Abstract:
In a system for optimizing television reception by a television receiver receiving a diversity of input signals from a plurality of antennas, one of a plurality of combiners and television tuners selects the strongest combination of input signals for viewing and another scans the various combinations of input signals searching for a combination significantly stronger than the combination being viewed. A plurality of multiplexers selects the output signal of one combiner and transmits the selected signal to one of the tuners. A signal evaluation module evaluates the strength of the combinations of input signals and compares the strength of each combination of input signals to the strength of the viewing signal, looking for a superior signal. A control processor controls the combination of input signals selected by the combiners and the combiner output signal selected by the multiplexers. The control processor converts any superior signal found into the signal being viewed, at which time the process starts over and repeats.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATION 
     This patent application is related in subject matter to U.S. patent application Ser. No. 09/201,376 filed Nov. 30, 1998, by M. L. Grabb, N. AI-Dhahir, R. L. Frey, J. E. Hershey, J. A. F. Ross, and N. A. VanStralen, for “System and Method for Mitigating Multipath Effects in Television Systems”, which is assigned to the instant assignee. The disclosure of application Ser. No. 09/201,376 is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH &amp; DEVELOPMENT 
     The U.S. Government may have certain rights in this invention pursuant to the NIST Contract Number 70NANB8H4078, awarded by the National Institute of Standards and Technology. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     This invention relates to digital television (DTV) and, more particularly, to optimization of television reception by selecting among, or combining, input signals from multiple antennas for mitigation of multipath. 
     2. Background Art 
     High definition television (HDTV) is an emerging technology that is capable of providing service either in an analog or digital format. In the United States, research on HDTV has focused on digital, rather than analog, technology. While digital HDTV is not currently available, Japanese companies have developed an HDTV system based on analog technology (known as Hivision) that has been in use since 1991. Because of the potential advantages of digital HDTV and many technical problems shared by both types of systems, research in digital HDTV has also been active in Japan. See, for example, David K. Kahaner in “HDTV Research in Japan”,  IEEE Micro , October 1993, pp. 49-53. 
     One of the most important prevalent problems in digital television (DTV) is that of multipath. In fact, it is useful to think of the DTV channel as multipath limited and not power limited. Multipath may arise from fixed structures, such as building walls, acting as reflectors in the transmission channel. Moving objects, such as airplanes, may also cause a multipath condition. Even microreflections in cabling can cause multipath. See, for example, P. T. Marhiopoulos and M. Sablatash, “Design of a Ghost Canceling Reference Signal for Television Systems in North America”,  Proceedings of Canadian Conference on Electrical and Computer Engineering , Vancouver, BC, Canada, 14-17 September 1993, pp. 660-663. 
     The effect of multipath is to create “ghosts” in the displayed TV image. The statistics of multipath ghosts have been studied and compiled by, among others, the BTA (Japan&#39;s Broadcasting Technology Association). A BTA survey reported that 92% of ghosts are within a −4 to 26 μsecond range, and when extended to −4 to 37 μseconds, almost all occasions of ghost creation are covered. 
     An adaptive equalizer has been proposed to “undo” the effects of the multipath. In its crudest form, an adaptive equalizer can be thought of as a signal processor that estimates the parameters of a hypothetical filter that best describes the channel. The signal processor adjusts the taps of the adaptive equalization filter to approximate an inverse of the hypothetical filter, thus inverting or undoing the effects of the multipath. 
     The BTA, and other concerns, designed a “ghost canceling reference (GCR)” transmitted signal to mitigate these multipath induced effects. The BTA GCR was found to be less than satisfactory in some cases. While homes with outdoor antennas displayed non-varying (stationary) ghosting conditions which could be largely corrected, those homes with indoor antennas experienced changing (dynamic) ghosts. These ghosting conditions were more prevalent when people were moving about the room or other moving objects were in the signal path. The BTA ghost canceller generally was unable to adequately compensate for these conditions. In fact, false ghosts were actually added to an already ghosted picture, leading to reduced picture quality. 
