JP2003092559A - Method for displaying transmission state information and ofdm receiver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To display even a delay wave at a small level without being buried in noise of a transmission path. SOLUTION: A receiver for receiving an OFDM (orthogonal frequency division multiplexing) signal including a guard interval obtains information representing a transmission state in the transmission path and displays the information by finding a difference between a baseband signal that is received and demodulated and a signal obtained by delaying the baseband signal by an effective symbol length, averaging respective sum of square in a symbol and further differentiating the results.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a system for displaying the status of a transmission line in a receiver of an OFDM transmission device.

[0002]

2. Description of the Related Art In recent years, as a modulation method suitable for mobile digital audio broadcasting and terrestrial digital television broadcasting, an orthogonal frequency division multiplex modulation method (orthogonal frequency division multiplexing modulation method) which is characterized by resistance to multipath fading and ghost Frequency Division Multiplex: OFD
M) is in the spotlight. This OFDM system is a type of multi-carrier modulation system, and is a transmission system in which a plurality of orthogonal carrier waves are digitally modulated. Here, since the receiver side has a function of displaying the status of the transmission path, particularly the level and delay time of the delayed wave such as multipath, the status of the transmission path can be grasped at a glance. The display of the delay wave level and the delay time is called a delay profile. In order to realize this function, as one conventional method, as shown in FIG. 5, a reference signal of a period R having a known pattern is included in a transmission signal sent from a transmitter to a receiver, and a period D There was a signal format for actual data transmission. This is because the signal in the period R is a signal of a known pattern,
By holding the same signal as this known pattern signal sent from the transmitter in the receiver and correlating the received signal with this known pattern signal, a signal representing the delay profile can be obtained and displayed. As a result, the status of the transmission line can be grasped.

The operation of generating a signal representing the delay profile will be described with reference to FIG. Note that, here, for simplification of description, it is assumed that the received signal includes a single delayed wave delayed by time τ. Further, the received signal is a combination of the main wave and the delayed wave, but is shown separately here. Since the received signal is composed of the main wave and the delayed wave, when the signal of the period R is used as a reference signal for correlation and the received signal is correlated at the position of the correlation reference (1), it is correlated with the main wave. Since the reference (1) matches, a large value is output as the correlation output. Also, if the received signal is correlated with the position delayed by time τ from that position, that is, the position of the correlation reference (2), the correlation with the delayed wave is obtained, and the delayed wave and the correlation reference are obtained.
Since (2) matches, a relatively large value is output as the correlation output. Normally, the delayed wave has a smaller level than the main wave, and therefore the correlation output can be obtained according to the level. At positions other than the above-mentioned correlation references (1) and (2), there is no part that coincides with the correlation reference, so that the output of the correlation becomes a small value in other positions. Therefore, by taking the correlation between the received signal and the correlation reference with the passage of time of the received signal, the waveform shown in the delay profile display of FIG. 5 is obtained.
By displaying this on a predetermined display circuit (oscilloscope, monitor, etc.), the state of the transmission line can be visually observed.

However, when transmitting an OFDM signal as shown in FIG. 6, it is not possible to include a predetermined known signal. This OFDM signal is a signal having a symbol structure in which a guard interval (G) for reducing the influence of a delayed wave is added to a signal having an effective symbol length (T _G ). That is, a signal having a symbol structure in which the latter half B ₁ portion of a signal having an effective symbol length (T _G ) is copied before the first half A ₁ portion and is inserted as a guard interval (G) is continuous. In this case, since the signals of the A ₁ and B ₁ parts both depend on the input signal in the transmitter, if the transmission data as the input signal changes, it is undefined data that changes accordingly, and at the receiver Unpredictable. That is, there is no signal that can be used as a correlation reference signal that correlates with the received signal as described above. Therefore, by utilizing the fact that the guard interval is the same signal as the latter half of the effective symbol, the delay profile is obtained by the following method.

