JP2002077101A - Ofdm signal receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an OFDM signal receiver equipped with an automatic gain controller which converts OFDM signals transmitted by switching the lengths of a plurality of kinds of effective symbol periods into digital signals at appropriate levels corresponding to the lengths of the effective symbol periods. SOLUTION: The OFDM signal receiver discriminates the effective symbol period of OFDM signals by means of a mode discriminating circuit 50 and switches a reference voltage switching circuit 120 to refer to an appropriate reference voltage circuit for receiving OFDM signals having the effective symbol length. Then the receiver measures the mean power of the received OFDM signals by means of a mean power measuring circuit 80 and generates a gain control signal for an AGC circuit 30 by comparing the measured mean power with the output of the reference voltage switching circuit 120 by means of an AGC voltage calculating circuit 130.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital broadcast receiving apparatus, and more particularly to a receiving apparatus for a signal modulated by using an orthogonal frequency division multiplexing digital modulation / demodulation system (OFDM system).

[0002]

2. Description of the Related Art Orthogonal frequency division multiplexing
(Ncy Division Multiplex, hereinafter referred to as OFDM) signal modulation method using technology, since digital information is distributed and transmitted to a number of carriers, QAM (quadrature amplitude modulation) to have digital information only on a single carrier Compared with other signal modulation methods, the symbol length is longer,
Less susceptible to delayed waves such as multipath.

In Europe, taking advantage of this feature, DVB-T (Digi
tal Video Broadcasting-Terrestrial)
Terrestrial digital television broadcasting using a modulation method has started. In Japan, ISDB-T (Int
egrated Services Digital Broadcasting-Terrestrial)
A terrestrial digital television broadcast called “Digital Terrestrial Digital Broadcasting” is being started.

[0004]

Here, an OFDM receiver will be examined.

FIG. 4 shows a block diagram of a general OFDM receiver. In FIG. 4, 10 is an OFDM RF signal input terminal, 2
0 is a tuner, 30 is an automatic gain control circuit (hereinafter, referred to as an AGC circuit), 40 is an analog-digital conversion circuit (hereinafter, AD
C), 50 is a mode determination circuit, 60 is a fast Fourier transform circuit (hereinafter referred to as an FFT circuit), 70 is a demodulation circuit, 8
0 is the average power measurement circuit, 91 is the reference voltage circuit, and 130 is
An AGC voltage calculation circuit, 140 is a digital-analog conversion circuit (hereinafter, referred to as DAC), and 200 is an output terminal.

[0006] The frequency division multiplexed OFDM signal propagates through a transmission path, is received at an OFDM RF signal input terminal 10, and is input to a tuner 20.

[0007] The tuner 20 is provided with an OF which exists in a desired band.
Only the DM signal is extracted and input to the AGC circuit 30.

The AGC circuit 30 is an amplifier that varies the gain according to the output signal of the DAC 140. The OFDM signal controlled to a desired level by the AGC circuit 30 is input to the ADC 40.

The ADC 40 converts an analog OFDM signal into a digital signal. The output signal of the ADC 40 is output to the mode determination circuit 50, the FFT circuit 60, the average power measurement circuit 80
Is input to

[0010] The mode determination circuit 50 detects the symbol period of the OFDM signal.

Before describing the mode determination circuit 50, the configuration of the OFDM signal will be described.

FIG. 2 shows the configuration of the OFDM signal. In FIG. 2, 1000 is an OFDM symbol, 1010 is a guard interval, 1020 is an effective symbol, and 1030 is a time axis.

The direction of the arrow on the time axis 1030 indicates the direction in which time advances.

As shown in FIG. 2, OFDM symbol 1000
Is the guard interval 1010 and the effective symbol 1020
It is composed of

[0015] The guard interval 1010 is a signal obtained by translating the signal portion behind the effective symbol 1020 in time. Therefore, the signal behind the guard interval 1010 and the portion after the effective symbol 1020 have a correlation.

