JP4331580B2 - OFDM modulated signal receiver - Google Patents

Description

  The present invention relates to an OFDM modulated signal receiving apparatus that receives an OFDM modulated signal modulated by an Orthogonal Frequency Division Multiplexing (OFDM) modulation scheme.

  In recent years, as a modulation method suitable for application to mobile digital transmission, terrestrial digital television broadcasting, wireless LAN, etc., an orthogonal frequency division multiplex modulation method (hereinafter, referred to as multipath fading and ghosting) (Referred to as OFDM).

  The OFDM system is a type of multi-carrier modulation system, and is a transmission system in which digital modulation is performed on n (n is several tens to several thousands) carrier waves (carriers) orthogonal to each other.

  A discrete code is assigned to each of the I-axis component and Q-axis component of each carrier as a modulated signal, and the code is updated every symbol period (for example, several μsec to several hundred msec).

  As a digital modulation method of the carrier, a four-phase differential phase shift keying (DQPSK) is also used, but multi-level modulation such as 16-QAM (16 Quadrature Amplitude Modulation) or 64QAM. The method is often used.

  In the 16QAM and 64QAM modulation schemes, information is given to the amplitude and phase of each carrier, and therefore the absolute amplitude and absolute phase of the received carrier must be accurately reproduced during demodulation.

  Therefore, the transmitter transmits a signal in which pilot carriers with specified amplitudes and phases are arranged every several carriers, and the receiver estimates the channel characteristics based on the amplitude and phase fluctuations of the received pilot carriers, A method of equalizing amplitude and phase is used.

  Also, in order to improve transmission performance, the amplitude of the pilot carrier is often set larger than that of a normal data carrier for transmission. For example, the amplitude ratio is set to 4/3 times that of the data carrier.

  These carriers are added so as to maintain an orthogonal relationship with each other, and an OFDM time axis waveform is generated.

  This addition processing can be realized by performing IFFT (Inverse Fast Fourie Transform) processing on each carrier.

  As shown in FIG. 4, the symbol structure of the OFDM signal is composed of an effective symbol that is a time-axis waveform after the IFFT processing, and a guard interval in which a part of the effective symbol is copied and added before the effective symbol. .

  By adding a guard interval, it is possible to avoid deterioration due to inter-symbol interference with respect to a delayed wave having a delay time within the guard interval period, so that it is possible to have strong resistance against multipath fading.

  The OFDM signal generated by the above processing is frequency-converted to a high frequency (RF) and then transmitted.

  In the conventional receiver shown in FIG. 5, after the signal received by the antenna 21 is amplified by the amplifier 22, the automatic gain control (Automatic Gain control) is performed so that the AGC unit 23 has an appropriate signal level with respect to the subsequent circuit. Control: hereinafter referred to as AGC).

  Thereafter, the frequency is converted to the baseband, and then sampled by the A / D converter 24.

  In the demodulation processing for the OFDM modulated signal, a time window having an effective symbol period length is provided on the obtained received sample sequence. The FFT unit 25 performs FFT calculation processing on the sample signal included in the provided time window, and converts the time axis signal into a frequency axis signal.

  Then, as shown in FIG. 6, the transmission path estimation unit 27 extracts only the pilot carrier from the signal after the FFT calculation process, and estimates the characteristics of the transmission path from the pilot carrier.

  Specifically, the frequency characteristics of the transmission path caused by multipath fading or the like are estimated by performing filter processing on the pilot carrier.

  The equalizer 51 equalizes the amplitude and phase of the received data carrier using the estimated channel characteristics. Finally, the demodulator 2a demodulates 16QAM, DQPSK, and the like based on the equalized data carrier amplitude and phase information, and completes OFDM reception.

  Such demodulation processing of the receiving apparatus is described in Non-Patent Document 1 below.

  By the way, because OFDM is suitable for mobile transmission, it should be used in a transmission system of an image radio transmission apparatus called FPU (Field Pickup Unit) that performs television relay such as marathon relay and emergency reporting. There is.

  In outdoor transmission such as a marathon relay, multipath communication in which there is a reflected wave that arrives with a delay time reflected from a building etc. in addition to the main wave coming directly from the transmitter according to the topography. A path may be formed.

  Under a multipath environment, in a frequency band in which the phases of the main wave and the reflected wave are close to the opposite phase, the signal is canceled and the amplitude is reduced, causing deterioration called frequency selective fading.

  In OFDM, since a band for each carrier is narrow, a transmission error frequently occurs in a carrier whose level is lowered due to frequency selective fading.

  Furthermore, in mobile transmission, there may be a Rayleigh fading environment in which the levels of the main wave and the reflected wave change from moment to moment.

