WO2008069512A1

WO2008069512A1 - Apparatus and method for acquiring frame synchronization and frequency synchronization simultaneously in communication system

Info

Publication number: WO2008069512A1
Application number: PCT/KR2007/006173
Authority: WO
Inventors: Pan-Soo Kim; Dae-Ig Jang; Deock-Gil Oh; Ho-Jin Lee
Original assignee: Electronics And Telecommunications Research Institute
Priority date: 2006-12-04
Filing date: 2007-12-03
Publication date: 2008-06-12

Abstract

An apparatus and method for acquiring reliable frame synchronization performance and frequency synchronization simultaneously in a communication system includes a differential correlator outputting a result value obtained by performing a differential operation based on an input signal having symbol timing synchronization and a value pre-defined with a sender sending the input signal, a frequency error calculator compensating for a frequency error based on the result value of the differential correlator and a frequency compensation control signal, and a frame synchronization determiner outputting the frequency compensation control signal if the result value of the differential correlator is greater than a threshold set having at least one value. By introducing a correlation scheme, i.e. Differential Generalized Post Detection Integration (DGPDI), in which differential information corresponding to a half of a symbol duration length is used within a duration from a frame start symbol to a UW symbol duration in a TDM/TDMA transmission structure in which a low SNR and a high frequency error must be considered, a more reliable frame synchronization estimation value can be acquired in comparison to a conventional frame synchronization scheme in which differential information between single neighbor symbols is used.

Description

APPARATUS AND METHOD FOR ACQUIRING ERAME SYNCHRONIZATION AND FREQUENCY SYNCHRONIZATION SIMULTANEOUSLY IN COMMUNICATION SYSTEM Technical Field

[1] The present invention relates to an apparatusand method for acquiring initial synchronization at a high data rate in a satellite communication/broadcasting system in which a low Signal to Noise Ratio (SNR) and a high freqμency difference exist.

[2] This work was supported by the IT R&D program of MIC/ITTA[2005-S-013-02, Development of Broadband Adaptive Satellite Communication and Broadcasting Convergence Technology]. Background Art

[3] In order to accomplish proper communication in communication systems, especially in Time Division Multiplexing (TDM)/Time Division Multiple Access (TDMA) digital transmission systems, estimation and compensation of a carrier frequency error occurring due to frame synchronization technology of determining a beginning point of a frame in a receiver end and imperfection of a local oscillator of a transmission and reception end are definitely recμired. In general, the performance of a frame and frecμency synchronization process performed right after symbol timing synchronization in an initial synchronization mode is an important parameter acting as the basic assumption for the performance reqired for the entire system.

[4] Recently, DVB-S2 (Digital Video Broadcasting-Satellite, Second Generation) and

DVB-RCS (Digital Video Broadcasting-Return Channel System), which are representative digital transmission standards using a TDM/TDMA scheme, satisfy a low bit error rate even in a poor channel environment, such as a low SNR, by introducing a robust channel code, such as a Low-Density Parity-Check (J-DFC) code or a turbo code, enabling communication with a high data rate and high quality. lor example, in a case of DVB-S2 system, which is a next generation satellite communication/ broadcasting system, when Quadrature Phase Shift Keying (QPSK) modulation method is used, a bit frame is 64,800, and a code rate is 1/4, a Packet Error Rate ^ER) is about 10^" at E /N =-2.35dB₅ i.e., a high qμality communication system can be achieved. In addition, Since a carrier freqαency usually corresponds to Ku/Ka band when satellite communication and a high frecμency are used, a frecμency error based on a bandwidth is relatively very high in comparison to other systems. Furthermore, a Doppler frecμency error occurring in a high-speed mobile environment, such as highspeed train or airplane, is also largely generated. Actually, in the case of DVB-S2 system, a considered frequency error is ± 5 MHz, and thus, when a bandwidth is considered as 25 MHz, a frecμency error of ± 20% based on the bandwidth is generated.

