JP2890104B2 - QAM demodulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a quadrature amplitude modulation (QA).
More particularly, the present invention relates to a QAM demodulator that can generate a synchronization symbol and a pilot symbol by a simple configuration and arithmetic processing.

[0002]

2. Description of the Related Art An MCA (Multi Channel Access) system is a system in which different users jointly use a plurality of channels, and is widely used for business use. In addition, this MCA system is used not only for voice wireless communication but also for various data communication.

The number of users in this MCA system has been rapidly increasing in recent years, and there are many demands for a system suitable for data transfer. Therefore, digital MC
The specifications of the A system were determined and put into practical use.

In such a system, multi-level QAM (Qua
For example, multi-level quadrature amplitude modulation in which four-value amplitude modulation is performed on carrier signals having phases different by 90 degrees to have a 16-value state is used.

As a modulation method of the digital MCA system, RCR (Research & Development Center fo
(R Radio Systems: Radio System Development Center) In the STD-32 digital MCA system defined as a standard, the M16QAM system is used and four subcarriers are used.

[0006] In wireless communication, the signal strength on the receiving side may change greatly due to a change in propagation conditions.
ontrol (AGC) is indispensable.

For example, an AGC circuit 1 for realizing AGC of a signal by frequency modulation or phase modulation is realized by a circuit as shown in FIG. That is, after the integration result of the integrator 3 is processed by the low-pass filter 4 so that the integration value of the output of the variable gain amplifier 2 becomes constant, the variable gain amplifier 2
To the gain control terminal to adjust the gain. In this case, since the amplitude of a signal obtained by frequency modulation or phase modulation is always constant, such processing can be realized.

By the way, since the QAM signal also has information on the amplitude value, the integrated value of the amplitude value is not always a stable value. Therefore, AGC cannot be realized using a value obtained by integrating the amplitude value of the signal as shown in FIG.

Here, the multilevel QAM modulated signal can be expressed by the following equation. I (t) = A (t) cosφ (t) Q (t) = A (t) sinφ (t) That is, this multi-level QAM modulation signal is modulated so as to have information on phase and amplitude.

As described above, AGC and phase correction for a multi-level QAM modulated signal having information also in an amplitude component are performed according to FIG.
In general, the following operation is performed using a circuit as shown in FIG.

First, the demodulation circuit 4 performs quadrature demodulation to separate an in-phase component I (T) and a quadrature component Q (T).
Then, the synchronization detector 5 detects a synchronization symbol from I (T) and Q (T), and the timing extractor 6 determines the timing of the synchronization symbol from the synchronization symbol. The phase / amplitude corrector 7 uses the amplitude value and the phase value of the synchronization symbol that are known in advance to obtain I (T) and Q (Q).
The amplitude correction (AGC) and the phase correction of (T) are performed.

In this case, synchronization detection must be performed by the synchronization detector 5 in a state where the amplitude value before performing AGC is indefinite, which is a very difficult process. That is, although the detection is performed using the fact that the synchronization symbol is determined by both the amplitude value and the phase angle, it is not possible to perform the detection in an undefined state. There was a problem that a complicated thing was required in terms of an algorithm.

That is, the detection of a synchronization symbol having a predetermined phase by the synchronization detector 5 is performed as follows. Here, assuming that the amplitude is A (T) and the phase is φ (T), tan φ (T) = (A (T) sin φ (t) / A (T) cos φ (t) = (Q (T) / I (T)).

Therefore, φ (T) = Arctan (Q (T) / I (T)).

Thus, I is independent of the amplitude A (T).
It is possible to obtain φ (T) from (T) and Q (T), and the process of detecting the phase φ (T) is performed by the phase detector 5a in FIG. Further, the phase φ (T) detected in this way is compared with the phase of the synchronization symbol read from the phase table 5c by the phase comparator 5b, and the position of the synchronization symbol is detected.

[0016]

In such a synchronization detection process, there is a problem that a division is required to remove the amplitude value A (T). A circuit for realizing this division is generally more complicated than a process such as integration. Furthermore,
Since the operation of Arctan is required to obtain the phase φ (T), the processing is complicated. Also, a large amount of processing time is required for processing such as table reference.

Further, in addition to the above-described phase-based synchronization detection,
Detection from amplitude may also be performed incidentally. In this case, the synchronization symbol having the maximum amplitude value is detected by the synchronization detector 5.
It is necessary to detect at least one frame of amplitude value information, and to search from the stored contents. Accordingly, the synchronization detection from the amplitude also has a problem that the circuit becomes large-scale.

Therefore, even in the case of realizing with a DSP, the arithmetic processing becomes troublesome and there is a problem that high-speed arithmetic is required.

