JPH10200500A - Receiver in multi carrier modulation system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a satisfactory reception characteristic even when a high frequency band limit filter has a group delay characteristic. SOLUTION: A receiver in a multi carrier modulation system which has a band limit filter that limits a high frequency component in a reception signal is provided with a voltage controlled oscillator 6B which performs orthogonal transformation of a multi carrier from a high frequency component, a demodulating part 10 which consists of plural demodulators that demodulate each multi carrier that is performed orthogonal transformation into data, a transversal filter 23 which is located between a band limit filter and the part 10 and inverse characteristic calculating parts 24 to 26 which output an output signal of the voltage controlled oscillator to the band limit filter before receiving, change the frequency of the voltage control oscillator within the range of the frequency limit of the band limit filter, calculate an inverse filter characteristic of the band limit filter and calculate the impulse response that is acquired by performing inverse Fourier transformation of the inverse characteristic as a filter coefficient that is set to the transversal filter.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a multi-carrier modulation type receiving apparatus, and more particularly to a multi-carrier type receiving apparatus capable of obtaining a good receiving characteristic irrespective of a group delay characteristic of an intermediate filter constituting the receiving apparatus. About.

[0002]

2. Description of the Related Art In recent years, a multi-carrier modulation scheme capable of stably transmitting a digital signal even in an environment where mobile reception is inferior, such as a multipath, has attracted attention. FIG. 5 is a diagram illustrating demodulation in a conventional multi-carrier modulation type receiving apparatus. In the multi-carrier modulation method, a multi-subcarrier modulated by a phase or the like is converted to a high frequency and transmitted. As shown in the figure, a received signal obtained via an antenna 1 is amplified by a high frequency (RF) processing unit 2 and converted to an intermediate frequency (IF). In the intermediate frequency (IF) filter 3, which is a band limiting filter, unnecessary frequencies are removed from the converted intermediate frequency signal. The multiplier 4 orthogonally transforms the intermediate frequency signal and demodulates the same into multi-subcarriers having the same phase. The multiplier 5 orthogonally transforms the intermediate frequency signal and demodulates the signal into multi-subcarriers of orthogonal components. Local oscillator (LO) 6
A outputs a signal of a frequency for orthogonally transforming the intermediate frequency signal to the multiplier 4, and further outputs the signal to the multiplier 5 via the π / 2 phase shifter 7. Low-pass filters (LPFs) 8 and 9 connected to the outputs of the multipliers 4 and 5, respectively, remove unnecessary frequency components after the multiplication. The demodulation unit 10 receives the outputs of the low-pass filters 8 and 9 and demodulates the phase and the like from the multicarrier and decodes the data into symbols (data).

FIG. 6 is a diagram for explaining the demodulation unit 10 of FIG. As shown in the figure, the demodulation unit 10 includes demodulators 10-1, 10-2,..., 10-n for the number of sub-multicarriers. For example, the demodulator 10-1 includes a subcarrier generator (LO) 10-11 that generates a reference subcarrier.
A multiplier 10-12 for multiplying the input multicarrier and a reference subcarrier from the subcarrier generator 10-11 to detect a phase modulated on one subcarrier among the input multicarriers; , Multiplier 10
-12, a low-pass filter (LPF) 10-13 for removing unnecessary frequency components from the output, and a low-pass filter 10-.
Clock recovery device 1 that recovers a clock from the output of
A data decoder 10-15 for synchronizing symbols with the output of the low-pass filter 10-13 and decoding the data into data based on the reproduced clock signal is provided.

A signal input to the demodulation unit 10 shown in FIG. 1 is an A / D converter (Analog to Digital Converter) not shown.
, And each subcarrier is finely sampled at a rate of m times the symbol rate. The demodulating unit 10 includes a clock reproducing device 10 for reproducing a clock.
-14, and a clock recovery device 10-14
A maximum value selector 10-14A for comparing before and after the sample value of the synchronization symbol to be input to and selecting the maximum value of the amplitude;
The sample value selected by the maximum value selection unit 10-14A and the reproduction clock from the clock reproduction device 10-14 are input, and the reproduction clock position is set as the reproduction timing position by the reproduction clock of the largest amplitude among the subcarriers.
15 is provided with a clock selection unit 10-14B for performing decoding.

FIG. 7 is a diagram showing a conventional multicarrier burst format. As shown in the figure, a symbol obtained by modulating each of the subcarriers 1, 2,..., N includes a leading synchronization symbol and a data symbol following the leading synchronization symbol. Generally, a synchronization symbol is formed by k symbols, and in this example, k = 3. FIG. 8 is a diagram illustrating the clock selector 10-14B of FIG. 6 in detail. As shown in the figure, when sampling the symbol rate by m times, the clock selection unit 10-14B looks at the amplitude of each sample for the k synchronization symbol, and the amplitude becomes maximum before and after the k synchronization symbol. Is synchronized with the synchronization symbol, and it is determined that the clock is reproduced by performing synchronization pull-in.

