JP2007258794A - Method and device for reducing noise in ofdm receiver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve precise diversity composition by improving an error in a calculated OFDM diversity composition ratio caused by noise included into a pilot carrier in environment having deteriorated C/N and the deterioration of the C/N after composition. <P>SOLUTION: In a device for diversity-receiving an OFDM signal using a discrete pilot, a reception signal is subjected to Fourier transform to average the pilot carrier extracted from a frequency axis signal in a time direction, and the composition ratio in the diversity composition is calculated from the transmission path estimation result of a frequency axis based on the pilot carrier after noise is reduced. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

The present invention relates to a reception apparatus of a data transmission apparatus modulated by an orthogonal frequency division multiplexing (hereinafter, abbreviated as OFDM) modulation scheme.

  In recent years, as a modulation method suitable for application to mobile digital transmission and terrestrial digital television broadcasting, an OFDM method characterized by resistance to multipath fading and ghost has been attracting attention. The OFDM system is a kind of multi-carrier modulation system, and is a transmission system in which tens to several hundreds of carriers (carriers) orthogonal to each other are digitally modulated.

  A discrete code is assigned to each of the I-axis component and Q-axis component of each carrier as a modulated signal, and the code is updated every symbol period (several μsec to msec). As a digital modulation system of the carrier, a 4-phase differential phase shift keying (DQPSK) is also used, but a 16-value quadrature amplitude modulation (16QAM) or A multilevel modulation system such as 64QAM is often used.

In the 16QAM and 64QAM modulation schemes, information is given to the amplitude and phase of each carrier, and therefore the absolute amplitude and absolute phase of the received carrier must be accurately reproduced during demodulation.
Therefore, a method is used in which pilot carriers having constant amplitude and phase are arranged for every several carriers, and the receiver estimates channel characteristics based on the pilot carriers and equalizes the amplitude and phase.

4 and 5 are examples of pilot carrier arrangement methods.
FIG. 4 shows a configuration in which pilot carriers are continuously arranged on the same carrier in time, and is called a continuous pilot (hereinafter abbreviated as CP). FIG. 5 shows a configuration in which the pilot carriers are arranged so as to be shifted for each symbol, and is called a discrete pilot (hereinafter abbreviated as SP).

Although the explanation on the equalization processing on the receiving side by the pilot carrier will be described later, the pilot arrangement shown in FIG. 4 is suitable for a transmission path with a large fluctuation such as high-speed mobile transmission, and conversely, in the pilot arrangement shown in FIG. Although the time responsiveness is low, the multipath delay time that can be equalized is long.
In order to improve transmission performance, the amplitude of the pilot carrier is often set larger than that of a normal data carrier. For example, the amplitude ratio is set to 4/3 times that of the data carrier.
These carriers are added so as to maintain an orthogonal relationship with each other, and an OFDM time axis waveform is generated. This addition process can be realized by performing an inverse Fourier transform (hereinafter referred to as IFFT) process on each carrier.

As shown in FIG. 6, the OFDM signal has a symbol structure in which an OFDM symbol is composed of an effective symbol that is a time axis waveform after the IFFT processing and a guard interval that is a part of the effective symbol copied and added before the effective symbol. Composed.
By adding a guard interval, it is possible to avoid deterioration due to inter-symbol interference with respect to a delayed wave having a delay time within the guard interval period, and thus can have strong resistance against multipath fading. .
The OFDM signal generated by the above processing is frequency-converted to a high frequency (RF) and then transmitted.

By the way, because OFDM is suitable for mobile transmission, it may be used in a transmission system of a video wireless transmission device called FPU (Field Pickup Unit) that performs television relay such as marathon relay and emergency news.
In mobile transmission, in addition to the main wave coming directly from the transmitter according to the terrain, a multipath communication path may be formed in which there is a reflected wave that is reflected from a building or the like and arrives with a delay time. is there.

Under a multipath environment, in a frequency band where the phases of the main wave and the reflected wave are close to opposite phases, degradation called frequency selective fading occurs in which the signal is canceled and the amplitude is reduced. In OFDM, since the band for each carrier is narrow, a transmission error frequently occurs in a carrier whose level is reduced by frequency selective fading.
Furthermore, in mobile transmission, there may be a Rayleigh fading environment in which the levels of the main wave and the reflected wave change from moment to moment. In a Rayleigh fading environment, the level of all carriers may be reduced, so that transmission errors frequently occur even for symbols with reduced levels.

