Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1998-05-01
2001-11-20
Chin, Stephen (Department: 2634)
Pulse or digital communications
Receivers
Automatic frequency control
C375S326000, C455S182200, C455S192200
Reexamination Certificate
active
06320917
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 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, phase error correction, and automatic frequency control. The demodulated and corrected signal may then be decoded in an FEC (forward error correction decoder) for recovery of data.
Automatic frequency control (AFC) is important, since frequency offsets may appear after down-conversion to an intermediate frequency, because of variations in local oscillator frequency. Such frequency offsets are lethal for frequency recovery, and must therefore be reduced to a minimum.
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.
GB-A-2307155 describes (see Sec 2.1, p.7 and FIG. 11) an arrangement for automatic frequency control wherein it is recognised that the phase of the retrieved signals in OFDM is proportional to frequency error, and therefore a signal representing phase can be used to control the frequency of a local oscillator.
In practice, the frequency offset arising from local oscillator frequency variations may extend over an interval of many carrier frequencies. One technique which has been used in other applications, using QPSK or QAM modulation, to recover such large frequency offsets, is frequency sweeping. Frequency sweeping involves sweeping over a first frequency range, to detect a frequency lock condition, and then waiting for some time to allow the frequency lock algorithm to converge for a reliable detection of frequency lock. If there is no convergence, then the sweep range is increased until lock is reliably detected.
Whilst this method is suitable for QPSK or QAM modulation because an AFC lock indication can be provided very quickly, a similar method applied to COFDM would take an unduly long time for convergence, because an OFDM symbol lasts 1 millisecond, which would be the basis for any frequency detection algorithm.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method of automatic frequency control in a receiver for COFDM signals.
The invention is based on the recognition that, although the phase variation between adjacent symbols in COFDM is random, for continual pilot signals, as defined in the ETSI Specification, the intended phase of the signals is the same in adjacent symbol intervals. The phase error in adjacent data symbols may be determined by measuring the phase difference in adjacent symbol intervals in the continual pilot signals. Whilst this phase difference is primarily of use for common phase error correction, nevertheless it may also be employed for automatic frequency control since frequency variations are proportional to the change of phase.
Further, in accordance with the invention, it is recognised that frequency control may be split into two separate controls, namely coarse control for frequency offsets of integral numbers of carrier spacing intervals, and fine frequency control for frequency offsets of fractions of a carrier spacing interval.
For frequency offsets less than one carrier interval, the phase change may be used as representing the fine frequency offset. For coarse frequency control, a signal is used representing rate of change of phase. Since the phase variation between adjacent symbol intervals in continual pilots is constant, a second difference of phase error representing rate of change of phase error should be zero. This therefore provides a means of locating the continual pilot where the coarse frequency offset is a plurality of carrier spacings from the nominal position.
In our copending application (GBP12427A), there is claimed 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, transform means for analysing the samples to provide a series of data symbol values for each carrier frequency, signal processing means for processing the series of data signal values including phase error correcting means, and automatic frequency control means for controlling the frequency of the signals input to the transform means,
wherein the automatic frequency control means includes coarse frequency control means for controlling the frequency in terms of increments of the carrier spacing frequency, and fine frequency control means for controlling the frequency for values less than a single carrier spacing frequency interval,
wherein the coarse frequency means filter means for assessing a group of a predetermined number (N) of carrier signals on either side of the nominal position of a plurality of predetermined continual pilot signals to determine which signal best represents the continual pilot signal, whereby to determine the coarse frequency error.
Problems arise in that where large frequency offsets may be encountered, the number (2N+1) of carrier signals that needs to be assessed becomes large, and the nominal position of more than one continual pilot may be present in the search range. Hence a false minimum value may be assigned as the continual pilot being located. A further problem is that because of the wide search range, the overall filtering effect is diminished and the immunity to noise is decreased.
The present invention provides 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, transform means for analysing the samples to provide a series of data symbol values for each carrier frequency signal processing means for processing the series of data symbol values including phase error correcting means, and automatic frequency control means for controlling the frequency of the signals input to the transform means,
wherein the automatic frequency control means includes coarse frequency control me
Clarke Christopher Keith Perry
Guyot Jean-Marc
Haffenden Oliver Paul
Lauret Régis
Mitchell Justin David
Chin Stephen
Ghayour Mohammad
LSI Logic Corporation
Oppenheimer Wolff & Donnelly
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