JP4647296B2 - Delay profile measuring device - Google Patents

Description

  The present invention relates to a delay profile measuring apparatus, and more specifically, for measuring transmission path characteristics in a communication system that performs communication using an OFDM (Orthogonal Frequency Division Multiplex) modulation method such as a terrestrial digital broadcasting system. The present invention relates to a delay profile measuring apparatus.

  In general, a signal transmitted from a transmitter includes a direct wave that directly reaches the receiver and a delayed wave (multipath signal) that arrives after being delayed by reflection, diffraction, or scattering on a building or the like. When a plurality of delayed waves are combined and received by the receiver, a multipath failure that causes image quality degradation due to waveform distortion occurs. In order to reduce such a multipath failure as much as possible, it is necessary to set the directivity of the antenna of the receiver, etc. by measuring the transmission path characteristic, that is, the delay profile.

In particular, the transition from analog terrestrial broadcasting to terrestrial digital broadcasting is currently underway. However, in terrestrial digital broadcasting systems that employ the OFDM modulation method, the transmission path characteristics can be improved by simply measuring the combined electric field of multiple waves. It is difficult to grasp. Therefore, in the terrestrial digital broadcasting system, it is particularly effective to measure the delay profile in order to grasp the transmission path characteristics.
Note that the delay profile is expressed as a time response characteristic of the transmission line. Since the time response characteristic and the frequency characteristic are in a Fourier transform pair relationship, the frequency characteristic of the transmission line is measured, and the frequency characteristic is inverse Fourier transformed. It can be obtained by converting.
Such delay profile measuring devices are described in Patent Documents 1 and 2 below.

  When delay profile measurement is performed in a terrestrial digital broadcasting system, a scattered pilot signal (hereinafter referred to as an SP signal) is demodulated from a received signal using the digital demodulator 1 having the configuration shown in the block diagram of FIG. The digital demodulator 1 includes a local oscillator 11, a multiplier 12 that multiplies a received signal and a local oscillation frequency to output an IF signal, an A / D converter 13 that converts the IF signal into a digital signal, and an IF signal (digital). A symbol synchronization processing circuit 14 that performs the symbol synchronization processing, a DFT circuit 15 (or FFT circuit) that performs Fourier transform processing on the synchronized signal, and a pilot signal separation circuit 16 that separates and extracts the SP signal from the output signal of the DFT circuit 15 I have.

The SP signal is a complex vector having a known constant amplitude and phase. In order to express the frequency characteristics of the transmission path to the terrestrial digital wave, the SP signal has a constant interval in the frequency direction within the band of the terrestrial digital broadcast wave. Has been placed. An original data signal is arranged between the SP signal and the SP signal. The demodulated SP signal is obtained in a distorted state due to the influence of the frequency characteristics of the transmission path, but since the state of the SP signal at the time of transmission is known, the SP signal at the time of transmission and the SP signal at the time of reception By comparing these, the frequency characteristics of the transmission line can be obtained.
Using the SP signal demodulated by the digital demodulator 1, the delay profile calculator 2 calculates a delay profile. The obtained delay profile is graphically displayed on the monitor 3 with the horizontal axis representing the delay time and the vertical axis representing the signal power level, and the signal power of the direct wave (Desired Wave) and the delayed wave (Undesired Wave). The characteristics of the transmission line are evaluated by the level ratio D / U.
JP 2000-115087 A JP-A-5-75568

  As described above, in the delay profile measurement device, the digital demodulator is used to demodulate the SP signal from the received signal, and the delay profile is calculated using the SP signal. As a result, various distortions occur. In particular, a shift in the local oscillation frequency, a shift in the sampling timing of the A / D converter, and a symbol synchronization timing cause phase rotation of the SP signal. Such phase rotation in the SP signal appears as a time shift of the entire delay profile, and eventually it is impossible to obtain an accurate delay profile.

  That is, inverse Fourier transform (IDFT) is normally used for the Fourier transform in the delay profile calculation unit 2 shown in FIG. 3, and therefore the time resolution is the number of SP signals, that is, the frequency data in the inverse Fourier transform. Limited by number. Therefore, when the time shift due to the phase rotation of the SP signal is an integral multiple of the time resolution, the delay profile can be measured with high accuracy because it can be detected correctly. If it occurs at an integral multiple, a component that would otherwise be detectable cannot be detected.

In particular, the direct wave is a signal component that appears at the time zero position of the delay profile, but when the time shift occurs at a non-integer multiple of the time resolution, the position becomes a position other than time zero. Therefore, the signal level at time zero on the obtained delay profile is lower than the signal level of the original direct wave. Since the evaluation of the delayed wave is expressed by the D / U ratio as described above, if the signal level of the direct wave is incorrect, the D / U ratio also becomes an incorrect value.
In addition, since the delayed wave appears as a very steep mountain shape on the delay profile, the detected signal level decreases if it is a non-integral multiple of the time resolution even if there is a slight time shift. As a result, the D / U ratio becomes an incorrect value.

