Abstract:
Conventional satellite communication systems operated under DAMA system have such disadvantages that assignment of communication lines becomes time-consuming in case of transmitting small quantity of data many times and that data transmission is impossible in case of no available communication line, because the line assignment is performed by DAMA system for each individual demand for line assignment regardless of data quantity to be transmitted. The invention provides a satellite communication system in which the time required for assigning and securing a communication line can be shortened and communication lines can be utilized more efficiently, because of using single or plural common signal channel lines instead of using ordinary communication lines in case that data quantity is below a certain level.

Description:
BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to a satellite communication system operated under Demand Assignment Multiple Access system (hereinafter referred to as DAMA system) for the purpose of communications between a hub station and a plurality of remote stations, and in which common signal channel line (hereinafter referred to as CSC line) from remote stations to a hub station is used for data transmission. 
     2. Background Art 
       FIG. 5  is a schematic diagram showing an arrangement of one of conventional satellite communication systems operated under the DAMA system. DAMA system stands for a system in which a line required for communication is assigned on demand, in a communication network consisting of a plurality of earth stations. For that purpose, the DAMA system is provided with two types of lines, i.e., CSC line and communication line respectively. The CSC line is used for demanding assignment of a communication line and for controlling assignment of an idle line, and the assigned communication line is used for actual communication. 
     Referring to  FIG. 5 , reference numeral  31  is a hub station engaged in total control of a network including assignment, supervision and control of lines by the DAMA system, in the mentioned conventional satellite communication system. Numeral  32  is a plurality of remote stations, and numeral  33  is a communication satellite launched into space for relaying communication lines. Numeral  34  are forward CSC lines from the remote stations  32  to the hub station  31  through the communication satellite  33 . In the operation of the DAMA system, the CSC lines  34  are used for transmission of demands for assignment of lines, supervisory signals and reply signals to the supervisory signals, or the like. Numeral  35  are backward CSC line from the hub station  31  to the remote stations  32  through the communication satellite  33 . Numeral  36  is communication lines used for actual communications. 
       FIG. 6  is a diagram showing an arrangement of frequency band and time slots of the forward CSC line  34  and backward CSC line  35 , fixed by the mentioned satellite communication system. Referring to  FIG. 6 , numeral  35  is a frequency band of the backward CSC line from the hub station  31  to the remote stations  32 , and numeral  34  is a frequency band of the forward CSC line from the remote stations  32  to the hub station  31 . Further, a different frequency band is allocated to the satellite communication lines  36 , though not shown in  FIG. 6 . 
       FIG. 7  is a block diagram showing an arrangement of the conventional hub station  31 . In the drawing, numeral  40  is a terminal unit. Numeral  41  is a line connection control device. This line connection control device  41  consists of: a DAMA control section  41   a  for detecting and assigning idle lines in response to demands for line assignment from the remote stations  32 ; and a supervision and control section  41   b  for transmitting supervisory signals to check whether or not the operation of remote stations is normal and receiving reply signals from the remote stations. Numeral  42  is a CSC modulator section for modulating radio signals such as instructions of line assignment from the line connection control device  41  to a frequency band of the backward CSC line  35 . Numeral  43  is a CSC demodulator section for demodulating received signals corresponding to the frequency of the forward CSC line  34  shown in  FIG. 6 . Numeral  44   a  is a communication line modulator/demodulator (modem) section of the hub station  31  for establishing a frequency on an idle communication line for actual communication in accordance with instructions from the DAMA control section  41   a,  and for transmitting/receiving radio signals of the mentioned frequency. Numeral  45  is a transmitter/receiver for transmitting radio signals outputted from the CSC modulator  42  into space after amplifying the signals at the time of transmission and, and for converting radio signals received from the satellite  33  into intermediate frequency signals and outputting those signals to the CSC demodulator  43  at the time of reception. 
