The invention relates to an antenna diversity receiver array comprising at least two antennas, a first receiver unit for processing antenna signals, a change-over switch via which one of the antennas can optionally be connected to the first receiver unit, a control unit coupled to the change-over switch, for selecting the antenna to be connected to the first receiver unit.

The invention further relates to a method of selecting one from a plurality of antennas of an antenna diversity receiver array in which the selected antenna is coupled to a first receiver unit.

An antenna diversity receiver array is known, for example, from EP 1 098 452 A2. Antenna diversity receiver arrays have a plurality of antennas for the reception of television signals, radio signals (FM) or data, of which one respective antenna can be coupled to a receiver unit to process the antenna signals. In this manner it is possible particularly with mobile applications to achieve a best possible, error-free reception in which the respective antenna having the best signal is coupled to the receiver unit. With respect to this, 454 describes a method in which a receiver and an interference detector are included in the receiver unit, the interference detector supplying pulses to an integrator in case of reception interference. In so far as the pulses integrated by the integrator exceed a certain threshold, a control unit is caused to couple the receiver unit with another antenna. A disadvantage of this method is that a new antenna is not changed over to until there is interference on the antenna currently coupled to the receiver unit.

Furthermore, methods are known in which a complete receiver unit is assigned to each of the antennas so that the best signal may be selected to be output. Such solutions, however, are very expensive because of the fact that demodulation units are to be included in the multiple receiver units (receivers).

Against this background it was an object of the present invention to provide a cost-effective antenna diversity receiver array and a method for its operation, which array and method ensure a high quality of the selected antenna signals at low cost.

This object is achieved by an antenna diversity receiver array having the characteristic features of claim 1 as well as a method having the characteristic features of claim 5. Advantageous embodiments are contained in the dependent claims.

The antenna diversity receiver array according to the invention comprises at least two antennas, a first receiver unit for processing antenna signals, a change-over switch via which one of the antennas can optionally be connected to the first receiver unit, as well as a control unit coupled to said change-over switch for selecting the antenna to be connected to the first receiver unit. The array is characterized in that it comprises a second receiver unit as well as a second change-over switch, which second receiver unit can be coupled via the second change-over switch to a respective one of the antennas as desired and is further connected to the control unit to transfer to it information about the signal strength of the antennas.

In the antenna diversity receiver array according to the invention the signal strength on the antennas are continuously monitored by means of a background receiving unit, so that information about the antenna currently having the best reception is available at any time. This information may then be used by the control unit to optionally connect the (current) receiver unit to the best antenna. The change to a new antenna can therefore be made if there are no controls yet at the antenna currently coupled to the receiver. This achieves a high reception quality. The system according to the invention is then very cost- effective, because it uses only one additional receiver for the background monitoring of the antennas. Besides, this receiver can be kept comparatively simple and cost-effective because, in essence, it is to establish only the amplitude of the antenna signals.

According to a further embodiment of the antenna diversity receiver array the second change-over switch is arranged for connecting the antennas to the background receiver unit time-sequentially. More particularly, the second change-over switch can cyclically couple the antennas to the background receiver unit for respective fixed connection times. In this way, it is ensured that the background receiver unit uniformly scans the signal strengths of all antennas and always knows the current receive ratios.

Furthermore, with the information obtained from the background receiver unit the control unit is preferably arranged for determining the antenna that has the best current

and/or pre-calculated signal strength. Since the background receiver unit quasi continuously monitors all the antennas, it knows the time-dependent variations of the respective signal strengths. Therefore, the control unit can both compare the current signal strengths of the individual antennas with each other and make a forecast about the further development of the signal strengths based on the curves. With such a forecast the control unit can then cause the change of the antenna coupled to a current receiver at an early stage if this antenna shows a tendency of signal deterioration and another antenna offers a better signal.

The antenna diversity receiver array described is preferably arranged for the mobile reception of radio and/or television signals. Especially when the receiver is moved, the receive ratios may change very rapidly, so that an efficient and high-quality method is important for the selection of the best antenna. Furthermore, mobile applications have the advantage that the antenna diversity receiver array makes due with only a single additional receiver and can therefore be designed cost-effectively and saving space and weight.

The invention further relates to a method of selecting one from a plurality of antennas of an antenna diversity receiver array, which antenna may be of the type described above. In the method the selected antenna is coupled to a receiver unit to process the antenna signals in the desired fashion. The method is characterized in that the signal strength of all the antennas is monitored by means of a background receiver unit and in that the selection of the antenna to be coupled to the receiver unit is made on the basis of the thus obtained monitoring results.

The described method thus has the advantage that always the receive ratios on the antennas are known, so that always the best antenna can be selected. However, it is then not necessary for each antenna to have its own receiver unit, but the monitoring is rather made by a single receiver unit which therefore can have a simple and cost-effective structure.

According to the method the antennas are preferably connected to the background receiver unit in a time-dependent sequence. In this way it is ensured that the receive ratios on the antennas are scanned regularly. The duration for which a single antenna is connected to the background receiver unit, as well as the sequential order of the antennas, may be irregular or following a program depending on the situation. In the simplest case the background receiver unit, however, is cyclically connected to the various antennas. Switching over from one antenna to the next may then take place typically with a frequency from 100 to 400 Hz.