  Therefore, a reception processing method called space diversity (hereinafter referred to as diversity) may be applied in order to reduce such fading degradation. Diversity refers to fading by placing multiple antennas spatially apart and appropriately combining the signals received by each antenna, or by selecting the one with the higher received strength from the multiple antennas and processing them. This is a method that can reduce the influence of deterioration.

  In the diversity of the OFDM scheme, appropriate switching can be performed even in a multipath environment by performing the above switching and combining processing for each subcarrier. Further, the maximum ratio combining method having excellent diversity gain is often used among the diversity methods, and this method will be described below.

FIG. 7 is a diagram showing a configuration of a diversity apparatus having two antennas.
Signals received by the two antennas are each subjected to processing such as frequency conversion, and then input to the A / D converters 1a and 1b to be converted into digital signals. The output signals of the obtained A / D converters 1a and 1b are input to the FFT units 2a and 2b. In the FFT units 2a and 2b, a time window having an effective symbol period length is provided on the received sample sequence. FFT processing is performed on the sample signal included therein to convert the time axis signal into a frequency axis signal.

  Output signals from the FFT units 2a and 2b are input to the pilot carrier extraction units 3a and 3b, and a pilot carrier is extracted from the received frequency axis signal. Thereafter, by performing interpolation in the frequency direction on the pilot carrier extracted by the interpolation filter units 5a and 5b, the transmission channel characteristics of the carrier without the pilot carrier are calculated.

The coefficient calculation unit 7 calculates a weighting coefficient to be multiplied by the output signals from the FFT units 2a and 2b based on the transmission path characteristics from the interpolation filter units 5a and 5b. For example, since the C / N is deteriorated for a carrier whose level is reduced due to multipath fading and a transmission error is likely to occur, the weight (importance) of the carrier is set low. Theoretically, the optimum weighting factor w _i (k) ( _i : antenna number, k: carrier number) for maximum ratio combining with M-branch receiving antennas is the transmission path characteristic from the transmitter to the i-th antenna. h ^* _i (k) ( ^* indicates a complex number)
₍₁₎
It is known that this is a coefficient that maximizes C / N.

In the multipliers 6 a and 6 b, the output signals from the FFT units 2 a and 2 b are multiplied by the weight coefficient w _i (k) calculated by the coefficient calculation unit 7, and the synthesis unit 8 performs addition synthesis processing.
By the processing of the multipliers 6a and 6b and the synthesis unit 8, the C / N is optimized so as to be maximized, and the amplitude and phase are also equalized at the same time.
The combined signal is input to the demodulator 9, and after symbol determination, an information code is output.
The principle of diversity described above is described in detail in Non-Patent Document 1, for example.

Next, the processing of the OFDM receiver when a single antenna is used will be described. FIG. 8 is a diagram showing the configuration of the OFDM receiver. The signal received by the antenna is converted into a digital signal by the A / D converter 1a, and subjected to FFT calculation processing in the FFT unit 2a to convert from a time axis signal to a frequency axis signal.
The output signal from the FFT unit 2a is input to the pilot carrier extraction unit 3a, where the pilot carrier is extracted, and the transmission channel characteristics of the carrier without the pilot carrier are calculated by the interpolation filter unit 5a.
The above processing is the same processing as that of the OFDM diversity receiver shown in FIG.

The equalization unit 21 receives the output signal from the FFT unit 2a and the transmission path characteristic signal calculated by the interpolation filter unit 5a, and multiplies the output signal from the FFT unit 2a by the coefficient shown in Expression (2). As a result, equalization of amplitude and phase is performed.
₍₂₎
The output signal of the equalizing unit 21 is input to the demodulating unit 9, and after symbol determination, an information code is output.

As described above, the reception processing of the OFDM signal can be completed by the OFDM diversity receiver or the OFDM receiver by the processing described above.
ITE Technical Report, Vol.23, No.35, PP13-18 (1999) JP 2004-112155 A

In the diversity processing in the prior art, when the C / N of each branch deteriorates, noise is mixed in the pilot carrier as well as the data carrier. As described above, in OFDM diversity, the weight coefficient w _i (k) based on the transmission path characteristic h _i (k) calculated from the pilot carrier is used. However, since noise is mixed in a pilot carrier that should be a reference signal, it is difficult to estimate transmission path characteristics with high accuracy. As a result, an error due to noise also occurs in the weighting coefficient, the ideal synthesis shown in the equation (1) cannot be performed, and the C / N after the synthesis also deteriorates.

