EP2235870A1 - Apparatus and method for inserting network clock reference packet in satellite communication system - Google Patents

Apparatus and method for inserting network clock reference packet in satellite communication system

Info

Publication number
EP2235870A1
EP2235870A1 EP08793490A EP08793490A EP2235870A1 EP 2235870 A1 EP2235870 A1 EP 2235870A1 EP 08793490 A EP08793490 A EP 08793490A EP 08793490 A EP08793490 A EP 08793490A EP 2235870 A1 EP2235870 A1 EP 2235870A1
Authority
EP
European Patent Office
Prior art keywords
ncr
frame
value
packet
modcod
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP08793490A
Other languages
German (de)
French (fr)
Inventor
Ki-Seop Han
Dae-Ig Chang
Ho-Jin Lee
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Electronics and Telecommunications Research Institute ETRI
Original Assignee
Electronics and Telecommunications Research Institute ETRI
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Electronics and Telecommunications Research Institute ETRI filed Critical Electronics and Telecommunications Research Institute ETRI
Publication of EP2235870A1 publication Critical patent/EP2235870A1/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/14—Relay systems
    • H04B7/15—Active relay systems
    • H04B7/204—Multiple access
    • H04B7/212—Time-division multiple access [TDMA]
    • H04B7/2125—Synchronisation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/14—Relay systems
    • H04B7/15—Active relay systems
    • H04B7/185—Space-based or airborne stations; Stations for satellite systems
    • H04B7/18578—Satellite systems for providing broadband data service to individual earth stations
    • H04B7/18589—Arrangements for controlling an end to end session, i.e. for initialising, synchronising or terminating an end to end link
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/32—Carrier systems characterised by combinations of two or more of the types covered by groups H04L27/02, H04L27/10, H04L27/18 or H04L27/26
    • H04L27/34—Amplitude- and phase-modulated carrier systems, e.g. quadrature-amplitude modulated carrier systems
    • H04L27/3405—Modifications of the signal space to increase the efficiency of transmission, e.g. reduction of the bit error rate, bandwidth, or average power
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04W—WIRELESS COMMUNICATION NETWORKS
    • H04W56/00—Synchronisation arrangements
    • H04W56/0055—Synchronisation arrangements determining timing error of reception due to propagation delay
    • H04W56/0065—Synchronisation arrangements determining timing error of reception due to propagation delay using measurement of signal travel time
    • H04W56/007—Open loop measurement
    • H04W56/0075—Open loop measurement based on arrival time vs. expected arrival time

