KR100673404B1 - Apparatus and method for demodulating the received data on the central station in the satellite spread multiple access system - Google Patents

Abstract

The present invention relates to a bidirectional satellite communication system in which forward link data transmitted from a central station to a satellite terminal uses a time division multiple access scheme, and reverse link data transmitted from the satellite terminal to the central station uses a code division multiple access scheme. In particular, in the apparatus for receiving and demodulating the reverse link data transmitted from the satellite terminal at the central station, the L-band data received from the satellite terminal is multiplied by a predetermined center frequency output from a local oscillator and converted into a center frequency band. Letting multiplier; An analog / digital converter for converting an analog signal output from the multiplier into a digital signal; A searcher function extracting unit extracting a position value of the satellite terminal from the digital signal output from the analog / digital converter within a predetermined window time interval; A finger function performer which performs tracking on the received digital signal with reference to the position value extracted from the searcher function performer; And a demodulator for demodulating the received signal after tracking by the finger function performing unit.

Two-way satellite communication system, central station, satellite terminal, reverse link, searcher, finger

Description

APPARATUS AND METHOD FOR DEMODULATING THE RECEIVED DATA ON THE CENTRAL STATION IN THE SATELLITE SPREAD MULTIPLE ACCESS SYSTEM}

1 is a view showing the overall configuration of a two-way satellite system to which the present invention is applied.

2 is a diagram showing a detailed structure of a central station in a bidirectional satellite system to which the present invention is applied.

3 is a diagram illustrating a detailed structure of a satellite terminal in a bidirectional satellite system to which the present invention is applied.

4 is a signal flow diagram of a receiver in a central station of a bidirectional satellite system according to an embodiment of the present invention;

5 is a diagram illustrating a detailed structure of a demodulation device in a central station of a bidirectional satellite system according to an embodiment of the present invention.

6 is a flowchart illustrating a procedure for compensating for a frequency offset from received data in a central station of a bidirectional satellite system according to an embodiment of the present invention.

7 is a flowchart illustrating a procedure for compensating for a time delay offset from received data in a central station of a bidirectional satellite system according to an embodiment of the present invention.

8 is a flowchart illustrating a procedure for optimizing the spreading code offset of received data in a central station of a bidirectional satellite system according to an embodiment of the present invention.

9A illustrates a structure of data transmitted from a satellite terminal to a central station in a spread code offset search process according to an embodiment of the present invention.

9B is a diagram showing the structure of data transmitted from a central station to a satellite terminal in the process of spreading code offset search according to an embodiment of the present invention.

<Explanation of symbols for the main parts of the drawings>

100: communication satellite 200: central station (base station)

210: central station antenna 220: wireless processing unit

230: central station transmitter (DVB-S) 231: modulator

232: multiplexer 233: IP encapsulation

240: central station receiving unit (W-CDMA)

241: medium frequency interface card assembly

242: Multi-rate channel card assembly

243: digital unit control card assembly

244: Alarm Control Processor Card Assembly

245 hub control processor card assembly

246: clock supply

250: Ethernet network 300: satellite terminal

310: baseband processor 320: terminal transmitter (W-CDMA)

330: terminal receiving unit (DVB-S) 501: multiplier

502: A / D conversion unit 503: search function performing unit

504: finger function performing unit 505: demodulation unit

506: frequency offset detection unit 507: control unit

508: local oscillator

The present invention relates to a two-way satellite system, and more particularly, to an apparatus and method for demodulating received data at a central station of a two-way satellite system.

In general, the satellite broadcasting system is unidirectional, so that it is possible to provide satellite broadcasting to a user, but a method of receiving a desired data or request from the user is not implemented.

In addition, the two-way satellite broadcasting service currently in service is not strictly referred to as a two-way communication using a satellite because the user's request is not transmitted using the satellite but the ground network.

On the other hand, in recent years, a two-way system using satellites has been developed, but the two-way satellite system, which has been developed and used in the past, uses the same communication method in which all communication methods are connected from the central station to the user and the user to the central station.

For example, both the forward link and the reverse link use Time Division Multiple Access (hereinafter referred to as 'TDMA') or Frequency Division Multiple Access (hereinafter referred to as 'FDMA'). Use the method.

Accordingly, since the user terminal is a terminal corresponding to one communication method, there is bound to be a limitation of the communication service that can be provided. That is, the data transmitted from the central station to the users are broadcast data that transmits the same large amount of data to multiple users in real time, whereas the data transmitted by each user is a small amount of data that is transmitted when needed, and the multiple users are irregular. Will be used. Therefore, in implementing a two-way system via satellite, when the same communication method is applied to the communication method connected from the central station to the user and from the user to the central station, many service constraints follow.

On the other hand, in order to receive a communication service by accepting a different communication method has a disadvantage that must have a plurality of terminals having a different communication method.

Therefore, in order to solve various problems that occur when performing communication using the above-described conventional satellite, the present applicant has a bidirectional satellite for transmitting and receiving data in a different communication method between a central station and a user in Korean Patent Application No. 2003-70510 (2003.10.10). A communication system is proposed.

Hereinafter, the proposed bidirectional satellite communication system will be briefly described.

In the satellite bidirectional communication system, a forward link (ie, a channel transmitted from a central station to a user terminal) and a reverse link or return link (ie, a channel transmitted from a user terminal to a central station) are different from each other. The system is adopted. For example, the forward link uses a DVB-S (Digital Video Broadcasting via Satellite) method using a TDMA method, and the reverse link uses a W-based code division multiple access (CDMA) method. Wideband CDMA (CDMA) is used.

1 is a view showing the overall configuration of a two-way communication system using a satellite.

Referring to FIG. 1, a central station (CS) 200 transmits broadcast data received from a predetermined broadcast information provider (eg, an Internet service provider (ISP) 110) through a communication satellite (Satellite) 100. The subscriber station transmits to the remote terminal 300. Information such as the broadcast data may be provided through the Internet 120, a content delivery network (CDN) 130, an application service 140, and the like.

