JP2002290487A - Multiple-value qam demodulation method and device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize high speed drawing in by detecting a DC offset value in a high speed at channel changeover time, etc. SOLUTION: An orthogonal demodulation output signal is divided into a signal for DC offset correction and a signal for area determination. DC offset values of an I component and a Q component are respectively obtained from the I component average and the Q component average of the orthogonal demodulation output signal for the DC offset correction at the detection timings of the maximum vector value and the minimum vector value based on area information determined by the area determination. The DC offset values of the I component and the Q component of the orthogonal demodulation output signal for area determination are corrected based on the respective DC offset values.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a multi-level QAM demodulation such as 16-QAM demodulation, in which a DC offset error caused by IF undersampling is reduced to a maximum value and a minimum value of an I (in-phase) component and a Q (quadrature) component, respectively. The present invention relates to a multi-level QAM demodulation method and apparatus which obtains from an average value and corrects the DC offset error based on the average value so that high-speed pull-in is performed even at the time of channel switching or the like.

[0002]

2. Description of the Related Art In recent years, multi-level QAM of digital radio system
The demodulation device is required to have a small size and high performance. For this reason, quasi-synchronous detection is performed by software processing using a DSP (Digital Signal Processor) in the demodulation device.

In the conventional 16QAM demodulator, when calculating a DC offset level generated by IF undersampling, the DC offset value is obtained by using a pilot symbol and a minimum vector value of an I component and a Q component.

[0004]

However, since the appearance probability of only the minimum vector value of the pilot signal and the I component and the Q component is low, when a high-speed pull-in is required at the time of channel switching or the like, the pilot signal and the I component and the Q component have a low probability. Pilot signal and I
It is necessary to wait for the appearance of the minimum vector value of the component and the Q component, and there is a problem that high-speed pull-in cannot be performed.

An object of the present invention is to use not only the pilot symbols, the minimum vector values of the I and Q components, but also the maximum vector values of the I and Q components in an initial state such as channel switching and the like. By using the value, the DC offset value is detected at high speed, and high-speed pull-in is enabled.

[0006]

According to a first aspect of the present invention, an intermediate frequency signal of a multilevel QAM signal is converted into a digital signal by undersampling, and then quadrature demodulated. In the multi-level QAM demodulation method of performing data determination by performing area determination of the above, the quadrature demodulated output signal is branched for DC offset correction and for the area determination, and a maximum based on the area information determined in the area determination is obtained. The DC offset values of the I component and the Q component are obtained from the average value of the I component and the average value of the Q component of the quadrature demodulation output signal for DC offset correction at the detection timing of the vector value and the minimum vector value, respectively. A multi-level QA, wherein a DC offset of an I component and a Q component of the quadrature demodulation output signal for area determination is corrected based on the value. And the demodulation method.

According to a second aspect of the present invention, in the first aspect, a phase error of the quadrature demodulation output signal is detected by detecting a pilot symbol included in the quadrature demodulation output signal. A multilevel QAM demodulation method is characterized in that a phase error of the quadrature demodulated output signal is corrected based on the error.

According to a third aspect of the present invention, in the second aspect of the present invention, the phase error correction is individually performed on both the DC offset correction quadrature demodulation output signal and the area determination quadrature demodulation output signal. Characteristic multi-valued QAM
The demodulation method was used.

According to a fourth aspect of the present invention, there is provided an A / D converter for converting an intermediate frequency signal of a multilevel QAM signal into a digital signal by undersampling, and an orthogonal demodulator for orthogonally demodulating an output signal of the A / D converter. And a multi-level QAM demodulation device comprising: an area determination unit that performs data decoding of an output signal obtained by the quadrature demodulation unit; the quadrature demodulation output signal is branched for DC offset correction and for the area determination. In addition, I of the quadrature demodulation output signal for DC offset correction at the detection timing of the maximum vector value and the minimum vector value based on the area information determined by the area determination unit.
DC offset value calculating means for obtaining the average value of the component and the average value of the Q component as a DC offset value, and a DC for correcting the DC offset of the I component and the Q component of the area determination quadrature demodulation output signal based on the respective average values. The multi-level QAM demodulator is provided with offset correction means.

According to a fifth aspect of the present invention, in the fourth aspect of the present invention, the DC offset value calculating means includes a maximum value / minimum value detecting section for detecting whether the vector value of the decoded data is a maximum value or a minimum value. A switch unit for opening a gate at a timing when the maximum value or the minimum value is detected by the maximum value / minimum value detection unit and passing the DC offset correction quadrature demodulation output signal, and the DC offset correction passing through the switch unit A first average calculator for calculating an average value of an I component of the quadrature demodulation output signal for use, and a second average for calculating an average value of a Q component of the DC offset correction quadrature demodulation output signal passed through the switch unit. And a value calculating unit.

