Abstract:
A D/U ratio is measured for desired and undesired signals in a wireless video transmission system at a shared channel frequency based on a received signal at a geographic location in proximity to regions within respective service areas for the desired and undesired signals. A video tuner demodulates the received signal to generate a baseband video signal. A leveling circuit normalizes the baseband signal. A video processor identifies horizontal sync pulses within the baseband signal, generates a sampled signal comprising the horizontal sync pulses, and removes components of the desired signal from the sampled signal to generate an undesired signal component. A D/U analyzer determines a Fourier transform having a plurality of bins in response to the undesired signal component, identifies at least one of the bins having a spectral peak corresponding to an undesired signal, and calculates the D/U ratio in response to a magnitude of the identified peak.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Not Applicable. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
   Not Applicable. 
   BACKGROUND OF THE INVENTION 
   The present invention relates in general to determining co-channel interference levels for wireless transmissions, and, more specifically, to measuring a desired/undesired ratio using portable equipment that does not require disruption of broadcasting or complicated test equipment or procedures. 
   In connection with obligations of licensees of wireless broadcasting services, it often becomes necessary to measure various signals at potential receiving locations in order to comply with requirements designed to minimize interference between different broadcasters. For example, in the Broadband Radio Service (BRS) authorized in a 2.5 GHz band by the Federal Communications Commission in the United States, a transition is being conducted wherein licensees of the Multipoint Distribution Service (MDS) are being reassigned to frequencies in the BRS. The BRS has been used to broadcast analog television (i.e., video) signals. Some new licensees in the BRS will operate on the same frequencies as existing licensees with the band. Licensees at the same frequencies will operate in respective service areas, but the potential for co-channel interference still exists and the FCC has specified certain interference requirements to be met. More specifically, the FCC requires that, as measured at a particular receiving site, the co-channel desired/undesired (D/U) ratio for a protected (i.e., previously existing) licensee must be at least the lesser of either 45 dB or the actual D/U ratio at the receiving site for the previously existing licensee prior to the transition minus 1.5 dB. 
   It is known that D/U ratio measurements can be done by first measuring the received power of a desired signal and then shutting off the desired transmitter and measuring the level of any undesired signal that may be present. This type of testing creates problems because it may be necessary to shut off the transmitter repeatedly or for noticeably long periods, resulting in interruption of programming to viewers being served by the BRS licensee (which may be a cable television provider, for example). In addition, the coordination required if multiple receive sites are being transitioned can be difficult and time consuming. 
   Difficulties arise when attempting to conduct measurements of desired and undesired power when both signals are present simultaneously. Typically, the undesired signal falls within a well-defined window relative to the desired signal. Considering the BRS service, the frequency difference between the two signals will be between 0 kHz and 11 kHz, and the D/U need only be measured down to 45 dB. As the frequency separation between the two transmitters approaches zero, extremely high resolving capability would be required in any measuring equipment. This situation can be improved by shifting the frequency of the desired transmitter to increase the separation, but even with frequency shifting the proximity of the desired and undesired carrier frequencies as well as the complex voltage of the active video signal makes the D/U measurement virtually impossible using standard test equipment. Relatively expensive equipment and/or highly skilled test operators have been required. 
   SUMMARY OF THE INVENTION 
   The present invention achieves an accurate and convenient system and method for determining D/U ratios without disrupting any broadcast signals and without requiring expensive test equipment or highly specialized training of test technicians. 
   In one aspect of the invention, a system is provided for measuring a D/U ratio for desired and undesired signals in a wireless video transmission system at a shared channel frequency based on a received signal at a geographic location in proximity to regions within respective service areas for the desired and undesired signals. A video tuner demodulates the received signal to generate a baseband video signal. A leveling circuit normalizes the baseband signal. A video processor identifies horizontal sync pulses within the baseband signal, generates a sampled signal comprising the horizontal sync pulses, and removes components of the desired signal from the sampled signal to generate an undesired signal component. A D/U analyzer determines a Fourier transform having a plurality of bins in response to the undesired signal component, identifies at least one of the bins having a spectral peak corresponding to an undesired signal, and calculates the D/U ratio in response to a magnitude of the identified peak. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram showing the spatial relationship between a receiving site and desired and undesired transmitters. 