The present invention has been made in view of the above points, and has as its object to provide an accurate AGC with a simple configuration and calculation.
To realize a QAM demodulator capable of demodulating a modulated wave signal.

[0020]

The inventor of the present application has conducted intensive studies to improve the disadvantages of the complexity of the arithmetic and processing circuits in AGC in a conventional QAM demodulator,
The present invention has been completed by finding a new technique for performing accurate AGC by focusing on the amplitude of the synchronization symbol.

Accordingly, the present invention, which is means for solving the above-mentioned problems, is configured as described below.

That is, the present invention, which is a means for solving the problem, comprises a frame composed of an arbitrary number of continuous symbols at a predetermined phase angle on a circle passing through a symbol having a maximum amplitude on a signal space coordinate. In a QAM demodulator for demodulating a QAM signal including a synchronization symbol and a pilot symbol inserted in the middle of a frame to facilitate phase correction, a QAM signal demodulated by a symbol timing reproduction signal substantially synchronized with a symbol timing on a transmission side. , A reproduction circuit for reproducing a symbol signal composed of orthogonal components I (T) and Q (T), and M synchronization and pilot symbols determined to have the maximum amplitude within a predetermined period. In this case, the upper M values of the amplitude value of the symbol signal in a predetermined period are extracted, and the extracted M
An AGC circuit for performing gain adjustment using an average value or an integrated value of the amplitude values of the symbol signals is provided.

[0023]

In a QAM demodulator which is means for solving the problem, when receiving a QAM signal in which a predetermined number M of synchronization symbols and pilot symbols are determined to have a maximum amplitude within a predetermined period, Focusing on the upper M amplitude values of the amplitude value during a predetermined period , storing the values, and calculating the error between the average or integrated value of the stored amplitude values and the value determined to be the maximum amplitude on the demodulation side. Is adjusted so that approaches zero.

[0024]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is a block diagram showing the principle configuration of the main part of the QAM demodulator according to the present invention, and FIG. 2 is the QAM demodulator according to the present invention.
FIG. 2 is a configuration diagram illustrating an overall configuration of a demodulation device. In FIGS. 1 and 2, the same components are denoted by the same reference numerals. In this embodiment, a case where the number of subcarriers is 4 will be described as an example.

First, the overall configuration of the QAM demodulator and its operation and processing procedure will be outlined with reference to FIG. 2, and then AGC, which is a feature of the present invention, will be described with reference to FIG.

The QAM demodulator shown in FIG. 2 is roughly divided into a quadrature demodulator 8 for performing subcarrier separation and quadrature demodulation.
A symbol extracting unit 9 for extracting symbol data;
An AGC circuit 10 that performs AGC on the amplitude value, a phase correction unit 20 that performs phase correction, and a data demodulation unit 30 that performs data demodulation. The reproducing circuit includes a quadrature demodulation unit 8 and a symbol extraction unit 9. The description will be made below for each part.

[Subcarrier Separation] In FIG. 2, a received signal received by an antenna (not shown) and converted into an electric signal is frequency-converted into a modulated wave signal having an intermediate frequency fR ′, Supplied to And
In the quadrature modulator 8, in-phase components (I signals: I1 (t), I2 (t), I3
(T), I4 (t)) and a quadrature component (Q signal: Q1
(T), Q2 (t), Q3 (t), Q4 (t)).

FIG. 3 is an explanatory diagram schematically showing the arrangement of the subcarriers. Four subcarriers (Sub1 to Sub4) are arranged around the carrier frequency f0. Here, the subcarriers are arranged at intervals of 4.5 kHz, and the preceding center frequency is f0-6.75.
kHz, f0 -2.25 kHz, f0 +2.25 kHz
z, f0 +6.75 kHz.

In order to demodulate a modulated wave signal having such subcarriers, in a conventional general device, the signals are supplied to four quadrature demodulators equal in number to the subcarriers. Each of the four quadrature demodulators has a frequency f of the intermediate frequency signal.
The frequency fL of the difference between R 'and the frequency of the desired subcarrier
(1) to quadrature signals of fL (4) (2
A local oscillation signal from a local oscillator that generates signals (sin component and cos component) is also supplied. Thus, quadrature demodulation and subcarrier separation are performed.

However, such a conventional general demodulation (subcarrier separation) method requires the same number of local oscillators as the number of subcarriers to separate the subcarriers.
There was also a problem that the circuit configuration became complicated.

