JP4555243B2 - OFDM signal transmission quality estimation circuit and method thereof - Google Patents

Description

  The present invention relates to a transmission quality of an orthogonal frequency division multiplexing signal (OFDM signal: Orthogonal Frequency Division Multiplexing signal) for estimating a transmission margin and a bit error rate at which a transmission signal from a digital FPU transmitter is broken in a digital FPU receiver. The present invention relates to an estimation circuit and a method thereof.

  In live broadcasting of sports and the like, a broadcasting technique is used in which a video signal captured by a television camera is transmitted from a relay site to a relay station using microwaves using an FPU (Field Pickup Unit) device. Conventionally, analog transmission has been the mainstream in FPU devices, but in recent years, digital transmission has become widespread due to less transmission degradation than analog transmission.

  In transmission using a digital signal, a value called a bit error rate (BER) is used to grasp the transmission quality of the signal. A digital signal has a signal form of 0 or 1, but may be switched over by exceeding the threshold value due to the influence of noise. BER is a numerical value obtained from the number of erroneously received bits with respect to the total number of bits. Therefore, it is used as a numerical value for grasping transmission quality.

  In an actual digital FPU device, although some correction is performed by convolution processing, which is an error correction technique, basically, it is difficult to grasp noise below a threshold value. That is, the received signal of a BER has a predetermined threshold value. It can be said that it is a numerical value that can be grasped only after reaching. Therefore, in the digital FPU device, when adjusting the direction of the antenna mounted on the mobile relay vehicle, fine adjustment of the antenna direction may not be performed due to the influence of noise. In addition, there is a problem in that it is impossible to grasp a margin (margin) for selecting an optimum transmission rate even during relay rehearsal.

  In recent years, in digital transmission technology, a modulation error ratio (MER) has been used to compensate for the above-mentioned drawbacks of BER. The smaller the influence of noise and distortion, the better the reception state is. Value. The MER is a value obtained by quantifying the variation in the coordinates, and will be described with reference to the data symbol coordinates in FIG. 4. The sum of the distance from the origin (0, 0) to the reference point (I, Q) and the reference It is a ratio with the sum of the distances from the point (I, Q) to the received signal point (δI, δQ), and the relational expression defining MER is shown in FIG. That is, while the BER is a value for grasping the number (ratio) of those exceeding the threshold, the MER can always grasp the transmission quality of the carrier wave of the received signal in the form of a margin (margin). is there. The BER can visually confirm the transmission quality by confirming the constellation display of the carrier wave.

  As described above, the digital transmission technique has a feature that it is difficult to grasp transmission quality, but an orthogonal frequency division multiplex carrier (hereinafter referred to as an OFDM carrier) that can minimize the influence of noise. There is a transmission method. An OFDM carrier is a carrier that generates a multicarrier signal obtained by multiplexing a plurality of carriers whose phases are orthogonal to each other. In mobile communication such as an FPU device, reflected waves from buildings, mountains, etc. This is input as a path (delayed wave) and this must be taken into consideration, and the OFDM carrier is used as a transmission system which is not easily affected by such influence.

  A conventional FPU device will be described with reference to the block diagram of FIG. FIG. 7A shows an outline of the FPU transmission / reception system, FIG. 7B shows a system for displaying the transmission quality in the digital modulation signal receiving apparatus of the FPU reception system, and FIG. 7C shows its main part. The details of the signal point distance versus noise calculation unit are shown, and this receiving apparatus estimates the code error rate using MER or the like. In this conventional example, although the technical term MER is not used, the distance between the signal point given from the received signal and the center position set for demodulation is set as described below. A noise level calculation means for calculating as a noise level is disclosed, and uses the same technology as MER.