       FIG. 2  is a waveform diagram showing an NTSC baseband television signal. 
       FIG. 3  shows horizontal sync pulses in the presence of an undesired signal. 
       FIG. 4  shows a sync-amplitude signal obtained by sampling and holding a sync level signal. 
       FIG. 5  is a flowchart showing one preferred method of the present invention. 
       FIG. 6  is a frequency-power spectrum of a sampled sync signal showing the presence of interfering undesired signals. 
       FIG. 7  is a block diagram showing main functionality of a test system of the present invention. 
       FIG. 8  is a block diagram showing one preferred hardware embodiment of the system of  FIG. 7 . 
       FIG. 9  is a schematic diagram of a level detector. 
       FIG. 10  is a schematic diagram of an RF attenuator for working together with the level detector in order to sustain an RF level at a predetermined magnitude. 
       FIG. 11  is a schematic diagram showing a video processor of the invention. 
       FIG. 12  is a flowchart showing operation of the testing system in greater detail. 
       FIG. 13  is a block diagram showing the main software components of the test system. 
       FIG. 14  is a screen shot of a user interface for initiating a test measurement of the present invention. 
       FIG. 15  is a screen shot from the user interface during capture of reception data. 
       FIG. 16  is a screen shot showing test results. 
   

   DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
   Referring now to the drawings,  FIG. 1  shows a transmitter  10  with a corresponding service area  11  having a receive site  12 . RF broadcast signals  13  propagate from transmitter  10  to receiving site  12 . Undesired transmitters  14  and  15  in other service areas radiate broadcast signals  16  and  17 , respectively, that also reach receiving site  12 . The present invention measures the D/U ratio corresponding to the various broadcasts to ensure compliance with FCC regulatory levels and to facilitate adjustments by the operators of the transmitters to achieve compliance. 
   A receiver  20  located at receiving site  12  is connected to a video distribution system  24 , such as a cable headend. Receiver  20  includes an antenna  21 , a downconverter  22 , and a demodulator  23 . A test system  25  of the present invention is connected to the output of downconverter  22 . 
   In order to be able to measure the much lower level undesired signals in the presence of the strong desired signals, the present invention takes measurements at a time during which the voltage of the desired video signal is substantially constant so that the measured variation in voltage is caused by the presence of undesired signals. In particular, measurements are made during horizontal sync pulses. During the horizontal sync pulses, the signal carrier is at its highest level. In addition, the sync pulses are evenly distributed over time, making them well suited for use as a measurement time because it is easier to detect variations in the horizontal sync pulses resulting from an interfering undesired signal. As shown in  FIG. 2 , regular horizontal blanking intervals  27  include respective horizontal sync pulses  28 . Each horizontal sync pulse lasts 4.7 μS, and consecutive horizontal sync pulses repeat at a period of 63.5 μS (i.e., they repeat at a frequency of 15.734 kHz). 
   As broadcast, each horizontal sync pulse is a square pulse having a predetermined, constant voltage level. As shown in  FIG. 3 , the received horizontal sync pulses deviate from the ideal pulse shape due to interfering transmissions from undesired transmitters. Thus, the amplitude or received signal strength corresponding to each horizontal sync pulse has deviations  29  superimposed on the ideal pulse shape as a result of energy from interfering sources having an overlapping frequency spectra. 