FIG. 9 shows a frequency spectrum of a multicarrier signal modulated at the time of transmission and a multicarrier signal processed by the intermediate filter 3 of FIG. 5 after reception. FIG. 10 shows the frequency and time of the multicarrier. FIG. As shown in FIG. 9A, the frequency band of one carrier is very narrow, and the spectrum has a rectangular shape as a whole. As described above, in the multi-carrier modulation scheme, the frequency occupied band of one transmission symbol is narrowed and the time width is widened by setting a large number of carriers on a wide frequency axis, and the influence of frequency selectivity and multipath delay is reduced. Can be avoided.

[0007]

When such a signal is received, the intermediate frequency filter 3 requires a sharp cutoff characteristic close to a rectangle in the frequency characteristic of the receiver. However, if a sharp cutoff characteristic is to be realized, actually, peaks occur on both sides of the spectrum due to the group delay characteristic as shown in FIG. 9B. That is, the frequency characteristic of the amplitude of the received signal does not become flat, and the delay time varies depending on the frequency. As a result, there is a problem that the reception characteristic is adversely affected. That is, as shown in FIG. 10A, when there is no group delay, the position of the synchronization symbol 1 of each subcarrier temporally matches.
As shown in FIG. 10B, when a group delay occurs, the position of the synchronization symbol 1 of each subcarrier does not match. In order to solve the above problem, each demodulator 10 of the demodulation unit 10
-1, 2,..., N are independent time axes for each subcarrier, that is, the clock recovery devices 10-14, 10-2.
4,..., 10-n4, it is necessary to perform demodulation, which causes a new problem that the processing amount becomes complicated.

Further, as described above, a plurality of synchronization symbols are provided for each subcarrier, because synchronization symbols may be destroyed due to the influence of mobile reception such as time selectivity. That is, if synchronization cannot be detected with one subcarrier, such as when a synchronization symbol is interleaved between subcarriers, demodulation cannot be performed at all. For this reason, there is a problem that the transmission efficiency is reduced by an extra set synchronization symbol.

The present invention has been made in order to solve the above problems.
It is an object of the present invention to provide a multi-carrier modulation type receiving apparatus capable of improving a group delay characteristic even if an intermediate frequency filter for limiting a band has a steep cutoff characteristic and a flat group delay characteristic cannot be obtained.

[0010]

In a receiving apparatus of a multicarrier modulation system having a band-limiting filter for limiting a high frequency component of a received signal modulated on a multicarrier, an orthogonal transform from the high frequency component to a multicarrier is performed. A voltage-controlled oscillator for demodulating, a demodulator comprising a plurality of demodulators for demodulating each of the orthogonally transformed multicarriers to data, a transverse filter located between the band-limiting filter and the demodulator, Before, the output signal of the voltage controlled oscillator is output to the band limiting filter, and the frequency of the voltage controlled oscillator is changed within the range of the frequency limit of the band limiting filter to change the inverse filter characteristic of the band limited filter. The impulse response obtained by inverse Fourier transform of this inverse characteristic is set in the transversal filter. Characterized in that it comprises an inverse characteristic calculation unit for obtaining the filter coefficients. By this means, even if the band-limiting filter has a steep cutoff characteristic and a flat group delay characteristic cannot be obtained, the group delay characteristic is canceled by the transverse filter, and the signal can be received without being affected.

The timing position of the clock is formed by the maximum value of the sample values of all the data demodulated by the plurality of demodulators of the demodulator. Since there is no delay time in the demodulation output of each subcarrier, this means allows the reproduction timing to be shared between the subcarriers, and enables decoding without being affected by frequency selectivity. The number of synchronization symbols located at the head of the symbol for performing modulation on each of the multicarriers is one in the time direction, all synchronization symbols for the multicarrier are set in a plurality of groups, and different synchronization words are set for each group. Perform frequency interleaving. By this means, the number of synchronization symbols becomes one and the transmission efficiency can be improved, and it is no longer impossible to demodulate all the subcarriers because synchronization of the subcarriers could not be drawn in as in the prior art.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram for explaining demodulation of a multi-carrier modulation type receiving apparatus according to the present invention.
FIG. 2 is a diagram showing the transversal filter 23 of FIG. As shown in FIG. 1, as a configuration different from FIG. 5, the switch 20 connects the input side of the high-frequency processing unit 2 to the antenna 1 (A
Side) or earth (B side). Adder 2
1 adds the output of the local oscillator 6 and the output of the high frequency processing unit 2. The switch 22 connects the input side of the converter 21 to the ground (A side) or the output (B) of the voltage controlled oscillator (VCO) 6B.
Side). Further, the transversal filter 23 is configured as shown in FIG. 2, and is provided between the low-pass filters 8 and 9 and the demodulation unit 10. The inverse filter characteristic calculation unit 24 receives the branch signals of the low-pass filters 8 and 9, passes through the inverse Fourier transform unit 25 and the inverse characteristic calculation unit of the intermediate filter 3 including the tap coefficient calculation unit 26, and outputs the transversal filter. 23 tap coefficients are formed. The calculation mode control unit 27 controls the switching of the switches 20 and 22, and controls the change of the oscillation frequency of the local oscillator 6.