According to a sixth aspect of the present invention, in the fourth aspect of the present invention, the DC offset correction means uses the signal of the average value of the I component to calculate the I / O of the quadrature demodulation output signal for area determination.
A first adder that corrects the component, and a second adder that corrects the Q component of the quadrature demodulation output signal for area determination based on the signal of the average value of the Q component. A multi-level QAM demodulator was used.

According to a seventh aspect of the present invention, in the fourth aspect of the present invention, there is provided a phase error calculating section for calculating a phase error of the quadrature demodulation output signal by detecting a pilot symbol included in the quadrature demodulation output signal. A phase error correction unit for correcting a phase error of the quadrature demodulation output signal based on the calculated phase error.
An M demodulator was used.

According to an eighth aspect of the present invention, in the seventh aspect of the present invention, the phase error calculating section and the phase error correcting section include a line of the DC offset correcting quadrature demodulation output signal and the area determining quadrature demodulation output. A multi-level QAM demodulator characterized by being provided individually for signal lines.

[0014]

FIG. 1 is a block diagram of a 16QAM demodulator according to one embodiment of the present invention. Reference numeral 1 denotes an A / D converter, which samples a 455 KHz intermediate frequency signal IF obtained by RF-amplifying and down-converting a received signal with a sampling clock CK and converts it into a digital signal. The frequency fs of this sampling clock CK
Is given by fs = f _IF = f _IF
f _IF / (4 / m). m is set to an odd number of 5 or more. Therefore, this sampling clock CK
Is lower than the frequency f _IF of the intermediate frequency signal IF. Due to such undersampling, the intermediate-frequency signal I
The speed of F can be adjusted.

Reference numeral 2 denotes a digital quadrature demodulation (detection) unit. The digitized intermediate frequency signal IF is demodulated by the quadrature demodulation unit 2 into an I (in-phase) component and a Q (quadrature) component. And a high-frequency component is removed.

The low-pass filter 3 includes an I-component low-pass filter 31, a Q-component low-pass filter 32, and a ROM 33 storing those filter coefficients.
Filtering is performed with characteristics set by coefficients read from the ROM 33 in accordance with the sampling time lag in the / D converter 1.

An AFC correction unit 4 mainly corrects a frequency deviation of the sampling clock CK in the A / D converter 1. In the AFC correction unit 4, the AFC calculation unit 6 calculates the AFC based on the phase error signal captured by the AFC filter 5 and described later, and the AFC correction control is performed.

The quadrature demodulation output signal output from the AFC correction unit 4 is branched for DC offset correction and area determination. The quadrature demodulation output signal branched for DC offset correction is taken into the DC offset correction phase error calculator 7. This phase error calculation unit 7
The phase error θ is detected from the I and Q components of the pilot symbol after the AFC correction by the correction unit 4. This phase error θ is also taken into the AFC filter 5 described above.

The pilot symbol is, for example, 16
In 16QAM in which one frame is composed of symbols, FIG.
The symbol PS is inserted at the head of the frame as shown in FIG. The phase error θ is an angle θ at which a pilot symbol that should be at a normal position indicated by a black circle is shifted to a position indicated by +, as shown in FIG. 2B.
FIG. 2C is a signal point arrangement diagram of 16QAM. The pilot symbol PS is arranged at the signal point of the maximum value “0000”, and this signal point “0000” is also used as data.

Reference numeral 8 denotes a phase error correction unit 8 for DC offset.
The data of the phase error θ obtained by the DC offset phase error calculator 7 is fetched, and the I and Q components of the quadrature demodulation output signal output from the AFC corrector 4 are corrected to normal phases. In this phase correction, the number of symbols per frame is used as a denominator, and an angle obtained by dividing the input phase error θ as a numerator is used as a correction amount per symbol. For example, when the number of symbols per frame is 16, When the phase error θ is 16 degrees, correction is performed once for each symbol.

Reference numeral 9 denotes a DC offset canceling level calculating unit which takes in the I component and the Q component whose phases have been corrected by the phase error correcting unit 8 at a timing described later, and as shown in FIG. The level of the origin shift between the axis and the Q axis: broken lines I ', Q') is detected, and the DC offset canceling level is calculated based on the detected level.

Reference numerals 10 and 11 denote adders inserted on the I component side and the Q component side of the quadrature demodulated output signal branched for the area determination.
The I component calculated by the C offset cancellation level calculator 9;
The DC offset cancellation level value of the Q component is added. Thereby, the I component and Q of the quadrature demodulation output signal for area determination
The DC offset value included in the component is canceled, and the I axis,
The Q axis is in a normal state shown by solid lines I and Q in FIG.

