JP5272893B2 - AFC circuit and AFC control method for digital radio - Google Patents

Description

  The present invention relates to an automatic frequency control circuit and an AFC control method for a digital radio, and in particular, an automatic frequency control circuit suitable for use in a digital radio communication apparatus and the like that detect and synchronize a known fixed data pattern. It relates to the control method.

2. Description of the Related Art In recent years, narrowing and digitization of the occupied frequency band per channel have been promoted in various wireless communication systems including a commercial radio (Land Mobile Radio) from the viewpoint of frequency utilization efficiency. For digitalization, for example, FSK (Frequency Shift Keying), PSK (Phase Shift Keying), etc. that are easy to coexist and share with conventional analog systems are adopted in commercial wireless systems, for example. Has been.
On the other hand, when the occupied frequency band is narrowed, the amount of frequency deviation and phase deviation modulated by the information (data) to be transmitted is reduced, so-called modulation becomes shallower, so that the transmission / reception frequency is slightly reduced. There is a greater tendency for data errors to occur due to deviations and fluctuations. In general, deviations and fluctuations in the frequency of transmission and reception are caused by fluctuations in the oscillation frequency of the local oscillator of the transceiver and fluctuations in the element frequency characteristics of the receiving and demodulating circuit due to changes in temperature and aging. The machine is equipped with an AFC (Auto Frequency Control) function. However, in a narrowband wireless device, a function of accurately controlling the frequency of the local oscillator is extremely important.
Various conventional AFC means have been proposed. For example, as shown in FIG. 8, in a wireless system in which a base station (Base Station) 101 and a portable wireless device 102 communicate with each other, a downlink frequency fc_b (Down) transmitted from the base station 101 and a portable wireless device Consider a case where there is a deviation of Δf between the uplink frequency fc_s (UP) + Δf transmitted from the terminal 102. Usually, the base station 101 is not strictly limited in terms of power consumption, shape, and the like, and is thus configured to generate a stable frequency signal using a high-accuracy temperature-compensated crystal oscillator with a thermostat. The portable radio device 102 that requires severe restrictions such as power consumption and downsizing, receives a stable frequency signal transmitted from the base station, and corrects the frequency of the local oscillator of the local station using the frequency information. Thus, the frequency stability equivalent to that of the base station 101 is obtained.

By the way, as a method of synchronizing the local oscillator on the portable wireless device 102 side with the transmission frequency signal from the base station 101, several methods are known. FIG. 9 is a block diagram for explaining an example of an AFC circuit and a control method thereof in a conventional digital wireless device. In the wireless device shown in this example, data to be transmitted is transmitted and received in frame units, and a synchronization signal (frame synchronization word: having a specific signal arrangement pattern at a predetermined position of each frame, for example, the frame head, is transmitted on the transmitter side. Frame Sync Word (hereinafter abbreviated as “SYNC”) is transmitted, and timing information for detecting and processing each information arranged in each frame by detecting this SYNC on the receiver side It is a system to get. In such a wireless communication system, after performing FM detection, when detecting the above-mentioned SYNC and performing synchronization processing, a DC offset component (hereinafter also abbreviated as “DC offset”) accompanying the frequency shift is detected by the SYNC unit DC calculation means. Is used to control a voltage controlled temperature compensated crystal oscillator (VC-TCXO) as a local oscillator so as to reduce the DC offset in accordance with the offset amount. It has been.
As another method, as shown in FIG. 10, as a means for extracting a DC offset component from the FM detection signal, a low-pass filter (LPF) is used instead of the SYNC section DC calculation means. .
Since the same applicant has proposed the synchronization word detection means shown in Patent Document 1, it can be used as a DC offset amount detection means of the present invention described later.

