JP3803149B2 - OFDM receiver - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a receiver that employs orthogonal frequency division multiplexing (OFDM).
[0002]
[Prior art]
Since the OFDM system is resistant to multipath, it has attracted much attention as a digital audio broadcasting (DAB) system, and research or broadcasting using this has been partially put into practical use.
[0003]
[Problems to be solved by the invention]
However, high precision is required for the oscillator of the down converter used in the receiving unit adopted in the OFDM system, and although it is strong against multipath, when receiving while moving at high speed, there is no adverse effect of multipath. I can't say I won't receive it.
Therefore, in view of the above problems, the present invention provides an OFDM receiver that can alleviate high-precision requirements for an oscillator and reduce the adverse effects of multipath.
[0004]
[Means for Solving the Problems]
In order to solve the above problem, the present invention provides an OFDM receiver that receives an OFDM signal phase-modulated for each of orthogonal quadrature carriers, a variable gain amplifier that amplifies the received OFDM signal, and the received signal An oscillator for down-converting the OMDM signal, and AGC / AFC control for automatically controlling the gain of the variable gain amplifier and controlling the frequency of the oscillator based on the result of demodulating the multi-carrier phase modulation The AGC / AFC control unit displays an arbitrary one phase of the demodulated multi-carrier in each quadrant of the complex plane and displays an ideal reception in each of these vectors and each quadrant. Subtract the vector in the state and rotate each vector obtained by the subtraction according to each quadrant of the complex plane to remove the phase modulation component. Forming a control vector, and detecting the length of the control vector and detecting the position of the tip of the control vector on the positive or negative side of the imaginary axis with respect to the real axis, The gain of the variable gain amplifier is controlled so that the length of the control vector becomes small, and the frequency of the oscillator is controlled so that the tip of the control vector is located on the real axis. By this means, it is possible to alleviate the high accuracy requirement for the oscillator and reduce the adverse effects of multipath.
[0005]
The AGC / AFC control unit periodically and randomly changes any one of the demodulated multicarriers to another carrier. By this means, it becomes possible to cope with the influence of wideband multipath.
Furthermore, a diversity antenna and an AGC / AFC / diversity control unit provided with a diversity control function for performing switching control of the diversity antenna in the AGC / AFC control unit are provided. In the AGC / AFC / diversity control unit, If the tip of the control vector is within a certain circle, the gain of the variable gain amplifier is controlled, and if it is outside the certain circle, diversity control is performed. Furthermore, a diversity antenna and an AGC / AFC / diversity control unit provided with a diversity control function for performing switching control of the diversity antenna in the AGC / AFC control unit are provided. In the AGC / AFC / diversity control unit, If the tip of the control vector is in a range surrounded by two straight lines sandwiching the real axis, the frequency of the oscillator is controlled, and if it is outside this range, diversity control is performed. By this means, not only AGC and AFC but also information for diversity control can be obtained from the position information of the carrier, so that information can be effectively used, and further a diversity function is added to AGC and AFC control. Better reception is possible.
[0006]
The AGC / AFC control unit obtains the control vector for each of a plurality of carriers in the band of the received OFDM signal, and further detects and controls the length of the control vector using a control vector obtained by averaging these vectors. Detect the tip of the vector. The AGC / AFC control unit obtains the control vector for each of a plurality of carriers in the band of the received OFDM signal, and should control the gain of the variable gain amplifier, control the frequency of the oscillator, or In the case of controlling the gain of the variable gain amplifier or the frequency of the oscillator when the diversity control is determined based on the principle of majority decision, the control amount of the gain of the variable gain amplifier or the frequency of the oscillator is controlled. Average the amount. Since a plurality of carriers in the band are observed, the amount of information is increased as compared with the case of observing only one carrier, and more reliable AFC and AGC control is possible.