Also, in the equalization process in the OFDM receiver with a single antenna, the equalization process is performed based on the channel characteristics h _i (k). Another drawback is that the accuracy deteriorates and ultimately the code error rate is deteriorated. This degradation amount is about 2.5 dB C / N degradation when the CP interval is 8 carriers and the CP amplitude ratio is 4/3.

In the present invention, in order to solve the above-described problems, in an OFDM receiver that receives a signal modulated by an OFDM modulation method in which pilot carriers having known amplitudes and phases are dispersedly arranged at least in the frequency direction, first, Means for diversity receiving the modulated wave with an antenna of M (M is an integer of 2 or more) branch, means for Fourier transforming the received signal and converting it to a frequency axis, and extracting a pilot carrier from the frequency axis signal; A filter is applied in the time direction of the extracted pilot carrier to reduce the pilot carrier noise, and the frequency axis transmission path is estimated based on the pilot carrier after the noise reduction, and diversity combining is performed based on the frequency axis transmission path estimation result. Provided is an OFDM receiver characterized by comprising means for calculating a composite ratio of time

  Second, the modulated signal is received, the received signal is Fourier-transformed and converted to the frequency axis, and a pilot carrier is extracted from the frequency axis signal, and a filter is applied in the time direction of the extracted pilot carrier. Means for reducing pilot carrier noise, and means for performing frequency axis transmission path estimation based on the pilot carrier after noise reduction, and performing amplitude and phase equalization processing based on the frequency axis transmission path estimation result An OFDM receiving apparatus is provided.

  Third, the modulated signal is received, the received signal is Fourier-transformed and converted to the frequency axis, and the pilot carrier is extracted from the frequency axis signal, and the fading frequency is set in the time direction of the extracted pilot carrier. A means to reduce the pilot carrier noise by applying a filter as a passband and the frequency axis transmission path estimation based on the pilot carrier after the noise reduction, and convert the frequency axis transmission path estimation result to the time axis transmission path characteristics An OFDM receiver characterized by comprising means for calculating a delay profile is provided.

  As described above, the noise mixed in the pilot carrier can be reduced by the pilot carrier averaging process according to the present invention. The maximum ratio combining ratio at the time of diversity combining can be set to an ideal combining ratio by calculating based on the pilot carrier signal with reduced noise. Therefore, even in an environment where C / N is deteriorated according to the present invention, it is possible to realize diversity combining with high accuracy.

Hereinafter, the receiver of the digital transmission apparatus according to the present invention will be described in detail with reference to the configuration of the first embodiment shown in FIG. In the following description, the CP shown in FIG. 4 is used as a pilot carrier.
FIG. 1 shows a configuration in which pilot carrier averaging units 4a and 4b are inserted between pilot carrier extraction units 3a and 3b and interpolation filters 5a and 5b in the configuration of the conventional OFDM diversity receiver of FIG. Other configurations and processing methods are the same as those in FIG.

The purpose of pilot carrier averaging sections 4a and 4b is to reduce noise mixed in the pilot carrier using a pilot carrier signal of an N symbol period.
The configuration of pilot carrier averaging sections 4a and 4b is shown in FIG. Pilot carrier signals r (t, k) from pilot carrier extraction units 3a and 3b are input to delay unit 91 (where t is a symbol number and k is a carrier number). The delay unit 91 outputs a signal r (t−1, k) obtained by delaying the pilot signal r (t, k) by one symbol. The output signal r (t−1, k) of the delay unit 91 is input to the delay unit 92 and to the multiplier 95. The operation of the delay unit 92 is the same as that of the delay unit 91 and outputs a signal r (t−2, k) delayed by two symbols. Similarly, the delay unit 93 and the delay unit 94 output a 3-symbol delayed signal r (t−3, k) and an N-symbol delayed signal r (t−N, k).

In the multipliers 95 to 99, the pilot carrier signals r (t, k), r (t-1, k), r (t-2, k), r (t-3, k) from the delay units 91 to 94 are provided. , R (t−N, k) are multiplied by a predetermined coefficient, and the adder 9a adds and synthesizes the results after multiplication. Here, the multiplicands of the multipliers 95 to 99 may be 1 / (N + 1) which is a simple moving average, but filters having different coefficients may be configured.
Further, there is no problem even if the pilot carrier averaging sections 4a and 4b are realized by a feedback type IIR (Infinite Impulse Response) filter as shown in FIG.

