Abstract:
A method of receiving an FM digital audio broadcasting signal including a first plurality of subcarriers in an upper sideband of a radio channel and a second plurality of subcarriers in a lower sideband of the radio channel comprises the steps of mixing the digital audio broadcasting signal with a local oscillator signal to produce an intermediate frequency signal, passing the intermediate frequency signal through a bandpass filter to produce a filtered signal, determining if one of the upper and lower sidebands of the digital audio broadcasting signal is corrupted, and adjusting the local frequency oscillator signal to change the frequency of the intermediate frequency signal such that the bandpass filter removes the subcarriers in the upper or lower sideband that has been corrupted. A receiver that processes a digital audio broadcasting signal in accordance with the method is also provided.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to methods and apparatus for receiving a Digital Audio Broadcasting (DAB) signal, and more particularly, to such methods and apparatus that mitigate adjacent channel interference in the DAB signal. 
     Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM DAB signals can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog AM or FM signal, or in an all-digital format without an analog signal. In-band-on-channel (IBOC) DAB systems require no new spectral allocations because each DAB signal is simultaneously transmitted within the spectral mask of an existing AM or FM channel allocation. IBOC systems promote economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners. Several IBOC DAB approaches have been suggested. 
     FM DAB systems have been the subject of several United States patents including U.S. Pat. Nos. 6,259,893; 6,178,317; 6,108,810; 5,949,796; 5,465,396; 5,315,583; 5,278,844 and 5,278,826. One FM IBOC DAB system uses a composite signal that includes orthogonal frequency division multiplexed (OFDM) subcarriers in the region from about 129 kHz to 199 kHz away from the FM center frequency, both above and below the spectrum occupied by an analog modulated host FM carrier. Some IBOC options (e.g., the All-Digital option) permit subcarriers starting as close as 100 kHz away from the center frequency. 
     The digital portion of the DAB signal is subject to interference, for example, by first-adjacent FM signals or by host signals in Hybrid IBOC DAB systems. The FM Digital Audio Broadcasting signal is designed to tolerate interference in a number of ways. Most significantly, the digital information is transmitted on both lower and upper sidebands. The digital sidebands extend out to nearly 200 kHz from the center carrier frequency. Therefore an intermediate frequency (IF) filter in a typical FM receiver must have a flat bandwidth of at least ±400 kHz. One proposed First Adjacent Canceller (FAC) technique requires an approximately flat response out to about ±275 kHz from the center for effective suppression of a first adjacent signal. This would normally require an IF filter with a flat bandwidth of at least 550 kHz. A first adjacent cancellation technique is disclosed in U.S. Pat. No. 6,259,893, which is hereby incorporated by reference. 
     DAB systems utilize a specially designed forward error correction (FEC) code that spreads the digital information over both the upper and lower sidebands. The digital information can be retrieved from either sideband. However, if both sidebands are received, the codes from both the upper and lower sidebands can be combined to provide an improved output signal. 
     FM stations are geographically placed such that the nominal received power of an undesired adjacent channel is at least 6 dB below the desired station&#39;s power at the edge of its protected contour or coverage area. Then the D/U (desired to undesired power ratio in dB) is at least 6 dB. There are exceptions to this rule, however, and listeners expect coverage beyond the protected contour increasing the probability of higher interference levels. 
     At a station&#39;s edge of coverage, a second adjacent&#39;s nominal power can be significantly greater (e.g. 40 dB) than the host&#39;s nominal power within the desired coverage area. This can present a problem for the IF portion of the receiver where dynamic range is limited. The IF is where the IBOC DAB signal is converted from analog to digital. The sample rate and number of effective bits in the analog-to-digital (A/D) converter limit the dynamic range of the IF section. 
     A B-bit A/D converter has a theoretical instantaneous dynamic range of about (1.76+6*B) dB (maximum sinewave to noise ratio in its Nyquist bandwidth). For this discussion, assume that a practical AID converter has a dynamic range of 6 dB per bit of resolution. Oversampling of the signal of interest can improve the effective dynamic range by spreading the quantization noise over the larger Nyquist bandwidth of the A/D. The effect is to increase the dynamic range by one bit for each quadrupling of the sample rate. On the other hand, some headroom must be allowed in the A/D sampling to control clipping to an acceptable level. 
