KR100611100B1 - Digital receiving apparatus using Low-IF method for ultra-narrowband - Google Patents

Abstract

1. TECHNICAL FIELD OF THE INVENTION

The present invention relates to a digital ultra narrowband receiver using a low-IF scheme.

2. The technical problem to be solved by the invention

In the present invention, when the low-IF structure is used in an ultra narrowband receiver having a bandwidth of 6.25 kHz, the in-phase (I) through the optimal selection of the intermediate frequency (IF) frequency while satisfying the frequency ultra narrow banding condition of 6.25 kHz or less Its purpose is to provide a digital ultra narrowband receiver using a low-IF method that can minimize the effects of gain and phase error of quadrature (Q) demodulator.

3. Summary of Solution to Invention

According to an aspect of the present invention, in a reception apparatus of a digital ultra narrowband terminal, an RF reception for demodulating a received RF signal into an IF signal having an in-phase (I) / quad phase (Q) component for variable gain amplification after filtering Way; Analog / digital conversion means for converting an intermediate frequency (IF) signal of the analog I / Q component into a digital signal, respectively; Amplifying means for amplifying variable gains of the IF signals of the digital I / Q components through automatic gain control (AGC), respectively; Frequency correction means for detecting a frequency offset (AFC) to vary the frequency of the local oscillator to compensate for the frequency offset; I / Q demodulation means for converting the IF signal of the I / Q component whose frequency offset is compensated into the original low frequency signal; Filtering means for low-pass filtering the IF signal of the I / Q component converted into the low-frequency signal to remove noise and adjacent channel signals; Symbol recovery means for recovering symbol timing of an IF signal of the lowpass filtered I / Q component; And differential detection means for differentially detecting the IF signal of the I / Q component whose symbol timing has been recovered and outputting a final symbol value.

4. Important uses of the invention

The present invention is used in digital ultra narrowband terminals and the like.

Ultra narrowband terminal, Low-IF, DSP, receiver

Description

Digital receiving apparatus using Low-IF method for ultra-narrowband}             

1 is a configuration example of a conventional FM and digital narrowband (12.5kHz) terminal,

2 is a block diagram of an embodiment of a digital ultra narrowband receiver using a Low-IF scheme according to the present invention;

3 is an exemplary diagram illustrating an IF selection process for minimizing the influence of an image signal in a digital ultra narrowband receiver according to the present invention.

* Explanation of symbols on the main parts of the drawing

200: RF receiving module 220,230: analog to digital converter

240: DSP module

The present invention relates to a digital ultra narrow band reception apparatus using a low-IF method, and more particularly, to satisfy in-phase super narrow banding conditions of 6.25 kHz or less, and to select in-phase (I: The present invention relates to a digital ultra narrowband receiver using a low-IF method that can minimize the effects of gain and phase error of an in-phase / quad-phase demodulator.

Currently, the land mobile radio system using the VHF (Very High Frequency) or UHF (Ultra High Frequency) band frequency mainly uses the FM method, but the utilization efficiency per unit frequency channel to improve the frequency shortage caused by the spectrum increase. Development of a frequency ultra narrowband technology that can be maximized (25 kHz → 12.5 kHz → 6.25 kHz) is underway.

The developed countries' ultra-bandwidth progress is shifting to digital rather than analog single sideband (SSB) technology, which provides 4 ~ 5 times more efficient use of frequency than 25KHz channel FM, while providing high quality service through network interworking. Promoting possible technology development. In addition, advanced foreign companies use SDR (Soft Defined Radio) technology to provide interoperability between existing analog and digital systems, public safety and many commercial systems (TETRA, APCO P25, MPT1327, GSM). It is focusing on infrastructure development and platform development between R-GM and GMRS.

In the case of conventional analog or digital receivers, heterotines using intermediate frequencies of several hundred kHz or more are mainly used. The receiver structure of this type has advantages such as sensitivity and selectivity, but has a disadvantage in that a complicated structure and a bulky element such as a filter are used. Referring to Figure 1, the configuration of a conventional FM and digital narrowband (12.5kHz) terminal is as follows.

1 is a VHF and UHF band terminal previously proposed for the conventional analog FM scheme and 12.kHz C4FM (Constant-envelope 4-level Frequency Modulation) digital scheme, using a superheterodyne receiver structure. It consists of 1 ^st IF (Intermediate Frequency) stage, 2 ^nd IF stage, analog baseband (I & Q) and frequency synthesizer.