Definitions

  • the present invention relates to an apparatus and method for inserting a network clock reference (NCR) packet in a network synchronization device of an interactive satellite communication system; and, more particularly, to an apparatus and method for inserting an NCR packet, which is capable of transmitting an NCR value while minimizing transmission capacity loss in a high-speed data communication and broadcast system that uses an adaptive coding and modulation (ACM) mode and a variable coding and modulation (VCM) mode, in which modulation scheme and channel coding scheme vary among digital video broadcasting (DVB)-S2 schemes, for a forward link, and uses a time division multiple access (TDMA)-based digital video broadcasting-return channel via satellite (DVB-RCS) scheme for a reverse link.
  • ACM adaptive coding and modulation
  • VCM variable coding and modulation
  • a TDM-based DVB-S2 scheme and a TDMA-based DVB-RCS scheme have been generally used for a forward link and a reverse link, respectively.
  • 42-bit timestamp value called a program clock reference (PCR) is generated by a 27-MHz clock, which is a reference clock of a base station, to recover a clock on a reception side.
  • the PCR is included in MPEG2-TS packet having a specific program identifier (PID) and periodically transmitted to the reception side.
  • the reception side recovers the 27-MHz clock, which is the reference clock of the central station, using the PCR value and period, and uses the 27-MHz clock as the reference clock of the reception side.
  • timestamp information is inserted into a field carrying an MPEG-PCR value to be periodically transmitted.
  • the timestamp information represents the reference time point of the central station necessary for maintaining a clock and a timing synchronization of the reverse link using this format and characteristic of the PCR value.
  • NCR network clock reference
  • a fundamental unit of transport data to be transmitted is not an MPEG2-TS packet but a DVS-S2 frame complying with the DVB-S2 scheme.
  • the DVS-S2 frame basically has a much greater length than the MPEG2-TS packet, and the length of a transport data is not fixed but variable in accordance with a transmission modulation scheme and a channel coding scheme.
  • the MPEG-TS packet including the NCR value for clock and network synchronization is temporarily stored in a buffer to configure one DVB-S2 frame for transmission and thereafter transmitted when the DVB -S2 frame is completed. Since the data length configuring the DVB-S2 frame is always variable, time duration while being stored in the buffer is also irregularly varied in accordance with the transmission modulation scheme and the channel coding scheme. Therefore, it is impossible to compensate for a delay factor with respect to an NCR value received at a receiving terminal, so that it is hard to find out an exact time point for a reverse transmission.
  • Korean Patent Registration No. 10-740872 discloses a network synchronization acquisition apparatus and a method for maintaining a more stable network synchronization through an NCR value insertion in order to use the DVB-S2 scheme for the forward link and use TDMA-based DVB-RCS scheme for the reverse link.
  • an amount of data capable of being included in a single physical layer (PL) frame is variable in accordance with a type of the PL frame and a low density parity check (LDPC) code rate used in a baseband (BB) frame.
  • LDPC low density parity check
  • BB baseband
  • the MPEG-TS packets converted from the amount of data and a transmission capacity waste rate of the PL frame can be represented as shown in Fig. 1.
  • Fig. 1 is a table illustrating a transmission capacity waste rate of the PL frame including the NCR value.
  • the number of the MPEG2-TS packets varies from about 2.042 to about 38.69.
  • the transmission capacity waste rate of the frame may reach up to about 48.95 %. If the NCR packet is inserted into each frame in case where a short frame is transmitted in a quarter of the LDPC code rate, about 48.95 % of the total transmission capacity may be used in synchronization maintenance.
  • An embodiment of the present invention is directed to providing an apparatus and method for inserting an NCR packet, which is capable of stably transmitting an NCR value while minimizing transmission capacity loss of a forward link in a high-speed data communication and broadcast system.
  • a method for inserting a network clock reference (NCR) packet for network synchronization in an interactive communication system including: selecting a predetermined type of a frame to which the NCR packet is to be inserted; if a timer value is equal to an arrival period of the NCR packet, inserting the NCR packet into a current frame; and if the timer value is smaller than the arrival period of the NCR packet and there is the selected frame among frames to be transmitted, inserting the NCR packet into the selected frame.