  In this conventional example, as shown in FIGS. 7B and 7C, the output from the demodulator 1 is input to the signal point distance-to-noise calculation unit 2, and further, the signal point distance-to-noise calculation is performed. The unit 2 will be described in detail. Among the outputs input from the demodulator 1, the received signal point R and the center position S are supplied to the noise level calculator 2a, and the output from the noise level calculator 2a An average calculation unit 2b that performs addition averaging over symbols, a noise level correction unit 2c that corrects an error amount that appears in the noise level averaged by the average calculation unit 2b, and the noise level of the output from the noise level correction unit 2c as signal points A division unit 2d that divides by the inter-distance and calculates a signal point distance to noise ratio, and a logarithmic conversion unit 2e that converts the noise ratio from the division unit 2d into decibels and performs logarithmic conversion. As described above, a digital modulation signal receiving apparatus is disclosed in the conventional example. Further, this conventional example discloses a technique for calculating a code error rate instead of decibel conversion in calculating the signal-point distance-to-noise ratio. Furthermore, a technique corresponding to the modulation scheme used in OFDM is also disclosed. These conventional examples disclose means for calculating a signal-to-signal point distance-to-noise ratio, but it is a technique equivalent to calculating the MER by dividing by the signal point distance instead of the signal point distance. Yes (see, for example, Patent Document 1).

Japanese Patent No. 3701851 (full description, full drawing)

  In the above conventional example, a technique for averaging error voltages such as MER by an average calculation unit using a plurality of symbols is disclosed, but this is effective for grasping a transmission line margin in a single carrier digital signal. This is not a very effective technique for OFDM signals. 5 and 6 are diagrams when noise is added to the transmission path of the OFDM signal, the horizontal axis indicates the frequency, the vertical axis indicates the signal strength, and the broken line indicates the signal spectrum and the dots. Represents an error (noise). FIG. 5 is related to general noise, but FIG. 6 is based on multipath. In OFDM, multipath has characteristics in which noise is periodically strong and weak with respect to frequency. ing.

  In this way, when a strong part such as noise due to multipath is periodically localized, in the MER obtained by averaging the error voltage, the influence of the part where the noise is localized acts on the entire symbol and is reduced. As a result, there is a risk of misunderstanding that there is a transmission margin over all symbols. Of course, in digital transmission, if a signal with a strong noise is localized, if the signal is broken, the signal cannot be received there, and the subsequent transmission is hindered. That is, the MER represents the average error level of a plurality of constellation points, and the error is averaged by a plurality of symbols. Therefore, only by monitoring the MER for measuring the transmission margin, the MER is measured near the valley of the spectrum. The BER cannot grasp that the reception state is bad, and there is a possibility that the digital transmission may be broken although the MER is monitored.

  The present invention has been made in view of the above-described problems, and can accurately estimate a transmission margin in a signal having a strong noise location with respect to multipath, such as an OFDM signal. An object of the present invention is to provide an OFDM signal transmission quality estimation circuit and method.

The present invention solves the above-mentioned problems, and the invention of claim 1 estimates transmission signal quality in an orthogonal frequency division multiplexing signal demodulator that demodulates a digitally modulated orthogonal frequency division multiplexing signal. A quality estimation circuit,
A demodulator for performing a Fourier transform on the quadrature demodulated signal and further demodulating the Fourier transformed signal;
A demapping unit for demapping the output of the demodulation unit;
An error correction unit that performs error correction from the output of the demapping unit and outputs a bit error rate;
An error power calculation unit that calculates error power of a reception signal point from a reception signal output of the error correction unit;
A maximum error power detection unit that detects a maximum value of the error power for each predetermined period from an output of the error power calculation unit;
A transmission margin estimation unit that estimates the transmission margin by a table that outputs the difference from the marginal error power when transmission is obtained in advance, using the output of the maximum error power detection unit as an input ,
An OFDM signal transmission quality estimation circuit comprising a display unit that displays the bit error rate obtained from the error correction unit and the transmission margin obtained from the transmission margin estimation unit on the same screen. .

The front Symbol predetermined period is a transmission quality estimation circuit O FDM signal you the 1OFDM symbol and cycle unit.

The invention of claim 2 is a transmission quality estimation method for demodulating a digitally modulated orthogonal frequency division multiplexed signal and estimating the quality of the transmission signal,
The orthogonally demodulated signal is Fourier-transformed, and the Fourier-transformed signal is demodulated,
Demapping the demodulated output signal,
Perform error correction from the demapping processing output and calculate the bit error rate,
Calculate the error power of the received signal point from the output after error correction,
The maximum value of the error power is detected every predetermined cycle from the error power,
Read the transmission margin due to the difference between the maximum error power and the limit error power when the transmission obtained in advance is unlocked from the table and estimate,
An OFDM signal transmission quality estimation method, wherein transmission quality is estimated by displaying the bit error rate and the transmission margin on the same screen .

