JP2010114883A - Ofdm receiver and relay unit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the following problems in conventional receivers: when high-accuracy extrapolation is achieved, equalization characteristics of the band edge are favorable, meanwhile, when a complicated transmission path characteristics are caused by a plurality of multipaths or when signal level of the band edge is reduced due to frequency selective fading caused by the multipaths, it is difficult to achieve high-accuracy extrapolation and equalization characteristics of the band edge are deteriorated. <P>SOLUTION: On a receiver side, a plurality of FIR filters having a different number of taps are provided for a pilot carrier extracted from a received signal, and frequency interpolation is performed using each filter. Transmission path estimation accuracy is improved by increasing the weight of the output signal from the short-tap FIR filter when estimating the transmission path characteristics of each band edge, and by increasing the weight of the output signal from the long-tap FIR filter when estimating the transmission path characteristics of each band center section. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a receiving apparatus or a relay apparatus of a data transmission apparatus modulated by an orthogonal frequency division multiplexing (OFDM) modulation scheme.

In recent years, the OFDM modulation method has been adopted for terrestrial digital broadcasting methods and the like. In the OFDM modulation method, in order to estimate the characteristics of the transmission path, pilot carriers, which are known signals, are inserted in advance at intervals of several carriers on the transmission side, and the characteristics of the transmission path are estimated on the reception side based on the pilot carrier. Demodulation processing is performed based on the result. The purpose of channel characteristic estimation is to obtain the channel characteristic of a data carrier in which no pilot carrier exists, and this is possible by interpolating the extracted pilot carrier. Interpolation can be performed by linear interpolation, polynomial interpolation using several points, etc., but it is necessary to accurately estimate the channel characteristics such as long delay multipath mixed. An interpolation method using an FIR (Finite Impulse Response) filter of several tens to several hundred taps is often used. A channel characteristic estimation method using this FIR filter will be described with reference to FIG.

1 is an antenna, 2 is a down converter, 3 is an A / D converter, 4 is an FFT (Fast Fourier Transform) unit, 5 is a pilot carrier extraction unit, 6 is a frequency extrapolation unit, and 12 is frequency interpolation. , 13 is a remodulator, 14 is a D / A converter, 15 is a down converter, and 16 is an antenna.

First, a transmitter (not shown) transmits an OFDM signal in which pilot carriers with known amplitudes and phases are inserted at equal intervals or non-equal intervals, and a receiver receives an OFDM signal.

A high-frequency reception signal received by the antenna 1 is input to the down converter 2 and converted into a reception signal having an intermediate frequency. The received signal converted to the intermediate frequency is sampled by the A / D conversion unit 11 and then input to the FFT unit 4 where a time window having an effective symbol period length is provided, and FFT processing is performed on the signal within the FFT time window. Is done. In general, the FFT time window is provided at a timing at which deterioration due to multipath does not occur. The signal after the FFT processing is input to the pilot carrier extraction unit 5, and the pilot carrier extraction unit 5 extracts the pilot carrier from the signal after the FFT processing. The extracted pilot carrier is input to the frequency extrapolation unit 6 and extrapolated and estimated. When the extrapolated signal is input to the frequency interpolation unit 12, the signal is interpolated and output to the remodulation unit 13 as an estimated transmission path characteristic H (k) signal. The remodulation unit 13 regenerates the waveform-shaped OFDM signal, and the digital-to-analog conversion is performed by the D / A converter 14 to output an analog signal to the up-converter 15 as an intermediate frequency relay signal. The intermediate frequency relay signal is converted into a high frequency signal by the up-converter 15 and transmitted from the antenna 16.

At this time, in order to interpolate the extracted pilot carrier with the FIR filter, the pilot carrier is required for the number of taps of the FIR filter. However, when estimating the channel characteristic at the band edge, the channel characteristic outside the band is unknown. For example, when the signal level of the channel characteristic outside the band is set to 0, the interpolation at the band edge is estimated. The error will increase. Therefore, there is a method of extrapolating and estimating the out-of-band transmission path characteristics using the band edge pilot carrier. (See Patent Document 1)

In this manner, the frequency extrapolation unit 6 extrapolates and estimates the out-of-band transmission line characteristics using the band edge pilot carrier, and the frequency interpolation unit 12 extrapolates and estimates the out-of-band transmission line characteristics. And the extracted pilot carrier is used as the transmission line characteristics in the band, and interpolation is performed in the frequency direction using an NIR (N> 1) FIR filter for these characteristics. Through the above processing, transmission path characteristics can be estimated, and equalization processing is performed based on the estimated transmission path characteristics.