  • NCR network clock reference
  • an apparatus for inserting a network clock reference (NCR) packet for network synchronization in an interactive communication system including: a MODCOD selector for selecting one modulation and coding (MODCOD) and providing information of the selected MODCOD; a timer for generating a timer value notifying an arrival period of the NCR packet; and a insertion determiner for comparing the timer value with a preset arrival period of the NCR packet, if the timer value reaches the arrival period of the NCR packet, inserting the NCR packet into a current frame, and if the timer value does not reach the arrival period of the NCR packet and a selected frame exists among frames to be transmitted, inserting the NCR packet into the selected frame, wherein the selected frame is a predetermined type of frame to which the NCR packet is to be inserted and is determined based on the selected MODCOD.
  • a MODCOD selector for selecting one modulation and coding (MODCOD) and providing information of the selected MODCOD
  • a timer for generating
  • a method for inserting a network clock reference (NCR) packet for network synchronization in an interactive communication system including: selecting a modulation and coding (MODCOD) frame with the lowest error rate as a frame to which the NCR packet is to be inserted, inserting the NCR packet into the selected MODCOD frame and transmitting the MODCOD frame; and inserting the NCR packet into a frame in accordance with the timer value and an arbitrarily set arrival period of the NCR packet.
  • MODCOD modulation and coding
  • a method for inserting a network clock reference (NCR) value for network synchronization in an interactive communication system including: selecting a predetermined type of a frame to which the NCR value is to be inserted; if a timer value is equal to an arrival period of the NCR value, inserting the NCR value into a current frame; and if the timer value is smaller than the arrival period of the NCR value and there is the selected frame among frames to be transmitted, inserting the NCR value into the selected frame.
  • NCR network clock reference
  • an apparatus for inserting a network clock reference (NCR) value for network synchronization in an interactive communication system including: a MODCOD selector for selecting one modulation and coding (MODCOD) and providing information of the selected MODCOD; a timer for generating a timer value notifying an arrival period of the NCR value; and a insertion determiner for comparing the timer value with a preset arrival period of the NCR value, if the timer value reaches the arrival period of the NCR value, inserting the NCR value into a current frame, and if the timer value does not reach the arrival period of the NCR value and a selected frame exists among frames to be transmitted, inserting the NCR value into the selected frame, wherein the selected frame is a predetermined type of frame to which the NCR value is to be inserted and is determined based on the selected MODCOD.
  • a MODCOD selector for selecting one modulation and coding (MODCOD) and providing information of the selected MODCOD
  • a timer for generating
  • An apparatus and method in accordance with embodiments of the present invention use a DVB-S2 scheme for a forward link and a TDMA-based DVB-RCS scheme for a reverse link, thereby efficiently transmitting an NCR packet for acquisition and maintenance of network synchronization and maximizing the usage efficiency of the transmission capacity in an interactive satellite communication system. Accordingly, the apparatus and method can provide a more efficient interactive satellite communication environment. [24]
  • Fig. 1 is a table illustrating a transmission capacity waste rate of the PL frame including the NCR value.
  • Fig. 2 is a table illustrating the error correction performance in accordance with an
  • FIG. 3 is a flowchart illustrating a method for inserting an NCR packet in accordance with an embodiment of the present invention.
  • a network clock reference (NCR) value is generated by a DVB-S2 modulator and inserted into a frame to be transmitted.
  • the NCR value may be included in a network clock reference (NCR) packet, and the NCR packet is inserted into the frame to be transmitted.
  • modulation and codings There are a variety of modulation and codings (MODCODs) for the frame unit in an adaptive coding and modulation mode (ACM) or a variable coding and modulation mode (VCM).
  • ACM adaptive coding and modulation mode
  • VCM variable coding and modulation mode
  • the MODCOD is a flag determining a modulation scheme and a code rate and it is included in a header of a PL frame.
  • Fig. 2 is a table illustrating the error correction performance presented in the DVB-
  • the maximum MODCOD for a demodulation is determined in accordance with an environment of a terminal.
  • the terminal can demodulate a MODCOD frame having a lower error rate than that of its own MODCOD.