[5] Moreover, in a case of DVB-RCS system, a symbol rate may be hundreds Ksps in general due to a variety of return link traffic. In this case, if a Doppler freqμency error of 1,000 Km/h is generated in an airplane, the maximum frecμency error of ± 10% based on a bandwidth is generated. In particular, in a case of CSC/ACQ/S YNC burst for initial connection and synchronization, it is expected that a frecμency error of more than several percentages is always generated. Due to these channel performance degradation causes, it is difficult to establish initial synchronization at a high data rate, and thus, the present invention siggests a method of simultaneously performing frame synchronization and frecμency synchronization in order to overcome this difficulty.

[6] In recent wireless communication systems including the DVB-S2/D VB-RCS systems, the development of robust channel code technology, such as LDPC, allows the systems to enable a broadband (high data rate) and high cμality service. However, a conventional initial synchronization scheme causes problems that initial synchronization is significantly delayed due to a low SNR and a high frecμency error in a channel and a demodulation circuit of a receiver end for the initial synchronization is complicated.

Disclosure of Invention Technical Problem

[7] The present invention provides an apparatus and method for simultaneously accμiring reliable frame synchronization performance and frecμency synchronization performance while overcoming a low SNR and a high frecμency error at a high data rate.

Technical Solution

[8] According to an aspect of the present invention, there is provided an apparatus for accμiring frame synchronization and frecμency synchronization simultaneously in a comminication system, the apparatus for accμiring synchronization in a Time Division Multiplexing (TDM)/Time Division Multiple Access (TDMA) demodulation system having a frame comprising a header having a constant length, the apparatus comprising: a differential correlator outputting a result value obtained by performing a differential operation based on an input signal having symbol timing synchronization and a value pre-defined with a sender transmitting the input signal; a frecμency error calculator compensating for a frecμency error based on the result value of the differential correlator and a frecμency compensation control signal; and a frame synchronization determiner cutputting the frecμency compensation control signal if the result value of the differential correlator is greater than a threshold set having at least one value.

[9] According to another aspect of the present invention, there is provided a method of accμiring frame synchronization and frecμency synchronization simultaneously in a commxnication system, the method of accμiring synchronization in a Time Division Multiplexing (TDM)/Time Division Multiple Access (TDMA) demodulation system having a frame comprising a Unicμe Word (UW) having a constant length, the method comprising: performing a differential operation based on an input signal having symbol timing synchronization and a value pre-defined with a sender sending the input signal; comparing the differential operation result value and a threshold set having at least one value, and if the differential operation result value is greater than the threshold set, generating a frecμency compensation control signal; and compensating for a frecμency error of the input signal based on the differential operation result value and the frecμency compensation control signal. Advantageous Effects

[10] According to the present invention , by introducing a correlation scheme, i.e. Differential Generalized lost Detection Integration (DGPDI), in which differential information corresponding to a half of a symbol duration length is used within a duration from a frame start symbol to a header (the same meaning as a Uniqμe Word (UW)) symbol duration in a TDM/TDMA transmission structure in which a low SNR and a • high frecμency error must be considered, a more reliable frame synchronization estimation value can be acψired in comparison to a conventional frame synchronization scheme in which differential information between single neighbor symbols is used can be acqired. Moreover, by compensating a frecμency error estimation value after frame synchronization is temporarily determined, cμick initial synchronization.

[11] The present invention relates to technology for performing initial synchronization, i.e. frame synchronization and freqμency synchronization, using a UW in a TDM/ TDMA transmission structure having a degraded channel environment. Ibr example, by simultaneously performing frame synchronization and frequency synchronization using multiple differential information in order to perform high-speed initial synchronization in an environment of a low SNR and a high freψency error (as an example of a concrete system, in a case of DVB-S2, a considered minimum SNR: E /N

=-2.35dB , a maxirmm frequency error: 20% based on a bandwidth), long initial syn- chronization time can be reduced.

[12] Since the present invention can simultaneously perform high-speed initial synchronization, frame synchronization, and frequency synchronization in communication systems having a TDM/TDMA transmission structure, which include DVB- S2/D VB-RCS transmission systems operating with a low SNR and a high frequency error, a demodulator Junction meeting recently required high-speed synchronization acqμisition and high-quality service can be performed, and a demodulator circuit and operation can be reduced by sharing a circuit for common parts.

