JP2007036639A - Ofdm demodulator and automatic gain control method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve amplitude variation tolerance in a whole burst without impairing throughput. <P>SOLUTION: In an OFDM (orthogonal frequency division multiplex) demodulator, an automatic gain control amplification means 1 performs gain control so that an output signal level becomes constant according to a level of an OFDM receiving signal. An OFDM signal power average means 7 averages an output signal of the automatic gain control amplification means for a long time, and obtains an AGC control value from the difference from a target. An automatic gain control amplification means control means 4 controls the automatic gain amplification means by an OFDM receiving signal, output of a burst detection means 3, an output of a packet signal detection means 6, and an output of the OFDM signal power average means; and composes an AGC circuit in which response calculating a AGC control value from a preamble for AGC in the OFDM receiving signal is fast, and an AGC circuit in which response to add an AGC control value obtained by the OFDM signal power average means to the calculated AGC control value is low. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an OFDM demodulator and an automatic gain control method, and more particularly to automatic gain control (AGC) for controlling the level of an OFDM received signal in an OFDM demodulator used in a digital radio communication system.

  Since OFDM modulation performs modulation such as QPSK on a plurality of subcarriers, the ratio between the average voltage value and the peak voltage value of the OFDM modulation signal is large. For this reason, in order to obtain the average voltage or average power of the OFDM modulation signal required for automatic gain control, it is necessary to take an average over a long interval. On the other hand, in wireless LAN and the like, it is necessary to reduce the useless bits such as the AGC preamble as much as possible in order to improve the throughput.

  Conventionally, this type of automatic gain control is performed by an AGC circuit that averages the AGC preamble signal in a short period of time in order to enable good reception signal gain control with a short AGC preamble signal. The AGC control value is fixed or stopped by the output of the burst detection means and the packet signal detection means (for example, see Patent Document 1). Specifically, when the AGC circuit is operated only for the AGC preamble signal portion and the packet signal is detected after detecting the burst, the OFDM data section continues to fix the gain obtained in the AGC preamble signal portion, and the packet signal Is not detected or the gain fixing is stopped when the end time of the packet signal is reached.

Japanese Patent Laid-Open No. 11-205278 (page 6, FIGS. 1 and 2)

  However, in the above-described conventional technique, since the AGC control value is obtained and fixed only at the beginning of the burst, it does not follow the amplitude fluctuation generated in the OFDM modulation wave portion of the OFDM data section at all, and as a result, particularly the burst When the length is long, the characteristics are remarkably deteriorated. That is, since this automatic gain control circuit includes only an AGC circuit that averages in a short time, a correct AGC control value cannot be obtained in the OFDM modulated wave portion due to the influence of a high peak voltage. If the AGC control gain is not optimal, saturation in the time domain, EVM degradation due to increased truncation, degradation due to non-optimal soft decision threshold, etc. occur, and throughput characteristics deteriorate.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an automatic gain control circuit and an automatic gain control method in which the amplitude fluctuation tolerance is improved and the throughput is improved by correcting the amplitude fluctuation generated in the OFDM modulation wave portion. .

  In order to achieve the above object, an OFDM demodulator according to the present invention comprises automatic gain control amplification means (1 in FIG. 1) for performing gain control so that the output signal level becomes constant according to the level of the input OFDM reception signal. A delay means for delaying the output signal of the automatic gain control amplifying means for a predetermined time (2 in FIG. 1), and a burst detection for performing a correlation operation between the output signal of the automatic gain control amplifying means and the output signal of the delay means to detect a burst. Means (3 in FIG. 1), an OFDM demodulating means (5 in FIG. 1) for demodulating an OFDM signal by subjecting the output signal of the automatic gain control amplifying means to discrete Fourier transform based on the output signal of the burst detecting means, and OFDM The packet signal detection means (6 in FIG. 1) for receiving the packet signal and detecting the end time of the packet signal from the output signal of the demodulation means and the output signal of the automatic gain control amplification means are averaged over a long period of time, and the difference from the target is obtained. AGC OFDM signal power averaging means for calculating the control value (7 in FIG. 1), and automatic gain amplifying means controlled by the OFDM reception signal, the output of the burst detection means, the output of the packet signal detection means and the output of the OFDM signal power averaging means A gain control amplifying means control means (4 in FIG. 1) is provided, and the automatic gain control amplifying means control means is an AGC circuit that calculates the AGC control value from the AGC preamble portion in the OFDM received signal, or a calculated AGC circuit. An AGC circuit having a slow response is configured by adding the AGC control value obtained by the OFDM signal power averaging means to the control value.

