JP2004112655A - Multilevel qam demodulation method and apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multilevel QAM demodulation method and apparatus for preventing erroneous decoding caused by erroneous demapping by monitoring a specific synchronizing word symbol. <P>SOLUTION: In the multilevel QAM demodulation method, a decoded signal is obtained by receiving a multilevel QAM signal, synchronizing frames by using the synchronizing word symbol and a pilot symbol, creating a threshold for determination from the average value of the amplitude information of the pilot symbol and determining the received multilevel QAM signal on the basis of the threshold for determination. It is determined whether the specific symbol in the synchronizing word symbol is right or not and when the errors continue equal with or longer than the predetermined number of times, the frame synchronization is canceled, and the threshold for determination is calculated on the basis of the amplitude information of the single pilot symbol. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a demodulation method and apparatus for receiving and demodulating a multi-level QAM signal.
[0002]
[Prior art]
2. Description of the Related Art In recent years, for a fixed wireless system such as a disaster prevention wireless system for municipalities, the application of a 16QAM system, which is a modulation system having high frequency use efficiency, is required. This 16QAM digital radio receiving apparatus performs frame synchronization by using a synchronization word symbol inserted at the beginning of a frame or a pilot symbol periodically inserted into an information field of the frame. , A received signal is determined based on the determined threshold, and a decoded signal is obtained (refer to Patent Documents 1 to 4 as demodulators using synchronization symbols and pilot symbols). In this decoding operation, accurate decoding cannot be performed unless the determination threshold is accurate.
[0003]
[Patent Document 1]
JP 2001-313683 A [Patent Document 2]
JP-A-6-85865 [Patent Document 3]
JP-A-10-75229 [Patent Document 4]
JP-A-8-331193
[Problems to be solved by the invention]
However, since this determination threshold is created from the average value of amplitude information of a plurality of pilot symbols in the past, a pilot symbol detected in an unstable operation state such as at power-on or at the time of frame mis-synchronization is determined. If used, the amplitude information of the pilot varies, so that a judgment threshold value greatly deviated from a normal value is created, and decoding by erroneous demapping may be performed.
[0005]
An object of the present invention is to provide a multilevel QAM demodulation method and apparatus which monitors a specific synchronization word symbol and prevents erroneous decoding due to erroneous demapping.
[0006]
[Means for Solving the Problems]
The invention according to claim 1 receives a multi-level QAM signal, synchronizes a frame using a synchronization word symbol and a pilot symbol, creates a determination threshold from an average value of the amplitude information of the pilot symbol, and In a multi-level QAM demodulation method for determining a received multi-level QAM signal based on a threshold value to obtain a decoded signal, it is determined whether a specific symbol among the synchronization word symbols is correct or not, and the frame synchronization is performed when an error continues for a predetermined number of times or more. And a multi-level QAM demodulation method characterized in that the determination threshold is calculated based on amplitude information of a single pilot symbol.
[0007]
According to a second aspect of the present invention, in the multi-level QAM demodulation method according to the first aspect, the specific symbol is a last symbol of a plurality of consecutive synchronization word symbols. Method.
[0008]
According to a third aspect of the present invention, a frame detector that receives a multi-level QAM signal and performs frame synchronization using a synchronization word symbol and a pilot symbol, and creates a determination threshold from an average value of the amplitude information of the pilot symbol. In a multi-level QAM demodulation apparatus comprising: a threshold calculating section; and a demapping section that determines a received multi-level QAM signal based on a determination threshold created by the threshold calculating section and obtains a decoded signal. An error detection unit which is enabled at the timing of a specific symbol, and compares a signal of the specific symbol with a regular symbol stored corresponding to the specific symbol, and increments when an error is detected by the error detection unit. And an integration counter that is reset when correctness is detected, and whether the component counter is counted up to a predetermined value. And a control unit that controls the frame detection unit to release frame synchronization when the determination unit counts to the predetermined value, and calculates the determination threshold based on amplitude information of a single pilot symbol. A multi-level QAM demodulator characterized by controlling the threshold value calculation unit to perform the above operation.