  In a Rayleigh fading environment, the level of the entire symbol may be lowered, and the level of all carriers included in the symbol is also lowered. As described above, due to Rayleigh fading, a symbol with a lowered level frequently causes transmission errors.

  In image transmission, the image decoding processing on the receiving side becomes impossible due to transmission errors caused by such fading, and the image may temporarily freeze and block noise may occur.

  In transmission such as live broadcasting, such image freeze is a matter that must be avoided in consideration of the influence on the viewer.

  For this reason, a reception processing method called space diversity (hereinafter referred to as diversity) may be applied in order to reduce such fading degradation.

  Diversity means that multiple antennas are arranged spatially apart, and the signals received by each antenna are appropriately combined, or the reception strength is selected and selected from the multiple antennas, The influence of fading deterioration can be reduced.

  Among the diversity methods, the maximum ratio combining method that excels in diversity effect is often used, and this method will be described below.

  FIG. 7 is a diagram showing a configuration of a diversity receiving apparatus having two antennas in the prior art.

  The radio wave transmitted from the transmitter receives distortion such as fading and is received by the two antennas 21. At this time, the antennas are generally arranged at a distance of half or more of the wavelength of the radio wave so that the correlation between the signals received by the respective antennas becomes low.

  The received signals are converted into digital signals by the A / D converter 24 through the amplifier 22 and the AGC unit 23, respectively, and converted from the time axis signal to the frequency axis signal by the FFT unit 25. The processing so far is the same as the reception processing of FIG.

  Similarly, the transmission path estimation unit 27 estimates the pilot carrier transmission path characteristics.

  Next, each weighting processing unit 28 multiplies the output signal from the FFT unit 25 by a coefficient based on the signal from the transmission path estimation unit 27.

  For example, since the C / N is deteriorated for a carrier whose level is lowered due to multipath fading and a transmission error is likely to occur, the weight (importance) of the carrier is set low.

Theoretically, the optimum weighting factor w _i (k) (i: antenna number, k: carrier number) of the maximum ratio combining with M receiving antennas indicates the transmission path characteristics from the transmitter to the i-th antenna. Assuming h _i (k), the following expression (1) is obtained, which is known to be a coefficient that maximizes C / N.

上記係数により重み付けされた各系統の信号は合成処理部２９により加算処理される。

The signals of each system weighted by the coefficients are added by the synthesis processing unit 29.

  The principle of diversity described above is described in detail in Non-Patent Document 2 below.

The Journal of the Institute of Image Information and Television Engineers vol. 53, no. 11, pp 1538-1549 (1999)

ITE Technical Report, Vol. 23, no. 35, PP 13-18 (1999) JP 2003-110521 A

  In the diversity processing in the prior art, the gain is automatically controlled by the AGC processing so that the optimum signal level is obtained even if the intensity of the received signal reaching each antenna becomes weak.

  However, since the thermal noise generated by the amplifier is also amplified at the same time as the signal level at this time, if the reception intensity for each antenna is different, the signal level is almost the same, but the C / N ratio is greatly different. Sometimes.

  When such a signal is diversity-combined, if the weighting by the coefficient of Expression (1) is performed without taking this C / N difference into consideration, there is a disadvantage that the combined result is deteriorated.

  For example, FIG. 8A shows a signal with a high reception strength and a good C / N, and FIG. 8B shows a signal with a low reception strength and a deteriorated C / N. It is controlled to almost the same level.

  Since the signal level is approximately the same, the coefficient of equation (1) is approximately the same, and the waveform after weighting by this coefficient is as shown in FIGS. 8C and 8D, and before the weighting process. The waveform does not change much.

  As a result of synthesizing these signals, as shown in FIG. 8 (e), the signal is strongly influenced by the signal having a deteriorated C / N (FIG. 8 (d)), and may be deteriorated more than before the synthesis. .

  FIG. 9 is a diagram showing the degree of C / N deterioration described above in a diversity apparatus having two antennas. The C / N of the system A is 0, 10, 20, 30, 40 [dB], and the C / N after the synthesis when the C / N of the system B is changed (horizontal axis) is synthesized Shown on the axis.

  For example, when the C / N of the system A is 20 [dB] and the C / N of the system B is also 20 [dB], the combined C / N is improved by about 3 [dB].

  However, even if the C / N of the system B is 26 [dB] or more, the synthesized result remains at about 26 [dB]. In this case, it is better to use only the system B without combining.

  Further, when the C / N of the system B is in the vicinity of 0 [dB], as a result of the synthesis, the influence of the system B becomes strong, and the synthesized C / N also deteriorates to about 10 [dB]. Even in this case, it is better to use only the system A without synthesizing.