[13] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by one of ordinary sMll in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Description of Drawings

[14] The features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which:

[15] HG. 1 is a structure of a transport frame applicable to the present invention;

[16] HG. 2 is a block diagram of an apparatus for acquiring frame synchronization and frequency synchronization similtaneously in a Time Division Multiplexing (TDM)/Time Division Multiple Access (TDMA) transmission scheme according to an embodiment of the present invention;

[17] HG. 3 is a detailed block diagram of the apparatus illustrated in HG. 2;

[18] HG. 4 is a block diagram of a TDM/TDMA receiver to which the apparatus illustrated in HGS. 2 and 3 are applied, according to an embodiment of the present invention;

[19] HG. 5 is a flowchart illustrating a method of acquiring frame synchronization and frequency synchronization simultaneously in a TDM/TDMA transmission scheme according to an embodiment of the present invention;

[20] HG. 6 is a graph for describing a process of detecting a frequency error according to a Uniψe Word (UW) length when the present invention is applied;

[21] HG. 7 is a graph for describing frecμency estimation error performance according to

E /N when the present invention is applied; and

[22] HG. 8 is a table illustrating a frame synchronization acquisition time when the present invention is applied. Mode for Invention

[23] The present invention will be described in detail by explaining embodiments of the invention with reference to the attached drawings.

[24] HG. 1 is a structure of a transport frame applicable to the present invention. HG. 2 is a block diagram of an apparatus for acquiring frame synchronization and freqμency synchronization simultaneously in a Time Division Multiplexing (TDM)/Time Division Multiple Access (TDMA) transmission scheme according to an embodiment of the present invention, HG. 3 is a detailed block diagram of the apparatus illustrated in HG. 2, and HG. 4 is a block diagram of a TDM/TDMA receiver to which the apparatus illustrated in HGS. 2 and 3 are applied, according to an embodiment of the present invention. HG. 5 is a flowchart illustrating a method of acqμiring frame synchronization and freςμency synchronization simultaneously in a TDM/TDMA transmission scheme according to an embodiment of the present invention. HG. 6 is a graph for describing a process of detecting a frecμency error according to a Unicμe Word (UW) length when the present invention is applied, and HG. 7 is a graph for describing frecμency estimation error performance according to E /N when the

S O present invention is applied. HG. 8 is a table illustrating a frame synchronization ac- cμisition time when the present invention is applied.

[25] ϊbr convenience of description and easiness of understanding, the apparatus and the method will be described together, and fractions or constructions, which can be easily understood by one of ordinary skill in the art, are not described in detail since they would obscure the invention in unnecessary detail. The present invention is applied to communication systems, and a TDM/TDMA transmission scheme will be described as an example. In addition, a header in the claims is described as a Unicμe Word (UW) hereinafter.

[26] The present invention performs frame synchronization and frecμency error estimation in the TDM/TDMA transport frame (burst) structure and in the construction of HGS. 2 and 3. In order to overcome a frecμency error in initial synchronization, a method of performing correlation using differential information corresponding to a half of the maximum symbol length in a UW symbol duration corresponding to the length L from a start symbol, detecting a frame start time by means of comparison

UW between a correlation value and a threshold by applying a Threshold Crossing (TC) scheme to the correlation, and compensating for a frequency error with a calculated frequency error compensation value is used.

[27] HG. 1 is a frame structure in a conventional TDM/TDMA transmission structure. In order to detect a start time of initial data at a receiver end in a TDM/TDMA transmission system, frame synchronization must be performed. In this case, the receiver end performs data correlation in a state in which the receiver end knows specific data transmitted from a transmitter end. In HG. 1, reference numeral 101 denotes a UW corresponding to the specific data, reference numeral 102 denotes traffic data, reference numeral 103 denotes a burst formed with the UW 101 and the traffic data 102, and reference numeral 104 denotes a distributed pilot transport frame structure having a pattern in which a UW and data are alternately repeated. A representative system having the transmission structure referred to as 103 is a DVB-RCS (Digital Video Broadcasting-Return Channel System), and a representative system having the transmission structure referred to as 104 corresponds to DVB-S2 (Digital Video Broadcasting-Satellite, Second Generation).