JP 2007-150472 A

However, the above-described conventional AFC circuit and its control method have the following problems.
As is well known, in the mobile radio communication system, the radio wave environment varies significantly, and the level of the incoming radio wave greatly fluctuates. Therefore, it is not always possible to accurately detect SYNC. In addition, in order to speed up the synchronization judgment, the synchronization detection result is compared with a synchronization threshold value that allows a certain range of errors (errors). Generally, it is determined that synchronization is detected. However, in such a system, even if it is determined that the synchronization state is detected, a certain amount of bit errors are included.
FIG. 11 shows the state of AFC control in the circuit shown in FIG. 9 described above. A DC offset corresponding to the frequency deviation is extracted from SYNC added to the head of each frame, and VC is selected according to the level. -Shows control of TCXO. In this case, if the incoming electric field strength deteriorates, the detection accuracy rate of the SYNC data bit decreases, and the DC offset value becomes inaccurate. As a result, the VC-TCXO control signal level varies from frame to frame. That is, even if the oscillation frequency matches the incoming signal from the base station within the allowable range, the DC offset amount may show a value different from the actual frequency deviation amount. Therefore, VC-TCXO control is performed for each frame. The signal level varies.

Further, as a method of extracting the DC offset amount included in the FM detection signal, when the LPF is used as shown in FIG. 10, the frequency control fluctuation accompanying the deterioration of the electric field strength as described above is reduced, but the low frequency component The LPF for extracting (DC component) has a large time constant, so that it takes a considerable time for the frequency to converge to a steady state as shown in FIGS. Accordingly, it is difficult to ensure quick synchronization, and normal communication cannot be performed until synchronization is acquired.
There is also a method of correcting (following) the frequency fluctuation of radio waves transmitted from mobile radios (such as mobile phones and in-vehicle radios) on the base station side. However, as the number of mobile radios increases, In addition to an increase in the equipment burden, there may be a decrease in call quality.
The present invention has been made in view of these various circumstances, and provides an AFC circuit and an AFC control method for a digital radio capable of stable frequency control even when the incoming electric field strength varies. The purpose is that.
For example, in a digital radio employing an FM modulation method, the frequency shift is detected as a DC offset amount included in the FM detection output, but the DC offset does not correspond to an accurate frequency shift when reception quality deteriorates. There is. The present invention provides an AFC circuit of a digital radio and a control method therefor that prevent frequency correction based on such an erroneous value.

In order to solve this problem, the AFC circuit of the digital radio according to claim 1 acquires synchronization word candidate symbol data from a received signal waveform, obtains an error variance value with known synchronization word data , and a synchronization determining means for determining synchronization by comparing the error distribution values and the synchronization determination threshold value, a controllable local oscillator means the oscillation frequency based on said synchronization word candidate symbol data, the frequency shift An offset amount detecting means for detecting a DC offset amount generated in response to the signal, and a DC effective dispersion threshold comparing means for judging whether or not the synchronization state is good. The DC effective dispersion threshold comparing means comprises the synchronization the second DC effective dispersion threshold as the threshold for judging means judges good synchronization state than the synchronization determination threshold value used for determining synchronization Is set, the in a state where it is determined that synchronization detection by the synchronization determining means, when said error variance is determined to better than DC effective dispersion threshold, the frequency of the local oscillator means in response to said DC offset amount There line correction, when said error variance is determined not to be better than DC effective dispersion threshold, characterized in that it does not take place the frequency correction of the local oscillation means corresponding to the DC offset.

According to a second aspect of the present invention, in the AFC circuit of the digital radio according to the first aspect, the synchronization determination means is a symbol of the synchronization word candidate symbol data (X _i ) and the known synchronization word symbol data (fs _i ). Symbol error calculation means for obtaining an error (Y _i ), symbol error average calculation means for obtaining a symbol error average value (E) for all symbols obtained by the symbol error calculation means, and a synchronization word obtained by the symbol error calculation means Symbol error average subtracting means for subtracting each symbol error (Y _i ) of the candidate from the symbol error average value (E), and the error variance value (from the square value of the symbol error average subtraction value for all symbols of the synchronization word candidate The error variance calculation means for calculating δ ² ) and the error variance value (δ ² ) are compared with a preset synchronization determination threshold value. By, which was equipped with a synchronization word determination means for its synchronous word candidate is determined whether a synchronization word, the DC effective dispersion threshold value comparing means includes: a synchronization detection by the synchronization word determination means In the determined state, the error variance value (δ ² ) is compared with a DC effective variance threshold value, and when it is determined that the synchronization state is better than the DC effective variance threshold value, the symbol error average value (E ) Is regarded as a DC offset amount, and the frequency of the local oscillating means is corrected in accordance with the DC offset amount.