     As a practical IBOC DAB example, assume an 8-bit AID with 48 dB instantaneous dynamic range in its Nyquist bandwidth. Further assume a headroom of 12 dB peak-to-average ratio in the AGC, and another 10 dB of margin for fading and AGC “slop”. An oversampling ratio of 256 can increase the effective dynamic range in the signal bandwidth by 12 dB (in effect canceling the A/D headroom loss). Then the effective IF dynamic range in the IBOC signal bandwidth would be about 48 dB minus the 10 dB margin for fading, resulting in about 38 dB. If an instantaneous signal dynamic range of 28 dB in the signal bandwidth is required to detect the IBOC DAB signal without fading, then there is a margin of about 10 dB in the IF and A/D. This margin could be consumed by a large second adjacent signal entering the analog IF filter prior to A/D conversion. 
     It is a reasonable assumption that a good selective IF filter would suppress the second adjacent analog FM signal at 400 kHz away from FM center frequencies, but its IBOC sideband at 200 to 270 kHz from center would pass through the filter. If a second adjacent interferer is more than about +20 dB, then the dynamic range requirement of the A/D is increased by the excess second adjacent signal level above 20 dB. For example, if the second adjacent interferer is +50 dB, then the increased requirement above the minimum dynamic range is 30 dB, or about 5 more bits of A/D resolution above the minimum. However, there are ways to deal with the dynamic range issue other than the brute force method of increasing the bits in the A/D. 
     When a second adjacent interferer is +30 dB higher than the signal of interest, then the out-of-band emissions from it will likely corrupt the digital sideband on that side. Since corruption at that level will render that sideband useless, it may be preferable to filter out that sideband prior to A/D conversion. Filtering out the large second adjacent signal will restore the effective dynamic range eliminating the need for more bits of resolution. One way to approach this problem is to provide a set of selectable filters having different passbands for IF filtering prior to the A/D/converter. 
     Although the use of multiple filters may provide a good technical solution, the cost of the receiver is increased by the additional filters and switches. Also the accuracy of the filters may have an effect on cost. 
     There is a need for an improved method of minimizing the effects of first adjacent interference in IBOC DAB signals. 
     SUMMARY OF THE INVENTION 
     This invention provides a method of receiving an FM digital audio broadcasting signal including a first plurality of subcarriers in an upper sideband of a radio channel and a second plurality of subcarriers in a lower sideband of the radio channel. The method comprises the steps of mixing the digital audio broadcasting signal with a local oscillator signal to produce an intermediate frequency signal, passing the intermediate frequency signal through a bandpass filter to produce a filtered signal, determining if one of the upper and lower sidebands of the digital audio broadcasting signal is corrupted, and adjusting the local frequency oscillator signal to change the frequency of the intermediate frequency signal such that the bandpass filter removes the subcarriers in the upper or lower sideband that has been corrupted. 
     The invention also encompasses a receiver for receiving an FM digital audio broadcasting signal including a first plurality of subcarriers in an upper sideband of a radio channel and a second plurality of subcarriers in a lower sideband of the radio channel. The receiver includes a mixer for mixing the digital audio broadcasting signal with a local oscillator signal to produce an intermediate frequency signal, a filter for filtering the go intermediate frequency signal to produce a filtered signal, means for determining if one of the upper and lower sidebands of the digital audio broadcasting signal is corrupted, means for adjusting the local frequency oscillator signal to change the frequency of the intermediate frequency signal such that the bandpass filter removes the subcarriers in the upper or lower sideband that has been corrupted, and means for processing the filtered signal to produce an output signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a hybrid FM DAB spectrum; 
         FIG. 2  is a schematic representation of an interference scenario showing a first adjacent signal at −6 dB relative to the signal of interest; 
         FIG. 3  is a schematic representation of an interference scenario with a second adjacent signal at +20 dB relative to the signal of interest; 
         FIG. 4  is a functional block diagram of a receiver constructed in accordance with the invention; and 
         FIG. 5  is a functional block diagram of the frequency offset control of the receiver of  FIG. 4 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings,  FIG. 1  is a schematic representation of the frequency allocations (spectral placement) and relative power spectral density of the signal components for a hybrid FM IBOC DAB signal  10 . The hybrid format includes the conventional FM stereo analog signal  12  having a power spectral density represented by the triangular shape  14  positioned in a center, or central, frequency band  16  portion of the channel. The Power Spectral Density (PSD) of a typical analog FM broadcast signal is nearly triangular with a slope of about −0.35 dB/kHz from the center frequency. A plurality of digitally modulated evenly spaced subcarriers are positioned on either side of the analog FM signal, in an upper sideband  18  and a lower sideband  20 , and are transmitted concurrently with the analog FM signal. All of the carriers are transmitted at a power level that falls within the United States Federal Communications Commission channel mask  22 . 
