JP2002300135A - Automatic gain control circuit for ofdm demodulation circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an automatic gain control circuit for an OFDM demodulation circuit, that can reduce a time constant of a level detector and the attenuator of which can set up correct synchronization. SOLUTION: In the demodulation circuit where a level of a received OFDM signal is controlled to be constant and a synchronization processing circuit 14 conducts synchronization processing, a BPF 17 to extract a known subcarrier in an OFDM signal is interposed between the attenuator 12 and a level detection circuit 15, a counter 18 to operate the synchronization processing circuit 14 is interposed between the level detector 15 and an analog/digital converter 13 after a gain is fixed, the gain of the received signal is controlled on the basis of the known subcarrier extracted by the BPF 17 and the synchronization processing circuit 14 conducts synchronization processing after the gain is fixed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an OFDM (orthogonal frequency division multiplexing) signal demodulation circuit in which a gain can be determined at an early stage of a synchronization symbol period, and thereafter, the gain control can be locked. The present invention relates to an automatic gain control circuit in an OFDM demodulation circuit.

[0002]

2. Description of the Related Art As shown in FIG. 5, an AGC circuit in a conventional OFDM signal demodulation circuit of this kind has an input terminal 1 as shown in FIG.
The received signal input to 0 is amplified by the amplifier 11, and the level of the signal passing through the attenuator 12 is changed as shown in FIG.
When the level is higher than the set level, the level is detected by the level detector 15, and the attenuation rate is determined from the detected signal level, and the attenuation rate is fed back to the attenuator 12 to be attenuated. By repeating this operation several times, the level of the received signal is controlled to a constant level.

Also, while the operation of controlling the received signal to a constant level is repeatedly performed, the A / D converter 13
/ D conversion, and a correlation operation is performed in the synchronization processing circuit 14 at the subsequent stage to perform synchronization processing.

[0004]

However, since the OFDM signal is a multi-carrier system composed of several tens of subcarriers, the received signal level greatly fluctuates. Therefore, the conventional AGC circuit requires a level detector having a very large time constant, which is not practical in an actual circuit configuration. Also, while the level control of the received signal is being performed, the correlation processing is performed in the synchronization processing circuit 14 and the synchronization processing is performed. There is a problem that incorrect synchronization may be established.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a level detector in which the time constant of the level detector can be reduced and the attenuator can establish correct synchronization.

[0006]

According to the present invention, the level of an OFDM reception signal passing through an attenuator 12 is detected by a level detector 15, an attenuation rate is determined from the detected signal level, and this is fed back to the attenuator 12. A demodulation circuit which controls the level of the received signal to a constant value and performs a synchronous process in a synchronous processing circuit 14 after A / D conversion by an A / D converter 13 on the received signal passed through the attenuator 12 In the above, the attenuator 12
A BPF 17 for extracting a known subcarrier of the OFDM signal is interposed between the A / D converter 13 and the level detector 15, and the synchronization is performed between the level detector 15 and the A / D converter 13 after the gain is fixed. The gain of the received signal is controlled based on the known subcarriers extracted by the BPF 17 through a counter 18 for causing the processing circuit 14 to operate, and after the gain is fixed, the synchronization processing is performed by the synchronization processing circuit 14. An automatic gain control circuit in an OFDM demodulation circuit characterized by the above.

[0007] Since the OFDM reception signal level is larger than the set level, the level detector 15 uses the BPF 1
Detecting the level of the known subcarrier extracted in step 7,
An attenuation rate is determined from the detected signal level, and is fed to the attenuator 12 to be attenuated. By repeating this operation for a specific, for example, the first five short training symbols, the level of the received signal is controlled to a constant value.

[0008] The counter 18 counts the duration or the number of times of gain control, and outputs an enable signal to the A / D converter 13 when the gain is fixed. The circuit 14 is operated to perform synchronization processing using the last five short training symbols.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described with reference to FIG. In the present invention, the BPF 17 is interposed between the attenuator 12 and the level detector 15, and the operation of the synchronization processing circuit 14 is interrupted during the gain control between the level detector 15 and the A / D converter 13. The counter 18 is interposed. More specifically, an attenuator 12 is connected to an input terminal 10 via an amplifier 11, and an A / D converter 13 and a BPF 1 are connected to the output side of the attenuator 12.
7 is connected. The BPF 17 includes a level detector 15
The level detector 15 is connected so as to be fed back to the attenuator 12, and
This level detector 15 is connected to the A
/ D converter 13. Further, the A / D
The converter 13 is connected to a synchronization processing circuit 14 and an output terminal 16 in order.