In this case, the central station 200 transmits broadcast information and the like to the plurality of subscriber stations 300 in a DVB-S format through a forward link, and each subscriber station 300 transmits to the central station 200 through a reverse link. Data is transmitted by the W-CDMA method.

Accordingly, the central station 200 manages each user who uses the service (that is, subscriber terminals 300), processes data transmitted in a W-CDMA manner from various users, and converts the data desired by the user into the DVB. It converts to the -S format and transmits to the user through the satellite 100.

In addition, each subscriber station 300 receives and processes the DVB-S multimedia data transmitted from the central station 200 through the satellite 100, and requests W- to the satellite 100 when necessary data is requested. CDMA transmission data is transmitted.

On the other hand, the two-way satellite communication system as described above may cause various problems in its implementation.

For example, when the CDMA scheme is applied to the satellite reverse link, the capacity of the subscriber stations 300 can be increased and data security can be enhanced. However, the CDMA data is transmitted through the satellite, thereby delaying time. Not only does the problem of delay occur, but also the problem of frequency shifting occurs. Accordingly, there is a need for an effective implementation method for solving the above problems caused when applying the CDMA scheme to a system for bidirectional satellite communication.

An object of the present invention relates to an apparatus and a method for demodulating received data in a central station of a two-way satellite system.

The present invention also relates to an apparatus and method for receiving and demodulating data transmitted in a code division multiple access scheme in a central station of a bidirectional satellite system.

The present invention also relates to an apparatus and method for receiving and demodulating time-delayed data by transmitting in a code division multiple access scheme in a central station of a bidirectional satellite system.

The present invention also relates to an apparatus and method for receiving and demodulating frequency shifted data by transmitting in a code division multiple access scheme in a central station of a bidirectional satellite system.

The apparatus of the present invention for achieving the above object; A satellite communication system includes a predetermined communication satellite, a central station transmitting predetermined broadcast information to a plurality of subscribers through the communication satellite, and a plurality of satellite terminals receiving information transmitted from the central station through the communication satellite. And the forward link data transmitted from the central station to the satellite terminal uses a time division multiple access scheme, and the reverse link data transmitted from the satellite terminal to the central station uses a code division multiple access scheme in a bidirectional satellite communication system. In the apparatus for receiving and demodulating the reverse link data transmitted from the satellite terminal in the central station, the L-band data received from the satellite terminal multiplied by a predetermined center frequency output from the local oscillator to convert to the center frequency band Multiplier; An analog / digital converter for converting an analog signal output from the multiplier into a digital signal; A searcher function extracting unit extracting a position value of the satellite terminal from the digital signal output from the analog / digital converter within a predetermined window time interval; A finger function performer which performs tracking on the received digital signal with reference to the position value extracted from the searcher function performer; And a demodulator for demodulating the received signal after tracking by the finger function performing unit.

The apparatus may further include a frequency offset detector configured to receive a digital signal output from the analog / digital converter and detect a carrier frequency component at a center frequency level.

The apparatus may further include: a controller configured to compare center frequency information of the received signal obtained from the frequency offset detector with a transmission center frequency of a pre-stored satellite terminal, and adjust the output frequency value of the local oscillator according to the comparison result; It characterized in that it further comprises.

The first method of the present invention for achieving the above object; A satellite communication system includes a predetermined communication satellite, a central station transmitting predetermined broadcast information to a plurality of subscribers through the communication satellite, and a plurality of satellite terminals receiving information transmitted from the central station through the communication satellite. And the forward link data transmitted from the central station to the satellite terminal uses a time division multiple access scheme, and the reverse link data transmitted from the satellite terminal to the central station uses a code division multiple access scheme in a bidirectional satellite communication system. A method for demodulating and receiving reverse link data transmitted from the satellite terminal at the central station, the method comprising: receiving data transmitted from the satellite terminal at a basic spreading code offset value of the satellite terminal; Confirming a position and energy value of the satellite terminal through the data received from the satellite terminal; Confirming whether an energy value identified for the acquired position value has an appropriate value within a reference value range preset by the central station; Tracking the received signal by referring to the found position value when the position of the satellite terminal is found by the received data; And acquiring an accurate spreading code offset value of the satellite terminal and performing a call processing process with the corresponding satellite terminal.

The second method of the present invention for achieving the above object; A satellite communication system includes a predetermined communication satellite, a central station transmitting predetermined broadcast information to a plurality of subscribers through the communication satellite, and a plurality of satellite terminals receiving information transmitted from the central station through the communication satellite. And the forward link data transmitted from the central station to the satellite terminal uses a time division multiple access scheme, and the reverse link data transmitted from the satellite terminal to the central station uses a code division multiple access scheme in a bidirectional satellite communication system. A method for demodulating and receiving reverse link data transmitted from the satellite terminal at the central station, the method comprising: detecting a carrier frequency component at an IF level of a received signal; Comparing the center frequency information of the obtained received signal with a transmission center frequency of the corresponding satellite terminal previously stored; Calculating offset values of the center frequency information of the obtained received signal and the transmission center frequency of the pre-stored satellite terminal; And compensating for the frequency offset value of the received signal by adjusting the output frequency value of the local oscillator according to the calculated value.

Hereinafter, a detailed description of a preferred embodiment of the present invention will be described with reference to the accompanying drawings. In the following description, detailed descriptions of well-known functions or configurations will be omitted when it is determined that the detailed descriptions of the known functions or configurations may unnecessarily obscure the subject matter of the present invention.

The present invention is a bidirectional satellite communication system in which the forward link and the reverse link are different from each other, particularly in a bidirectional satellite communication system in which the reverse link is applied by Code Division Multiple Access (hereinafter, referred to as 'CDMA'). An apparatus and method for demodulating data received through the reverse link in a central station is provided.