  • a terminal capable of demodulating up to MODCOD QPSK 9/10 may also demodulate QPSK 1/4, QPSK 1/3, QPSK 2/5, QPSK 1/2, QPSK 3/5, QPSK 2/3, QPSK 3/4, QPSK 4/5, QPSK 5/6, QPSK 8/9, and 8PSK 3/5, which have a lower error rate than that of the MODCOD QPSK 9/10. Accordingly, a terminal using all MODCODs can demodulate the NCR packet from the MPEG2-TS packet where the NCR packet is inserted into a frame using a MODCOD with the lowest error rate.
  • a terminal may maintain synchronization with a central station by correcting a clock error.
  • the time period when the NCR packet must arrive is called t_period.
  • the NCR packet insertion is determined in accordance with a process shown in Fig. 3.
  • An apparatus for inserting an NCR packet in accordance with an embodiment of the present invention includes a MODCOD selector, a timer, and an NCR insertion determiner.
  • the MODCOD selector detects all MODCODs of the terminal and selects an MODCOD with the lowest error rate or one of MODCODs under the control of a modulator. Thereafter, the MODCOD selector transmits the selected MODCOD information to the NCR insertion determiner.
  • the timer generates a timer value for notifying the arrival period of the NCR packet and transmits the timer value to NCR insertion determiner.
  • the NCR insertion determiner inserts the NCR packet into the MODCOD frame with the lowest error rate at an early stage. Thereafter, the NCR insertion determiner compares the timer value with a preset arrival period t_period of the NCR packet. If the timer value reaches the arrival period t_period of the NCR packet, the NCR packet is inserted into the current frame. If the timer value does not reach the arrival period t_period of the NCR packet and the selected MODCOD frame exists, the NCR value is inserted into the selected MODCOD frame.
  • the NCR packet includes the NCR value.
  • FIG. 3 is a flowchart illustrating a method for inserting NCR packet in accordance with an embodiment of the present invention.
  • the method may be divided into a first execution stage and a second execution stage.
  • the first execution stage means that the NCR packet is inserted into the MODCOD frame with the lowest error rate, which is to be transmitted.
  • the timer value is compared with the arrival period t-period. If the timer value reaches the arrival period t_period of the NCR packet, the NCR packet is inserted into the current frame. If the timer value does not reach the arrival period t_period and the selected MODCOD frame exists, the NCR packet is inserted into the selected MODCOD frame. [41] Referring to Fig. 3, all MODCOD frames of the terminal are read in operation SlOl.
  • operation S 102 the MODCOD with the lowest error rate is selected.
  • operation S 103 the NCR packet is inserted into the selected MODCOD frame to be transmitted. The above operations belong to the first execution stage.
  • the timer is reset to operate.
  • the timer value is increased.
  • the timer value is compared with a critical value If the timer value is smaller than the critical value, the timer value continuously increases.
  • the critical value is determined by multiplying the arrival period t_period of the NCR packet by n (0 ⁇ n ⁇ l) in order to initiate the NCR packet insertion.
  • the timer value is equal or larger than the critical value
  • the timer value is compared with the arrival period t_period of the NCR packet in operation S 107. If the timer value is equal to or larger than the arrival period t_period of the NCR packet, this means a time point at which the NCR packet must be transmitted. Accordingly, in operation S 108, the NCR packet is inserted into the current frame regardless that the current frame is the selected MODCOD frame or not.
  • the timer value is equal or larger than the critical value and smaller than the arrival period t_period of the NCR packet, it is determined if the current frame is the selected MODCOD in operation S 109. If the current frame is the selected MODCOD, the NCR packet is inserted into the selected MODCOD frame in operation Sl 10. If the current frame is not the selected MODCOD frame, the timer value continuously increases. That is, if the timer value is equal or larger than the critical value and smaller than the arrival period t_period, the NCR packet is inserted into the existing selected MODCOD frames.
  • NCR packet is inserted into the frame in every arrival period of the NCR packet.
  • the NCR value may be inserted into the frame in every arrival period of the NCR value.
  • the methods of the present invention may be programmed in a computer language.
  • Codes and code segments constituting the computer program may be easily inferred by a computer programmer skilled in the art.
  • the computer program may be stored in a computer-readable recording medium including all kinds of media such as CD-ROM, RAM, ROM, floppy disk, hard disk and magneto-optical disk, and read and executed by a computer to embody the methods.