Reference numeral 12 denotes a phase error calculator for area determination, and 1
Reference numeral 3 denotes a phase error correction unit for area determination,
The same process as that of the DC offset correction phase error calculator 7 and the phase error corrector 8 is performed on the I component and the Q component output from 0 and 11. This phase error calculator 12
The processing by the phase error correction unit 13 is a correction processing for more accuracy.

Reference numeral 14 denotes a timing shift detecting unit which detects a timing shift of symbol data, that is, a cycle shift of a baseband signal, based on the I component of the quadrature demodulation output signal for area determination after phase error correction.

Reference numeral 15 denotes a zero-crossing point detecting unit which takes in the data of the timing deviation detected by the timing deviation detecting unit 14 and the I component and the Q component after the phase error correction.
By detecting a zero point (zero cross point) on the I axis, a time lag between an actual sample point and an ideal sample point is detected. This zero-cross detection signal is used for reading out coefficient data from the ROM 33 of the low-pass filter 3.
As a result, the sampling points of the filtering by the low-pass filters 31 and 32 are equivalently moved, and appropriate filtering is performed.

Reference numeral 16 denotes a pilot symbol detector.
The symbol indicating the maximum value in the frame (“00” in FIG.
00 "). The signal detected in this way becomes a pilot symbol PS. When the pilot symbol PS is detected in this manner, a timing signal of the detection is input as an enable signal or the like to the phase error calculation units 7 and 12 and an area determination unit 18 described later.

Reference numeral 17 denotes an amplitude threshold value calculation unit which uses pilot symbols after DC offset correction processing, and
In the case of 6QAM, threshold values Ia and Ib on the I axis and threshold values Qa and Qb on the Q axis are obtained as shown in FIG.

Numeral 18 denotes an area judging unit, which uses the thresholds obtained by the amplitude threshold calculating unit 17 to determine whether the input I component and Q component are, for example, the I and Q thresholds in the signal point arrangement diagram of FIG. It is determined to which of the areas delimited by and the data is decoded. The data obtained by the area determination unit 18 is taken into the DC offset cancellation level calculation unit 9, where the maximum value and the minimum value are detected.

FIG. 3 is a specific block diagram of the DC offset canceling level calculator 9. A maximum / minimum value detection unit 91 detects the maximum vector value and the minimum vector value of the data input from the area determination unit 18, and FIG.
In the signal point constellation diagram, the maximum vector value is “0000” in the first quadrant and the minimum vector value is “101” in the third quadrant.
0 ", which is detected. The pilot symbol always has the maximum vector value “0000”, but even the data may have the maximum vector value “0000”.
The maximum value / minimum value detection unit 91 performs the maximum value even when the data input from the area determination unit 18 is not determined to be the maximum value / minimum value by the area determination unit 18 in the initial stage where the DC offset cancellation is not sufficient. This is for determining the value / minimum value, and when the DC offset cancellation has a sufficient effect, the same determination as the maximum / minimum value determination performed by the area determination unit 18 is performed.

Each time a maximum / minimum value detecting section 91 detects a maximum vector value or a minimum vector value, a switch 92 opens a gate and outputs a phase-corrected I signal from the phase error correcting section 8. Component and the Q component.

Numerals 93 and 94 denote an average value calculating unit. The average value and the Q component of the I component (in this case, either the maximum vector value or the minimum vector value) taken in when the switch unit 92 opens the gate. At this time, the average value of either the maximum vector value or the minimum vector value is calculated. The obtained average value of the I component is a signal indicating the origin of the I axis, and the average value of the Q component is a signal indicating the origin of the Q axis. If the signal indicating both origins is zero, the DC offset amount is zero, but if only one of them is other than zero, there is a DC offset. The DC offset amount is inverted in phase by multipliers 95 and 96 and applied to the adders 10 and 11 as a DC offset correction signal.

As described above, in the present embodiment, the DC offset error caused by the undersampling is obtained by the lines of the phase error calculating section 7, the phase error correcting section 8, and the DC offset canceling level calculating section 9, and the adder 10 , 11 to cancel. At this time, the DC offset cancellation level calculation unit 9 calculates the DC offset cancellation level from the average value of the maximum vector value and the minimum vector value based on the area information determined by the area determination unit 18.
Three data of the maximum vector value as the pilot symbol, the maximum vector value other than the pilot symbol, and the minimum vector value are used for calculating the DC offset cancellation level, and any one of these three data is used at the time of channel switching or the like. Since the appearance probability of one is considerably increased, high-speed pull-in is possible.

In the embodiment described above,
Although the phase error calculation unit and the phase error correction unit are provided on both the quadrature demodulation output signal line for DC offset correction and the quadrature demodulation output signal line for area determination, the AFC correction unit 4
Can be provided immediately after the above and before branching to the quadrature demodulation output signal line and the quadrature demodulation output signal line for area determination. Further, in the above embodiment, the functions of the circuit portion after the output unit of the A / D converter 1 can be realized by software processing by the DSP.