  The invention of claim 5 is the OFDM signal transmission quality estimation method according to claim 4, wherein the predetermined period is one OFDM symbol as a period unit.

  The invention according to claim 6 is characterized in that the margin of the transmission signal and the bit error rate are estimated with reference to table data based on a modulation scheme and a convolutional coding rate. This is a method for estimating the transmission quality of an OFDM signal.

According to the first or second aspect of the present invention, the received OFDM signal is orthogonally demodulated, the Fourier-transformed signal is further demodulated and demapped, the error power of the received signal point is calculated, and a predetermined period is obtained. maximum error power detects, by estimating the maximum error power transmission margin and bit error rate than (BER), because the arithmetic processing step of averaging processing errors such as MER is not present, mobile communication Thus, it is useful when the communication path changes every moment. In particular, it is effective for fine adjustment of the antenna direction and determination of transmission parameters.

  The antenna fine adjustment is used to finely adjust the antenna in the direction with the largest margin while observing the margin display after initial adjustment (coarse adjustment) of the antenna direction using input electric field display or gray display. In addition, the transmission parameter is determined during the relay rehearsal, and the transmission rate is determined based on the margin. Since the display can be performed over a wider range (the degree of constellation disturbance) than the conventional BER display, it is easy to estimate the margin for noise on the transmission line. Furthermore, although the present invention is intended for transmission using OFDM signals, the present invention can also be applied to a single carrier as long as it is digital transmission that performs mapping processing.

Further, according to the present invention, by the detection of the maximum error power so as to detect at 1OFDM symbol unit enables setting by quite a long time from a few OFDM symbols, the application spreads the effect of the transmission quality estimation circuit is there.

Further, according to the present invention, since the calculation can be performed using a table based on the modulation scheme and the convolutional coding rate, it can be configured with a memory element such as a ROM, so that the circuit configuration is simplified. Further, since the table output is a value obtained by logarithmic conversion, that is, decibel (dB), it has an effect that it can be calculated by addition / subtraction even when calculating the transmission margin.

  Embodiments of an OFDM signal transmission quality estimation circuit and method according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is a diagram for explaining the calculation of error power in this embodiment, and FIGS. 3 (a) and 3 (b) are OFDM signal transmissions. It is a figure which shows the example of a display of a quality estimation circuit.

  First, the transmission quality estimation circuit of this embodiment will be described with reference to FIG. The transmission quality estimation circuit includes an input terminal 10, a demodulation unit 11, a demapping unit 12, an error correction unit 13, an error power calculation unit 14, a maximum error power detection unit 15, a transmission margin estimation unit 16, and a display unit 17. ing. The output voltage of the received signal is output from the error correction unit 13. In addition, the inside of the dotted line of the figure is a block diagram which shows the principal part of the transmission quality estimation circuit of this invention.

  An input terminal 10 receives an OFDM signal transmitted from a digital FPU transmitter by a receiving antenna, converts it into digital data and orthogonally demodulates it, and inputs a signal obtained by Fourier transforming each output. Is done.

  The demodulator 11 demodulates the Fourier-transformed signal, inputs the demodulated signal to the demapping unit 12, and outputs a timing control signal to the error correction circuit 13 and the maximum error power detector 15.

  The demapping unit 12 performs demapping processing on the point on the coordinate axis from the demodulated signal. The demapping process is the reverse of the mapping process on the transmission side.

  The error correction circuit 13 performs error correction on the demapping output, and outputs a BER by a timing control signal from the demodulator 11. The estimation of BER is well known.

  The error power calculation unit 14 calculates and outputs the distance of the reference point closest to the received signal point from the output of the demapping unit 12 as error power. Specifically, as shown in the constellation display of FIG. 2, the reference points are A and B, and A1 and A2 are both received signal points with respect to the reference point A. Since A1 is close to the reference point A, the distance L1 from the reference point A is calculated. Since A2 is closer to the reference point B than the reference point A, the distance L2 from the reference point B is calculated.

  The maximum error power detection unit 15 detects the maximum error power from the output of the error power calculation unit 14 based on the cycle of the control signal based on the timing control signal from the demodulation unit 11. The period is a symbol period signal of one OFDM signal. That is, the maximum error power detection unit 15 detects the maximum error power for each symbol.