The frequency conversion of the input signal was carried out by HSI (High Side Injection) to 45.15MHz for the VHF band (138 to 174MHz) and 73.35MHz for the UHF and 800MHz bands, and then the final frequency conversion to 450kHz.

The transmitter simplifies the configuration of the transmitter by performing direct-modulation, ie, direct frequency conversion, using the constant amplitude characteristics of the FM modulation.

In conclusion, the above-mentioned terminals have many filters required by using the heterodyne method to maintain frequency selectivity and sensitivity, and are difficult to integrate with other devices due to the difficulty in integrating with other devices. RF miniaturization and low price implementation are difficult, and future implementation of terminals for multiband / multimode services is practically impossible.

In order to solve this problem, in particular, in case of integrating the RF part of the receiver, a direct conversion method or a low-IF method that has a simple structure and does not require a bulky device such as a filter is preferred.

The low-IF method has a disadvantage in that the structure is somewhat complicated and the performance of removing the image signal is lower than that of the direct conversion method, but the DC-offset component, which is a disadvantage of the direct conversion method, can be easily removed.

Therefore, in the case of digital ultra narrowband receivers, since the signal bandwidth is very narrow at 6.25 kHz, a receiver structure using a low-IF method that processes almost all receiver functions with a DSP signal processing technology by setting the IF low is selected in terms of performance and complexity. Can be considered

As such, for the technical development trend of advanced countries and the domestic digital ultra narrowband technology to be developed later, frequency using direct conversion method or low-IF structure, rather than the super heterodyne method generally applied to LMR systems. There is a need for a method capable of increasing the flexibility of the terminals, in particular, by processing the signal control and driving of the terminals using DSP technology while satisfying the ultra narrow bandwidth condition. One of these methods, Low-IF, has excellent performance such as reception sensitivity and selectivity, but solves the problems of the conventional heterodyne method in which the structure is complicated and bulky devices such as filters are used. Although it is easy to integrate, it is possible to implement a multi-band terminal, but it is a compromise between two methods of improving the disadvantages of the direct conversion method in which the reception performance is somewhat degraded and the system performance is weak, such as DC-offset and local power leakage power. However, this approach eliminates the effects of image signals occurring at frequencies apart (2 x IF) by gain and phase error between the I / Q paths of quadrature demodulators that convert RF signals to IF signals. There is a difficult problem.

The present invention has been proposed to solve the above problems, and when using a low-IF structure in an ultra narrowband receiver having a bandwidth of 6.25 kHz, an intermediate frequency (IF) frequency is satisfied while satisfying a frequency ultra narrow banding condition of 6.25 kHz or less. It is an object of the present invention to provide a digital ultra narrowband receiver using a low-IF method which can minimize the effects of gain and phase error of an in-phase (I) / quad phase (Q) demodulator through the optimal selection of.

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.

The present invention for achieving the above object, in the receiving device of the digital ultra narrow band terminal, the received radio frequency (RF) signal is demodulated by the IF signal of the in-phase (I) / quadrature (Q) component variable after filtering RF receiving means for gain amplification; Analog / digital conversion means for converting an intermediate frequency (IF) signal of the analog I / Q component into a digital signal, respectively; Amplifying means for amplifying variable gains of the IF signals of the digital I / Q components through automatic gain control (AGC), respectively; Frequency correction means for detecting a frequency offset (AFC) to vary the frequency of the local oscillator to compensate for the frequency offset; I / Q demodulation means for converting the IF signal of the I / Q component whose frequency offset is compensated into the original low frequency signal; Filtering means for low-pass filtering the IF signal of the I / Q component converted into the low frequency signal to remove noise and adjacent channel signals, respectively; Symbol recovery means for recovering symbol timing of an IF signal of the lowpass filtered I / Q component; And differential detection means for differentially detecting the IF signal of the recovered I / Q component and outputting a final symbol value.

The present invention proposes an IF selection method for minimizing the influence of an image signal generated when a low-IF structure is used in an ultra narrowband receiver having a bandwidth of 6.25 kHz and provides an ultra narrowband receiver function using DSP signal processing technology. I would like to present an implementation plan.

To this end, the present invention proposes a low-IF structure for implementing a digital ultra narrowband receiver of 6.25 kHz or less, and uses automatic signal control (AGC) and automatic frequency control as performance factors when demodulating a received signal using DSP signal processing technology. (AFC), symbol timing recovery, and differential detection of π / 4 DQPSK modulation scheme are implemented digitally, and the image signals inevitably generated when low-IF is applied to digital ultra narrowband receivers. Present IF selection methods to minimize impacts.

