JPH0657022B2 - Multilevel demodulation method - Google Patents

Description

1. Field of the Invention The present invention relates to a demodulation method in which a level variation from a demodulator input to a discriminator is reduced in a multilevel demodulation method such as used in a digital radio.

(2) Prior art and problems Since a digital radio line uses a 4-phase PSK (phase shift keying) signal in recent years, a 16-value QAM type signal has come to be used for processing a large amount of information.
In the case of a four-phase PSK signal, the demodulation board in the receiver converts the intermediate frequency signal into a baseband signal and then demodulates it using the carrier wave obtained from the carrier wave reproduction circuit, and then outputs the bit pattern data by the discriminator. I am taking it out. In the case of 4-phase PSK, the eye pattern is binary, so it is relatively resistant to level fluctuations in the discriminator, but in the case of multi-level QAM modulation such as 16-level QAM, the signal point becomes an eye pattern containing AM components. Therefore, except for the identification of the central eye, it becomes very strict about level fluctuation. Fig. 1 shows the conventional 16
The block block diagram of the value QAM type demodulation board is shown. A fading equalizer is inserted in the next stage of the main intermediate frequency amplifier MIFA, and an amplifier IFA with an automatic gain control circuit is used in the initial stage of the demodulation board DM surrounded by a chain line. The recovered carrier from the carrier recovery circuit CR is applied to the quadrature demodulator MIX, and the output of the baseband amplifier BBA is the discriminator DS.
C, applied to the clock recovery circuit BTR. However, even with this configuration, the gain variation / DC drift of the pre-stage amplification system and the level variation of the baseband system in the demodulation board DM cannot be completely compensated, and the output of the discriminator DSC is inaccurate. It should be noted that the amplifier with automatic gain control circuit in the demodulation board has a drawback in that its output is fed back and the gain fluctuation of the baseband system cannot be compensated.

(3) Object of the Invention It is an object of the present invention to improve the above-mentioned drawbacks and to provide a demodulation method capable of performing accurate identification operation by reducing the level fluctuation from the input of the demodulator to the identification device. .

(4) Structure of the Invention The structure of the present invention for achieving the above-mentioned object is such that a QAM type digital signal in which amplitude modulation is added to phase shift keying modulation is received, amplified by an amplifier, and then a carrier recovery circuit is used. In the multilevel demodulation method using the obtained carrier wave for demodulation / discrimination, a decision feedbag type automatic gain control circuit is used which controls the gain by using the output of the discrimination circuit as the amplifier, and the discrimination circuit uses a QAM eye pattern. The fluctuation value is detected at the convergence points on both outer sides, and the gain of the amplifier is controlled by the detection signal.

(5) Embodiment of the Invention FIG. 2 is a block diagram showing the configuration of an embodiment of the present invention.
Only the demodulation board GM is shown, and the same reference numerals as those in FIG. 1 are the same. The discriminator DSC is provided with two more discriminating circuits DS than those in FIG.
It is applied as the A control signal. This type of automatic gain control system is called a decision feed bag type AGC. This gain control method enables more accurate identification than before.

Next, the decision feed bag type AGC will be described. In FIG. 3 showing the multilevel QAM eye pattern and the threshold value of the identification signal, the horizontal axis represents time and the vertical axis represents amplitude,
The demodulated signal in the case where the amplitude variation occurs is indicated by, and the threshold levels of certain two discriminating circuits DS are indicated by a and b,
The one-dot chain line is the demodulated signal when there is no fluctuation. The threshold levels of the other two identification circuits DS in FIG.
Put as intermediate level X, Y. The identification circuit DS is composed of, for example, a flip-flop whose input has an offset in order to take out each signal of a, b, X, and Y.

A baseband eye pattern signal is applied in parallel to each flip-flop and a clock signal is applied to operate. The data that can be output from the discrimination circuit is as shown in the following table by obtaining "1" when the input signal is large with respect to the threshold level and "0" when it is small.

Next, the configuration of the control circuit CTL will be described. First, when the demodulated signal has such a large amplitude that it passes through the convergence points at both ends of the eye pattern of the 16-value QAM type signal (X and Y have the same sign), the gain control signal C'is obtained.

The truth table is as follows.

Further, it is an intermediate portion between the convergence point at the end and the convergence point adjacent to the convergence point at the end. When the demodulation signal changes between the discrimination levels X and Y for edge discrimination (when X and Y have different signs), that is, when the amplitude is small, the gain control signal C'is prohibited and the Hold the state.

For such prohibiting / holding operation, the output of the signal C'is input to the D terminal of the flip-flop, and the result of the EX-NOR operation of the X and Y signals and the AND operation value of the clock are calculated. By inputting to the C terminal of the flip-flop, the flip-flop output Q becomes the normal gain control signal C. This signal C is the output of the control circuit CTL.

Therefore, the control circuit CTL is configured as shown in FIG.

(6) Effect of the Invention Thus, according to the present invention, when there is a level change at the input of the discriminator, it is detected accurately and the gain control is performed for the amplifier, so that the discriminating operation is accurate. Have.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a conventional 16-value QAM demodulation board, FIG. 2 is a block diagram showing a configuration of an embodiment of the present invention, and FIG. 3 shows a multi-value QAM eye pattern and an identification signal C. FIG. FIG. 4 is a specific circuit diagram of the control circuit CTL in FIG. MIFA ... Main intermediate frequency amplifier DM ... Demodulator IFA ... Amplifier with automatic gain control circuit MIX ... Quadrature demodulator BBA ... Baseband amplifier DSC ... Discriminator CR ... Carrier regeneration circuit BTR ... Clock regeneration circuit DS ... Discrimination circuit

Claims (1)

[Claims] 1. A multi-level demodulation system which receives a QAM type digital signal obtained by adding amplitude modulation to phase shift keying modulation, amplifies it by an amplifier, and then demodulates and identifies it by using a carrier obtained by a carrier regeneration circuit. The decision feedback type automatic gain control circuit for controlling the gain by using the output of the discrimination circuit as the amplifier, wherein the discrimination circuit detects a variation value at the convergence points on both outer sides of the QAM eye pattern, The multi-level demodulation method is characterized in that the gain of the amplifier is controlled by the following.