[0009]
According to a fourth aspect of the present invention, in the multi-level QAM demodulator according to the third aspect, the specific symbol is a last symbol of a plurality of consecutive synchronization word symbols. The device.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present embodiment, as shown in FIG. 4A, a format has a ramp signal 31 of about four symbols at the head, followed by a known synchronization word (SW) symbol field 32, followed by an information field 33. 16QAM frame 30 is received. Then, by detecting the timing of the end of the synchronization word symbol field 32 (the start point of the information field 33) by correlation processing, provisional symbol synchronization is achieved, and thereafter, the pilot symbols included in the information field 33 are periodically updated. Frame synchronization is achieved by receiving the data.
[0011]
As shown in FIG. 4B, the synchronization word symbol SW is composed of ten symbols SW ₁ to SW ₁₀ , and the value of 16QAM of each synchronization word symbol is 0, 8, A, or 2 Have been chosen. FIG. 4C is a signal point arrangement diagram showing the arrangement position of the 16QAM value. Symbols 0, 8, A, and 2 are on the maximum amplitude envelope, and the amplitude (absolute value, the same applies hereinafter) is Same at peak value. Note that any of the symbols (any of 0, 8, A, and 2) on the maximum amplitude envelope in FIG. 4C is used as the pilot symbol. For example, when the same symbol 0 as the synchronization word symbol SW ₁₀ is used as the pilot symbol, the synchronization word symbol SW ₁₀ can function as the first pilot symbol of the information field 33.
[0012]
Then, on the transmission side, the synchronization word symbols are included in the synchronization word symbol field 32 of the frame 30 in the order of SW ₁ to SW ₁₀ and sent, and on the reception side, the synchronization word symbols SW _{1 to} SW ₁₀ are correlated and detected. frame by the time the peak is obtained (that is the last synchronization time the word symbol SW ₁₀ is detected) information field 33 is tentatively detected as the timing point for starting, then periodically receives pilot symbols Synchronization is performed, and determination thresholds (+ Ith, -Ith, + Qth, and -Qth in FIG. 4C) for decoding the 16QAM signal are set using the average value of the amplitude information of the pilot symbols.
[0013]
However, when it could not be decoded synchronization word symbol SW ₁₀ continues for a predetermined number of times, with re-synchronization frame, to create the determination threshold without using the average value of the amplitude information of the pilot symbols, single pilot symbol Use the amplitude information of When made properly decode the sync word symbol SW ₁₀ is to create the determination threshold by using the average value of the amplitude information of the pilot symbols so that stable decoding is performed. The details will be described below.
[0014]
FIG. 1 is a block diagram of a main part of a 16QAM demodulator according to one embodiment. Reference numeral 1 denotes a signal that is received and converted into an IF (intermediate frequency) signal is converted into a digital signal by undersampling with a sampling signal fs having a lower frequency (for example, 260 kHz or 140 kHz when the IF signal is 455 kHz). An A / D converter 2 and a quadrature detector 2 extract an in-phase component (I component) and a quadrature component (Q component) from the digitized IF signal by quasi-synchronous detection.
[0015]
FIR filters 3 and 4 remove high-frequency components from in-phase and quadrature components. The FIR filters 3 and 4 follow the zero-cross points of the in-phase component and the quadrature component, and follow the phase shift amount between the sample point closest to the Nyquist point (the widest part of the eye pattern) and the theoretical value (Nyquist point). It is configured to perform a product-sum operation based on the contents of the filter table 5.
[0016]
Reference numeral 6 denotes a complex correlator, which is a complex between a received synchronization word symbol (SW ₁ to SW _{10 in} FIG. 4) and a conjugate value of a synchronization word symbol (same as SW ₁ to SW _{10 in} FIG. 4) stored in advance therein. performs the operation, the symbol position of the last sync word symbol SW ₁₀ from the correlation peak values (timing value: the sample points) to the roughly detected.
[0017]
Reference numeral 7 denotes a pilot detection unit, which repeatedly accumulates and stores the power values of the received symbols in a circular memory having a memory space for one cycle interval of the pilot symbols, calculates the average power of each received symbol, and obtains the maximum average power value. Is detected as a timing point of a pilot symbol, when the memory address indicating is a predetermined specific memory address.
[0018]
Reference numeral 8 denotes a frame detection unit that performs frame synchronization based on the timing position of the synchronization word symbol SW ₁₀ detected by the complex correlation unit 6 and the timing position of the pilot symbol detected by the pilot detection unit 7.