A third aspect of the present invention relates to an AFC control method for a digital radio, obtains synchronization word candidate symbol data from a received signal waveform, obtains an error variance value with known synchronization word data , and calculates the error variance value. a synchronization determination process for determining synchronization by comparing the synchronization determination threshold value, the oscillation control process for controlling the oscillation frequency using the local oscillation means, based on the synchronization word candidate symbol data, corresponding to the frequency deviation An offset amount detection process for detecting a DC offset amount generated in response to this, and a DC effective dispersion threshold comparison process for determining whether or not the synchronization state is good. The DC effective dispersion threshold comparison process includes the synchronization determination. processing DC effectively dispersed Ki as a second threshold value for determining good synchronization state than the synchronization determination threshold value used for determining synchronization Value is set, the in a state where it is determined that synchronization detection by the synchronization determination process, the error variance DC effective dispersion corresponding to the DC offset amount if it is determined that the better the threshold the oscillation control process perform frequency correction against, characterized in that the error variance is if it is found not to be better than DC effective dispersion threshold, the frequency correction is not performed with respect to the oscillation control process corresponding to the DC offset amount And

According to a fourth aspect of the present invention, in the AFC control method for a digital wireless device according to the third aspect, the synchronization determination processing is performed between the synchronization word candidate symbol data (X _i ) and the known synchronization word symbol data (fs _i ). Symbol error calculation processing for obtaining symbol error (Y _i ), symbol error average calculation processing for obtaining symbol error average value (E) for all symbols obtained by symbol error calculation processing, and synchronization word obtained by symbol error calculation processing Symbol error average subtraction processing for subtracting each candidate symbol error (Y _i ) and symbol error average value (E), and the error variance value (δ) from the square value of the symbol error average subtraction value for all the synchronization word candidate symbols. ² ) Compare the error variance calculation process to calculate the error variance value (δ ² ) with a preset threshold value for synchronization determination. And by, which includes synchronization word determination process its synchronous word candidate is determined whether a synchronization word, the DC effective dispersion threshold comparison process, it is determined that the synchronization detection by the synchronization word determination process In this state, the error variance value (δ ² ) is compared with the DC effective variance threshold value, and if the synchronization state is determined to be better than the DC effective variance threshold value, the symbol error average value (E) is DC offset. It is regarded as a quantity, and includes a process of performing frequency correction on the oscillation control process corresponding to the DC offset quantity.
The “all symbols” in the symbol error average calculation means (process) for obtaining the symbol error average value (E) for all symbols obtained by the symbol error calculation means (process) needs to be limited to all symbols in one frame. However, the number may be any number as long as a target symbol error average can be obtained, and may be a predetermined number of symbols according to the communication system.

As described above, the present invention detects the DC offset amount generated according to the frequency shift based on the synchronization word candidate symbol data and determines whether the reception quality is good or the synchronization state is good in the AFC circuit of the radio. Only when the DC effective dispersion threshold comparison means determines that the synchronization state is better than the DC effective dispersion threshold, the frequency of the local oscillation means is corrected corresponding to the DC offset amount, or Since it is controlled, even in the synchronization detection state, if the synchronization state, that is, the reception quality state is not equal to or greater than the threshold value, it is determined that the DC offset amount obtained at that time does not indicate an accurate frequency deviation, Does not perform frequency correction.
Therefore, when correcting the oscillation frequency based on the DC offset in the digital radio, if the DC offset does not correspond to an accurate frequency deviation, the frequency correction is prevented from being performed erroneously and accurate and stable frequency control is performed. It is possible to realize the AFC circuit and control of the obtained digital radio.