JP 2002-009726 A

In the conventional receiver, when the extrapolation estimation can be realized with high accuracy, the band edge equalization characteristics are good, but the transmission path characteristics are complex such that there are multiple multipaths. In an environment where the signal level at the band edge becomes small due to frequency selective fading due to multipath, etc., it is very difficult to perform highly accurate extrapolation estimation, etc. There is a disadvantage that the conversion characteristics deteriorate.

An object of the present invention is to provide an OFDM receiver capable of eliminating these drawbacks and performing highly accurate transmission path estimation even in an environment where multipath exists.

In order to achieve the above-mentioned object, the present invention modulates a pilot carrier having a known amplitude and phase in an orthogonal frequency division multiplexing (OFDM) modulation system by inserting the carrier in a frequency domain at regular intervals or non-uniform intervals. In the OFDM receiving apparatus for receiving the received signal, the apparatus comprises pilot carrier extracting means for extracting the pilot carrier from the received signal and a filter having a predetermined number of taps, and the filter performs a filter operation process on the extracted pilot carrier. A first interpolation means for performing interpolation in the frequency domain, and a filter having a smaller number of taps than a filter constituting the first interpolation means, and the filter for the extracted pilot carrier Is fi Second interpolation means for performing interpolation in the frequency domain by performing data calculation processing, weight coefficient control means for generating first and second weight coefficients that change according to the carrier position, and the first First multiplying means for multiplying the signal interpolated by the interpolation means with the first weighting factor, and the second interpolation means for the signal interpolated by the second interpolation means. Second multiplying means for multiplying the weighting factor; and addition combining means for adding and combining the multiplication result signal obtained by the first multiplication means and the multiplication result signal obtained by the second multiplication means; It is characterized by providing.

Further, in order to achieve the above object, the present invention is modulated by an orthogonal frequency division multiplexing (OFDM) modulation system in which pilot carriers having known amplitudes and phases are inserted at equal intervals or non-equal intervals. In the OFDM receiving apparatus for receiving the received signal, a pilot carrier extracting means for extracting the pilot carrier from the received signal and a plurality of filters having different tap numbers, and the plurality of filters for each of the extracted pilot carriers. A plurality of interpolation means for performing interpolation by performing filter calculation processing, a weight coefficient control means for variably controlling the weight coefficient according to the carrier position, and a signal interpolated by the plurality of interpolation means The weight coefficient system A plurality of multiplying means for multiplying the weighting coefficient input respectively from the unit, characterized in that it comprises, an addition synthesis section for adding and combining the multiplied signal by the plurality of multiplication means.

In order to achieve the above object, the present invention is characterized in that, in the above-mentioned OFDM receiver, the weighting factor control means controls on the basis of the ratio of the main power and the total power of the multipath.

In order to achieve the above object, the present invention is characterized in that, in the above-mentioned OFDM receiver, the weighting factor control means performs control based on a standard deviation of frequency characteristics at each band end.

In order to achieve the above object, the present invention provides the above-described OFDM receiver, wherein the weighting factor control means uses a first-order function or a second-order or higher-order function to gently change the transition between each band edge and the band center. It is characterized by performing.

As described above, according to the OFDM receiver according to the present invention, a plurality of FIR filters having different tap numbers and tap coefficients are provided for pilot carriers extracted from a received signal, and frequencies are obtained using the respective filters. When interpolating and estimating the channel characteristics at each band edge, the weight of the output signal from the short tap FIR filter is increased, and when estimating the channel characteristics at the center of the band, By increasing the weight of the output signal from the long tap FIR filter, the transmission path estimation accuracy can be improved.