To this end, the inventor of the present application has already proposed a method of separating subcarriers with a local oscillator having a smaller number of subcarriers on April 21, 1995, and uses this technique. It is possible to

That is, in a QAM demodulator for performing quadrature demodulation on a modulated wave signal using a plurality of subcarriers, a first quadrature demodulation is performed on the modulated wave signal before separating the subcarriers, and an in-phase component and a quadrature component are separated. And a second quadrature demodulation for the in-phase component and the quadrature component using the frequency of the difference between the center frequency of the modulated wave signal and the subcarrier, and a signal from a local oscillator smaller than the number of subcarriers. Using a quadrature demodulator,
An arithmetic processing circuit for arithmetically processing the result obtained by the second quadrature demodulation; and an LPF for filtering the arithmetic processing result with the bandwidth of the subcarrier. Demodulation with orthogonal components is performed.

By doing so, it becomes possible to separate subcarriers with a smaller number of oscillators than the number of subcarriers in a modulated wave signal using a plurality of subcarriers. As described above, the number of oscillators can be reduced, and the circuit configuration and scale can be reduced. It is also advantageous when processing is performed by a DSP. Furthermore, each frequency of the local oscillator used to separate the subcarriers is a simple integer ratio. This is because the frequency intervals of normal subcarriers are equal. As a result, the original frequency used for frequency division, multiplication, etc. can be shared. This also works advantageously in the case of processing by the DSP.

[Symbol extraction] On the transmitting side, FIG.
As shown in (1), both the in-phase component I (T) and the quadrature component Q (T) of the orthogonal code are generated as signals having a plurality of stepwise levels. Here, four levels are shown. When this is demodulated by the quadrature demodulator 8, FIG.
The analog signal has a gentle slope as shown in FIG. Therefore, the symbol extraction section 9 performs symbol timing reproduction to reproduce the symbol clock (FIG. 4C). Then, the symbols are extracted from the orthogonal code of the analog signal using the reproduced symbol clock (FIG. 4D). In this way, the symbols are extracted and the orthogonal codes (I1 (T), I2 (T),
I3 (T), I4 (T), Q1 (T), Q2 (T), Q
3 (T), Q4 (T)).

[AGC] The AGC, which is a feature of the present invention, will be described with reference to FIG.

A variable gain amplifier 11 for performing amplification with a variable gain is arranged in the AGC circuit 10, and the gain is adjusted by a control signal, which will be described later, supplied to a gain control terminal so that AGC is realized. It has become. Here, for the sake of explanation, only one of the signal processing systems for four subcarriers is shown in FIG.
In a considerable overall circuit configuration, a number of processes equal to the number of subcarriers are performed.

First, the amplitude value extractor 12 receives the output signal of the variable gain amplifier 12 and extracts an amplitude value. The extraction of the amplitude value in this case is performed by I ² (T) + Q ² (T) or (I
^{2 (T) + Q 2 (} T)) carried out by ^1/2.

When a QAM signal having M synchronization symbols and pilot symbols determined to have the maximum amplitude during a predetermined period is received, the error extraction unit 1
3, receives amplitude information from the amplitude extraction section 12, a predetermined
The upper M amplitude values of the amplitude values during the period are stored. Then, an average value or an integrated value of the stored amplitude values is obtained.
The variable gain amplifier 11 is controlled so that the error between the average value or the integrated value and the value determined as the maximum amplitude on the demodulation side approaches zero.
A control signal for adjusting the gain of the control signal is generated.

Further, the integrator 14 integrates the control signal generated by the error extractor 13 and multiplies the time constant so that the control system of the AGC circuit operates stably.

Therefore, the variable gain amplifier 11 receives the control signal adjusted as described above, performs gain adjustment, and generates an orthogonal code whose amplitude value has been corrected. That is, such processing is performed for each subcarrier, and the orthogonal code (I
1 (T), I2 (T), I3 (T), I4 (T), Q1
(T), Q2 (T), Q3 (T), Q4 (T)).

According to such an AGC process, the process can be performed without going through a synchronization detection procedure which is conventionally required, and the amplitude can be optimally corrected for the QAM signal. In addition, since the proper amplitude correction is performed here, the process of synchronization detection and phase correction at the subsequent stage becomes easy.

[Phase Correction] The phase corrector 20 corrects the phase of the orthogonal code whose amplitude has been corrected by the AGC circuit 10.

Here, the carrier angular frequency on the transmitter side is ω
As c, the difference from the demodulation angular frequency on the receiver side is θ (t)
In this case, if both the frequency on the transmitter side and the frequency on the receiver side are stable, the above θ (t) becomes proportional to the time t.

In the normal case, when the value of the nominal vector is known in advance, that is, when the pilot symbols Ip and Q
When p is transmitted, the correction of θ (t) is performed. In this case, a conventional general phase correction unit obtains a phase difference between data actually received during the pilot symbol period and the pilot symbol, and performs correction (phase estimation) based on the phase difference data. ing.

