GB2242807A

GB2242807A - Diversity antenna switching in a multi-channel radio communication system

Info

Publication number: GB2242807A
Application number: GB9107217A
Authority: GB
Inventors: Anthony Peter Hulbert
Original assignee: Roke Manor Research Ltd
Current assignee: Roke Manor Research Ltd
Priority date: 1990-04-07
Filing date: 1991-04-05
Publication date: 1991-10-09
Also published as: GB9007927D0; GB9107217D0

Abstract

A base station for a radio communication system has a plurality of transmitter/receiver units connected to a signal combiner, two radio communications antennas a, b which are connected through a switch means to the combiner, the switch means being operable to select one or other of said antennas, and means for providing a switch actuation signal if a deterioration in the conditions for radio channel operation should be detected by any one of the T/R units. When a T/R unit detects unacceptable conditions, it will pulse its "C" pulse line high thereby causing change of state of a flip-flop F/F resulting in a change of position of the antenna selector switch. The arrangement may alternatively be implemented in a computer programme. <IMAGE>

Description

RADIO COMMUNICATION SYSTEM This invention relates to a radio communication system and it is especially applicable to the provision of a base station diversity arrangement for common air interface (CAI) telepoint systems.

Two branch selection diversity is a common technique employed to mitigate the effects of deep transmission fades due to multipath signal propagation. For a cordless telephone system it is desirable to implement the diversity in the base station part of the system to avoid increasing the size or complexity of the handset part.

The use of a time division duplex mode ensures path reciprocity so that antenna switch decisions made at the base station on the basis of received signal strength will also effect improvement in the signal strength received at the base station.

There are two approaches to combining transmitter receiver units to create a multi-channel base station, namely the multiple antenna depicted in Figure 1 of the drawing and the combiner/single antenna of Figure 2. For the former approach, base station diversity can be achieved by a doubling up on the antenna function provided for each unit. However, for the combiner approach only one antenna can be connected to all of the base stations at a time.

It is an object of the present invention to provide a combiner system type base station which includes a measure of diversity.

According to the invention, there is provided a base station for a radio communication system, the base station comprising a plurality of transmitter receiver units connected to a signal combiner, two radio communications antennas which are connected through a switch means to said combiner, the switch means being operable to select one or other of said antennas, and means for providing a switch actuation signal if a deterioration in the conditions for radio channel operation should be detected by any one of said transmitter receiver units.

Preferably, each transmitter receiver unit includes a 'C' pulse line, each of which lines is connected to a respective input terminal of an OR gate logic device, such that an output terminal of said device will produce the required switch actuation signal.

In one embodiment, the said OR gate output terminal is connected to a CK input terminal of a 'D' type bistable switch, an output signal from said bistable switch then serving to effect the required antenna connection change.

The base station may form part of a radio communication system having an increased diversity in operation.

An exemplary embodiment of the invention will now be described with reference being made to the accompanying drawing, in which: Figures 1 and 2 are block schematic diagrams of typical multichannel base stations; and, Figure 3 is a block schematic diagram of a modified form of multi-channel base station in accordance with the present invention.

As described above, diversity is commonly employed to mitigate the effects of multipath signal propagation. However, deep fades will only cause problems to a given link when the average signal to interference + noise ratio is not greater than required by a significant margin. For any system of communications involving a frequency re-use this required ratio is referred to as the protection ratio. The benefit of diversity is that it reduces the required protection ratio permitting a more frequent spatial re-use of channels. However in such a system, performance is generally determined by the effects from the worst placed terminals, that is, those communicating from the edge of the cell. It will commonly be the case that such terminals will be making frequent switches on the diversity antenna whilst those terminals which are nearer to the base station will not need to do so.

On this basis it might be expected that a significant benefit may arise as a result of providing diversity only to the worst situated equipment in a cell. Thus the combiner would feed two antennas and switch between them according to demands made by whichever link is currently experiencing a communications difficulty. Obviously this would give rise to conflicts but it is likely that, bearing in mind the distribution of users, a significant overall improvement would nevertheless result. Moreover, the increase in total equipment complexity for the base station would be minimal.

The basic approach to the above proposed antenna switching is as shown in Figure 3.

In the embodiment of Figure 3, when any of the transmitter receiver units T/R, during an active call via antenna 'a' detects unacceptable channel conditions on its call (for example, inadequate signal strength, bit timing or error rates) it will pulse its 'C' line high.

Each of the 'C' pulse lines of the transmitter receiver units are connected to a respective input terminal of an OR gate logic device.

An output signal from the OR gate, when actuated, will be applied to a CK input terminal of 'D' type bistable switch. When thus operated, the 'D' type flip flop bistable switch F/F is caused to toggle its state thereby selecting the alternative antenna 'b'. The technique could equally well be implemented in a software computer programme.

The above approach is simple in concept but it could be subject to oscillation ('hunting') when two transmitter receiver units simultaneously have inadequate signal to noise ratios on the alternative antennas. This situation would arise in the condition where two channels are making competing demands for antennas (for example, channel 3 may work well with antenna 'a' but not with antenna 'b' whereas channel 5 works well with antenna 'b' but not with antenna 'a'). The effect would be for the switch state to move repeatedly between the two antennas. The rate at which this happens would be limited by the time taken to detect the occurrence of inadequate signal to interference + noise, usually several time division duplex cycles. The effect would therefore be to share fade free performance between the two conflicting channels on a time basis. This behaviour is acceptable and it may even be desirable, being 'democratic' in nature. Moreover, the period of conflict is unlikely to be significant and therefore the possible oscillation duration may be only short lived.

Claims

1. A base station for a radio communication system, the base station comprising a plurality of transmitter receiver units connected to a signal combiner, two radio communications antennas which are connected through a switch means for said combiner, the switch means being operable to select one or other of said antennas, and means for providing a switch actuation signal if a deterioration in the conditions for radio channel operation should be detected by any one of said transmitter receiver units.

2. A base station as claimed in Claim 1, in which each transmitter receiver unit includes a 'C' pulse line, each of which lines is connected to a respective input terminal of an OR gate logic device, such that an output terminal of said device will produce the required switch actuation signal.

3. A base station as claimed in Claim 2, in which the said OR gate output terminal is connected to a CK input terminal of a 'D' type bistable switch, an output signal from said bistable switch then serving to effect the required antenna connection change.

4. A base station for a radio communication system, substantially as hereinbefore described with reference to Figure 3 of the accompanying drawing.

5. A radio communication system including a base station as claimed in any one of Claims 1 to 4.