[0007]
A diversity antenna for receiving the OFDM signal and a variable gain amplifier for amplifying the received OFDM signal in an OFDM receiver that receives a π / 4 shift DQPSK phase-modulated OFDM signal for each of the orthogonal multicarriers And an oscillator for down-converting the received OMDM signal, and based on a result of demodulating the phase modulation of the multicarrier, automatically controlling the gain of the variable gain amplifier, automatically controlling the frequency of the oscillator, An AGC / AFC / diversity control unit that controls switching of diversity antennas, and the AGC / AFC / diversity control unit displays any one phase of the demodulated multicarrier in each quadrant of the complex plane as a vector display Subtract the previous vector from the current vector , Rotate to obtain a control vector representing a temporal phase change, and if the tip of the control vector is within a certain circle centered on the ideal state, control of the gain of the variable gain amplifier and the oscillator The frequency is controlled, and if it is outside a certain circle, the diversity antenna is switched. The AGC / AFC / diversity control unit obtains two adjacent phase differences of the demodulated multicarrier, and controls the frequency of the oscillator so that the phase difference is close to the ideal phase difference. Also good. By this means, it is possible to alleviate the high accuracy requirement for the oscillator and reduce the adverse effects of multipath.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Embodiments of the present invention will be described below with reference to the drawings.
FIG. 1 is a diagram showing main blocks of an OFDM receiver according to an embodiment of the present invention. As shown in the figure, an OFDM signal is received by the antenna 1. The radio frequency (RF) receiver 2 connected to the antenna includes a variable gain amplifier 2A that amplifies the received signal, an oscillator 2B that downconverts the high frequency, and the like. IQ generation converts the output signal of the RF receiver 2 into an in-phase component (I) and a quadrature component (Q). An FFT (Fast Fourier Transform) demodulator 4 inputs a digital signal obtained by changing the output signal of IQ generation 3 from analog, performs Fast Fourier Transform, and forms a demodulated signal. The AFC / AGC control unit 5 forms an AFC (automatic frequency control) control signal and an AGC (automatic gain control) control signal based on the demodulated signal obtained by the FFT demodulating unit 4, and the frequency of the oscillator 2B of the high frequency receiving unit 2 The gain of the variable gain amplifier 2A is controlled.
[0009]
FIG. 2 is a diagram for explaining the amplitude frequency characteristics of the received OFDM signal. As shown in the figure, the OFDM signal is composed of, for example, 1536 orthogonal multicarriers, and its transmission bandwidth is 1.5 MHz. Each carrier is modulated by, for example, QPSK (quaternary phase modulation), but demodulated by the FFT demodulator 4.
FIG. 3 is a diagram showing the carrier phase in the ideal reception state obtained by the FFT demodulator 4. In the complex plane shown in the figure, in an ideal reception state when there is no adverse effect such as multipath, the phases of the carriers calculated by the FFT demodulator 4 are concentrated at four points. On the other hand, in an ordinary state, an ideal reception state is not achieved under conditions such as transmission quality such as multipath and oscillator accuracy for the down converter, and variation occurs from the ideal state on the above phase diagram.
[0010]
FIG. 4 is a diagram for explaining a processing example of the control operation of the AFC / AGC control unit 5. As shown in this figure, in the AFC / AGC control unit 5, when the reception map position (black circle) from the FFT demodulation unit is in the first quadrant of the complex plane, the vector of the reception map position and the ideal reception position (dotted line circle) Subtraction with the vector of
Next, the buckle obtained by the subtraction is translated to the origin to form a dotted line vector. Then, the dotted line vector is rotated clockwise by π / 4 to obtain a solid line vector. In this way, the same processing is performed in the second quadrant, the third quadrant, and the fourth quadrant, but the dotted subtraction vector is 3π / 4 in the second quadrant, and in the third quadrant. In the case of the fourth quadrant by 5π / 4, it rotates clockwise by 7π / 4 to obtain a solid line subtraction vector. Since the received signal is in one of the four quadrants, the influence of the quadrant is removed in this way, that is, the phase modulation component is removed so that processing can be performed for each reception, thereby improving the responsiveness.
[0011]
FIG. 5 is a diagram for explaining the characteristics of a solid line vector. As shown in the figure, regarding the subtraction vectors OA ′ and OB ′ of the solid line with respect to the vectors OA and OB at the two reception map positions, the subtraction vector OA ′ is on the positive side of the imaginary axis with respect to the real axis (real). Yes, the subtraction vector OB 'is on the negative side of the imaginary axis with respect to the real axis. The angle α of the reception map position vector OA with respect to the real axis (real), the angle β of the reception map position vector OA with respect to the real axis (real), and the angle π / of the ideal map position with respect to the real axis (real) of the vector OS. 4
α> π / 4> β
Are in a relationship.
