JPWO2009044431A1 - Receiver and receiving method for receiving orthogonal frequency division multiplexed signal by Zero-IF method - Google Patents

Abstract

An object of the present invention is to provide an OFDM receiver and an OFDM signal receiving method capable of suppressing S / N degradation of a signal near zero frequency passing through a device operating at a low voltage such as a CMOS device. An OFDM receiver comprising a high-frequency amplifier (50) for amplifying the OFDM signal and down-converting the OFDM signal passed through the high-frequency amplifier (50) into a baseband, wherein the high-frequency amplifier (50) is connected to the OFDM signal. Are amplified (W1) with different gains depending on the subcarrier frequency.

Description

The present invention relates to a receiver and a reception method for receiving a signal that is orthogonal frequency division multiplexed by the Zero-IF method.
The present invention is suitable when a device operating at a low voltage such as a CMOS device is provided as an electronic device.

In general, signal processing in a wireless receiver is performed by frequency-converting a high-frequency signal transmitted from a wireless transmitter to a low frequency.
FIG. 17 is a block diagram of a superheterodyne radio receiver that is one of the Zero-IF radio receivers. The frequency conversion operation in the conventional radio receiver will be described with reference to FIG. That is, when a high-frequency signal is received by the antenna 10, the high-frequency signal is amplified by a high-frequency amplifier (RF amplifier) 12 and then output to a frequency converter 13, and the frequency converter 13 Multiplying to generate an intermediate frequency signal. Next, the intermediate frequency signal output from the frequency converter 13 is amplified by an intermediate frequency amplifier (IF amplifier) 15 and then output to a quadrature detector (orthogonal demodulator) 16. The quadrature demodulator 16 includes two frequency converters 16-1 and 16-2, and signals amplified by the intermediate frequency amplifier 15 are input to the respective frequency converters 16-1 and 16-2. Furthermore, a 90-degree phase shifter 16-3 for shifting the phase of the signal from the second local oscillator 17 by 90 degrees is provided, and a signal from the second local oscillator 17 and a signal obtained by shifting the phase by 90 degrees are provided. Input to separate frequency converters 16-1, 16-2. In this way, the quadrature demodulator 16 multiplies the respective signals by the frequency converters 16-1 and 16-2 and outputs them as I-phase and Q-phase baseband signals. The frequency converter 13, the first local oscillator 14, and the intermediate frequency amplifier 15 can be omitted. In this case, the direct conversion radio receiver is particularly used, and a high-frequency signal is transmitted. Direct conversion to baseband signal without going through intermediate frequency signal. The I-phase signal (I signal) and the Q-phase signal (Q signal) demodulated by the quadrature demodulator 16 are converted into digital signals through the A / D converters 18-1 and 18-2 at the subsequent stage, and the digital signal processor 19 And the digital data is demodulated.

  Thus, in the conventional radio receiver, the high frequency signal is converted into a baseband signal by dropping the frequency, and this is digital signal processed. In such a configuration, the conversion speeds of the A / D converters 18-1 and 18-2 are suppressed to the minimum necessary (theoretically, the same speed as the signal bandwidth), and peripheral circuits and signal processing are performed. It is possible to reduce the burden.

  Incidentally, it is generally known that 1 / f noise may not be negligible when a device operating at a low voltage such as a CMOS device is used as the electronic device of the receiver. The 1 / f noise generated in the electronic device has an amount of noise that is inversely proportional to the magnitude of the frequency, and becomes very large particularly at low frequencies. Therefore, 1 / f noise can be ignored when a signal having a relatively high frequency such as a high frequency signal or an intermediate frequency signal passes, but in a baseband signal including a zero frequency component, 1 / f noise is ignored. Since the value becomes very large and S / N deterioration occurs, it cannot be ignored.

Therefore, conventionally, a method (so-called low-IF method) of down-converting to a frequency band in which the 1 / f noise can be ignored, that is, a frequency band outside the vicinity of the frequency zero has been mainstream. In addition, when adopting the zero-IF method, for example, by adopting a signal with a relatively small influence of a low frequency component in a wideband signal such as a wireless LAN signal defined in IEEE802.11a / g Therefore, the influence of 1 / f noise is avoided (see, for example, Non-Patent Document 1 to Non-Patent Document 3).
A Bluetooth radio in 0.18- / spl mu / m CMOSvan Zeijl, P .; Eikenbroek, J.-WT; Vervoort, P.-P .; Setty, S .; Tangenherg, J .; Shipton, G .; Kooistra, E .; Keekstra, IC; Belot, D .; Visser, K .; Bosma, E .; Blaakmeer, SC; Solid-State Circuits, IEEE Journal of Volume 37, Issue 12, Dec. 2002 Page (s): 1679-1687 A dual-mode 802.11b / bluetooth radio in 0.35- / spl mu / m CMOSDarabi, H .; Chiu, J .; Khorram, S .; Hea Joung Kim; Zhimin Zhou; Hung-Ming; Chien; Ibrahim, B .; Geronaga, E .; Tran, LH; Rofougaran, A .; Solid-State Circuits, IEEE Journal of Volume 40, Issue 3, Mar 2005 Page (s): 698-706 A 0.13 / spl mu / m CMOS front-end, for DCS1800 / UMTS / 802.11bg with multiband positive feedback low-noise amplifierLiscidini, A .; Brandolini, M .; Sanzogni, D .; Castello, R .; Solid-State Circuits , IEEE Journal of Volume 41, Issue 4, April 2006 Page (s): 981-989

  In orthogonal frequency division multiplexing (hereinafter abbreviated as OFDM) transmission schemes, pre-processing such as error correction codes is applied to the data to be transmitted, and the data is multiplexed on a plurality of subcarriers having different bands for each symbol, and a time axis signal (OFDM signal) is transmitted through the transmission medium.