The BPF 17 is a narrow band filter such as a crystal filter for extracting only a sub-carrier of a known frequency from a plurality of sub-carriers constituting an OFDM signal. The counter 18 counts the duration of the gain control performed by the attenuator 12, the BPF 17, and the level detector 15 or the number of times of the gain control. An enable signal is output to the converter 13 to drive the A / D converter 13 and the subsequent synchronization processing circuit 14.

The operation of the above configuration will be described.
The received signal input to the input terminal 10 is amplified by the amplifier 11 and sent to the BPF 17 through the attenuator 12. In an initial state in which the signal has just been received and the gain control has not been performed, the A / D converter 13 and the synchronization processing circuit 14 are shut off. The signal passing through the attenuator 12 is sent to the BPF 17. The frame configuration of the OFDM signal has, for example, as shown in FIG. 3, 10 short training symbols S1, S2,...
2. SIGNAL containing information such as long training symbols LT1, LT2, guard interval GI, modulation method, transmission rate, guard interval GI, DAT
A1, a guard interval GI, and DATA2.

The short training symbol shown in FIG. 3 has a spectrum as shown in FIG. 2A, and is a known subcarrier for estimating a frequency error.
It is inserted in the DM signal. Therefore, BPF17
Has a narrow band transfer function as shown in FIG. 2B, whereby only known subcarriers are extracted.

When the OFDM signal is received, its level is higher than the set level as shown in FIG. 4, so that the level detector 15 detects the level of the known subcarrier extracted by the BPF 17. The attenuation factor is determined from the detected signal level, and the attenuation factor is fed to the attenuator 12 to be attenuated. This operation is 1
The level of the received signal is controlled to a constant value by repeating the first half, for example, five of the zero short training symbols.

The counter 18 includes an attenuator 12, a BP
F17, the duration of the gain control performed by the level detector 15 or the number of times of the gain control is counted, and when a certain level is reached, an enable signal is output to the A / D converter 13 which has been shut off so far, and A The A / D converter 13 is driven, and the synchronization processing circuit 14 connected to the A / D converter 13 is driven.
Is operated to perform synchronization processing using the latter five of the ten short training symbols.

As described above, when an OFDM signal is received, first, the level of the received signal is controlled to a constant value, and when the gain is fixed, a correlation operation is performed by the synchronization processing circuit 14 to establish correct synchronization.

In the above embodiment, when the enable signal of the counter 18 is output, the A / D which has not been driven until then is output.
The D converter 13 is driven, and the synchronization processing circuit 14 connected to the A / D converter 13 is operated. However, the present invention is not limited to this. By providing a switch circuit between the A / D converter 13 and the synchronization processing circuit 14 and controlling the switching of the switch circuit by an enable signal of the counter 18, the A / D The D converter 13 may be set to be always in the driving state, and when the enable signal of the counter 18 is output, close the switch circuit and operate the synchronization processing circuit 14 connected to the switch circuit to perform the synchronization processing. .

In the above embodiment, BP is used for level detection.
Although the short training symbol is used as the symbol to be extracted in F17, this is because the short training symbol has a smaller number of subcarriers in the band as compared with the symbol that follows, and therefore, each carrier by the guard interval is used. This is because the side lobes can be neglected, and the short training symbols are open between each other, so that the bandwidth accuracy of the BPF 17 does not matter much. However, the present invention is not limited to short training symbols as long as a specific carrier can be extracted using a BPF 17 having excellent accuracy.

[0018]

According to the first aspect of the present invention, the level of the OFDM reception signal passing through the attenuator 12 is detected by the level detector 15, and the attenuation rate is determined from the detected signal level. In the demodulation circuit in which the level of the received signal is controlled to a constant value by feeding back to the attenuator 12 and the level detector 1
5, a BPF 17 for extracting a known subcarrier of the OFDM signal is interposed, and the gain of the received signal is controlled based on the known subcarrier, so that only a specific known subcarrier is detected. This suffices to reduce the time constant of the level detector, which is suitable for level detection of an OFDM signal whose reception level fluctuates greatly.