It is a block diagram which shows the internal structure of the receiver which is one Embodiment of this invention. It is a signal diagram which shows the transmission-path characteristic estimation process in the receiver which is one Embodiment of this invention. It is a figure which shows the relationship between a tap number, a transmission-line estimation error, and a weighting coefficient in the receiver which is one Embodiment of this invention. It is a block diagram which shows the internal structure of the receiver which is other embodiment of this invention. It is a block diagram which shows the internal structure of the conventional receiver.

Hereinafter, embodiments according to the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of a receiving apparatus according to an embodiment of the present invention. 5 is a pilot carrier extraction unit, 6 is a frequency extrapolation unit, 7-1 is a long tap FIR filter frequency interpolation unit, 7-2 is a short tap FIR filter frequency interpolation unit, and 8-1 and 8-2 are multiplication units. , 9 is a weighting coefficient control unit, and 10 is an addition synthesis unit.

The processing in the previous stage of the pilot carrier extraction unit 5 is the same as that described in FIG. The signal after the FFT processing is input to the pilot carrier extraction unit 5 in the OFDM symbol period, and the pilot carrier extraction unit 5 performs the FFT processing as shown in FIG. 2A based on the pilot arrangement inserted on the transmitter side. The pilot carrier is extracted from the signal. The extracted pilot carrier is input to the frequency extrapolation unit 6, and as shown in FIG. 2 (b), the out-of-band (upper and lower) transmission line characteristics are extrapolated based on the pilot carrier at each band end. Presumed. This is because, as described above, if the channel characteristics outside the band (upper side and lower side) are unknown, the channel estimation characteristics at each band end are deteriorated by the frequency interpolation processing in the subsequent stage. Even when extrapolation estimation is not performed, that is, when the out-of-band characteristic is 0, it is possible to reduce the above-described characteristic deterioration due to the unknown out-of-band characteristic. The extrapolated signal is input to the long tap FIR filter frequency interpolation unit 7-1 and the long tap FIR filter frequency interpolation unit 7-2. The long tap FIR filter frequency interpolation unit 7-1 is an N1 tap FIR filter, and the long tap FIR filter frequency interpolation unit 7-2 is an N2 tap FIR filter. At this time, N1 and N2 are integers, have a relationship of N1> N2, and corresponding to these, the filter coefficients are also different. In order to perform OFDM demodulation correctly, it is necessary to estimate the channel characteristics with high accuracy. However, as shown in FIG. 2 (c), these FIR filters have the channel characteristics of carriers in which pilot carriers are not arranged. Interpolate.

In the present embodiment, the FIR filter is used in the method of interpolating in the frequency direction in order to estimate transmission path characteristics with high accuracy even when there is a multipath having a long delay time. For example, when the guard interval length is 1/8 of the effective symbol length, in order to equalize the multipaths within the guard interval period, the pilot carrier interval may be 8 intervals. By using it, the delay time to the limit of the guard interval period can be equalized.

Here, the relationship between the pass bandwidth of the FIR filter and the pilot carrier interval will be described. When a guard interval having a ratio of 1 / R (for example, R = 8) is added to the effective symbol length and the pilot carrier interval is the R carrier, interpolation in the frequency direction is performed based on the Nyquist sampling theorem. By setting the passband width of the filter to 1 / R and the other period as the stop band, the transmission path characteristics can be completely estimated. In order for this FIR filter to obtain a steep characteristic as shown in FIG. 3A, it is desirable to increase the number of taps of the FIR filter, but as the number of taps is increased, the out-of-band characteristics are unknown. There is a relationship that the estimated deterioration due to the existence increases. This is because, for example, when estimating the channel characteristic at the center of the band, the FIR filter output is determined only from the known channel characteristic within the band, so a highly accurate estimation characteristic can be obtained. When estimating the transmission line characteristics of the FIR filter, the output of the FIR filter is determined from the known transmission line characteristics in the band and the unknown transmission line characteristics outside the band. This is because accuracy deteriorates.