First, the overall configuration of the receiver will be described with reference to FIG. Regarding this configuration and operation, for example,
Since it is disclosed in detail in the following documents, a brief description will be given here. "Digital transmission" p117-p119,
Issued in 1998 by Ohmsha Co., Ltd. In a receiver as shown in FIG. 3, a modulated OFDM signal is received, passed through a band limiting filter 1, and then an A / D converter 2
Convert to digital signal. Here, a sine wave equal to the carrier frequency of the modulated reception signal is generated by the carrier generator 8 and this signal cosω is generated by the multiplier 3.
Multiply t. On the other hand, the signal output from the carrier wave generator 8 is shifted by π / 2 by the phase shifter 7, and the signal s
The multiplier 4 performs multiplication as inωt. Then, the components I and Q of the demodulated baseband OFDM signal are obtained by passing through the low-pass filters 5 and 6 for removing harmonic components, respectively. In addition,
I represents the in-phase component, and Q represents the quadrature component, which is output as a so-called complex signal. Then, this output is converted into a parallel signal by the serial / parallel converter 9, and this signal is reproduced here by the FFT (Fast Fourier Transform) calculator 10 by the inverse FFT operation on the transmission side. Then, the output of each frequency of the FFT calculator 10 is discriminated by the discriminator 11 and then output as serial data by the parallel / serial converter 12. This output signal is a signal reproduced from the signal transmitted from the transmitting side. Further, the demodulated baseband signals I and Q are detected by the delay wave detector 13 to detect the level and delay time of the delay wave, and the signals are input to the display circuit 14,
Display the delay profile visually.

The delayed wave detector 13 has, for example, the configuration shown in FIG. 4, and delays the guard interval (G) of the received signal and the received signal by the time of the effective symbol length (T _G ). The delay profile is detected by calculating the cross-correlation with the B ₁ portion of the signal and output to the display circuit 14. That is, the baseband signals I and Q and these signals are delayed by the delay units 130-1 and 130-.
The signals delayed by the time corresponding to the effective symbol length in 2 are respectively subjected to cross-correlation in correlators 130-3 and 130-4. Then, in order to process these cross-correlation values as a scalar quantity, a signal obtained by squaring a complex in-phase component (output signal of the correlator 130-3) by the multiplier 130-5 and a quadrature component (correlator 130-3) are used. -4 output signal) and the sum of the squared signals of the multiplier 130-6
30-7, the inter-symbol averaging unit 130-8 performs averaging, and outputs it to the display circuit 14.

The correlators 130-3 and 130-4 can be realized, for example, by the configuration shown in FIG. Input IN
Signal input to the correlator 130-3 (130-4) from 1
(I, Q) is delayed by a plurality of stages of delay devices 130-3-1.
The digital discrete signals are sequentially shifted at intervals Ts. Then, for each delay device 130-3-1,
It is output to the input of the multiplier 130-3-2. On the other hand, like the input IN1, the signal input from the input IN2 is
Each delay device 130-3-4 similar to the delay device 130-3-1
Is sequentially shifted, and its output is multiplied by the multiplier 13
It is input to the other input of 0-3-2. For each of the delay devices 130-3-1 and 130-3-4, the respective outputs are input to the respective multipliers 130-3-2, and the multiplication outputs are summed by the adder 130-3-3. Is done. Here, the number of multipliers 130-3-2 is equal to or less than the number of data of the B ₁ portion of the OFDM signal. The correlators 130-3 and 130-4 are configured to take a general correlation, and if the correlation between the signal input to the input IN1 and the signal input to the input IN2 is strong, the output value will be a large value. Is output. When there is almost no correlation, the output value is small.

