KR20060055225A - Automatic frequency correction apparatus and method for digital ultra-narrowband terminal, and receiving system using its - Google Patents

Abstract

1. TECHNICAL FIELD OF THE INVENTION

The present invention relates to a frequency offset automatic compensation device for a digital ultra narrowband terminal, a method thereof, and a receiving system using the same.

2. The technical problem to be solved by the invention

The present invention obtains symbol timing and frequency information using only data information without reference frequency information such as a pilot channel while the frequency band has ultra narrow band characteristics, and then estimates a frequency offset to automatically compensate for the error thereof. An object of the present invention is to provide an offset automatic compensation device and a method thereof, and a receiving system using the same.

3. Summary of Solution to Invention

The present invention provides a frequency offset automatic compensation device for a digital ultra narrowband terminal, and extracts and accumulates a phase difference between input data and delay data by the number of symbols used for frequency offset estimation, and then uses the average of the accumulated phase difference. First frequency offset correction means for extracting frequency offset information and correcting the frequency offset similar to the symbol transmission rate by using the extracted frequency offset information; Means for filtering only desired data and compensating for timing errors to extract data of correct timing every symbol period; And differential detection of data whose primary frequency offset is corrected, and extracting and correcting phase error information for each symbol, accumulating phase error information, and correcting a frequency offset of the next data. Include.

4. Important uses of the invention

The present invention is used in a digital ultra narrowband terminal.

Digital ultra narrowband terminal, frequency offset compensation, AFC, CQPSK, frequency error

Description

Frequency offset automatic compensation device for digital ultra narrowband terminal and method and receiving system using same             

1 is an exemplary configuration diagram of a receiver using fine tuning of an oscillator according to a conventional frequency offset compensation device;

2 is a configuration diagram of an automatic frequency offset automatic compensation device for a digital ultra narrowband terminal according to the present invention;

3 is a flow chart of an embodiment of a frequency offset automatic compensation method for a digital ultra narrowband terminal according to the present invention;

4 is an explanatory diagram illustrating a variance of a normalized frequency offset estimation error in the first frequency offset compensation according to the present invention;

FIG. 5 is an exemplary explanatory diagram showing a histogram of a normalized frequency offset estimation error in first frequency offset compensation according to the present invention; FIG.

6 is an exemplary configuration diagram of a receiving system that does not require fine tuning of an oscillator according to the frequency offset automatic compensation device of the present invention.

* Explanation of symbols for the main parts of the drawings

21: frequency offset correction unit 22: primary frequency offset information extraction unit

23: Receive filter 24: Timing synchronizer

25: sampling unit 26: differential detector

27: secondary frequency offset information extraction unit

The present invention relates to a frequency offset automatic compensation device for a digital ultra narrowband terminal, a method thereof, and a receiving system using the same. More particularly, the frequency band has a super narrowband characteristic and uses only data information without reference frequency information. The present invention relates to a frequency offset automatic compensation device, a method thereof, and a receiving system using the same, by implementing an AFC (Automatic Frequency Correction) function by calculating the AFC.

The frequency offset component due to the frequency difference between the base station transmission signal and the terminal reception signal is mainly generated by the Doppler frequency according to the frequency stability of the Voltage Controlled Temperature Compensated Crystal Oscillator (VCTCXO) used as the reference clock in the terminal and the terminal moving speed.

In particular, these components have the effect of rotating the constellations of in-phase and quadrature signals in the case of digital modulation, making it difficult to accurately demodulate the signal, thereby reducing the system's bit error rate (BER). ) Deteriorates performance. In addition, the frequency offset component is generated according to the bandwidth of the modulated signal, that is, in the case of a relatively wide bandwidth of the signal, such as IMT-2000, even if a slight frequency offset occurs, the degree of spectrum shift in frequency compared to the bandwidth. On the contrary, in the case of ultra narrow bands where the signal bandwidth is narrow, even if a slight frequency offset occurs, the movement of the signal spectrum has a relatively large effect, resulting in considerable system performance degradation.

Therefore, to minimize such performance degradation, most commercial systems, such as Code Division Multiple Access (CDMA) mobile communications and 8-Vestibular SideBand (VSB) digital broadcasting, use AFC (reference channel) signals such as pilot channels. Automatic frequency correction function to correct frequency error.

