JP2005117394A - Automatic gain control circuit of radio communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an automatic gain control circuit for preventing a burst signal from being erroneously detected. <P>SOLUTION: This automatic gain control circuit is provided with: a variable gain amplifier circuit part 16 which can independently control each of the gains of a received signal divided into I and Q components; receiving power calculating parts 25a and 25b for calculating the received power of each of the I and Q components obtained by applying A/D conversion to the I and Q components; a gain control circuit part 26 for calculating a gain on the basis of the calculated received power and feeding the gain back to the variable gain amplifier circuit part; a delaying part 28 for delaying the I and Q components; and a burst signal detecting part 29 for applying correlation processing to the delayed I and Q components to perform burst detection. Predetermined gains to the I and Q components are set in the variable gain amplifier circuit part 16 in advance, and when the burst signal detecting part 29 detects burst signals of the I and Q components when a radio signal is received, a correction gain is calculated on the basis of the power value of the burst signal and is fed back to the variable gain amplifier circuit part 16. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an automatic gain control circuit applied to a receiver or the like of a radio communication system, and in particular, is modulated by, for example, an orthogonal frequency division multiplexing (OFDM) modulation method, and is added to the head of this modulated packet signal. The present invention relates to an automatic gain control circuit applied to a radio communication system or the like that receives a radio signal to which a burst signal including a preamble signal is added.

  In recent years, transmission systems using orthogonal frequency division multiplexing (OFDM) technology are used in mobile digital audio broadcasting and digital terrestrial television broadcasting. OFDM is one of the multi-carrier modulation schemes in which a plurality of digital modulation waves are added. Since data is distributed and sent over many carriers, the duration of one symbol is long, and the guard interval on the time axis. Because of the addition of, there is little deterioration in characteristics even in a multipath environment, and it is resistant to frequency selective fading by combining an appropriate error correction code and interleaving on the frequency axis, and the signal waveform is close to random noise It has features such as being less susceptible to interference with other services and having relatively good bandwidth utilization efficiency. Taking advantage of these features, it has come to be used for data transmission such as wireless LAN. For example, a 5 GHz band wireless LAN system employs an OFDM modulation method in order to realize excellent communication performance over a wide band.

In a wireless communication system employing such an OFDM modulation scheme, the transmission side performs serial / parallel conversion of transmission data and performs inverse high-speed discrete Fourier transform (IFFT) to collectively modulate a large number of orthogonal subcarriers. . A burst signal, which is a training signal for synchronization called a preamble signal, is added to the head of the modulated signal subjected to IFFT processing in this way for transmission.
In order to suppress the fluctuation level of the received signal in the transmission path, it is conceivable to use an automatic gain control circuit on the input stage side of the demodulator. However, in order to converge the automatic gain control circuit, a preamble signal is required. Become.

On the other hand, on the receiving side, an automatic gain control circuit, frequency offset correction, FFT (Fast Fourier Transform) timing generation, and the like are performed using this preamble signal, and FFT calculation is performed based on the generated FFT timing. Is called.
Since the OFDM modulated signal is considered as a composite signal of a plurality of modulated waves, the ratio of the peak amplitude to the average amplitude is large and the amplitude fluctuation is large. In a digital demodulator, since the dynamic range of an A / D converter that performs analog / digital (A / D) conversion is limited, a digital signal that is A / D converted beyond this dynamic range includes signal distortion. Will be output in the form.

  Normally, in a wireless communication system such as a wireless LAN, a user does not always transmit and receive signals at the same reception level. Different levels of signals are received for each user, such as the distance from the transmitting antenna is short or the distance is long. Therefore, suppressing the level of the received signal within the dynamic range of the A / D converter is an essential technique for the radio communication system. In the demodulator, this operation is performed by the automatic gain control circuit, and the automatic gain control circuit is also essential in the OFDM demodulator.