  FIG. 18 is a simplified block diagram of a wireless transmitter that transmits an OFDM signal. In the wireless transmitter 2, first, the error correction code unit 20 preprocesses the data. For example, error correction encoding processing is performed on the data, and the error correction encoded data is serial / parallel converted and assigned to a plurality of subcarriers (1st to nth subcarriers) having different bands. Then, after performing primary modulation by QPSK, QAM or the like, each data is output to the inverse fast Fourier transform (IFFT) unit 21 and multiplexed into a time axis signal, which is D / A converted by the transmission circuit 22 Then, the high-frequency signal up-converted is transmitted to the receiver side through the antenna 23.

  The OFDM signal is already adopted in the wireless LAN signal defined in the above-mentioned IEEE802.11a / g. An OFDM signal is also used as an OFDMA (Orthogonal Frequency Division Multiple Access) -PUSC (Partial Usage of Subchannel) signal defined in IEEE 802.16. Each signal is similarly defined in that the error-correction-encoded data is transmitted by being distributed to a plurality of subcarriers having different bands. However, they are different as follows in the interval between different subcarriers.

  In the wireless LAN signal, 48 subcarriers obtained by dividing the 20 MHz band into 64 are assigned as distribution destinations of data subjected to error correction coding. Four of the remaining subcarriers are allocated for pilots for specifying communication channel parameters. The other 12 are guard subcarriers that are not used for communication. As described above, since the 20 MHz band is defined to be divided into 64, the subcarrier interval is 312.5 kHz, and the center frequency of the lowest frequency subcarrier is 312.5 kHz.

  On the other hand, in the OFDMA-PUSC signal, a band is divided into 2048 to constitute subcarriers. Since the band limitation is not particularly defined, when considering the same 20 MHz as the wireless LAN signal, the subcarrier interval is about 10 kHz, and the center frequency of the subcarrier with the lowest frequency is about 10 kHz.

  Here, the frequency at which the noise amount of 1 / f noise decreases with increasing frequency and falls below the thermal noise level (so-called cutoff frequency) is considered to be about 100 kHz at most in a typical example. Therefore, let us consider the influence of 1 / f noise on each signal on the premise of this.

  First, in the wireless LAN signal, as described above, the subcarrier interval is 312.5 kHz, which is much larger than the bandwidth from the zero frequency to the cutoff frequency of 100 kHz. Therefore, the influence of 1 / f noise per subcarrier can be regarded as relatively small and can be ignored.

  On the contrary, in the OFDMA-PUSC signal, the subcarrier interval is about 10 kHz as described above, which is much smaller than the bandwidth from the zero frequency to the cutoff frequency 100 kHz. More specifically, a total of about 20 subcarriers, each of about 10 on one side, provided at symmetrical positions about DC on the frequency axis are below the cutoff frequency. For this reason, the influence of 1 / f noise is generated as severe deterioration of S / N as shown in FIG. 19 as an example in a subcarrier below the cut-off frequency, particularly in the vicinity of DC (S / N depends on the frequency). / F Noise cannot be ignored. Such characteristics of the OFDMA-PUSC signal become more pronounced as the subcarrier spacing on the frequency axis becomes narrower than the bandwidth from the zero frequency to the cutoff frequency.

  As already mentioned, Zero-IF radio receivers have the advantage of reducing the burden on peripheral circuits and signal processing. However, when a device that operates at a low voltage, such as a CMOS device, is used for the baseband stage (especially used for the quadrature demodulator for the baseband here), sub-frequency When the carrier interval is narrow, the baseband signal includes a signal component near zero frequency, and the noise amount of 1 / f noise in the vicinity thereof increases. Therefore, there arises a problem that severe deterioration of S / N occurs. Furthermore, there is a problem that the data error rate after decoding increases in a subcarrier having a large 1 / f noise amount.

  For this reason, in a conventional Zero-IF OFDM receiver using a device that operates at a low voltage such as a CMOS device, an OFDM signal having a narrow subcarrier interval such as the above OFDMA-PUSC signal is practical. Could not be used.

  An object of the present invention is to provide an OFDM receiver and an OFDM signal receiving method capable of suppressing S / N degradation of a signal near zero frequency passing through a device operating at a low voltage such as a CMOS device. It should be noted that an OFDM signal with a narrow subcarrier interval is not limited to an OFDM-PUSC signal, but is clearly applicable to other systems using an OFDM signal with a narrow subcarrier interval.

  One aspect of the OFDM receiver of the present invention is an OFDM receiver that includes a high-frequency amplifier that amplifies a received OFDM signal and down-converts the OFDM signal that has passed through the high-frequency amplifier into a baseband. The high-frequency amplifier is configured to amplify the OFDM signal by changing the gain according to the frequency of the subcarrier (amplify with a different gain according to the frequency).

  Note that the high-frequency amplifier preferably makes the gain of the signal in the frequency band that is down-converted to the zero frequency and the vicinity of the zero frequency of the OFDM signal higher than the reference gain.

  Alternatively, it is preferable that the high-frequency amplifier has a gain of a signal other than the zero frequency of the OFDM signal and a signal other than a frequency band down-converted near the zero frequency lower than a reference gain.

In the superheterodyne receiver, the high-frequency amplifier is preferably configured as an RF amplifier of an RF stage or an IF amplifier of an IF stage.
Further, it is preferable that a correction means for correcting a signal intensity difference in each frequency band generated in the RF amplifier or the IF amplifier is provided in the baseband stage.

The correction means is preferably a baseband amplifier having a frequency characteristic opposite to the frequency characteristic of the gain in the RF amplifier or the IF amplifier.
Further, it is preferable to further include means for obtaining an S / N ratio of the OFDM signal and automatically calculating a frequency characteristic of a gain to be set in the high frequency amplifier based on the S / N ratio.

