Abstract:
A modulation/demodulation receiver having a reference signal in a digital communication system. A method for signal processing and co-channel interference signal removal in the receiver includes the steps of processing an input signal to be at a predetermined multilevel, feeding the processed signal to an adaptive equalizer, determining the type of multilevel from the signal applied to the adaptive equalizer and selecting an operation mode from at least two multilevel operation modes, for the adaptive equalizer and blocks downstream from the adaptive equalizer, and causing the adaptive equalizer to adaptively equalize the signal at the predetermined multilevel in the selected operation mode and remove co-channel interference.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a digital communication system, and in particular, to a modulation/demodulation receiving system having a reference signal. 
     2. Description of the Related Art 
     A digital communication system is known in which a transmitter transmits a reference signal along with an original signal to a receiver so that the receiver can remove or minimize problems caused by interference, multipath, and the like on a transmission channel. The aim of transmitting the reference signal, for example, a PN (Pseudo-Noise) sequence is to enable the receiver to sufficiently reflect channel characteristics. 
     A co-channel interference processing method has been disclosed in “Guide to the Use of the Digital Television Standard for HDTV Transmission”, United States Advanced Television System Committee, Apr. 12, 1995, pp. 104-107. In the above method, an HDTV (High Definition TeleVision) signal in VSB (Vestigial Side Band) is described by way of example. 
     A receiving system suggested in the method is similar to that shown in FIG.  1 . FIG. 1 is a partial block diagram of a conventional receiver (see FIG. 1 in U.S. Pat. No. 5,594,496). 
     For simultaneous broadcast of a VSB signal and a signal of the currently used broadcast system, namely, NTSC (National Television System Committee) on the same channels, a VSB transmission system implements an NRF (NTSC Rejection Filter)-related operation. The major NRF processing in VSB is to employ a comb filter  16  for removing NTSC carriers where energy is concentrated. 
     More specifically, if an HDTV signal in GA-VSB (Grand Alliance-VSB) and an NTSC signal are present together in the same channel, a relationship exists between them, as shown in FIGS. 3A and 3B. Here, FIGS. 3A to  3 D are identical to FIGS. 4A to 4D of U.S. Pat. No. 5,546,132, respectively. FIGS. 3A and 3B respectively illustrate the RF spectra of an HDTV signal and an NTSC signal, and FIGS. 3C and 3D respectively illustrate frequency characteristics of an NTSC rejection and an NTSC extraction filter. 
     Common methods of removing co-channel interference include removal of carriers (e.g., picture carrier, color carrier, and audio carrier) where energy is concentrated. GA-VSB uses comb filters (i.e., NTSC rejection filters)  140  and  150  having delays  141  and  151  for delaying 12 symbols and subtracters  142  and  152  for obtaining the difference between an undelayed symbol from a delayed symbol in order to remove co-channel interference. Except for slight changes to the reference numerals, FIG. 2 is identical to FIG. 3 of U.S. Pat. No. 5,546,132 and FIG. 10.8 of the aforementioned volume, p.106. 
     Since NTSC carriers are present around the null point as shown in FIG. 3C, if a received signal passes through the comb filters  140  and  150  of FIG. 2, much of the NTSC interference is removed. The NTSC interference can also be removed by use of a notch filter in which NTSC carriers are present around the null point. 
     As described above, the conventional receivers include a switching portion (switch  19  of FIG.  1  and multiplexer  230  of FIG. 2) for selecting an NRF-processed signal at 15 levels and a non-NRF processed signal at 8 levels on the basis of the selection of an NRF block for removing co-channel interference and adjusting blocks for driving the NRF block. Therefore, the input of an adaptive or channel equalizer downstream from the NRF block depends upon whether the NTSC rejection filter is operated or not. That is, the equalizer is configured to accommodate both the 8-level and 15-level signals. Because the 15-level signal is incremented in the number of signal levels from the 8-level signal due to comb filtering, the equalizer should have an input limit ranging up to 15 levels. 