According to the present invention, it is possible to improve terminal flexibility by providing an interoperability function that enables a call using an existing FM terminal and one receiver structure, and to use it in the design and implementation of ultra narrowband terminal for multi-band service later. This can be applied to the terminal system having the characteristics of miniaturization, low cost and light weight of the LMR terminals in the future.

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 block diagram of an embodiment of a digital ultra narrowband receiver using a Low-IF scheme according to the present invention.

The digital ultra narrowband receiver using the low-IF scheme includes an RF receiving module 200, an analog / digital converter (ADC) 220, 230, and a DSP module 240 for processing these signals.

The DSP module 240 uses DSP signal processing technology to differential detection of performance factors such as automatic gain control (AGC), automatic frequency control (AFC), symbol timing recovery, and π / 4 DQPSK modulation schemes when demodulating received signals. Digitally implement key features such as

As shown in FIG. 2, the digital ultra narrowband receiver using the Low-IF method according to the present invention converts a received radio frequency (RF) signal into an IF signal of in-phase (I) / quad phase (Q) component. RF receiving module 200 for demodulating and variable gain amplification after filtering, analog / digital converters (ADCs) 220 and 230 for converting intermediate frequency (IF) signals of analog I / Q components into digital signals, and digital Variable gain amplifiers 241 and 242 for variable gain amplifying the IF signal of the I / Q component through an automatic gain controller (AGC) 243, and a frequency offset is detected (AFC) to determine the frequency of the variable frequency local oscillator 247. An automatic frequency controller (AFC) 248 for compensating for the frequency offset by varying the frequency offset, and a second I / Q demodulator 244 for converting the IF signal of the frequency offset compensated I / Q component into the original low frequency signal. And IF signal of I / Q component converted to low frequency signal Low pass filters 245 and 246 for low pass filtering to remove noise and adjacent channel signals, and a symbol timing recoverer 251 and an interpolator for recovering symbol timing of IF signals of low pass filtered I / Q components, respectively. 249 and 250, and a differential detector 252 for differentially detecting the IF signals of the I and Q components whose symbol timing is recovered and outputting a final symbol value.

The RF receiving module 200 includes an antenna 201, a pre-selection bandpass filter 202, a low noise amplifier (LNA) 203, a bandpass filter 204 for suppressing an image signal, and a first I / Q demodulator 205. ), Low pass filters 206, 207, local oscillator 208, variable gain amplifiers (VGA) 209, 210, and the like.

The weak RF signal received by the antenna 201 passes the in-band signal through the band pass filter 202 and the out-of-band signal is suppressed and then amplified low noise through the low noise amplifier 203, and then removes the image signal filter 204. Go through). Thereafter, the first I / Q demodulator 205 converts the IF signal of the in-phase (I) component and the quadrature (Q) component. Thereafter, the IF signal is passed through a low pass filter (206, 207) for the purpose of aliasing, and then amplified through the variable gain amplifiers (209, 210) and then converted into a digital signal by the analog-to-digital converter (220, 230) for most of the receiver functions. It is delivered to the DSP module 240 to perform the.

DSP module 240 includes variable gain amplifier (VGA) 241, 242, automatic gain controller (AGC) 243, variable frequency local oscillator 247, automatic frequency controller (AFC) 248, second I / Q demodulator 244, low pass filters 245 and 246, interpolators 249 and 250, symbol timing recovery unit 251, differential detector 252 and the like.

Devices corresponding to the functions of the DSP module 240 are implemented and performed using a DSP algorithm.

The IF signal converted to a digital signal is subjected to variable gain amplifiers 241 and 242 whose gain is varied by an automatic gain controller (AGC) 243 to ensure optimal performance and dynamic range of the receiver. At this time, the automatic gain controller (AGC) 243 may be configured by an algorithm, and may be obtained from an IF signal or from a symbol value that has undergone baseband symbol timing recovery.

The control of the signal magnitude in the variable gain amplifiers 241 and 242 can be sufficiently maintained even if it is simply implemented in the bit shift operation in the DSP module 240.