1 is a block diagram illustrating an embodiment of a wireless device including an AFC circuit of the present invention. It is a figure for demonstrating the example of control of the AFC circuit of this invention, (a) is a frame data block diagram, (b) is a control signal waveform diagram of VC-TCXO (local signal oscillator). The flowchart which shows the example of AFC control of this invention. The flowchart which shows the modification of AFC control of this invention. The image figure which shows the example of frequency control of the conventional AFC circuit. It is a figure which shows the example of frequency control in this invention, (a) is a figure which shows the relationship between a SYNC synchronous detection dispersion | distribution value and DC effective dispersion | distribution threshold value, (b) is a frequency control image figure. The figure which shows the relationship between the DC effective dispersion value which shows the specific example of the frequency control in this invention, and the correction | amendment frequency variation range with a graph. 1 is a schematic diagram illustrating an example of a wireless system. The block diagram which shows an example of the radio | wireless machine provided with the conventional AFC circuit. The block diagram which shows the other example of the radio | wireless machine provided with the conventional AFC circuit. It is a figure explaining the example of control of the conventional AFC circuit, (a) is a frame data block diagram, (b) is a control signal waveform diagram of a local oscillator. It is a figure explaining the other example of the conventional AFC circuit control, (a) is a frame data block diagram, (b) is a control signal waveform diagram of a local signal oscillator.

Hereinafter, the present invention will be described in detail with reference to embodiments shown in the drawings. However, the components, types, combinations, shapes, relative arrangements, and the like described in this embodiment are merely illustrative examples and not intended to limit the scope of the present invention only unless otherwise specified. .
FIG. 1 is a schematic block diagram of a digital mobile radio (portable radio) showing an embodiment of the present invention. The portable radio shown in this example includes a receiving antenna 1, a first mixer (MIX) 2, a first band filter (BPF) 3 for extracting a first intermediate frequency (First IF frequency), and a second mixer (MIX). 4) Second band filter (BPF) 5 for extracting the second intermediate frequency (Second IF frequency), FM detector 6 for detecting the output signal of the second band filter, symbol detection for detecting the symbol value from the detected signal A frame synchronization detection unit (SYNC synchronization detection) 10 that detects the SYNC (frame synchronization word) at the beginning of the frame and performs frame synchronization is a main configuration block of the reception unit (RX).

Further, a VC-TCXO (voltage controlled temperature compensated crystal oscillator) 11 that is a target of AFC control and generates a reference frequency signal in the present invention is based on a SYNC DC offset signal supplied from the frame synchronization detection unit 10. The oscillation frequency is controlled. The oscillation frequency signal of the VC-TCXO 11 is supplied to a voltage controlled oscillator (Voltage Controlled Oscillator: VCO) 13 through a phase lock loop (PLL) circuit 12 having a function of converting to a desired channel frequency. A part of the output of the VCO 13 is supplied to a first local oscillator 14 that generates a first local oscillation signal to the first mixer 2.
The transmission unit TX includes the VCO 13 as a part of the configuration block, generates transmission data and the like by a transmission control unit (not shown), and is supplied to the digital modulation unit (Digital mod) 15 to become a digital modulation signal. . The signal is supplied to the VCO 13 and the output signal is amplified to a required level and emitted from the transmission antenna 15 as a transmission radio wave. The second mixer 4 includes a second local oscillator 17 that supplies a second local signal.

What is characteristic in this embodiment according to the present invention is the configuration and processing portion of the SYNC synchronization detection unit 10, which is configured and controlled as follows.
That is, the SYNC synchronization detecting unit 10 includes a SYNC unit distributed calculation processing unit 18, a synchronization determination processing unit 19 that performs synchronization determination or reception quality pass / fail determination based on the result of the distributed value calculation processing, and a synchronization determination signal, or A SYNCDC calculation processing unit 20 that generates a DC offset signal based on a reception quality determination signal and a signal from the SYNC unit dispersion processing unit 18, and signals from the SYNC unit dispersion calculation processing unit 18 and the SYNCDC calculation processing unit 20 And a DC determination processing unit 21 that compares the supplied dispersion value with a preset DC effective dispersion threshold.

The SYNC unit dispersion processing unit 18 has a function of calculating a dispersion value between a frame synchronization word that is a known fixed pattern (ideal symbol pattern) and a waveform pattern of a received signal. The dispersion value calculated here corresponds to the incoming field strength, that is, the quality of reception quality, as shown in Patent Document 1, and the presence or absence of synchronization or the reception state at that time (reception quality is good or bad) ) Can be used as a means of determining. Therefore, the calculated dispersion value is supplied to the DC determination unit 21 and compared with a preset DC effective dispersion threshold value to determine the reception state (reception quality is good or bad) at that time. When the reception quality is better than a preset threshold value, the frequency correction processing of the local oscillator VC-TCXO 11 is performed based on the average error value indicating the DC offset amount calculated at that time.
That is, when the variance value is large, it is determined that the incoming field strength is small and the DC offset amount at that time does not accurately represent the frequency deviation amount, and the frequency correction processing is not performed, and the variance value is a constant value. The frequency correction process is performed only in the following cases.

