Demodulators – Phase shift keying or quadrature amplitude demodulator
Reexamination Certificate
2001-07-10
2004-04-13
Pascal, Robert (Department: 2817)
Demodulators
Phase shift keying or quadrature amplitude demodulator
C375S324000, C375S326000
Reexamination Certificate
active
06720824
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a demodulation method and apparatus applicable to e.g., digital broadcast in accordance with the orthogonal frequency division multiplexing system (OFDM).
2. Description of Related Art
Recently, a modulation system, termed the orthogonal frequency division multiplexing system (OFDM), has been proposed as a system for transmitting digital signals. This OFDM system is such a system in which a large number of orthogonal sub-carriers are provided in a transmission band and data are assigned to the amplitudes and phases of the respective sub-carriers to effect digital modulation in accordance with the PSK (phase shift keying) or QAM (quadrature amplitude modulation).
This OFDM system has a feature that, although the band per sub-carrier is narrow to slow down the modulation rate due to the splitting of the transmission band with a large number of sub-carriers, the total transmission rate is unchanged from that in the conventional modulation system. Moreover, the OFDM system has a feature that the symbol rate is lowered due to parallel transmission of a large number of sub-carriers. Consequently, this OFDM system is less susceptible to multipath interference because the multipass time duration relative to the symbol time duration can be shorter. In addition, this OFDM system has a feature that, since data are assigned to plural sub-carriers, it is possible to construct a transmission/reception circuit by employing a calculation circuit performing inverse fast Fourier transform (IFFT) during modulation and also by employing fast Fourier transform (FFT) during demodulation.
In light of the above characteristics, the possibility of application of the OFDM system to terrestrial digital broadcast susceptible strongly to multipass interference is scrutinized extensively. For the terrestrial digital broadcast, to which is applied the OFDM system, such standards as DVB-T (Digital Video Broadcasting-Terrestrial) or ISDB-T (Digital Video Broadcasting-Terrestrial) or ISDB-T (Integrated Services Digital Broadcasting-Terrestrial) have been proposed.
The reception apparatus for digital television broadcast in accordance with the OFDM system is now explained.
FIG. 1
is a block diagram showing a conventional OFDM reception apparatus.
In
FIG. 1
, if the signals transmitted between the blocks are complex or real number signals, signal components are expressed by thick and fine lines, respectively.
Referring to
FIG. 1
, a conventional OFDM reception apparatus
100
includes an antenna
101
, a tuner
102
, a bandpass filter (BPF)
103
, an A/D converter
104
, a digital quadrature demodulation circuit
105
, an fc correction circuit
106
, an FFT calculation circuit
107
, a fine range fc error calculation circuit
108
, a wide range fc error calculation circuit
109
, a numerical control oscillator (NCO)
110
, an FFT window synchronization circuit
111
, a CPE cancellation circuit
112
, a CPE calculating circuit
113
, an equalizer
114
, a detection error correction circuit
115
and a transmission control information demodulating circuit
116
.
The digital television broadcast waves, aired from a broadcasting station, are received by the antenna
101
of the OFDM reception apparatus
100
, and thence supplied as RF signals to the tuner
102
.
The RF signals, received by the antenna
101
, are frequency-converted into IF signals by the tuner
102
, comprised of a local oscillator
102
a
and a multiplier
102
b,
and thence supplied to the BPF
103
. The IF signals are filtered by the BPF
103
and digitized by the A/D converter
104
so as to be sent to the digital quadrature demodulation circuit
105
.
The digital quadrature demodulation circuit
105
demodulates the digitized IF signals, using carrier signals of a pre-set frequency (carrier frequency or fc) to output base-band OFDM signals. The base-band OFDM signals, output by this digital quadrature demodulation circuit
105
, are so-called time-domain signals prior to FFT calculations. So, the baseband signals prior to the FFT calculations following the digital quadrature demodulation are hereinafter called OFDM time domain signals. The OFDM time domain signals, quadrature demodulated, are complex signals comprised of real-axis components (I-channel signals) and imaginary-axis components (Q-channel signals). The OFDM time domain signals, output by the digital quadrature demodulation circuit
105
, are sent to the fc correction circuit
106
.
The fc correction circuit
106
executes complex multiplication of the carrier frequency error correction signal, output from the NCO
110
, with the OFDM time domain signals, to correct the carrier frequency error of the OFDM time domain signals. The carrier frequency error is an error in the center frequency position of the OFDM time domain signals, produced due e.g., to the deviation in the reference frequency output from e.g., a local oscillator
102
a.
If this error is increased, an error rate of output data is increased. The OFDM time domain signals, corrected for carrier frequency errors, are fed to the FFT calculation circuit
107
and to the fine range fc error calculation circuit
108
.
The FFT calculation circuit
107
performs FFT calculations on the OFDM time domain signals to extract data quadrature demodulated in each sub-carrier to output the extracted data. Output signals of the FFT calculation circuit
107
are so-called frequency domain signals following the FFT. So, the signal following the FFT calculations are referred to below as OFDM frequency domain signals.
Meanwhile, the OFDM time domain signals are transmitted in terms of an OFDM symbol as a unit of transmission, as shown in FIG.
2
. This OFDM symbol is made up of an effective symbol, as a signal period during which IFFT occurs during transmission, and a guard interval during which the waveform of a latter portion of this effective symbol has been copied directly. This guard interval is provided in the former portion of the OFDM symbol. In the OFDM system, multipass durability is improved by provision of this guard interval. In the DVB-T standard (2K mode), for example, 2048 sub-carriers are contained in the effective period, with the sub-carrier interval being 4.464 kHz. It is in 1705 of the 2048 sub-carriers in the effective symbol that the data is modulated. The guard interval is the signal having one quarter time duration of the effective symbol. Meanwhile, in the OFDM reception apparatus, the OFDM symbol is quantized in accordance with the DVB-T standard (2K mode) by the A/D converter
104
using clocks sampling the effective symbol of the OFDM time domain signals and the guard interval with 2048 and 512 samples, respectively.
The FFT calculation circuit
107
extracts signals within the range of the effective symbol length, such as 2048 samples, from one OFDM sample, that is it eliminates the range of the guard interval from one OFDM symbol, and executes FFT calculations on the so-extracted 2048 samples of the OFDM time domain signals. Specifically, the position of starting the calculations is an optional position between the boundary of the OFDM symbol (position A of
FIG. 1
) and the end position of the guard interval (position B in FIG.
2
), as shown in FIG.
2
. This range of calculations is termed an FFT window.
Thus, similarly to the OFDM time domain signals, the OFDM frequency domain signals, output from the FFT calculation circuit
107
, are complex signals comprised of real components (I-channel signals) and imaginary components (Q-channel components). The OFDM frequency domain signals are sent to the fc error calculation circuit
109
, CPE cancellation circuit
112
and to the CPE calculating circuit
113
.
The fine range fc error calculation circuit
108
and the wide range fc error calculation circuit
109
calculate the carrier frequency error contained in the OFDM time domain signals following digital quadrature demodulation by the digital quadrature demodulation circuit
105
. Specifically, the fine range fc error calcul
Hyakudai Toshihisa
Okada Takahiro
Frommer William S.
Frommer & Lawrence & Haug LLP
Glenn Kimberly E
Kessler Gordon
Pascal Robert
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