Regarding the above description, the characteristics of transmission path characteristic errors when the number of taps of the FIR filter is large and small are summarized below. FIG. 3B shows transmission path estimation errors when the number of taps of the FIR filter is large and small. When the number of taps is large, a steep cut-off characteristic can be realized, so that the noise contained in the filter band can be reduced and the estimation error due to the noise can be reduced, while the estimation error at the band edge is reduced. As for, since the number of taps is large, a region with a large estimation error spreads to the inside of the band. When the number of taps is small, it is difficult to realize a steep cutoff characteristic, so the noise included in the filter band increases more than the amount of noise when the number of taps is large, while the estimation error at the band edge Since the number of taps is small, the region where the estimation error is large is only in the vicinity of the band edge. Therefore, the transmission path estimation result by the long tap FIR filter frequency interpolation section 7-2 is used for the transmission path estimation at the band edge, and the long tap FIR filter frequency interpolation section 7- is used for the transmission path estimation at the band center section. A better estimation accuracy can be obtained by using the transmission path estimation result of 1.

The long tap FIR filter frequency interpolation unit 7-1 inputs the output signal H _L (k) to the multiplication unit 8-1, and the long tap FIR filter frequency interpolation unit 7-2 multiplies the output signal H _S (k). Each is input to part 8-2. At this time, k is a carrier number. The weighting factor α (k) is input from the weighting factor control unit 9 to the multiplication unit 8-1, and is multiplied with the output signal of the long tap FIR filter frequency interpolation unit 7-1. Also, the weighting factor β (k) is input from the weighting factor control unit 9 to the multiplication unit 8-2 and multiplied with the output signal of the long tap FIR filter frequency interpolation unit 7-2. At this time, since the tap number of the long tap FIR filter frequency interpolation unit 7-1 and the long tap FIR filter frequency interpolation unit 7-2 are different from each other, it is necessary to match the phases of the respective filter outputs. A method for calculating the weighting factors α (k) and β (k) will be described later. The multiplication unit 8-1 and the multiplication unit 8-2 input the multiplication result signal to the addition synthesis unit 10. The adder / synthesizer 10 adds and synthesizes the multiplication result signals from the multipliers 8-1 and 8-2, and outputs the resultant signal to the subsequent stage (not shown) as an estimated transmission path characteristic H (k) signal. Here, the calculation method of the estimated transmission line characteristic H (k) is shown in Expression (1).

Thus, the weight of the long tap FIR filter frequency interpolation unit 7-2 is increased at each band end, and the weight of the long tap FIR filter frequency interpolation unit 7-1 is increased at the center of the band. The transmission path estimation accuracy can be improved.

Next, a method for calculating the weighting factors α (k) and β (k) by the weighting factor control unit 9 will be described. Here, it is assumed that the weighting coefficients α (k) and β (k) are normalized so that α (k) + β (k) = 1.

As a first method for calculating the weighting factors α (k) and β (k), as shown in FIG. 4C, the weighting factor α (k) is set for a predetermined number of carriers k0 at the band edge. The weighting factor α (k) is set to 1 for 0 and other carriers, the weighting factor β (k) is set to 1 for the predetermined number of carriers k0 at each band edge, and the weighting factor β (k) for the other carriers. ) Is 0.

According to this first method, k0 in FIG. 3C is determined so that the transmission path estimation error shown in FIG. 3B is minimized. Note that the transmission path estimation error is the smallest when the transmission path estimation error (solid line) in the case of using a filter with a large number of taps in FIG. 4B and the transmission path in the case of using a filter with a small number of taps. This is the value of the intersection of the estimation errors (dotted lines).

As a second method, in the first method, there is a method of adaptively controlling the predetermined number of carriers k0 at each band edge according to the transmission path characteristics. For example, in a simple AWGN (Additive White Gaussian Noise) environment where there is no multipath, there is no significant variation in the frequency characteristics of each band end, and therefore high-precision extrapolation estimation by the frequency extrapolation unit 6 Is possible. Further, in an environment where no significant fluctuation occurs in the frequency characteristics at the band edge, the estimation of the channel characteristics at each band edge in the long tap FIR filter frequency interpolation unit 7-1 after extrapolation estimation can be performed with high accuracy. That is, in such an environment, it is more accurate to increase the weight of the long tap FIR filter frequency interpolation unit 7-1 and reduce the predetermined number of carriers k0 at each band edge even at the band edge. Transmission path estimation can be performed. Therefore, this method adaptively controls the predetermined number of carriers k0 at each band edge in accordance with the amount of fluctuation of the band edge frequency characteristics, that is, the extrapolation estimation accuracy, so that the transmission path can be compared with the first method. The accuracy of characteristic estimation can be improved. There are various methods for determining whether or not the frequency characteristics at the end of each band are varied. For example, a delay profile detection unit is provided to calculate a delay profile, and the main power and multipath. There is a method of using the ratio of the total electric power as a criterion. When the delay profile is DPF (t) and DPF (0) is the main wave component, the above criterion J is expressed by Equation (2).