However, the Association of Public Safety Communications Officials (APCO) P25 Complex Quadrature Phase Shift Keying (CQPSK) modulation scheme, which is suitable as a digital ultra narrowband scheme for a simple radio station, has a super narrow band (6.25 kHz) characteristic and a reference frequency such as a pilot. Since there is no information, only the data information should be used to compensate for the frequency error. In addition, in the CQPSK modulation scheme, the phase difference between symbols during one symbol period (+ 3π / 4, + π / 4, -π / 4, -3π / 4) causes phases between symbols during one symbol period of a transmission signal. If the difference is received more than ± π / 4, that is, if the frequency offset size is more than 12.5% of the symbol transmission rate, the system BER characteristic is rapidly increased.

For reference, the configuration of the receiver using the fine tuning of the oscillator according to the conventional frequency offset compensation device is as shown in FIG.

As shown in FIG. 1, in a receiver using an oscillator fine tuning according to a conventional frequency offset compensator, frequency offset component information is obtained using a transmitted reference pilot channel, and then the frequency error thereof is estimated to estimate the VCTCXO 12. By fine-tuning the voltage of the oscillator (11) by changing the frequency characteristics of the frequency error is automatically compensated for.

At this time, the frequency error can be automatically compensated to the locking range of the local oscillator 11 using the reference pilot channel, but if the frequency characteristic according to the voltage of the VCTCXO 12 is not constant for each part, the initial voltage for each terminal It should be set and narrow the control signal interval to increase the frequency error accuracy. In addition, generation of a pulse density modulation (PDM) signal or a pulse width modulation (PWM) signal for the VCTCXO control is required, and an interface must be implemented in hardware.

Therefore, the digital ultra narrowband terminal is urgently required to perform a digital AFC function by calculating the frequency error using only the data information without reference frequency information. By doing so, it is easy to implement, no oscillation frequency fine tuning is required and no adjustment signal is required for its control.

The present invention has been proposed to meet the above-mentioned requirements, and the frequency band has ultra narrowband characteristics, and the symbol timing and frequency information are obtained using only the data information without reference frequency information such as a pilot channel, and then the frequency offset is estimated. An object of the present invention is to provide a frequency offset automatic compensation device for automatically compensating an error digitally, a method thereof, and a receiving system using the same.

Other objects and advantages of the present invention can be understood by the following description, and will be more clearly understood by the embodiments of the present invention. Also, it will be readily appreciated that the objects and advantages of the present invention may be realized by the means and combinations thereof indicated in the claims.

In order to achieve the above object, the present invention provides a frequency offset automatic compensation device for a digital ultra narrowband terminal, wherein the phase difference between the input data and the delay data is extracted and accumulated by the number of symbols used for frequency offset estimation. First frequency offset correction means for extracting frequency offset information using the average of the differences and correcting the frequency offset similar to the symbol transmission rate using the extracted frequency offset information; Means for filtering only desired data and compensating for timing errors to extract data of correct timing every symbol period; And second-order frequency offset correction means for differentially detecting the data whose primary frequency offset is corrected, extracting and correcting phase error information for each symbol, and accumulating phase error information to correct frequency offset of the next data. Characterized in that made.

In addition, the present invention provides a frequency offset correction method applied to a frequency offset automatic compensation device for a digital ultra narrowband terminal, comprising: extracting and accumulating a phase difference between input data and delay data by the number of symbols used for frequency offset estimation; A frequency offset correction step of extracting frequency offset information by using the average of the accumulated phase difference and correcting the frequency offset by using the extracted frequency offset information similar to a symbol transmission rate; Differentially detecting data whose frequency offset is corrected, and extracting and correcting phase error information for each sampled symbol; And accumulating the phase error information and using the frequency offset correction for the next data.

Meanwhile, the present invention provides a symbol timing and frequency using only data information without reference frequency information in a digital signal processing (DSP) means in a receiving system using a frequency offset corrected by the frequency offset automatic compensation apparatus and method. After the information is obtained, the frequency offset is estimated and automatically compensated for the error thereof, so that a control voltage for local oscillator frequency correction is not required.

