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 xe2x80x9cGuide to the Use of the Digital Television Standard for HDTV Transmissionxe2x80x9d, 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 3D 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.
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.