The selection of the antenna to be coupled to the receiver unit may be made according to various programs while taking the information into account supplied by the

background receiver unit. A very simple and at the same time efficient method then provides that for a connection to the receiver unit the antenna is selected that currently has the highest signal strength. As required, a certain hysteresis may be added to avoid an undesired, frequent changing in the case of a plurality of antennas having the same signal strength.

According to a preferred embodiment of the method the variation of the signal strengths of the antennas provide a forecast for the further variation of the signal strengths and the selection of the antenna to be connected to the receiver unit is made while this forecast is taken into account. Since the signal strengths on the individual antennas are continuously monitored, their complete variation over time is known. Therefore, it is also possible to forecast the further variation or calculate it in advance. This in its turn allows to abandon an antenna currently coupled to the receiver unit if the signal strength starts to deteriorate, and to change to a better antenna. The change can therefore be made before an interference in the reception that can be noticed by the user has occurred.

For the forecast of the further development of the signal strength of an antenna there are various possibilities. In the simplest case the time-dependent derivation of signal strength (tangent to the instantaneous variation) is calculated and used for the forecast.

Accordingly, negative derivations point at a deterioration of the signal strength and positive derivations point at signs of improvement of the signal strength.

In so far as television signals are received by the antennas, the change-over to a new antenna to be coupled to the receiver unit is preferably made at a time in which there is a line flyback of the television signal. In this manner it is ensured that an interference caused by the change-over does not affect the television picture.

These and other aspects of the invention are apparent from and will be elucidated, by way of non-limitative example, with reference to the embodiment (s) described hereinafter.

In the drawings: Fig. 1 shows a circuit diagram of an antenna diversity receiver array according to the invention ; Fig. 2 shows typical curves of the signal strengths on various antennas and the curve of the signal strengths when the antennas are selected with the method according to the invention.

Fig. 1 shows an antenna diversity receiver array as can be used, for example, for the reception of radio or television signals in a motor vehicle. A plurality of preferably spatially separated antennas 1 a, 1 b and 1 c are provided by means of which a sufficiently good reception is to be ensured in case of difficult and spatially strongly varying receive conditions. The array further comprises a receiver unit (receiver unit proper or main receiver unit) 2a for amplifying and processing the antenna signals. The processed signals can then be shown on a display 3 coupled to the receiver unit 2a. The display 3 may be, for example, a monitor for television signals and a loudspeaker for radio signals.

Furthermore, a change-over circuit 4 is provided which comprises a first change-over switch 5a. Via the first change-over switch 5a the receiver unit 2a can be connected to the antennas 1 a, 1 b or 1 c as desired. The selection of the position of the change- over switch 5a is made by a control unit 6. This control unit can receive on an input the synchronization signal of received television signals, so that it can synchronize a change-over operation herewith and put it particularly in the time of a line flyback.

With the embodiment according to the invention of the antenna diversity receiver array a second background receiver unit 2b is provided which can be connected via a second change-over switch 5b to one of the antennas 1 a, 1 b or 1 c as desired. Preferably, the second change-over switch 5b is then set so that it cyclically rotates between the antennas la, lb and lc, so that they are frequently coupled to the background receiver unit 2b. It is also possible that more than the three antennas shown are provided. If during a car ride at 120 km/h signal interruptions typically having a width of at least 0.3 m are certainly to be detected, the scanning frequency of the background receiver unit can be selected to be accordingly high. With, for example, a 400 Hz frequency the signal rise would be determined in four intervals along the signal interruption. The background receiver unit 2b can quasi continuously follow the curve of the signal strengths of the antennas 1 a-1 c in this way and render this information available to the control unit 6. The background receiver unit 2b may then be arranged as a simple and cost-effective tuner with an amplitude detector.

By means of the information rendered available by the background receiver unit 2b, the control unit 6 can decide which of the antennas 1 a-1 c has the best signal and compare it with the antenna that is currently coupled to the main receiver unit 2a via the first change-over switch 5a. The control unit 6 can further make its decision also based on a forecast of the signal strength variation. For example a large negative rise of the signal strength of an antenna indicates that there is a signal interruption (dip). In that case the

control unit 6 can timely change the signal receive path to an antenna having a constant or rising signal level, so that the upcoming interference cannot first have its effect on the reception.

A change-over operation at the first switch 5a is preferably carried out at an instant at which it is ensured that no error can occur in the signal or data transmission. With a television signal this can be guaranteed, as already observed before, more particularly by a change-over during the line flyback.

Fig. 2 shows in its upper diagram exemplary curves of the signal strength (vertical axis) of the three antennas la, Ib and lc plotted against time (horizontal axis). With the method according to the invention all three curves are known to the control unit 6 as a result of the continuous monitoring by the background receiver unit 2b. The control unit 6 can therefore at any instant change the first change-over switch 5a to the antenna that currently has the highest signal strength. As a result, there will be a variation of the signal strength at the input of the main receiver unit 2a in accordance with the lower diagram of Fig. 2, in the example shown the antennas 1 a, I c and I b and la being used one after the other.

REFERENCE CHARACTERS: la, la, Ic antennas 2a (main) receiver unit 2b background receiver unit 3 display 4 change-over unit 5a first change-over switch 5b second change-over switch 6 control unit