The frequency offset caused by the local oscillator frequency difference of the transceiver greatly reduces the receiver performance and requires a technique to compensate for this. Therefore, in the present invention, the automatic frequency controller (AFC) 248 uses a method of taking four samples in one symbol interval to detect a frequency offset, which is suitable for estimating a wide range of frequency offsets. . In this case, the estimated frequency offset varies the frequency of the variable frequency local oscillator 247 implemented in the DSP module 240 to compensate for the frequency offset.

Thereafter, the frequency offset-compensated IF signal is converted into the original low frequency signal by the second I / Q demodulator 244 and goes through the low pass filters 245 and 246 to remove noise and adjacent channel signals.

The symbol timing recoverer 251 is easy to implement and uses a forward symbol timing method using four samples in a symbol period. At this time, the symbol timing recovery is performed by the interpolators 249 and 250 using four sample values by the DSP program, and then outputs the final symbol value using the differential detector 252.

Although the low-IF scheme has a simple receiver structure and can avoid a DC-offset problem, the I / Q path-to-path gain and phase error of the first I / Q demodulator 205 converting an RF signal into an IF signal is reduced. There is a structural problem in that the image signal suppression performance that occurs at (2 x IF) away frequency is poor. Therefore, in order to remove the influence of the image signal, it is important to select an appropriate IF frequency, and as shown in FIG. 3, it is necessary to select an IF frequency to minimize the influence of the image signal.

In FIG. 3, an image signal corresponds to a frequency band signal below (2 x IF) in a selected channel, and occurs when the I / Q demodulator has a general performance of about -30 to -35 dB compared to a desired signal. Therefore, in order to select an IF frequency for minimizing the influence of the image signal, select an IF satisfying Equation 1 below and arrange a channel such that a signal component does not exist in the middle of the image band.

(2 x IF) = (n + 0.5) x BW

In Equation 1, BW represents a channel bandwidth of 6.25 kHz, and IF is set to 1, the IF becomes 4.6875 kHz.

Since the DSP module 240 uses four samples in the symbol interval for frequency offset estimation and symbol timing recovery, the sampling rate of the analog-to-digital converters 220 and 230 is 19.2ksps, assuming that the symbol transmission rate is 4.8ksps. do. Therefore, in this case, the cutoff frequencies of the low pass filters 245 and 246 for the purpose of preventing aliasing are 9.6 kHz, which is 1/2 of the sampling rate.

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.

As described above, the present invention proposes an IF selection method for minimizing the influence of the image signal generated when the low-IF method is applied to a 6.25 kHz digital ultra narrow band receiver and demodulates the received signal using DSP signal processing technology. Optimizing and improving reception performance by digitally implementing key features such as automatic gain control (AGC), automatic frequency control (AFC), symbol timing recovery, and differential detection with π / 4 DQPSK modulation This easy implementation of the digital receiving modem for the ultra narrowband terminal is possible.

Claims (3)

In the receiving device of a digital ultra narrowband terminal, RF receiving means for demodulating the received radio frequency (RF) signal into an IF signal of in-phase (I) / quad phase (Q) component to perform variable gain amplification after filtering; Analog / digital conversion means for converting an intermediate frequency (IF) signal of the analog I / Q component into a digital signal, respectively; Amplifying means for amplifying variable gains of the IF signals of the digital I / Q components through automatic gain control (AGC), respectively; Frequency correction means for detecting a frequency offset (AFC) to vary the frequency of the local oscillator to compensate for the frequency offset; I / Q demodulation means for converting the IF signal of the I / Q component whose frequency offset is compensated into the original low frequency signal; Filtering means for low-pass filtering the IF signal of the I / Q component converted into the low frequency signal to remove noise and adjacent channel signals, respectively; Symbol recovery means for recovering symbol timing of an IF signal of the lowpass filtered I / Q component; And Differential detection means for differentially detecting the IF signal of the I / Q component whose symbol timing has been recovered and outputting a final symbol value Digital ultra narrowband receiver using a low-IF method comprising a. The method of claim 1, In order to minimize the influence of the image signal inevitably generated when using the low-IF structure, the IF frequency is selected so that the signal component does not exist in the middle of the image band by selecting an IF satisfying the following Equation. Digital ultra narrowband receiver using a low-IF method characterized in that the channel is arranged. [Equation] (2 x IF) = (n + 0.5) x BW (However, BW represents a channel bandwidth of 6.25 kHz, and if the integer N is selected as 1, the IF becomes 4.6875 kHz.) The method of claim 2, Cutoff frequency of the filtering means, A digital ultra narrowband receiver using a low-IF method, characterized in that 9.6kHz, which is half the sampling rate.

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