  In this way, when the C / N of each system is different when diversity combining is performed, the influence of the system having deteriorated C / N becomes large, and as a result of combining, the characteristics are deteriorated. .

  In order to reduce such influence, generally, the weighting coefficient is set based on the RSSI (Received Signal Strength Indicator) indicating the received signal level of the antenna and the control signal at the time of AGC processing. It is variable.

  For example, when a weak signal is greatly amplified by AGC processing, noise is also amplified at the same time. In this case, a method is used in which the weighting factor is reduced to reduce the influence during synthesis.

  However, devices such as the FPU described above are often mounted on a relay vehicle or a steel tower, and the high frequency parts such as the antenna 21, the amplifier 22, the AGC part 23, and the baseband part for diversity synthesis are separated from each other. It is often installed in.

  In addition, in order to simplify the system, the connection cable to the high-frequency unit is only one cable that transmits the IF signal, so it is very difficult to effectively use the AGC control signal and RSSI of the RF unit.

  An object of the present invention is to provide an OFDM modulation signal receiving apparatus capable of detecting C / N in a baseband unit and calculating a weighting factor corresponding thereto.

  Another object of the present invention is to provide an OFDM modulation signal receiving apparatus capable of observing a transmission path condition or knowing a code error rate by using a noise level detecting apparatus for detecting a noise level in a baseband unit. It is to provide.

  An OFDM modulated signal receiving apparatus according to claim 1 of the present invention is an OFDM modulated signal receiving apparatus for diversity receiving a signal modulated by an orthogonal frequency division multiplexing (OFDM) modulation system having a guard interval period with a plurality of antennas. Delay means for delaying the effective symbol period with respect to the received sample value series for each received system, subtracting means for subtracting the delayed signal and the non-delayed received sample value series, and converting the result of the subtraction into an absolute value Noise level detecting means comprising: absolute value converting means that detects the noise level based on the result of the absolute value conversion; and a received signal of each system that is mixed based on the detected result Noise normalization means for normalizing the noise level, and processing for multiplying a predetermined weighting factor based on the received signal of each system after the normalization processing Comprising a weighting processing means for, and further comprising a synthesis processing means for the adding processing signals resulting from multiplying the weight coefficient for each channel.

  The OFDM modulated signal receiving apparatus according to claim 2 of the present invention is the OFDM modulated signal receiving apparatus according to claim 1, wherein the noise normalization means is configured to perform all the operations when the noise levels of the received signals of the respective systems are equal. If the received signal of each system is not equal in level, the received signal with the lowest noise level is multiplied by the specified value and the other received signals are On the other hand, the received signal is multiplied by a coefficient such that the noise level of the received signal matches the multiplication result of the smallest noise level and the predetermined value.

  According to a third aspect of the present invention, there is provided an OFDM modulated signal receiving apparatus for receiving a signal modulated by an OFDM modulation scheme having a guard interval period, wherein the effective symbol period is delayed with respect to the received sample value sequence. A subtracting means for subtracting the delayed signal and the non-delayed received sample value sequence, an absolute value converting means for converting the result of the subtraction into an absolute value, and a noise level from the result of the absolute value conversion. And a noise level detecting means having a detecting means for detecting.

  An OFDM modulated signal receiving apparatus according to claim 4 of the present invention is the OFDM modulated signal receiving apparatus according to any one of claims 1 to 3, wherein the detecting means uses a signal waveform as a result of the absolute value conversion. Differentiating, setting positive and negative thresholds for the result of differentiation, comparing the level with the result of differentiation, and the timing that is the latest among the timings when the result of differentiation becomes smaller than the negative threshold , Providing a temporal window in the interval with the fastest timing in the timing when the result of differentiation is greater than the positive threshold, and the signal included in the temporal window in the absolute value result The noise level is detected by calculating an average value.

An OFDM modulated signal receiving apparatus according to claim 5 of the present invention is the OFDM modulated signal receiving apparatus according to any one of claims 1 to 4, wherein a transmission path condition or a code error rate is determined based on a result of the noise level detecting means. It is characterized by comprising estimation means for estimating.
The noise level is detected by the detection means based on the above.

  According to a sixth aspect of the present invention, there is provided the OFDM modulated signal receiving apparatus according to the fifth aspect, wherein the estimated transmission path condition or code error rate is visually determined by a meter, a lamp, a video display device, or the like. It is characterized by comprising expression means, auditory display means by converting into sound volume, scale, etc., or conversion means for converting into electric signals or optical signals.