       FIG. 8  is a block diagram showing an arrangement of the conventional remote station  32 . In the drawing, numeral  50  is a terminal unit. Numeral  51  is an IDU (In Door Unit). This IDU  51  consists of a CSC modulator/demodulator (modem) section  51   a  corresponding to the frequency of the forward CSC line  34 , a communication line modulator/demodulator (modem) section  44   b  of the remote station  32  for establishing a frequency on an idle line at the time of communication and transmitting/receiving radio signals of the mentioned frequency, and an IDU control section  51   c  for controlling the CSC modulator/demodulator (modem) section  51   a  at the time of demanding a line, and for controlling the communication line modulator/demodulator (modem) section  44   b  so as to establish a frequency on an idle line in accordance with the instruction from the hub station  31  at the time of communication after the demand. Numeral  52  is an ODU (Out Door Unit) for transmitting into space radio signals outputted from the IDU  51  after amplifying those signals at the time of transmission, and for converting radio signals received from the satellite  33  into signals of an intermediate frequency and outputting those signals to the IDU  51 , at the time of reception. Numeral  53  is an antenna. 
     The mentioned conventional system operates as described hereunder. First, in case of conducting a communication between a remote station  32   a  and another remote station  32   b,  the CSC modulator/demodulator (modem) section  51   a  of the remote station  32   a  modulates the frequency by the control of the IDU control section  51   c  to that of the forward CSC line  34  in response to a call from the terminal unit  50 , and selects a time slot and transmits radio signals to the ODU  52  demanding a line connection (line assignment) to the hub station  31 . The ODU  52  amplifies the mentioned radio signals and transmits them into space through the antenna  53 . 
     The hub station  31  receives through the antenna  46  the signals demanding the line connection relayed through the satellite  33 , and outputs the mentioned signals to transmitter/receiver  45 . The transmitter/receiver  45  converts the signals demanding line assignment into intermediate frequency signals and outputs those signals to the CSC modulator/demodulator (modem) section  43 . The CSC modulator/demodulator (modem) section  43  demodulates the signals demanding line connection and outputs them to the line connection control device  41 . The DAMA control section  41   a  detects an idle communication line to be used for actual communication, and outputs a line assignment signal to the CSC modulator section  42 . The mentioned signal is modulated at the CSC modulator  42 , amplified at the transmitter/receiver  45 , transmitted into space through the antenna  46 , thus an available communication line (frequency and bandwidth to be used) is instructed to the remote stations  32   a  and  32   b  through the satellite  33 . 
     Then, the remote stations  32   a  and  32   b  adjust the frequency of their respective modulator/demodulator (modems) sections  44   b  in tune with the instructed frequency band of the satellite communication line, and perform the communication. In case of conducting a communication between a remote station  32   a  and the hub station  31 , the same steps as the foregoing are taken. That is, frequency of the communication line modulator/demodulator (modem) section  44   a  of the hub station  31  and that of the communication line modulator/demodulator (modem) section  44   b  of the remote station  32  are adjusted in tune with each other. 
     Supervision and control of each of the remote stations  32  by the hub station  31  is performed as described hereunder. The hub station  31  transmits a supervisory signal tuned to the frequency band of the backward CSC line  35  from the supervision and control section  41   b  into space through the CSC modulator  42 , transmitter/receiver  45  and the antenna  46 . 
     The supervisory signal received by a remote station  32  through the satellite  33  is modulated into an intermediate frequency at the ODU  52 , demodulated at the CSC modulator/demodulator (modem) section  51   a  of the IDU  51 , and then recognized as a supervisory signal destined to the remote station itself. At this time, if the remote station is normally operating, this remote station returns a reply signal to the hub station  31  through the CSC modulator/demodulator (modem) section  51   a,  the ODU  52  and the antenna  53 , using the forward CSC line  34 . The returned reply signal is recognized at the supervision and control section  41   b  in the line connection control device  41 , whereby it is acknowledged that the remote station  32  is normally operating. The hub station  31  repeating the mentioned steps for each of the remote stations one after another conducts the supervision of all the remote stations  32 . 