Further, it is preferable to further comprise means for obtaining a data error rate of the OFDM signal and automatically setting a gain in the high-frequency amplifier based on the data error rate.
Another aspect of the OFDM receiver of the present invention includes a high-frequency amplifier that amplifies the received OFDM signal, and a down-conversion circuit that down-converts the OFDM signal passed through the high-frequency amplifier to a baseband. An OFDM receiver comprising two down-conversion circuits, wherein the first high-frequency amplifier on one side of the two down-conversion circuits has a higher gain than the second high-frequency amplifier on the other side; and The first high-frequency amplifier is provided with a band-pass filter that passes a received OFDM signal in a frequency band that takes zero frequency or near zero frequency when down-converted to baseband.

  One aspect of the OFDM signal reception method of the present invention is an OFDM signal reception method including a subcarrier component whose S / N depends on the frequency, and the OFDM signal is received in accordance with the frequency of the subcarrier in a high frequency amplifier. The gain is changed and amplified (amplified with a different gain depending on the frequency), and the amplified signal is down-converted to a baseband signal.

According to the present invention, it is possible to reduce S / N degradation of a signal near zero frequency that passes through a device operating at a low voltage such as a CMOS device.
Also, by providing the correction means, it is possible to have substantially the same S / N in all bands, and it is possible to reduce the increase in bit error rate.

  Furthermore, since a low-rate A / D converter and a simple demodulation circuit can be configured, the power consumption of the receiver is reduced.

It is explanatory drawing of the method of suppressing S / N degradation of the signal of the zero frequency vicinity by Example 1. FIG. 1 is a configuration example of an OFDM receiver according to Embodiment 1. 2 is a specific configuration example of an RF amplifier 50. FIG. 3 is a spectrum diagram of signals in the first embodiment. It is arrangement | positioning explanatory drawing of an attenuator. 6 is a configuration example of an OFDM receiver according to a second embodiment. FIG. 6 is a spectrum diagram of signals in the second embodiment. 10 is a configuration example of an OFDM receiver according to Embodiment 3. It is explanatory drawing of the signal strength in Example 3. FIG. FIG. 10 is a block diagram of a digital signal processing unit in Embodiment 4. FIG. 10 is a diagram illustrating a configuration of a processing unit related to measurement of an S / N ratio in a digital signal processing unit according to a fifth embodiment. 10 is an operation flow of a processing unit according to the fifth embodiment. FIG. 10 is a configuration diagram of a digital signal processing unit in Embodiment 6. 14 is an operation flow of a processing unit according to the sixth embodiment. 10 is a configuration example of an OFDM receiver according to Embodiment 7. 10 is a modification of the OFDM receiver according to the seventh embodiment. It is a block diagram of the conventional superheterodyne system radio receiver. It is a simple block diagram of the wireless transmitter which transmits the conventional OFDM signal. It is the graph which showed S / N in the subcarrier near conventional DC.

Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to the drawings.
In general, the receiver configuration of the zero-IF method differs between the superheterodyne method and the direct conversion method. However, since the direct conversion type receiver is equivalent to the super heterodyne type receiver excluding the IF stage circuit, the following methods can be easily applied to both types of receivers regardless of the receiver. It is. Therefore, in each of the following embodiments, the configuration of a superheterodyne receiver among them will be given in order to show various applications of the present invention. Further, in each of the following embodiments, an OFDM transmission system signal transmitted from the wireless transmitter in FIG. 18 is handled. Unless otherwise specified, the digital processing unit configured in the receiver has a conventional configuration. It is assumed that the OFDM signal is demodulated by the reverse procedure of the wireless transmitter.

Example 1
In the first embodiment, a method for suppressing S / N degradation of a signal near zero frequency when an OFDM signal is down-converted into a baseband signal in an RF amplifier will be described.

FIG. 1 is an explanatory diagram of the method.
FIG. 1A shows “1 / f noise” generated in a quadrature detector (orthogonal demodulator) configured by a device operating at a low voltage such as a CMOS device, and “thermal noise” generated in an amplifier or the like. It is a general frequency characteristic graph of both. According to this graph (anti-noise intensity-frequency characteristic graph), the noise intensity of 1 / f noise decreases in inverse proportion to the increase in frequency from the zero frequency (0 Hz) indicating the maximum value. When the cutoff frequency (xHz) is exceeded, the thermal noise intensity expressed as being substantially constant is reduced.

  FIG. 1B is a graph of the relationship between the gain and the input frequency in the RF amplifier, created in correspondence with the anti-noise intensity-frequency characteristic graph of FIG. As shown in this graph (vs. gain-frequency characteristic graph), the RF amplifier is configured to amplify with the maximum gain when the input frequency (the frequency of the OFDM signal) takes the center frequency (RFHz). Furthermore, when the input frequency reaches from the center frequency (RFHz) to the frequency (RFHz + xHz) obtained by adding this to the cut-off frequency (xHz), the gain is gradually decreased and the gain is increased at the frequency (RFHz + xHz) or higher. Is fixed (flattened), for example, fixed to a gain that can be set in a frequency band where 1 / f noise can be ignored (flattened). In addition, about the gain below a center frequency (RFHz), what is necessary is just to give it so that a versus gain-frequency characteristic graph may become left-right symmetric centering on a center frequency (RFHz).

  FIG. 1 (c) is a graph showing the relationship between the signal strength and frequency of the I and Q signals obtained at the baseband stage when the RF amplifier is configured to have the frequency characteristics shown in FIG. 1 (b). It is. As shown in this graph (vs. signal strength-frequency characteristic graph), the signal intensity of the I signal and the Q signal in the baseband stage behaves in the same manner as the noise intensity-frequency characteristic graph of FIG. To come. That is, it takes a maximum value at zero frequency (0 Hz), and then decreases in inverse proportion to the increase in frequency. Above the cut-off frequency (xHz), the signal intensity is constant because the 1 / f noise is not considered. As is apparent from the graph of FIG. 1D described later, the signal intensity level on the vertical axis is different from that in FIG. 1A, and the signal intensity at zero frequency (0 Hz) in FIG. It becomes higher than the noise intensity at the zero frequency (0 Hz) in FIG.