In this case, since the data transmitted by the CDMA method through the reverse link is transmitted over a very long distance, a serious time delay phenomenon (generally about 300 ms) occurs unlike the CDMA method applied to a general mobile communication system. In addition, shifting of a carrier frequency occurs. Therefore, functions that can compensate for the time delay and frequency shift phenomena in the demodulation device of the central station should be implemented.

Before describing the present invention, the bidirectional satellite communication system to which the present invention is applied and the configurations of the central station and the subscriber station in the bidirectional satellite communication system will be described.

The bi-directional satellite communication system to which the present invention is applied is a DVB-enabled device that enables internet, e-mail, satellite broadcasting, video conferencing, video transmission and reception, and other high-speed multimedia services using an artificial satellite. S satellite broadcasting technology (hereinafter referred to as 'DVB-S') and broadband code division multiple access (hereinafter referred to as 'W-CDMA') is a satellite bidirectional communication system through integration.

More specifically, in case of a two-way communication using a satellite between a remote terminal (RT) receiving a service and a central station (CS) providing a service, a forward link and a reverse link are used. It consists of a link (Reverse link or Return link), and implements the integration of satellite broadcasting technology (DVB-S) and W-CDMA wireless communication technology by adopting different communication methods, DVB-S and W-CDMA. . In addition, the W-CDMA reverse link enables W-CDMA circuit switching and packet switching communication.

Meanwhile, two-way satellite communication methods using code division multiple access schemes in the reverse link, respectively, to which the present invention is applied, are called satellite spread multiple access (SSMA) schemes. It will be called. In addition, a base station apparatus for transmitting broadcast data to a plurality of users through a forward link in the SSMA system will be referred to as a 'Central Station' (hereinafter referred to as a 'CS'), and the SSMA system will be referred to from the CS. A subscriber station apparatus that receives broadcast data and transmits data to the CS through a reverse link will be referred to as a "remote terminal" (hereinafter referred to as "RT").

Hereinafter, detailed configurations of the CS and the RT constituting the SSMA communication system will be described with reference to FIGS. 2 and 3.

2 is a diagram illustrating a detailed structure of a central station in a bidirectional satellite system to which the present invention is applied.

Referring to FIG. 2, the central station 200 of the bidirectional satellite system includes a satellite antenna 210, a radio frequency (RF) processor 220, a transmitter 230, and a receiver 240. That is, the central station 200 modulates and transmits the transmission unit 230 in the same manner as the DVB-S, and demodulates the reception unit 240 in the same manner as the W-CDMA.

In more detail, the transmitter 230 modulates data to be transmitted to each satellite terminal 300 by a satellite broadcast transmission method such as DVB-S, and then modulates the modulated data to the radio processor 220. In the transmission (Tx) radio (RF) processing unit 221 and transmits the radio through the antenna 210. The transmitter 230 includes an Internet Protocol (IP) encapsulator 233, a multiplexer 232, a modulator 231, and the like. The IP encapsulation unit 233 performs a function of adding an IP header by receiving broadcast data received from a network such as Ethernet 250. The multiplexer 232 receives a plurality of data from the IP encapsulation unit 233 and multiplexes the data. In addition, the modulator 231 receives the multiplexed data from the multiplexer 232 and encodes the data by a method such as Forward Error Correction (FEC) / Digital Video Broadcasting (DVB) / Quadrature Phase Shift Keying (QPSK). Modulates the role.

That is, an IP encapsulation is received through a multimedia stream server (not shown) that generates a multimedia stream for a user or data received from an external network or multimedia stream data generated by the multimedia stream server through an internal communication interface Ethernet 250. The unit 233 converts the IP data into a format for IP communication and converts the IP data converted by the IP encapsulation unit 233 into a data such as MPEG-2 DVB-S (Digital Video Broadcasting-Satellite) scheme by the modulation unit 231. Will be modulated by

The broadcast data supplied to the transmitter 230 may be provided through an application server 251 or a database server 252, or may be provided with a gateway 254 and a router 256. It may be provided from an external content provider in connection with the Internet 257.

In addition, the central station 200 manages its state and the like through a network management system (hereinafter, referred to as 'NMS') 253 connected to the Ethernet 250.

Meanwhile, the receiver 240 of the central station 200 receives and demodulates W-CDMA format data received from the satellite terminals 100. That is, signals transmitted by the satellite terminals 100 are received through the satellite antenna 210, and radio signals processed by the Rx radio processor 222 of the RF processor 220. After that, the demodulation is performed at the receiver 240.

The receiving unit 240 is composed of a plurality of channel cards to enable processing of signals received from a plurality of subscribers subscribed to the system. In addition, each channel card provides a digital unit for demodulating directly received data (hereinafter, referred to as a 'DU') and a hub control processor for supplying a clock to the DU and managing the receiver 240. Control Processor Unit (hereinafter referred to as "HCPU").

The DU is an intermediate frequency interface card assembly (IF interface card assembly) for converting an intermediate frequency (hereinafter, referred to as IF) signal received from the RF processor 220 into a digital signal corresponding thereto. IFCA '241), a multi-rate channel card assembly (hereinafter referred to as' MCCA') for QPSK demodulation of the digital IF signal converted in the intermediate frequency interface card assembly 241; The digital unit control for transmitting the data output from the 242 and the MCCA 242 through the Ethernet 250 in synchronization with a clock supplied from a hub system clock (hereinafter, referred to as “HBSC”) 246. It consists of a digital unit control card assembly (hereinafter referred to as "DCCA") (243).

In addition, the HCPU is a clock supply unit (HBSC) 246 for generating and supplying a required clock (Clock) in the unit (Unit) in the central unit 200 based on the clock synchronized to the GPS receiver, the alarm in the central station 200 ( An alarm control processor card assembly (hereinafter referred to as 'ACPA') 244 which collects signals and reads state information of each unit, and is connected to the ACPA 244 and the HBSC 246. Hub control processor card assembly (hereinafter referred to as 'HCPA') that operates and manages the reverse link as a whole and controls each unit to perform call setup and release, abnormal state management, and maintenance functions. 245).