  The automatic gain control amplification means control means (4 in FIG. 1) is a control circuit (40 in FIG. 5) for calculating the AGC control value from the AGC preamble portion in the OFDM received signal at the burst detection timing, and output from the control circuit Flip-flop (41 in FIG. 5) that holds the AGC control value to be controlled by control of the control circuit, and an adder (in FIG. 5) that adds the AGC control value held by the flip-flop and the output of the OFDM signal power averaging means 43) and a selector (42 in FIG. 5) that outputs either the output of the flip-flop or the output of the adder to the automatic gain control amplification means as an AGC control value in response to the select signal from the control circuit. The

  The control circuit switches the input to the selector from the output side of the flip-flop from the output side of the flip-flop by a select signal after a lapse of a certain time from the detection of the burst signal, and transmits the transmission packet based on the number of transmission bytes and transmission rate information obtained from the packet signal. The total length may be obtained, and the polarity of the selector and the value held in the flip-flop may be initialized at the end of the packet.

  The automatic gain control method of the present invention uses the OFDM demodulator described above, and two types of AGC circuits having different response speeds and burst detection means start operating simultaneously before the OFDM reception signal standby is started, and the arrival of the OFDM reception signal. When the burst detection means detects the burst later, the gain of the automatic gain control amplification means is set to the AGC control value obtained by the AGC circuit with a fast response. If the packet signal cannot be detected, the AGC processing is reset and the AGC operation and the packet signal are performed again. When a packet signal is detected, the automatic gain control amplification means gains with the AGC control value obtained by the AGC circuit with a slow response when a sufficiently convergent time has elapsed even with an AGC circuit with a slow response. And when the time longer than the packet length elapses, the AGC processing is reset and the AGC operation and the packet signal detection are performed again.

  According to the present invention, an AGC circuit that responds quickly but is not smoothed is applied to the preamble part of the OFDM received signal, and the response is slow but sufficiently smoothed to the data part of the OFDM received signal. The gain control of the amplification means follows the amplitude fluctuations that occurred in the data part of the OFDM received signal because the two types of AGC circuits with different convergence times are used separately, such as applying a generalized AGC circuit. As a result, it is possible to obtain the effect that the amplitude fluctuation tolerance of the entire burst is improved without impairing the throughput of the OFDM demodulator.

  An OFDM demodulator according to the present invention includes an automatic gain control amplification unit that performs gain control so that an output signal level becomes constant according to the level of an input OFDM reception signal, and delays an output signal of the automatic gain control amplification unit for a certain period of time. A delay means, a burst detection means for detecting a burst by performing a correlation operation between the output signal of the automatic gain control amplification means and the output signal of the delay means, and an output signal of the automatic gain control amplification means based on the output signal of the burst detection means An OFDM demodulator for demodulating an OFDM signal by discrete Fourier transform; a packet signal detector for receiving a packet signal and detecting an end time of the packet signal from an output signal of the OFDM demodulator; and an automatic gain control amplifying unit. An OFDM signal power averaging means that averages the output signal for a long time and obtains the AGC control value by the difference from the target, OFDM received signal, burst detection means output, packet signal detection An automatic gain control amplification means control means for controlling the automatic gain amplification means by the output of the output means and the output of the OFDM signal power averaging means is provided.

  Then, the automatic gain control amplification means control means is an AGC circuit having a quick response for calculating an AGC control value from the AGC preamble portion in the OFDM received signal, or the AGC obtained by the OFDM signal power averaging means for the calculated AGC control value. An AGC circuit with a slow response for adding control values is configured.

  Two types of AGC circuits with different response speeds and burst detection means start operating simultaneously before starting the reception of OFDM reception signals. When the burst detecting means detects a burst after the arrival of the OFDM reception signal, the gain of the automatic gain control amplifying means is set to the AGC control value obtained by the AGC circuit having a quick response.