  As in FIG. 9, the pilot carrier signal extracted from the pilot carrier extraction units 3 a and 3 b is input, and the pilot carrier signal is multiplied by α by the attenuator 101. The signal from the attenuator 101 and the signal from the attenuator 103 are input to the adder 102 and added. The signal after the addition is delayed by one symbol length in the delay unit, and the delay result is output as an average result of the pilot carrier and fed back to the attenuator 103. The attenuator 103 inputs the signal multiplied by (1−α) to the adder 102. The process of FIG. 10 described above is called a first-order IIR filter, and has the characteristics of a low-pass filter.

Various conditions need to be considered in selecting the filter coefficient in FIG. 9 or the coefficient α related to the attenuator in FIG. 10, but a large factor that affects the characteristic is time variation. For example, in mobile transmission, time variation occurs at the fading frequency fd determined by the moving speed and the carrier frequency. If the averaging symbol period N is set large in such a fading environment that fluctuates at a high speed, an error occurs between the transmission path characteristics that fluctuate at a high speed and the averaging result, and degradation occurs during diversity combining. End up. Therefore, it is desirable to select a filter coefficient that forms a filter having a fading frequency to be used as a pass band.
Specifically, considering operation with a symbol period of about 56 μsec and a fading frequency fd of about 130 Hz, it is desirable to select about N = 9 for a simple moving average.

The noise components mixed in the pilot carrier can be reduced by the pilot carrier averaging units 4a and 4b described above.
Output signals from the pilot carrier averaging units 4a and 4b are input to the interpolation filters 5a and 5b to perform interpolation in the frequency direction.

  Signals that have been subjected to pilot carrier noise reduction, that is, noise reduction of estimated transmission path characteristics in each branch, are input to the coefficient calculation unit 7 to calculate the synthesis coefficient described in Equation (1) and perform maximum ratio synthesis. Thus, even in an environment where C / N is deteriorated, it is possible to perform diversity combining with high accuracy.

  Further, as shown in FIG. 11, the order of the pilot carrier averaging units 4a and 4b and the interpolation filters 5a and 5b may be reversed.

Next, the receiver of the digital transmission apparatus according to the present invention will be described in detail with reference to the second embodiment shown in FIG.
FIG. 2 is a diagram showing a configuration of an OFDM receiver in a single branch using the present invention. Also in the second embodiment, FIG. 2 shows a configuration in which a pilot carrier averaging unit 4a is inserted between the pilot carrier extraction unit 3a and the interpolation filter 5a with respect to the configuration of FIG. The method is the same as in FIG.

A pilot carrier is extracted by the pilot carrier extraction unit 3a from the signal on the frequency axis that has passed through the A / D converter 1a and the FFT unit 2a. The extracted pilot carrier is input to the pilot averaging unit 4a, and the noise mixed in the pilot carrier is reduced as described in the first embodiment. Thus, using the pilot carrier with reduced noise, the interpolation filter unit 5a performs interpolation in the frequency direction to estimate the transmission path characteristics.
The estimated transmission line characteristic signal from the interpolation filter unit 5a is input to the equalization unit 21, and is calculated based on the signal from the FFT unit 2a and the estimated transmission line characteristic signal shown in Expression (2). Multiplying with the coefficient, the amplitude and phase are equalized.
The equalized signal is subjected to symbol determination by the demodulator 21 and outputs an information code.

  As in the first embodiment, as shown in FIG. 12, the order of the pilot carrier averaging unit 4a and the interpolation filter unit 5a may be reversed.

As described above, by reducing the pilot carrier noise, the equalization error caused by the noise during the equalization process can be reduced.
For example, when the CP interval is 8 carriers, the CP amplitude ratio is 4/3, and the average number of times N = 9, this improvement degree has a C / N improvement effect of about 1.6 dB.

  Finally, the receiver of the digital transmission apparatus according to the present invention will be described in detail with reference to the third embodiment shown in FIG.