That is, in the OFDM signal, only the signal in the part before the OFDM symbol and the signal in the part after the OFDM symbol have a correlation. Therefore, the result of the autocorrelation function with the signal obtained by delaying the OFDM signal by the symbol period has a peak value in the same period as the period of the OFDM symbol.

In an actual OFDM signal, the length of an effective symbol and the length of a guard interval are variable. Therefore, "OFDM
The “type of signal symbol period” includes only “(type of length of effective symbol) × (type of length of guard interval)”.

Here, the description returns to FIG.

The mode determination circuit 50 calculates a correlation function between the output signal of the ADC 40 and a signal obtained by delaying the output signal of the ADC by the symbol period of the OFDM signal, and detects a peak value. The symbol period of the OFDM signal is estimated from the period of the peak value.

FIG. 3 shows an example of the mode determination circuit 50.
3, reference numeral 2000 denotes an input terminal, 2010 denotes a correlation function calculation circuit, 2020 denotes an output terminal, 2030 denotes a delay circuit, and 2040 denotes a delay time switching signal input terminal.

The digitized OFDM signal input from input terminal 2000 is input to delay circuit 2030 and correlation function calculation circuit 2010.

The delay time of delay circuit 2030 is set by a signal from delay time switching signal input terminal 2040.

The signal at the input terminal 2000 and the delay circuit 20
The output signal of 30 is input to a correlation function calculation circuit 2010 to calculate a correlation function.

When the delay time of the delay circuit 2030 set by the signal from the delay time switching signal input terminal 2040 matches the symbol period of the digitized OFDM signal input from the input terminal 2000, a correlation function calculating circuit 20
The calculation result of 10 has a peak value.

The cycle of this peak value is determined by the input terminal 2000
Coincides with the symbol period of the digitized OFDM signal input from.

The output signal of the mode decision circuit 50 and the ADC 4
The output signal of 0 is input to the FFT circuit 60.

The FFT circuit 60 performs fast Fourier transform (hereinafter, referred to as FFT) only on the effective symbol period in the digitized OFDM signal, which is the output signal of the ADC 40.

The FFT of the FFT circuit 60 includes, for example, two types of 2048-point FFT and 8192-point FFT in DVB-T, and 2048-point FFT, 4096-point FFT and 81 in ISDB-T.
There are three types of 92 point FFT. 2048 point FFT, 4
The 096-point FFT and the 8192-point FFT have the effective symbol period of the OFDM signal at equal intervals, each at 2048 points,
4096 points and 8192 points are sampled and FFT is performed.

The output signal of the FFT circuit 60 is input to the demodulation circuit 70, subjected to demodulation processing, and output from the output terminal 200.

The signal output from the output terminal 200 is input to an MPEG (Moving Picture Coding Experts Group) -2 decoder (not shown) in the case of a digital television broadcast receiver, for example.

Next, the operation of the AGC circuit 30 will be described.

The output signal of the ADC 40 is input to the average power measuring circuit 80.

The average power measuring circuit 80 calculates the average power value of the digitized OFDM signal, which is the output signal of the ADC 40. The average power value and the reference voltage value output from the reference voltage circuit 91 are input to the AGC voltage calculation circuit 130, and the control voltage of the AGC circuit 30 is calculated.

Specifically, the AGC voltage calculation circuit 130
ADC from the reference voltage value which is the output from the reference voltage circuit 91
If the average power value of the digitized OFDM signal, which is the output signal of 40, is large, a control signal for reducing the gain of the AGC circuit 30 is generated. Conversely, if the average power value of the digitized OFDM signal, which is the output signal of the ADC 40, is smaller than the reference voltage value output from the reference voltage circuit 91, a control signal for increasing the gain of the AGC circuit 30 is output. appear.

