Abstract:
A technique for reducing multipath distortion in an FM receiver, with a plurality of switchable antennas, provides a fast distortion detector that monitors a received signal for significant distortion events of less than about 15 microseconds in duration. In response to a multipath event, the output of the fast distortion detector initiates a search for a lower distortion (better quality) antenna. To prevent frequent antenna searches from causing an audible disturbance, a threshold is introduced to desensitize the fast distortion detector. Threshold decay is a function of an overall received RF signal level. A slow distortion detector is also provided that measures distortions of the received signal relating to signal quality.

Description:
TECHNICAL FIELD 
   The present invention is generally directed to a receiver and, more specifically, to a technique for reducing multipath distortion in a mobile FM receiver having a single analog front-end. 
   BACKGROUND OF THE INVENTION 
   As is well known, multipath interference is caused when two or more signal rays of an original transmitted signal converge upon a receiving antenna of a receiver at significantly different times. This misalignment or superposition of several delayed signals, which are replicas of the original signal, may cause distortion in audio recovered from the signals. Distortion caused by the multipath interference may be attributable to long delay (e.g., greater than five microseconds between signals) multipath interference or short delay (e.g., less than five microseconds between signals) multipath interference. 
   In a typical urban environment, RF signals experience changes in amplitude and phase due to short delay multipath. This amplitude and phase shift may result in broadband signal fades of up to 40 dB, as the receiver and its associated motor vehicle change locations. At typical highway speeds, signal fluctuation rates in the range of 100 to 1 kHz may occur. In general, long delay multipath (or frequency selective multipath) is found in areas where reflectors are greater than four to five miles away. Typically, long delay multipath occurs in cities with large buildings and in mountainous regions. 
   Typically, long and short delay multipath coexists and creates frequency selectivity and broadband fading, simultaneously. For example, an FM demodulated signal may contain a 1 kHz tone with a 75 kHz deviation. In such a situation, a reflected signal may have an amplitude of, for example, 0.9 units while a direct signal has, for example, an amplitude of 1 units. In the case where the time delay of the reflected signal is about 30 microseconds, the distortion attributable to the time delay may be on the order of approximately twelve percent. 
   In various receiver systems, antenna diversity has been implemented in conjunction with an FM receiver to reduce degraded reception performance caused by multipath interference. Antenna diversity has been accomplished through the use of two or more uncorrelated antennas. Prior art antenna diversity reception for mobile communication systems has been achieved by a number of different implementations. For example, antenna diversity has been accomplished with equal gain combiner (EGC) systems, maximal ratio combiner (MRC) systems and antenna diversity systems, such as the adaptive reception system (ARS) disclosed in U.S. Pat. No. 5,517,686, the disclosure of which is hereby incorporated herein by reference in its entirety. 
   EGC and MRC systems utilize signals from all antennas through a variety of combining techniques that attempt to optimize certain characteristics of the received signal. In a switched antenna diversity system, only one antenna is utilized for reception at any instant in time and, thus, the non-selected antennas do not contribute to the demodulated signal. EGC and MRC systems generally outperform switched antenna diversity systems. However, EGC and MRC systems tend to be more expensive to implement, as they require multiple receiver analog front-ends. 
   What is needed is an economical technique for further reducing multipath distortion in a mobile FM receiver having a single analog front-end. 
   SUMMARY OF THE INVENTION 
   One embodiment of the present invention is directed to a technique for reducing multipath distortion in an FM receiver, with a plurality of switchable antennas. The technique includes providing a fast distortion detector that monitors a received signal for distortion events less than about fifteen microseconds in duration, which indicates a multipath disturbance. A slow distortion detector is also provided that measures distortions of the received signal related to the signal quality. In response to a multipath disturbance, an output of the fast distortion detector initiates a search for a lower distortion (better quality) antenna. The search involves selecting a trial antenna and comparing its measured signal quality (provided by an output of the slow distortion detector) to that previously measured for the antenna that initiated the search (i.e., a reference antenna). An antenna having better signal quality is accepted for continued use and the search is ended. An antenna having a worse signal quality is rejected and the search is continued by selecting another trial antenna. 
   To prevent frequent searches that can result in audible switching noise, a threshold is introduced that desensitizes the fast distortion detector for a period following an antenna search. The threshold is decayed at a rate dependent on the overall RF signal level to provide a longer desensitized period for weak signals, which are more susceptible to disturbances. The slow distortion detector uses an averaging time that is a function of the received overall RF signal level, since, in weak signal conditions, the distortion being measured is more corrupted by noise. The averaging time may typically range between twenty-five microseconds for large signal levels to five hundred microseconds when the overall RF signal level is below a predetermined RF level. Antennas are ranked for trial selection based on their recently measured RF level. This approach helps to minimize antenna switching since an antenna having a larger signal level, which is more likely to be lower in distortion, is selected as the next trial antenna. 