  If the packet signal cannot be detected, the AGC process is reset and the AGC operation and the packet signal are detected again. When a packet signal is detected, even if it is an AGC circuit with a slow response, when a sufficiently converging time has elapsed, the gain of the automatic gain control amplification means is controlled with the AGC control value obtained by the AGC circuit with a slow response. When the time longer than the packet length elapses, the AGC processing is reset, and AGC calculation and packet signal detection are performed again.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an example of an OFDM demodulator according to the present invention.

  This OFDM demodulator is implemented with two types of AGC circuits that respond at high speed but are not smoothed, and AGC circuits that are slow in response but smoothed sufficiently. The side that responds fast is adjusted so that it converges in about 1 OFDM symbol time using the AGC preamble, and the one that responds slower is not the AGC preamble, but converges even with an OFDM signal with a large ratio of amplitude average value to peak voltage. In this way, the circuit takes an average amplitude over several tens of OFDM symbols.

  An AGC circuit with a fast response is fast to detect, but has the characteristic of generating errors by following large amplitude fluctuations, such as an OFDM signal. On the other hand, if the response is slow, the response is slow but well averaged. Even signals with large amplitude fluctuations, such as OFDM signals, can be controlled correctly.

  This OFDM demodulator has automatic gain control amplification means 1, delay means 2, burst detection means 3, automatic gain control amplification means control means 4, OFDM demodulation means 5, packet signal detection means 6 and OFDM signal power averaging means 7. .

  The automatic gain control amplification means 1 amplifies the incoming OFDM reception signal a1, and the amplification degree is controlled by the AGC control signal a8 from the automatic gain control amplification means control means 4. The structure of the OFDM received signal a1 consists of an AGC preamble suitable for AGC calculation, followed by a synchronization preamble that repeats an OFDM modulated wave for a certain fixed pattern twice, followed by a unique word, the number of transmitted bytes, and transmission rate information. It is assumed that it is composed of an OFDM-modulated header portion including information on equal packets and a data portion. The amplification means output a2 is output to the delay means 2, the burst detection means 3, the OFDM demodulation means 5, and the OFDM signal power averaging means 7.

  The configuration of the delay means 2 and the burst detection means 3 is shown in FIG. The delay means 2 includes a shift register 21 that uses the fact that the synchronization preamble in the amplifying means output a2 is continuously transmitted twice at a constant period and delays the amplifying means output a2 by one period. . The burst detection means 3 performs burst detection of the OFDM reception signal a1 from the amplification means output a2 and the delay means output a3. That is, the correlator 31 of the burst detection means 3 correlates the amplification means output a2 and the delay means output a3, the power converter 32 converts the correlator output into electric power, and the comparator 33 outputs the power converter output. By comparing with a predetermined threshold value a34, a burst detection signal a4 is output.

  A circuit configuration example of the OFDM demodulating means 5 is shown in FIG. In the OFDM demodulating means 5, the window control circuit 50 controls the FFT window using the burst detection signal a4 as a trigger. The shift register 51 delays the amplifying means output a2 and matches the phase of the signal (output a50 of the window control circuit 50) and the control signal (amplifying means output a2), thereby reducing the processing delay caused by the FFT Window control or the like. To compensate.

  The guard interval removal circuit 52 removes the guard interval added to the OFDM reception signal a1 by the output a50, and after the serial / parallel conversion by the serial / parallel conversion circuit 53, the FFT circuit 54 performs the Fourier transform. The FFT output signal a54 is processed by the subcarrier demodulation circuit 55 for correcting phase rotation or the like generated for each subcarrier, parallel-serial converted by the parallel-serial conversion circuit 56, and output demodulated data a5.

  The packet signal detection means 6 detects the packet from the header in the burst based on the packet information included in the demodulated data a5. At the time of packet detection, a packet detection signal a6 (including the number of transmission bytes and transmission rate information) is output to the OFDM signal power averaging means 7 and the automatic gain control amplification means control means 4.