That is, the received radio processor 222 wirelessly processes the data received and outputs an L-band (eg, a 950 to 1450 MHz band) signal, and the L-band signal is an intermediate frequency signal by the IFCA 241. (For example, a 70 MHz band), and a digital signal by an A / D converter.

The digital signal output from the IFCA 241 is input to the MCCA 242, and demodulated to the DCCA 243 by demodulating the input digital signal, that is, reverse channel data. Meanwhile, the DCCA 243 controls the entire DU and transfers the clock received from the HBSC 246 to each unit. In addition, the demodulated data is received from the MCCA 242, and the received demodulated data is output by a clock inputted from the HBSC 246.

Meanwhile, the central station 200 manages user information on the RTs 300 and the like through a satellite subscriber management system (hereinafter referred to as 'RSMS') 256 connected to the Ethernet 250. do.

3 is a diagram illustrating a detailed structure of a satellite terminal (RT) 300 in a bidirectional satellite system to which the present invention is applied.

Referring to FIG. 3, the satellite terminal 300 of the bidirectional satellite system includes a baseband processor 310, a transmitter 320, a receiver 330, and the like. That is, in the satellite terminal 300, the transmitter 320 modulates and transmits the signal in the same manner as the W-CDMA, and the receiver 330 demodulates the signal in the same manner as the DVB-S.

Meanwhile, the baseband processor 310 provides a memory area for operating a terminal and a space for a buffer used to process data, and a memory 313 for providing a storage space of an operating system for operating a terminal. A control unit connected to 313 and controlling reception and processing operations of data transmitted through a satellite through a forward link, and converting data requested by a user into transmission data and transmitting the transmission data to the satellite via a reverse link ( 314, a data processing unit 311 connected to the control unit 314 for demodulating MPEG-2 data on DVB-S transmitted through a forward link, and a 7-segment for information such as a state of a terminal. The display unit 312 and the base band processor 310 output in a form of an external interface unit 315 for communicating with an external network, etc. are configured.

On the other hand, the baseband processor 310 is connected to the control unit 314 through the external interface unit 315 is connected to the serial communication unit 318, the control unit 314 to provide a serial communication interface of up to 1Mbps And an analog audio / video interface unit 319 providing an analog video / audio interface for satellite broadcasting and an Ethernet 317 providing a 10 base-T Ethernet interface.

The receiving unit 330, when receiving the DVB-S method of the forward link, the DVB-S demodulation and QPSK demodulation function to perform the function of the receiving tuner 333, the signal transmitted from the receiving tuner 333 digital data The analog / digital conversion and data error correction unit (ADC &FEC; 332) and the analog / digital conversion and data error correction unit 332 for converting the digital data into the digital data and correcting the converted digital data are performed. The demultiplexer 331 may be demultiplexed and temporarily stored in the memory 313.

In addition, the transmitter 320 may include a clock generator 321 that provides a clock for W-CDMA modulation and a clock required for data transmission, and transmits W data according to a clock supplied from the clock generator 321. W-CDMA modulator 322, which modulates CDMA scheme data (i.e., multiplies a predetermined spreading code and QPSK modulates), modulates W-CDMA scheme data modulated by W-CDMA modulator 322. And a transmit tuner 323 for performing L-band direct conversion and automatic gain control.

In the bidirectional satellite communication system applied to the present invention configured as described above, the forward link and the data transmitted from the terminal 300 receive data transmitted from the central station 200 to the terminal 300 via the satellite 100. It operates as a reverse link transmitted to the central station 200 via the &lt; RTI ID = 0.0 &gt;

First, the operation of the forward link is as follows.

When the user requests a multimedia service after connecting to the central station 200 through the satellite 100, the user can access the network through a network address transfer (NAT) (not shown) under the control of a web server (not shown). The website is linked.

Thereafter, the stream data generated by the multimedia stream server (not shown) or the like for data provided to the user on the website or the multimedia service is transferred to the IP encapsulation unit 233 through the Ethernet 250.

The IP encapsulation unit 233 receives the multimedia stream data generated by the multimedia stream server or the like received from an external network through Ethernet 250, an internal communication interface, and converts the multimedia stream data into a format for IP (Internet Protocol) communication. The signal is transmitted to the modulator 231 through the multiplexer 232.

The modulator 231 modulates the IP data converted by the IP encapsulator 233 into MPEG-2 DVB-S data and transmits the data to the satellite 100 through the RF processor 220.

The forward link signal thus transmitted is transmitted to the satellite terminal 300 via the satellite 100, and the satellite terminal 300 receives and processes it.

That is, the control unit 314 in the satellite terminal 300 is responsible for the overall control of the receiver 330 when the DVB-S data of the forward link is received through the satellite 100, and the reception tuner in the receiver 330. In operation 333, the DVB-S demodulation and QPSK demodulation functions are performed and transmitted to the analog / digital conversion and error correction unit 332.

The analog / digital conversion and error correction unit 332 in the receiver 330 converts a signal transmitted from the reception tuner 333 into digital data and corrects an error of the converted data. 331).

The demultiplexer 331 in the receiver 330 demultiplexes the data stream transmitted from the analog / digital conversion and error correction unit 332 and demultiplexes the data from the data processor 311 for processing. Temporarily store the data stream.

The controller 314 controls the demultiplexer 331 to transmit data to the data processor 311 at a point in time where the user desires data, and the data processor 311 demodulates the received MPEG-2 data. .

Through the above-described processes, data of the forward link is received and processed. Next, the operation of the reverse link for transmitting data from the terminal 300 to the central station 200 through the satellite 100 is as follows.

First, when the terminal 300 wants to request necessary data through the satellite 100, the transmitter 320 generates and transmits corresponding transmission data under the control of the controller 314.

