Pulse or digital communications – Bandwidth reduction or expansion
Reexamination Certificate
1998-04-29
2001-04-10
Kelley, Chris (Department: 2713)
Pulse or digital communications
Bandwidth reduction or expansion
Reexamination Certificate
active
06215819
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a receiving apparatus and a receiving method and, more particularly, to a receiving apparatus and a receiving method based on an OFDM method.
2. Description of the Related Art
Modulation methods called orthogonal frequency division multiplexing (OFDM) have recently been proposed as a method for transmitting a digital signal. In an OFDM method, a number of subcarriers orthogonal to each other are provided in a transmission band, data items are respectively assigned to the amplitudes and phases of the subcarriers, and digital modulation is performed by phase shift keying (PSK) or quadrature amplitude modulation (QAM). This method uses a reduced band for one subcarrier since the transmission band is divided with respect to a number of subcarriers, so that the modulation speed is reduced. This method, however, achieves the same total transmission speed as other conventional modulation methods because the number of carriers is large.
In this OFDM method, the symbol speed is reduced since a number of subcarriers are transmitted parallel to each other, so that a multipath period relative to the length of a symbol with respect to time can be reduced. Thus, an OFDM method can be expected as a method ensuring high resistance to multipath interference.
Because of the above-described feature, OFDM methods have attracted attention particularly with respect to transmission of digital ground wave signals susceptible to the influence of multipath interference. For example, Digital Video Broadcasting-Terrestrial (DVB-T) is well known as such digital signal transmission by ground waves.
With the recent progress of the semiconductor technology, it has become possible to achieve discrete Fourier transform (hereinafter referred to as FFT (fast Fourier transform)) and discrete inverse Fourier transform (hereinafter referred to as IFFT (inverse fast Fourier transform)) by hardware. If these transforms are used, modulation and demodulation in accordance with an OFDM method can easily be performed. This has also contributed to the increase of attention to OFDM methods.
FIG. 10
is a block diagram showing the configuration of an example of an OFDM receiver. A receiving antenna
101
captures an RF signal. A multiplication circuit
102
calculates the product of the RF signal and a signal which is output from a tuner
103
and which has a predetermined frequency. A bandpass filter
104
extracts the desired IF signal from an output from the multiplication circuit
102
. An A/D (analog to digital) conversion circuit
105
converts the IF signal extracted by the bandpass filter
104
into a digital signal.
A demultiplexer
106
separates and extracts an I channel signal and a Q channel signal from the digitized IF signal. Lowpass filters
107
and
108
respectively convert the I channel signal and the Q channel signal into baseband signals by removing unnecessary high-frequency components contained in the I channel signal and the Q channel signal.
A complex multiplication circuit
109
removes a carrier frequency error in the baseband signals by a signal of a predetermined frequency supplied from a numerical control oscillation circuit
110
, and thereafter supplies the baseband signals to a fast Fourier transform circuit
112
, which frequency-decomposes the OFDM time signals to form I and Q channel received data.
A correlation value calculation circuit
113
calculates a shift average of guard intervals by calculating the product of the OFDM time signal converted into the base band and the OFDM signal delayed by the effective symbol period to obtain a correlation value of the two signals, and makes the fast Fourier transform circuit
112
start calculating when the correlation value becomes maximized.
A carrier frequency error calculation circuit
114
calculates a carrier frequency error by detecting a frequency power deviation and outputs the calculation result to an addition circuit
111
. The addition circuit
111
calculates the sum of the outputs from the carrier frequency error calculation circuit
114
and the correlation value calculation circuit
113
and outputs the calculation result to the numerical control oscillation circuit
110
.
A clock frequency reproduction circuit
115
forms a control signal by referring to the I channel data and Q channel data to control the frequency of oscillation of the clock oscillation circuit
116
. The clock oscillation circuit
116
forms and outputs a clock signal in accordance with the control signal supplied from the clock frequency reproduction circuit
115
.
The operation of the above-described example of the conventional apparatus will next be described.
The multiplication circuit
102
calculates the product of an RF signal captured by the receiving antenna
101
and the signal supplied from the tuner
103
and having a predetermined frequency. The bandpass filter
104
extracts the IF signal from the signal output from the multiplication circuit
102
.
The A/D conversion circuit
105
converts the IF signal output from the bandpass filter
104
into a digital signal in synchronization with the clock signal output from the clock oscillation circuit
116
, and supplies the digital signal to the demultiplexer
106
. The demultiplexer
106
separates and extracts an I channel signal and a Q channel signal from the digitized signal and supplies these signals to the lowpass filters
107
and
108
. The lowpass filters
107
and
108
respectively convert the I channel signal and the Q channel signal into baseband signals by removing aliasing components which are unnecessary high-frequency components contained in the I channel signal and the Q channel signal.
The complex multiplication circuit
109
removes a carrier frequency error in the baseband signals by a signal of a predetermined frequency supplied from the numerical control oscillation circuit
110
, and thereafter supplies the baseband signals to the fast Fourier transform circuit
112
. The fast Fourier transform circuit
112
frequency-decomposes the OFDM time signal to form I and Q channel received data.
The correlation value calculation circuit
113
calculates a value representing a correlation between the OFDM time signal converted into the base band and the OFDM signal delayed by the effective symbol period and makes the fast Fourier transform circuit
112
start calculating when the correlation value becomes maximized. Consequently, the fast Fourier transform circuit
112
can accurately extract data contained in the I channel signal and Q channel signal sent from the transmitting side.
There are various synchronization requirements for correctly demodulating the OFDM signal on the receiving side. For example, it is necessary to synchronize the frequency of oscillation in the numerical control oscillation circuit
110
with the corresponding frequency on the transmitting side in order to convert the OFDM signal in the IF band into the OFDM signal in the base band. It is also necessary to synchronize the clock signal, which is a reference for all the processings, with that on the transmitting side.
A clock reproduction method already proposed, which is used as a method for the latter synchronization of the clock signal with that on the transmitting side, will now be described.
According to the method described below, on the transmitting side, a predetermined number of particular signals prescribed in amplitude and phase (hereinafter referred to as pilot signals other than information to be transmitted are inserted and transmitted with respect to each of symbols. On the receiving side, pilot signals inserted on the transmitting side are extracted from the OFDM signal processed by FFT calculation, and the extracted pilot signals are processed by Costas calculation or the like described below to reproduce the clock signal.
FIG. 11
shows the configuration of a conventional clock reproduction circuit for reproducing a clock signal by using Costas calculation in the case where pilot signals are modulated by
Hyakudai Toshihisa
Ikeda Yasunari
Okada Takahiro
Kelley Chris
Lerner David Littenberg Krumholz & Mentlik LLP
Sony Corporation
Vo Tung T
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