Abstract:
An apparatus determines an optimal antenna and frequency combination by selecting the optimal antenna from among a plurality, N, of antennas associated with a receiver and also selects the optimal frequency from among a plurality, M, of alternative receiving frequencies transmitted from a transmitter. This is achieved by determining all N×M possible antenna/frequency combinations, by determining the received signal quality at each combination, by comparing the N×M received signal qualities, and by selecting the one antenna/frequency combination having the optimal received signal quality.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to an antenna diversity system, and in particular to selecting one of a plurality of antennas and one of a plurality of alternative receiving frequencies to determine a desirable antenna and frequency combination. 
     An antenna and frequency diversity receiving apparatus typically includes a receiver connected to one of several spatially separated antennas and tunable to one of several alternative receiving frequencies. Such an apparatus is oftentimes used with a mobile receiver, for example, a radio receiver in a motor vehicle. As such, a plurality of antennas are integrated within the windows of the motor vehicle. A selection circuit selects according to prescribable criteria one of the antennas to be connected to the receiver (e.g., the antenna receiving a signal with the greatest field strength). Signals to be received may emanate from various types of transmitters, for example, a radio broadcasting apparatus, a television broadcasting apparatus or telephone equipment. An evaluation circuit selects one of the alternative receiving frequencies to which the receiver will be tuned. In order to achieve the best possible signal reception, it is necessary to determine the best antenna and frequency combination. 
     What is needed is a technique for determining the best antenna and frequency combination as quickly and reliably as possible from both the plurality of available antennas and the plurality of available receiving alternative frequencies. 
     SUMMARY OF THE INVENTION 
     An optimal antenna is selected from among a plurality (e.g., N) of antennas associated with a receiver and the optimal frequency is selected from among a plurality, M, of alternative receiving frequencies transmitted from a transmitter. This is achieved by determining N×M possible antenna/frequency combinations, by determining the received signal quality at each combination, by comparing the N×M received signal qualities, and by selecting the one antenna/frequency combination having the best received signal quality. 
     Each of the N antennas is connected to the input of a corresponding Intermediate Frequency (IF) circuit stage, while the output of each IF stage is connected to one of the corresponding N inputs of a multiplex switch. The output of the multiplex switch is connected to an analog-to-digital converter that provides digitized data to a digital signal processor (DSP), the output of which is connected to a demodulator which can be part of the DSP or separated therefrom. Each IF stage provides a signal representing the quality of the transmitted signal received by the associated antenna, the N quality signals are provided to the DSP. The DSP provides one or more frequency control signals to a corresponding controlling input of each IF stage for setting the receiving frequency thereof. A controlling output of the DSP is connected with the controlling input of the multiplex switch for selecting the antenna with the optimal received signal quality to pass from the associated IF stage through the multiplex switch to the DSP. 
     The received signal quality from each of the N antennas can be measured at each of the M available receiving frequencies. Therefore, all N×M possible antenna frequency combinations are accounted for and the received signal quality will be determined for each of these combinations. Then the one antenna/frequency combination that provides the best received signal quality will be selected. 
     Each of the N IF stages will be tuned to each of the M receiving frequencies, This can occur such that all of the IF stages are tuned to the same one of M receiving frequencies at any one moment in time. In the alternative, the IF stages can be tuned to different ones of the M receiving frequencies at any one moment in time. The receiving quality will be determined for each receiving frequency and analyzed in the digital signal processor to determine the antenna/frequency combination having the best received signal quality. The digital signal processor switches the selected antenna to the input of the analog-to-digital converter via the multiplex switch and tunes the corresponding IF stage to the optimal receiving frequency found. 
     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustration of an antenna diversity system; and 
         FIG. 2  is a block diagram of the system of  FIG. 1  with alternative embodiments of certain features. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , an antenna diversity apparatus  10  includes a plurality (e.g., N) of antennas  15 - 17  which each provides an associated received signal to a corresponding IF (Intermediate Frequency) stage  25 - 27 , respectively. Each IF stage  25 - 27  may comprise well-known components, for example, a mixer, a phase loop control, and an oscillator. Each of the IF stages provides an IF output signal (e.g., lines  30 - 32 ) to a multiplex switch  35 . The switch  35  provides a switch output signal on a line  40 , indicative of a selected one of the IF input signals on the lines  30 - 32 . The switch output signal on the line  40  is input of an analog-to-digital converter (ADC)  45  which provides a digitized signal on a line  50  to a digital signal processor (DSP)  55 . The DSP  55  provides a DSP output signal on a line  60  to a demodulator  65 . 