[0008]

However, this delay wave detection method has the following problems. Figure 7
Will be explained. In FIG. 7, the received signal and the delay device 1
A delayed output signal delayed by the effective symbol length (T _G ) by 30-1 and 130-2 is decomposed into a main wave and a delayed wave for the purpose of explanation. Actually, they are the received signal and the delayed output signal respectively synthesized. here,
The level of the delayed wave is usually lower than the level of the main wave. For example, if the level of the main wave is -20 dB, the two are multiplied by the cross-correlation, and the result is a very low value of -40 dB. Will be. This is a level equivalent to the noise level in the transmission line, and is difficult to detect as a delayed wave. At lower levels of delayed waves, they are completely undetectable because they are buried in noise. Explaining in FIG. 7, in the periods a and b, the correlation between the main waves is
Since the levels are high, there is no problem, but the correlation between the delayed waves is in the periods b and c. Since this is a delayed wave with a small level, when they are multiplied by each other, the level becomes a smaller value, which is buried in noise and cannot be detected as a delayed wave. SUMMARY OF THE INVENTION It is an object of the present invention to eliminate the above-mentioned drawbacks and enable display of a delayed wave having a small level without being buried in the noise of the transmission line.

[0009]

In order to achieve the above object, the present invention provides a baseband signal received and demodulated in a receiver for receiving an OFDM signal including a guard interval and an effective symbol length of the baseband signal. The difference from the delayed signal is calculated, and the information indicating the transmission state on the transmission path is obtained from the difference calculation result.
Further, in a receiver that receives an OFDM signal including a guard interval, the difference between the received and demodulated baseband signal and the signal obtained by delaying the baseband signal by the effective symbol length is calculated, and the sum of squares of each is calculated within the symbol. By averaging and further differentiating the result, information indicating the transmission state in the transmission path is obtained and displayed. In addition, OF including the guard interval
In a receiver that receives a DM signal, the difference between the received and demodulated baseband signal and the signal obtained by delaying the baseband signal by an effective symbol length is calculated, and the sum of squares of each is averaged within the symbol, and the inter-symbol But on average,
Further, by differentiating the result, information indicating the transmission state on the transmission path is obtained and displayed.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, FIG. 1 shows a configuration of an embodiment of a delayed wave detector 13A according to the present invention.
The delay wave detector 13 of FIG. 3 showing the entire configuration of the demodulator of the transmission apparatus will be described in detail, using the delay wave detector 13A of the present invention, together with the time chart of the signals of the respective parts of FIG. In FIG. 3, the received and demodulated OFDM signal is input to the serial / parallel converter 9 for performing the FFT calculation process and also to the delayed wave detector 13A. In FIG. 1, the demodulated in-phase component I of the baseband signal is input to the adder 13-3. Also,
The in-phase component I is also input to the delay device 13-1, where
The signal is output after being delayed by a time equal to the effective symbol length (T _G ) of the OFDM signal. This output is input to the other side of the adder 13-3 to take the difference. That is, when delayed by the time of the effective symbol length, when the input signal is B ₁ in the latter half of the effective symbol length, the delay unit 13-1
The output of B is the guard interval B ₁ .
Assuming that there is no delayed wave, preceding wave, or noise on the transmission path, these two signals are exactly the same signal, so the difference between the two signals is zero.

Therefore, the output of the adder 13-3 becomes 0 while the B ₁ portion is being input. In the other periods, the difference signals are completely different, so the adder 1
A random value is output as the output of 3-3. Quadrature component Q
In the same manner as in the in-phase component I, the difference between the input signal and the output of the delay device 13-2 is calculated by the adder 13-4. Here, since the complex type signal represented by the in-phase component and the quadrature component is represented in two dimensions, in order to easily process the signal, the respective adders 13-3 and 13-4 are used. The output of is squared by the multipliers 13-5 and 13-6,
The sum is taken by the adder 13-7. Then, the value becomes a one-dimensional value (scalar amount). As above, B ₁
Periods other than the period that the part is input have random values. By averaging this by the intra-symbol averaging unit 13-8, a signal having a smooth predetermined value from a random value (differential input signal shown in FIG. 8) is obtained. When this differentiating input signal is differentiated by the differentiating circuit 13-9, as shown as a differentiating output signal in FIG. 8, a large pulse is relatively output at the position of the main wave and delayed at the position delayed by time τ. A pulse corresponding to the wave level is output. Since these pulse signals are output in the transmission symbol period, by synchronizing with the symbol time on the display circuit 14 and displaying with the position of the main wave as a reference, a delay profile can be obtained even for a delayed wave with a small level. Can be displayed.

