Pulse or digital communications – Receivers – Automatic frequency control
Reexamination Certificate
1998-05-01
2001-05-29
Chin, Stephen (Department: 2734)
Pulse or digital communications
Receivers
Automatic frequency control
C375S340000, C375S147000
Reexamination Certificate
active
06240146
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 carrier 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. Naturally, various practical problems arise in demodulation in a receiver, firstly in down-converting the transmitted signal in a tuner to a frequency at which demodulation can be carried out, and secondly by accurately demodulating the data from a large number of carriers in a demodulator which is not overly complex or expensive.
In the receiver, frequency offsets may appear after the tuner down-conversation due to oscillator tolerance. Such frequency offset is lethal for signal recovery and frequency has therefore to be tracked by Automatic Frequency Control (AFC). In addition, oscillator phase noise introduces a so-called Common Phase Error (CPE) term, which is a phase offset all carriers bear, and that varies randomly from symbol to symbol. This effect has also to be compensated. Finally, the channel response may not be flat, due to echoes, interferers, and a Channel Equalizer is required to correct for such channel imperfections.
An important consideration in designing a demodulator for incorporation in an integrated circuit chip is reducing the requirements for memory. Bearing in mind the chip may only contain about 1 M Bit of memory, and that signal values for up to about 7000 carrier frequencies may be processed in the chip, this requires tight control over the use of available memory. Certain operations such as Fourier transformation and symbol interleaving require fixed amounts of memory (about 50% of the total). However, other operations such as timing synchronization, common phase error (CPE) correction, and Channel Equalization (CE) require some memory but the amount of memory is not fixed.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides an apparatus for demodulating a digital video broadcast signal comprising data modulated on a multiplicity of spaced carrier frequencies, the apparatus including:
transform means for analyzing the broadcast signal to provide a series of symbol values for each of the multiplicity of carrier frequencies,
automatic frequency control means for controlling the frequency of said series of signal values in dependence on a common phase error signal from said series of symbol values,
first and second delay means coupled in series to receive said series of symbol values from said transform means, and means for deriving from symbol values in the first and second delay means a common phase error signal,
first and second common phase error correction (CPE) means for receiving said common phase error signal, for correcting said symbol values from said transform means, the output of the first delay means being applied to the first CPE means and the output of the second delay means being applied to said second CPE means,
channel equalization means for compensating for communication channel impairments for receiving directly the phase error corrected signals from said first CPE means, and third and fourth delay means connected in series and connected to receive the output from said second CPE means and applying delayed versions of the same to the channel equalization means.
In a more specific aspect, the invention provides apparatus for demodulating digital video broadcast signals comprising data modulated on a multiplicity of spaced carrier frequencies, including:
down-conversion means for converting an input broadcast signal to a frequency sufficiently low to enable analog to digital conversion of the signal;
analog to digital conversion means for converting the broadcast signal to a series of digital samples, real to complex conversion means for converting each digital sample to a complex number value, Fourier Transform means for analyzing the complex number values to provide a series of symbol values for each carrier frequency, frequency control means, comprising means responsive to the output of said Fourier Transform means for producing a signal for controlling the frequency of the signal formed by said complex number values, wherein the frequency control means derives a common phase error signal from said series of symbol values,
first and second delay means coupled in series to receive said series of symbol values from said transform means, and for providing delayed versions to said automatic frequency control means,
first and second common phase error correction (CPE) means for receiving said common phase error signal, for correcting said symbol values from said transform means, the output of the first delay means being applied to the first CPE means and the output of the second delay means being applied to said second CPE means,
channel equalization means for compensating for communication channel impairments for receiving directly the phase error corrected signals from said first CPE means, and third and fourth delay means connected in series and connected to receive the output from said second CPE means and applying delayed versions of the same to the channel equalization means.
In accordance with the invention, the input broadcast signal which is normally a UHF signal, say 700 MHz, is down converted, preferably in two stages, firstly to about 30-40 MHz and secondly to about 4.5 MHz. Since the bandwidth of the signal is about 7.6 MHz, an IF frequency of 4.5 MHZ represents essentially a DC or base band signal which can then be sampled by means of an analog to digital converter. Subsequent to analog to digital conversion, the sampled signal is converted to complex number values, in order to represent a true DC signal centred on 0 Hz. This facilitates the operation of the Fourier transform device which as mentioned above is normally an FFT performing a DFT on each carrier signal. The result of the transform is a series of data symbol values for the symbols encoded on each carrier wave.
The data is processed, principally for channel equalization and for weighting the contribution of each channel by the derived Channel State Information.
Another signal processing employed is correction for common phase error. As will become clear below, phase error in COFDM signals is present in two components, a random component and a component which is common to all carriers, arising from local oscillator phase noise. Such common phase error may be removed by a technique as described in more detail below.
The process of demodulation requires very accurate tracking of the input signal and to this end automatic frequency control and timing control are desirable. Timing control is necessary in order to ensure that the timing window for the FFT is correctly positioned in relation to the input waveforms. Thus, the sampling by the ADC must be synchronized with the input wave forms. For an input signal centred on 4.57 MHz, an ADC operating frequency of 18.29 MHz (4.57×4) is preferred. The ADC is maintained in synchronization by a l
Clarke Christopher Keith Perry
Guyot Jean-Marc
Haffenden Oliver Paul
Mitchell Justin David
Regis Lauret
Chin Stephen
Jiang Lenny
LSI Logic Corporation
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