It is also necessary to determine whether the received in-band signal is a burst signal. This is because only the burst signal is subjected to reception processing after the automatic gain control circuit, and no reception processing is performed except for the burst signal. Therefore, burst signal detection is also indispensable.
The automatic gain control circuit controls the amplification gain based on the reception level of the preamble signal. However, in wireless packet communication such as a wireless LAN, if the preamble signal is long, the throughput of the wireless section is reduced, so it is necessary to control the AGC circuit. It is desirable that the length of the preamble signal is short. Therefore, in the 5 GHz band wireless LAN system, it is necessary to perform level adjustment, signal detection, and synchronization detection within a preamble signal of 10 to 20 μs.

  In the AGC control method, for example, when an in-band signal is detected using an RSSI signal from the RF unit, the signal power is calculated from the AD input signal, and the gain is maximized in the first burst signal waiting state. It is conceivable that the AGC gain is gradually reduced until it falls within the AD dynamic range with reference to the signal power value.

  In addition, burst detection must be performed simultaneously with the control processing of the automatic gain control circuit. As a method of detecting this burst, the preamble signal is correlated with a signal after a predetermined time, and the burst is detected by detecting the peak of the correlation value. A method of detecting the error can be considered.

  As these conventional technologies, as shown in FIG. 4, an automatic gain control amplification unit (AGCAMP) 101, an A / D converter (ADC) 102, an OFDM demodulation unit (DEMOD) 103, a delay unit (DLY) 104, burst detection Unit (BDT) 105, packet detection unit (PDT) 106, and amplification gain control unit (AGCTL) 107.

  In demodulation apparatus 10 having such a configuration, OFDM reception signal RS received by an antenna (not shown) is input to automatic gain control amplification section 101. In the automatic gain control amplification unit 101, the reception signal RS is subjected to automatic gain control and output to the A / D converter 102 as an optimum signal level. The automatic gain control amplification unit 101 controls the case where automatic gain control is performed and the case where the control gain is fixed based on a control signal S107 from the amplification gain control unit 107. In the A / D converter 102, the input received signal is converted from an analog signal to a digital signal, and the digital received signal S102 is output to the OFDM demodulator 103, the delay unit 104, and the burst detector 105. (For example, Patent Document 1).

On the other hand, when the burst detection start signal is received, a gain control signal is output to the automatic gain control amplification unit so as to amplify with the maximum value, and when the burst detection unit receives the first burst synchronization detection signal, the received signal power observation unit The second gain is calculated based on the received signal power value detected in step (a), the gain control signal is output to the automatic gain control amplifier so as to amplify with the second gain, and the digital signal amplified with the second gain is amplified. The received signal is integrated to obtain a received signal power value, and when the second burst synchronization detection signal is received by the burst detector, a third gain is calculated based on the obtained received signal power value. There is provided an amplification gain control unit that outputs a gain control signal to an automatic gain control amplification unit (for example, Patent Document 2).
Japanese Patent Laid-Open No. 11-205278 JP 2003-8676 A

However, in the conventional technique as described above, since the gain is set to the maximum at the start of burst detection, the output signal of the A / D converter is distorted.
An ideal preamble signal that is not affected by noise or fading can detect bursts by correlation processing even if the signal is distorted by clipping, but in general such an ideal situation If there is an influence of noise or the like, the correlation value does not increase if the maximum peak is distorted by clipping.
SUMMARY OF THE INVENTION An object of the present invention is to provide an automatic gain control circuit for a wireless communication system that prevents erroneous detection of a burst signal due to noise or fading.