The output signal of the AGC voltage calculation circuit 130 is a DAC
The signal is converted into an analog signal by 140 and input to the AGC circuit 30. In this way, the gain of the AGC circuit 30 is controlled, and the output signal of the tuner 20 becomes a desired level and is input to the ADC 40.

An OFDM signal has a large ratio of peak power to average power. On the other hand, since the rated input voltage range of the ADC 40 is finite, a signal that is necessarily saturated
This is input to the FFT circuit 60. This degrades the probability of erroneously receiving digital data (hereinafter referred to as BER) (deterioration means an increase in BER).

In order to prevent this, it is necessary to appropriately set the signal level by the AGC circuit 30 and input the signal level to the ADC 40. However, the appropriate signal level depends on the length of the effective symbol, that is, the number of FFT points.

[0038]

In order to solve the above problems, an OFDM receiver according to the present invention is configured as follows.

An input terminal for inputting a frequency division multiplexed OFDM signal, and of the frequency division multiplexed OFDM signals input from the input terminal, selectively amplify and output only an OFDM signal present in a desired band. Tuner, an AGC circuit that amplifies the output of the tuner to a desired level, an ADC that converts the output of the AGC circuit into a digital signal, stores the output of the ADC, and outputs a signal delayed for an arbitrary time A delay circuit, a delay time switching signal for switching a delay time of the delay circuit, an output signal of the ADC, a correlation function calculation circuit for calculating a correlation function of the output signal of the delay circuit, and the correlation function calculation circuit A peak cycle measuring circuit for measuring a peak cycle of an output of the ADC; a mode determining circuit for determining an effective symbol period of the OFDM signal from the peak cycle measuring circuit; Using the output, a FFT circuit for fast Fourier transform only effective symbol period,
A demodulation circuit that receives the output of the FFT circuit and performs symbol determination processing, error correction processing, and the like; an average power measurement circuit that calculates average power from the output of the ADC; and an output of the average power measurement circuit and a reference voltage. And an AGC voltage calculation circuit that calculates the control voltage of the AGC circuit, and a DAC that converts the output of the AGC voltage calculation circuit into an analog signal and converts the output into an analog signal for controlling the gain of the AGC circuit. In the OFDM signal receiving apparatus, the reference voltage switches according to the length of the effective symbol period of the OFDM signal.

[0040]

Embodiments of the present invention will be described.

FIG. 1 shows a block diagram of a television signal receiving apparatus according to the first embodiment of the present invention.

In FIG. 1, 10 is an OFDM RF signal input terminal, 20 is a tuner, 30 is an AGC circuit, 40 is an ADC, 50
Is a mode determination circuit, 60 is an FFT circuit, 70 is a demodulation circuit,
80 is an average power measurement circuit, 90 is a first reference voltage circuit,
100 is a second reference voltage circuit, 110 is a third reference voltage circuit, 120 is a reference voltage switching circuit, 130 is an AGC voltage calculation circuit, and 140 is a digital-analog conversion circuit (hereinafter, D
AC), and 200 is an output terminal.

The frequency division multiplexed OFDM signal propagates through a transmission path, is received at an OFDM RF signal input terminal 10, and is input to a tuner 20.

[0044] The tuner 20 controls the OF which exists in the desired band.
Only the DM signal is extracted and input to the AGC circuit 30.

The AGC circuit 30 is an amplifier that varies the gain according to the output signal of the DAC 140. The OFDM signal controlled to a desired level by the AGC circuit 30 is input to the ADC 40.

The ADC 40 converts an analog OFDM signal into a digital signal. The output signal of the ADC 40 is output to the mode determination circuit 50, the FFT circuit 60, the average power measurement circuit 80
Is input to

The mode determination circuit 50 detects the symbol period of the OFDM signal. Here, the configuration of the OFDM signal is as described with reference to FIG. Therefore, the mode determination circuit 5
0 calculates the correlation function between the output signal of the ADC 40 and the signal obtained by delaying the output signal of the ADC by the symbol period of the OFDM signal, and detects the peak value. The symbol period of the OFDM signal is estimated from the period of the peak value.