     Thus, multipath behavior of the DTV channel is important for two different regimes, the outdoor antenna propagation channel and the indoor antenna propagation channel. The former is well-studied and understood. The latter regime still presents a problem. The chief difference is the presence of significant reflectors near the indoor receiving antenna, the presence of which implies that there will be multipath whose delay occasions it to fall within a symbol period. In order to resolve multipath differences of such limited extent, special techniques must be employed or the channel diagnostic signal must have a very wide effective bandwidth. According to S. Salous in “Indoor and Outdoor UHF Measurements with a 90 MHZ Bandwidth”,  IEEE Colloquium on Propagation Characteristics and Related System Techniques for Beyond Line - of - Sight Radio , 1997, pp. 8/1-8/6, the extent of multipath delays of outdoor environments can be a few tens of μseconds, whereas in indoor environments, it is on the order of a few hundred nanoseconds. While multipath components can be adequately resolved with a 10 to 40 MHZ bandwidth for outdoor environments, the resolution of multipath for indoor environments requires 90 to 100 MHZ bandwidth. 
     BRIEF SUMMARY OF THE INVENTION 
     In order to receive weak television signals in rapidly changing mutlipath interference environments, such as encountered in a house which does not have an external (outdoor) television antenna, a plurality of indoor antennas are attached to a television receiver. These indoor antennas are located, constructed or oriented in a manner to provide a diversity of input signals to the television receiver. This diversity is such that if one antenna, or a particular combination of more than one antenna, does not provide an acceptable television signal, then another antenna, or a different combination of more than one antenna, may provide an acceptable television signal. In a preferred embodiment, the television receiver is provided with more than one television tuner, so that, while one tuner is receiving the signal being viewed from one antenna or a particular combination of more than one antenna, the second tuner scans the signals from all other antennas and other combinations of antennas. The second tuner assigns a quality measure to each of these signals. When the quality measure of the signal producing the viewed image is significantly inferior to some other input signal combination, as happens when the signal multipath interference environment changes due, for example to people moving about in the television viewing room, the television receiver changes the input signal for viewing to whatever other antenna, or particular combination of antennas, has the highest quality measure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of one embodiment of the invention; 
     FIG. 2 is a schematic diagram showing one implementation of the combiner used in the embodiment of FIG. 1; 
     FIG. 3 is a block diagram showing an alternative implementation of the combiner used in the embodiment of FIG. 1; and 
     FIG. 4 is a block diagram of an alternative embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows one embodiment of the invention wherein three antennas  101 ,  102  and  103  are attached to the front end of a television receiver  112 . These antennas may be dipoles embedded in the mechanical structure of the television receiver, or external devices such as “rabbit ear” antennas placed in convenient locations and/or orientations in the vicinity of the television receiver. Each antenna is coupled to two signal combiners  104  and  105 , the output signals of which are selected by multiplexers  106  and  107  as input signals to a respective one of ATSC (Advanced Television Standards Committee) tuner  108  and NTSC (National Television Standards Committee) tuner  109 . ATSC tuner  108  is required for processing DTV signals, while NTSC tuner  109  is used to process conventional analog television signals. 
     Most digital television (DTV) receivers will have one NTSC tuner and one ATSC tuner, since there will be a transition period during which both analog and digital program material will be transmitted. During reception, only one tuner is in use decoding the program, the tuner being used corresponding to whether the channel being viewed is digital or analog. This invention makes use of the fact that there are two tuners in the television receiver. It will be understood by those skilled in the art, however, that the two tuners may be of the same type, e.g., both ATSC tuners, if the receiver is designed to receive only one type of transmission, e.g., DTV signals. 
     Each of combiners  104  and  105 , multiplexers  106  and  107  and tuners  108  and  109  are controlled by a control processor  110 . The control processor configures combiners  104  and  105  based on evaluations of the output signals of tuners  108  and  109  made by a signal evaluation module  111 . Depending on the signal evaluation from module  111 , multiplexers  106  and  107  are controlled to send the output signal of either combiner  104  or combiner  105  to ATSC tuner  108  or, correspondingly, the output signal of either combiner  104  or combiner  105  to NTSC tuner  109 . 