The invention according to claim 1 is an automatic gain control circuit of a radio communication system that performs gain control of a received signal in which a burst signal including at least a preamble signal is added to a head portion.
An IQ separation circuit for separating the received signal into an I component and a Q component;
A gain variable amplification circuit unit capable of independently controlling gains for the I component and the Q component after separation by the IQ separation circuit unit;
A received power calculation unit that A / D-converts the I component and the Q component amplified by the variable gain amplifier circuit unit, and calculates respective received power from the converted I component and Q component digital signals. When,
A gain control circuit unit that calculates a gain based on the calculated received power and feeds back to the variable gain amplifier circuit unit;
A delay unit that delays the digital signal of the I component and the Q component for a certain time;
An automatic gain control circuit comprising a burst signal detection unit for performing burst detection by correlating digital signals of I component and Q component delayed by a predetermined time by the delay unit,
Predetermined gains for the I component and the Q component are set in advance in the variable gain amplification circuit unit, the received signal is received, and the burst signal detection unit detects the burst signal of the I component or the Q component. When the burst signal is detected, a correction gain is calculated based on the power value of the I component or the Q component in which the burst signal is detected, and the correction gain is fed back to the variable gain amplification circuit unit. .

According to the first aspect of the present invention, in the automatic gain control circuit, predetermined gains for the I component and the Q component are respectively set in advance in the variable gain amplification circuit unit, and the received signal is received and the burst signal detection unit performs When the burst signal of the component or Q component is detected, the correction gain is calculated based on the power value of the I component or Q component from which the burst signal is detected, and the correction gain is fed back to the variable gain amplification circuit unit. Therefore, by setting preset gains for the I component and the Q component as different values in the gain variable amplification circuit unit in advance, the gain is set to the maximum without being separated into the I component and the Q component at the start of burst detection. Thus, the output signal of the A / D converter is not distorted. In addition, since the scheduled gains for the I component and the Q component are preset as different values in the variable gain amplification circuit unit, the maximum peak of the preamble signal affected by noise and fading is not distorted by clipping.
Therefore, it is possible to provide an automatic gain control circuit of a wireless communication system that prevents erroneous detection of a burst signal due to noise or fading.

  According to a second aspect of the present invention, in the automatic gain control circuit according to the first aspect, the scheduled gain of the I component and the scheduled gain of the Q component are set differently.

  According to the second aspect of the present invention, the expected gain of the I component and the expected gain of the Q component are set different from each other, so that the above action can be performed more clearly.

  According to a third aspect of the present invention, in the automatic gain control circuit according to the second aspect of the invention, the burst signal detection unit has the power value of the smaller number of clips of the I component and the Q component within a predetermined time. And calculating the corrected gain.

  According to the invention of claim 3, since the correction gain is calculated based on the power value of the smaller number of clips of the I component and the Q component within a predetermined time in the burst signal detection unit, the influence of distortion due to the clip. And the correction gain can be calculated more accurately.

According to the first aspect of the present invention, in the automatic gain control circuit, the predetermined gains for the I component and the Q component are respectively set in advance in the variable gain amplification circuit unit, and the received signal is received and the burst signal detection unit is set. When the burst signal of the I component or Q component is detected, the correction gain is calculated based on the power value of the I component or Q component from which the burst signal is detected, and the correction gain is supplied to the variable gain amplifier circuit unit. Since feedback is performed, the gain for the I component and the Q component is preset as different values in the variable gain amplification circuit unit, so that the gain is set to the maximum without being separated into the I component and the Q component at the start of burst detection. The output signal of the A / D converter is not distorted as in the prior art. In addition, since the scheduled gains for the I component and the Q component are preset as different values in the variable gain amplification circuit unit, the maximum peak of the preamble signal affected by noise and fading is not distorted by clipping.
Therefore, it is possible to provide an automatic gain control circuit of a wireless communication system that prevents erroneous detection of a burst signal due to noise or fading.
In addition, by providing separate gains for the I component and the Q component only in the first stage of the signal for detecting the burst signal, it is possible to detect the burst signal with less influence of waveform distortion due to the AD dynamic range, Thereby, burst signal detection errors due to noise and fading can be prevented.

  According to the second aspect of the present invention, the scheduled gain of the I component and the scheduled gain of the Q component are set differently, so that the above action can be performed more clearly.

  According to the third aspect of the present invention, since the correction gain is calculated based on the power value of the smaller number of clips of the I component and the Q component within a predetermined time in the burst signal detection unit, the distortion is caused by the clip. The correction gain can be calculated more accurately.