As described above, in the present invention, as shown in FIG. 1, synchronization word candidate symbol data is acquired from the received signal waveform and compared with known synchronization word data to determine synchronization, and the oscillation frequency can be controlled. In an AFC circuit of a digital radio having a local oscillation means,
An offset amount detection means for detecting a DC offset amount generated according to a frequency shift when performing synchronization determination based on the synchronization word candidate symbol data;
A threshold comparison unit for determining whether the synchronization state is good or not provided in the synchronization detection determination unit, and a local portion corresponding to the frequency offset amount when the synchronization state is determined to be better than the threshold by the threshold comparison unit It is characterized in that the frequency correction of the oscillation means is performed.
Accordingly, it is possible to provide an AFC circuit or an AFC control method capable of solving the problem caused by performing frequency correction based on the DC offset and performing accurate frequency control in a state where the reception quality is poor as in the prior art.

  FIG. 2 is a diagram illustrating an example of the effect of the present invention. For the first to third frames shown in FIG. 2A, if the above-described symbol error average value E is obtained with respect to the leading SYNC data, the DC offset amount, that is, the frequency shift amount in each frame can be detected. At the same time, the synchronization state in each frame, that is, the quality of the reception quality is judged, and the frequency correction process is executed only when the synchronization detection state or the reception quality is better than a preset threshold value. As a result, as shown in FIG. 2B, when the reception quality state in the second frame is bad, the frequency correction processing is not performed on the frame, so as described in FIG. 11 and the related description, It is possible to solve the problem that the frequency varies depending on the frame.

Hereinafter, specific processing examples of the present invention will be described in detail. The same applicant has proposed means for determining frame synchronization using a variance value as disclosed in Patent Document 1, and in this example, Although not limited, this means can also be used in the present invention. Note that Patent Document 1 focuses on synchronization determination, and the calculation of the DC offset signal is performed in the process. However, in the present invention, when only the DC offset calculation part is used, Both cases of use including determination are conceivable.
That is, in the present invention, the amount of frequency deviation is determined as the DC offset amount, but the frequency deviation calculation (DC calculation) is obtained in the process of performing symbol pattern dispersion calculation as disclosed in Patent Document 1. An average symbol pattern error can be used. In this case, the formula is
E = (1 / N) Σfs _i −X _i (Equation 1)
Here, E is an error average value (frequency deviation) of FS (synchronization word: SYNC), fs _i is a symbol value of the synchronization word FS, N is the number of FS symbols, X _i is a detection signal, and _i is from 0 to N It takes a value of -1.
Further, in order to determine the synchronization state, an error variance value (δ ² ) shown in the following (Equation 2) is obtained.
δ ² = (1 / N) Σ (Y _i −E) ² Y _i = fs _i −X _i (Expression 2)
Here, δ ² is the error variance (FS correlation value) with FS, Y _i is the error between the FS symbol and the detected signal, and _i takes a value from 0 to N−1.
That is, (Equation 1) is obtained by calculating the average error value of the required bits for the difference between the symbol value of the known synchronization word FS and the detection signal after reception detection. In the FM detector, the frequency deviation is DC ( DC) value, so that it can be regarded as an average value of frequency deviation.

A method of synchronous detection using (Expression 2) is disclosed in Patent Document 1. The error variance value (δ ² ) of the received detection signal is a value corresponding to the FS correlation value, whereas the correlation value has a maximum value in the synchronized state, whereas the error variance value (δ ² ) is (Equation 2). As is clear from the above, the minimum value is taken in an ideal synchronization state, and the presence or absence of synchronization can be determined by the value as in the case of the correlation value.
Here, the error average value (frequency deviation) from FS is E, the symbol value of FS is fs, the number of symbols to be calculated is N, the detection signal is X, the error between the FS symbol and the detection signal is Y, FS. Is expressed as δ ² , but in Patent Document 1, it is indicated by D, Foff, n, a, s,... You can refer to it. In addition, the calculation formula shown in Patent Document 1 includes a case where the division by the number of symbols n is not performed in order to obtain an average, but this (1 / n) is a simple constant value, so that it can be omitted. The meaning of is not different.