The above-described delay profile measuring device of the conventional example cannot solve the problem of the phase rotation of the SP signal generated inside the digital demodulator, and the present invention has been made in view of such a problem. .
Accordingly, an object of the present invention is to measure the delay profile of a communication system employing an OFDM modulation method such as terrestrial digital broadcasting with high accuracy while reducing the influence of SP signal distortion caused by internal factors of the digital demodulator. Is to do so.

In order to achieve the above object, the present invention provides a delay profile measuring apparatus for measuring a delay profile in a communication system employing an OFDM modulation scheme.
It received signal Scattered Pilot signal demodulated (hereinafter, SP signals) sample sequence of the input by adding a plurality of samples of zero value at the end of the sample sequence of the SP signals, the number of samples is extended Sample expansion means for outputting an expanded SP signal;
An inverse Fourier transform means for performing an inverse Fourier transform of the extended SP signal and outputting a transmission line impulse response;
A maximum power level in the transmission line impulse response, and a maximum level detecting means for detecting the position thereof;
Ideal appearance position of the direct wave, determined as the start position of the output data from the inverse Fourier transform unit calculates the difference between the ideal occurrence position of the position of the maximum power level and the direct wave, minute difference Only a barrel shifter to barrel shift the transmission line impulse response,
And a means for detecting a power level at a time other than time 0 in the barrel-shifted transmission line impulse response and calculating a D / U ratio that is a ratio with the maximum power level detected by the maximum level detecting means. A delay profile measuring device is provided.

In a preferred embodiment of the delay profile measuring apparatus according to the present invention described above , the delay profile measuring apparatus further includes a digital demodulator for demodulating the SP signal from the received signal.

Since the delay profile measuring apparatus of the present invention is configured as described above, it is possible to measure a delay profile with high accuracy even if distortion is included in the SP signal demodulated from the received signal.
At this time, since it is not necessary to measure the amount of phase rotation of the SP signal generated by the digital demodulator that demodulates the SP signal, the deviation of the local oscillation frequency inside the demodulator, the sampling timing of the A / D converter, and It is not necessary to measure a symbol synchronization timing shift or the like and use these values for delay profile correction. Therefore, even a digital demodulator that does not include a means for acquiring internal demodulation information can perform delay profile measurement simply by connecting the delay profile measurement device of the present invention to the digital demodulator. . Therefore, according to the present invention, a highly accurate delay profile measurement can be performed even in combination with an existing digital demodulator.

FIG. 1 is a block diagram showing a configuration of a delay profile measuring apparatus 20 according to the present invention. The delay profile measurement apparatus 20 includes a delay profile calculation unit 210, which includes a sample extension unit 211, an IFFT unit (or IDFT) 212, a maximum level detection unit 213, a barrel shifter 214, and a D / U calculation unit 215. It is configured. The delay profile measurement device 20 further includes a monitor 220. Reference numeral 10 denotes an SP signal demodulating digital demodulator having the configuration shown in FIG.
Hereinafter, the operation of the delay profile measuring apparatus 20 shown in FIG. 1 will be described in detail.

The sample extension unit 21 of the delay profile calculation unit 210 has a function for making the number of samples used for calculation in the IFFT unit 212 larger than the number of SP signals, and the SP signal input from the digital demodulator 10. A plurality of samples having zero values (hereinafter referred to as zero value samples) are added to the sample sequence, and the resulting SP signal with the expanded number of samples is output to the IFFT unit 212 as an extended SP signal. Extended SP signals, especially from adding a plurality of zero value samples at the end of the input SP signal can be obtained.

The IFFT unit 212 receives the extended SP signal, performs inverse fast Fourier transform processing based on the extended SP signal, and outputs a transmission path impulse response. Accordingly, the IFFT unit 212 can perform inverse fast Fourier transform with a high time resolution. The transmission path impulse response is supplied to the maximum level detector 213 and the barrel shifter 214.
The maximum level detection unit 213 detects the maximum power level in the supplied transmission line impulse response, supplies this to the D / U calculation unit 215 as the direct wave signal level, that is, the D level, and the appearance of the maximum level. The direct wave appearance position, which is the position (appearance time), is supplied to the barrel shifter 214.