     Each of the plurality of IF stages  25 - 27  provides an associated quality signal on a line  70 - 72 , respectively, to the DSP  55 . Each quality signal indicates the quality of the received signal from the corresponding antenna  15 - 17 . For example, the quality may represent the field strength of the signals received by the antennas  15 - 17  from the various transmitters. The DSP  55  provides frequency control signals on lines  75 - 77  to an associated one of each of the IF stages  25 - 27 , respectively. In addition, the DSP  55  also provides a switch control signal on line  80  to the switch  35 . 
     The DSP  55  tunes each of the N IF stages  25 - 27  to each of the M alternative receiving frequencies, so that all N×M possible antenna/frequency combinations can be determined. Therefore, N×M quality signals are generated and compared with each other in the DSP  55 . 
     The best receiving can be ascertained, for example, in a well-known manner by quality evaluation of the IF quality signals  70 - 72  for the alternative receiving frequencies. In the analysis, the DSP  55  ascertains, with the help of the N×M quality signals, the one antenna/frequency combination that provides the best received signal quality. The DSP  55  then provides the switching control signal on the line  80  in order to switch the switch  35  to the selected optimal antenna, and the DSP  55  also provides the frequency control signal on a line  75 - 77  to tune the corresponding IF stage to the receiving frequency found. The DSP  55  then receives the signal  50  from the selected one antenna/frequency combination with that signal  50  being distinguished by the best received signal quality among all possible antenna/frequency combinations. 
     A feedback of the IF signals is not necessary, because the receiving quality will be ascertained already in the IF stages  25 . The digital processing of the IF signal within the DSP  55  provides the advantage that generating of a returning signal will be comparatively easy. Furthermore, interferences caused by returning operations can be suppressed or resampled in an easy manner. In a preferred embodiment, the IF stages  25 - 27  are integrateable in a module as integrated circuits. Preferably, the IF stages  25 - 27 , the switch  35 , the analog-to-digital converter  45 , the digital signal processor  55 , and the demodulator  65  are integrated in a single module. 
     In  FIG. 2 , the plurality, N, of quality signals  70 - 72  are input to a second multiplex switch  100 . The output of the second multiplexer  100  on a line  105  is input to the DSP  55 . The DSP  55  controls the second multiplexer  100  by a signal on a line  110 . In this alternative embodiment, it is not necessary to provide the DSP  55  with a plurality, N, of quality signal inputs. 
     According to another aspect, a frequency output signal on a line  115  is provided by the DSP  55  to a third multiplex switch  120 . To control the third multiplexer  120 , the DSP  55  provides a signal on a line  125  to the multiplexer  120 . This control signal  125  controls the operation of the third multiplexer  120  to switch the frequency signal  115  to the corresponding IF stages  102 - 104 . 
     Each IF stage  102 - 104  includes memory  130 - 132  for storing the frequency value to be used by the corresponding IF stage  102 - 104  and sent by the DSP  55  until receipt of another frequency value from the DSP  55 . Therefore, it is possible to use a frequency value determined as a best frequency value in an earlier determining cycle. 
     The third multiplexer  120  may instead comprise a frequency splitter that splits the frequency output signal on the line  115  from the DSP  55  into a plurality of frequency lines, thereby connecting the frequency signal on the line  115  to the frequency inputs of the IF stages  102 - 104 . In this embodiment, the DSP would provide all of the IF stages  102 - 104  at one moment with the same frequency value. 
     Therefore, the DSP  55  provides a frequency control output signal  75 - 77  to each one of the plurality, N, of IF stages  102 - 104 , respectively, via a plurality of N frequency output signals from the DSP  55  or, alternatively, via only one frequency output signal on the line  115  from the DSP  55  and a frequency control device, i.e., the third multiplexer  120 . The frequency control device may be the third multiplexer  120  or only a simple line splitter. 
     As described above, it is possible to provide a plurality, N, of frequency output signals to the corresponding plurality, N, of IF stages, wherein each IF stage has a single or the same frequency value. Thus, within a plurality, M, of steps every IF stage will be provided with the M possible frequency values to determine the M quality values of the received signals  20 . 
     In an alternative embodiment, it is possible to provide all or individual IF stages with an amount, P, or possible frequency signal values that is less than all of the N×M total amount of the possible frequency values. As such, only a corresponding amount, P, of quality signals would be determined. To then achieve all of the N×M possible antenna/frequency combinations, it would be necessary to calculate any missing values, for example, by interpolation. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.