FIG. 3 is a flowchart showing an outline example of the AFC control procedure according to the present invention, and a control example will be described with reference to the block diagram of FIG. In FIG. 3, when processing is started, synchronization detection is performed in the SYNC synchronization detection unit 10 based on the signal detected by the FM detector 6 as described above. Thereafter, the variance value δ ² shown in (Equation 2) is calculated, and it is determined whether or not the calculated variance value δ ² is smaller than a preset DC effective variance threshold value (S1). This DC effective dispersion threshold is a threshold for determining whether the synchronization state is good or bad. In this determination, if the synchronization state is better than the preset DC effective dispersion threshold (S1, Yes), the error average value E shown in (Equation 1) is regarded as the DC offset amount indicating the frequency deviation. Thus, the oscillation frequency of the VC-TCXO 11 is corrected (S2).
On the other hand, if the variance value δ ² is larger than the preset DC effective variance threshold value in step S1, the synchronization state is bad, and the DC offset value at that time includes a bit error exceeding a certain value. Therefore, the frequency control based on the symbol error average value (frequency deviation) obtained at the time of frame synchronization detection at that time is not performed, and the previous state is maintained (No in S1).
In this example, whether or not the synchronization state is good is determined based on whether or not the dispersion value δ ² is larger or smaller than a preset DC effective dispersion threshold value, and only obtained when the synchronization state is good. The frequency control of the local oscillator is performed based on the obtained symbol error average value (frequency deviation). As the synchronous detection means in this example, a conventionally known method may be used. However, as described above, if the invention of Patent Document 1 that is the same applicant is used, the present invention is more efficiently performed. The invention can be implemented.

FIG. 4 is a flowchart for explaining another example of AFC control according to the present invention, which uses the synchronization means disclosed in Patent Document 1. In this example, when processing is started, reception / detection processing is performed (S10), symbol data (fs _i ) of a synchronization word candidate is captured from the received signal (S11), and symbol error Y _i (= fs _i −X) _i ) is calculated (S12). In this equation, ( _i ) is the SYNC symbol number of each frame, and here it is the total number of bits of SYNC. However, the number of bits is not necessarily the number of bits as long as it is the number of bits necessary for synchronization status determination and reception status determination. Need not be.
Next, an average of symbol errors is calculated for the number N of SYNC symbols (S13), and using this symbol error average E (= (1 / N) Σfs _i −X _i ), error variance δ ² shown in (Expression 2) (= (1 / N) Σ (Y _i −E) ² ) is calculated (S14). As described in detail in Patent Document 1, this error variance value can be used to determine the synchronization state. Therefore, following the method, it is determined whether or not the synchronization state is established by comparing with a preset synchronization determination threshold value (synchronization threshold value) (S15). If synchronization is detected in this determination (S15, Yes), then, similarly, the error variance value δ ² indicating the synchronization state is set to “DC valid” which is the second threshold value for determining the quality of the synchronization state. It is compared with “dispersion threshold” (S16). In this determination, if the error variance value δ ² is smaller than the DC effective variance threshold value, it is determined that the synchronization state is good (S16, Yes), and the already calculated error average E is set to the DC offset amount ( The frequency of the local oscillator VC-TCVO is corrected assuming that the frequency deviation amount (S17).

If the error variance value δ ² is larger than the synchronization threshold value in the process S15, it is determined that the state is an asynchronous state, and the process is terminated, or the process returns to S10 and symbol data of a new synchronization word candidate The same processing is repeated for. If the error variance value δ ² is larger than the DC effective variance threshold in the process S16, it is determined that the synchronization is obtained but the synchronization state or the reception quality state is not good (S16, No). However, without performing the frequency correction process based on the DC offset obtained at that time (S18), the process returns to S10 and the same process is repeated for the new synchronization word candidate symbol data, or the process ends.
In the example shown in FIG. 4, two threshold values are set in S15 and S16, but the first threshold value is a threshold value for synchronization determination, which is necessary for quick synchronization acquisition. In general, an allowable range for a bit error rate is set relatively wide so that synchronization can be acquired with a minimum communication quality. On the other hand, the DC effective dispersion threshold value, which is the second threshold value, is used to determine whether or not the error average E at that time indicates an accurate DC offset amount. Therefore, the magnitude relationship is set such that the threshold for synchronization> DC effective dispersion threshold.
In this way, the target frequency correction function is stabilized by further correcting the oscillation frequency after determining whether the synchronization state is good or not in the synchronized state.