   The slow distortion detector may implement a filter that passes frequency components of the received RF signal that are higher than about 60 kHz. According to another aspect of this embodiment of the present invention, the filter passes frequency components of the received RF signal that are less than about 100 kHz. According to one aspect of the present invention, the slow distortion detector implements a rectifier and a low-pass filter. In at least one embodiment, the slow distortion detector functions as an ultra sonic noise (USN) detector. 
   These and other features, advantages and objects of the present invention will be further understood and appreciated by those skilled in the art by reference to the following specification, claims and appended drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  is a block diagram of an exemplary radio with a single analog front-end and a digital signal processor (DSP); 
       FIG. 2A  is a block diagram of a receiver system implementing a classic switched diversity system; 
       FIG. 2B  is a graph depicting an RF signal level and an FM demodulator output signal (MPX) for the receiver system of  FIG. 2A ; 
       FIG. 3A  is an exemplary graph of an FM baseband spectrum for an FM receiver; 
       FIG. 3B  is a block diagram of a relevant portion of an FM receiver system, including a slow distortion detector for detecting high-frequency components in the signals of the graph of  FIG. 3A ; 
       FIG. 4  is a system block diagram for an FM receiver implementing switched diversity, according to one embodiment of the present invention; 
       FIGS. 5A-5B  are graphs depicting regions of operation for the system of  FIG. 4 ; 
       FIG. 6A  is a high-level flow chart of an exemplary process for reducing multipath distortion in an FM receiver, with a plurality of switchable antennas, according to one embodiment of the present invention; and 
       FIG. 6B  is a lower-level flow chart of an exemplary process for reducing multipath distortion in an FM receiver, with a plurality of switchable antennas, according to another embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Today, manufacturers of automotive radios have moved from analog receiver systems to receiver systems that have increasingly incorporated more digital components within the receiver systems. As a general rule, the functions that are performed by these digital components are being increasingly implemented in digital signal processing (DSP) software. 
   With reference to  FIG. 1 , an exemplary receiver system  100  is shown, which includes a plurality of antennas A 1 , A 2  through AN, which are coupled to a single analog front-end  106  (of an FM receiver  104  incorporated within a radio  102 ) by a different one of a plurality of switches SW 1 , SW 2  through SWN. The output of the front-end  106  is provided to an input of an analog-to-digital converter (ADC)  108 , which converts the received analog signal to a digital signal. An output of the ADC  108  is coupled to an input of a digital signal processor (DSP)  110 A, which digitally processes the digital signal to provide an audio signal. 
   According to one aspect of the present invention, as is discussed further below, software algorithms (see  FIGS. 6A-6B ) executed by a DSP implement switched antenna diversity for the receiver system  100 . According to another aspect of the present invention, an FM demodulator (not shown separately in  FIG. 1 ) outputs an MPX signal, which is directed to the DSP  110 A, which implements a switched antenna diversity routine  150  (see  FIG. 4 ). In general, the routine  150  improves FM reception by reducing multipath distortion by choosing a least distorted antenna signal from one of a plurality of antennas. As noted above, switched antenna diversity is generally the simplest algorithm to implement among antenna diversity systems. In essence, the switched antenna diversity system selects the antenna with the best signal-to-noise ratio (SNR). However, because only one antenna can truly be selected at a time, the diversity algorithm must generally make the antenna selection based on incomplete knowledge. 
     FIG. 2A  depicts an FM receiver system  180  that implements a classic switched diversity system using a fast distortion detector  160  that detects spikes, in an FM demodulator output (MPX) signal provided by an FM receiver  104 A, with a spike filter  162 . The detector  160  also detects negative dips, in a received RF level signal, with a dip filter  164 . The outputs of the spike filter  162  and dip filter  164  are provided to threshold comparators  166 A and  166 B, respectively. Outputs of the threshold comparators  166 A and  166 B are provided to inputs of a decision logic block  168 , which determines when an antenna switch  107  should be switched to another antenna, i.e., a next one of the antennas A 1 , A 2 , A 3  and A 4 . In general, the logic  168  causes a next antenna to be selected when a spike is detected in the MPX signal coincident with a negative dip in the RF level signal, i.e., when the occurrence of spikes and dips are correlated. 
   With reference to  FIG. 2B , a graph  200  includes an exemplary RF level signal  202  and an exemplary FM demodulator output (MPX) signal  204 . As the RF signal level  202  becomes weaker (decreases in magnitude), the received SNR degrades and the spike and dip detection may be corrupted by noise. In this case, the system  180  may increase antenna switching erroneously, which tends to cause audible switching noise in an audio signal. As such, the system  180  may fail to settle on an appropriate antenna, i.e., ‘thrash’ between antennas or select an antenna that does not provide the best received signal. 