[28] Referring to HGS. 2 and 5, a differential correlator 210 performs a differential operation based on an input signal r in which symbol timing synchronization is achieved and a value c pre-defined with a sender transmitting the input signal r in i i operation S510. The differential correlator 210 includes a delay calculator 211, which multiplies the input signal r by a conjugate complex value of the value c pre-defined i i with the sender transmitting the input signal r , performs a conjugate complex operation after delaying the multiplication result by a one-symbol unit, and multiplies the conjugate complex operation result by the multiplication result, and a first adder 213 summing an output of the delay calculator 211 by the length of a UW. [29] A frequency error calculator 220 compensates for a frequency error of the input signal r by increasing or decreasing the frequency error according to a result value i output from the differential correlator 210 and a frequency compensation control signal output from a frame synchronization determiner 230 in operation S530. [30] The frame synchronization determiner 230 sequentially compares the result value output from the differential correlator 210 and a set of thresholds set in a threshold comparator 231, i.e. different thresholds, and outputs the frequency compensation control signal according to the comparison result. In this case, if the result value output from the differential correlator 210 is equal to or less than a threshold, the result value exists within an allowed error range, and thus, compensation is unnecessary. However, if the result value output from the differential correlator 210 is greater than a threshold, compensation is necessary, and thus, the frecμency compensation control signal is enabled, performing the compensation. This compensation is performed for each of the different thresholds in operation S520.

[31] The apparatus illustrated in HG. 2 will now be described in more detail with reference to HG. 3. r 301 is a received signal for which a symbol timing syn- i chronization function has been performed and is represented using Eψation 1. [32] [Math.l]

[33] (Eψation 1)

[34] Here, r denotes a received signal of a k -symbol time, a denotes a data symbol k k stream, which is the same as c in a UW length duration and corresponds to a k modulation method in a data duration except the UW, [35] [Math.2]

[36] denotes the amplitude of a frecμency error normalized to a symbol rate, and

[37] [Math.3]

[38] denotes a phase error.

[39] c 302 denotes a UW symbol stream known by a receiver end. c 302 is a conjugate i i complex number of a UW symbol stream and is multiplied by r 301. i

[40] A block 310 is a detailed configuration of a delay calculator 211 and performs one- symbol unit delay, a conjugate complex operation, and multiplication. The apparatus illustrated in HG. 3 performs an addition operation corresponding to a relevant UW length with respect to a differential correlation operation corresponding to a half of a maximum UW symbol. Blocks 310 and 320 correspond to the differential correlator 210 illustrated in HG. 2 as logics sharing for frame synchronization and frequency synchronization. Br the frame synchronization, the frame synchronization determiner 230 of a block 330 is necessary. Rr frecμency error estimation, a block 340 is additionally applied, and a block 345 compensates for an estimated frecμency error. [41] [Math.4]

[42] multiplied in the block 340 is a value for implementing optimal performance, and it is preferable that an optimal value be selected by using a trial and error method. A compensation Junction of the block 345 is performed by multiplying a value output from the differential correlator 210 by an integer value (even thαgh the integer value is 2 in HG. 3, it is preferable that an optimal value be selected by using the trial and error method.), obtaining an absolute value of the multiplication result, and generating a control signal only if a correlation value, which is the absolute value, is greater than a threshold. The threshold comparator 231 can perform the comparison function with various thresholds. In the current embodiment, three thresholds 333, 335, and 337 are used as examples, wherein an operator may variably use each threshold according to retirements of channel information, an initial synchronization time, and a false alarm probability.