FIG. 3 is a diagram for explaining the demodulation unit 10 of FIG. A signal input to the demodulation unit 10 shown in FIG. 1 is digitized by an A / D converter (not shown), and each subcarrier is finely sampled at a rate of m times the symbol rate. The demodulation unit 10 includes a clock recovery device 10-1.
, 10-n4,..., 10-n4, a maximum value selection unit 30 that compares the sample values before and after the synchronization symbol input before and after to select the maximum value of the amplitude, and is selected by the maximum value selection unit 30. Sample value and clock recovery device 10-14, 10
-24,..., 10-n4, and the data decoder 10- as the reproduction timing position by the reproduction clock of the sub-carrier having the largest amplitude in each subcarrier.
, 10-n5 are provided with a clock selection unit 40 for performing decoding. In this way, the reproduction timing is shared between the subcarriers.

Hereinafter, the operation will be described in detail. Prior to the reception, the calculation mode control unit 27 moves the switch 20 to the ground position on the B side and moves the switch 22 to the position of the voltage controlled oscillator 6B on the B side. Further, the calculation mode control unit 2
7 designates the frequency band of the intermediate filter 3 as f0-fd to f0 +
In the range of fd, the frequency is changed at every minute frequency interval Δf (see FIG. 9A). Now, assuming that the oscillation frequency of the voltage controlled oscillator 6B is fc, the oscillation signal of the voltage controlled oscillator 6B is represented by the following equation (1).

[0015]

(Equation 1)

Here, A is an amplitude value, and θ is an initial phase. If this is input to the intermediate frequency filter 3, the output will be represented by the following equation (2) because of the delay depending on the oscillation frequency of the voltage controlled oscillator 6B due to its group delay characteristic.

[0017]

(Equation 2)

Here, the amplitude and delay are determined by the voltage controlled oscillator 6B.
A (fc), τ (f
c). Since this signal is subjected to quadrature demodulation by the oscillation signal from the intermediate frequency filter 3, the signals passing through the low-pass filters 8 and 9 are represented by the following equations (3) and (4).

[0019]

(Equation 3)

Therefore, if these are represented by complex numbers, they are represented by the following equation (5).

[0021]

(Equation 4)

Here, G (fc) indicates the amplitude characteristic at the frequency fc, and φ (fc) indicates the phase characteristic. If these operations are performed over the frequencies f0 -fd to f0 + fd, the frequency characteristics of the intermediate frequency filter 3 are given by the following equation (6).

[0023]

(Equation 5)

Therefore, the inverse characteristic H ^-1 (f) of this characteristic is obtained by the inverse filter characteristic calculation unit 24, and this is subjected to inverse Fourier transform by the inverse Fourier transform unit 25, and an impulse response is obtained. This impulse response is formed as a tap coefficient of the transversal filter 23 by the tap coefficient calculator 26. Therefore, the transversal filter 23 cancels the frequency characteristic H (f) of the intermediate frequency filter 3.

That is, the frequency characteristic of the received signal is represented by R (f)
Then, the frequency characteristic of the signal passing through the intermediate frequency filter 3 is R (f) H (f). This is represented by the frequency characteristic H ^-1
Since the signal passes through the transversal filter 23 of (f), the obtained signal is R (f) H (f) H ^-1 (f) = R
(F), and the effect of H (f), that is, the effect of group delay can be canceled. When the above expression (6) is obtained and the transversal filter 23 is ready, the calculation mode control unit 27 enters the reception mode by setting the switches 20 and 22 to the A side position.

Therefore, the delay characteristic of the signal input to the demodulation unit 10 is flattened, so that there is no delay time difference between the subcarriers, so that the reception characteristic is improved. Alternatively, there is no need to perform independent demodulation on only the time axis for each subcarrier as in the related art, and two-dimensional processing on the frequency axis and the time axis becomes possible, and the processing can be simplified. Since such a result is obtained, the number of synchronization symbols can be reduced and the transmission efficiency is improved as described below.

