Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1998-05-01
2001-11-13
Pham, Chi (Department: 2631)
Pulse or digital communications
Receivers
Automatic frequency control
C375S362000
Reexamination Certificate
active
06317473
ABSTRACT:
This invention relates to demodulating digital video broadcast (DVB) signals.
There are currently two major types of DVB, namely, terrestrial broadcasting and satellite/cable broadcasting. The invention is particularly, though not exclusively concerned with terrestrial broadcasting, which has special problems, particularly in communication channel impairment, arising from adjacent television channels, multipath and co-channel interference, for example. A type of transmission which has been developed to meet these problems is known as Coded Orthogonal Frequency Division Multiplexing (COFDM)—see for example “Explaining Some of the Magic of COFDM” Stott, J. H.—“Proceedings of 20th International Television Symposium”, Montreux, June 1997. In COFDM, transmitted data is transmitted over a large number of carrier frequencies (1705 or 6817 for DVB), spaced (by the inverse of the active symbol period) so as to be orthogonal with each other; the data is convolutionally coded, to enable soft-decision (Viterbi) decoding. Metrics for COFDM are more complex than those single carrier modulation in that they include Channel State Information (CSI) which represents the degree of confidence in each channel for reliably transmitting data.
Modulation and Demodulation of the carriers may be carried out by a Fast Fourier Transform (FFT) algorithm performing Discrete Fourier Transform operations.
Subsequent to demodulation, signal processing corrections are carried out such as channel equalisation, channel state information correction, and phase error correction. The demodulated and corrected signal may then be decoded in an FEC (forward error correction decoder) for recovery of data.
In regard to phase error correction, a principal problem is that of local oscillator phase-noise. The addition of local oscillator phase noise to an COFDM signal has two notable effects;
1) To rotate the received constellation by an amount which is the same for all the carriers within any one OFDM symbol, but varying randomly from symbol to symbol. This is called the Common Phase Error (CPE), and primarily results from the lower-frequency components of the phase-noise spectrum; and
2) To add Inter-Carrier Interference (ICI) of a random character similar to additive thermal noise. ICI primarily results from the higher-frequency components of the phase-noise spectrum. ICI cannot be corrected and must be allowed for in the noise budget. It can be kept small in comparison with thermal noise by suitable local oscillator design.
It is an object of the present invention to provide an improved means of removing the common phase error in digital video broadcast signals.
The present invention provides in a first aspect, apparatus for demodulating digital video broadcast signals comprising data modulated on a multiplicity of spaced carrier frequencies, including:
analog to digital conversion means for providing a series of digital samples of the broadcast signal, real to complex conversion means for converting each digital sample to a complex number value, Fourier Transform means for analysing the complex number values to provide a series of data signal values in complex number format for each carrier frequency, and signal processing means for processing the series of data signal values including phase error correcting means,
the phase error correcting means including means for converting the data signal values from a complex number format to a phase angle format, means for determining a common phase error by assessing the phase of continual pilot signals in the broadcast signals and determining the variation in phase of the continual pilot signals between consecutive symbols in the broadcast signals, and means for subtracting the common phase error from the data signal values.
In accordance with the invention, an improved means is provided for accurately demodulating digital video broadcast signals which relieves the necessity for a very accurate down-conversion of the received broadcast signal to intermodulate frequencies.
As preferred, a plurality of said continual pilot signals (there are 45 or 177 available) are arranged to determine the phase error. In addition, a weighting means is employed to give more significance in the averaging process of those values near the average.
In a further aspect, the invention provides an apparatus for demodulating digital broadcast signals comprising data modulated on a multiplicity of spaced carrier frequencies, including:
analog to digital conversion means for providing a series of digital samples of the broadcast signal, Fourier Transform means for analysing the samples to provide a series of data signal values for each carrier frequency, and signal processing means for processing the series of data signal values including phase error correcting means,
the phase error correcting means including, means for determining a common phase error by assessing the phase of continual pilot signals in the broadcast signals and determining the variation in phase of the continual pilot signals between consecutive symbols in the broadcast signals, and means for subtracting the common phase error from the data signal values, wherein the common phase error determining means includes means for averaging the phase of a plurality of said continual pilot signals, and weighting means for applying a weighting to the pilot signals so that more significance is accorded to pilot signals near the average value of phase error.
Thus, it is possible to remove the common phase-error component caused by phase noise added in the down-converter by digital processing in the chip. This processing is performed by the common-phase-error correction block in the architecture.
The common-phase-error correction block is able to remove the common phase error because all carriers within a given symbol suffer the same common phase error. By measuring the continual pilots, whose intended phase is the same from symbol to symbol, the common phase error is determined and then subtracted from the phase of all the data cells in the same symbol. There are sufficient continual pilots (which in any case are transmitted with a power approx. 2.5 dB greater than data cells) that the effect of thermal noise on this measurement can be rendered negligible by averaging. There are essentially three components required to implement common-phase-error correction in the chip. These are:
1) A one-symbol data delay; since the common phase error varies randomly from symbol to symbol, it must be applied to the symbol from which it was calculated. Furthermore, it is not possible to calculate the common phase error until the whole symbol has been received.
2) The digital circuitry required to calculate the common phase error based on the received data.
3) A phase-to-complex-number look-up table. This is required since the common phase error value that is calculated will be a phase value. In order to apply the correction to the signal, the signal must be multiplied by a complex number equal to the complex representation of the phase.
These three factors, which together form the “cost” of implementing the feature on the chip, must be balanced against the cost of improving the performance of the down-converter so that the phase-noise it introduces is negligible. In our architecture we decided that the cost of including a common phase error correction circuit was substantially less than the cost of eliminating phase-noise in the down-converter, and so the chip includes circuitry to perform common-phase-error correction.
Naturally, strict control is required over the frequencies of the incoming video signals, and to this end automatic frequency control (AFC) is desirable. An AFC signal is preferably derived from the series of data signal values output from the Fourier Transform means, either for control of analog intermediate frequency local oscillator in a down conversion stage, or for digital phase adjustment via a direct digital frequency synthesis (DDFS) unit applied to the input of the Fourier Transform device.
The AFC preferably comprises two parts, a coa
Clarke Christopher K. P.
Guyot Jean-Marc
Haffenden Oliver
Lauret Régis
Mitchell Justin
Bayard Emmanuel
LSI Logic Corporation
Oppenheimer Wolff and Donnelly
Pham Chi
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