Communication engineering continually must deal with the problem of restoring a signal which has been altered by the communication path over which the signal was transmitted. Signal restoration often can be achieved if the communication path is fully characterized, at least as to those parameters which contribute to the signal alteration. Thus, a frequently essential component of the signal restoration problem is that of identifying the characteristics of the communication path or channel.
One approach to the channel identification problem is to transmit a cancellation reference signal having a known characteristic, over the channel, and to receive the transmitted signal after it has passed through the channel. The originally transmitted signal is compared with the received signal, and a model of the channel characteristics is developed based on the comparison. One type of cancellation reference signal useful for correcting echo interference is known as an echo cancellation reference (“ECR”) signal. Echo interference, which is also known as multipath or dispersive interference, affects analog and digital communications signals. An example of a system and ECR for improved echo cancellation in analog television receivers is described in U.S. Pat. No. 5,121,211, issued Jun. 9, 1992 to David Koo. Another system and architecture for echo cancellation suitable for television receives is described in U.S. Pat. No. 5,278,872, issued Jan. 11, 1994 to Craig B. Greenberg; and in U.S. Pat. No. 5,396,299, issued Mar. 7, 1995 to Craig B. Greenberg. An ECR also has been specified by the Advanced Television Systems Committee (“ATSC”) of the United States, and is described in the following document: Advanced Television Systems Committee, Standard A/49: Ghost Canceling Reference Signal for NTSC, Approved Aug. 14, 1992 and Modified May 13, 1993.
A digital Advanced Television System (“ATV”) has now been specified for the United States. The characteristics of the digital ATV are documented in various standards of the Advanced Television Systems Committee (“ATSC”) and are available from the ATSC, 1750 K Street N.W., Suite 1200 Washington, D.C. 20006. Basically, the Digital Television Standard describes a system designed to transmit high quality video and audio and ancillary data over a single 6 MHz channel. The system can deliver reliably about 19 Mbps of throughput in a 6 MHz terrestrial broadcasting channel and about 38 Mbps of throughput in a 6 MHz cable television channel. Although the RF/Transmission subsystems described in the Digital Television Standard are designed specifically for terrestrial and cable applications, the objective is that the video, audio, and service multiplex/transport subsystems be useful in other applications. Further general information about the Digital Television Standard is presented in the following publication: Advanced Television Systems Committee, Standard A/54: Guide to the Use of the ATSC Digital Television Standard, Oct. 4, 1995.
One component of the digital ATV is known as “RF/Transmission,” which refers to channel coding and modulation. The channel coder takes the data bit stream and adds additional information that can be used by the receiver to reconstruct the data from the received signal which, due to transmission impairments, may not accurately represent the transmitted signal. The modulation (or physical layer) uses the digital data stream information to modulate the transmitted signal. One mode of the modulation subsystem is the terrestrial broadcast mode, also known as “8 VSB,” which uses vestigial sideband modulation with 8 discrete amplitude levels.
As more fully described in the above-referenced Standard A/54, the VSB signal contains features which allow design of receivers that perform the functions of acquiring and locking to the transmitted signal. The equalization of the signal for channel frequency response and echoes uses a training signal, and is facilitated by the inclusion of specific features in the Data Field Sync. Utilization of these features is made more reliable by the provision of means to first acquire and synchronize to the VSB signal, particularly by the Segment Sync. The Data Field Sync then can be used both to identify itself and to further perform equalization of linear transmission distortions. The VSB signal may also be equalized by databased or blind equalization methods which do not use the Data Field Sync. Blind equalization methods are more fully described in the above-referenced Standard A/54.
A standard VSB receiver is shown in FIG. 1, and includes a tuner 1, an IF filter and synchronous detector 3, synchronization and timing circuits 5, an NTSC rejection filter 7, an equalizer 9 for equalization of the signal for channel frequency response and echoes, a phase tracker 11, a trellis decoder 13, a data de-interleaver 15, a Reed-Solomon decoder 17, and a data de-randomizer 19. Standard VSB receivers also are available from various manufacturers, including the IEEE 1394/Device Bay ATSC DTV receiver reference design available from Philips Semiconductors, Philips Electronics N.V., 811 East Arques Avenue, Sunnyvale, Calif., 94088-3409 (incorporates a Philips type TDA8960 demodulator/decoder single chip component).
Despite significant advancements in today's digital telecommunications, echo interference remains one of the most damaging distortions within the communication links. Strong echo interference can collapse digital communication links altogether. The collapse of the digital communication link is due to an intermediate distortion scenario called inter-symbol interference (“ISI”), which results from the interactions between the echo conditions and the transmitted data itself.
Various proposals and methods are know or are in practice today to reduce the impact of echo interference which use equalizers whose weights are primarily from adaptation and mainly based on statistical data information obtained from the multipath or dispersive communication channels. Some of these proposals and methods are described in the following article: David Koo, “Ghost Cancellation with ITU System-C standard Ghost Cancellation Reference Signal,” Journal of the Society of Motion Picture and Television Engineers, June 1995, PP. 370-376. Nonetheless, the current ATSC 8 VSB digital ATV system remains heavily susceptible to echo interference, even though it is not noise limited. It is therefore desirable to improve the performance of the ATSC 8 VSB digital ATV system as well as other similar digital communications systems in the presence of echoes while maintaining their advantage in a white noise environment.