   The horizontal sync pulses contain no program material, occur every 63.5 μS at the highest RF signal level, and as seen at the receiver have a substantially constant received signal strength. Variations in signal strength seen at the times of the horizontal sync pulse are thus due to contributions by undesired signals. By limiting the measurement to coincide with the horizontal sync pulses, the invention can remove the active video portion of the signal so that the measured variations are due to undesired signals which are automatically separated from the desired sync pulses. By constructing a sampled waveform containing only data from horizontal sync pulses, a frequency analysis can be employed to distinguish between desired and undesired signal contributions. Since the expected signal strength of the desired signal alone is substantially constant, the desired signal contribution shows up as the portion of the frequency spectra at around 0 Hz (i.e., DC). Other signal contributions correspond either to undesired broadcasts on the same shared channel frequency or noise. 
     FIG. 4  shows a preferred embodiment for constructing a sampled sync signal containing only signal components present during the horizontal sync pulses. Thus, a sampled sync signal  30  has a magnitude determined by sampling an average signal strength level of an individual horizontal sync pulse at  31  and holding that sample value at  32  until the occurrence of the next horizontal sync pulse. The sample interval  31  may last for about 3 μS during a center portion of a horizontal sync pulse, for example. Hold period  32  then comprises the remaining 60.5 μS. Thus, sampled sync signal  30  represents the varying energy content derived from the undesired signals. 
     FIG. 5  shows an overall method of the present invention wherein test equipment is set up at a test location in step  35 . Typically, the test system equipment is installed in association with a fixed receiver within the area being served by the desired signal and outside the service area for the undesired signal. In step  36 , the carrier frequency of the desired transmitter is offset by a predetermined offset frequency from the shared channel frequency assigned by the FCC. As explained below, the use of a frequency offset allows energy contributions from undesired signals to be distinguished from noise or other non-video sources. 
   In step  37 , the test equipment is tuned in order to receive the desired broadcast signal. The predetermined offset frequency introduced for the desired transmitter is sufficiently small (e.g., about 2-3 kHz) that tuning to and receiving the desired broadcast signal is not significantly affected. 
   In order to reliably compare the desired signal level to an undesired signal level, the received signal seen by the reception antenna is normalized to a predetermined level in step  38 . The tuner demodulates the desired signal and samples of the normalized demodulated signal are collected during the horizontal sync pulses in step  39 . 
   In step  40 , a fast Fourier transform (FFT) is calculated for each sampled sync signal derived from a respective one of a plurality of measuring periods. A measuring period lasting about 200 mS, for example, is input to a spectrum analyzer to calculate all the frequency components of each FFT. In step  41 , peaks are identified in any particular FFT that correspond to an undesired broadcast signal. Once the frequency of an undesired signal is identified according to the peaks, then the maximum level of the FFT at the identified peak frequencies is determined over the plurality of measuring periods. Due to the phase relationship between the horizontal sync pulses of the desired signal and the frequency content of the interfering portion of an undesired signal, the energy of the interfering signal oscillates between constructive interference and destructive interference. By identifying a maximum FFT level, the maximum constructive interference can be found which corresponds to the true D/U ratio. Typically, the plurality of measuring periods covers a time span of about 35 seconds to ensure that the appropriate maximum has occurred. In step  43 , the D/U ratio is calculated for each undesired signal&#39;s carrier frequency identified by a corresponding peak. If the worst D/U ratio is below the regulatory limit, then calculation of D/U ratios for other interfering undesired signals would be unnecessary. 
     FIG. 6  shows a calculated Fourier transform corresponding to the frequency spectra for the sampled sync signal over one measuring period. The carrier frequency of the desired transmitter is offset so that the carrier frequency of any undesired transmitters on the shared channel frequency will be seen in this frequency spectrum as being offset from zero Hz. Due to variations in the precision of the frequency reference used by various transmitters, some small offset may normally be seen between the desired and any undesired frequency signals. By deliberately adding an additional offset of around 5 kHz, the undesired frequencies are easier to identify and can be positively identified as an interfering signal in the following manner. 
