for the reduction of multi-path propagation effects.

In practice, when terrain features modify the coverage of a cell site antenna, compared to the coverage that it would have over regular terrain, the present invention may be used to compensate for the effect by means of an antenna with a compensatory irregular radiation pattern. This situation is shown in Figures 1 and 2. In Figure 1, line 3 represents the pattern of an omni-directional antenna 10 and the coverage area it would provide over flat, even terrain. However in direction x, the coverage is reduced due to hilly terrain in that direction. In direction y, coverage is extended due to ducting along a valley. In direction Z, coverage is as expected, over regular terrain. The overall irregular coverage boundary is shown as line 4 in Figure 1.

The reduced coverage in direction x provides poor service quality for users in that area. while the extended radiation in direction y is undesirahle because it could cause interference to subscribers using a distant cell which shares the same frequencies.

In order to compensate an antenna with the inverse irregular radiation pattern is required, as shown in Figure 2. Such an antenna arrangement may be provided according to the present invention so that increased radiation is provided in direction X and reduced radiation is provided in direction Y. The overall radiation pattern (i.e. measured over flat terrain or in a test chamber) would need to approximate to line 5 in Figure 2.

An antenna arrangement, according to the present invention, produces the desired effect of such a combination process is shown in Figure 3, whereby three antennas with individual patterns shown by lines 34, 35 and 36 combine to produce an overall radiation pattern shown by line 37.

This approximates to the required irregular pattern required by the example (line 5) shown in Figure 2.

Thus, FIG. 3 shows a plurality of antennas each having different directional antenna patterns or coverage areas combined to provide a desired covered area.

An antenna arrangement, according to the above aspect of the present invention is shown in Figure 4. A radio receiver 41 is provided including two or more antennas 42-44 each having different directional antenna patterns, an equaliser 46 for combining components of a received signal, a combiner 48 is provided for combining the signals received at each of the antennas and coupling the combined signal to the equaliser 46 and separate delay means 21-23 are provided in the receive path of each of the antennas for delaying signals at that antenna differently with respect of signals received at the other antennas so as to eliminate the probability of interference between the signals from the different antennas.

In accordance with a further aspect of the invention, as shown in Figure 5, a radio transmitter 50 is provided for communication with a receiver 51 having an equaliser 52 for combining components of a received symbol which are separated in time wherein that the transmitter comprises two or more antennas having different directional antennas. Splitter means 59 for splitting the signal to be transmitted and coupling it to the different antennas 53-55 and delay means 56-58 provided in the transmit path of each of the antennas for delaying the signals transmitted by that antenna by more than the pre-determined minimum delay with respect to signals transmitted bv any of the other antennas so as to eliminate the prohal)ilitv of interference between the signals from the different antennas.

The present invention provides an inexpensive and simple method in which multiple antenna sets (transmit and receive) can be combined without mutual interference in order to provide irregular cell site coverage or alternatively to compensate for the irregular cell site coverage which would otherwise occur for a single antenna set due to terrain features. A particular advantage is that the irregular cell coverage, so produced, is a single cell rather than separate cells, as would be the case if separate transmitter/receivers operating on different frequencies were connected to each antenna. The arrangement described herein is much simpler in terms of equipment and the requirement for control channels than the alternative of a site having separate multiple cells.

According to the present invention, the shape and amplitude of the individual antenna patterns required to make up the pattern of the combined cell site coverage of the irregular cell sites may be controlled by many factors. These include:1 The horizontal beam width of the individual antennas; 2 The gain of the individual antennas; 3 The direction of pointing of each antenna; 4 The height of each antenna; 5 The down tilt of each antenna.

Path loss prediction programs exist to determine the coverage pattern of the individual antennas for the actual terrain area over which they radiate (taking into account terrain features, free space antenna pattern, antenna height and down tilt etc.) Such programmes can be used to determine the combined coverage for the actual terrain.

The method of operation of the antenna combining scheme is similar to the method of operation of the antenna combining scheme of UK Patent Application No. 9119194.0. The difference between the two inventions is that in the case of combining two (or more) antennas to provide diversity reception the antennas would normally have the same radiation patterns and direction of radiation but are spaced apart by at least several wavelengths. In the present invention, the antennas have different radiation patterns and/or different directions of radiation in order to produce an irregular radiation pattern when combined and the individual antennas may or may not be co-located.

In both of the above described embodiments of the present invention.

for a cellular mobile telephone system, normally both transmitter combining and receiver combining would be used at a site. The antenna arrangements may be used particularly in either TDMA or CDMA communications systems.

A further embodiment of the present invention includes extending the present invention to a microcellular embodiment. In microcellular networks or in networks comprising of a cluster of microcells within a boundary of an overlaid macrocell, it may be advantageous, in some circumstances to make the cluster of microcells appear to certain mobiles as a single cell.

A particular example is when fast moving mobiles pass through the microcellular cluster. The mobile may not have time to decode the system information (broadcast on the control channel) for an individual microcell before it passes out of that microcell and passes on to the next one. It will not properly acquire the cells and will therefore not be able to make or receive calls.

If, however the microcells are combined (as far as the control channel information is concerned) then the fast moving mobiles will not "see" the boundaries between the individual microcells and so will have time to acquire the microcells as a combined group, appearing to be a single cell.