An example of the mode determination circuit 50 described with reference to FIG. 3 can be considered.

The output signal of the mode decision circuit 50 and the ADC 4
The output signal of 0 is input to the FFT circuit 60.

The FFT circuit 60 performs FFT only on the effective symbol period in the digitized OFDM signal, which is the output signal of the ADC 40.

The FFT of the FFT circuit 60 includes, for example, two types of 2048-point FFT and 8192-point FFT in DVB-T, and 2048-point FFT, 4096-point FFT and 81 in ISDB-T.
There are three types of 92 point FFT. 2048 point FFT, 4
The 096-point FFT and the 8192-point FFT have the effective symbol period of the OFDM signal at equal intervals, each at 2048 points,
4096 points and 8192 points are sampled and FFT is performed.

The output signal of the FFT circuit 60 is input to the demodulation circuit 70, subjected to demodulation processing, and output from the output terminal 200.

The signal output from the output terminal 200 is input to, for example, an MPEG (Moving Picture Coding Experts Group) -2 decoder (not shown) in the case of a digital television broadcast receiver.

Next, the operation of the AGC circuit 30 will be described.

The output signal of the ADC 40 is input to the average power measuring circuit 80.

The average power measuring circuit 80 calculates the average power value of the digitized OFDM signal, which is the output signal of the ADC 40. The average power value and the reference voltage value output from the reference voltage switching circuit 120 are calculated by the AGC voltage calculation circuit 130.
To calculate the control voltage of the AGC circuit 30.

The reference voltage switching circuit 120 determines the length of the effective symbol period of the OFDM signal from the output of the mode determination circuit 50, and thereby determines the first reference voltage circuit 90, the second reference voltage circuit 100, and the third reference voltage circuit. Among the three reference voltage circuits 110, a reference voltage corresponding to a desired effective symbol period length is input to the AGC voltage circuit 130.

The AGC voltage calculation circuit 130 determines the gain of the AGC circuit 30 if the average power value of the digitized OFDM signal, which is the output signal of the ADC 40, is larger than the reference voltage value output from the reference voltage switching circuit 120. Generates a control signal to reduce the Conversely, if the average power value of the digitized OFDM signal, which is the output signal of the ADC 40, is smaller than the reference voltage value output from the reference voltage switching circuit 120, a control signal for increasing the gain of the AGC circuit 30 Occurs.

Here, in FIG. 1, based on the ISDB-T system,
Although three reference voltage circuits are provided, an OFDM signal receiving apparatus for receiving an OFDM signal having N types of effective symbol periods has N types of reference voltages.

The output signal of the AGC voltage calculation circuit 130 is
The signal is converted into an analog signal by 140 and input to the AGC circuit 30. In this way, the gain of the AGC circuit 30 is controlled, and the output signal of the tuner 20 becomes a desired level and is input to the ADC 40.

[0061]

According to the present invention, an OFDM signal transmitted by switching the lengths of a plurality of types of effective symbol periods can be input to the ADC at an appropriate level according to the length of the effective symbol periods, A receiving device capable of performing digital demodulation processing with little deterioration of the signal error rate can be configured.

[Brief description of the drawings]

FIG. 1 is a block diagram illustrating a configuration of an OFDM receiver according to a first embodiment of the present invention.

FIG. 2 is a diagram illustrating a configuration of an OFDM signal.

FIG. 3 is a block diagram illustrating an example of a mode determination circuit.

FIG. 4 is a block diagram showing a general OFDM receiving apparatus.

