Pulse or digital communications – Equalizers – Automatic
Reexamination Certificate
1998-11-17
2001-04-17
Pham, Chi (Department: 2631)
Pulse or digital communications
Equalizers
Automatic
C375S326000
Reexamination Certificate
active
06219379
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The subject invention relates to receiving digital signals, and more particularly, to a vestigial-sideband (VSB) receiver for operating in a less than ideal environment.
The digital terrestrial television transmission standard for the U.S. is a trellis coded 8-VSB system as described in detail in ATSC Digital Television Standard, Doc. A/53, http://www.atsc.org/stan&rps.html, September 1995. Briefly, three trellis-coded bits are encoded into one of eight symbols ±2, ±3, ±5, ±7. This real symbol stream, at 10.76 Msyms/s, has a double-sided spectrum in the digital frequency domain. One of the sidebands is filtered with a square-root-raised-cosine(SQRC) filter to give a vestigial-sideband (VSB) signal which is then up-converted to RF and transmitted.
2. Description of the Related Art
At the receiver, the conventional method of demodulation consists of first recovering the carrier frequency and phase, matched filtering, and then recreating the double-sideband real signal prior to equalization with a real equalizer operating at the symbol rate of 10.76 Msyms/s. In fact, one of the so-called “advantages” of a VSB system is the fact that, unlike quadrature amplitude modulation (QAM), complex equalization is not necessary.
However, the above receiver structure with a real equalizer is optimal only in the case of an ideal channel with perfect carrier and timing phase recovery. When the signal suffers multipath distortion, processing only the real part of the signal at the receiver causes information about the channel at the band edges to be lost. The situation is analogous to QAM where it has been shown that fractionally-spaced equalization can compensate for band-edge distortions in the channel that symbol-spaced equalization cannot. In VSB system, taking the real part of the signal causes aliasing of the band-edges in the same way that sampling a QAM signal at the symbol rate does.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide a receiver structure for a VSB system which reduces the loss of information at the band edges in severe multipath by using a blind complex equalizer in conjunction with a real equalizer. This object is achieved because the complex VSB signal at the symbol rate (i.e., 10.76 Msyms/s) is not aliased, and any band-edge distortions can be equalized with a complex equalizer.
It is another object of the invention to equalize the band edges of the complex baseband signal prior to recreating the double sideband signal.
It is yet a further object of the invention to simplify the complex equalizer by using a complex equalizer with a small number of taps to just equalize the band edges and then recover the field synchronization signal from the output of this complex equalizer for training a longer equalizer that equalizes the rest of the band.
It is even a further object of the invention to use a composite equalizer which has a feedback section having real taps and a forward section having an inner section with complex taps to compensate for the band edges, and an outer section with real taps.
The invention accordingly comprises the methods and features of construction, combination of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.
REFERENCES:
patent: 4334313 (1982-06-01), Gitlin et al.
patent: 5642382 (1997-06-01), Juan
patent: 5648987 (1997-07-01), Yang et al.
patent: 5712873 (1998-01-01), Shiue et al.
patent: 5793821 (1982-06-01), Norrel et al.
patent: WO9526075 (1995-09-01), None
ATSC 8-VSB Demodulator and Decoder, TDA 8960.
“Philips Semiconductors VSB Turner Front End and Channel Decoder Chipset for Digital TV.”.
“ATSC Digital Television Standard”, Doc. A/53, Sep. 1995.
“Self-Recovering Equalization and Carrier Tracking in Two-Dimensional Data Communication Systems”, by D.N. Godard, IEEE Transactions on Communications, vol. 28, No. 11, Nov. 1980..
“A Method of Self-Recovering Equalization for Multilevel Amplitude-Modulation Systems”, Y. Sato, IEEE Communications, Vo. Com-23, pp. 679-682, Jun. 1975.
“A Sign-Error Algorithm for Blind Equalization of Real Signals”, Monisha Ghosh, ICASSP, May 1998.
Corrielus Jean B.
Goodman Edward W.
Pham Chi
Philips Electronics North America Corporation
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