FIG. 1D is a graph showing the frequency characteristics of the signal-to-noise power ratio (S / N ratio) of the I signal and Q signal obtained in the baseband stage.
The relationship between the anti-noise intensity-frequency characteristic graph of FIG. 1A and the anti-signal intensity-frequency characteristic graph of FIG. 1C is not clear from the figure, but the signal intensity at each point on the frequency axis is It becomes a constant multiple of the noise intensity corresponding to each frequency. Accordingly, as shown in FIG. 1D, the S / N ratio takes a constant value for each frequency (so-called frequency flat state), and the 1 / f noise near the zero frequency and the zero frequency. It is possible to prevent a decrease in the S / N ratio due to.

  However, the value of the S / N ratio in FIG. 1 (d) needs to be within a range of a level at which a signal can be correctly detected by a subsequent circuit. In addition, it is necessary to set the maximum gain value in a linear region where the signal is not saturated in the A / D converter at the subsequent stage. If it is within the range, the level of the vertical axis of the graph of gain vs. frequency characteristics in FIG. 1B may be changed as appropriate. For example, 0 dB may be given to the maximum gain of the center frequency (RFHz), and a negative gain may be given at other frequencies, that is, the signal may be attenuated.

FIG. 2 is a configuration example of a superheterodyne OFDM receiver according to the first embodiment. In addition, the same number as FIG. 17 is attached | subjected to the location equivalent to the structure of FIG.
The receiver 5 in FIG. 2 amplifies the OFDM signal received by the antenna 10 by changing the gain according to the subcarrier frequency (amplifies with a different gain according to the subcarrier frequency) and outputs the amplified signal to the frequency converter 13. An RF amplifier 50 is provided.

This RF amplifier 50 operates to amplify the OFDM signal input from the antenna 10 with a gain suitable for each subcarrier frequency.
Here, on the assumption that the relationship between the frequency characteristics shown in FIGS. 1 (a) to 1 (d) is established, the graph waveform of FIG. 2 has the frequency characteristics of the gain of FIG. 1 (b). An RF amplifier 50 having a frequency characteristic such as W1 is configured.

FIG. 3 shows a specific configuration example of the RF amplifier 50.
The RF amplifier 500 in FIG. 3A is configured by connecting an LC resonance circuit in parallel to the subsequent stage of the amplifying element in the broken line in the figure for amplifying an input signal with a constant gain. Here, L and C are set so that the resonance frequency of the LC resonance circuit matches the center frequency of the input signal.

  In this RF amplifier 500, when an OFDM signal is input, the signal is amplified with the maximum gain for the signal whose frequency matches the LC resonance frequency, and the frequency is close to the frequency near the LC resonance frequency. The signal is amplified with a smaller gain as it is farther from the LC resonance frequency. When the frequency of the OFDM signal becomes far from the LC resonance frequency, for example, by about the cut-off frequency, the signal is amplified with a constant gain set in the amplifying element in the broken line in FIG.

  In this configuration, the minimum gain of the waveform in FIG. 1B is set to take 0 dB or more. The minimum gain, that is, a constant gain at (RFHz + xHz) or higher in FIG. 1B is set as the gain of the amplifying element.

  In the RF amplifier 500, an RFC (Radio Frequency Choke) coil is configured as a variable device. Therefore, when the center frequency of the OFDM signal and the resonance frequency do not match, the inductance of the RFC coil is adjusted so that the frequencies coincide with each other. For example, when receiving an OFDM signal of another center frequency, the resonance frequency can be matched with the center frequency by adjusting the inductance of the RFC coil. In addition to the RFC coil, a MEMS (Micro-Electro-Mechanical Systems) element or a varactor (variable capacitance diode) may be used as the variable device.

Moreover, it is good also as a structure which changes to the adjustment of RFC, and changes the frequency of a local oscillator according to the center frequency of another OFDM signal.
The RF amplifier 502 in FIG. 3B is configured by connecting a frequency filter F to the subsequent stage of the amplifying element in the broken line in the figure for amplifying an input signal with a constant gain. The frequency filter F is configured by combining filter devices such as a SAW filter, and the signal whose frequency matches the center frequency of the input signal is best passed, and the signal transmittance is inversely proportional as the distance from the frequency increases. This is a filter that attenuates a signal having a lower frequency, that is, a frequency far from the center frequency of the input signal.

  In this RF amplifier 502, when an OFDM signal is input, the signal at the center frequency passes through the filter F without reducing the gain obtained at the amplifying element, and the signal at a frequency near the center frequency is obtained at the amplifying element. Pass the filter F to such an extent that the gain is slightly reduced. When the frequency of the OFDM signal becomes far from the center frequency, for example, by about the cut-off frequency, the signal is greatly attenuated to a predetermined level and passes through the filter F. If the gain of the amplifying element is 0 dB, the maximum gain of the waveform in FIG. 1B is 0 dB, that is, a frequency band that is not affected by 1 / f noise (zero frequency and near zero frequency). Lower the gain).

  The RF amplifier 502 is configured with an RFC (Radio Frequency Choke) coil as a variable device. For this reason, when the relationship between the frequency characteristic of the frequency filter F and the center frequency of the OFDM signal is not the above-described relationship, the inductance of the RFC coil is adjusted. For example, when receiving an OFDM signal having another center frequency, the inductance of the RFC coil is adjusted to obtain the above relationship. In addition to the RFC coil, a MEMS (Micro-Electro-Mechanical Systems) element or a varactor (variable capacitance diode) may be used as the variable device.

Moreover, it is good also as a structure which changes to the adjustment of RFC, and changes the frequency of a local oscillator according to the center frequency of another OFDM signal.
FIG. 4 is a diagram showing a spectrum of a signal at each stage of the receiver 5 provided with the RF amplifier 500 of FIG.