  ADVANTAGE OF THE INVENTION According to this invention, the OFDM modulation signal receiver which can detect C / N in a baseband part and can calculate the weighting factor according to it can be obtained. Further, according to the present invention, an OFDM modulation signal receiving apparatus capable of observing a transmission path condition or knowing a code error rate by using a noise level detecting apparatus for detecting a noise level in a baseband unit is obtained. be able to.

  First, an OFDM modulated signal receiving apparatus according to the present invention will be described in detail with reference to the first embodiment shown in FIGS.

  FIG. 1 is a configuration diagram of a first embodiment of an OFDM modulated signal receiving apparatus according to the present invention. FIG. 2 is a block diagram of an embodiment of a noise level detection apparatus in an OFDM modulation signal receiving apparatus according to the present invention.

  FIG. 1 is different from FIG. 7 in that each noise normalization unit 26 is inserted into the output of each FFT unit 25 and each noise level detection unit 30 is connected to the output of each A / D converter 24. In this configuration, the output of each noise level detection unit 30 is connected to the other input of each noise normalization unit 26.

  That is, each noise level is detected from the output of the A / D converter 24 in the preceding stage of the FFT unit 25, and each noise standardizing unit 26, as will be described later, uses each noise level based on the detected noise level. The level is standardized. Other configurations and processing methods are the same as those in FIG.

  As shown in FIG. 2, the detailed configuration of the noise level detection unit 30 is such that the received sample value series from the A / D converter 24 (FIG. 1) is input to the delay unit 31 and the subtraction unit 32 plus (+). Connected to. The output of the delay unit 31 is connected to the minus (−) input of the subtraction unit 32.

  The output of the subtraction unit 32 is input to the absolute value conversion unit 33, the output of the absolute value conversion unit 33 is connected to the input of the filter unit 34, and the output of the filter unit 34 is input to the floor level detection unit 35. The output of the floor level detector 35 is the output signal of the noise level detector 30.

  Next, the operation of the noise level detection unit 30 will be described with reference to FIG. In addition, since complex signals are handled basically, complex numbers are used unless otherwise specified.

  First, the received sample value series from the A / D converter 24 (FIG. 1) is input to the noise level detection unit 30. The delay unit 31 delays the effective symbol length as shown in FIG. 10B with respect to the received sample value series shown in FIG.

  As described above, the OFDM modulation signal has a structure in which the second half period of the effective symbol (the shaded area of the lattice in FIG. 10A) is copied at the beginning of the symbol (shaded area) as a guard interval.

  Therefore, if the delay unit 31 delays the effective symbol length, the symbol second half period (lattice shaded area) and the guard interval period (shaded shaded area) coincide with each other in time as shown in the thick line in FIG.

  These signals are input to the subtractor 12, and the subtracter 12 subtracts the signal at the minus (−) input terminal from the signal at the plus (+) input terminal.

  Ideally, since the signal in the second half of the symbol (hatched shade) and the guard interval (shaded shaded) match, the result of subtraction is 0 as shown in FIG. Since there is no correlation, it becomes a noise-like waveform with a large amplitude.

  However, the subtraction signal in the guard interval period does not become 0 due to fading distortion and noise actually generated in the transmission path.

  FIG. 10D shows an output signal of the subtracting unit 32 when multipath fading occurs in the transmission path and noise also occurs. The signal indicated by g in the figure is a component due to multipath fading.

  This component increases in the time direction as the multipath delay time increases, and increases in the amplitude direction as the multipath level increases.

  Next, the signal indicated by h is a component due to noise. This increases in the amplitude direction as the noise component increases.

  The output signal of the subtracting unit 32 is converted into an absolute value by the absolute value converting unit 33. Various methods are conceivable for the absolute value processing. For example, as shown in the following equation (2), there is a method in which a real axis signal and an imaginary axis signal are squared and added, and then square root processing is performed.

また、次式（３）示すように、実数軸信号と虚数軸信号をそれぞれ絶対値化した後、それぞれを加算する方式も考えられる。

Further, as shown in the following equation (3), a method may be considered in which the real axis signal and the imaginary axis signal are converted into absolute values and then added to each other.

式（２）の方式は正確な絶対値の演算であるが２乗処理や平方根処理などにより回路規模が大きくなってしまうという欠点がある。それに対して式（３）の方式は式（２）の方式と比較して回路規模を大幅に削減することが出来る。

The method of equation (2) is an accurate absolute value calculation, but has a drawback that the circuit scale becomes large due to square processing or square root processing. On the other hand, the method of the formula (3) can greatly reduce the circuit scale as compared with the method of the formula (2).

  Further, if the calculation of Expression (3) is performed on a signal having a normal distribution, a result proportional to the accurate calculation result of Expression (2) can be obtained. Since noise and OFDM signals are signals having a substantially normal distribution, there is no problem even if the calculation of Expression (3) is used.