According to the second aspect of the present invention, the level of the OFDM reception signal passing through the attenuator 12 is detected by the level detector 15, the attenuation factor is determined from the detected signal level, and this is fed back to the attenuator 12. A demodulation circuit which controls the level of the received signal to a constant value and performs a synchronous process in a synchronous processing circuit 14 after A / D conversion by an A / D converter 13 on the received signal passed through the attenuator 12 In the above, the attenuator 12
A BPF 17 for extracting a known subcarrier of the OFDM signal is interposed between the A / D converter 13 and the level detector 15, and the synchronization is performed between the level detector 15 and the A / D converter 13 after the gain is fixed. The gain control is performed based on the known subcarriers extracted by the BPF 17 through a counter 18 for causing the processing circuit 14 to operate, and the synchronization processing is performed by the synchronization processing circuit 14 after the gain is fixed. Therefore, the correlation processing by the synchronization processing circuit 14 is not performed while the level control of the received signal is performed, and the attenuator 12 may establish erroneous synchronization with a large-level signal in the initial stage. And correct synchronization can be established.

According to the third aspect of the present invention, the BPF 17
May extract one or more known subcarriers.

According to the fourth aspect of the present invention, the BPF 17
Is designed to extract the first half of the short training symbols as known subcarriers of the OFDM signal. Therefore, the number of subcarriers existing in the band is smaller than that of the symbols following the short training symbols. The side lobe of each carrier due to the guard interval can be neglected, and the short training symbols have an effect that the accuracy of the bandwidth of the BPF 17 does not matter so much because they are open.

According to the fifth aspect of the present invention, as the known subcarrier of the OFDM signal, the gain can be controlled by the first plurality of short training symbols, and the synchronization processing can be performed by the second half of the short training symbols.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of an automatic gain control circuit in an OFDM demodulation circuit according to the present invention.

2 (a) is a spectrum of a short training symbol of an OFDM signal, FIG. 2 (b) is an explanatory diagram of a transfer function of a BPF 17, and FIG. 2 (c) is a waveform diagram of a known subcarrier after passing through the BPF 17. .

FIG. 3 is a diagram illustrating a frame configuration of an OFDM signal.

FIG. 4 is a waveform diagram until a gain is fixed by a known subcarrier.

FIG. 5 is a block diagram of an automatic gain control circuit in a conventional OFDM demodulation circuit.

[Explanation of symbols]

10 input terminal, 11 amplifier, 12 attenuator,
13 ... A / D converter, 14 ... Synchronization processing circuit, 15 ... Level detector, 16 ... Output terminal, 17 ... BPF, 18 ... Counter.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5J100 JA01 KA05 LA03 LA12 QA02 SA02 5K022 DD01 DD13 DD17 DD19 DD42 5K061 AA11 BB06 BB07 CC25 CC52 CD04

Claims (5)

[Claims] 1. A level detector 15 detects the level of an OFDM reception signal passing through an attenuator 12, determines an attenuation rate from the detected signal level, and feeds it back to the attenuator 12 to keep the level of the reception signal constant. In a demodulation circuit controlled to a value, a BPF 17 for extracting a known subcarrier of the OFDM signal is interposed between the attenuator 12 and the level detector 15, and a BPF 17 of the received signal is extracted based on the known subcarrier. An automatic gain control circuit in an OFDM demodulation circuit, wherein gain control is performed. 2. The level of an OFDM reception signal passing through an attenuator 12 is detected by a level detector 15, an attenuation rate is determined from the detected signal level, and this is fed back to the attenuator 12 to keep the level of the reception signal constant. In the demodulation circuit which controls the received signal which has passed through the attenuator 12 and performs A / D conversion by the A / D converter 13, the synchronization processing circuit 14 performs synchronization processing on the received signal. BP for extracting a known subcarrier of the OFDM signal between the
F17, and the level detector 15 and the A / D
A counter 18 for operating the synchronization processing circuit 14 after the gain is fixed is provided between the converter 13 and the converter 13 to control the gain of the received signal based on the known subcarrier extracted by the BPF 17, An automatic gain control circuit in an OFDM demodulation circuit, wherein synchronization processing is performed by a synchronization processing circuit after a gain is fixed. 3. The automatic gain control circuit in an OFDM demodulation circuit according to claim 1, wherein the BPF 17 is configured to extract one or a plurality of known subcarriers. 4. The automatic gain control circuit in an OFDM demodulation circuit according to claim 1, wherein the BPF 17 extracts a plurality of short training symbols in the first half as known subcarriers of the OFDM signal. 5. A BPF 17 extracts a first half of a short training symbol as a known subcarrier of an OFDM signal, and a second half of the short training symbol.
3. The automatic gain control circuit in an OFDM demodulation circuit according to claim 2, wherein the synchronization processing is performed by: