JP2638294B2 - Automatic gain control circuit - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an automatic gain control circuit, and more particularly to an automatic gain control circuit used for demodulating a received burst signal in digital satellite communication for transmitting a digitally modulated burst signal.

2. Description of the Related Art Generally, in a multiple access satellite communication system that performs communication on a common channel via a communication satellite, for example, a TDMA (time division multiple access) communication system, a pure Aloha communication system, a slotted Aloha communication system, and the like, the communication system is used. It is necessary to reproduce digitally modulated burst signals from each of the earth stations scattered in various places participating in the satellite.

As shown in FIG. 2, the burst signal usually includes a carrier, a carrier recovery sequence 20 for reproducing bit timing, and a head portion composed of a bit timing reproduction sequence.
21 and a unique word 22 and data 23 following this head
In order to faithfully reproduce the unique word and data composed of the above, it is necessary to establish quick synchronization between the carrier wave reproduction sequence at the head and the bit timing reproduction sequence for each burst signal transmitted from each station. . Therefore, it is desirable to control the received power to be constant. Note that the “other” portion 24 in the drawing may include a post angle for setting the receiving side to the initial state for the next burst signal.

In the communication method as described above, transmission power control or the like may be performed in order to keep the reception power of the burst signal constant, but such a complicated device is reduced as much as possible in order to reduce the size and cost of the earth station. There is a tendency. In this case, a large difference occurs in the received power of the burst signal due to variations in the transmission output of earth stations scattered in various places, differences in weather conditions in various places, and the like. A closed loop is configured as shown in FIG. 3 so that the received power at the input of the demodulator is at a predetermined level with respect to the difference in received power, and a circuit that constantly performs automatic gain control regardless of the presence or absence of a burst signal has been devised. Have been.

In the figure, a conventional automatic gain control circuit includes a demodulator 1 for demodulating a digitally modulated wave, a power calculation circuit 2 for calculating power from the output of the demodulator 1, and a power calculated by the power calculation circuit 2. A subtraction circuit 3 for outputting an error from a reference value, a filter circuit 4 for filtering the output of the subtraction circuit, and a variable attenuation disposed before the demodulator and controlling an attenuation of an input signal by an output control voltage of a gain control circuit. vessel
The automatic gain control is performed according to the reference value input to the subtraction circuit 3.

In the above-described conventional automatic gain control circuit, when the control circuit is configured by a primary closed loop, the response speed until the desired output power is obtained when the received power changes is inversely proportional to the noise bandwidth of the closed loop. Therefore, since the demodulator is operated under the condition that the carrier-to-noise bandwidth is low (low C / N) as in the satellite communication, the response speed decreases when the noise bandwidth is reduced.

On the other hand, the receiving side must wait for a burst signal to be transmitted and respond to a different reception power on the receiving side with respect to the burst signal transmitted from the earth station in each place. In other words, it is necessary to control the received power, which differs depending on the burst signal arriving from the standby no-signal state, to be the predetermined power.

However, the fact that the response speed is slow means that the control of the gain is not completed between the carrier recovery sequence and the bit timing reproduction sequence which are the head of the burst signal and receive the burst signal from the standby no-signal state. In some cases, it may be difficult to establish synchronization. This has the disadvantage that the burst error is reduced and the bit error rate (BER) is degraded because the power is not the predetermined power in the data portion even if the burst signal is received incorrectly.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned conventional disadvantages, and an object of the present invention is to provide an automatic gain control circuit having a fast gain control response.

The automatic gain control circuit according to the present invention includes a variable attenuator capable of controlling the amount of attenuation, a receiving unit that receives data via the variable attenuator, and a receiving unit that receives the data when the receiving unit is receiving data. Means for calculating an average value of an error of a reception level with respect to a predetermined reference value, and control means for controlling the amount of attenuation of the variable attenuator according to the average value when the receiving means is not receiving data. I do.

Examples Hereinafter, the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing a configuration of an automatic gain control circuit according to the present invention, and portions equivalent to those in FIG. 3 are denoted by the same reference numerals. In the figure, an automatic gain control circuit according to one embodiment of the present invention includes a demodulator 1 for demodulating a digitally modulated wave, a power calculation circuit 2 for calculating power from the output of the demodulator 1, and a power calculation circuit 2 And a filter circuit 4 for filtering an output from the subtraction circuit 3 to output an error from the reference value and the power calculated in step (1).

The automatic gain control circuit according to the present embodiment detects a carrier wave (the head of a burst signal) in response to the output of the demodulator 1 and outputs a control signal indicating that the carrier wave has been detected, and a carrier wave detection circuit. 5, an aperture generation circuit 6 for opening an aperture for unique word detection by the control signal output from the control unit 5, and a unique word wait state when the aperture generation circuit 6 is opening the aperture, and a unique word detection when a unique word is detected. When it is determined that the signal for the burst length is being received by the unique word detection circuit 7 that outputs a signal and the burst detection signal that is the output of the carrier wave detection circuit 5, and the output of the unique word detection circuit 7 is received. And a signal reception determination circuit 8 that determines that the burst length is being received accurately.

Furthermore, the automatic gain control circuit according to the present embodiment includes an average value circuit 9 for averaging the output value of the filter circuit 4 only when the signal reception determination circuit 8 determines that the signal is being received. A gain control circuit 10 for switching between outputting the output of the filter circuit 4 as a gain control voltage or outputting the output of the averaging circuit 9 as a control voltage 100 depending on the output, and a preceding stage of the demodulator 1. The variable attenuation device 11 controls an attenuation amount of an input signal in accordance with an output control voltage 100 of the gain control circuit 10.

In such a configuration, a loop for automatic gain control is formed by the demodulator 1, the power calculation circuit 2, the subtraction circuit 3, the filter circuit 4, the gain control circuit 10, and the variable attenuator 11.

The variable attenuator 11 controls the amount of attenuation of the input digital modulation signal by the applied control voltage to make the signal level appropriate. The demodulator 1 demodulates the digital modulation signal whose gain has been controlled by the variable attenuator 11 and outputs demodulated data. The power calculation circuit 2 calculates the power of the demodulated signal from the demodulated data,
The subtraction circuit 3 calculates the difference between the power of the demodulated signal and the reference value and outputs the difference to the filter circuit 4. The filter circuit 4 removes unnecessary components of harmonics with a reduction pass filter.

The carrier detection circuit 5 receives the output of the demodulator 1 to detect the carrier (the head of the burst signal), and outputs a control signal indicating the detection to the aperture generation circuit 6 and the signal reception determination circuit 8. The aperture generation circuit 6 opens an aperture for controlling the operation of the unique word detection circuit 7 by the control signal output from the carrier wave detection circuit 5. The unique word detection circuit 7 performs a unique word search when the aperture generation circuit 6 has opened the aperture, and outputs a unique word detection signal when a unique word is detected.

Here, an aperture for detecting a unique word will be described.

Generally, there is a case where a received signal is demodulated by a low C / N satellite line and erroneously detected as always searching for a unique word. This is because the lower the C / N, the higher the bit error rate (BER), and the higher the probability of erroneously detecting a unique word.

In order to suppress such erroneous detection, the detection of the unique word may be performed only during the time when the unique word is expected to be detected, and the time when the unique word is expected to be detected at this time may be used. Is called an aperture, and a unique word is detected only when the aperture is open.

That is, the head of the burst signal received in a predetermined format is detected, the time at which a unique word is detected is estimated from the detection timing, and an aperture is provided. If the time width of the aperture is wide, the effect of the aperture is lost and the rate of erroneous detection increases. Conversely, when the time width of the aperture is reduced, the rate of non-detection that misses the detection where a unique word is detected increases.

Therefore, the time period for opening the normal aperture may be set so that both the ratio of the erroneous detection and the ratio of the non-detection are reduced.

The signal reception determination circuit 8 determines that a signal corresponding to the burst length is being received based on a burst detection signal output from the carrier wave detection circuit 5, and furthermore, when there is an output from the unique word detection circuit 7, the signal is determined in advance. The end of the reception burst is accurately determined from the current burst length, and it is determined that the burst length is being received. The average value circuit 9 calculates the average value of the output of the filter circuit 4 while the signal reception determination circuit 8 determines that the signal is being received.

The gain control circuit 10 knows whether or not the signal is being received from the output of the signal reception determination circuit 8, and if the signal is being received, outputs the output of the filter circuit 4 as a gain control voltage and conversely receives the signal. If not, switching control is performed such that the output of the average value circuit 9 is output as a control voltage. At the same time, the gain control circuit 10 controls the attenuation of the input signal by applying the control voltage 100 to the variable attenuator 11.

In the automatic gain control circuit that operates as described above, when waiting for a burst signal transmitted from each earth station, the automatic gain control loop is opened, and the output of the average value circuit 9 is used to output the signal from the gain control circuit 10. The control voltage is set to the variable attenuator 11.

When the burst signal is detected in the carrier detection circuit 5 (for example, the carrier recovery sequence at the head of the burst signal is detected), the reception is transmitted to the signal reception determination circuit 8 and the input to the gain control circuit 10 is filtered. Switch to the output of circuit 4 and close the loop. In the burst signal reception state,
Automatic gain control is performed in this loop.

When the signal indicating that the signal is being received, which is output from the signal reception determination circuit 8, is completed, that is, the automatic gain control loop is re-opened by the gain control circuit 10 in response to the control signal indicating that the reception has been completed, and the output value of the average value circuit 9 is output. In other words, the output control voltage in which the amount of attenuation during reception of the burst signal is averaged is added to the variable attenuator 11,
Prepare for the next transmitted burst signal.

When such automatic gain control is performed, the burst signal is transmitted because the variable attenuator 11 has already set the average value of the attenuation during reception of the burst signal when there is no signal waiting for the burst signal. Therefore, the amount to be controlled is small, and a predetermined demodulator input level can be quickly obtained.

Therefore, as in the conventional circuit, when the automatic gain control is always performed, the variable attenuator 11 has the maximum gain when there is no signal during standby, and when a burst signal arrives there, the variable attenuator 11 receives a predetermined signal at the demodulator input. Since the variable attenuator 11 is controlled so as to obtain power, a response time is required (especially, the larger the power difference between the no-signal state and the burst signal reception state, the longer the response time). It is advantageous in terms of establishing synchronization, and burst signal reception errors due to synchronization delay,
The deterioration of the error rate characteristic can be prevented. In addition,
The same effect can be obtained by a method of calculating the average value of the input power of data itself, instead of calculating the output value of the filter circuit 4, that is, the average value of the error from the reference value.

In other words, in the conventional circuit, since the automatic gain control is always performed, the gain of the variable attenuator is always in the maximum state in the state of no signal waiting for the transmitted burst signal, and the large power In order to obtain a predetermined power at the demodulator input when the signal arrives, a long response time is required to attenuate the received signal power by the variable attenuator. On the other hand, in the control circuit of the present invention, while the burst signal to be transmitted is waiting, the amount of attenuation in the variable attenuator is set by the average value of the input power in the burst signal receiving state. Even so, since the automatic gain control is started from the set attenuation amount, the control amount is smaller than in the past, and the gain can be controlled to a predetermined reception power level in a short response time. It is clear that the present invention can be applied to fields other than digital satellite communication.

As described above, according to the present invention, the demodulation is performed by determining the attenuation of the variable attenuator based on the average value of the attenuation during signal reception in the no-signal state and switching to the automatic gain control during signal reception. Control response time for setting the input signal to the input signal to a predetermined input level can be shortened, which is advantageous for establishing phase synchronization, and a burst signal reception error due to a delay in synchronization establishment, that is, deterioration of error rate characteristics. There is an effect that can be prevented.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an automatic gain control circuit according to an embodiment of the present invention, FIG. 2 is a conceptual diagram of received data, and FIG.
FIG. 1 is a block diagram showing a configuration of a conventional automatic gain control circuit. Explanation of Signs of Main Parts 5 ... Carrier Wave Detection Circuit 6 ... Aperture Generation Circuit 7 ... Unique Word Detection Circuit 8 ... Signal Reception Judgment Circuit 9 ... Average Circuit 10 ... Gain Control Circuit 11 ... Variable Attenuator

 ──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-48-66910 (JP, A) JP-A-60-68732 (JP, A) JP-A-63-202110 (JP, A) JP-A 60-68710 22815 (JP, A) JP-A-57-168513 (JP, A) JP-A-60-33719 (JP, A)

Claims (1)

(57) [Claims] 1. A variable attenuator having a controllable attenuation, receiving means for receiving data via the variable attenuator, and a receiving level of the data with respect to a predetermined reference value when the receiving means is receiving data. An automatic gain control circuit comprising: means for calculating an average value of an error; and control means for controlling the amount of attenuation of the variable attenuator according to the average value when the receiving means is not receiving data.