     Also, even when the input of the equalizer falls in the required range by reducing the gain of a 15-level signal, constraints may be imposed on the resolution at the input terminal of the equalizer or bias errors can be generated due to rounding. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a device and method for processing a multilevel input of a predetermined type in one or more multilevel operation modes in a modulation/demodulation receiver of a digital communication system. 
     Another object of the present invention is to provide a receiver having an equalizer capable of operating both in an 8-level mode and in a  15- level mode for an 8-level input in VSB. 
     A further object of the present invention is to provide a device and method for removing or reducing multipath on a modulation/demodulation transmission channel with a reference signal. 
     Still another object of the present invention is to provide a device and method for removing co-channel interference on a modulation/demodulation transmission channel with a reference signal. 
     A still further object of the present invention is to provide a device and method for implementing equalization and NRF operation together without requiring an extra component. 
     To achieve the above objects, there is provided a method for signal processing and co-channel interference signal removal in a modulation/demodulation receiver having a reference signal in a digital communication system. In the method, an input signal is processed to be at a predetermined multilevel and fed to an adaptive equalizer, the type of the multilevel is determined from the signal applied to the adaptive equalizer, an operation mode is selected from at least two multilevel operation modes, for the adaptive equalizer and blocks downstream from the adaptive equalizer, and the adaptive equalizer adaptively equalizes the signal at the predetermined multilevel in the selected operation mode and removes co-channel interference. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above objects and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which: 
     FIG. 1 is a partial block diagram of a conventional receiver; 
     FIG. 2 is a block diagram of a general NTSC interference detector; 
     FIG. 3A is a graph showing the RF spectrum of an HDTV signal; 
     FIG. 3B is a graph showing the RF spectrum of an NTSC signal; 
     FIG. 3C is a graph showing a frequency characteristic of an NTSC rejection filter; 
     FIG. 3D is a graph showing a frequency characteristic of an NTSC extraction filter; 
     FIG. 4 is a partial block diagram of a receiver according to an embodiment of the present invention; 
     FIG. 5 is a block diagram of an embodiment of an adaptive equalizer according to the present invention; 
     FIG. 6 is a block diagram of another embodiment of an adaptive equalizer according to the present invention; and 
     FIG. 7 is a block diagram of a third embodiment of an adaptive equalizer according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Preferred embodiments of the present invention will be described in detail with reference to the attached drawings. It is to be noted that like reference numerals denote the same components in the drawings and a detailed description of a known function or structure of the present invention will be omitted if it obscures the subject matter of the present invention. The following description is conducted on a receiver in a GA-VSB digital communication system for better understanding of the present invention. 
     FIG. 4 is a partial block diagram of a receiver according to an embodiment of the present invention. The difference between FIG.  1 . and FIG. 4 is that comb filter  16 , field comb circuit  30 , comparator  36 , comb filter  38 , and a timing circuit  44  of FIG. 1 are replaced by a mode selection controller  50  in FIG.  4 . That is, the output of a block  14 , identifying an IF amplifier demodulator and A/D converter, is directly fed to an adaptive equalizer  22 . In the embodiment of the present invention, the adaptive equalizer  22  performs NTSC interference rejection as well as equalization, as compared to the conventional comb filters ( 16  and  38  of FIG. 1 and 140 and  150  of FIG. 2) dedicated to NTSC interference rejection. 
     According to the features of the present invention, the adaptive equalizer  22  can be limited to the function of equalization in which a multi-level input of a predetermined type is processed in one or more multi-level operation modes. 
     Considering that the adaptive equalizer  22  operates in an LMS (Least Mean Square) algorithm, it is preferable that the adaptive equalizer  22  accommodates an input at a predetermined multilevel rather than an input at a multilevel changed by a comb filter. A conventional adaptive equalizer selectively operates for 8- and 15-level inputs. 