   Due to the presence of the desired signal, a frequency peak is seen at 15.734 kHz corresponding to the repetition frequency of the horizontal sync pulses in the desired signal. If an undesired signal is present, then the energy of the horizontal sync pulses contained in the undesired signals are likewise shown as spectral peaks in the Fourier transform. As a result of folding during demodulation, however, spectral peaks corresponding to the horizontal sync pulses of any particular undesired signals are shown at the corresponding frequency offset both above and below the spectral peak at 15.7 kHz. Thus, a frequency-power peak  45  appearing at a difference frequency equal to DELTA 1  corresponds with a symmetrical peak  46  at a difference frequency equal to negative DELTA 1  from 15.734 kHz. Since spectral peaks  45  and  46  are symmetrically spaced above and below the frequency of the horizontal sync pulses, it can be concluded that frequency DELTA 1  identifies the frequency of a spectral peak  47  of the corresponding undesired signal carrier. Likewise, a second undesired signal appears at a frequency DELTA 2  as confirmed by symmetrical placed spectral peaks corresponding to the horizontal sync pulses of the second undesired signal. Another spectral peak  48  in the vicinity of the peak at 15.7 kHz is found not to correspond to an undesired signal because it has no matching spectral peak symmetrically placed on the opposite side of 15.7 kHz. Therefore, it can be concluded that spectral peak  48  is due to noise or some other radiated admission source which does not need to be considered in order to determine the D/U ratio. 
     FIG. 7  shows a portable test system based on a laptop PC  50 . An auxiliary battery  51  and charger  52  are provided for allowing prolonged periods of use without continuous connection to an active power source. A DC power distribution and USB hub  53  is coupled to laptop  50 , a DDU measurement block  54 , and an RF measurement block  55 . RF measurement block  55  receives the VHF/UHF RF input also seen by the receiver installed at the test site. Digital attenuation, logarithmic detection, and level measurement of horizontal sync pulses are performed in RF measurement block  55 . DDU measurement block  54  performs tuning, demodulation, and signal processing and sampling. 
     FIG. 8  shows a preferred embodiment of the test system in greater detail. A USB video tuner and capture card  57  is controlled by laptop  50  and provides a demodulated video/audio signal to laptop  50  through a USB hub  58 . The demodulated signal may also be stored by the capture feature in card  57  under control of laptop  50 . The demodulated signal is also provided along with an intermediate frequency signal to a custom circuit block  60  which generates a sampled sync signal as will be described below. The sampled sync signal may be stored in a D/U waveform capture card  61 . A USB A/D converter  62  is coupled to custom circuit block  60  and to USB hub  58  for providing conversion of a level detection signal as will be described below. Custom circuit block  60  receives an RF signal from the downconverter which is part of the normal installation at the receive site. 
   In order to automatically calibrate the sampled sync signal according to the level of the desired signal, the IF signal generated in USB video tuner and capture card  57  is provided to a level detector circuit  65  within custom circuit block  60  as shown in  FIG. 9 . A lowpass filter  66  receives the IF signal from an input connector  67  and couples the filtered IF signal to an input of a logarithmic amplifier  68  which comprises an integrated circuit AD8310 available from Analog Devices, Inc. of Norwood, Mass. Logarithmic amplifier  68  is connected to perform a signal level determination in accordance with published configurations of the AD8310. When a switch  69  is configured to supply a high DC voltage level to an enable input of the AD8310, an output voltage level is provided at an output terminal  70  which is proportional to the received signal strength of the input IF signal. The received signal strength signal is provided to A/D converter  62  and the digitized IF level is provided to an attenuator circuit  72  which is part of the custom circuit block  60  as shown in  FIG. 10 . In the embodiment of  FIG. 9 , level detector circuit  65  is implemented using an evaluation board with support components as recommended by Analog Devices. 