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  • Engineering & Computer Science (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Signal Processing (AREA)
  • Physics & Mathematics (AREA)
  • Astronomy & Astrophysics (AREA)
  • Aviation & Aerospace Engineering (AREA)
  • General Physics & Mathematics (AREA)
  • Synchronisation In Digital Transmission Systems (AREA)
  • Data Exchanges In Wide-Area Networks (AREA)

Abstract

Provided is an apparatus and method for inserting a network clock reference (NCR) packet for network synchronization in an interactive satellite communication system. The method includes selecting a predetermined type of a frame to which the NCR packet is to be inserted; if a timer value is equal to an arrival period of the NCR packet, inserting the NCR packet into a current frame; and if the timer value is smaller than the arrival period of the NCR packet and there is the selected frame among frames to be transmitted, inserting the NCR packet into the selected frame.

Description

Description
APPARATUS AND METHOD FOR INSERTING NETWORK CLOCK REFERENCE PACKET IN SATELLITE COMMUNICATION SYSTEM
Technical Field
[1] The present invention relates to an apparatus and method for inserting a network clock reference (NCR) packet in a network synchronization device of an interactive satellite communication system; and, more particularly, to an apparatus and method for inserting an NCR packet, which is capable of transmitting an NCR value while minimizing transmission capacity loss in a high-speed data communication and broadcast system that uses an adaptive coding and modulation (ACM) mode and a variable coding and modulation (VCM) mode, in which modulation scheme and channel coding scheme vary among digital video broadcasting (DVB)-S2 schemes, for a forward link, and uses a time division multiple access (TDMA)-based digital video broadcasting-return channel via satellite (DVB-RCS) scheme for a reverse link.
[2] This work was supported by the IT R&D program of MIC/IITA [2006-S-020-02,
"Development of Satellite and Terrestrial Convergence Technology for Internet Service on High-speed Mobile Vehicles"].
[3]
Background Art
[4] In a high-speed interactive satellite communication system, a TDM-based DVB-S2 scheme and a TDMA-based DVB-RCS scheme have been generally used for a forward link and a reverse link, respectively. In a satellite broadcast and communication system, 42-bit timestamp value called a program clock reference (PCR) is generated by a 27-MHz clock, which is a reference clock of a base station, to recover a clock on a reception side. The PCR is included in MPEG2-TS packet having a specific program identifier (PID) and periodically transmitted to the reception side. The reception side recovers the 27-MHz clock, which is the reference clock of the central station, using the PCR value and period, and uses the 27-MHz clock as the reference clock of the reception side. In the interactive satellite communication system complying with DVB- RCS standard for configuring a reverse link via a satellite, timestamp information is inserted into a field carrying an MPEG-PCR value to be periodically transmitted. The timestamp information represents the reference time point of the central station necessary for maintaining a clock and a timing synchronization of the reverse link using this format and characteristic of the PCR value.
[5] The value inserted into the MPEG-PCR field to maintain the clock and the timing synchronization of the reverse link is designated as a network clock reference (NCR). A terminal recovers the 27 -MHz clock, which is the reference clock of the central station, using a time interval between the periodically received NCR values as describe above, and also maintains absolute time synchronization with the central station using the NCR values itself at the very time when the NCR values are received.
[6] When using the DVB-S2 scheme for the forward link instead of a DVB-S scheme that is a conventional broadcasting scheme, a fundamental unit of transport data to be transmitted is not an MPEG2-TS packet but a DVS-S2 frame complying with the DVB-S2 scheme. The DVS-S2 frame basically has a much greater length than the MPEG2-TS packet, and the length of a transport data is not fixed but variable in accordance with a transmission modulation scheme and a channel coding scheme.
[7] Accordingly, the MPEG-TS packet including the NCR value for clock and network synchronization is temporarily stored in a buffer to configure one DVB-S2 frame for transmission and thereafter transmitted when the DVB -S2 frame is completed. Since the data length configuring the DVB-S2 frame is always variable, time duration while being stored in the buffer is also irregularly varied in accordance with the transmission modulation scheme and the channel coding scheme. Therefore, it is impossible to compensate for a delay factor with respect to an NCR value received at a receiving terminal, so that it is hard to find out an exact time point for a reverse transmission.
[8] To solve this problem, Korean Patent Registration No. 10-740872 discloses a network synchronization acquisition apparatus and a method for maintaining a more stable network synchronization through an NCR value insertion in order to use the DVB-S2 scheme for the forward link and use TDMA-based DVB-RCS scheme for the reverse link.
[9] However, although the above patent discloses the method for inserting and transmitting the NCR value, it does not describe a transmission period of the NCR value.
[10] In the DVB-S2 standard, an amount of data capable of being included in a single physical layer (PL) frame is variable in accordance with a type of the PL frame and a low density parity check (LDPC) code rate used in a baseband (BB) frame. Furthermore, the MPEG-TS packets converted from the amount of data and a transmission capacity waste rate of the PL frame can be represented as shown in Fig. 1.
[11] Fig. 1 is a table illustrating a transmission capacity waste rate of the PL frame including the NCR value.
[12] As shown in Fig. 1, when converting the amount of data capable of being included in a single PL frame into the MPEG2-TS packet, the number of the MPEG2-TS packets varies from about 2.042 to about 38.69. When one MPEG-TS packet is consumed to the NCR in case of a low capacity frame, the transmission capacity waste rate of the frame may reach up to about 48.95 %. If the NCR packet is inserted into each frame in case where a short frame is transmitted in a quarter of the LDPC code rate, about 48.95 % of the total transmission capacity may be used in synchronization maintenance.
[13]