     Therefore, the embodiment of the present invention is characterized by input of a predetermined type, for example, an 8-level signal, and operation of the adaptive equalizer in operation modes corresponding to one or more types (e.g., 8 level and 15 level) according to the presence or absence of NRF processing. In FIG. 4, the mode selection controller  50  determines whether an input signal contains co-channel interference. Upon the presence of the co-channel interference, it provides a signal for operating the adaptive equalizer  22 , a phase tracker  24 , and a trellis decoder  26  in a 15-level mode. On the other hand, upon the absence of the co-channel interference, it provides a signal for operating them in an 8-level mode. For this purpose, the mode selection controller  50  compares MSEs (Mean Square Errors) at the front and back of an NRF block as in U.S. Pat. No. 5,546,132, obtains NTSC components as in U.S. Pat. No. 5,446,132, detects NTSC synchronization, or uses a GCR (Ghost Cancellation Reference) signal. 
     FIGS. 5,  6 , and  7  are block diagrams of different embodiments of the adaptive equalizer  22  according to the present invention. 
     Referring to FIG. 5, the adaptive equalizer  22  includes a first adaptive filter portion  60  of an FIR (Finite Impulse Response) type, having a first filter  62  and a filter coefficient calculator  64 , an error calculator  66  for obtaining an error in the adaptive equalizer output, and a reference signal generator  68  for generating a reference signal to the error calculator  66 . A signal REFSEL for selecting a reference signal is applied to the reference signal generator  68 , and a window pulse WNP indicating a reference signal period is applied to the first adaptive filter portion  60 . 
     The error calculator  66  includes only a subtracter, and the reference signal generator  68  has a multiplexer (MUX 1 )  74  for selectively outputting a training sequence TS_ 1   70  as a first reference signal and a training sequence TS_ 2   72  as a second reference signal according to the signal REFSEL. 
     It is assumed here that the first reference signal TS_ 1   70  is for an 8-level signal and the second reference signal TS_ 2   72  is for a 15-level signal converted from an 8-level signal due to comb filtering for removing co-channel interference. 
     An input signal of the receiver shown in FIG. 4 can be divided into a reference signal period such as a PN sequence period and a random data period. The input signal is directly fed to the adaptive equalizer  22  from the block  14  through a tuner  10  and a SAW (Surface Acoustic Wave) filter  12 , without comb filtering. Thus, the signal is always an 8-level signal. 
     The 8-level signal received in the adaptive equalizer  22  is filtered using a filter coefficient renewed by an error obtained during the reference signal period and output as an adaptive equalizer output signal. The error calculator  66  receives the adaptive equalizer output signal and a reference signal selected by the signal REFSEL (the 8-level reference signal TS_ 1   70  is selected at an initial stage), and calculates the error between the selected reference signal and the adaptive equalizer output signal. The first filter coefficient calculator  64  applies a filter coefficient renewed by the calculated error to the first filter  62 . That is, the first adaptive filter portion  60  filters the 8-level input signal using the filter coefficient. For the random data period, a filter coefficient is calculated from the error between an adaptive equalizer output corresponding to random data and a selected reference signal, and the first adaptive filter portion  60  filters an input signal using the filter coefficient. 
     In a normal state in which an NTSC co-channel interference signal is not present, the mode selection controller  50  of FIG. 4 causes the 8-level reference signal TS_ 1   70  to be selected by the signal REFSEL. In a state requiring comb filtering due to the presence of an NTSC co-channel interference signal, the mode selection controller  50  causes the 15-level reference signal TS_ 2   72  to be selected and then causes the adaptive filter portion  60  to change from an 8-level mode to a 15-level mode. Therefore, the NTSC co-channel interference signal contained in an HDTV signal is removed by the adaptive equalizer  22  of FIG.  5 . 
     For a signal including a reference signal such as an HDTV signal, the reference signal for a 15-level signal changed from comb filtering is fed to the first adaptive filter portion  60 , which operates in the 15-level mode without changing bits at the 8-level input terminal, preventing a bias error encountered in the prior art. 