FIG. 4 is a diagram showing a multicarrier burst format according to the present invention. As shown in the figure, a synchronization group (Di, j) including j synchronization symbols and i different synchronization words is prepared for n subcarriers. These are interleaved on the frequency axis in the first symbol of the burst. This rule is known on the receiving side. Here, i may be selected to be the same as the number of multiplex channels in the data symbol, and j ≦ n / i. In the clock selection unit 40 of the demodulation unit 10, the maximum sample value is compared with j known synchronization symbols for each group. In other words, since synchronization confirmation can be performed i times with one symbol, the synchronization pull-in can be performed with at least the number of synchronization symbols. Also, the j synchronization symbols in one group are interleaved on the frequency, so that the influence of frequency selectivity can be avoided.

Next, when symbol synchronization is achieved as described above, decoding of data symbols starts. Here, the clock selection unit 40 determines whether each of the data decoders 10-15, 10-25,.
Clock recovery of 10-n5 is performed collectively. This is done for each subcarrier, due to the frequency selectivity,
The timing of a certain subcarrier is reproduced correctly,
This is because the timing of the subcarrier whose amplitude has dropped can not be reproduced, and the deterioration of the reception characteristics can be improved. Then, the sample value at which the amplitude before and after the sample value becomes the maximum and the clock position (reproduced clock) thereof are received, and the reproduced clock having the largest amplitude among the subcarriers is demodulated by the demodulators 10-1 and 10-1 of each subcarrier.
,..., 10-n, and decoding is performed using this as a reproduction timing position. This allows stable reception without being affected by frequency selectivity. That is, according to the present invention in which a delay time difference does not occur in the output of each subcarrier, this stable reception can be realized.

[0029]

As described above, according to the present invention, even if the band-limiting filter has a steep cutoff characteristic and a flat group delay characteristic cannot be obtained, the group filter characteristic is canceled by the transverse filter. You can receive without being affected. Reproduction timing can be shared between subcarriers, and decoding can be performed without being affected by frequency selectivity. Since the transmission efficiency can be improved and the group delay has been eliminated, it is no longer impossible to demodulate all of the subcarriers because the subcarriers cannot be synchronized.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating demodulation of a multi-carrier modulation type receiving apparatus according to the present invention.

FIG. 2 is a diagram showing a transversal filter 23 of FIG.

FIG. 3 is a diagram illustrating a demodulation unit 10 of FIG. 1;

FIG. 4 is a diagram illustrating a multicarrier burst format according to the present invention.

FIG. 5 is a diagram illustrating demodulation in a conventional multi-carrier modulation type receiving apparatus.

FIG. 6 is a diagram illustrating a demodulation unit 10 of FIG. 1;

FIG. 7 is a diagram showing a conventional multicarrier burst format.

FIG. 8 is a diagram illustrating in detail a clock selection unit 10-14B of FIG. 6;

9 is a diagram illustrating a frequency spectrum of a multicarrier signal modulated during transmission and a multicarrier signal processed by the intermediate filter 3 of FIG. 5 after reception.

FIG. 10 is a diagram illustrating the relationship between the frequency and time of a multicarrier.

[Explanation of symbols]

 3 ... Intermediate filter 4, 5 ... Multiplier 6B ... Voltage controlled oscillator 7 ... π / 2 phase shifter 10 ... Demodulator 23 ... Transversal filter 24 ... Inverse filter characteristic calculator 25 ... Inverse Fourier transformer 26 ... Tap coefficient calculation Unit 30: Maximum value selection unit 40: Clock selection unit

Claims (3)

[Claims] 1. A multicarrier modulation type receiving apparatus having a band limiting filter for limiting a high frequency component of a received signal modulated on a multicarrier, a voltage control for performing orthogonal transformation from the high frequency component to a multicarrier. An oscillator, a demodulator comprising a plurality of demodulators for demodulating each of the orthogonally transformed multicarriers into data, a transversal filter located between the band-limiting filter and the demodulator, The output signal of the voltage controlled oscillator is output to the band limiting filter, and the frequency of the voltage controlled oscillator is changed within the range of the frequency limit of the band limiting filter to obtain an inverse filter characteristic of the band limited filter. A filter for setting an impulse response obtained by inverse Fourier transform of characteristics to the transversal filter. Receiver of a multicarrier modulation scheme, characterized by an inverse characteristic calculation unit for obtaining the coefficients. 2. The multi-carrier modulation method according to claim 1, wherein a clock timing position is formed by a maximum value of sample values of all the data demodulated by the plurality of demodulators of the demodulation unit. System receiver. 3. A synchronization symbol located at the head of a symbol for performing modulation on each of the multicarriers is set to be one in the time direction.
The multi-carrier modulation scheme according to claim 1, wherein all the synchronization symbols for the multi-carrier are set in a plurality of groups, and a different synchronization word is set for each group to perform a frequency interleaving process. Receiver.