  FIG. 10E shows a signal obtained by making the absolute value of the signal mixed with multipath and noise as shown in FIG.

  Next, the output signal of the absolute value converting unit 33 is averaged by the filter 34 unit.

  The signal of the absolute value converting unit 33 has a variation in value due to noise and OFDM components. The subsequent floor level detection unit 35 detects the noise level from this signal. However, if the signal variation is large, accurate detection cannot be performed.

  For this reason, the filter unit 34 performs filtering in advance to suppress variations as shown in FIG. The filter processing method can be easily realized by a general digital filter such as an FIR filter or an IIR filter.

  The output signal of the filter unit 34 is input to the floor level detection unit 15. The level indicated by i in FIG. 10F is due to noise, and the value of this level changes according to the magnitude of the noise. Therefore, the amount of noise can be detected by detecting the level indicated by i in FIG.

  Care must be taken when detecting this level so that the level caused by multipath is not erroneously detected as a noise level.

  As a method for avoiding this and detecting the noise level, there is a method of detecting the lowest level among the signal components and outputting this as the noise level. Further, in consideration of the variation of the value, the detection result may be filtered to suppress the variation of the value.

  In addition, as shown in FIG. 11, the signal component of FIG. 10 (f) is differentiated, positive and negative threshold values are provided for the differential result, and the differential result is positive from the position where the differential result exceeds the negative threshold value. The average value of the period until the threshold is exceeded may be used as the noise level.

  Through the above processing, the level of noise generated by the amplifier or the like is detected, and the detected signal is output from the noise level detection unit 30.

  As another noise level detection method, a method of using a constellation error as a noise level is also considered, which is described in detail in Japanese Patent Application Laid-Open No. 2003-110521.

  FIG. 12 is a diagram illustrating a method for detecting a noise level using a 16QAM reception constellation. Reference numerals C1 to C4 are received in an ideal state where there is no deterioration such as noise. Symbols TH1 to TH4 are thresholds for demodulation. For example, a signal received in an area between TH1 and TH2 and between TH3 and TH4 is a C1 signal on the transmission side. The code corresponding to C1 is demodulated assuming that it has been transmitted.

  Here, it is assumed that received signals that vary due to noise are D1 to D3, and distances from C1 are L1, L2, and L3 ', respectively. If the noise level increases, the distance also increases. Therefore, the noise level can be detected by calculating this distance.

  However, since the received signal D3 is received in an area exceeding the threshold value TH2, it is demodulated erroneously at point C2. At the same time, the noise level detection also calculates a distance L3 from C2 and an erroneous noise level. When the noise is relatively small, noise level detection by the above method can be performed with high accuracy, but if the amount of noise increases, the probability that the threshold value will be exceeded increases, and the calculated noise level value will contain an error. Will occur.

  In this way, the fact that the noise level cannot be accurately detected means that, as described in the section of the problem, “the C / N after the synthesis is deteriorated due to the influence of the C / N deteriorated system. ”Again results in the fault.

  The present invention solves the problem in detecting the noise level from the received constellation, and has a feature that the noise level can be detected with high accuracy even when the amount of noise is large.

  FIG. 13 is a diagram illustrating the noise level detected by the noise level detection unit 30, where the horizontal axis indicates C / N and the vertical axis indicates the noise level. Further, it is assumed that AGC processing is performed as a condition.

  From this figure, it can be seen that when the C / N is improved, the noise level is decreased, and when the C / N is deteriorated, the noise level is increased.

  Next, a method for solving the above-described problem using the calculated noise level will be described with reference to FIG.

  FIG. 1 is a configuration diagram of a diversity receiving apparatus having two antennas as described in FIG.

  As in FIG. 7, the received signals received by the two antennas are converted into digital signals by the A / D converter 24 through the amplifier 22 and the AGC unit 23, respectively, and the FFT unit 25 converts the frequency from the time axis signal to the frequency. Converted to axis signal. The processing so far is the same as the reception processing of FIG.

  Furthermore, in order to detect noise accurately from the received constellation, an averaging process over a long period of time is required in order to reduce noise variation. On the other hand, the present invention can detect noise levels with high accuracy in a short period of time, and is therefore suitable for an environment in which fluctuations in transmission path characteristics such as a moving body are severe.

  The output signal of each A / D converter 24 is input to each noise level detection unit 30, and the noise level detection unit 30 outputs a value corresponding to the noise level.

  The signals from the respective FFT units 25 are input to the respective noise normalization units 26, and the output signal of the noise level detection unit 30 is connected to the other input of the noise normalization unit 26.