According to the present invention, the frequency band has ultra narrowband characteristics, and without using reference frequency information such as pilot, the frequency error is reduced to ± π / 4 or less through symbol timing and frequency information using only data information. It satisfies the defined frequency stability characteristics (VHF & UHF band: ± 2.5ppm, 800MHz band: ± 2ppm) and is applied to digital ultra narrowband terminals.

Therefore, in the present invention, the frequency band has a super narrowband characteristic, and there is no need for reference frequency information such as a pilot channel to estimate the frequency offset using only the data information and digitally compensate for the error thereof, thereby fine tuning the oscillation frequency. Therefore, even if the frequency characteristics according to the voltage of VCTCXO is not constant for each part, the frequency error can be compensated automatically, and the interface signal for VCTCXO control is not required. It is easy. In addition, the present invention can be applied not only to the VHF band of the digital ultra narrow band terminal, but also to the 800 MHz band, and can also be used to implement the AFC function of the ultra narrow band terminal for digital TRS.

The above objects, features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings, whereby those skilled in the art may easily implement the technical idea of the present invention. There will be. In addition, in describing the present invention, when it is determined that the detailed description of the known technology related to the present invention may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

2 is a configuration diagram of an automatic frequency offset automatic compensation device for a digital ultra narrowband terminal according to the present invention.

As shown in FIG. 2, the frequency offset automatic compensation apparatus for a digital ultra narrowband terminal according to the present invention extracts and accumulates a phase difference between input data and delay data by the number of symbols used for frequency offset estimation, and then accumulates the accumulated signal. Primary frequency offset correction unit (frequency offset correction unit 21, primary) for extracting frequency offset information using the average of the phase difference and correcting the frequency offset similar to the symbol transmission rate using the extracted frequency offset information. Frequency offset information extracting section 22 and means for extracting data of correct timing every symbol period by filtering only desired data and compensating for timing errors (receive filter 23, timing synchronizer 24, sampling) Section 25), and differentially detect data whose primary frequency offset is corrected, extract phase correction information for each symbol, correct it, and accumulate phase error information. And a second frequency offset correction section (differential detector 26, a second frequency offset information extraction unit 27) for correcting the frequency offset of the data.

Here, the reception filter 23, the timing synchronizer 24, and the sampling unit 25 for sampling timing after every symbol period after filtering only desired data are compensated for a general configuration in an apparatus for compensating for frequency errors. In this invention, since it is only too much, it is a matter of discussion.

The frequency offset automatic compensation device (receiver) for digital ultra narrowband terminals uses two levels of frequency offset correction (primary frequency offset correction and secondary frequency offset correction) according to the magnitude of the frequency error. It satisfies the characteristic (<2.5ppm).

First, the first frequency offset correction unit 21, 22 of the preceding stage is a device for correcting an approximate frequency offset similar to the symbol transmission rate, and can be up to twice the maximum symbol transmission rate. The first frequency offset information extractor 22 extracts the frequency offset information, and the frequency offset corrector 21 corrects the frequency offset using the frequency offset information.

If there is no frequency offset when the random data is input to the primary frequency offset correction unit (21, 22), the average value of the phase difference between the input signal and the delayed signal for a predetermined time is 0 degrees, otherwise the direction and magnitude of the frequency offset Accordingly, the average value of the phase difference between the input signal and the delayed input signal is different, where the average value of the phase difference is the frequency offset amount.

The data whose frequency offset is compensated to a degree similar to the symbol transmission rate by the first frequency offset correctors 21 and 22 is low-pass filtered through the reception filter 23 and then the timing synchronizer 24 and the sampler 25 are adjusted. Through the second frequency offset correction unit 26, 27 is applied. In this case, the secondary frequency offset correction units 26 and 27 digitally compensate for the frequency error generated by using the symbol determination data within ± / 4.

After passing through the differential detector 26, the frequency offset component of the signal input from the secondary frequency offset correcting units 26 and 27 is converted into a phase error component.

Looking at the frequency offset correction process (primary frequency offset correction process (301 ~ 306), secondary frequency offset correction process (307 ~ 311) with reference to Figure 3), the primary frequency offset correction unit (21, 22) After extracting L phase differences between the delayed data and the delayed data (305), the L phase difference information is averaged (306). The frequency information obtained in this way is used to multiply the frequency having the opposite magnitude of the frequency offset measured first, thereby correcting the first frequency offset (306).