Disclosure of Invention Technical Problem
[14] An embodiment of the present invention is directed to providing an apparatus and method for inserting an NCR packet, which is capable of stably transmitting an NCR value while minimizing transmission capacity loss of a forward link in a high-speed data communication and broadcast system.
[15] Other objects and advantages of the present invention can be understood by the following description, and become apparent with reference to the embodiments of the present invention. Also, it is obvious to those skilled in the art of the present invention that the objects and advantages of the present invention can be realized by the means as claimed and combinations thereof.
[16]
Technical Solution
[17] In accordance with an aspect of the present invention, there is provided a method for inserting a network clock reference (NCR) packet for network synchronization in an interactive communication system, the method including: selecting a predetermined type of a frame to which the NCR packet is to be inserted; if a timer value is equal to an arrival period of the NCR packet, inserting the NCR packet into a current frame; and if the timer value is smaller than the arrival period of the NCR packet and there is the selected frame among frames to be transmitted, inserting the NCR packet into the selected frame.
[18] In accordance with another aspect of the present invention, there is provided an apparatus for inserting a network clock reference (NCR) packet for network synchronization in an interactive communication system, the apparatus including: a MODCOD selector for selecting one modulation and coding (MODCOD) and providing information of the selected MODCOD; a timer for generating a timer value notifying an arrival period of the NCR packet; and a insertion determiner for comparing the timer value with a preset arrival period of the NCR packet, if the timer value reaches the arrival period of the NCR packet, inserting the NCR packet into a current frame, and if the timer value does not reach the arrival period of the NCR packet and a selected frame exists among frames to be transmitted, inserting the NCR packet into the selected frame, wherein the selected frame is a predetermined type of frame to which the NCR packet is to be inserted and is determined based on the selected MODCOD. [19] In accordance with another aspect of the present invention, there is provided a method for inserting a network clock reference (NCR) packet for network synchronization in an interactive communication system, the method including: selecting a modulation and coding (MODCOD) frame with the lowest error rate as a frame to which the NCR packet is to be inserted, inserting the NCR packet into the selected MODCOD frame and transmitting the MODCOD frame; and inserting the NCR packet into a frame in accordance with the timer value and an arbitrarily set arrival period of the NCR packet.
[20] In accordance with another aspect of the present invention, there is provided a method for inserting a network clock reference (NCR) value for network synchronization in an interactive communication system, the method including: selecting a predetermined type of a frame to which the NCR value is to be inserted; if a timer value is equal to an arrival period of the NCR value, inserting the NCR value into a current frame; and if the timer value is smaller than the arrival period of the NCR value and there is the selected frame among frames to be transmitted, inserting the NCR value into the selected frame.
[21] In accordance with another aspect of the present invention, there is provided an apparatus for inserting a network clock reference (NCR) value for network synchronization in an interactive communication system, the apparatus including: a MODCOD selector for selecting one modulation and coding (MODCOD) and providing information of the selected MODCOD; a timer for generating a timer value notifying an arrival period of the NCR value; and a insertion determiner for comparing the timer value with a preset arrival period of the NCR value, if the timer value reaches the arrival period of the NCR value, inserting the NCR value into a current frame, and if the timer value does not reach the arrival period of the NCR value and a selected frame exists among frames to be transmitted, inserting the NCR value into the selected frame, wherein the selected frame is a predetermined type of frame to which the NCR value is to be inserted and is determined based on the selected MODCOD.
[22]
Advantageous Effects
[23] An apparatus and method in accordance with embodiments of the present invention use a DVB-S2 scheme for a forward link and a TDMA-based DVB-RCS scheme for a reverse link, thereby efficiently transmitting an NCR packet for acquisition and maintenance of network synchronization and maximizing the usage efficiency of the transmission capacity in an interactive satellite communication system. Accordingly, the apparatus and method can provide a more efficient interactive satellite communication environment. [24]
Brief Description of the Drawings
[25] Fig. 1 is a table illustrating a transmission capacity waste rate of the PL frame including the NCR value.
[26] Fig. 2 is a table illustrating the error correction performance in accordance with an
MODCOD.
[27] Fig. 3 is a flowchart illustrating a method for inserting an NCR packet in accordance with an embodiment of the present invention.
[28]
Best Mode for Carrying Out the Invention
[29] The advantages, features and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. Therefore, those skilled in the field of this art of the present invention can embody the technological concept and scope of the invention easily. In addition, if it is considered that detailed description on a related art may obscure the points of the present invention, the detailed description will not be provided herein. The preferred embodiments of the present invention will be described in detail hereinafter with reference to the attached drawings.
[30] Generally, a network clock reference (NCR) value is generated by a DVB-S2 modulator and inserted into a frame to be transmitted. Also, the NCR value may be included in a network clock reference (NCR) packet, and the NCR packet is inserted into the frame to be transmitted. There are a variety of modulation and codings (MODCODs) for the frame unit in an adaptive coding and modulation mode (ACM) or a variable coding and modulation mode (VCM). The MODCOD is a flag determining a modulation scheme and a code rate and it is included in a header of a PL frame.