Here, the intra-symbol averaging unit 13-8 is
For example, it can be realized by a configuration as shown in FIG. The output of the adder 13-7, the delay unit 13-8-1 symbols in averager 13-8 is sequentially shifted by T _S intervals for each data. Then, the adders 13-8-2 sum the outputs of the respective delay devices 13-8-1 and the multiplier 13-
In 8-3, the coefficient is multiplied by 1/4, and the data of four samples are averaged. Here, the number of delay devices 13-8-1 does not necessarily have to be three, and the number may be reduced or increased depending on the required averaging. Also, instead of averaging
A low-pass filter with the necessary frequency characteristics,
The output signal may be a smooth signal.

Further, an embodiment of the differentiating circuit 13-9 can be realized by a structure as shown in FIG. Input signal is sequentially shifted by T _S intervals for each data delay unit 13-9-1. Then, the difference circuit 13-9 can be realized by taking the difference between the shifted signal and the input signal in the adder 13-9-2. In this example, the delay device 13-
Although four 9-1 are connected in succession, the minimum number may be one, or four or more. This delay device 13-9-1
When the number of is small, the resolution is improved, but conversely, all minute changes are output, resulting in a large ripple. In addition, when the number of delay devices 13-9-1 is large, ripples are small and a smooth waveform is obtained.
The resolution is reduced. When there are a plurality of delayed waves and their delay times are almost equal, there is a possibility that the delayed waves cannot be distinguished when the resolution is reduced. Therefore, the number of delay devices 13-9-1 is set to the number of the transmission lines used. Decisions should be made taking into account the circumstances.

Next, another embodiment of the delayed wave detector 13A of the present invention is shown in FIG. The delayed wave detector 13B has the same configuration as that shown in FIG. 1 up to the intra-symbol averaging unit 13-8, and the inter-symbol averaging unit 13-10 is inserted in the preceding stage of the differentiating circuit 13-9. The only difference is that. By performing averaging between symbols by the inter-symbol averaging device 13-10, there is an effect of suppressing the noise component of the transmission path, and it is possible to display a delayed wave of a very low level. The inter-symbol averaging unit 13-10 can be realized by the configuration shown in FIG. Intra-symbol averaging device 13
-8 output is serial / parallel converter 13-10-1
Is converted into a parallel signal for each data. Then, it is averaged by the inter-symbol averaging filter 13-10-2 in the inter-symbol cycle T _B. The inter-symbol averaging filter 13-10-2 can be realized by a configuration as shown in FIG. 10, for example. The output of the serial / parallel converter 13-10-1 is output to each delay device 13-10-2- in each inter-symbol averaging filter 13-10-2.
At 1, they are shifted at intervals of T _B. The output of each delay unit 13-10-2-1 is added to the adder 1
3-10-2-2 calculates the sum, and the multiplier 13-10-2
-3 is set to 1/5, and the data for each symbol is averaged. Similar to the intra-symbol averaging unit 13-9, this filter does not necessarily have to have four delay units 13-10-2-1 and may have a configuration with less or more than that. Further, instead of averaging, a smooth signal may be obtained by using a low-pass filter having a necessary frequency characteristic.

[0015]

As described above, according to the present invention, even a delayed wave having a low level which could not be displayed in the past can be displayed as a delay profile. By increasing the inter-symbol average, noise can be suppressed and the profile of the delayed wave at a lower level can be displayed. Furthermore, the present invention can be realized with a simpler structure than that which has been conventionally used, and can be made compact and inexpensive.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a delayed wave detector of the present invention.