     FIG. 2 shows an implementation of a combiner unit wherein the signal from each antenna connection thereto passes through a wideband buffer amplifier  201 ,  202  and  203 , respectively. These wideband amplifiers isolate the incoming signal from the corresponding antenna so that no processing of the signal done by the combiner will be reflected back into the antenna to generate possible interference with the other combiner. The antenna signals from buffer amplifiers  201 ,  202  and  203  are passed through respective tapped delay lines  204 ,  205  and  206 . Switches  207 ,  208  and  209  at the outputs of respective delay lines  204 ,  205  and  206  select among one of the delayed signals from the antenna, or the ground connection (no signal). Switches  207 ,  208  and  209  are connected to the inputs of a summing amplifier  210 , which provides an output signal to each of the two multiplexers  106  and  107 , shown in FIG.  1 . 
     Another method of implementing variable delay is to use a continuously variable delay circuit where the amount of delay is selected by controlling voltage instead of using discrete switched steps. External control of the switches or control voltages allows the output signal of the summing amplifier to be the sum of the antenna signals with independently varied delays. 
     FIG. 3 shows another implementation of the combiner. As in the FIG. 2 implementation, each antenna is connected to the combining unit through a wideband buffer amplifier  301 ,  302  and  303 , respectively, which isolates the incoming signal from the corresponding antenna for the same reason as in the FIG. 2 implementation. In place of the tapped delay lines, however, the antennas are coupled to respective variable phase shifters  304 ,  305  and  306  which, in turn, are coupled to respective attenuators  307 ,  308  and  309 . Each of the phase shifters and attenuators can be varied continuously with a control voltage, as shown, or in discrete steps with switches. External control of the switches or control voltages allows the output signal of a summing amplifier  310  to be the sum of the antenna signals with independently varied phases. Providing a phase shift of 180° allows summing amplifier  310  to perform a subtraction of signals, resulting in an output signal proportional to the difference between two antenna signals. 
     In both implementations shown in FIGS. 2 and 3, the combiner circuit can be configured to produce an output signal that is a summation of the different antenna signals with independently determined amounts of delay and phase shift, or with no contribution at all from one or more antennas. This makes possible the implementation of various diversity algorithms by the control processor  110  shown in FIG.  1 . Spatial diversity combining is well known in the art for providing enhanced results in many difficult signal reception environments, such as rapidly changing multipath interference environments previously mentioned. Traditional methods of achieving enhanced reception comprise minimization of mean square error such as described in “Optimum Diversity Combining and Equalization to Digital Data Transmission with Applications to Cellular Mobile Radio—Part I: Theoretical Considerations” by P. Balaban and J. Salz,  IEEE Transactions on Communications , Vol. 40, No. 5, pp. 885-894, May 1992. More recent work entails such techniques as blind maximum likelihood sequence estimation and its variants such as described in “Blind MLSE with Spatial Diversity Combining and Its Simplified Algorithm” by X. Biao and D. Min,  Intemational Conference on Communication Technology , pp. S47-05-1 to SA47-05-5, 1998. 
     In operation, a television receiver with multiple indoor antennas, viewed in a house where movement of persons inside the house, or other environmental changes inside or outside the house, causes the multipath signal environments for all of the TV channels to change with time. This television receiver is provided with the circuitry shown in FIG. 1, having two tuners which may be two NTSC tuners, two ATSC tuners, or one NTSC tuner and one ATSC tuner, the latter being typical of DTV receivers capable of receiving both analog and digital programs. The channel being observed is received by one of these tuners receiving a signal from either combiner  104  or  105 , depending on the selection by multiplexers  106  and  107 . 
     When combiner  104  is providing, via multiplexer  106 , a signal to ATSC tuner  108 , which is the signal being viewed, control processor  110  uses a signal evaluation module  111  to measure the strength and quality of the signal coming from ATSC tuner  108  (the signal being viewed), and configures multiplexer  107  so that the signal from combiner  105  is sent to NTSC tuner  109 . Control processor  110  also configures the circuitry of combiner  105  in any one way, for example, with antenna  101  coupled to the summing amplifier ( 210  in FIG. 2 or  310  in FIG. 3) with no delay or phase shift, and no input signal (i.e., switches,  208  and  209  in FIG. 2 set to ground) from antennas  102  and  103 . Control processor  110  then uses signal evaluation module  111  to measure the strength and quality of the signal produced by the summing amplifier. If this measurement is significantly higher (e.g., 1 dB) than the measurement made from the output of ATSC tuner  108 , multiplexer  106  is reconfigured by control processor  110  so that ATSC tuner  108  now receives the signal from combiner  105 . 