The data approximately frequency corrected through the primary frequency offset process is again passed through the secondary frequency offset correction units 26 and 27.

Unlike the second frequency offset correction unit 26 or 27 accumulating L data information in the first frequency offset correction unit 21 and 22, the phase information is extracted from the phase information for each data after the differential detection, and then the phase offset is immediately obtained. It is used for correction (307-309). Only error information is accumulated (310) and used for the next data phase offset correction (307 to 309).

Here, since the secondary frequency offset correction units 26 and 27 cannot correct a frequency offset of 1/8 (π / 4) or more of the symbol frequency, the primary frequency offset correction units 21 and 22 are at least 1 / second. A signal having a frequency offset of 8 (π / 4) or less must be transmitted to the secondary frequency offset correction units 26 and 27. Therefore, if the normalized frequency error range is less than 1/8 (π / 4), the frequency offset appears as a phase error after differential detection, so the secondary frequency offset correction units 26 and 27 use it to input the frequency offset. Correct the frequency offset of the signal.

If the frequency offset of the frequency error corresponds to a symbol frequency of 1/8 (π / 4) or more, the frequency offset is a value obtained by multiplying the delayed value with the original signal as shown in Equation 1 below. Find the magnitude of the frequency offset.

In the present invention, the delay time is set to 1 / 4T, which is the first reason that the sampled data can be used as it is without any processing since the reception sampling uses 1: 4 oversampling, and the frequency estimation range can be maximized. Is the second reason. In this way, if the delay time is set to 1 / 4T, it is possible to correct the frequency offset of twice the symbol frequency.

는 추정하고자 하는 주파수 옵셋을, T는 심볼 주기를, x는 심볼 주기의 4배 오버샘플링한 입력 신호를 나타내고, L₀는 주파수 옵셋 추정에 사용되는 심볼 수를 나타낸다.here,

Denotes the frequency offset to be estimated, T denotes a symbol period, x denotes an input signal oversampled four times the symbol period, and L ₀ denotes the number of symbols used for frequency offset estimation.

4 illustrates the variance of the normalized frequency offset estimation error of the primary frequency offset correction units 21 and 22 through computer simulation.

Here, if the phase difference L value of the delayed data is large (L = 100 → L = 200), accurate frequency offset compensation is possible, but it requires a lot of memory due to long convergence time, so the processing speed of the DSP chip needs to be faster, so it is better Proper trade-off of performance is required.

A histogram of the normalized frequency estimation error for the primary frequency offset correction units 21 and 22 is shown in FIG. 5 by computer simulation.

5 is a histogram of the frequency offset estimation error when L ₀ is 100 and E _b / N _o is 10 dB. Here, the probability that the frequency offset is greater than 1/8 (π / 4) of the symbol frequency may be applied to a voice signal transmission that allows a BER of 10 ⁻³ with 1.58 × 10 ⁻⁵ . That is, the probability that the estimated frequency offset becomes more than 1/8 (π / 4) of the symbol frequency after the first frequency offset correction is very small, and thus can be applied to an actual ultra narrowband terminal.

Therefore, in the hardware configuration of the present invention, when viewed in the receiver using the oscillator fine adjustment according to the conventional frequency offset compensation apparatus of FIG. 1, the oscillation frequency fine adjustment by the DSP module is not necessary and thus the VCTCXO control signal is not necessary. This will be described with reference to FIG. 6.

FIG. 6 is an exemplary configuration diagram of a receiving system in which oscillator fine tuning is not required according to the frequency offset automatic compensation device of the present invention, and illustrates a structure of a receiver simplified by the present invention.

The receiving system of the present invention may appear more clearly through comparison with a receiver requiring fine tuning of the oscillator of FIG.

In FIG. 1 having a conventional frequency offset compensation device, a local oscillator frequency is corrected by using a signal converted to a baseband signal after passing through a low pass filter. At this time, the frequency of the local oscillator 11 should be corrected, but this requires a separate control voltage, which makes the structure somewhat complicated.