  5 and 6 are diagrams for explaining the effect of the present invention. First, as shown in FIG. 5, considering an example in which the oscillation frequency is corrected based on the DC offset amount at that time only on the condition that synchronization is detected in each frame, an allowable bit error rate in synchronization determination is considered. Is large, it may deviate from the frequency stability (allowable range Δf) required by the system. In FIG. 5, the portion surrounded by a broken line corresponds to this. If transmission is performed at a frequency that deviates from the allowable value in this way, the base station that receives the transmission increases time and processing load for frequency tracking, and the symbol error rate may deteriorate.

On the other hand, in the present invention, such a problem can be eliminated as shown in FIG. That is, FIG. 6A is a diagram showing the relationship between the dispersion value δ ² in the SYNC synchronization detection and the DC effective dispersion threshold, and δ ² becomes smaller as the communication quality (synchronization state) is better. (The level is zero in the ideal state). Therefore, if a DC effective dispersion threshold value (solid line) having a level smaller than that of the synchronization threshold value (one-dot chain line) is set, as shown in FIG. However, if the communication quality is poor and it is not preferable for maintaining the frequency stability required by the system, it is possible to determine that fact and control the frequency not to be corrected. Therefore, according to this circuit and the control method, transmission and reception can be performed while guaranteeing the frequency accuracy required in the system between the base station and the portable wireless device.
FIG. 7 shows an example of calculating the relationship between the frequency variation range (Hz) and the DC effective dispersion threshold when the present invention is actually implemented. This example is a simulation using a system frame based on the APCO P25 standard. In an environment where BER = 5%, variation (corresponding to 5σ) of DC value (frequency deviation calculated value) for each DC effective dispersion value is shown. Show. Note that the DC effective dispersion threshold indicates a dispersion value calculated using symbol values (± 1, ± 3).
As is apparent from this figure, the frequency variation range varies depending on the DC effective dispersion threshold. For example, when the allowable range of the frequency stability is 160 Hz, the DC effective dispersion threshold is set to 1, and when the allowable range is 200 Hz, the DC effective dispersion threshold is set to 4. .

The present invention need not be limited to the examples described above, and various modifications are possible. For example, the timing to correct the oscillation frequency can be avoided by avoiding synchronization detection and data detection, thereby preventing errors in data being detected due to frequency fluctuations due to frequency control. It will be possible. If the previous frequency error is within the allowable range, and the frequency error in the current processing is very small (less than a certain value) compared to the frequency error in the previous frequency correction, the frequency correction If processing is not performed, it will be useful in reducing the occurrence of bit errors due to frequency correction.
Further, the “all symbols” in the symbol error average calculation means (process) for obtaining the symbol error average value (E) need not be limited to all symbols in one frame, but can be a number that can obtain a target symbol error average. However, it may be effective to perform quick AFC control by further configuring or controlling the number of symbols to be the minimum necessary number according to the reception quality state.
Furthermore, if the AFC circuit or the AFC control method of the above-described embodiment is programmed, the present invention can be implemented by installing these programs and data in a communication device equipped with a DSP or CPU. is there.

6 FM detector, 7 symbol detector, 10 frame sync detector (SYNC sync detector), 11 local oscillator (VC-TCXO), 13 voltage controlled oscillator (VCO), 18 SYNC variance value calculator, 19 sync determiner , 20 SYNC unit DC calculation unit, 21 DC determination unit

Claims (4)