  The OFDM signal power averaging means 7 averages the power of the OFDM modulated signal, and comprises a square circuit 71, a moving average filter 72, and a subtractor 73 as shown in FIG. The square circuit 71 averages the amplification means output a2, and the moving average filter 72 adjusts the time constant so that an OFDM modulated signal having a large amplitude fluctuation is sufficiently averaged. The moving average filter 72 is reset by the packet detection signal a6. The subtracter 73 subtracts the filter output a72 and the convergence target value a74, and the subtraction result is output to the automatic gain control amplification means control means 4 as the power average signal a7.

  The automatic gain control amplification means control means 4 generates an AGC control signal a8 from the packet detection signal a6, the burst detection signal a4, the power average signal a7 and the OFDM reception signal a1, and outputs it to the automatic gain control amplification means 1. The configuration is as shown in FIG. 5 and comprises a control circuit 40, a flip-flop (F / F) 41, a selector 42 and an adder 43. The side that obtains the AGC control signal a8 by the control circuit 40 constitutes an AGC circuit having a quick response, and the side that obtains the AGC control signal a8 by the power average signal a7 constitutes an AGC circuit having a slow response.

  The control circuit 40 calculates the AGC control gain from the AGC preamble portion in the OFDM reception signal a1 at the detection timing of the burst detection signal a4, and controls the AGC control gain to be held by the F / F 41. In addition, the control circuit 40 adds the input to the selector 42 from the F / F output a41 side to the AGC operation in order to reflect the operation result obtained by the AGC circuit with a slow response after the lapse of a certain time from the burst detection signal a4. Switch to the output a42 side.

  Further, the control circuit 40 obtains the total length of the transmission packet from the number of transmission bytes obtained from the packet detection signal a4 and the transmission rate information, and initializes the polarity of the selector 42 and the value held in the F / F 41 at the end of the packet.

  The F / F 41 holds the AGC control gain output from the control circuit 40 and outputs it to the selector 42 and the adder 43. The adder 43 adds the AGC control gain (F / F output a41) held by the F / F 41 and the power average signal a7, and outputs an adder output a42 to the selector 42.

  In response to the select signal a40 from the control circuit 40, the selector 42 outputs either the F / F output a41 or the adder output a42 to the automatic gain control amplification means 1 as the AGC control signal a8.

  The operation of this OFDM demodulator will be described with reference to the flowchart shown in FIG. Before starting the reception of the OFDM reception signal, the two types of AGC circuits having different response speeds and the burst detection circuit 3 start operating simultaneously (step S1 in FIG. 6), and wait for the arrival of the OFDM reception signal a1 (step S2). ).

  Burst detection is performed by the burst detection means 3 using a synchronization preamble, and the burst detection signal a4 is used for FFT window control necessary for OFDM demodulation. When a burst is detected (step S3), the gain of the automatic gain control amplification means is set to the AGC control gain obtained by the fast response AGC circuit (step S4). By using the gain obtained by the AGC preamble section, there is an advantage that the characteristics are improved by the detection processing of the packet signal that arrives in a short time.

  Immediately after the initial AGC control gain is determined, detection of the packet signal is started. The packet signal is detected by detecting a header included in the OFDM transmission signal. If a burst is detected by correlation but the header cannot be detected within a certain time (step S5), it is determined that the burst detection itself is an error, and all AGC processing is reset (step S8). Perform detection. The objects of reset are the F / F 41 and selector 42 in the automatic gain control amplification means control means 4 and the moving average filter 72 in the OFDM signal power averaging means 7.

  On the other hand, when the header is detected within a certain time (step S5), the packet length is obtained from the number of transmission bytes and the transmission rate information. Even during packet information detection, the AGC circuit with slow convergence continues to operate. At this time, the output of the automatic gain amplifying means control means 4 is in a state in which the AGC control value previously obtained by the AGC preamble is fixed, and is not affected by the AGC circuit having a slow response yet (step S6).

  Even after a slow response AGC circuit, after sufficient time for convergence has elapsed (step S6), it occurred after fixing the control amount and gain of the automatic gain amplification means control means output determined by the AGC preamble section. The addition of the amplitude variation is allowed, the amplitude variation from the beginning of the burst is corrected, and the gain is controlled by the AGC control value obtained by the AGC circuit having a late response until the packet end time (step S7). After the elapse of a time longer than the obtained packet length, the above-described AGC process is reset (step S8).