   Attenuator circuit  72  in  FIG. 10  can provide two-step attenuation using an analog fixed attenuator  73 , and a digitally controlled variable attenuator  74 . In particular, variable attenuator  74  is comprised of a digital step attenuator integrated circuit DAT-3175-PP available from Mini-Circuits Laboratory of Brooklyn, N.Y. Fixed attenuator  73  is a known commercial device. The digitized IF level from A/D converter  62  is provided at a connector  75 . When a most significant bit  76  of the digitized IF signal is a 1, then fixed analog attenuator  73  is switched on in order to introduce a predetermined attenuation such as 20 dB. Other bits of the parallel control word from the A/D converter  62  are coupled to respective inputs of variable attenuator  74  through respective buffer circuits. The support circuits for integrated circuit DAT-3175-PP are as shown for evaluation board TB-337 also available from Mini-Circuits. An attenuated RF signal is provided at output connector  79  such that the RF signal has been attenuated by the amount necessary to maintain the level of the IF signal at a predetermined level. 
   The properly attenuated RF signal is passed to the USB video tuner and capture card  57  which provides a baseband video signal after demodulation to a video processing circuit  80  as shown in  FIG. 11 . Video processing circuit  80  is included in custom circuit block  60  and is built around a Sync Separator integrated circuit EL4583 available from Intersil Americas Inc. of Milpitas, Calif. The baseband video signal is applied to pin 4 of the EL4583 and after filtering is applied to a sample and hold circuit  81 . Pin 9 is a level output which is an analog voltage equal to twice the horizontal sync pulse amplitude of the video input signal applied to pin 4. In a normal video receiver, the level output of pin 9 would be used to provide an indication of signal strength. In the present invention, variations in the level output signal are instead used to characterize signal contributions from undesired signals. Thus, the level output signal from pin 9 is applied to D/U waveform capture card  61  for storage and also for use in an analysis performed by software programs loaded on laptop PC  50 . More particularly, the level output at pin 9 provides the sampled sync signal. 
   A preferred method for identifying carrier frequencies of undesired broadcast signals and for characterizing the D/U ratio is shown in  FIG. 12 . In step  84 , the software components within laptop PC  50  and the states of the hardware elements are all initialized. In step  85 , a control program in laptop PC  50  sets the video tuner to the desired shared channel frequency to be tested and then activates the level detecting circuit to measure the IF level. In step  86 , the measured IF level is used to set the attenuation of the RF signal in order to obtain a predetermined target IF level which results in calibration of the sampled sync signal so that the D/U ratio can be directly determined from the sampled sync signal level. 
   In step  87 , horizontal sync pulses of the desired signal are detected using the sync separator in the video processing circuit. The amplitude of each horizontal sync pulse is sampled and held until the occurrence of the next horizontal sync pulse in order to construct a sampled sync signal that varies in accordance with the amplitude of each horizontal sync pulse. An FFT of the horizontal sync pulse level from the sampled sync signal is computed in step  89 . Based on this first FFT, a search is conducted for the spectral peaks within the FFT in order to identify the presence of undesired broadcast signals that may interfere with the desired signal. A spectral peak may be defined as a frequency bin or bins in the Fourier transform have an amplitude greater than surrounding bins as is known in the art. 
   In step  90 , a search is conducted for peaks within the FFT between 15.734 kHz (the peak corresponding to the horizontal sync frequency of the desired signal) and an upper limit corresponding to a worse case separation of the desired signal and other undesired broadcast signals. The worst case frequency is determined according to the largest frequency error that may be inadvertently present in the transmission of an undesired signal from its assigned frequency plus the predetermined offset frequency deliberately introduced in the transmission of the desired signal for purposes of this test. A typical upper bound may be about 22 kHz, for example. 
   For each spectral peak found in the search, a check is made to determine whether there is a symmetrical peak at the negative DELTA frequency of such peak in step  91 . If no such corresponding symmetrical peaks are found for any peak about 15.7 kHz, then no undesired signals are detected and the test system indicates a failure at step  92 . 