Here, as an example, it is assumed that an OFDM signal having a center frequency of 2400 MHz is received from the antenna 10.
In this case, the RF amplifier 50 can obtain a signal (signal spectrum RF1) in which the level of the frequency band in the vicinity of 2400 MHz is emphasized.

  Further, the frequency converter 13 obtains a signal (signal spectrum IF1) in which the level of the frequency band of 70 MHz and the vicinity thereof is emphasized in the intermediate frequency signal of the center frequency 70 MHz down-converted by the 2330 MHz signal of the local oscillator 14. Be able to.

  Then, in the I signal and Q signal output from the quadrature demodulator 16, a signal whose level is enhanced as it approaches the zero frequency, particularly a signal (signal spectrum BB1) whose level is emphasized near the zero frequency and near the zero frequency is obtained. This corresponds to the frequency characteristic of the signal intensity shown in FIG. 1C, and the S / N ratio of the I signal and the Q signal becomes a frequency flat corresponding to FIG. 1D.

  In this case, the signal is obtained by emphasizing the level of the frequency band of 2400 MHz and its vicinity from the RF amplifier 50, but this emphasis increases the overall signal power entering the subsequent element, thereby reducing the linear region of the element. May exceed. For example, the signal may be saturated in the A / D converter, and the output signal may be distorted. Therefore, when such a problem occurs, an attenuator (attenuator) 505 is configured at the subsequent stage of the IF amplifier 500 as shown in the conceptual diagram of FIG. Since this attenuator 505 uniformly attenuates the signal amplified with frequency characteristics as shown in FIG. 5 (b) over the entire frequency in the direction of the arrow in FIG. 5 (b), the influence of 1 / f noise is reduced. While increasing the gain at the receiving frequency (particularly the frequency before down-conversion corresponding to the zero frequency and the vicinity of the zero frequency), it is possible to decrease the gain at the frequency that is not affected by the 1 / f noise. Therefore, by selecting an attenuator having an attenuation rate such that the entire signal power entering the subsequent element falls within the linear region of the element, saturation in the element is prevented.

(Example 2)
In the second embodiment, a method for suppressing S / N degradation of a signal near zero frequency when an OFDM signal is down-converted to a baseband signal in an IF amplifier will be described.

FIG. 6 is a configuration example of a superheterodyne OFDM receiver according to the second embodiment. In addition, the same number as FIG. 17 is attached | subjected to the location equivalent to the structure of FIG.
The receiver 8 in FIG. 6 includes an IF amplifier 80 that amplifies the OFDM intermediate frequency signal output from the frequency converter 13 and outputs the amplified signal to the quadrature demodulator 16.

  The IF amplifier 80 operates to amplify the OFDM intermediate frequency signal output from the frequency converter 13 with a gain suitable for the frequency of each subcarrier (specifically, the frequency of the subcarrier down-converted to the intermediate frequency). To do.

  Here, as described in the first embodiment, on the assumption that the respective frequency characteristics shown in FIGS. 1A to 1D are established, the gain frequency characteristics shown in FIG. An IF amplifier 80 having a frequency characteristic like the graph waveform W2 of FIG. 6 is configured. However, since the IF amplifier 80 is configured in an IF stage, the stage is different from the RF amplifier 50 of the RF stage of the first embodiment. For this reason, the principle can be said to be substantially the same as in FIG. 1, but it is assumed in the graph of FIG. 1B that the center frequency of the high frequency signal and its peripheral band are the center frequency of the intermediate frequency signal and its peripheral band. .

  As a specific configuration of the IF amplifier 80, the configuration of FIG. 3A or FIG. 3B is applied. However, the center frequency of the OFDM signal received by the antenna and the frequency band affected by 1 / f noise around this frequency are different from the center frequency in the intermediate frequency band where the signal is down-converted. The center frequency band. Therefore, when each amplifier of FIG. 3A or FIG. 3B is used, setting is performed by changing the center frequency of the OFDM signal to the center frequency of the intermediate frequency signal in the first embodiment. . Further, if necessary, the frequency bandwidth for emphasizing the gain is also changed and set.

  When the IF amplifier 500 of the type shown in FIG. 3A is used as the IF amplifier 80, the overall signal intensity entering the subsequent element increases as in the first embodiment, which may exceed the linear region of the element. There is. In such a case, the attenuator is configured after the IF amplifier according to the first embodiment.

Since other than that is clear from the description in the first embodiment, the description is omitted.
FIG. 7 is a diagram showing a spectrum of a signal at each stage when the IF amplifier 80 of the receiver 8 is configured by the IF amplifier 502 of the type shown in FIG.

Here, as an example, it is assumed that an OFDM signal having a center frequency of 2400 MHz is received from the antenna 10 as in the first embodiment.
In this case, since the gain of the RF amplifier 12 is constant, a signal of a constant level (signal spectrum RF1 ′) amplified with a constant gain over the entire frequency is obtained at the output section.

  Further, the frequency converter 13 outputs a signal (signal spectrum IF1′-1) having a constant level over the entire frequency in the intermediate frequency signal of the center frequency 70 MHz down-converted by the 2330 MHz signal of the local oscillator 14. Is obtained.

Further, the IF amplifier 80 obtains a signal (signal spectrum IF1′-2) in which the level of the frequency band excluding the frequency band including 70 MHz and the vicinity thereof is attenuated.
Then, in the I signal and Q signal output from the quadrature demodulator 16, a signal whose level is enhanced as it approaches the zero frequency, particularly a signal (signal spectrum BB1 ′) whose level is emphasized near the zero frequency and the zero frequency is obtained. . This also corresponds to the frequency characteristics of the signal strength shown in FIG. 1C, as in the first embodiment, and the S / N ratio of the I signal and the Q signal corresponds to FIG. 1D. The frequency becomes flat.