[0012]
In such a case, first, the length of the subtraction vectors OA ′ and OB ′ represents the distance on the phase diagram between the reception carrier and the ideal reception carrier.
Next, from the viewpoint of the position of the tip of the subtraction vector OA ′, since the tip of the subtraction vector OA ′ is on the positive side of the imaginary axis with respect to the real axis, the angle α of the reception map position vector OA with respect to the real axis is ideal. This indicates that the map position is larger than the angle π / 4 with respect to the real axis of the vector OS. Since the tip of the subtraction vector OB ′ is on the negative side of the imaginary axis with respect to the real axis, the angle β of the received map position vector OB with respect to the real axis is the angle β of the ideal map position vector OS with respect to the real axis (real). It is smaller than π / 4.
[0013]
Utilizing such a property of the subtraction vector, the AFC / AGC control unit 5 performs the vector calculation as described above by paying attention to any one carrier of multicarriers of the received OFDM signal. Then, the AFC / AGC control unit 5 controls the variable gain amplifier 2A so that the length of the subtraction vector described above becomes small for the selected carrier.
[0014]
In addition, when the tip of the subtraction vector is on the positive side of the imaginary axis with respect to the real axis, it moves in the clockwise direction on the phase diagram, and on the contrary, when it is on the negative side, it is counterclockwise. Control to move in the direction. By such control, the tip of the subtraction vector is positioned on the real axis. The amount of control is determined in relation to the distances AHA and BHB from the tip of the subtraction vector to the real axis.
[0015]
Therefore, according to the present invention, it is possible to alleviate the high accuracy requirement for the oscillator, which is the intended purpose, and to reduce the adverse effects of multipath.
In addition, you may make it change the carrier which pays attention to the above regularly, for example at random. Moreover, you may have the table of the carrier to which attention is paid in order to change at random. It becomes possible to deal with the effects of broadband multipath.
[0016]
FIG. 6 is a diagram showing another main block of the OFDM receiver according to the embodiment of the present invention. The components different from FIG. 1 are provided with diversity antennas 1-1 and 1-2 in place of the antenna 1, an antenna switching unit 6 in front of the RF receiving unit 2, and a diversity control unit 7 for controlling the antenna switching unit. Along with these additions, instead of the AFC / AGC control unit 5, a diversity control unit 7, a variable gain control unit 2A of the RF reception unit 2, and an AFC / AGC / diversity control unit 5A for outputting a control signal to the oscillator 2B are provided. Is to be.
[0017]
FIG. 7 is a diagram for explaining an example of the control operation of the AFC / AGC / diversity control unit 5A in FIG. As shown in this figure, if the length of the subtraction vector is within the dotted circle in the complex plane of the selected carrier, the AFC / AGC / diversity control unit 5A outputs a control signal to perform AGC control. If it is outside the dotted circle, a diversity control signal is output. The dotted circle indicates the limit of AGC control.
[0018]
FIG. 8 is a diagram for explaining an example of the control operation of the AFC / AGC / diversity control unit 5A in FIG. As shown in this figure, if the distances AHA and BHB (see FIG. 5) from the leading end of the subtraction vector to the real axis are within the range surrounded by two straight lines between which the real axis is sandwiched, the AFC / AGC / diversity control unit 5A A control signal is output, and a diversity control signal is output if the angle exceeds the range surrounded by the two straight lines that sandwich the real axis. The angle in the range surrounded by the two dotted lines across the real axis indicates the limit of AFC control.
[0019]
Therefore, according to the present invention, not only AGC and AFC but also information for diversity control can be obtained from the position information of the carrier, so that the information can be effectively used, and the diversity function is further added to AGC and AFC control. By being added, better reception becomes possible.
In the above description, attention is paid to one carrier in the received OFDM signal. However, the above processing is simultaneously performed on a plurality of appropriate carriers in the OFDM signal band, and the vector is averaged to calculate AGC, AFC, or diversity. Control may be performed.