  A pilot carrier is extracted by the pilot carrier extraction unit 3a from the signal on the frequency axis that has passed through the A / D converter 1a and the FFT unit 2a. The extracted pilot carrier is input to the pilot averaging unit 4a, and the noise mixed in the pilot carrier is reduced as described in the first embodiment. Thus, using the pilot carrier with reduced noise, the interpolation filter unit 5a performs interpolation in the frequency direction to estimate the transmission path characteristics.

  The IFFT unit 31 converts the frequency axis transmission line characteristic obtained by the interpolation filter unit 5a into a time axis transmission line characteristic by performing inverse Fourier transform. The transmission path characteristic on the time axis indicates a delay profile representing the complex impulse response of the transmission path.

  If the pilot carrier signal is not averaged, the accuracy of the delay profile signal also deteriorates due to the influence of noise mixed in the pilot carrier in an environment where the C / N deteriorates. For example, in an environment in which a reflected wave with a large D / U (a low level) is mixed, the reflected wave is buried in noise, making it difficult to detect. Therefore, by performing the pilot carrier averaging process according to the present invention, a highly accurate delay profile can be generated even in an environment where C / N is deteriorated.

The block diagram which shows the structure by 1st Example of this invention. The block diagram which shows the structure by the 2nd Example of this invention. The block diagram which shows the structure by the 3rd Example of this invention. Schematic diagram showing the carrier arrangement of CP Schematic diagram showing SP carrier arrangement Schematic diagram showing symbol structure of OFDM signal Block diagram showing the configuration of a conventional OFDM diversity receiver The block diagram which shows the structure of the OFDM receiver by a prior art Block diagram showing configuration of pilot carrier averaging sections 4a and 4b Block diagram showing the configuration of pilot carrier averaging sections 4a and 4b using IIR filters The block diagram which shows the other structure in 1st Example of this invention. The block diagram which shows the other structure in the 2nd Example of this invention.

Explanation of symbols

1a: A / D converter, 2a: FFT unit, 3a: pilot carrier extraction unit, 4a pilot carrier averaging unit, 5a: interpolation filter unit, 6a: multiplier, 7: coefficient calculation unit, 8: synthesis unit, 9: Demodulation unit, 1b: A / D converter, 2b: FFT unit, 3b: Pilot carrier extraction unit, 4b Pilot carrier averaging unit, 5b: Interpolation interpolation filter unit, 6b: Multiplier, 21: Equalization unit, 31: IFFT unit, 91-94: delay unit, 95-99: multiplier, 9a: adder, 101: attenuator, 102: adder, 103: attenuator, 104: delay unit

Claims (3)

In an OFDM receiver that receives a signal modulated by an OFDM modulation method in which pilot carriers having known amplitudes and phases are distributed at least in the frequency direction, the modulated wave is divided into M (M is an integer of 2 or more) branches. Means for receiving diversity by the antenna, means for Fourier-transforming the received signal and converting it to the frequency axis, extracting a pilot carrier from the frequency axis signal, and applying a filter to the extracted pilot carrier in the time direction Means for reducing carrier noise, and means for estimating a frequency axis transmission path based on the pilot carrier after noise reduction, and calculating a combining ratio at the time of diversity combining based on the frequency axis transmission path estimation result An OFDM receiver characterized by the above. In an OFDM receiver that receives a signal modulated by an OFDM modulation method in which pilot carriers with known amplitudes and phases are distributed at least in the frequency direction, the modulated signal is received, the received signal is Fourier transformed, Based on the means for converting to the frequency axis, the means for extracting the pilot carrier from the frequency axis signal, filtering the extracted pilot carrier in the time direction to reduce the noise of the pilot carrier, and the pilot carrier after the noise reduction An OFDM receiver characterized by comprising means for performing frequency axis transmission path estimation and performing amplitude and phase equalization processing based on the frequency axis transmission path estimation result. In an OFDM receiver that receives a signal modulated by an OFDM modulation method in which pilot carriers having known amplitudes and phases are distributed at least in the frequency direction, the modulated signal is received, and the received signal is Fourier transformed. Means for converting to a frequency axis, means for extracting a pilot carrier from the frequency axis signal, applying a filter with a fading frequency as a pass band in the time direction of the extracted pilot carrier, and reducing noise of the pilot carrier; An OFDM receiver comprising means for calculating a delay profile by estimating a frequency axis transmission path based on a pilot carrier of the frequency and converting a frequency axis transmission path estimation result into a time axis transmission path characteristic .