   With reference to  FIG. 3B , a receiver system  190  includes an FM receiver  104 A, whose output is coupled to an input of a slow distortion detector  170 . It should be appreciated that the detector  170  may be implemented in hardware or software. The detector  170  includes a filter  172 , which may be, for example, a bandpass filter that passes frequencies between about 60 kHz and 100 kHz. In general, the detector  170  provides an indication of signal quality for weak RF signals, long delay multipath or adjacent channel interference. When the RF signal is weak (or in the presence of adjacent channel interference), high-frequency components  302  appear in the FM baseband spectrum, as is shown in graph  300  of  FIG. 3A . 
   The slow distortion detector  170  averages energy of the components  302 , with a relatively long-time constant, to provide an indication of the received signal quality. The less high-frequency component energy present, the better the antenna signal quality. In general, the high-frequency components can be thought of as ultrasonic noise (USN). With reference again to  FIG. 3B , an output of the filter  172  may be rectified and low-pass filtered by DSP routines. According to one aspect of the present invention, higher noise levels (associated with weak signal reception) require longer time averaging for reliable statistics. This, in turn, reduces both ‘thrashing’ among antennas and poor antenna selection under weak signal conditions. 
   With reference to  FIG. 4 , a receiver system  400 , configured according to one embodiment of the present invention, exhibits robust operation over a full dynamic range of a received signal. In this embodiment, implemented, for example, in software, the digitized signal output of the ADC  108  represents the pre-detected FM signal. The FM demodulator  110  performs FM detection on this signal to recover the FM multiplex (MPX) signal. The ADC signal is also level detected (AM detected) by the RF level detector  112  to obtain the received signal strength, referred to as Level. Stereo decoding and de-emphasis of the MPX signal is performed by audio processor  114  to recover the left and right audio signals. Multipath disturbances are generally manifested as distortion of the MPX signal, and dynamic variations (AC component) of the Level signal that is otherwise essentially constant for FM. The distortion of the MPX signal results in a distortion of the recovered audio. Though the audio processor  114  may employ techniques to suppress or conceal audio distortion, the function of the antenna diversity system is to minimize distortion of the MPX signal, which correspondingly minimizes audio distortion. 
   A separate level average calculation block  403 , LevelA(n), is maintained for each antenna (n=1 to N) as a measure of its average received signal strength. The level average calculation block  403  averages the Level signal (using approximately a 6 mS time constant) to provide an update of LevelA(n) for the currently selected antenna. As a measure of the overall received signal strength, an overall average calculation block  402  is produced by averaging the LevelA(n) signals across all antennas to provide a LevelC signal. The LevelC signal is then used by a decay τ H  calculation block  410  to determine a decay time τ H  for an event trigger threshold and an average T A  calculation block  408  to provide an averaging time T A  for the quality measurement (see  FIGS. 5A and 5B ). 
   An event trigger is provided by an event trigger function block  414  and is based on an implementation of a fast distortion detector that correlates between RF level dip and an MPX signal spike, as shown in  FIG. 2B . An event threshold provided by a threshold function calculation block  412  is introduced to slow down antenna switching, to minimize audible disturbance (“thrashing”), when excessive events, which are more frequent with weak RF signals, occur. The event trigger initiates a search for a less distorted (better quality) antenna signal, which then becomes the new favored (reference) antenna. The threshold calculation is based on prior antenna event levels that triggered the search, which provides desensitization to reduce switching. This threshold decays at a rate provided by an average decay rate function block  410  that is determined from the combined average RF level, LevelC. A slower decay (longer desensitization) is used at weak signal levels where distortion events are expected to occur more frequently. 
   A quality measure function block  406  derives a received signal quality, based on the MPX and RF level signals. The quality measure may include signal strength (DC or low-frequency components), AM level (AC or high-frequency components) and ultrasonic noise (USN), i.e., energy beyond the known MPX bandwidth. A quick determination of signal quality is desirable with the switched antenna system to minimize the time possibly connected to a poor antenna. However, a sufficient averaging time is needed for a confident measurement. The quality measurement averaging time is based on the combined RF level, LevelC, provided by the overall average calculation function block  402 . It should be appreciated that lower RF levels require longer averaging time to obtain reliable quality statistics, due to more noise. 
   The decision logic function block  404  compares the quality statistics of the trial antenna (currently connected antenna) to that of the reference antenna (i.e., the reference antenna, before the search was trigged by the event trigger). The search terminates when the system  400  finds an antenna signal with better quality than the reference antenna. This selected antenna becomes the new reference antenna. By performing the quality comparison to accept a new antenna, an antenna is chosen which is less likely to encounter distortion events that would lead to another antenna search. 