Here, T is a time range for evaluating the delay profile. According to this method, for example, when the criterion J represented by the equation (2) is large, the value of the criterion J is increased so that k0 is increased with a small amount of fluctuation at each band edge. The predetermined number of carriers k0 can be adaptively controlled.

As another method based on the determination criteria, there is a method using the standard deviation of the frequency characteristics at each band edge as a determination criterion. According to this method, when there is a large variation in the frequency characteristics at the band edge, the standard deviation is large, and it is determined that the extrapolation estimation accuracy is degraded. When the standard deviation is small, each band is The predetermined number of carriers k0 can be adaptively controlled such that k0 is increased with a small amount of edge fluctuation.

As a third method, as shown in FIG. 3 (d), there is a method of gradually transitioning between each band edge and the band center. This is a gradual transition between each band edge and the band center in the first method. The function in the transition region may be a linear function, but may be a higher-order function.

As a fourth method, there is a method in which the transition position of the third method is adaptively controlled according to the determination criterion of the second method.

Another embodiment of the present invention will be described with reference to FIG. In the above description, the case where two filters of the long tap FIR filter frequency interpolation unit 7-1 and the long tap FIR filter frequency interpolation unit 7-2 are used has been described. The configuration shown in FIG. 1 is expanded to m types of filters each having a different number of taps. This is a configuration for the purpose of further subdividing and optimizing each band edge using a plurality of filters in the configuration of FIG. In addition, the block of the same code | symbol shall have the same function.

5 is a pilot carrier extraction unit, 6 is a frequency extrapolation unit, 11-1 is an N1 tap FIR filter frequency interpolation unit, 11-2 is an N2 tap FIR filter frequency interpolation unit, and 11-3 is within an N3 tap FIR filter frequency. , 11-m is an Nm tap FIR filter frequency interpolation unit, 8-1, 8-2, 8-3,..., 8-m are multiplication units, and 10 is an addition synthesis unit. .

The processing in the previous stage of the pilot carrier extraction unit 5 is the same as that described in FIG. The signal after the FFT processing is input to the pilot carrier extraction unit 5 at the OFDM symbol period, and the pilot carrier extraction unit 5 performs the FFT processing as shown in FIG. 2A based on the pilot arrangement inserted on the transmitter side. The pilot carrier is extracted from the signal. The extracted pilot carriers are input to the frequency extrapolation unit 6, and as shown in FIG. 2B, extrapolation is performed for transmission path characteristics outside the band (upper side and lower side) based on the pilot carrier at each band end. Presumed. The frequency extrapolation unit 6 outputs N1 tap FIR filter frequency interpolation unit 11-1, N2 tap FIR filter frequency interpolation unit 11-2, N3 tap FIR filter frequency interpolation unit 11-3,. , Nm tap FIR filter frequency interpolation unit 11-m. At this time, N1, N2, N3,..., Nm are integers, and have a relationship of N1>N2>N3>...> Nm, and correspondingly, the filter coefficients are also different. To do. N1-tap FIR filter frequency interpolation unit 11-1, N2-tap FIR filter frequency interpolation unit 11-2, N3-tap FIR filter frequency interpolation unit 11-3,..., Nm-tap FIR filter frequency interpolation unit 11- m represents N1, N2, N3,..., Nm tap FIR filters, and N1, N2, N3,. N1-tap FIR filter frequency interpolation unit 11-1, N2-tap FIR filter frequency interpolation unit 11-2, N3-tap FIR filter frequency interpolation unit 11-3,..., Nm-tap FIR filter frequency interpolation unit 11- m represents the output signals H ₁ (k), H ₂ (k), H ₃ (k),..., H _m (k), respectively, multiplying unit 8-1, multiplying unit 8-2, multiplying unit 8 -3,..., Are input to the multiplication unit 8-m. The weight coefficients γ, δ, ε,..., Ζ are respectively sent from the weight coefficient control unit 9 to the multiplication unit 8-1, the multiplication unit 8-2, the multiplication unit 8-3,. N1 tap FIR filter frequency interpolation unit 11-1, N2 tap FIR filter frequency interpolation unit 11-2, N3 tap FIR filter frequency interpolation unit 11-3,..., Nm tap FIR filter frequency interpolation It is multiplied with the output signal of the section 11-m. The multiplication unit 8-1, the multiplication unit 8-2, the multiplication unit 8-3,... The adding and synthesizing unit 10 adds and synthesizes the multiplication result signals from the multiplying unit 8-1, the multiplying unit 8-2, the multiplying unit 8-3,..., The multiplying unit 8-m, and the estimated transmission line characteristic H ( k) Output as a signal to a subsequent stage (not shown).