However, to perform such a correction, the calculation of the trigonometric function must always be performed. For this purpose, it is necessary to always perform series expansion or table reference,
There was a problem that processing became complicated. Therefore, even when the processing is realized by the DSP, the calculation processing becomes troublesome and a high-speed calculation is required.

To this end, the inventor of the present application has already obtained a fixed coefficient on April 12, 1995,
A method of performing phase correction according to this coefficient has been proposed.
It is possible to use this technique.

That is, when demodulating a multilevel QAM modulated signal into which pilot symbols Ip, Qp having predetermined phases and amplitudes are inserted, when the pilot symbols Ip, Qp of the multilevel QAM modulated signal are received, Sinβ and cosβ corresponding to the phase difference β between the pilot symbols Ip and Qp and the true values Io and Qo are determined as fixed-value phase difference coefficients, and the phase difference coefficients sinβ and c
The true values Io and Qo can be obtained by performing phase correction by integration and addition / subtraction using osβ and the in-phase component Ir and quadrature component Qr in each symbol of the received data. Thus, by a simple operation of addition and addition / subtraction,
Highly accurate phase correction can be performed.

Even if any of the above-described techniques is used for the phase correction, since the amplitude has already been corrected, the configuration and processing contents of the processing circuit are simplified, and the phase correction can be performed accurately. Will be possible.

[Data Demodulation] As described above, the orthogonal code whose amplitude and phase have been corrected is demodulated by the data demodulator 30 in accordance with a predetermined algorithm to obtain bit stream output digital data.

[Effect: Comparison with Conventional Example] As described above, when a QAM signal having M synchronization and pilot symbols determined to have the maximum amplitude during a predetermined period is received. Meanwhile, the error extracting unit 13 receives the amplitude information from the amplitude value extracting unit 12,
Focusing on the upper M amplitude values of the amplitude value during a predetermined period , storing the values, and calculating the error between the average or integrated value of the stored amplitude values and the value determined to be the maximum amplitude on the demodulation side. Is 0
, The gain of the variable gain amplifier 11 is adjusted.

As a result, the AGC process itself can be performed easily and accurately. Further, the synchronization detection can be performed in a state where the amplitude value is corrected by the AGC, and the synchronization detection processing is facilitated. Therefore, it is possible to use a simple configuration in terms of hardware of a processing circuit and a processing algorithm as compared with a conventional device.

[0053]

As described in detail above, in the present invention,
When receiving a QAM signal in which the number of synchronization symbols and pilot symbols determined to have the maximum amplitude in a predetermined period is M, pay attention to the upper M amplitude values of the amplitude values in the predetermined period. Then, this is stored, and the gain of the variable gain amplifier is adjusted so that the error between the average or integrated value of the stored amplitude values and the value determined as the maximum amplitude on the demodulation side approaches zero. did.

As a result, the AGC process itself can be easily and accurately performed, and furthermore, the synchronization detection can be performed with the amplitude value corrected by the AGC, and the synchronization detection process is facilitated. . Therefore, it is possible to use a simple configuration in terms of hardware of a processing circuit and a processing algorithm as compared with a conventional device. Accordingly, it is an object of the present invention to realize a QAM demodulator capable of demodulating a modulated wave signal by performing accurate AGC with a simple configuration and calculation.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing a configuration of a main part of a QAM demodulation device of the present invention.

FIG. 2 is a configuration diagram showing an overall configuration of a QAM demodulation device of the present invention.

FIG. 3 is an explanatory diagram showing an arrangement of four subcarriers.

FIG. 4 is an explanatory diagram showing a state of symbol extraction.

FIG. 5 is a configuration diagram showing a configuration of a conventional AGC circuit.

FIG. 6 is a configuration diagram showing a configuration of a conventional circuit for performing amplitude correction and AGC.

FIG. 7 is a configuration diagram showing a conventional phase detection circuit.

[Explanation of symbols]

 8 Quadrature demodulation unit 9 Symbol extraction unit 10 AGC circuit 20 Phase correction unit 30 Data demodulation unit

Claims (1)

(57) [Claims] 1. A synchronization symbol of a frame composed of an arbitrary number of continuous symbols at a predetermined phase angle on a circle passing through a symbol having a maximum amplitude on signal space coordinates.
In a QAM demodulator for demodulating a QAM signal including a pilot symbol inserted in the middle of a frame in order to facilitate phase correction, a quadrature component I is obtained by a symbol timing reproduction signal substantially synchronized with a symbol timing on a transmission side. (T) and Q
(T) a reproducing circuit for reproducing a symbol signal comprising: (M) a synchronizing symbol and a pilot symbol determined to have a maximum amplitude within a predetermined period; An AGC circuit for extracting the upper M values within a predetermined period and performing gain adjustment using the average value or integrated value of the amplitude values of the extracted M symbol signals. QAM demodulator.