[43] A TDM/TDMA demodulator will now be described with reference to HG. 4. A received signal illustrated in HG. 4 is a digital data sample stream obtained by assigning a channel in a transmitter end, down-converting the channel to an Intermediate Freςμency (DP) in a receiver end, converting the IF to a base band, and passing the base band throtgh an Analog-to-Digital Converter (ADC). A matching filter 410 is a filter having the same structure as a pulse type filter of the transmitter end. In the current embodiment, the matching filter 410 acts as a Low Pass Hlter (LPF) so that the digital data sample stream has the maximum SNR. A symbol timing synchronizer 420 performs a symbol timing synchronization function for seeking an optimal point (a point having the maxirmm SNR) in a symbol duration by adjusting clock timing of symbol streams of an input signal. After performing the timing synchronization, the digital signal can be operated with one sample per symbol by a decimation function. Thereafter, by detecting a start point of a frame (burst), a distorted signal in the channel is compensated for using known data (UW) transmitted from the transmitter end. A frame synchronizer 431 detects a start point of a frame (burst), and a frequency synchronizer 433 performs a channel demodulation fraction by receiving frame start timing information from the frame synchronizer 431. If a block 430 is re-configμred, the block 430 includes the differential correlator 210, the frequency error calculator 220, and the frame synchronization determiner 230 as described with reference to HGS. 2 and 3.

[44] HG. 6 is a graph showing a frequency error estimation value output as a result obtained by controlling an operation of the frequency error calculator 220 using synchronization flag control, i.e. a control signal generated by the frame synchronization determiner 230 as a result of comparison with each threshold, when the present invention is used, showing that accuracy of estimation is determined according to a UW length.

[45] HG. 7 is a graph showing accuracy of a frequency error estimation value calculated by flag control using a control signal generated by performing threshold comparison when initial frame synchronization is performed, showing differences of accuracy according to a value N in a case of UW 64 symbol. As N increases, frequency error estimation accuracy increases. Reference numeral 701 denotes a curve of which N is 8. Reference numeral 702 denotes a Modified Cramer-Rao Bound (MCRB) curve showing an ideal limit value of frequency estimation performance in the UW 64 symbol.

[46] HG. 8 is a table illustrating a mean acquisition time expected when a frame synchronization scheme using miltiple differential information used in the present invention is applied.

[47] The invention can also be embodied as computer readable codes on a computer readable recording medium. The computer readable recording medium is any data storage device that can store data which can be thereafter read by a computer system. Examples of the computer readable recording medium include read-only memory (ROM), random-access memory (RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storage devices, and carrier waves (such as data transmission through the Internet). The computer readable recording medium can also be distributed over network coupled computer systems so that the computer readable code is stored and executed in a distributed fashion.

[48] As described above, according to the present invention , by introducing a correlation

SUBSTITUTE SHEETTRULE 26) scheme, i.e. Differential Generalized lost Detection Integration (DGPDI), in which differential information corresponding to a half of a symbol duration length is used within a duration from a frame start symbol to a header (the same meaning as a Unicμe Word (UW)) symbol duration in a TDM/TDMA transmission structure in which a low SNR and a high frecμency error mast be considered, a more reliable frame synchronization estimation value can be accμired in comparison to a conventional frame synchronization scheme in which differential information between single neighbor symbols is used can be accμired. Moreover, by compensating a frecμency error estimation value after frame synchronization is temporarily determined, quick initial synchronization.

[49] The present invention relates to technology for performing initial synchronization, i.e. frame synchronization and frecμency synchronization, using a UW in a TDM/ TDMA transmission structure having a degraded channel environment. Rr example, by simultaneously performing frame synchronization and frecμency synchronization using multiple differential information in order to perform high-speed initial synchronization in an environment of a low SNR and a high frecμency error (as an example of a concrete system, in a case of DVB-S2, a considered minimum SNR: E /N

=-2.35dB , a maximum frecμency error: 20% based on a bandwidth), long initial syn- chronization time can be reduced.

[50] Since the present invention can sirmltaneously perform high-speed initial synchronization, frame synchronization, and frecμency synchronization in communication systems having a TDM/TDMA transmission structure, which include DVB- S2/D VB-RCS transmission systems operating with a low SNR and a high frecμency error, a demodulator function meeting recently recμired high-speed synchronization accμisition and high-qμality service can be performed, and a demodulator circuit and operation can be reduced by sharing a circuit for common parts.