[Explanation of symbols]

10: OFDM RF signal input terminal, 20: tuner, 30: A
GC circuit, 40 ADC, 50 mode judgment circuit, 60 FFT
Circuit, 70 demodulation circuit, 80 average power measurement circuit, 90
... First reference voltage circuit, 91 ... Reference voltage circuit, 100 ...
Second reference voltage circuit, 110... Third reference voltage circuit, 1
20: reference voltage switching circuit, 130: AGC voltage calculation circuit,
140 DAC, 200 output terminal, 1000 OFDM symbol, 1010 guard interval, 1020 effective symbol, 1030 time axis, 2000 input terminal, 20
10: Correlation function calculation circuit, 2020: Output terminal, 203
0: delay circuit, 2040: delay time switching signal input terminal

Continuing from the front page (72) Inventor Hiroyuki Mizukami 292 Yoshida-cho, Totsuka-ku, Yokohama-shi, Kanagawa Prefecture Hitachi, Ltd. Digital Media Development Division F-term (Reference) 5C025 AA25 AA28 BA11 BA14 BA25 BA27 DA01 5C026 BA01 BA12 5K022 DD01 DD13 DD19 DD33 DD34 5K061 AA11 BB06 BB07 CC52 CD04

Claims (3)

[Claims] A tuner for receiving an OFDM signal and selecting and outputting only the OFDM signal present in a desired band; a variable gain amplifying unit for amplifying an output of the tuner to a desired level; Calculating a correlation function of the output signal of the unit and a signal obtained by delaying the output signal, a peak period measuring unit that measures a peak period, and a mode determining unit that determines an effective symbol period of the OFDM signal from the peak period measuring unit. A fast Fourier transform unit for fast Fourier transforming only an effective symbol period from an output signal of the variable gain amplifying unit using an output of the mode decision unit; and a symbol decision process for an output of the fast Fourier transform unit. A demodulator that performs an error correction process, an average power measuring unit that calculates an average power from the output of the variable gain amplifying unit, and the validity of the OFDM signal determined by the mode determining unit. A voltage calculating unit that compares an output of the average power measuring unit with one of a plurality of types of reference voltages and calculates a control voltage of the variable gain amplifying unit according to a length of the symbol period. OFDM receiver. 2. The OFDM receiving apparatus according to claim 1, wherein the voltage calculation unit switches and compares a plurality of reference voltage units that hold the plurality of types of reference voltages. 3. Receiving a frequency division multiplexed OFDM signal
In the OFDM receiver, an input terminal for inputting a frequency division multiplexed OFDM signal, and selectively amplify only an OFDM signal present in a desired band among frequency division multiplexed OFDM signals input from the input terminal A tuner that amplifies the output of the tuner to a desired level (hereinafter, referred to as an AGC circuit); and an analog-to-digital converter that converts the output of the AGC circuit into a digital signal.
A digital conversion circuit (hereinafter referred to as an ADC), a delay circuit that stores an output of the ADC and outputs a signal delayed by an arbitrary time, a delay time switching signal for switching a delay time of the delay circuit, A correlation function calculating circuit that calculates a correlation function of the output signal of the ADC and the output signal of the delay circuit; a peak cycle measuring circuit that measures a peak cycle of an output of the correlation function calculating circuit; and a peak cycle measuring circuit. A mode determination circuit that determines an effective symbol period of an OFDM signal; and an FF that performs fast Fourier transform only on the effective symbol period from an output OFDM signal of the ADC using an output of the mode determination circuit.
A T circuit, a demodulation circuit that receives an output of the FFT circuit and performs symbol determination processing, error correction processing, and the like; an average power measurement circuit that calculates average power from the output of the ADC; The output and the reference voltage are compared and the AGC
An AGC voltage calculation circuit for calculating a control voltage of the circuit; and converting an output of the AGC voltage calculation circuit into an analog signal.
O having a digital-analog converter (hereinafter referred to as DAC) for converting to an analog signal for controlling the gain of the GC circuit
An OFDM signal receiving apparatus, wherein the reference voltage is switched according to the length of an effective symbol period of the OFDM signal.