[31] It is determined at which MODCOD the frame is sent, depending on the MODCOD flag. An error correction performance in accordance with the MODCOD is presented in the DVB -S2 standard.
[32] Fig. 2 is a table illustrating the error correction performance presented in the DVB-
S2 standard in accordance with the MODCOD.
[33] As shown in Fig. 2, the error correction performance varies in accordance with the
MODCOD of a frame. In an interactive satellite communication system using the DVB-S2 and a DVB-RCS, the maximum MODCOD for a demodulation is determined in accordance with an environment of a terminal. The terminal can demodulate a MODCOD frame having a lower error rate than that of its own MODCOD.
[34] For example, as described with reference to Fig. 2, a terminal capable of demodulating up to MODCOD QPSK 9/10 may also demodulate QPSK 1/4, QPSK 1/3, QPSK 2/5, QPSK 1/2, QPSK 3/5, QPSK 2/3, QPSK 3/4, QPSK 4/5, QPSK 5/6, QPSK 8/9, and 8PSK 3/5, which have a lower error rate than that of the MODCOD QPSK 9/10. Accordingly, a terminal using all MODCODs can demodulate the NCR packet from the MPEG2-TS packet where the NCR packet is inserted into a frame using a MODCOD with the lowest error rate.
[35] However, as described with reference to FIG. 1, the MODCOD with the lower error rate represents a higher transmission capacity waste rate due to its low transmission capacity. Accordingly, a method for solving this problem will be presented herein.
[36] When an NCR packet is received within a predetermined time period, a terminal may maintain synchronization with a central station by correcting a clock error. The time period when the NCR packet must arrive is called t_period. The NCR packet insertion is determined in accordance with a process shown in Fig. 3.
[37] An apparatus for inserting an NCR packet in accordance with an embodiment of the present invention includes a MODCOD selector, a timer, and an NCR insertion determiner. The MODCOD selector detects all MODCODs of the terminal and selects an MODCOD with the lowest error rate or one of MODCODs under the control of a modulator. Thereafter, the MODCOD selector transmits the selected MODCOD information to the NCR insertion determiner. The timer generates a timer value for notifying the arrival period of the NCR packet and transmits the timer value to NCR insertion determiner.
[38] The NCR insertion determiner inserts the NCR packet into the MODCOD frame with the lowest error rate at an early stage. Thereafter, the NCR insertion determiner compares the timer value with a preset arrival period t_period of the NCR packet. If the timer value reaches the arrival period t_period of the NCR packet, the NCR packet is inserted into the current frame. If the timer value does not reach the arrival period t_period of the NCR packet and the selected MODCOD frame exists, the NCR value is inserted into the selected MODCOD frame. Herein, the NCR packet includes the NCR value.
[39] Fig. 3 is a flowchart illustrating a method for inserting NCR packet in accordance with an embodiment of the present invention.
[40] The method may be divided into a first execution stage and a second execution stage.
The first execution stage means that the NCR packet is inserted into the MODCOD frame with the lowest error rate, which is to be transmitted. At the second execution stage, the timer value is compared with the arrival period t-period. If the timer value reaches the arrival period t_period of the NCR packet, the NCR packet is inserted into the current frame. If the timer value does not reach the arrival period t_period and the selected MODCOD frame exists, the NCR packet is inserted into the selected MODCOD frame. [41] Referring to Fig. 3, all MODCOD frames of the terminal are read in operation SlOl.
In operation S 102, the MODCOD with the lowest error rate is selected. In operation S 103, the NCR packet is inserted into the selected MODCOD frame to be transmitted. The above operations belong to the first execution stage.
[42] The second execution stage will be described below. In operation S 104, the timer is reset to operate. In operation S 105, the timer value is increased. In operation S 106, the timer value is compared with a critical value If the timer value is smaller than the critical value, the timer value continuously increases. Herein, the critical value is determined by multiplying the arrival period t_period of the NCR packet by n (0<n<l) in order to initiate the NCR packet insertion.
[43] On the contrary, the timer value is equal or larger than the critical value, the timer value is compared with the arrival period t_period of the NCR packet in operation S 107. If the timer value is equal to or larger than the arrival period t_period of the NCR packet, this means a time point at which the NCR packet must be transmitted. Accordingly, in operation S 108, the NCR packet is inserted into the current frame regardless that the current frame is the selected MODCOD frame or not.
[44] If the timer value is equal or larger than the critical value and smaller than the arrival period t_period of the NCR packet, it is determined if the current frame is the selected MODCOD in operation S 109. If the current frame is the selected MODCOD, the NCR packet is inserted into the selected MODCOD frame in operation Sl 10. If the current frame is not the selected MODCOD frame, the timer value continuously increases. That is, if the timer value is equal or larger than the critical value and smaller than the arrival period t_period, the NCR packet is inserted into the existing selected MODCOD frames.
[45] In the above embodiment, it is described that NCR packet is inserted into the frame in every arrival period of the NCR packet. However, it is possible that the NCR value may be inserted into the frame in every arrival period of the NCR value.
[46] The methods of the present invention may be programmed in a computer language.
Codes and code segments constituting the computer program may be easily inferred by a computer programmer skilled in the art. Furthermore, the computer program may be stored in a computer-readable recording medium including all kinds of media such as CD-ROM, RAM, ROM, floppy disk, hard disk and magneto-optical disk, and read and executed by a computer to embody the methods.
[47] The present application contains subject matter related to Korean Patent Application
No. 2007-0132496, filed in the Korean Intellectual Property Office on December 17, 2007, the entire contents of which are incorporated herein by reference.
[48] While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modi- fications may be made without departing from the spirit and scope of the invention as defined in the following claims.