However, since the frequency offset correction of the present invention is processed by the DSP algorithm itself, no interface signal with the RF transceiver module is required. That is, when using the present invention, since a separate control voltage is not required as shown in FIG. 6, it is possible to implement a receiving system with a simple structure.

As described above, the method of the present invention may be implemented as a program and stored in a recording medium (CD-ROM, RAM, ROM, floppy disk, hard disk, magneto-optical disk, etc.) in a computer-readable form. Since this process can be easily implemented by those skilled in the art will not be described in more detail.

The present invention described above is capable of various substitutions, modifications, and changes without departing from the technical spirit of the present invention for those skilled in the art to which the present invention pertains. It is not limited by the drawings.

In the present invention as described above, the frequency band has a super narrowband characteristic and obtains symbol timing and frequency information using only data information without reference frequency information such as a pilot channel, and then estimates a frequency offset to automatically compensate for an error thereof. It can be effective.

In addition, the present invention is capable of compensating for frequency error over a range of about twice the symbol data (in the case of APCO P25, the operating range is (-9.6 kHz, +9.6 kHz)), and fine tuning of the oscillator frequency is not required. All of them can be implemented digitally without a signal, making it easy to implement digital ultra narrowband terminals and easily improving performance.

Claims (8)

A frequency offset automatic compensation device for a digital ultra narrowband terminal, After extracting and accumulating the phase difference between the input data and the delay data by the number of symbols used for frequency offset estimation, the frequency offset information is extracted using the average of the accumulated phase difference, and the symbol transmission rate is extracted using the extracted frequency offset information. Similarly to primary frequency offset correction means for correcting the frequency offset; Means for filtering only desired data and compensating for timing errors to extract data of correct timing every symbol period; And Secondary frequency offset correction means for differentially detecting data whose primary frequency offset is corrected, extracting and correcting phase error information for each symbol, and accumulating phase error information to correct frequency offset of the next data. Frequency offset automatic compensation device for a digital ultra narrowband terminal comprising a. The method of claim 1, The primary frequency offset correction means, A frequency offset automatic compensation device for a digital ultra narrowband terminal, characterized in that for transmitting a signal having a frequency offset of less than 1/8 (π / 4) range of frequency error to the secondary frequency offset correction means. The method of claim 1, The primary frequency offset correction means, When the delay time is set to 1 / 4T, the frequency offset automatic compensation device for a digital ultra narrowband terminal, characterized in that the frequency offset of twice the symbol frequency can be corrected. The method according to any one of claims 1 to 3, The primary frequency offset correction means, And automatically correcting the first frequency offset by multiplying a frequency having an opposite magnitude of the first frequency measured using the frequency offset information. The method of claim 4, wherein In the primary frequency offset correction means, A frequency offset automatic compensation device for a digital ultra narrowband terminal, characterized in that for measuring the frequency offset based on the following equation.

(here,

Denotes a frequency offset to be estimated, T denotes a symbol period, x denotes an input signal oversampled four times the symbol period, and L ₀ denotes the number of symbols used for the frequency offset estimation. A frequency offset correction method applied to a frequency offset automatic compensation device for a digital ultra narrowband terminal, Extracting and accumulating the phase difference between the input data and the delay data by the number of symbols used for frequency offset estimation; A frequency offset correction step of extracting frequency offset information by using the average of the accumulated phase difference and correcting the frequency offset by using the extracted frequency offset information similar to a symbol transmission rate; Differentially detecting data whose frequency offset is corrected, and extracting and correcting phase error information for each sampled symbol; And Accumulating the phase error information and using the frequency offset correction for the next data. Frequency offset automatic compensation method comprising a. The method of claim 6, In the frequency offset correction step, Frequency offset correction of twice the symbol frequency is possible, and the frequency offset can be compensated to 1/8 (π / 4) or less by estimating the frequency offset using only data information without reference frequency information. Frequency offset automatic compensation method. A receiving system using a frequency offset corrected by any one of claims 1 to 3, or 6 or 7, The digital signal processing (DSP) means itself obtains symbol timing and frequency information using only the data information without reference frequency information, and then estimates the frequency offset and automatically compensates for the error thereof, thereby providing a control voltage for local oscillator frequency correction. Receiving system, characterized in that it is not necessary.

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