JPH07283807A - Symbol head detector for ofdm signal - Google Patents

Abstract

PURPOSE:To obtain precise start time position information for a valid symbol block of an input signal by providing a subtractor means subtracting an original OFDM signal from an output signal from a signal delay means, a detection means and a leading edge detection means to the detector. CONSTITUTION:A guard interval Tg is provided to each symbol block Ts of a received signal. Part of the received signal is given to a delay device 11, from which a signal delayed by Tg is outputted. The output signal is given to a delay device 12 and a subtractor 13 and an output signal of the delay device 12 is given to the subtractor 13 in which subtraction is processed. The output signal from the subtractor 13 is converted into a signal representing an envelope in an envelope detector 14 and becomes a clock signal having an interval of one symbol period Ts rising at the start of a valid symbol period T of the input signal in a comparator 15. Moreover, the output signal from the comparator 15 is given to a PLL circuit 16 and the PLL is locked by rising information.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an OFDM (Orthogonal Frequency Division Multiplexing) signal symbol head detecting apparatus, and more particularly to an OFDM signal symbol head detecting apparatus used in an OFDM modulator / demodulator for digital audio broadcasting.

[0002]

2. Description of the Related Art Radio broadcasting has been carried out by AM broadcasting using amplitude modulation for many years since the broadcasting started, so that the sound quality was not so good, but FM broadcasting started. As a result, the sound quality has improved remarkably, and it has become possible to enjoy music broadcasting with relatively good sound quality. However, nowadays when digital communication has become popular, there is an increasing demand for radio broadcasting with even better sound quality. Against this background, the development of a digital audio broadcasting system that has excellent sound quality of CD class and is capable of broadcasting to mobiles has been attracting attention in various European countries, and various researches have been conducted. It is being appreciated.

One of them is OFD as a modulation method.
This is a method using M (Orthogonal Frequency Division Multiplexing), which is already in the field experiment stage in Europe.

OFDM employs a system of digital modulation using a large number of carriers (subcarriers) orthogonal to each other. The schematic configuration of the OFDM modulator / demodulator is shown in FIG.

As shown in FIG. 3, in OFDM, serial transmission data is first converted into serial-parallel conversion. The degree of parallelism at this time is the same as the number of subcarriers. Next, the serial-parallel converted signals are digitally modulated. This modulation process applies the inverse fast Fourier transform (Inverse Fast) to each signal.
It is realized by performing Fourier Transform (IFFT) and setting a guard interval.

The modulated signal is expressed as the sum of a large number of subcarriers, and the signal in one symbol section (effective symbol section) is expressed by the following equation (1).

[0007]

[Equation 1]

In the above equation 1, C _n is the transmission signal alphabet of the nth subcarrier, N is the number of subcarriers, f ₀ is the transmission frequency, T is the symbol time, and Re
[•] indicates the real part of the complex number [•], and i
Is (-1) ^1/2 .

Then, the signals represented by the above formula 1 are connected every T time to form a modulated OFDM transmission signal.

It is known that this signal can be easily demodulated by performing FFT processing including DFT (discrete Fourier transform) and parallel-serial conversion processing.
In the FT processing, an operation (synchronization) for accurately adjusting the time position of the signal for one symbol is required.

[0011]

FIG. 4 is a timing chart showing a conventional method of synchronizing input modulation signals, in which A is a received signal and B is a signal after envelope detection. C is a signal indicating the start time position (t ₂ , t ₃ , t ₄ ...) Of each symbol from the reference position (t ₁ ) obtained by detecting the rising edge with an arrow. is there.

Conventionally, when a received signal as indicated by A is input, it is envelope-detected, and a rising edge is detected from the output signal after envelope detection indicated by B, and its time (t ₁₎ the start position of the symbol relative to the (t _2,
t _3, t ₄ ···) and had been estimated. However, when such a synchronization method is used, if the position of the head is shifted,
There is a problem that the estimation of all subsequent symbol positions also becomes erroneous and an erroneous signal is output.

The present invention has been made in view of the above problems, and it is possible to obtain accurate information of a symbol head (symbol head position) for each symbol of an OFDM-modulated input signal, and to use this device as a receiver. An object of the present invention is to provide a symbol head detecting device that can always output accurate received data when used as a component.

[0014]

In order to achieve the above object, a symbol head detecting apparatus according to the present invention is an OFDM (Orthogonal Frequency Division Multiplexing) in which a guard interval is set.
A signal symbol head detecting apparatus, comprising: a signal delay means for delaying an OFDM signal by one symbol section; and an original OF.
It is characterized in that it is provided with subtraction means for performing subtraction processing of the DM signal and the output signal from the signal delay means, detection means, and rising edge detection means.

[0015]

According to the symbol head detecting device having the above structure,
A symbol head detector for an OFDM (Orthogonal Frequency Division Multiplexing) signal with a guard interval set, comprising:
Since the FDM signal is provided with the signal delay means for delaying by one symbol section, the subtraction means for performing the subtraction processing of the original OFDM signal and the output signal from the signal delay means, the detection means, and the rising edge detection means. The subtracting means cancels the signal in the guard interval portion to 0, and then the rising edge is detected for the signal subjected to envelope detection or the like by the detecting means. Therefore, information on the accurate start time position of each effective symbol section of the OFDM-modulated input signal is obtained, and by incorporating this device into the OFDM receiver, it is possible to accurately output the received signal.

[0016]

EXAMPLES AND COMPARATIVE EXAMPLES Examples of the symbol head detecting apparatus according to the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of a symbol head detecting apparatus according to this embodiment, and FIG. 2 is a timing chart showing the operation of each circuit in the apparatus.