Claims

Claims
[1] A method for inserting a network clock reference (NCR) packet for network synchronization in an interactive communication system, the method comprising: selecting a predetermined type of a frame to which the NCR packet is to be inserted; if a timer value is equal to an arrival period of the NCR packet, inserting the NCR packet into a current frame; and if the timer value is smaller than the arrival period of the NCR packet and there is the selected frame among frames to be transmitted, inserting the NCR packet into the selected frame.
[2] The method of claim 1, wherein the NCR packet includes a network clock reference (NCR) value.
[3] The method of claim 1, wherein the selected frame is a modulation and coding
(MODCOD) frame with the lowest error rate.
[4] The method of claim 3, wherein said inserting the NCR packet into the selected frame comprises: if the timer value is larger than a critical value and smaller than the arrival period of the NCR packet, checking whether the current frame is the selected frame; if the current frame is not the selected frame, increasing the timer value; and if the current frame is the selected frame, inserting the NCR packet into the selected frame.
[5] The method of claim 4, wherein the critical value is set to a value smaller than the arrival period of the NCR packet.
[6] An apparatus for inserting a network clock reference (NCR) packet for network synchronization in an interactive communication system, the apparatus comprising: a MODCOD selector for selecting one modulation and coding (MODCOD) and providing information of the selected MODCOD; a timer for generating a timer value notifying an arrival period of the NCR packet; and a insertion determiner for comparing the timer value with a preset arrival period of the NCR packet, if the timer value reaches the arrival period of the NCR packet, inserting the NCR packet into a current frame, and if the timer value does not reach the arrival period of the NCR packet and a selected frame exists among frames to be transmitted, inserting the NCR packet into the selected frame, wherein the selected frame is a predetermined type of frame to which the NCR packet is to be inserted and is determined based on the selected MODCOD.
[7] The apparatus of claim 6, wherein the MODCOD selector detects entire
MODCODs of terminals at an early stage, selects an MODCOD frame with the lowest error rate as the selected frame and transmits the information of the selected frame to the insertion determiner.
[8] The apparatus of claim 7, wherein the insertion determiner increases the timer value when the timer value is larger than a critical value and smaller than the arrival period of the NCR packet, and the current frame is not the selected frame.
[9] The apparatus of claim 8, wherein the critical value is set to a value smaller than the arrival period of the NCR packet.
[10] A method for inserting a network clock reference (NCR) packet for network synchronization in an interactive communication system, the method comprising: selecting a modulation and coding (MODCOD) frame with the lowest error rate as a frame to which the NCR packet is to be inserted, inserting the NCR packet into the selected MODCOD frame and transmitting the MODCOD frame; and inserting the NCR packet into a frame in accordance with the timer value and an arbitrarily set arrival period of the NCR packet.
[11] The method of claim 10, wherein said inserting the NCR packet into a frame comprises: if a timer value is equal to the arrival period of the NCR packet, inserting the NCR packet into a current frame; and if the timer value is smaller than the arrival period of the NCR packet and there is the selected MODCOD frame among frames to be transmitted, inserting the NCR packet into the selected MODCOD frame.
[12] The method of claim 11, wherein said inserting the NCR packet into the selected
MODCOD frame comprises: if the timer value is larger than a critical value and smaller than the arrival period of the NCR packet, checking whether the current frame is the selected MODCOD frame; if the current frame is not the selected MODCOD frame, increasing the timer value; and if the current frame is the selected MODCOD frame, inserting the NCR packet into the selected MODCOD frame.
[13] A method for inserting a network clock reference (NCR) value for network synchronization in an interactive communication system, the method comprising: selecting a predetermined type of a frame to which the NCR value is to be inserted; if a timer value is equal to an arrival period of the NCR value, inserting the NCR value into a current frame; and if the timer value is smaller than the arrival period of the NCR value and there is the selected frame among frames to be transmitted, inserting the NCR value into the selected frame.
[14] The method of claim 13, wherein the selected frame is a modulation and coding
(MODCOD) frame with the lowest error rate.
[15] The method of claim 14, wherein said inserting the NCR value into the selected frame comprises: if the timer value is larger than a critical value and smaller than the arrival period of the NCR value, checking whether the current frame is the selected frame; if the current frame is not the selected frame, increasing the timer value; and if the current frame is the selected frame, inserting the NCR value into the selected frame.
[16] The method of claim 15, wherein the critical value is set to a value smaller than the arrival period of the NCR value.
[17] An apparatus for inserting a network clock reference (NCR) value for network synchronization in an interactive communication system, the apparatus comprising: a MODCOD selector for selecting one modulation and coding (MODCOD) and providing information of the selected MODCOD; a timer for generating a timer value notifying an arrival period of the NCR value; and a insertion determiner for comparing the timer value with a preset arrival period of the NCR value, if the timer value reaches the arrival period of the NCR value, inserting the NCR value into a current frame, and if the timer value does not reach the arrival period of the NCR value and a selected frame exists among frames to be transmitted, inserting the NCR value into the selected frame, wherein the selected frame is a predetermined type of frame to which the NCR value is to be inserted and is determined based on the selected MODCOD.
[18] The apparatus of claim 17, wherein the MODCOD selector detects entire
MODCODs of terminals at an early stage, selects an MODCOD frame with the lowest error rate as the selected frame and transmits the information of the selected frame to the insertion determiner.
[19] The apparatus of claim 18, wherein the insertion determiner increases the timer value when the timer value is larger than a critical value and smaller than the arrival period of the NCR value, and the current frame is not the selected frame.
[20] The apparatus of claim 19, wherein the critical value is set to a value smaller than the arrival period of the NCR value.
EP08793490A 2007-12-17 2008-08-26 Apparatus and method for inserting network clock reference packet in satellite communication system Withdrawn EP2235870A1 (en)

Applications Claiming Priority (2)

Application Number Priority Date Filing Date Title
KR1020070132496A KR100902335B1 (en) 2007-12-17 2007-12-17 Method and apparatus for inserting network clock reference packet in bidirectional communication system
PCT/KR2008/004985 WO2009078539A1 (en) 2007-12-17 2008-08-26 Apparatus and method for inserting network clock reference packet in satellite communication system

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US7515566B2 (en) * 2004-12-09 2009-04-07 Viasat, Inc. Partial mesh communication in hub based system
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