In the above, this invention was demonstrated based on embodiment. It is to be understood by those skilled in the art that these embodiments are exemplifications, and that various modifications can be made by combinations of these components, and such modifications are also within the scope of the present invention.

DESCRIPTION OF SYMBOLS 1,16: Antenna, 2: Down converter, 3: A / D conversion part, 4: FFT part, 5: Pilot carrier extraction part, 6: Frequency extrapolation part, 7-1: Long tap FIR filter frequency interpolation part 7-2: Short tap FIR filter frequency interpolation unit, 8-1, 8-2,..., 8-m: Multiplication unit, 9: Weight coefficient control unit, 10: Addition synthesis unit, 11-1: N1-tap FIR filter frequency interpolation unit, 11-2: N2-tap FIR filter frequency interpolation unit, 11-3: N3-tap FIR filter frequency interpolation unit, ..., 11-m: Nm-tap FIR filter frequency interpolation unit Section: 12: frequency interpolation section, 13: remodulation section, 14: D / A conversion section, 15: down converter.

Claims (3)

In an OFDM receiving apparatus that receives a pilot carrier having a known amplitude and phase that are inserted at equal intervals or non-equal intervals in a frequency domain and modulated by an orthogonal frequency division multiplexing (OFDM) modulation scheme,
Pilot carrier extracting means for extracting the pilot carrier from the received signal;
A first interpolation unit that comprises a filter having a predetermined number of taps, and performs interpolation calculation in the frequency domain by performing filter calculation processing on the extracted pilot carrier;
The second interpolation is implemented by a filter having a smaller number of taps than the filter constituting the first interpolation means, and the filter performs a filter calculation process on the extracted pilot carrier to perform interpolation in the frequency domain. Interpolating means;
Weighting factor control means for generating first and second weighting factors that change according to the carrier position;
First multiplying means for multiplying the first weighting factor by the signal interpolated by the first interpolation means;
Second multiplication means for multiplying the second weighting factor by the signal interpolated by the second interpolation means;
Addition synthesis means for adding and synthesizing the multiplication result signal obtained by the first multiplication means and the multiplication result signal obtained by the second multiplication means;
An OFDM receiving apparatus comprising: In an OFDM receiver that receives a signal modulated by an orthogonal frequency division multiplexing modulation system in which pilot carriers having known amplitudes and phases are inserted at equal intervals or non-equal intervals,
Pilot carrier extracting means for extracting the pilot carrier from the received signal;
A plurality of interpolation means for interpolating each of the plurality of filters having different tap numbers, wherein the plurality of filters perform filter calculation processing on the extracted pilot carrier,
Weight coefficient control means for variably controlling the weight coefficient according to the carrier position;
A plurality of multiplication means for multiplying the signals interpolated by the plurality of interpolation means by the weighting coefficients respectively input from the weighting coefficient control means;
Addition synthesis means for adding and synthesizing signals multiplied by the plurality of multiplication means;
An OFDM receiving apparatus comprising: 6. A relay device using the OFDM receiver according to claim 1

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