[0019]
Reference numeral 9 denotes an AFC rotation correction unit that corrects a phase rotation with respect to a steady frequency error generated between transmission and reception from the phase rotation amount of the synchronization word symbol and the pilot symbol. Reference numeral 10 denotes a phase error calculation unit that operates only when receiving a pilot symbol and calculates a phase shift of the pilot symbol from a normal position. A phase error correction unit 11 corrects the phases of the in-phase component and the quadrature component to normal phases based on the phase error calculated by the phase error calculation unit 10. Reference numeral 12 denotes a frequency error calculator, which calculates a steady-state frequency error generated between transmission and reception from a pilot symbol and sends it to the AFC rotation corrector 9.
[0020]
Reference numeral 13 denotes a threshold value calculation unit that calculates an average value of amplitude values of a plurality of past pilot symbols and determines a threshold value for decoding a 16QAM signal (+ Ith, -Ith, + Qth, and -Qth in FIG. 4C). ) Is set.
[0021]
Reference numeral 14 denotes a demapping unit which uses the determination threshold value obtained by the threshold value calculation unit 13 to determine which signal point arrangement position 0 to F the received symbol corresponds to in FIG. Then, the result is output as a multi-bit decoded signal.
[0022]
Reference numeral 15 denotes a zero-cross detecting unit which receives the in-phase signal and detects a positive-to-negative transition point and a negative-to-positive transition point as a zero-cross point. Reference numeral 16 denotes a timing correction unit which detects whether the polarity code (positive → negative, negative → positive) of the zero-cross point detected by the zero-cross detecting unit 15 is the same as the code of the previous symbol or the code of the current symbol. The timing is synchronized by performing a random walk filter process. In this random walk filter, the threshold is switched so that the pull-in is accelerated with a small threshold before the frame synchronization and is stabilized with the large threshold after the frame synchronization. Information on the zero-cross point detected by the zero-cross detection unit 15 is sent to the filter table 5, and the filter coefficients for the FIR filters 3 and 4 are selected.
[0023]
A clock reproducing unit 17 reproduces and outputs a clock corresponding to the information field 33 of the frame 30 in FIG. 4A based on the frame synchronization signal detected and determined by the frame detecting unit 8.
[0024]
Reference numerals 18 and 19 denote FIFOs, which operate in synchronization with a clock obtained by the clock reproducing unit 17. The level of the pilot symbol used in the threshold value calculation unit 13 is fetched from the FIFO 18 and an RSSI (received signal field strength signal) is output. The FIFO 19 outputs the signal decoded by the demapping unit 14. WE indicates a write enable, and RE indicates a read enable.
[0025]
Reference numeral 20 denotes an operation enable signal generation unit which takes in the frame synchronization signal detected by the frame detection unit 8 and operates the units (the units other than the quadrature detection units 2, the FIR filters 3, 4, and the FIFOs 18, 19). Generate an enable signal.
[0026]
21 is a frame synchronization monitoring unit captures the decoded signal of the sync word symbol SW ₁₀ of the decoded signal decoded by the demapping section 14, determines whether it is correct, If invalid frames The detection unit 8 cancels the frame synchronization and restarts the frame synchronization. At the same time, the averaging process in the threshold calculation unit 13 is stopped, and the threshold calculation is performed only with a single pilot symbol. The portion surrounded by a broken line can be constituted by a DSP.
[0027]
FIG. 2 is a block diagram of a specific configuration of the frame synchronization monitoring unit 21 described above. The frame synchronization monitoring unit 21 includes a ROM 211 storing a normal synchronization word symbol SW ₁₀ , an error detection unit 212 which is enabled at a specific timing and operates only at that time, and increments or resets based on a detection result of the error detection unit 212. An integration counter 213 to be performed, and a determination unit 214 for performing error determination according to the count value of the integration counter 213.