     In one example of a hybrid FM IBOC modulation format,  95  evenly spaced orthogonal frequency division multiplexed (OFDM) digitally modulated subcarriers are placed on each side of the host analog FM signal occupying the spectrum from about 129 kHz through 198 kHz away from the host FM center frequency as illustrated by the upper sideband  18  and the lower sideband  20  in  FIG. 1 . In the hybrid system, the total DAB power in the OFDM digitally modulated subcarriers in each sideband is set to about −25 dB relative to its host analog FM power. 
     Signals from an adjacent FM channel (i.e. the first adjacent FM signals), if present, would be centered at a spacing of 200 kHz from the center of the channel of interest.  FIG. 2  shows a spectral plot of a hybrid DAB signal  10  with an upper first adjacent interferer  24  centered 200 kHz above the center of signal  10 , and having an analog modulated signal  26  and a plurality of digitally modulated subcarriers in sidebands  28  and  30 , that are at a level of about −6 dB relative to the signal of interest (the digitally modulated subcarriers of signal  10 ).  FIG. 2  shows that the DAB upper sideband  18  is corrupted by the analog modulated signal in the first adjacent interferer. 
       FIG. 3  is a schematic representation of an interference scenario with a second adjacent signal  32  centered 400 kHz above the center of the signal of interest, and at +20 dB with respect to the signal of interest. The second adjacent signal includes an analog modulated signal  34  and a plurality of digitally modulated subcarriers in a lower sideband  36 . The upper sideband of the second adjacent signal is not shown in this Figure. 
       FIG. 4  is a block diagram of a receiver  100  constructed in accordance with the invention. Antenna  102  serves as a means for receiving an in-band on-channel digital audio broadcast signal including a signal of interest in the form of an analog modulated FM carrier and a plurality of OFDM digitally modulated subcarriers located in upper and lower sidebands with respect to the analog modulated FM carrier. The receiver includes a front end circuit  104  that is constructed in accordance with well known techniques. The signal on line  106  from the front end is mixed in mixer  108  with a signal on line  110  from a local oscillator  112  to produce an intermediate frequency (IF) signal on line  114 . The IF signal passes through a bandpass filter  116  and is then digitized by an analog-to-digital converter  118 . A digital down converter  120  produces in-phase and quadrature baseband components of the composite signal. The composite signal is then separated by FM isolation filters  122  into an analog FM component on line  124  and upper and lower DAB sideband components on lines  126  and  128 . The analog FM stereo signal is digitally demodulated and demultiplexed as illustrated in block  130  to produce a sampled stereo audio signal on line  132 . 
     The upper and lower DAB sidebands are initially processed separately after the isolation filters. The baseband upper sideband DAB signal on line  126  and the baseband lower sideband DAB signal on line  128  are separately processed by a first adjacent canceller as illustrated by blocks  134  and  136 , to reduce the effect of first adjacent interference. The resulting signals on lines  138  and  140  are demodulated as illustrated in blocks  142  and  144 . 
     After demodulation, the upper and lower sidebands are combined for subsequent processing and deframed in deframer  146 . Next the DAB signal is FEC decoded and de-interleaved as illustrated by block  148 . An audio decoder  150  recovers the audio signal. The audio signal on line  152  is then delayed as shown in block  154  so that the DAB stereo signal on line  156  is synchronized with the sampled analog FM stereo signal on line  132 . Then the DAB stereo signal and the sampled analog FM stereo signal are blended as shown in block  158 , to produce a blended audio signal on line  160 . 