(Example 3)
In the third embodiment, a method for correcting a difference in signal intensity in a baseband signal caused by changing a gain according to a frequency in a high-frequency amplifier (RF amplifier or IF amplifier) will be described.

FIG. 8 is a configuration example of a superheterodyne OFDM receiver according to the third embodiment. In addition, the same number as FIG. 17 is attached | subjected to the location equivalent to the structure of FIG.
The receiver 110 of FIG. 8 is configured by further providing a baseband amplifier 1100 in the baseband stage on the premise of the RF amplifier 50 of the first embodiment.

As described in the first embodiment, the RF amplifier 50 functions to amplify the OFDM signal input from the antenna 10 with a gain suitable for each subcarrier frequency.
On the other hand, the baseband amplifier 1100 makes the signal strength of the signal whose gain is set low in the RF amplifier 50 for the I and Q signals output from the quadrature demodulator higher than the signal strength of other signals. Work to amplify. In particular, as shown in FIG. 8, the inverse frequency characteristic W3 of the frequency characteristic W1 of the RF amplifier 50 is set in the baseband amplifier 1100. In this example, the center frequency in which the gain is emphasized in the frequency characteristic W1 of the RF amplifier 50 is zero frequency in the baseband. Since the gain decreases as the frequency becomes higher than the center frequency in the frequency characteristic W1 of the RF amplifier 50, the baseband amplifier has a frequency characteristic in which the signal strength increases as the frequency increases from zero frequency so as to have the opposite characteristic. Configure.

FIG. 9 is a diagram for explaining the signal strength of a signal output when the baseband amplifier 1100 has an inverse frequency characteristic of the frequency characteristic of the RF amplifier 50.
FIG. 9A shows the frequency characteristics given to the baseband amplifier. Here, the signal strength is reduced at the zero frequency emphasized in the RF amplifier 50 and in the vicinity of the zero frequency, and the signal strength is maintained otherwise. The setting is

FIGS. 9B and 9C are diagrams respectively showing the signal strengths of the input signal and the output signal of the baseband amplifier of this setting.
FIG. 9B shows a frequency characteristic that is just the opposite of the frequency characteristic shown in FIG. 9A. For this reason, in the input signal, the signal enhancement portion near the zero frequency is absorbed in the baseband amplifier, An output signal having a flat signal intensity as shown in FIG. 9C is obtained.

  In this way, when the high frequency amplifier is configured with the gain having the frequency characteristic, the frequency characteristic opposite to the previous frequency characteristic is given to the output signal as a correcting means for the output signal. Here, the A / D converter Although the baseband amplifier provided in the preceding stage is provided as the correction means, the correction means may be provided in the digital signal processing unit after A / D conversion, so that the signal intensity can be flattened in frequency. . Therefore, when the data is demodulated from the I signal and the Q signal in the digital signal processing unit, it is not necessary to change the threshold for data determination based on the signal amplitude for each carrier. Further, the signal amplitude of the subcarrier can be used as it is for the soft decision value used for decoding the error correction code.

  In this way, a constant baseband signal can be obtained together with the amplitude and S / N regardless of each subcarrier, and the demodulation operation can be simplified.

Example 4
In the fourth embodiment, an example will be described in which the correction unit is configured in a baseband stage, here in the digital signal processing unit after A / D conversion.

FIG. 10 is a block diagram of a digital signal processing unit in the fourth embodiment.
The FFT processing unit 1300 in the figure converts a baseband signal from a discrete time waveform to a discrete frequency waveform and outputs a signal of each subcarrier (such as a QPSK modulation signal or a QAM modulation signal) in parallel. Each signal output from FFT processing section 1300 is multiplied by a predetermined coefficient by multiplier 1302, and the multiplication result is converted to a serial signal and output to data demodulator 1303. The data demodulator 1303 uses a set of threshold values to determine data from the signal amplitude of the signal, and performs error correction processing or the like to demodulate the data.

Note that the coefficient of the multiplier 1302 is determined based on the frequency characteristics of the RF amplifier, for example, so that the signal intensity changed for each frequency by the RF amplifier becomes frequency flat.
In this way, since each signal output from the FFT processing unit 1300 is multiplied by an appropriate coefficient in the multiplier, the variation in the signal intensity changed for each frequency is absorbed, and a signal with a flat intensity is obtained. .

(Example 5)
In the fifth embodiment, a method for automatically calculating the gain to be set in the high frequency amplifier will be described.

FIG. 11 shows the configuration of a processing unit related to the measurement of the S / N ratio in the digital signal processing unit.
As shown in the figure, the digital signal processing unit is an FFT operation unit 1400, an S / N measurement unit 1401, a control unit 1402, an input unit 1403, an output unit 1404, and a timer as processing units for measuring the S / N ratio. 1405.

  The FFT operation unit 1400 converts the baseband signal from a discrete time waveform to a discrete frequency waveform and outputs a signal of each subcarrier (such as a QPSK modulation signal or a QAM modulation signal) to the S / N measurement unit 1401.

S / N measurement section 1401 measures the S / N for each subcarrier component based on the signal output of each subcarrier from FFT operation section 1400.
The input unit detects that the digital signal processing unit is turned on or detects a signal for starting an S / N measurement inspection and transmits an instruction signal for starting an S / N measurement to the control unit.

The output unit outputs various calculation results to a monitor or the like.
The control unit controls the entire processing unit and has an S / N automatic measurement processing function.
FIG. 12 is an operation flow of this apparatus.

  When there is an input to start S / N measurement inspection at the time of shipment from the factory, or when the digital signal processing unit is turned on during normal use, the input unit first detects this and the control unit detects S / N. A measurement start instruction signal is output (S1). When the timer is set to output an S / N measurement start instruction signal to the control unit at a predetermined time (inspection interval timing), the timer periodically starts the S / N measurement. Is output to the control unit as an instruction signal.