In more detail, the clock generator 321 in the transmitter 320 generates a clock for W-CDMA modulation and a clock required for data transmission to the CDMA modulator 322 and the transmit tuner 323. Each will be provided. For example, the clock generator 321 generates a 32.768 MHz clock and a 10 MHz system clock for data modulation, and supplies the same to each block.

Next, the W-CDMA modulator 322 modulates the transmission data into W-CDMA scheme data according to a clock supplied from the clock generator 321 (ie, multiplies a predetermined spreading code and performs QPSK modulation). ), The transmit tuner 323 converts the W-CDMA QPSK modulated digital data modulated by the W-CDMA modulator 322 into a corresponding analog signal, and L-band direct conversion. And auto gain control to transmit the data to the satellite 100 through the satellite transmitter.

As described above, the backward signal transmitted to the satellite 100 is transmitted to the central station 200 through the satellite 100.

Meanwhile, the IFCA 241 of the central station 200 converts the intermediate frequency (IF) signal received from the RF portion into a digital signal corresponding thereto and transmits the converted digital signal to the MCCA 242.

The multi-rate channel card assembly 242 QPSK demodulates the digital IF signal converted in the IFCA 241 and delivers the QPSK demodulated signal to the DCCA 243.

On the other hand, HBSC 246 generates the necessary clock in the central station unit based on the clock synchronized to the GPS receiver and supplies to each block, ACPA 244 collects the alarm signals in the central station and each unit State information is read and forwarded to the HCPA 245.

The HCPA 245 is connected to the ACPA 244 and the HBSC 246, operates and manages the reverse link as a whole, and controls each unit to perform call setup and release, abnormal state management, and maintenance functions. do.

In addition, the DCCA 243 transmits the data output from the MCCA 2428 to the transmitter through the Ethernet 250 in synchronization with the clock supplied from the HBSC 246 under the control of the HCPA 245.

In the above, detailed structures of the central station 200 and the satellite terminal 300 in the bidirectional satellite system have been described in detail with reference to FIGS. 2 and 3. Hereinafter, with reference to FIG. 4, the signal flow between the respective components constituting the receiver 240 of the central station 200 will be described.

4 is a diagram illustrating a signal flow of a receiver in a central station of a bidirectional satellite system according to an embodiment of the present invention.

Referring to FIG. 4, the IFCA 241 is an L-band signal from a low noise block down converter (hereinafter referred to as 'LNB') of the satellite antenna 210 of the central station 200. Down conversion to an intermediate frequency (IF) band (eg, 70 MHz), and extracts In phase / Quadrature (I / Q) data. In addition, the IFCA 241 performs the operation according to the system clock provided by the HBSC 244.

The MCCA 242 performs a function of demodulating the I / Q data received from the IFCA 241 and performs the operation according to the system clock provided by the HBSC 244 like the IFCA 241.

The DCCA 243 receives demodulated digital data from the MCCAs 242 and transmits the demodulated digital data to the Ethernet. On the other hand, the function of the DCCA (243) can be implemented to be performed in the MCCA (242), each MCCA 242 can be implemented to transmit data directly to the Ethernet (Ethernet). In addition, although only one MCCA 242 is shown in FIG. 4, several MCCAs 242 may be actually mounted in the system according to the number of subscribers.

The HBSC 244 generates system clocks (eg, 10 MHz, 32.768 MHz) that are synchronized with the Global Positioning System (hereinafter, referred to as 'GPS'). The generated clocks are supplied to the IFCA 241, the MCCA 242, and the like.

The ACPA 246 aggregates and manages alarms generated in an operation state of each unit and transmits the result to the HCPA 245. At this time, the HCPA 245 monitors the state of each unit in the system, and controls the units with reference to the information received from the ACPA 246.

Meanwhile, in the bidirectional satellite communication system using the reverse link as described above, the CDMA data transmitted through the reverse link is transmitted over a satellite over a long distance, and thus CDMA applied to a general mobile communication system. Unlike the scheme, a serious time delay phenomenon (generally about 300 ms) and shifting of a carrier frequency occur. Accordingly, according to the present invention, the demodulation device of the central station 200 described above, in particular, the MCCA 242, is implemented to compensate for the time delay and frequency shift phenomena.

Hereinafter, an apparatus and method for receiving and demodulating a signal modulated by a W-CDMA method in a demodulation device (particularly, MCCA 242) of the central station 200 according to the present invention will be described.

5 is a diagram illustrating a detailed structure of a demodulation device in the central station 200 of the bidirectional satellite system according to an embodiment of the present invention.

Referring to FIG. 5, the demodulation device of the central station 200 according to an embodiment of the present invention includes an IFCA 241 for converting received data into an intermediate frequency signal and converting the received data into a digital signal and the received data converted into the digital signal. Is composed of an MCCA 242 for W-CDMA demodulation processing.

The IFCA 241 is a local oscillator 508 that generates a predetermined intermediate frequency (IF) signal, and an intermediate frequency output from the local oscillator 508 by outputting an L-band signal received from the wireless processor 222. A multiplier 501 for multiplying the signal and an analog / digital converter for converting the received analog signal processed from the intermediate frequency signal output from the multiplier 501 into a digital signal (Analog / Digital Converter; 502).

In addition, the MCCA 242 includes a demodulator 505 that receives a digitally converted signal from the IFCA 241 and demodulates the signal according to the W-CDMA scheme. Meanwhile, since the received W-CDMA modulated signal is a signal received through the satellite 100, a signal time delay phenomenon and a carrier frequency shifting phenomenon occur.

Accordingly, the MCCA 242 further includes a search function performing unit 503 and a finger function performing unit 504 to compensate for the time delay problem according to the present invention, and to compensate for the carrier frequency shifting problem. A frequency offset detector 506 is further provided. In addition, the controller 507 may further include a controller 507 for controlling the local oscillator 508 according to the reception center frequency information detected by the frequency offset detector 506.