     If the strength and quality measurement of the NTSC output signal is not significantly greater than that for the ATSC output signal, then control processor  110  configures combiner  105  in a different manner. Processor  110  continues to try different configurations for combiner  105  until a superior signal is found. If and when a superior signal is found, processor  110  causes multiplexer  106  to send the superior signal to ATSC tuner  108 . When this happens, processor  110  changes multiplexer  107  to evaluate the output signal of combiner  104 . Processor  110  configures combiner  104  and evaluates different signal combinations therein in a manner similar to that previously described. 
     There are many possible methods by which processor  110  can cycle through the various combinations. One way is simply to exhaust all possible discrete settings. Another, and generally faster, way is to vary one parameter at a time to see if it leads to an increase in measured signal strength. If it does, that value of the parameter is adopted as the new setting. If it does not, another parameter is varied. If no single parameter results in an improvement, then two parameters are varied at a time, and so on. This type of sequential refinement is commonly known in the art as “hill climbing”. 
     Periodically, processor  110  uses signal evaluation module  111  to re-evaluate the output signal of ATSC tuner  108  to update the quality measurement which may have changed since it was last examined. When this is done, the new value is used for evaluation as described above. Whenever the channel is changed by the viewer, the process begins again. 
     There are many possible metrics to use to evaluate signal strength and quality, and any of these may be used in this invention. One metric is the strength of the incoming signal, as indicated by an automatic gain control (AGC) signal that is present in the ATSC and NTSC tuners. Another metric is the flatness of a power spectrum measurement of the signal over the 6 MHz channel bandwidth. 
     In an alternative embodiment, the combiner circuitry may be intelligently driven by a wideband signal overlay correlator. Such a wideband signal overlay correlator is disclosed in copending application Ser. No. 09/201,376, cited above. This alternative embodiment is illustrated in FIG.  4 . 
     The system of FIG. 4 comprises a transmitter subsystem  410  and receiver subsystem  420 . The transmitter subsystem includes a first overlay signal generator  411 , an adder  412  and a transmitter  413 . A digital television signal to be transmitted is provided to adder  412 . Also provided to adder  412  is a first overlay signal generated by a first overlay signal generator  411 . Adder  412  combines the first overlay signal and the digital television signal to provide a combined signal to transmitter  413  for transmission via a transmitting antenna  417 . Receiver subsystem  420  includes receiving antennas  421 ,  422  and  423 , combiners  424  and  425 , multiplexers  426  and  427 , ATSC tuners  428  and  429 , a correlator  430 , second overlay signal generator  431 , phase adjuster  432 , and control processor  433 . 
     At the transmitter subsystem, the first overlay signal is a wide band, relatively low power random signal, having an autocorrelation property that produces periodic correlation peaks but a low cross-correlation property with the digital television signal. In one embodiment, overlay signal generator  411  is a sequence generator which produces an m-sequence as described in  Data Transportation and Protection  by John E. Hershey and R. K. Rao Yarlagadda, Chapter 8, pp. 273-308, Plenum Press, 1986. 
     The combined signal transmitted by transmitter  413  is received by antennas  421 ,  422  and  423  and supplied to combiners  424  and  425 , in a manner similar to the FIG. 1 embodiment. The output signal of one of the combiners is selected by multiplexer  426  and supplied to tuner  428  and the output signal of the other combiner is selected by multiplexer  427  and supplied to tuner  429 , which produces a demodulated signal at an intermediate frequency (IF). The IF signal is supplied to correlator  430  and to second overlay signal generator  431 . The output signal of correlator  430  is supplied to phase adjuster  432  which provides a phase control feedback signal to second overlay signal generator  431 . The output signal of phase adjuster  432  is also provided to processor  433  which generates control signals for combiners  424  and  425  and multiplexers  426  and  427 . 
     While only certain preferred features of the invention have been illustrated and described, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.