[51] While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by one of ordinary sMll in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims.

Claims

[1] An apparatus for acqiring frame synchronization and frecμency synchronization simultaneously in a comminication system having a frame comprising a header having a constant length, theapparatus comprising: a differential correlator outputting a result value obtained by performing a differential operation based on an input signal having symbol timing synchronization and a value pre-defined with a sender transmitting the input signal; a frecμency error calculator compensating for a frequency error based on the result value of the differential correlator and a frecμency compensation control signal; and a frame synchronization determiner outputting the frecμency compensation control signal if the result value of the differential correlator is greater than a threshold set having at least one value.

[2] The apparatus of claim 1, wherein the differential correlator comprises: a delay calculator multiplying the input signal by a conjugate complex value of the value pre-defined with the sender transmitting the input signal, performing a conjugate complex operation after delaying the multiplication result by a one- symbol unit, and multiplying the conjugate complex operation result by the multiplication result; and a first adder summing an output of the delay calculator by the length of the header.

[3] The apparatus of claim 2, wherein the delay calculator calculates a differential correlation value corresponding to a half of the length of the header, and the first adders corresponding to the half of the length of the header are parallel connected.

[4] The apparatus of claim 1, wherein the frame synchronization determiner obtains a correlation value by rmltiplying a value output from the differential correlator by an integer value and obtaining an absolute value of the multiplication result and compares the correlation value with the threshold.

[5] The apparatus of claim 4, wherein the frame synchronization determiner comprises at least one threshold comparator having different thresholds, comparing the correlation value with each threshold, and enabling a frecμency compensation Sanction of the freqμency error calculator if the correlation value is greater than each threshold.

[6] The apparatus of claim 1, wherein the threshold varies according to system reqirements comprising channel information, an initial synchronization time, and a false alarm probability .

[7] A method of acqiring frame synchronization and frecμency synchronization simultaneously in a commmication system having a frame comprising a header having a constant length, the method comprising: performing a differential operation based on an input signal having symbol timing synchronization and a value pre-defined with a sender transmitting the input signal; comparing the differential operation result value and a threshold set having at least one value, and if the differential operation result value is greater than the threshold set, generating a freqμency compensation control signal; and compensating for a frecμency error of the input signal based on the differential operation result value and the frecμency compensation control signal.

[8] The method of claim 7, wherein the performing of the differential operation comprises delaying a value obtained by multiplying the input signal by the pre-defined value in a one-symbol unit and calculating a differential correlation value corresponding to a half of the length of the header.

[9] The method of claim 8, wherein the calculating of the differential correlation value comprises: obtaining a first result value by rrultiplying the input signal by a conjugate complex value of the pre-defined value; obtaining a second result value by obtaining a conjugate complex number after delaying the first result value by a one-symbol unit and rrultiplying the conjugate complex number by the first result value; and summing the second result value by the length of the header.

[10] The method of claim 7, wherein the compensating of the frecμency error of the input signal comprises: setting at least one threshold; obtaining a correlation value by multiplying a differential correlation result in the performing of the differential operation by an integer value and obtaining an absolute value of the multiplication result, and comparing the correlation value with the threshold; and if the correlation value is greater than the threshold, enabling the freqμency compensation control signal.

[11] The method of claim 10, wherein the threshold has at least one value varying according to system reqirements comprising channel information, an initial synchronization time, and a false alarm probability .

[12] A computer readable recording medium storing a computer readable program for executing a method of acquiring frame synchronization and frecμency synchronization sirmltaneously in a comrrunication system having a frame comprising a header having a constant length, the method comprising: performing a differential operation based on an input signal having symbol timing synchronization and a value pre-defined with a sender sending the input signal; compensating for a frecμency error of the input signal based on the differential operation result value and a frecμency compensation control signal; and comparing the differential operation result value and a threshold set having at least one value, and if the differential operation result value is greater than the threshold set, generating the frecμency compensation control signal.