  The noise normalization unit 26 divides the output signal from the FFT unit 25 by the output signal from the noise level detection unit 30. FIGS. 14A and 14B illustrate the output result of the FFT unit 25 when the C / N of each system is different as described in FIG.

  Moreover, the arrow in the figure indicates the noise level value output from the noise level detector. Although this noise level value may differ from the absolute noise level, there is no problem as long as they are in a relative relationship.

  The noise normalization unit 26 performs division processing according to the noise level, and the noise levels of the respective systems become substantially equal as shown in FIGS.

  Further, the signal component level also fluctuates at the same time by this division processing, and the level of the signal in FIG. 14 (b) having a large noise level becomes a small level as shown in FIG. 14 (d) as a result of the division.

  Thus, the signal divided by the noise level is output from the noise normalization unit 26.

  A configuration of the noise normalization unit 26 different from the configuration of FIG. 2 will be described with reference to FIG.

  FIG. 15 shows a configuration in which two systems of noise normalization units 26 in the configuration of FIG.

  In the noise normalization unit 26 of FIG. 2, the signals of the respective systems are divided as they are according to the noise level.

  On the other hand, the noise ratio normalization unit 131 in FIG. 15 performs a process of multiplying a signal of a system with a high noise level by a coefficient that matches the level of a system with a low noise level.

  Details of the noise ratio normalization unit 131 will be described with reference to FIG.

System signals of two systems respectively A, and system B, and the signal from the noise level detecting unit 30 of the system A N _A, the signal from the noise level detecting unit 30 of the system B and N _B.

N _A and N _B are input to the noise level comparator 141, which compares their magnitudes and inputs the comparison result to the coefficient setting unit 142.

  The coefficient multiplier A143 and the coefficient multiplier B144 multiply the respective FFT unit output signals by the coefficients set by the coefficient setting unit 142.

The coefficient setting unit 142 sets coefficients according to the comparison result from the noise level comparison unit 141 and the values of N _A and N _B.

  This coefficient is set according to the following three conditions.

As a first condition, when the value of N _A and N _B are equal, sets the coefficients of each strain to 1.

As a second condition, in the case towards the N _B than N _A is large, the coefficient is set in the system A to 1, it sets the coefficients of the system B to N _{A /} N _B.

As a third condition, when towards the N _A is larger than the N _B sets the coefficients of the system B to 1 and sets the coefficients of the system A to N _{B /} N _A.

FIG. 17 shows a summary of this coefficient setting in a flowchart. Here, _{K A} is the coefficient value of the system A, _{K B} denotes the coefficient values of the system B.

First, in step 171, it is determined whether the noise levels N _A and N _B of the two received signals are equal. If they are equal, in step 172, the coefficient values K _A and K _{B are obtained} for the two received signals. Is set to 1.

If the noise levels N _A and N _{B of} the two received signals are not equal at step 171, it is determined at step 173 whether the noise level N _B is greater than N _A.

If larger, at step 174, set to 1 the coefficient K _A is the received signal of the channel A having a noise level N _A, relative to the received signal of the system B having a noise level N _B The coefficient value K _B is set to N _A / N _B.

If not larger Conversely, at step 175, for the reception signal of channel A having a noise level _{N A} obtained by setting the coefficient _{K A} to _N B / _{N A,} system B having a noise level _{N B} It sets a coefficient value K _B to 1 for the received signal.

  Since the noise normalization unit 26 shown in FIG. 2 requires a division operation, the configuration shown in FIG. 2 has a drawback in that a division operation is required for each system, resulting in an increase in circuit scale.

  On the other hand, in the noise ratio normalization unit 131 of FIG. 15, the coefficient of one system is 1, so that there is only one system for multiplication or division, and the circuit scale can be reduced.

  As described above, even if the noise levels are different, the noise levels are matched by the above processing. Therefore, when the addition is performed in the subsequent synthesis processing unit 29, the noise influence levels of the respective systems become equal.

  The processing of FIGS. 2 and 15 will be described again.

  Output signals from the noise normalization unit 26 or the noise ratio normalization unit 131 are input to the transmission path estimation unit 27 and the weighting processing unit 28, and the output signals of the respective weighting processing units 28 are output from the synthesis processing unit 29. The addition processing is performed as shown in FIG. 14E and demodulated by the demodulator 2a.

  These series of processes are the same as those of the conventional receiving apparatus.

  By performing the synthesis after matching the noise levels by the above processing, optimum diversity synthesis can be performed even if the C / N of each system is different.

  FIG. 18 shows the effect of the above processing.