  The transmission margin estimation unit 16 refers to the modulation scheme and the convolutional coding rate from the maximum error power and estimates the margin in transmission. The margin in transmission is the level of difference between the limit error power when breaking and the maximum error power obtained. The difference between the limit error power and the maximum error power is calculated from table data (table ROM). Also, table data based on the modulation scheme and the convolutional coding rate is provided, and the margin of the transmission signal and the bit error rate are estimated from this table data.

  The display unit 17 displays the margin (margin), which is the output of the transmission margin estimation unit 16, and the BER as shown in FIG. FIGS. 3A and 3B show an example in which the transmission margin (margin) and the BER are displayed side by side on the display unit 17. FIG. 3A is a display example based on BER OVER with 0 dB of transmission margin as a reference, and it is possible to grasp how many dB remaining OVER. FIG. 3B is a display example based on BER UNDER with a transmission margin of 0 dB as a reference. Similarly to OVER, it is possible to grasp how many dB it will become UNDER later.

  The present invention relates to a transmission quality estimation circuit that solves the problems inherent in MER, and the transmission quality estimation circuit can display the output on the display unit 17, and in actual operation in a relay vehicle equipped with a digital FPU receiver. An engineer can adjust the direction of the antenna while viewing both displays on the display unit 17. By monitoring the BER and margin of the display unit 17, the OFDM signal can be received without being broken. The display examples in FIGS. 3A and 3B are switched by providing a switch (not shown) so that one of the displays in FIGS. 3A and 3B can be selected depending on the operating person or application. You may do it.

  The present invention can also be processed by a program using a CPU. Although it depends on the CPU processing speed and the transmission speed, it is possible when the CPU processing speed is faster than the transmission speed. As for the order of processing, it is possible to configure a program in the order of the symbols in FIG.

  As an application example of the present invention, it can be used in a digital receiver of a digital FPU device.

It is a block diagram which shows one Embodiment of the transmission quality estimation circuit of this invention. , It is a figure explaining calculation of error power in this embodiment. (A), (b) is a figure which shows the example of a display of the transmission quality estimation circuit of an OFDM signal. It is explanatory drawing of MER. It is a spectrum figure when general noise is added to the transmission path of the OFDM signal. It is a spectrum figure when multipath noise is added to the transmission path of the OFDM signal. (A) is a block diagram showing an outline of an FPU transmission / reception system, (b) is a block diagram of a digital modulation signal receiving device of an FPU reception system, and (c) is a detail of a signal point distance-to-noise calculation unit showing its main part. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Input terminal 11 Demodulation part 12 Demapping part 13 Error correction part 14 Error power calculation part 15 Maximum error power detection part 16 Transmission margin estimation part 17 Display part

Claims (2)

A transmission quality estimation circuit for estimating the quality of a transmission signal in an orthogonal frequency division multiplexing signal demodulator that demodulates a digitally modulated orthogonal frequency division multiplexing signal,
A demodulator for performing a Fourier transform on the quadrature demodulated signal and further demodulating the Fourier transformed signal;
A demapping unit for demapping the output of the demodulation unit;
An error correction unit that performs error correction from the output of the demapping unit and outputs a bit error rate;
An error power calculation unit that calculates error power of a reception signal point from a reception signal output of the error correction unit;
A maximum error power detection unit that detects a maximum value of the error power for each predetermined period from an output of the error power calculation unit;
A transmission margin estimation unit that estimates the transmission margin by a table that outputs the difference from the marginal error power when transmission is obtained in advance, using the output of the maximum error power detection unit as an input ,
A transmission quality estimation circuit for an OFDM signal, comprising: a display unit for displaying the bit error rate obtained from the error correction unit and the transmission margin obtained from the transmission margin estimation unit on the same screen . A transmission quality estimation method for demodulating a digitally modulated orthogonal frequency division multiplexed signal and estimating the quality of the transmission signal,
The orthogonally demodulated signal is Fourier-transformed, and the Fourier-transformed signal is demodulated,
Demapping the demodulated output signal,
Perform error correction from the demapping processing output and calculate the bit error rate,
Calculate the error power of the received signal point from the output after error correction,
The maximum value of the error power is detected every predetermined cycle from the error power,
Read the transmission margin due to the difference between the maximum error power and the limit error power when the transmission obtained in advance is unlocked from the table and estimate,
An OFDM signal transmission quality estimation method, wherein the bit error rate and the transmission margin are displayed on the same screen to estimate transmission quality.

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