Hereinafter, an automatic gain control circuit used in an OFDM modulation type radio communication system according to the present invention will be described with reference to embodiments.
FIG. 1 is a control block diagram of an automatic gain control circuit showing an example of the present invention. Since the configuration other than the variable gain amplification circuit unit and the control unit, which will be described later, is a well-known configuration conventionally used in an automatic gain control circuit, the description of the configuration will be simplified and only the portion related to the present invention will be described in detail. .

  The automatic gain control circuit according to the present invention includes an antenna 11 and a transmission / reception changeover switch 21 and, on the receiving side, an RF unit includes a band pass filter (hereinafter referred to as “BPF”) 12, a Low Noise Amp (hereinafter referred to as “ LNA ”) 13, mixer 14, etc., IF section includes BPF 12, IQ separation circuit section 15, etc., and baseband includes variable gain amplification circuit section 16, two low-pass filters (hereinafter“ LPF ”). ") 17a, 17b, a control unit 18 and the like. On the transmission side, the baseband includes two LPFs 17, the IF unit includes a mixer 14, a transmission power control amplifier 19, a BPF 12, and the like, and the RF unit includes a mixer 14, a BPF 12, an amplifier 20, and the like.

  The IQ separation circuit unit 15 further includes a mixer 14, a transmitter 22, and the like. The variable gain amplification circuit unit 16 has 2 components corresponding to the I component and the Q component separated by the IQ separation circuit unit 15. Two gain control amplifiers 23a and 23b are provided, and LPFs 17a and 17b are provided on the downstream side. Then, two A / D converters 24a and 24b corresponding to the I component and the Q component, received power calculation units 25a and 25b, a gain control circuit unit 26, a delay unit 28, a burst signal are provided on the reception side in the control unit 18. A detection unit 29, a demodulation processing unit 30 and the like are provided, and a modulation processing unit 31 and two D / A converters 27a and 27b are provided on the transmission side. A feedback circuit 32 is provided from the gain control circuit unit 26 toward the variable gain amplification circuit unit 16.

  That is, in the automatic gain control circuit of this example, the IQ separation circuit unit 15 that separates the received signal from the antenna 11 into the I component and the Q component, and the I component after being separated by the IQ separation circuit unit 15 Gain variable amplification circuit section 16 that can control the gain of each Q component independently, A / D conversion of each of the I component and Q component, and each received power from the converted I component and Q component digital signals And a gain control circuit unit 26 that calculates a gain based on the received power calculated thereby and feeds back the gain to the variable gain amplifier circuit unit 16 via the feedback circuit 32. A delay unit 28 that delays the digital signals of the I component and the Q component for a fixed time, and the digital signals of the I component and the Q component that are delayed by the delay unit 28 for a fixed time Comprising a burst signal detector 29 for performing burst detection and correlation process.

Next, the reception operation of the automatic gain control circuit of this example will be described.
The radio signal received by the antenna 11 is sent to the mixer 14 via the two BPFs 12 of the RF unit and the LNA 13 between them, and is down-converted to the intermediate frequency band (IF unit) by the mixer 14. Then, the signal is converted into a preamble signal separated into an I component and a Q component by the IQ separation circuit unit 15 via the BPF 12 of the IF unit.

  The preamble signals separated into the I component and the Q component are amplified by the respective gain control amplifiers 23. Here, as initial values, for example, a maximum gain value for the I component and a minimum gain value for the Q component are preset as scheduled gains, and in this state, reception of a reception signal is awaited. When the preamble signal at the head of the received signal is input to the variable gain amplifier circuit 16 in a state where the preamble signal is separated into an I component and a Q component, the preamble signal is amplified with different gains for the I component and the Q component. The preamble signals of the I and Q components digitally converted by the A / D converters 24a and 24b via the LPFs 17a and 17b are sent to the power calculation unit 25 having the I component and the Q component, respectively. The signal is sent to the delay unit 28 and the burst signal detection unit 29. The received power calculation units 25a and 25b obtain the power calculation and maximum waveform value of each of the I component and Q component, and confirm the influence of distortion due to the A / D dynamic range. Then, it is determined which of the I component and the Q component is less affected by the distortion due to the A / D dynamic range, and the component determined to be less affected by the distortion is used for the subsequent gain for the variable gain amplifier circuit unit 16.