The frequency offset detector 506 included in the MCCA 242 may be provided in the IFCA 241 according to its implementation. That is, each of the assemblies constituting the receiver 240 of the central station 200 (ie, IFCA 241, MCCA 242, DCCA 243, ACPA 244, HCPA 245 and HBCS 246). )) Is a unit that bundles chips that perform one or more functions, and according to the implementation, a block that performs a specific function included in a predetermined assembly may be modified to be implemented in another adjacent assembly.

Meanwhile, the search function performing unit 503 and the finger function performing unit 504 included in the MCCA 242 are provided in a receiver of a conventional spread multiple access (for example, W-CDMA). The searcher and the finger perform similar functions, but the searcher function performer 503 and the finger function performer 504 actually process data received through a satellite environment, not a conventional mobile wireless environment. Therefore, in the input / output data format and its processing method, it is different from the conventional search and finger. Detailed description thereof will be described later.

Hereinafter, a procedure of performing time delay compensation and frequency shift compensation on data received through the respective functional blocks illustrated in FIG. 5 will be described in detail with reference to FIGS. 6 to 8. First, the frequency shift compensation procedure will be described with reference to FIG. 6.

6 is a flowchart illustrating a procedure of compensating a frequency offset from received data at a central station of a bidirectional satellite system according to an embodiment of the present invention.

Referring to FIG. 6, first, the frequency offset detector 506 of the MCCA 242 is configured with I (In Phase) data and Q (Quadrature) output through the A / D converter 402 of the IFCA 241. ), The carrier frequency component at the IF level of the received signal is detected (step S601).

Then, the carrier frequency information at the IF level of the received signal obtained by the frequency offset detector 506 is transmitted to the controller 507 (step S602).

On the other hand, the control unit 507 has in advance information on the center frequency component transmitted by the satellite terminal 300, the center frequency information of the received signal obtained from the frequency offset detection unit 506 is Compared to the transmission center frequency of the stored satellite terminal 300 (step S603).

At this time, the control unit 507 is an offset that is a difference between the center frequency information of the received signal obtained from the frequency offset detection unit 506 and the transmission center frequency of the pre-stored satellite terminal 300 through the comparison process as described above. (Offset) the value is calculated (step S604).

Finally, the controller 507 compensates the frequency offset value of the received signal by adjusting the output frequency value of the local oscillator 508 of the IFCA 241 based on the calculated value (step S605).

7 and 8, a procedure for compensating for time delay by the MCCA 242 implemented according to the present invention will be described in detail.

As described above, in the MCCA 242 whose primary function is data demodulation of a central station in a satellite bidirectional system to which the present invention is applied, a time that may occur by applying a spread multiple access scheme to communication using satellites. Software algorithms are added to compensate for the delay.

That is, the signal transmitted from the satellite terminal 300 to the central station 200 via the satellite 100 must go much further than the distance that the conventional terrestrial wave passes. Therefore, this time delay is bound to occur. This time delay is also the most important problem in general satellite communication. Therefore, for reliable satellite communication, the central station 200 according to the present invention compensates for the time delay by the following procedure for mutual communication with the satellite terminal 300.

Particularly, in the CDMA scheme, since spreading codes (eg, PN (Pseudo Noise) codes) on the transmitting and receiving side must match, demodulation, the present invention using the code division multiple access scheme in the reverse link is a transmission side according to a time delay. The spreading codes at the satellite terminal 300 and the central station 200 at the receiving end may not match so that demodulation may not be performed properly.

On the other hand, the spreading code used in the conventional mobile communication system is used to distinguish the base station, each base station has a unique PN offset value, each terminal confirms this to perform normal data communication between the base stations. However, the spreading code applied to the present invention is not intended to distinguish the base stations, and since the spreading data is reached over a considerable distance through the satellite, a time delay problem is not considered in a general mobile communication system. Accordingly, there is a problem that accurate demodulation cannot be performed in the central station 200 according to the mismatch of the spreading codes according to the time delay. Accordingly, the central station 200 should be implemented to compensate for the time delay in order to solve the spreading code mismatch problem caused by the time delay.

7 is a flowchart illustrating a procedure for compensating for a time delay offset from received data in a central station of a bidirectional satellite system according to an embodiment of the present invention.

Referring to FIG. 7, the satellite terminal 300 first initializes to a predetermined access mode (step S701) in order to transmit data to the central station 200.

Meanwhile, the satellite terminal 300 includes a spreading code offset value corresponding to a time delay that may occur at a distance connected to the satellite terminal 300, the satellite 100, and the central station 200. The satellite terminal 300 transmits the basic spreading code offset value, which the satellite terminal 300 has, to the central station 200 in the process of accessing the base station through the initialization of the satellite terminal 300. do.