[0034]

As described above, according to the present invention, it is possible to detect a DC offset value at the time of channel switching or the like at a high speed, to perform a high-speed pull-in, to improve the performance in digital communication, and to improve the speed. It greatly contributes to the development of

[Brief description of the drawings]

FIG. 1 is a block diagram of a 16QAM demodulator according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a pilot symbol and the like.

FIG. 3 is a block diagram of a DC offset canceling level calculator of FIG. 1;

[Explanation of symbols]

1: A / D converter, 2: digital quadrature demodulation unit, 3: low-pass filter, 4: AFC correction unit, 5: AFC filter, 6: AFC calculation unit, 7: phase error calculation unit, 8: phase error correction unit , 9: DC offset canceling level calculator, 1
0, 11: adder, 12: phase error calculator, 13: phase error corrector, 14: timing shift detector, 15: zero cross point detector, 16: pilot symbol detector, 1
7: amplitude threshold value calculation unit, 18: area determination unit.

Claims (8)

[Claims] 1. A multi-level QAM demodulation method for converting an intermediate frequency signal of a multi-level QAM signal into a digital signal by undersampling and then quadrature demodulating the data to determine the area of the quadrature demodulated output signal and decoding the data. The quadrature demodulation output signal is branched for DC offset correction and for the area determination, and the D at the timing of detecting the maximum vector value and the minimum vector value based on the area information determined by the area determination is determined.
The DC offset values of the I component and the Q component are respectively obtained from the average value of the I component and the average value of the Q component of the quadrature demodulation output signal for C offset correction, and the quadrature demodulation output signal for area determination is determined based on the DC offset values. A multi-level QAM demodulation method comprising correcting a DC offset between an I component and a Q component. 2. The quadrature demodulation output signal according to claim 1, wherein a phase error of the quadrature demodulation output signal is detected by detecting a pilot symbol included in the quadrature demodulation output signal, and the quadrature demodulation output is detected based on the detected phase error. Multi-value Q characterized by performing signal phase error correction
AM demodulation method. 3. The multi-level QAM demodulation according to claim 2, wherein the phase error correction is individually performed on both the DC offset correction quadrature demodulation output signal and the area determination quadrature demodulation output signal. Method. 4. An A / D converter for converting an intermediate frequency signal of a multi-level QAM signal into a digital signal by undersampling, an orthogonal demodulator for orthogonally demodulating an output signal of the A / D converter, and the orthogonal demodulator. A multi-level QAM demodulation device comprising: an area determining unit that performs data decoding of an output signal obtained by the unit; and a step of branching the quadrature demodulated output signal for DC offset correction and the area determination. DC for obtaining the average value of the I component and the average value of the Q component of the DC offset correction quadrature demodulation output signal at the detection timing of the maximum vector value and the minimum vector value based on the area information determined by the section as a DC offset value
A multi-valued QAM comprising: an offset value calculating means; and a DC offset correcting means for correcting a DC offset of an I component and a Q component of the quadrature demodulation output signal for area determination based on each of the average values.
Demodulator. 5. The DC offset value calculating means according to claim 4, wherein said DC offset value calculating means detects a maximum value / maximum value for detecting that a vector value of the decoded data is a maximum value or a minimum value.
A minimum value detection unit, a switch unit that opens a gate at a timing when the maximum value or the minimum value is detected by the maximum value / minimum value detection unit, and passes the DC offset correction quadrature demodulation output signal, and passes through the switch unit. A first average value calculating unit for calculating an average value of the I component of the quadrature demodulation output signal for DC offset correction, and D which has passed through the switch unit.
A multi-level QAM demodulation device comprising: a second average value calculation unit that calculates an average value of the Q component of the quadrature demodulation output signal for C offset correction. 6. The Q adder according to claim 4, wherein the DC offset correction means corrects an I component of the area determination quadrature demodulation output signal using a signal of an average value of the I component; A multi-level QAM demodulator comprising: a second adder for correcting a Q component of the area determination quadrature demodulation output signal using a signal of an average value of the components. 7. The phase error calculator according to claim 4, wherein a phase error of the quadrature demodulation output signal is calculated by detecting a pilot symbol included in the quadrature demodulation output signal. A phase error correction unit for correcting a phase error of the quadrature demodulated output signal based on the quadrature demodulated output signal. 8. The system according to claim 7, wherein the phase error calculating section and the phase error correcting section
A multi-level QAM demodulator, wherein a line for an offset correction quadrature demodulation output signal and a line for the area determination quadrature demodulation output signal are separately provided.