Synchronization determination means for acquiring synchronization word candidate symbol data from a received signal waveform, obtaining an error variance value with known synchronization word data , and comparing the error variance value with a synchronization determination threshold value. When,
Local oscillation means capable of controlling the oscillation frequency ;
An offset amount detecting means for detecting a DC offset amount generated according to a frequency shift based on the synchronization word candidate symbol data;
DC effective dispersion threshold value comparison means for judging the quality of the synchronization state,
The DC effective variance threshold value comparison means includes:
The synchronization determination means sets the DC effective dispersion threshold as a second threshold value for determining good synchronization state than the synchronization determination threshold value used for determining synchronization, the sync word determined in the state in which it is determined that the synchronization detection by means, when the error distribution is determined to better than DC effective dispersion threshold, have rows frequency correction of the local oscillator means in response to the DC offset, the An AFC circuit of a digital radio , wherein when the error variance is determined not to be better than the DC effective variance threshold, the frequency of the local oscillation means corresponding to the DC offset amount is not corrected . In the AFC circuit of the digital wireless device according to claim 1,
The synchronization determination means is obtained by a symbol error calculation means for obtaining a symbol error (Y _i ) between the synchronization word candidate symbol data (X _i ) and the known synchronization word symbol data (fs _i ), and the symbol error calculation means. Symbol error average calculating means for obtaining a symbol error average value (E) for all symbols,
Symbol error average subtracting means for subtracting each symbol error (Y _i ) of the synchronization word candidate obtained by the symbol error calculation means and the symbol error average value (E), and for all symbols of the synchronization word candidate, the symbol The error variance calculation means for calculating the error variance value (δ ² ) from the square value of the error average subtraction value and the error variance value (δ ² ) are compared with a preset synchronization determination threshold value to thereby determine the synchronization. A synchronization word determination means for determining whether or not the word candidate is a synchronization word;
The DC effective dispersion threshold comparison means, in a state where it is determined that the synchronization detection by the synchronization word determination means, the error distribution value ([delta] ²⁾ compared to the DC effective dispersion threshold, the synchronization status DC The symbol error average value (E) is regarded as the DC offset amount when it is determined to be better than the effective dispersion threshold, and the frequency of the local oscillating means is corrected in accordance with the DC offset amount. AFC circuit of digital radio. Synchronization determination processing for acquiring synchronization word candidate symbol data from a received signal waveform, obtaining an error variance value with known synchronization word data , and comparing the error variance value with a threshold value for synchronization determination. When,
An oscillation control process for controlling the oscillation frequency using a local oscillation means ;
Based on the synchronization word candidate symbol data, an offset amount detection process for detecting a DC offset amount generated according to a frequency shift;
DC effective dispersion threshold comparison processing for determining whether the synchronization state is good or bad,
The DC effective variance threshold value comparison process is:
The synchronization determination process is DC effective dispersion threshold as a second threshold value for determining the synchronization determination for better synchronization state than the threshold used for synchronization determination is set, the synchronization determination processing If the error variance is determined to be better than the DC effective variance threshold value in the state where the synchronization detection is determined by the above , frequency correction is performed on the oscillation control process corresponding to the DC offset amount, and the error A frequency control is not performed for the oscillation control process corresponding to the DC offset amount when it is determined that the variance is not better than the DC effective variance threshold , the AFC control method for a digital radio device . In the AFC control method of the digital radio according to claim 3,
The synchronization determination process is obtained by a symbol error calculation process for obtaining a symbol error (Y _i ) between the synchronization word candidate symbol data (X _i ) and the known synchronization word symbol data (fs _i ), and the symbol error calculation process. Symbol error average calculation processing for obtaining a symbol error average value (E) for all symbols,
Symbol error average subtraction processing for subtracting each symbol error (Y _i ) of the synchronization word candidate obtained by the symbol error calculation processing and the symbol error average value (E), and for all symbols of the synchronization word candidate, the symbol An error variance calculation process for calculating an error variance value (δ ² ) from the square value of the error average subtraction value and the error variance value (δ ² ) are compared with a preset synchronization determination threshold value to thereby determine the synchronization Including a synchronization word determination process for determining whether a word candidate is a synchronization word,
The DC effective dispersion threshold comparison processing, in a state where it is determined that the synchronization detection by the synchronization word determination processing, the error distribution value ([delta] ²⁾ compared to the DC effective dispersion threshold, the synchronization status DC When it is determined that the value is better than the effective dispersion threshold, the symbol error average value (E) is regarded as the DC offset amount, and the oscillation control processing is frequency corrected in accordance with the DC offset amount. A method for controlling an AFC of a digital radio device.

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