[0020]
Further, the above processing may be performed simultaneously on a plurality of carriers, and AGC, AFC, and diversity control may be determined in a majority manner based on these processing results. At this time, when AGC and AFC are executed, the control amount may be an average value of control amounts obtained from the respective vectors.
Therefore, according to the present invention, by observing a plurality of carriers in the band, the amount of information is increased as compared to observing only one carrier, and more reliable control of AFC, AGC, etc. can be performed.
[0021]
Next, a case where each carrier of the received OFDM signal is subjected to π / 4 shift DQPSK modulation (eg, Eureka 147, DAB) will be described. When each carrier of the received OFDM signal is subjected to π / 4 shift DQPSK modulation, the phase state of a certain carrier rotates by π / 4 from time to time.
FIG. 9 is a diagram for explaining a state of phase change in π / 4 shift DQPSK modulation. As shown in the figure, the reception map position at time t + 1 is obtained with respect to the reception map position at time t. Therefore, paying attention to a certain carrier, the phase change vector is formed by subtracting and rotating the vector (on the phase) of the carrier received at time t from the vector (on the phase) of the carrier received at time t + 1. .
[0022]
FIG. 10 is a diagram for explaining the phase change vector. As shown in the figure, the phase change vector is the center of a dotted circle in the ideal state. Even if the phase change vector deviates from the ideal state, if the phase change vector is within the dotted circle, the AFC / AGC / diversity control unit 5A outputs the AGC / AFC control signal, and if it is out of the circle, the diversity control signal is output.
[0023]
Further, when the OFDM signal received at a certain time is demodulated by π / 4 shift DQPSK, in an ideal reception situation, any phase change of 0, π, ± π / 2 occurs in adjacent carrierization. In an actual reception situation, a phase change of ± γ occurs due to various factors. Using this fact, the phase change amount between the carriers of the OFDM signal may be examined, and AFC control may be performed so that the change amount is close to the ideal state.
[0024]
Therefore, according to the present invention, better reception is possible for π / 4 shift DQPSK.
In the above description, QPSK has been described. However, the present invention is not limited to this, and is applicable to, for example, 16-value QAM.
[Brief description of the drawings]
FIG. 1 is a diagram showing main blocks of an OFDM receiver according to an embodiment of the present invention.
FIG. 2 is a diagram for explaining amplitude frequency characteristics of a received OFDM signal.
FIG. 3 is a diagram showing a carrier phase in an ideal reception state obtained by the FFT demodulator 4;
FIG. 4 is a diagram for explaining a processing example of a control operation of an AFC / AGC control unit 5;
FIG. 5 is a diagram for explaining characteristics of a solid line vector;
FIG. 6 is a diagram showing another main block of the OFDM receiver according to the embodiment of the present invention.
7 is a diagram illustrating an example of a control operation of an AFC / AGC / diversity control unit 5A in FIG.
8 is a diagram illustrating an example of a control operation of an AFC / AGC / diversity control unit 5A in FIG.
FIG. 9 is a diagram for explaining a state of phase change in π / 4 shift DQPSK modulation.