As shown in FIG. 1, in the symbol head detecting apparatus according to the embodiment, the signal line for inputting the received signal is connected to the delay device 11, and the other end of the delay device 11 is connected to the delay device 12. The subtractor 13 is connected to the subtracter 13, and the other end of the subtractor 12 is connected to the subtractor 13 so that the subtraction operation is performed.

The output side of the subtractor 13 is connected to an envelope detector 14 for extracting the envelope of the modulated wave which is the input signal, and the binary signal is sequentially inputted from the envelope detector 14 using the input signal as a reference value. Comparator 15 for converting the analog signal into a digital signal, and a PLL (Phase Lock-ed) for outputting an edge detection signal synchronized with the rising position of the input signal
Loop) circuit 16 is connected.

Next, the operation of each circuit of the symbol head detecting device will be described with reference to FIG.

In each symbol section Ts of the received signal,
As shown in FIG. 2, a guard interval Tg is provided. This guard interval Tg is a quarter of the latter half of each effective symbol section T actually used as a signal.
Is added before each effective symbol section T, and by providing this guard interval Tg, intersymbol interference due to ghost or multipath is prevented. The relationship between the symbol section Ts, the guard interval Tg, and the effective symbol section T is expressed by the following mathematical formula 2.

[0021]

## EQU00002 ## Ts = T + Tg represents a received signal, and a part of this received signal is first input to the delay device 11. And, as shown in,
The delay device 11 outputs a signal delayed by the guard interval section Tg from the input signal.

Next, the output signal of the delay device 11 is the delay device 12
Is also input to the subtractor 13, and the output signal of the delay unit 12 is also input to the subtractor 13 to be subjected to subtraction processing. The output signal from the delay unit 12 is delayed by the effective symbol period T as compared with the output signal of the delay unit 11 as shown in, and is a signal delayed by one symbol period Ts with respect to the received signal finally shown. Is the delay device 1
It is output from 2. In the subtractor 13, the output signal from the delay device 11 and the output signal from the delay device 12 are subjected to subtraction processing. As can be seen from the timing chart shown in and, the guard interval Tg of the output signal from the delay device 12 is Means that the timing coincides with the latter quarter of the effective symbol period T of the output signal from the delay device 11. Since the signal in the latter half of the effective symbol section T and the signal in the guard interval Tg are exactly the same as described above, the subtraction processing cancels them out, and the portion becomes 0.

Further, it is known that the waveform of the signal in the effective symbol section T expressed by the equation 1 is close to Gaussian noise, and if the characteristic that the linear operation result of Gaussian process is Gaussian process is used, Subtractor 1 for other parts
The output signal from 3 has a waveform close to Gaussian noise. As a result, the output signal from the subtractor 13 is
The output of the portion corresponding to the guard interval Tg of the received signal is 0, and the other portions are signals having a waveform close to Gaussian noise. Note that the signal indicated by a rectangle in and also has a waveform close to Gaussian noise as shown in the actual waveform.

The output signal from the subtractor 13 is converted into a signal having the envelope shown in the envelope detector 14 and further shaped into the waveform shown in the comparator 15 to obtain the effective symbol section of the input signal. The clock signal has an interval of one symbol period Ts which rises at the beginning of T.

Further, by inputting the output signal from the comparator 15 to the PLL circuit 16, if the PLL is locked by this rising information, the stable symbol indicating the beginning of the effective symbol section T is obtained for each symbol section Ts of the input signal. It is possible to obtain accurate symbol start point (symbol head) information.

Therefore, by performing OFDM demodulation of the input signal using this information as a synchronization signal, it becomes possible to output accurate received data without error.

The delay devices 11 and 12 used in this embodiment are as follows.
A circuit using an analog delay line can be used as the circuit of, and a DSP (Digital Signal Processor)
Can be used, and when the circuit using the DSP is used, the delay time can be controlled with higher accuracy.

In another embodiment, the delay device 11 in the above embodiment may be omitted. In this case, as information,
Since information that is earlier than the start point of the effective symbol section T of the input signal by the guard interval Tg is obtained, the time after the guard interval Tg after this point may be set as the symbol start point.

[0029]

As described in detail above, the symbol head detecting apparatus according to the present invention is a symbol head detecting apparatus for an OFDM (Orthogonal Frequency Division Multiplexing) signal in which a guard interval is set. Since the signal delay means for delaying by one symbol period, the subtraction means for performing the subtraction processing of the original OFDM signal and the output signal from the signal delay means, the detection means, and the rising edge detection means are included, the subtraction is performed. The signal in the guard interval portion is canceled by the means to become 0, and then the rising edge can be detected for the signal subjected to the envelope detection or the like in the detection means. Therefore, OF
The information of the exact start time position of each effective symbol section of the DM-modulated input signal is obtained, and the apparatus is OF
By incorporating it in a DM receiver, it is possible to output a correct received signal.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of a symbol head detection device according to an embodiment of the present invention.

FIG. 2 is a timing chart showing the operation of each circuit in the symbol head detection device according to the embodiment.

FIG. 3 is a configuration diagram showing a schematic configuration of an OFDM modulator / demodulator.

FIG. 4 is a timing chart showing a conventional method for synchronizing an input modulated signal.

[Explanation of sign]

 11, 12 Delay device 13 Subtractor 14 Envelope detector 15 Comparator 16 PLL

Claims (1)

[Claims] 1. An OFD in which a guard interval is set
A symbol head detection device for M (orthogonal frequency division multiplexing) signals, which is a signal delay means for delaying an OFDM signal by one symbol interval, and a subtraction process for performing subtraction processing between an original OFDM signal and an output signal from the signal delay means. A symbol head detecting device for an OFDM signal, comprising: means, detecting means, and rising edge detecting means.