1 is a block diagram showing an example of an OFDM demodulator according to the present invention. Detailed block diagram of delay means 2 and burst detection means 3 in FIG. Detailed block diagram of the OFDM demodulation means 5 in FIG. Detailed block diagram of OFDM signal power averaging means in FIG. Detailed block diagram of automatic gain amplification means control means 4 in FIG. 1 is a flowchart illustrating an automatic gain control method of an OFDM demodulator according to the present invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Automatic gain control amplification means 2 Delay means 3 Burst detection means 4 Automatic gain control amplification means control means 5 OFDM demodulation means 6 Packet signal detection means 7 OFDM signal power averaging means
21 Shift register
31 Correlator
32 Power converter
33 Comparator
40 Control circuit
41 flip-flops
42 Selector
43 Adder
50 Window control circuit
51 Shift register
52 Guard interval elimination circuit
53 Series-parallel converter
54 FFT circuit
55 Subcarrier demodulation circuit
56 Parallel to serial converter
60 Header information extraction circuit
71 Square circuit
72 Moving average filter
73 Subtractor

Claims (3)

Automatic gain control amplification means for performing gain control so that the output signal level becomes constant according to the level of the input OFDM reception signal;
Delay means for delaying the output signal of the automatic gain control amplifying means for a fixed time;
Burst detection means for detecting a burst by performing a correlation operation between the output signal of the automatic gain control amplification means and the output signal of the delay means;
OFDM demodulating means for demodulating an OFDM signal by performing discrete Fourier transform on the output signal of the automatic gain control amplifying means based on the output signal of the burst detecting means;
Packet signal detection means for receiving a packet signal from the output signal of the OFDM demodulation means and detecting the end time of the packet signal;
OFDM signal power averaging means for averaging the output signal of the automatic gain control amplification means for a long time and obtaining an AGC control value by a difference from a target;
Automatic gain control amplification means control means for controlling the automatic gain amplification means by the OFDM reception signal, the output of the burst detection means, the output of the packet signal detection means and the output of the OFDM signal power averaging means,
The automatic gain control amplification means control means is an AGC circuit having a quick response for calculating an AGC control value from an AGC preamble portion in the OFDM received signal, or the OFDM signal power averaging means has obtained the calculated AGC control value. An OFDM demodulator comprising an AGC circuit with a slow response for adding an AGC control value. The automatic gain control amplification means control means comprises:
A control circuit that calculates an AGC control value from an AGC preamble portion in the OFDM reception signal at the detection timing of the burst signal;
A flip-flop that holds the AGC control value output from the control circuit under the control of the control circuit;
An adder for adding the AGC control value held by the flip-flop and the output of the OFDM signal power averaging means;
In response to a select signal from the control circuit, it comprises a selector that outputs either the output of the flip-flop or the output of the adder to the automatic gain control amplification means as an AGC control value,
The control circuit switches the input to the selector from the output side of the flip-flop from the output side of the flip-flop by a select signal after a predetermined time has elapsed from the detection of the burst signal, and the number of transmission bytes obtained from the packet signal and transmission 2. The OFDM demodulator according to claim 1, wherein the total length of the transmission packet is obtained from the rate information, and the polarity of the selector and the value held in the flip-flop are initialized at the end of the packet. Using the OFDM demodulator according to claim 1,
Two types of AGC circuits with different response speeds and the burst detection means start operating simultaneously before starting the reception of OFDM reception signals,
When the burst detection means detects a burst after the arrival of the OFDM received signal, the gain of the automatic gain control amplification means is an AGC control value obtained by the AGC circuit having a fast response,
When the packet signal cannot be detected, the AGC process is reset and the AGC operation and the packet signal are detected again.
When a packet signal is detected, even when the AGC circuit has a slow response, when the time that can be sufficiently converged has elapsed, the automatic gain control amplification means is gain-controlled with an AGC control value obtained by the AGC circuit with a slow response. ,
An automatic gain control method characterized by resetting the AGC process and performing AGC calculation and packet signal detection again when a time longer than the packet length elapses.

Images