In order to achieve this, it is necessary for all the microcells in the group to broadcast the same system information, synchronously on the same control channel frequency. If this was attempted according to conventional practice, severe interference would be experienced between the microcells, particularly near their borders, resulting in severe fading and distortion (producing unacceptable error rates for the data transmission). However, applied according to the present invention, whereby delay means are introduced between the transmitters and the antennas, the interference between different microcell transmissions is removed (in cases where the corresponding receiver incorporates a multipath equaliser (such as the GSM system).Such is a particular example of the combined antenna pattern (in this case, combined cell coverage area, defined by the "combined" cell antennas) described in this invention.

The above has described the situation for the downlink (base to mobile) control channel. The same technique could be used in reverse for the uplink control (e.g. for channel access requests by the mobile, but is less practical or necessary. Alternatively the uplink control may be received and processed by the microcellular base stations individually, since this data is much less time critical than the cell acquisition and re-acquisition process on the downlink.

By this means, the microcells can be combined as an apparently single cell for fast moving mobiles and simultaneously can appear as individual microcells to slow moving hand portable units if they each transmit a second individual control channel on an individually assigned frequency. Typical system operation is that fast moving mobiles will acquire the common microcellular group control channel (Broadcast Control CHannel), and if they wish to make a call, will detected as fast moving and be re-directed to the macrocellular overlay, which is better suited to handling them. Slow moving hand portables will be directed to individual microcells, which collectively have the high capacity required to handle the high density of calls in those areas.

FIG. 6 shows a macrocell overlay 61 and microcell underlay 63 including four microcells 66, 67, 68, 69 shown as three sector cells The combined, identical microcellular control information (data) may be functionally sourced from a common Base Station Controller (BSC) 65 via transmission lines 71, 72, 73, 74 to respective radio equipment of the cells 66, 67, 68, 69.

FIG. 7 shows a typical embodiment of the invention at a cell site 63 In this example, a three sector cell site is described actually consisting of three co-located cells, with the coverage of each being defined by three directional antennas 53, 54, 55. For this embodiment, the delay means 56, 57, 58 should be time varying delay means as described in UK Patent 91191940. Also for this embodiment, additional delay means 80, 81, 82 may be required for some microcellular deployments between Base Station Controller and microcells in order to compensate for the different delays introduced by the transmission lines. This compensating delay would bring the average delays, produced by the variable main delay means 56, 57 58, the transmission line delays and the radio propagation paths into the range which can be processed by the receiver equalisers 52.

Claims

1. A radio receiver comprising: at least a first antenna and a second antenna each having different coverage areas; a variable delay means for delaying signals received at each antenna with respect to signals received at the other antennas; a combiner for combining the signals received at each antenna and outputting a received signal; and an equaliser for combining components of the received signal.

2. A radio receiver according to claim 1 wherein the signals received at the antennas are divided into discrete frames and the delay means vary the delay from frame to frame.

3. A radio receiver according to any one of the preceding claims wherein the delay means further comprises frequency shift means for shifting the frequency of the signal received at one antenna with respect to the frequency of the signal received at the other antenna.

4. A radio receiver according to any one of the preceding claims wherein the delay means further comprises phase shift means for shifting the phase of the signal received at one antenna with respect to the phase of the signal received at the other antenna.

5. A radio receiver according to any one of the preceding claims wherein the radio includes a radio frequency stage and the delay means are located in that stage.

6. A radio receiver according to any of the preceding claims wherein the radio includes an intermediate frequency stage and the delay means are located in that stage.

7. A radio receiver according to any one of the preceding claims wherein the delay means cause a delay of a least a quarter of a bit period.

8. A radio receiver according to any one of the preceding claims wherein the delay means cause a delay of at least a full bit period.

9. A radio receiver according to any one of the preceding claims wherein the delay means in the receive path of each antenna causes signals received at each antenna to be separated in time by more than a predetermined minimum delay with respect to signals received at each other antenna.

10. A radio receiver add-on unit for use with a radio receiver having an equaliser for combining components of a received symbol which are separated in time, the add-on unit comprising: a plurality of antennas coupled together each having different directional antenna patterns; a combiner and a variable delay means coupled between the combiner and each of the antennas for delaying signals received at each antenna with respect to signals received at the other antennas so as to significantly reduce or eliminate the probability of destructive interference between the signals from the different antennas.

11. An add-on unit according to claim 10 wherein the delay means comprises down-converter means and up-converter means wherein the delay means are located between the down-converter means and the up-converter means.

12. A radio receiver or add-on unit according to any one of the preceding claims wherein the delay means comprise a surface acoustic wave element.

13. A radio transmitter for communication with a receiver having an equaliser for combining components of a received symbol which are separated in time, the transmitter comprising: a plurality of antennas each having different coverage area combinable to provide a desired combined coverage area; splitter means for splitting a signal to be transmitted and coupling it to each of the antennas; and delay means provided in the transmit path of each of the antennas for delaying signals transmitted by that antenna by more than the predetermined minimum delay with respect to signals transmitted by the other antennas so as to significantly reduce the probability of destructive interference between the signals from the different antennas.

14. A plurality of radio transmitters arranged for communication with a receiver having an equaliser for combining components of a received symbol which are separated in time, the plurality of radio transmitters comprising: an antenna means for each radio transmitter where each antenna means has a different coverage area so that in combination all transmitters and antennas provide a desired total coverage area; a common source of data transmission which is intended for reception by the receiver located anywhere in the desired total coverage area; delay means provided in the transmit path of each of the antennas for delaying signals transmitted by that antenna by more than the predetermined minimum delay with respect to signals transmitted by the other antennas so as to significantly reduce the probability of destructive interference between the signals from the different antennas.