[0028]
The error detection unit 212 is enabled at the symbol timing of the synchronization word symbol SW ₁₀ detected by the frame detection unit 8, and receives the decoded signal of the synchronization word symbol SW ₁₀ input from the demapping unit 14 and the normal signal stored in the ROM 211 in advance. comparing the synchronization word symbol SW ₁₀ performs error detection. The integration counter 213 is incremented when the error detection unit 212 performs the error detection, and is reset when the error detection unit 212 does not perform the error detection. When the count value A of the integration counter 213 is A = 0 (reset), the determination unit 214 issues a signal for causing the threshold value calculation unit 13 to operate normally, and the count value A exceeds a predetermined comparison value B (A ≧ A). B) At the time, the frame synchronization flag is released for the frame detection unit 8, and further, the threshold value calculation unit 13 stops the average value processing for calculating the average value of the determination threshold value from the past plurality of pilot symbols, and receives the signal. The threshold calculation unit 13 is controlled to calculate the threshold using a single pilot symbol.
[0029]
Next, the operation of the frame synchronization monitoring unit 21 will be described with reference to the flowchart of FIG. When synchronization becomes when symbol timing of the word symbols _{SW 10,} the decoded signal and the previously stored synchronous word symbol _{SW 10} in ROM211 synchronization word symbol _{SW 10} for input from demapping section 14 in the error detection unit 212 are compared, both one If so, the integration counter 213 is reset, and as a result A = 0, the threshold calculation unit 13 continues normal operation by the determination unit 214. In such a normal operation, the integration counter 213 continues to be reset, and the threshold value calculation unit 13 continues the operation of the average value processing (update of the threshold for determination).
[0030]
On the other hand, when a system abnormality or the like occurs in the RF unit or the like and the two do not match in the error detection unit 212, the integration counter 213 is incremented. Then, this discrepancy is generated every timing of each of the sync word symbol SW _10, the integrated value A exceeds the comparison value B and (A ≧ B), it is determined that the erroneous synchronization frame, the frame synchronization of the flag Is released, and the frame detection unit 8 searches again for the position of the synchronization word and controls the frame synchronization operation to resume. Further, the threshold value calculation unit 13 stops the averaging process, and calculates a threshold value using the received single pilot symbol.
[0031]
【The invention's effect】
As described above, according to the present invention, whether or not the frame synchronization is normal is constantly monitored. If the frame synchronization is not normal, the frame synchronization is restarted and the average value processing is not used for the determination threshold value. As a result, it is possible to prevent the determination threshold for demapping from fluctuating greatly, and to perform accurate demapping.
[Brief description of the drawings]
FIG. 1 is a block diagram of a 16QAM demodulator according to one embodiment.
FIG. 2 is a block diagram of a frame synchronization monitoring unit in FIG.
FIG. 3 is a flowchart of an operation of a frame synchronization monitoring unit.
FIG. 4 is an explanatory diagram of a synchronization word symbol.

Claims (4)

A multi-level QAM signal is received, frame synchronization is performed using a synchronization word symbol and a pilot symbol, a determination threshold is created from the average value of the amplitude information of the pilot symbol, and the received multi-level QAM signal is formed by the determination threshold. In a multilevel QAM demodulation method of determining and obtaining a decoded signal,
The correctness of a specific symbol among the synchronization word symbols is determined, and when the error continues for a predetermined number of times or more, the frame synchronization is released, and the determination threshold is calculated based on amplitude information of a single pilot symbol. Characteristic multi-level QAM demodulation method. The multi-level QAM demodulation method according to claim 1,
The multi-level QAM demodulation method, wherein the specific symbol is the last symbol of a plurality of consecutive synchronization word symbols. A frame detection unit that receives a multi-level QAM signal and performs frame synchronization using a synchronization word symbol and a pilot symbol, a threshold calculation unit that creates a determination threshold from an average value of amplitude information of the pilot symbols, And a demapping unit for determining a received multilevel QAM signal based on a determination threshold created by the unit and obtaining a decoded signal.
An error detection unit that is enabled at the timing of a specific symbol of the synchronization word symbol and compares a signal of the specific symbol with a normal symbol stored in correspondence with the specific symbol; An integration counter that is incremented when is detected and reset when correctness is detected, and a determination unit that determines whether the component counter is counted up to a predetermined value. And counting the frame detection unit to release frame synchronization when the count is counted up, and controlling the threshold calculation unit to calculate the determination threshold based on amplitude information of a single pilot symbol. Value QAM demodulator. The multi-level QAM demodulator according to claim 3,
The multi-level QAM demodulator, wherein the specific symbol is the last symbol of a plurality of consecutive synchronization word symbols.

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