FIG. 2 is a block diagram showing another embodiment of the delayed wave detector of the present invention.

FIG. 3 is a block diagram showing an overall configuration of a receiver of an OFDM transmission device.

FIG. 4 is a block diagram showing a configuration of a conventional delayed wave detector.

FIG. 5 is an explanatory diagram of a conventional delay profile display when a correlation reference is used.

FIG. 6 is an explanatory diagram of an OFDM signal.

FIG. 7 is an explanatory diagram of a conventional method when a delayed wave exists.

FIG. 8 is an explanatory diagram of a delay profile display of the present invention when a delayed wave exists.

FIG. 9 is a block diagram showing an embodiment of an inter-symbol averaging device of the present invention.

FIG. 10 is a block diagram showing an embodiment of an averaging device of the present invention.

FIG. 11 is a block diagram showing an embodiment of an intra-symbol averaging device of the present invention.

FIG. 12 is a block diagram showing an example of a conventional correlator.

FIG. 13 is a block diagram showing an embodiment of a differentiating circuit of the present invention.

[Explanation of symbols]

1: band-limited filter, 2: A / D converter, 3,
4, 13-5, 13-6, 13-9-3, 13-10-
2-3: Multiplier, 5, 6: Low-pass filter, 7: Phase shifter, 8: Carrier wave oscillator, 9, 13-10-1: Serial / parallel converter, 10: FFT calculator, 11: Discriminator, 12 : Parallel / serial converter, 13, 13
A, 13B, 130: Delayed wave detector, 13-1, 13-
2: delay device, 13-3, 13-4, 13-7, 13-9
-2, 13-10-2-2: Adder, 13-8: In-symbol averaging device, 13-8-1, Delay device, 13-10: Inter-symbol averaging device, 14: Display circuit.

Claims (5)

[Claims] 1. A receiver for receiving an OFDM signal including a guard interval calculates a difference between a baseband signal received and demodulated and a signal obtained by delaying the baseband signal by an effective symbol length, and the difference calculation result. A method for displaying transmission status information, characterized in that information indicating a transmission status on a transmission path is obtained from the. 2. A receiver for receiving an OFDM signal including a guard interval, wherein an in-phase component signal and a quadrature component signal of a baseband signal received and demodulated and a signal obtained by delaying each of these signals by an effective symbol length are provided. Transmission state information characterized by calculating the difference, averaging the respective square sums within the symbol, and further differentiating the result to obtain information indicating the transmission state in the transmission path and displaying the information. Display method. 3. A receiver for receiving an OFDM signal including a guard interval, takes a difference between a received and demodulated baseband signal and a signal obtained by delaying the baseband signal by an effective symbol length, and sums the respective squares. Is averaged within a symbol, averaged between symbols, and the result is further differentiated to obtain information indicating the transmission state on the transmission path, and the information is displayed. 4. In a receiver for receiving an OFDM signal including a guard interval, a quadrature demodulation means for quadrature demodulating an in-phase component signal and a quadrature component signal of a baseband signal from the received signal, and the in-phase component signal and the quadrature component signal. When,
The difference between each of these signals and the signal delayed by the effective symbol length is calculated, each sum of squares is averaged within the symbol, and the result is differentiated to calculate the information indicating the transmission state in the transmission path. An OFDM receiver having means for displaying and information for displaying the calculated transmission state. 5. In a receiver for receiving an OFDM signal including a guard interval, a quadrature demodulation means for quadrature demodulating an in-phase component signal and a quadrature component signal of a baseband signal from the received signal, and the in-phase component signal and the quadrature component signal. When,
Differences between these signals and signals delayed by the effective symbol length are calculated, and the sums of squares of the signals are averaged within the symbols and also between the symbols. An OFDM receiver comprising means for calculating information indicating a transmission state and means for displaying the calculated information indicating the transmission state.