  FIG. 18 shows the C / N after diversity combining in the two systems of diversity receivers as in FIG. C / N of system A is 0, 10, 20, 30, 40 [dB], C / N of system B is changed (horizontal axis), and C / N after synthesis when the synthesis is performed is plotted on the vertical axis It is shown in

  For example, when the C / N of the system A is 20 [dB], the combined C / N converges to 20 [dB] even if the C / N of the system B deteriorates.

  Further, when the C / N of the system B is 20 [dB], the combined C / N is improved by about 3 [dB], and when the C / N of the system B is 20 [dB] or more, the system A Thus, the combined C / N does not deteriorate more than the C / N of the system B.

Next, the configuration of the second embodiment shown in FIG. 3 of the OFDM modulated signal receiving apparatus according to the present invention will be described in detail. As described above, the OFDM system is used for mobile transmission such as FPU. Sometimes.

  Since the FPU is often used for applications that require high reliability such as television relaying, it is necessary to avoid broadcasting of video with degraded image quality due to transmission errors.

  As an effective means for this problem, there is a method of observing a propagation path condition or a code error rate and switching a video signal used for television broadcasting to another video signal when a situation where image quality deterioration is likely to occur. It is used.

  As a second embodiment using the present invention for solving this problem, an apparatus for observing the state of a transmission line using a noise level will be described.

  3 shows the configuration of the conventional receiving apparatus shown in FIG. 5, in which the output signal of the A / D converter 24 is input to the noise level detector 30, and the output signal of the noise level detector 30 is input to the code error rate estimator 36. In this configuration, the output signal of the code error rate estimation unit 36 is connected to the display unit 37.

  As described above, the operation of the noise level detection unit 30 detects the noise level and outputs a value corresponding to the magnitude.

  The error rate estimation unit 36 converts the detected noise level into a code error rate. The noise level detected by the “noise level detection unit” 30 is generally within the transmission band, and a certain amount of noise is mixed into all carriers. This signal level varies depending on the carrier due to the influence of multipath or the like. As described above, this signal level can be calculated by interpolating the pilot carrier. If the ratio between the signal level and the noise level is calculated for each carrier, the C / N for each carrier can be calculated.

  In the digital modulator / demodulator, various modulation schemes are used depending on the transmission path characteristics from modulation schemes capable of transmitting multiple bits such as 64QAM and 32QAM to low bit transmission modulation schemes such as QPSK and BPSK.

In general, a modulation scheme capable of transmitting multiple bits has a high probability of causing a demodulation error in a poor environment, but a modulation scheme of low bit transmission has a low probability of causing a demodulation error even in a poor environment. In this way, even in the same transmission path environment depending on the modulation method used, the demodulation error rate differs. Therefore, the demodulation error rate relative to the noise level differs depending on the modulation method used, and this should be measured in advance by computer simulation or actual equipment. Thus, the demodulation error rate can be estimated.

  This error rate estimation unit 36 is easily configured by a ROM table or the like in which the modulation method, noise level or C / N for each carrier is used as an input signal, the demodulation error rate corresponding thereto is used as an output signal, and the corresponding relationship is recorded. It is possible.

  Further, error correction processing may be used in combination with a digital modem. In this case as well, the code error rate after error correction can be easily estimated by calculating the correction capability of the error correction method in advance, similarly to the condition of the modulation method.

  The display unit 37 changes the level of the meter according to the code error rate, changes the brightness of a lamp, LED, etc. according to the code error rate, or converts it into a video signal and displays it on a monitor or the like. The display is performed so that the error rate can be visually recognized.

  Alternatively, as other representation methods, a method of expressing aurally by changing the volume or scale of the scale according to the code error rate, or converting the code error rate into a signal in a data format that can be imported into a computer or the like. There is also a way to output.

Furthermore, when the code error rate at which image quality deterioration is predicted, a warning is displayed with the brightness, color, buzzer sound, etc. of the lamp or LED, and a video signal used for television broadcasting based on the estimated code error rate. Switching to another video signal is also conceivable.
As described above, by using the second embodiment of the present invention, the state of the transmission path is observed based on the noise level, and the noise level is converted into the code error rate. It becomes possible to estimate, and it becomes possible to easily know the limit point of image quality degradation.