FIG. 10 is a diagram illustrating a phase change vector.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1, 1-1, 1-2 ... Antenna 2 ... RF receiving part 3 ... IQ generation 4 ... FFT demodulation part 5 ... AFC / AGC control part 5A ... AFC / AGC / diversity control part 6 ... Antenna switching part 7 ... Diversity control Part

Claims (8)

In an OFDM receiver that receives a phase modulated OFDM signal for each of the orthogonal multicarriers,
A variable gain amplifier for amplifying the received OFDM signal;
An oscillator for down-converting the received OMDM signal;
An AGC / AFC controller that automatically controls the gain of the variable gain amplifier and automatically controls the frequency of the oscillator based on the result of demodulating the phase modulation of the multicarrier,
The AGC / AFC control unit displays an arbitrary one phase of the demodulated multicarrier as a vector in each quadrant of the complex plane, and influences such as transmission quality and transmission / reception device accuracy in each quadrant and each quadrant Subtracting with the vector in the ideal reception state that does not receive, rotating each vector obtained by the subtraction according to each quadrant of the complex plane to remove the phase modulation component to form a control vector, The length of the control vector is reduced by detecting the length of the control vector and detecting the position of the tip of the control vector on the positive or negative side of the imaginary axis with respect to the real axis. An OFDM receiver characterized by controlling the gain of the variable gain amplifier and controlling the frequency of the oscillator so that the tip of the control vector is located on the real axis.   The OFDM receiver according to claim 1, wherein the AGC / AFC control unit changes any one of the demodulated multicarriers to another carrier periodically and randomly.   Furthermore, a diversity antenna and an AGC / AFC / diversity control unit provided with a diversity control function for performing switching control of the diversity antenna in the AGC / AFC control unit are provided. In the AGC / AFC / diversity control unit, 2. The OFDM receiver according to claim 1, wherein the gain of the variable gain amplifier is controlled when the tip of the control vector is within a certain circle, and diversity control is performed when the tip of the control vector is outside the certain circle.   Furthermore, a diversity antenna and an AGC / AFC / diversity control unit provided with a diversity control function for performing switching control of the diversity antenna in the AGC / AFC control unit are provided. In the AGC / AFC / diversity control unit, 2. The OFDM according to claim 1, wherein the frequency of the oscillator is controlled if the tip of the control vector is within a range surrounded by two straight lines sandwiching the real axis, and diversity control is performed if the tip is outside this range. Receiving machine.   The AGC / AFC control unit obtains the control vector for each of a plurality of carriers in the band of the received OFDM signal, and further detects and controls the length of the control vector using a control vector obtained by averaging these vectors. 2. The OFDM receiver according to claim 1, wherein detection of a tip of a vector is performed. The AGC / AFC control unit obtains the control vector for each of a plurality of carriers in the band of the received OFDM signal, and should control the gain of the variable gain amplifier, control the frequency of the oscillator, or In the case of determining whether to perform diversity control based on the principle of majority voting, and controlling the gain of the variable gain amplifier or the frequency of the oscillator, the control amount of the gain of the variable gain amplifier or the control of the frequency of the oscillator The OFDM receiver according to claim 3 or 4, characterized in that the quantity is averaged. In an OFDM receiver that receives a π / 4 shift DQPSK phase modulated OFDM signal for each of the orthogonal multicarriers,
A diversity antenna for receiving the OFDM signal;
A variable gain amplifier for amplifying the received OFDM signal;
An oscillator for down-converting the received OMDM signal;
An AGC / AFC / diversity control unit that automatically controls the gain of the variable gain amplifier, automatically controls the frequency of the oscillator, and controls switching of the diversity antenna based on the result of demodulating the phase modulation of the multicarrier; With
The AGC / AFC / diversity control unit displays an arbitrary one phase of the demodulated multicarrier as a vector in each quadrant of the complex plane, subtracts the previous vector from the current vector, rotates, and temporally If the tip of the control vector is within a certain circle centered on an ideal reception state that is not affected by transmission quality, transmission / reception device accuracy, etc., the variable gain is obtained. An OFDM receiver characterized by controlling the gain of an amplifier and controlling the frequency of the oscillator, and performing switching control of the diversity antenna if it is outside a certain circle. In an OFDM receiver that receives a π / 4 shift DQPSK phase modulated OFDM signal for each of the orthogonal multicarriers,
A diversity antenna for receiving the OFDM signal;
A variable gain amplifier for amplifying the received OFDM signal;
An oscillator for down-converting the received OMDM signal;
An AGC / AFC / diversity control unit that automatically controls the gain of the variable gain amplifier, automatically controls the frequency of the oscillator, and controls switching of the diversity antenna based on the result of demodulating the phase modulation of the multicarrier; With
The AGC / AFC / diversity control unit obtains a phase difference between two demodulated adjacent multicarriers, and the phase difference in an ideal reception state in which the phase difference is not affected by transmission quality, transmission / reception device accuracy, and the like. An OFDM receiver characterized by controlling the frequency of the oscillator so as to be close to (0, π, or ± π / 2) .

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