즉, 어떤 임의의 시간 T 동안에 두 개의 확산코드를 상호 곱했을 때 같은 확산코드를 곱해준 경우에만 '1'이 되고, 다른 확산코드나 타이밍이 틀린 코드를 곱해줬을 경우에는 '0'이 되는 것을 보여준다. 즉, 단말기가 보낸 확산코드와 기지국이 가지고 있는 확산코드가 시간상 동기가 되어 일치하게 되면 두 코드를 통해 계산한 값이 최대치가 되고 서로 어긋나게 되면 계산한 값은 아무 쓸모 없는 값으로 존재하게 된다. 여기서 두 코드를 통해 계산한 값의 결과가 바로 '에너지 값'을 말하는 것이며, 송신한 신호와 수신한 신호가 기본적으로 동일하고 시간상 일치하게 되면 될수록 상기 '에너지 값'은 높아지게 되어 중심국이 단말기의 신호를 더 잘 복조해 낼 수 있는 것이다.In the search function performing unit 503, which is additionally provided in the MCCA 242 of the central station 200, the IDU (In Door Unit), that is, the data of the satellite terminal 300, is received through the data received from the satellite terminal 300. The location is checked (step S702). That is, the central station 200 receives the signal transmitted by the basic spreading code offset value that the satellite terminal 300 initially has, and performs a search function function unit provided in the MCCA 242 of the central station 200. In operation 703, a process of finding a position and an energy value of the satellite terminal 300 within a predetermined reference time (for example, t ms) is performed.
Here, the 'energy value' is an arithmetic value obtained in the process of demodulating the received signal. The degree of correlation between the spreading code component of the received signal and the same spreading code used for demodulation is calculated through mathematical calculation and integration. The calculated value. That is, the 'energy value' may be an index indicating how well the central station demodulates the signal of the terminal. If the signal transmitted by the terminal is well demodulated, the energy value is high. This has a lower result. A detailed explanation of why these 'energy values' should be used is as follows.
In the spreading system, since different terminals use different spreading codes, the central station demodulates the signals of the terminals as well as determining whether each terminal is present through the same spreading code used by each terminal. However, even if the same spreading code is important, the same two codes are recognized as different codes if the time synchronization is not identical. For example, with simple correlation instead of complex mathematical calculations, the following shows the operation of the received signal and the code used for demodulation (assuming "1" if two bits are equal in time, "0" if different).
000111000111000111000111 (receive signal),
If you calculate 000111000111000111000111 (demodulation spread code),
The result is 111111111111 (only one sequence is computed). That is, adding the values of the result results in the highest value in the bitwise operation having 0 and 1.
However, if the time synchronization is not identical and is wrong, for example
000111000111000111000111 (receive signal),
If you calculate 11000111000111000111000111 (demodulation spread code),
The result is 001001001001001. In other words, when compared to the first result, the value of the bit operation is significantly lower.
If the above content is expressed by a simple equation, it is as follows.

That is, when two spreading codes are multiplied with each other for a certain time T, it becomes '1' only when multiplying the same spreading code, and '0' when multiplying another spreading code or an incorrect timing code. Shows. That is, if the spreading code sent from the terminal and the spreading code of the base station are synchronized with each other in time, the calculated values through the two codes become the maximum value, and if the spreading codes are shifted from each other, the calculated values exist as useless values. Here, the result of the value calculated by the two codes means the 'energy value'. The more the signal transmitted and the received signal are basically identical and match in time, the higher the 'energy value' becomes and the central station receives the signal from the terminal. To better demodulate.

On the other hand, following the process (step S703), the energy value checked for the position value acquired by the search function performing unit 503 in the process (step S703) is a threshold value preset by the central station 200. ) Check if the value is within the range. If there is an appropriate energy value for the acquired position value, the position is found (Position Lock). On the contrary, if the position value is not found if the position value is not found (Position). Unlock).

In this case, if the location of the satellite terminal 300 is not found by the received data, the checking process is repeated based on the received data. That is, the satellite terminal 300 confirms that the central station 200 has not found its location (for example, does not receive the Ack signal), and retransmits data with its default spreading code offset value. .

On the other hand, when the location of the satellite terminal 300 is found by the received data, the searched position value is transmitted to the finger function performing unit 504 of the MCCA 242 (step S704).

The finger function performing unit 504, which has received the position value from the searcher function performing unit 503, starts tracking by the received signal (step S705). In this case, the finger function performing unit 504 is different from a finger used in a general mobile communication system, and does not use a rake receiver.

After the tracking function of the finger function performing unit 504, the central station 200 having obtained the correct spreading code offset value of the satellite terminal 300 is then called with the satellite terminal 300. ) Step (S706).

The central station 200 that has performed the call processing process starts a communication with the satellite terminal 300 by switching to a traffic mode (step S707).

Meanwhile, in order to more efficiently optimize the time delay compensation method described above with reference to FIG. 7, the method as shown in FIG. 8 may be added.

8 is a flowchart illustrating a procedure for optimizing a spreading code offset of received data in a central station of a bidirectional satellite system according to an embodiment of the present invention.

Referring to FIG. 8, in the MCCA 242 of the central station 200, an algorithm for optimizing a spreading code offset value generated by a time delay for reliable communication with the satellite terminal 300 is described with reference to FIG. 7. Can be added to the method.

First, the search function performing unit 503 of the MCCA 242 receives the data including the spreading code offset from the corresponding IDU, that is, the satellite terminal 300 (step S801), which is previously stored in the central station 200. Through the basic spreading code offset, the spreading code offset value previously owned by the satellite terminal 300 is searched for in the data transmitted by the satellite terminal 300 (step S802).

Thereafter, the spreading code offset value acquired by the search function performing unit 503 is compared with the spreading code offset value already owned by the searched satellite terminal 300 (step S803).

Finally, if a difference occurs between the spreading code offsets as a result of the comparison, the result value is transmitted to the satellite terminal 300 so that data can be transmitted using a more accurate spreading code offset value (step S804).

On the other hand, the application range of the spreading code offset value in the search function performing unit 503 is preferably processed to obtain a position value within a few ms time in order to obtain a signal transmitted by the satellite terminal 300 as soon as possible. .

In order to implement this, the search function performing unit 503 of the MCCA 242 basically has a search window of about 1 ms, and for a reliable communication, it is accurate through several 1 ms search processes. The position value can be obtained. In this case, if the search window is large, a large time delay must be taken for reliable communication, and therefore, the use of a 1ms search window that can find a position value in a short time is effective in the application of the present invention.

FIG. 9A illustrates a structure of data transmitted from a satellite terminal to a central station in a spread code offset search according to an embodiment of the present invention.

Referring to FIG. 9A, as described above with reference to FIG. 7, the IDU spread code offset 910 value is included in the data transmitted from the satellite terminal for the spread code offset search.

Accordingly, the central station 200 receiving the data demodulates the received data to confirm the IDU spread code offset 910 value included in the received data to correct the spread code offset of the satellite terminal 300. do.