  Here, when the signal strength of the received radio wave is strong, the I component signal is amplified by the gain control amplifier 23 set to the maximum gain and is digitally converted by the A / D converter 24a. Clips beyond the range. In this case, the burst signal detection unit 29 may fail to detect the preamble signal because of the distorted I signal, and may not be able to accurately detect the synchronization timing based on the correlation value.

On the other hand, the Q component signal is amplified by the gain control amplifier 23 set to the minimum gain and is digitally converted by the A / D converter 24b, so that it is input within the input range of the A / D converter 24b. Therefore, since it is not affected by nonlinear distortion due to clipping, the preamble correlation is strong, the detection accuracy in burst signal detection is increased, and the synchronization timing detection accuracy is improved.
In this example, the I component signal is set to the maximum gain and the Q component signal is set to the minimum gain. However, the present invention is not limited to this, and the I component scheduled gain and the Q component Any setting may be used as long as the planned gain is set differently.

Further, when the signal strength of the received radio wave is small, contrary to the case where the signal strength of the received radio wave is strong, the I component preamble signal after A / D conversion is easily clipped because there is no clip, but the Q component is easy to detect. This preamble signal has a poor S / N ratio, which adversely affects correlation detection.
Here, the maximum gain value and the minimum gain value are determined from, for example, the maximum received power and the minimum received power standardized in the standard document, and the input ranges of the A / D converters 24a and 24b.

  The burst signal detector 29 selects and detects one of the input I-component and Q-component preamble signals with less distortion by a method described later. In the signal detection processing, for example, correlation processing with a delayed signal is performed, and signal detection is performed when the correlation value becomes a certain threshold value or more. If both the I-component and Q-component preamble signals are not distorted, the higher received power, that is, the better S / N ratio is selected and detected.

At the same time, when the burst signal is detected by correlation processing and it is determined that the burst signal is detected, the second gain is obtained from the power value of the first gain from the received power of the selected I component and Q component, and the gain The gain in the control amplifier 23 is determined, and the second gain is set in the variable gain amplification circuit unit 16 via the feedback circuit 32. At this time, the second gain uses the same gain for both the I component and the Q component. Further, since both the I component and the Q component are amplified at the same level and A / D input, only the power value of the gain is calculated, the third gain is calculated, and the third gain is obtained via the feedback circuit 32. Is set in the variable gain amplifier circuit section 16.
The flow for determining the optimum gain in this way is repeated a predetermined number of times between the control unit 18 and the variable gain amplification circuit unit 16 via the feedback circuit 32 to determine the optimum gain. That is, predetermined gains for the I component and the Q component are set in advance in the variable gain amplification circuit unit 16, respectively, and the received signal is received and the burst signal detection unit 29 detects the burst signal of the I component or the Q component. Sometimes, the correction gain is calculated based on the power value of the I component or Q component from which the burst signal is detected, and the correction gain is fed back to the variable gain amplification circuit unit 16.

Here, a description will be given of a method in which the burst signal detection unit 29 selects the one with less distortion among the preamble signals of the I and Q components converted by the A / D converters 24a and 24b.
When the A / D converted preamble signal of either I or Q component reaches the input range, the waveform is clipped, so it can be easily determined, but both the I and Q components are clipped. If so, the following criteria are required.

That is, in the above example, it is assumed that the received signal has a standardized maximum input and a minimum input. For example, in the 802.11a standard, the maximum input power assumed at the end of the antenna 11 is −30 dBm, It is specified that the minimum input power guarantees performance at -82 dBm (minimum).
Although depending on how the initial gain values of the I component and the Q component are set, when the received power is intermediate between the maximum input power and the minimum input power, both are waveforms after AD conversion. Can be clipped.