     To remove adjacent channel interference, receivers constructed in accordance with this invention include a frequency offset control  162 . The frequency offset control estimates the relative powers in the upper and lower DAB sidebands, and then makes a decision as to whether to invoke a frequency offset in the tunable local oscillator. The offset, if any, is applied to the tunable local oscillator as shown by line  164  and the negative of this offset is applied to the digital down converter as shown by line  166 . 
       FIG. 5  shows an example of the implementation of the frequency offset control  162 . The input signals on lines  126  and  128  are the upper and lower DAB sidebands out of the isolation filters  122 . 
     The frequency offset control uses a squaring and lowpass filtering (LPF) technique to measure the relative powers of the inputs. The upper DAB sideband signal on line  126  is squared as illustrated in block  168  and low pass filtered as illustrated in block  170  to produce a filtered upper sideband signal U on line  172 . The lower DAB sideband signal on line  128  is squared as illustrated in block  174  and low pass filtered as illustrated in block  176  to produce a filtered upper sideband signal L on line  178 . The low pass filters could be simple lossy integrators with a time constant on the order of one second. 
     The frequency offset Δf is then determined by comparing the filtered upper and lower sideband signal power as illustrated in block  180 . For example, if the filtered upper sideband signal power is greater than 1000 times the filtered lower sideband signal power, the frequency offset is set to 100 kHz. If the filtered lower sideband signal power is greater than 1000 times the filtered upper sideband signal power, the frequency offset is set to −100 kHz. If the filtered upper sideband signal power is less than 1000 times the filtered lower sideband signal power, and the filtered lower sideband signal power is less than 1000 times the filtered upper sideband signal power, then frequency offset is set to zero. The method for establishing the value of Δf involves thresholds and hysteresis as shown in the example of  FIG. 5 . The hysteresis used in setting thresholds prevents frequent changes in the adjustments of Δf. 
     One implementation of the invention applies a frequency offset to the local oscillator, thereby changing the intermediate frequency signal such that the skirt of the IF filter  116  suppresses the second adjacent on the appropriate sideband. Although this effectively places the second adjacent interferer in the stop band of the IF filter, the resulting frequency offset for subsequent signal processing may be undesirable. The frequency offset can be removed by offsetting the detuning in the digital frequency tracking after the down conversion process by the same (negative) frequency offset. A digital numerically controlled oscillator is already present in the previous receiver designs, so no additional hardware cost would be incurred in the receiver. Although the offset IF tuning allows a wider bandwidth on the “good” sideband, it is unlikely this will result in a dynamic range problem. This is because the likelihood of very strong second adjacent signals on both sides of the signal of interest simultaneously is very small. The IBOC DAB receiver would detect the presence of a large second adjacent interferer, and then provide the appropriate IF filtering. 
     The presence of a large interferer can be detected by measuring the level of the desired signal. If the level is significantly below the level expected to be set by the automatic gain control, then a large interferer is likely. It is very unlikely that the large interferer is a first adjacent signal due to intentional geographic protection. A very large first adjacent signal (−20 dB D/U or worse) would be unrecoverable anyway. Third adjacent interferers would be out of the filter passband. So the large interferer is assumed to be a second adjacent. A detection algorithm can detect the presence of a large power of the second adjacent&#39;s digital sideband. This detection algorithm would also determine whether the large interferer is an upper or lower second adjacent signal. A frequency offset control signal is created after appropriate filtering and possibly hysteresis on the relative interference power to prevent false detection. This control signal instructs the local oscillator  112  to detune by 100 kHz in the appropriate direction while the digital local oscillator in block  120  is offset by 100 kHz in the opposite direction such that the resulting digital signal output from the digital down converter still appears at baseband. 
     While the present invention has been described in terms of what is believed at present to be the preferred embodiments thereof, it will be appreciated by those skilled in the art that various modifications to the disclosed embodiments may be made without departing from the scope of the invention as set forth in the appended claims.