9B illustrates a structure of data transmitted from a central station to a satellite terminal in a spreading code offset search process according to an embodiment of the present invention.

Referring to FIG. 9B, when the central station 200 calculates a difference value of spreading code offsets from the data as shown in FIG. 9A, the spreading code offset value 920 compensated by the calculation result is used. By transmitting to 300, the data is then implemented to be transmitted by the correct spreading code offset value.

On the other hand, in the embodiment of the present invention has been described with respect to specific embodiments, various modifications are possible without departing from the scope of the invention. Therefore, the scope of the present invention should not be limited to the described embodiments, but should be defined not only by the appended claims, but also by those equivalent to the claims.

According to the present invention, by applying the CDMA scheme to the reverse link of the two-way satellite communication system, there is an advantage that the capacity of subscriber stations can be increased and data security can be enhanced. In addition, according to the present invention it is possible to implement an effective two-way satellite communication by solving the problem of time delay and frequency shifting caused by transmitting the CDMA data through the satellite.

Claims (13)

A satellite communication system includes a predetermined communication satellite, a central station transmitting predetermined broadcast information to a plurality of subscribers through the communication satellite, and a plurality of satellite terminals receiving information transmitted from the central station through the communication satellite. And the forward link data transmitted from the central station to the satellite terminal uses a time division multiple access scheme, and the reverse link data transmitted from the satellite terminal to the central station uses a code division multiple access scheme in a bidirectional satellite communication system. In the apparatus for receiving and demodulating the reverse link data transmitted from the satellite terminal at the central station, A multiplier for multiplying L-band data received from the satellite terminal by a predetermined center frequency output from a local oscillator and converting the L-band data into a center frequency band; An analog / digital converter for converting an analog signal output from the multiplier into a digital signal; A searcher function extracting unit extracting a position value of the satellite terminal from the digital signal output from the analog / digital converter within a predetermined window time interval; A finger function performer which performs tracking on the received digital signal with reference to the position value extracted from the searcher function performer; And And a demodulator configured to demodulate the received signal after tracking by the finger function performer. 2. The method of claim 1, The device, And a frequency offset detector for receiving a digital signal output from the analog / digital converter and detecting a carrier frequency component at a center frequency level. The method of claim 2, The device, And a control unit for comparing the center frequency information of the received signal obtained from the frequency offset detection unit with the transmission center frequency of the pre-stored satellite terminal and adjusting the output frequency value of the local oscillator according to the comparison result. A data demodulation device for a central station in a bidirectional satellite system. The method of claim 1, The data demodulation device of the central station in the bidirectional satellite system, characterized in that the search time is shortened by setting a predetermined window time interval in the search function performing unit. The method of claim 1, And extracting the position value of the satellite terminal from the search function performing unit and reflecting the position value of the satellite terminal to compensate for the demodulation unit. The method of claim 1, And the reverse link data is a wideband code division multiple access (W-CDMA) scheme. A satellite communication system includes a predetermined communication satellite, a central station transmitting predetermined broadcast information to a plurality of subscribers through the communication satellite, and a plurality of satellite terminals receiving information transmitted from the central station through the communication satellite. And the forward link data transmitted from the central station to the satellite terminal uses a time division multiple access scheme, and the reverse link data transmitted from the satellite terminal to the central station uses a code division multiple access scheme in a bidirectional satellite communication system. A method for demodulating and receiving reverse link data transmitted from the satellite terminal at the central station, Receiving data transmitted from the satellite terminal at a basic spreading code offset value of the satellite terminal; Confirming a position and energy value of the satellite terminal through the data received from the satellite terminal; Confirming whether an energy value identified for the acquired position value has an appropriate value within a reference value range preset by the central station; Tracking the received signal by referring to the found position value when the position of the satellite terminal is found by the received data; And And acquiring an accurate spreading code offset value of the satellite terminal and performing a call processing process with the corresponding satellite terminal. The method of claim 7, wherein Receiving data including a spreading code offset from the satellite terminal; Retrieving a spreading code offset value transmitted by the satellite terminal in data transmitted by the satellite terminal through a pre-stored default spreading code offset; Comparing a spreading code offset value obtained from the received data with a spreading code offset value transmitted by the searched satellite terminal; And And transmitting a result value to a corresponding satellite terminal when a difference occurs between spreading code offsets as a result of the comparison. The method of claim 7, wherein And a window time interval for checking the position and energy value of the satellite terminal through the data received from the satellite terminal is set to 1 ms or less, thereby shortening the search time. The method of claim 7, wherein And extracting the position value of the satellite terminal from the data received from the satellite terminal and reflecting the demodulated position value during demodulation to compensate for the time delay of the received data. The method of claim 7, wherein And the reverse link data is a wideband code division multiple access (W-CDMA) scheme. A satellite communication system includes a predetermined communication satellite, a central station transmitting predetermined broadcast information to a plurality of subscribers through the communication satellite, and a plurality of satellite terminals receiving information transmitted from the central station through the communication satellite. And the forward link data transmitted from the central station to the satellite terminal uses a time division multiple access scheme, and the reverse link data transmitted from the satellite terminal to the central station uses a code division multiple access scheme in a bidirectional satellite communication system. A method for demodulating and receiving reverse link data transmitted from the satellite terminal at the central station, Detecting a carrier frequency component at the IF level of the received signal; Comparing the center frequency information of the obtained received signal with a transmission center frequency of the corresponding satellite terminal previously stored; Calculating offset values of the center frequency information of the obtained received signal and the transmission center frequency of the pre-stored satellite terminal; And Compensating the frequency offset value of the received signal by adjusting the output frequency value of the local oscillator according to the calculated value; Data demodulation method of the central station in the two-way satellite system, characterized in that it further comprises. The method of claim 12, And the reverse link data is a wideband code division multiple access (W-CDMA) scheme.

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