When the instruction signal is input, the control unit drives the S / N measurement unit to execute the S / N measurement process (S2).
The S / N measurement unit obtains the S / N of each subcarrier for each symbol based on the modulation signal of each subcarrier from the FFT operation unit, and controls the data string indicating the S / N for each subcarrier. To the output.

  When receiving the data from the S / N measurement unit as described above, the control unit outputs the information to the output unit (S3 of case 1). Alternatively, the frequency characteristic of the gain to be set in the high frequency amplifier is calculated and then output to the output unit (S3-1 and S3-2 in case 2). Note that the method of determining the frequency characteristic in Step S3-1 of Case 2 is determined so that the S / N is constant in all subcarriers. For example, the frequency of the RF band (or IF band) of the subcarrier corresponding to the peak value in the obtained S / N is obtained, and the frequency corresponding to the peak value is emphasized with reference to a predetermined frequency flat gain.

  Then, the adjuster adjusts the frequency characteristic of the high-frequency amplifier based on the result obtained from the output unit (S4). For example, a high frequency amplifier is assembled from the beginning to have the above frequency characteristics. Alternatively, if the amplifier as shown in FIG. 3A is already configured, the resonance frequency is adjusted to the frequency at which the S / N peak value is obtained by adjusting the RFC coil.

(Example 6)
In the sixth embodiment, a method for automatically setting a gain for a high-frequency amplifier will be described.

  FIG. 13 shows a configuration of a digital signal processing unit based on the configuration of the fifth embodiment shown in FIG. 11. Compared with the configuration of FIG. 11, a data demodulation unit 1600 and a data error rate measurement unit 1601 are further provided. . Further, the amplifier 1602 having a plurality of types of frequency characteristics in the RF stage or IF stage is configured to be selectively switched. The amplifier 1602 has a plurality of preset frequency characteristics (frequency characteristics (1), (2), (3),...) In response to a switching instruction signal from the output section 1404 of the digital signal processing section. One of the amplifiers is selected by a switching means such as a switch, and after switching, an RF signal or IF signal is input to the selected amplifier. Become.

The data demodulator 1600 receives the output result from the FFT processor 1400 and demodulates the transmission data there.
Data error rate measurement section 1601 calculates the data error rate for each subcarrier based on the data demodulated by data demodulation section 1600 under the control of control section 1402.

FIG. 14 is an operation flow of this apparatus.
Similar to the operation of the fifth embodiment shown in FIG. 12, first, an S / N measurement start instruction signal is input to the control unit 1402 (SS1).

  When the S / N measurement start instruction signal is input, the control unit 1402 drives either one or both of the S / N measurement unit 1401 and the data error rate measurement unit 1601 and / or S / N measurement processing and / or Data error rate measurement processing is executed (SS2).

  The S / N measurement unit 1401 obtains the S / N of each subcarrier for each symbol based on the modulation signal of each subcarrier from the FFT operation unit 1400, and generates a data string indicating the S / N for each subcarrier. To the control unit 1402. The data error rate measurement unit 1601 calculates the data error rate by demodulating binary data based on the modulation signal of each subcarrier from the FFT operation unit 1400, performing soft decision processing, and the like. Is output to the control unit.

  When the control unit receives the above-described various data from the driven S / N measurement unit and / or error rate measurement unit, the control unit calculates the frequency characteristic of the gain to be set in the high-frequency amplifier and then outputs it to the output unit (Case 1). SS3-1, SS3-2). As a method of determining the frequency characteristics based on the data error rate, the frequency error rate of all subcarriers is determined to be equal to or less than a predetermined threshold. For example, the frequency of a subcarrier having a data error rate that greatly exceeds the threshold value is obtained, and the above frequency is emphasized with reference to a predetermined frequency flat gain.

  The output unit selects an optimum amplifier from the amplifiers provided from the calculation result of the frequency characteristic of the gain, and outputs a switching signal to the amplifier to the switching unit. For example, the gain data sequence changed according to the frequency corresponding to the equipped amplifier and the gain data sequence as the calculation result are compared for each frequency, and the frequency characteristic close to the calculation result is obtained. Select the amplifier you have. Then, the switching signal to the amplifier is output to the switching means.

  Thereafter, the control unit repeatedly acquires the S / N of the S / N measurement unit or the data error rate of the data error rate measurement unit, and outputs the gain data string to the output unit. The output unit again selects an optimum amplifier from the equipped amplifiers from the calculation result, and outputs a switching signal to the amplifier to the switching means. At this time, when it is determined that the currently selected amplifier is not optimal in the output unit, switching to another optimal amplifier is performed (SS4 to SS6 in case 1).

  When the output unit is a monitor or the like, the result output from the control unit is output as it is or after calculating the frequency characteristics of the gain to be set in the high frequency amplifier (SS3 in case 2), and then the adjuster The above-mentioned frequency characteristic acquired from the output unit is given to the high-frequency amplifier (SS4 in case 2).

(Example 7)
The seventh embodiment shows an example in which the S / N deterioration of a signal near zero frequency when an OFDM signal is down-converted into a baseband signal is configured without using a high-frequency amplifier having frequency characteristics.

FIG. 15 is a configuration example of a superheterodyne OFDM receiver according to the seventh embodiment. In addition, the same number as FIG. 17 is attached | subjected to the location equivalent to the structure of FIG.
The receiver 180 of FIG. 15 is configured by providing another down-conversion circuit in addition to the down-conversion circuit 10-17 of FIG. 17 shown in the prior art.

  This separate down-conversion circuit includes a band pass filter 1800, an RF amplifier 1801, a frequency converter 1802, an IF amplifier 1803, and a quadrature demodulator 1804, and is connected to the subsequent stage of the antenna. However, here, as the frequency converter, IF amplifier, and quadrature demodulator, the frequency converter, IF amplifier, and quadrature demodulator configured in the down-conversion circuit 10-17 shown in FIG. Suppose you are. Moreover, the local oscillator which outputs a local signal to each mixer shall be used also as the mixer of the down-conversion circuit 10-17 of FIG. 17 shown in the prior art.