The barrel shifter 214 monitors the data sequence of the transmission path impulse response supplied from the IFFT unit 212, and determines the appearance position of the head of the data sequence as an ideal digital demodulator (that is, a digital signal that does not cause phase rotation of the SP signal). It is detected as an appearance position of a direct wave (direct wave ideal appearance position) when a demodulator is used. Then, the barrel shifter 214 compares the direct wave ideal appearance position with the direct wave appearance position from the maximum level detection unit 213, and calculates the appearance position difference.
Next, the barrel shifter 214 shifts the transmission line impulse response from the IFFT unit 212 by the entire time axis, that is, barrel shift, by the appearance position difference, and matches the direct wave appearance position to the time axis 0 of the transmission line impulse response. Let Note that a time shift due to an internal factor of the digital demodulator 10 occurs in a constant amount in the entire delay profile, so that the time shift of the entire delay profile can be corrected by barrel shift. The barrel-shifted transmission line impulse response is supplied to the D / U calculator 215 and also to the monitor 220.

The D / U calculation unit 215 detects the power level of the impulse other than the time axis 0 in the barrel-shifted transmission line impulse response as the power level of the delayed wave. Then, the D / U ratio is calculated between the maximum signal level supplied from the maximum level detector 23, that is, the D level, with the maximum power level of the delay wave power level as the U level. The obtained D / U ratio is supplied to the monitor 220.
The monitor 220 displays the transmission path impulse response from the barrel shifter 214 and also displays the D / U ratio value from the D / U calculation unit 215.

FIGS. 2A and 2B exemplarily compare the delay profiles generated by the conventional delay profile measuring apparatus and the delay profile measuring apparatus of the present invention. Note that the vertical axis represents the power level of the signal in decibels, and thus the D / U ratio on the graph is represented as the difference between the direct (D) wave and the delayed (U) wave.
Since the delay profile measuring apparatus of the conventional example has a lower time resolution than the present invention, as shown in FIG. 2A, the original direct wave appearance position is determined from the time axis 0 due to an internal factor of the digital demodulator. In the case of deviation, the D level on the time axis 0 of the delay profile is greatly lowered, and the U level of the delayed wave is also lowered. As a result, the measured D / U ratio is greatly different from the original D / U ratio.
On the other hand, in the delay profile measuring apparatus of the present invention, the number of samples is expanded by the sample expansion unit 211 to increase the time resolution, and the maximum level appearance position is detected by the maximum level detection unit 213 and the barrel shifter 214. The barrel shift is performed with the direct wave appearance position. Accordingly, as shown in FIG. 2B, the measured D / U ratio and the original D / U ratio are almost the same.
Therefore, according to the present invention, it can be seen that an accurate D / U ratio can be measured in comparison with the conventional example.

  In the present invention, as the time resolution is increased, an accurate delay profile is obtained, but the amount of calculation of IFFT (or IDFT) increases. Therefore, it is necessary to determine the time resolution in consideration of the balance between the delay profile calculation processing load and the correction degree.

  In the delay profile measuring apparatus according to the present invention, it is not necessary to measure the amount of phase rotation of the SP signal generated in the digital demodulator. Therefore, the local oscillation frequency shift in the demodulator, the sampling timing of the A / D converter, Further, it is not necessary to measure a deviation of the symbol synchronization timing and use these values for correcting the delay profile. Therefore, the delay profile measurement can be performed simply by connecting the present invention to the digital demodulator. Therefore, the delay profile measurement apparatus of the present invention has a digital demodulator having the configuration shown in FIG. It may or may not be built-in. Of course, the delay profile measuring apparatus of the present invention may incorporate a digital demodulator for separating the SP signal.

It is a block diagram which shows the structure of the delay profile measuring apparatus which concerns on this invention. It is explanatory drawing which compares and shows the delay profile and D / U ratio which were produced | generated by the delay profile measuring apparatus of the prior art example, and the delay profile measuring apparatus which concerns on this invention. It is a block diagram which shows the structure of the digital demodulator for demonstrating the problem in the delay profile measurement of a prior art example.

Claims (2)

In a delay profile measuring apparatus for measuring a delay profile in a communication system employing an OFDM modulation scheme,
It received signal Scattered Pilot signal demodulated (hereinafter, SP signals) sample sequence of the input by adding a plurality of samples of zero value at the end of the sample sequence of the SP signals, the number of samples is extended Sample expansion means for outputting an expanded SP signal;
An inverse Fourier transform means for performing an inverse Fourier transform of the extended SP signal and outputting a transmission line impulse response;
A maximum power level in the transmission line impulse response, and a maximum level detecting means for detecting the position thereof;
The ideal appearance position of the direct wave is determined as the head position of the output data from the inverse Fourier transform means , the difference between the position of the maximum power level and the ideal appearance position of the direct wave is calculated, and the difference is calculated. Only a barrel shifter to barrel shift the transmission line impulse response,
And a means for detecting a power level at a time other than time 0 in the barrel-shifted transmission line impulse response and calculating a D / U ratio that is a ratio with the maximum power level detected by the maximum level detecting means. A delay profile measuring apparatus. 2. The delay profile measuring apparatus according to claim 1, further comprising a digital demodulator for demodulating the SP signal from the received signal.

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