   In step  93 , for each peak wherein a corresponding peak is found at the negative DELTA frequency, such DELTA frequency is added as a detected undesired frequency in a table. With the carrier frequencies of the undesired signals having been identified, a complete data collection is performed in step  94  sufficient to allow characterization of the D/U ratio. Thus, the method collects and stores additional sample and hold data for a plurality of measuring periods. In a preferred embodiment, each measuring period lasts about 200 mS and the sum of measuring period spans about 35 seconds (i.e., including about 175 measuring periods). In step  95 , Fourier transforms are computed for each respective measuring period. FFT values at the undesired signal carrier frequencies shown in the table are stored. Due to the shifting phase relationship between the desired and undesired broadcast RF signals, interference between the desired and undesired signals varies between constructive interference and destructive interference. It is necessary to identify the occurrence of constructive interference in order to accurately determine the D/U ratio. First, however, the data from the plurality of measuring periods is filtered for spurious data and checked for validity in step  96 . Then the maximum level for each table frequency is found in step  97 . Based on the maximum levels, the D/U ratio is calculated in step  98  and the results are displayed. 
   The software for the laptop PC of the present invention is comprised of two main blocks, namely a user interface and automation block and a D/U processing block. The user interface is designed to lead a test technician through each of the steps required to make D/U signal level measurements. In connection with transitioning transmitters getting reassigned channel allocations in the BRS service, the user interface may be adapted to collecting both pre-transition and post-transition measurements. The user interface deals with channel selection, recording and storing of video samples, acquiring and storing data from the D/U module, evaluation of pre- and post-transition measurements for conformity with FCC requirements, and other miscellaneous tasks. 
   The D/U processing software operates as previously described in connection with the flowcharts. All of the applications for the laptop PC are based on ActiveX and DLL, taking advantage of the .Net framework and DirectX.  FIG. 13  shows main software blocks based within the .Net framework together with various standard libraries of functions  101 - 103 . An RF module  104  is a customized software module for supervising operation of the hardware components as previously described. The tuning/recording module  105  provides the user interface and supervision for selecting the appropriate channel to be characterized. An FFT and display module  106  controls the processing of sampled sync signal data as described in the previous flowcharts. Preferably, a spectrum analyzer within block  106  is comprised of an audio spectrum analyzer of a type that comprises commercially available software. 
     FIG. 14  shows a screen shot during set up and initialization of the test equipment after connection to a downconverter at a site being tested. An instruction window  110  assists a technician in conducting the necessary operations to obtain a D/U measurement. A window  111  allows the technician to enter identifying information of a particular site and to select a channel frequency for testing, as well as identifying pre- and post-transition measurements. A window  112  is used to indicate a path for storing test data and captured video on the laptop PC. 
     FIG. 15  shows a screen shot during data capture of the present invention. A message window  113  informs the technician of parameters and events during the testing. A video window  114  displays the current desired signal being detected in order to allow the technician to confirm the identity of the desired signal being measured and to show the overall video quality at the time of testing. 
     FIG. 16  is a screen shot showing a results screen. The screen layout is similar to that shown by a conventional audio spectrum analyzer which may be conveniently used in the present invention since the frequency spectrum of the sampled sync signal falls within the audio frequency range. An FFT frequency-power spectrum  120  is plotted as confirmation to the technician that acceptable data has been gathered. For example, visual inspection can confirm the presence of a spectral peak at 15.734 kHz corresponding to the horizontal sync frequency of the desired signal. Based on the data represented in  FIG. 16 , a DELTA frequency of 2.5 kHz has been identified wherein a D/U ratio of −32 DB is calculated, as shown in window  121 . As seen in plot  120 , the spectral peak at 2.5 kHz is best identified by the DELTA frequency method rather than by simple visual inspection of spectral peaks. The method of the present invention is sufficiently simple and repeatable to be implemented by software that does not require significant expertise of the test technician in order to operate successfully.