     Set forth below is an equation expressing the operation of the adaptive equalizer  22  according to an LMS algorithm for a clearer understanding of how the input signal is processed while fixed at 8 levels. 
     With the input, output, and filter coefficient of the first filter  62  given as X T (n), z(n), and W(n), the LMS algorithm is expressed as 
     (Equation 1) 
     
       
           z ( n )= W   T ( n− 1) X   T ( n )  (1) 
       
     
     
       
           e ( n )= d ( n )− Z ( n )  (2) 
       
     
     
       
           W ( n )= W ( n− 1)+2 μe ( n ) X   T ( n )  (3) 
       
     
     where X T =[x(n), x(n−1), . . . , x(n−N+1), W T =[w 0  w 1 , . . . , w N−1 ], and d(n) is a reference signal value. 
     The LMS algorithm is performed toward minimization of E[(e 2 (n)]. 
     Referring to equation 1, as the input level of the conventional adaptive equalizer  22  is changed (e.g., from 8 levels to 15 levels or vice versa), x(n) in both (1) and (3) should be changed to (x(n) — 8) for an 8-level signal or (x(n) — 15) for a 15-level signal. Hence, the conventional adaptive equalizer  22  selectively receives (x(n) — 8) and (x(n) — 15). In this case, a number of input lines are required which is equal to the number of bits required to operate the conventional adaptive equalizer  22  in both modes, thereby complicating the structure of the conventional adaptive equalizer  22 . On the contrary, an input signal is always fixed at 8 levels (x(n) — 8) regardless of NTSC interference and a reference signal is selected by the signal REFSEL in the embodiment of the present invention. Due to the fixed input level of the adaptive equalizer  22 , bit assignment is easy and there is no need for changing x(n) in (1) and (3) of equation 1. 
     FIG. 6 illustrates another embodiment of the adaptive equalizer  22  involving a decision process generally used in digital communication. This is similar to the structure of FIG. 5, but the difference lies in that a multiplexer (MUX 2 )  78  for selecting a reference signal and a decision signal by the window pulse WNP and a decision portion  76  are added for adaptive equalization by decision made for random data. The decision portion  76  decides an error as a value approximate to a corresponding 8- or 15-level value (e.g., 7, 5, 3, 1, −1, −3, −5, and −7 for 8 levels, and 14, 12, 10, 8, 6, 4, 2, 0, −2, −4, −6, −8, −10, =12, and −14 for 15 levels) by selectively operating an 8-level decider or a 15-level decider according to the signal REFSEL. The multiplexer  78  selects a reference signal selected by the reference signal generator  68  during the reference signal period and a decision signal generated from the decision portion  76  during the random data period according to the window pulse WNP. The error calculator  66  calculates the error between the output signal of the multiplexer  78  and the adaptive equalizer output signal and feeds its output signal to the filter coefficient calculator  64 . 
     FIG. 7 illustrates a DFE (Decision Feedback Equalizer) most generally used for digital communication. This equalizer is the same as that of FIG. 6, except for addition of a second adaptive filter portion  80  of an IIR (Infinite Impulse Response) type including a second filter  82  and a second filter coefficient calculator  84 , and a subtracter  86 . 
     While two reference signals are employed in the equalizers of FIGS. 5,  6 , and  7 , it should be understood that the number of reference signals can be increased. 
     As described above, the adaptive equalizer of the present invention can operate in plural modes for signals of different multilevel types without the need of physical operation of an input signal, and perform an NRF operation. Furthermore, the equalizer can be constituted with a reduced number of hardware components because there is no need for an NRF block and its adjusting blocks used in the prior art. 
     While the present invention has been described in detail with reference to the specific embodiments, they are mere exemplary applications. Thus, it is to be clearly understood that many variations can be made by anyone skilled in the art within the scope and spirit of the present invention.