  As the bandpass filter, use a filter that is set to pass the signal of the frequency band measured in advance and take the zero frequency and the vicinity of the zero frequency when converted into the baseband signal, and attenuate the other signals. Yes. As the RF amplifier, an amplifier having a higher gain than the RF amplifier (low gain amplifier) of the down-conversion circuit 10-17 shown in FIG. 17 shown in the prior art is used. The gain of the high gain amplifier is determined, for example, so that the difference between the gains of the high gain amplifier and the low gain amplifier matches the difference between the thermal noise and 1 / f noise when the 1 / f noise is maximum. Yes.

    A low-pass filter 1805 is provided in the first down-conversion circuit and a high-pass filter 1806 is provided in the second down-conversion circuit to limit the band of the I and Q signals output from the respective quadrature demodulators. ing.

  Each of the two A / D converters includes a sum signal of the I signal output from the low-pass filter of the first down-conversion circuit and the I signal output from the high-pass filter of the second down-conversion circuit, The sum signal of the Q signal output from the low-pass filter of the first down-conversion circuit and the Q signal output from the high-pass filter of the second down-conversion circuit is input.

  In such a configuration, since only a signal in a certain frequency band passes through the band-pass filter 1800, even if the overall signal power increases due to the high gain of the RF amplifier, the signal is amplified in the entire frequency band of the signal received by the antenna. The overall signal power is small compared to the received signal, and the linearity of the elements in the subsequent quadrature demodulator can be maintained.

(Example 8)
In the eighth embodiment, a modification of the OFDM receiver of the seventh embodiment is shown.

  The receiver 190 of the eighth embodiment shown in FIG. 16 includes a bandpass filter 1900 as a separate down-conversion circuit in the subsequent stage of the frequency converter of the RF stage of the down-conversion circuit 10-17 shown in FIG. An IF amplifier 1901 and the like are provided.

  The band pass filter 1900 is configured to pass a zero frequency and a previously measured intermediate frequency band signal that takes zero frequency and the vicinity of the zero frequency when converted into a baseband signal, and attenuates other signals. is doing. Further, as the IF amplifier 1901, an amplifier having a gain higher than that of the IF amplifier (low gain amplifier) of the down-conversion circuit 10-17 shown in FIG. 17 shown in the prior art is used. The gain of the high gain amplifier is determined, for example, so that the difference between the gains of the high gain amplifier and the low gain amplifier matches the difference between the thermal noise and 1 / f noise when the 1 / f noise is maximum. Yes.

  The configuration shown in the first to eighth embodiments is only one example. Therefore, the OFDM receiver of the present invention may be configured not only by the above configuration but also by arbitrarily combining the configurations of the embodiments.

  As described above, according to the OFDM receiver and OFDM signal receiving method of the present invention, it is possible to reduce S / N degradation of a signal near zero frequency that passes through a device operating at a low voltage such as a CMOS device.

Further, by providing the correction means, it is possible to have substantially the same S / N in all bands, and it is possible to reduce the increase in bit error rate.
Furthermore, since a low-rate A / D converter and a simple demodulation circuit can be configured, the power consumption of the receiver is reduced.

Claims (10)

An OFDM receiver (5, 8) comprising a high-frequency amplifier (50, 80) for amplifying a received OFDM signal and down-converting the OFDM signal passed through the high-frequency amplifier (50, 80) to baseband. ,
The high-frequency amplifier (50, 80) amplifies the OFDM signal with different gains depending on the frequency of subcarriers.
An OFDM receiver. The high-frequency amplifier (50, 80) increases a gain of a signal in a frequency band that is down-converted to and near zero frequency in the OFDM signal higher than a reference gain.
The OFDM receiver according to claim 1. The high-frequency amplifier (50, 80) lowers the gain of a signal other than the zero frequency of the OFDM signal and a frequency band other than the frequency band down-converted near the zero frequency from a reference gain.
The OFDM receiver according to claim 1. In the superheterodyne receiver (5, 8),
The high-frequency amplifier (50, 80) is configured as an RF amplifier (50) of an RF stage or an IF amplifier (80) of an IF stage.
The OFDM receiver according to claim 1. The baseband stage includes correction means for correcting a signal intensity difference in each frequency band generated in the RF amplifier (50) or the IF amplifier (80).
The OFDM receiver according to claim 4. The correction means is a baseband amplifier (1100) having a frequency characteristic opposite to the frequency characteristic of the gain in the RF amplifier (50) or the IF amplifier (80).
The OFDM receiver according to claim 5. A means (1400-1405) for obtaining an S / N ratio of the OFDM signal and automatically calculating a frequency characteristic of a gain to be set in the high-frequency amplifier (50, 80) based on the S / N ratio;
The receiver according to claim 1. Means (1600-1602) for obtaining a data error rate of the OFDM signal and automatically setting a gain in the high-frequency amplifier (50, 80) based on the data error rate;
The receiver according to claim 1. An OFDM receiver (180, 190) comprising a high-frequency amplifier for amplifying a received OFDM signal and a down-conversion circuit for down-converting the OFDM signal passed through the high-frequency amplifier to a baseband,
With two down conversion circuits,
The first high-frequency amplifier (1801, 1901) on one side of the two systems of down-conversion circuits has a higher gain than the second high-frequency amplifier (12, 15) on the other side, and
A band-pass filter (1800, 1900) that passes a received OFDM signal in a frequency band that takes zero frequency or near zero frequency when down-converted to baseband is provided in front of the first high-frequency amplifier (1801, 1901). Prepare
An OFDM receiver. A method for receiving an OFDM signal including a subcarrier component whose S / N depends on a frequency,
Amplifying the OFDM signal with a different gain according to the frequency of the subcarrier in a high frequency amplifier,
Downconvert the amplified signal to a baseband signal;
An OFDM signal receiving method characterized by the above.

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