Here, as shown in FIGS. 2A and 2B, when the preamble signal is 802.11a, only the phase of the I component and the Q component is shifted by a repetitive signal having the same wavelength of 0.8 μsec.
Therefore, when both the I component and the Q component are clipped, when determining the influence of distortion, the ratio of the data indicating the maximum and minimum clip ranges of the preamble signals of the converted I component and Q component, respectively. Decide.

  That is, as shown in FIGS. 3A and 3B, the portions indicating the maximum and minimum ranges are included in the preamble signal waveforms of the A / D converted I component and Q component input at a predetermined time t. Let's determine how much is contained. In this example, with respect to the preamble signals of the I and Q components of the waveforms shown in FIGS. 2A and 2B, the gain variable amplification circuit unit 16 amplifies +40 dB into the I component and the Q component. In addition, an amplification of +20 dB was performed, and two wavelengths of 0.8 μsec wavelength of I component and Q component were sampled, and a predetermined time t = 1.6 μsec. Then, the number of clip data of the I component and the number of clip data of the Q component in the range of t are compared, and it is determined that the Q component has less distortion, and the gain of the Q component is adopted. In other words, the burst signal detector 29 calculates the correction gain based on the power value of the smaller number of clips of the I component and the Q component within a predetermined time.

It is a control block diagram which shows an example of the automatic gain control circuit of the radio | wireless communications system of this invention. (A) is a figure which shows the waveform of I component of a preamble signal, (b) is a figure which shows the waveform of Q component of a preamble signal. (A) is the figure which added + 40dB amplification to the waveform of I component of a preamble signal, (b) is the figure which added + 20dB amplification to the waveform of Q component of a preamble signal. It is a control block diagram of the automatic gain control circuit of the conventional radio | wireless communications system.

Explanation of symbols

11 Antenna 12 Band pass filter (BPF)
13 Low Noise AMP (LNA)
14 mixer 15 IQ separation circuit 16 variable gain amplification circuit 17a, 17b low pass filter (LPF)
DESCRIPTION OF SYMBOLS 18 Control part 19 Transmission power control amplifier 20 Amplifier 21 Switch 22 Transmitter 23 Gain control amplifier 24a, 24b A / D converter 25a, 25b Reception power calculation part 26 Gain control circuit part 27a, 27b D / A converter 28 Delay part 29 Burst Signal detection unit 30 Demodulation processing unit 31 Modulation processing unit 32 Feedback circuit

Claims (3)

In an automatic gain control circuit of a radio communication system that performs gain control of a reception signal to which a burst signal including at least a preamble signal is added at the beginning,
An IQ separation circuit for separating the received signal into an I component and a Q component;
A gain variable amplification circuit unit that can independently control gains for the I component and the Q component after separation by the IQ separation circuit unit;
A received power calculation unit that performs A / D conversion on the I component and the Q component amplified by the variable gain amplification circuit unit, and calculates respective received power from the converted digital signals of the I component and the Q component. When,
A gain control circuit unit that calculates a gain based on the calculated received power and feeds back to the variable gain amplifier circuit unit;
A delay unit that delays the digital signal of the I component and the Q component for a certain time;
An automatic gain control circuit comprising a burst signal detector for performing burst detection by correlating digital signals of I component and Q component delayed for a certain time by the delay unit,
Predetermined gains for the I component and the Q component are set in advance in the variable gain amplification circuit unit, the received signal is received, and the burst signal detection unit detects the burst signal of the I component or the Q component. When a correction is made, a correction gain is calculated based on the power value of the I component or the Q component in which the burst signal is detected, and the correction gain is fed back to the variable gain amplification circuit unit. Automatic gain control circuit.   2. The automatic gain control circuit according to claim 1, wherein the scheduled gain of the I component and the scheduled gain of the Q component are set differently. 3. The correction gain according to claim 2, wherein the burst signal detection unit calculates the correction gain based on the power value of the smaller number of clips of the I component and the Q component within a predetermined time. Automatic gain control circuit.

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