It is a block diagram of 1st embodiment of the OFDM modulation signal receiver by this invention. It is a block diagram of embodiment of the noise level detection apparatus in the OFDM modulation signal receiver by this invention. It is a block diagram of 2nd embodiment of the OFDM modulation signal receiver by this invention. It is a figure which shows the symbol structure of an OFDM signal. It is a figure which shows the structure of the receiver in a prior art. It is a figure which estimates the characteristic of the transmission line from a reception pilot carrier. It is a figure which shows the structure of the diversity receiver which has two systems of antennas in a prior art. It is a figure which shows the diversity synthetic | combination in a prior art when C / N for every system | strain differs. It is a figure which shows the diversity synthetic | combination result in a prior art when C / N for every system | strain differs. It is a figure explaining operation | movement of the noise level detection part of FIG. It is a figure explaining the process at the time of using a differential processing algorithm in the floor level detection part in the noise level detection part of FIG. It is a figure explaining calculation of a noise level from a reception constellation. It is a figure which shows the noise level detected by the noise level detection part of FIG. It is a figure which shows the diversity synthesis | combination by this invention when C / N for every system | strain differs. FIG. 3 is a diagram showing a configuration in which two systems of noise normalization units are aggregated by a noise ratio normalization unit in the first embodiment of the OFDM modulated signal receiving apparatus according to the present invention of FIG. 2. It is a figure which shows the structure of the noise ratio normalization part of FIG. FIG. 16 is a flowchart for explaining the operation of a coefficient setting unit in the noise ratio normalization unit of FIG. 15. It is a figure which shows the diversity synthetic | combination result by this invention when C / N for every system | strain differs.

Explanation of symbols

  21: Antenna, 22: Amplifier, 23: AGC unit, 24: A / D converter, 25: FFT unit, 26: Noise normalization unit, 27: Transmission path estimation unit, 28: Weighting processing unit, 29: Synthesis processing , 2a: demodulation unit, 30: noise level detection unit, 31: delay unit, 32: subtraction unit, 33: absolute value conversion unit, 34: filter unit, 35: floor level detection unit, 36: code error rate estimation unit 37: display unit, 131: noise ratio normalization unit, 141: noise level comparison unit, 142: coefficient setting unit, 143: coefficient multiplication unit A, 144: coefficient multiplication unit B.

Claims (5)

In an OFDM modulated signal receiving apparatus that receives a signal modulated by an orthogonal frequency division multiplexing (OFDM) modulation method having a guard interval period with a plurality of antennas,
Delay means for delaying the effective symbol period with respect to the received sample value series for each diversity reception system, subtracting means for subtracting the delayed signal and the non-delayed received sample value series, and the result of the subtraction as an absolute value The absolute value converting means, the differential signal waveform resulting from the absolute value conversion , the positive and negative thresholds are provided for the differential result, the level comparison is performed with the differential result, and the differential result A time window is provided in the interval between the latest timing that is smaller than the negative threshold and the earliest timing that the derivative result is greater than the positive threshold. a noise level detecting means that detect the noise level by calculating the average value of the signal included in the temporal window in the results of the absolute value of the received signal of each system on the basis of a result of the detected Noise normalizing means for normalizing the mixed noise level, and weighting processing means for performing a process of multiplying a predetermined weighting coefficient based on the received signal of each system after the normalization processing, An OFDM-modulated signal receiving apparatus comprising: a synthesis processing unit that performs addition processing on signals obtained by multiplying the weighting coefficients of the respective systems.   2. The OFDM modulated signal receiver according to claim 1, wherein the noise normalization means multiplies all received signals by a predetermined value when the noise levels of the received signals of each system are equal, When the noise level of the received signal is not equal, the received signal with the lowest noise level is multiplied by a predetermined value, and for the other received signals, the noise level of the received signal is the lowest noise. An OFDM modulation signal receiving apparatus, wherein a coefficient that matches a multiplication result of a level and the predetermined value is multiplied. In an OFDM modulated signal receiving apparatus that receives a signal modulated by an OFDM modulation scheme having a guard interval period,
Delay means for delaying the effective symbol period with respect to the received sample value series, subtracting means for subtracting the delayed signal and the non-delayed received sample value series, and absolute value converting means for absoluteizing the result of the subtraction And differential processing the signal waveform resulting from the absolute value conversion, providing positive and negative threshold values for the differentiation result, performing level comparison with the differentiation result, and the differentiation result being smaller than the negative threshold value. A time window is provided in the interval between the latest timing in the timing and the fastest timing in the timing when the result of differentiation is greater than the positive threshold value. OFDM modulation signal receiving apparatus characterized by comprising a noise level detecting means that detect the noise level by calculating the average value of the signal included in the temporal window in the results. 4. The OFDM modulation signal receiving apparatus according to claim 1, further comprising: an estimation unit that estimates a transmission path condition or a code error rate based on a result of the noise level detection unit. Modulation signal receiver. 5. The OFDM modulation signal receiving apparatus according to claim 4 , further comprising converting the estimated transmission path condition or code error rate into a visual expression means such as a meter, a lamp, a video display device, etc., or a volume, a scale, etc. An OFDM-modulated signal receiving apparatus, comprising: a display unit according to a target, or a conversion unit for converting into an electric signal or an optical signal.

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