   Switching between antennas creates some disturbance in the detected audio as a result of discontinuity between received antenna signals and from selecting an antenna with a poor signal quality. To minimize the audible disturbance, the decision logic block  404  selects trial antennas (other than the currently favored) in order of larger LevelA(n) signals recorded at the time of the triggering event. Since a larger signal level is more likely to provide better quality, a new favored antenna can be found with a minimum of antenna switching and less chance of trying a poor antenna. Reselecting the presently favored antenna, only after all other antennas have been tried, prevents exclusion of antennas from the search. 
   The system  400  utilizes short-term statistics (events) of the received signal, as detected by a fast distortion detector, to trigger a search for an antenna with a signal having better long-term statistics (quality) as detected by a slow distortion detector. To prevent frequent antenna searches from causing an audible disturbance, a threshold is introduced to desensitize the fast distortion detector. The threshold attacks on a triggering event value and then decays at a rate ( FIG. 5A ) that ranges from about 100 mS for signal strengths less than an RF level reference (e.g., a 5 μV level) to about 25 μS for strong signals. To maintain confidence in the long-term statistics, the averaging time used with the slow distortion detector ( FIG. 5B ) transitions from about 500 μS for signal strengths less than the RF level reference  506  to about 25 μS for strong signals. 
   With reference to  FIGS. 5A and 5B , these operation modes are further depicted in graphs  500  and  510 . The operation modes include a transition region  508  that is located between a strong signal mode  502  and a weak signal mode  504 . The RF level reference  506  (e.g., a 5 μV level) defines a point where the operation mode transitions between the strong signal mode  502  and the weak signal mode  504 . 
   With reference to  FIG. 6 , an exemplary antenna switching routine  600 , implemented according to one embodiment of the present invention, is depicted. In step  602 , the DSP  110 A (implementing routine  150 ) monitors a present reference antenna for fast distortion events (i.e., a dip in an RF level signal and an MPX signal spike). Next, in decision step  604 , the DSP  110 A determines whether an event trigger has occurred. If an event trigger has occurred, control transfers to step  606 . Otherwise, control returns from step  604  to step  602 . In step  606 , the DSP  110 A stores the quality statistics of the present antenna, to use as a reference. Next, in step  608 , the DSP  110 A searches for an antenna with better quality statistics. Then, in decision step  610 , the DSP  110 A determines whether an antenna with better quality has been located. If so, control transfers to step  612 , where the antenna with better signal quality becomes the reference antenna, at which point control transfers to step  602 . If an antenna with better signal quality is not located in step  610 , control returns to step  608 , where the DSP  110 A continues to search for an antenna with quality statistics that are better than the current reference antenna. 
   With reference to  FIG. 6B , a routine  600 A is depicted that provides a more detailed process flow for implementing various embodiments of the present invention. As is shown, steps  602 A and  630  receive an MPX signal value and steps  602 A,  630  and  620  receive an RF level signal value. The step  602 A represents a routine that monitors a present reference antenna for distortion events, which are indicated when an event threshold, provided in step  626 , is exceeded. In decision step  604 A, when an event trigger occurs, control transfers to step  606 A, where a quality measure of the present reference antenna is stored. Next, in step  608 A, a trial antenna is selected. 
   Then, in step  609 , the quality of a signal received by the trial antenna is compared to the quality of a signal received by the current reference antenna. Next, in decision step  610 A, it is determined whether the quality of the signal received by the trial antenna is better than that of the current reference antenna. If the quality of the signal provided by the trial antenna is better, the trial antenna becomes the new reference antenna in step  612 A and control returns to step  602 A. If the quality of the signal provided by the trial antenna is not better than that of the signal provided by the current reference antenna in step  610 A, control transfers to decision step  611 . In step  611 , it is determined whether the quality of the signal provided by the trial antenna is better than the quality of the signal provided by the current reference antenna. If so, control transfers to step  612 A, where the trial antenna becomes the new reference antenna. Otherwise, control transfers to step  608 A, where a next trial antenna is selected. 
   The quality of the signals received by the antennas is determined by a quality measure calculation in step  630 . The average level for a current antenna is determined by a calculation in step  620 . The average level is provided to another calculation in step  622 , which combines the average level of all antennas to provide a combined average of all antennas signal ‘LevelC’. The LevelC signal value is used in step  624  to calculate an event threshold decay time τ H , which is used in step  626  to calculate the event threshold. The LevelC signal is also used in step  628  to calculate an averaging time T A , which is used in step  630  to calculate a quality measure. 
   The above description is considered that of the preferred embodiments only. Modifications of the invention will occur to those skilled in the art and to those who make or use the invention. Therefore, it is understood that the embodiments shown in the drawings and described above are merely for illustrative purposes and not intended to limit the scope of the invention, which is defined by the following claims as interpreted according to the principles of patent law, including the doctrine of equivalents.