     In the mentioned conventional system, frequency bandwidth and information rate of the forward CSC line  34  were fixed at the time of demanding line connection and performing the supervision and control. 
     That is, in the foregoing prior art, the line assignment is performed regardless of data quantity to be transmitted. Since detection and assignment of a communication line is performed in response to each individual demand for line assignment, a problem exists in that in case of transmitting a small data quantity, the mentioned conventional communication system is time-consuming and inefficient. A further problem exists in that it is impossible to transmit data if there is no idle communication line. 
     SUMMARY OF THE INVENTION 
     The present invention was made to solve the above-discussed problems, and has an object of providing a satellite communication system in which in case that data quantity is below a certain level, a common signal channel line (CSC line) for data transmission is utilized instead of demanding, assigning and securing a communication line, thereby time from demanding data transmission until starting transmission being shortened without any need to assign and secure a communication line, more idle communication lines remaining available due to the use of the CSC line for data transmission, thus more efficient utilization of communication lines being achieved. 
     To accomplish the foregoing object, a satellite communication system according to the invention is operated under Demand Assignment Multiple Access system (DAMA system); in which a specific earth station performing line assignment control and a plurality of other earth stations are connected by a common signal channel line through a communication satellite; a calling earth station demands assignment of a communication line through the common signal channel line from the mentioned specific earth station at the time of performing a communication between the earth stations including the specific earth station; the mentioned specific earth station assigns a communication line in response to the demand for line assignment; and the calling earth station and the called earth station perform a communication using the mentioned assigned communication line, and release the communication line upon completing the communication; 
     the satellite communication system being characterized in that the mentioned common signal channel line is used for transmission of data whose quantity is below a certain level. 
     It is preferable that the mentioned plurality of other earth stations are provided with transmission band selection means for selecting a transmission bandwidth of the common signal channel line depending on data quantity to be transmitted from the mentioned earth stations. 
     It is also preferable that the mentioned plurality of other earth stations are provided with time slot selection means for selecting a plurality of continuous or discontinuous TDMA slots of the mentioned common signal channel line depending on data quantity to be transmitted from the mentioned earth stations. 
     It is also preferable that the mentioned plurality of other earth stations are provided with both transmission band selection means for selecting a transmission bandwidth of the common signal channel line depending on data quantity to be transmitted from the mentioned earth stations, and time slot selection means for selecting a plurality of continuous or discontinuous TDMA slots of the mentioned common signal channel line depending on the data quantity to be transmitted from the mentioned earth stations. 
     It is also preferable that in case of transmitting data to the mentioned specific earth station, the mentioned plurality of other earth stations demand transmission bandwidth and TDMA slots of the common signal channel line necessary for data transmission from the mentioned specific earth station, and the mentioned specific earth station assigns a transmission bandwidth and TDMA slots of the common signal channel line to the calling earth station to serve as exclusive communication line for the mentioned calling earth station. 
     In the satellite communication system of the foregoing arrangement, the time required for access to communication line can be shortened because of using the CSC line instead of assigning a communication line under DAMA system in case that data quantity to be transmitted is below a certain level. As a result, it becomes possible to obtain a satellite communication system in which efficient use of the communication line is improved. 
     Further, the time required for access to communication line can be shortened because of using the CSC line and making bandwidth and time slots of the CSC line variable, instead of assigning a communication line under DAMA system in case of transmitting a data quantity below a certain level. As a result, it becomes possible to obtain a satellite communication system in which efficient use of the communication line is improved. 
     Furthermore, by allocating temporarily single or plurality of time slots of the forward CSC line exclusively for data transmission of a specific remote station, it becomes possible to prevent data loss due to collision of different data being transmitted. As a result, it becomes possible to obtain a satellite communication system of high quality in which efficient use of the communication line is improved without communication line assignment under DAMA system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing an arrangement of a satellite communication system according to Embodiment 1 of the present invention. 
         FIG. 2  is a schematic diagram showing an arrangement of frequency and time slots of a CSC line in the satellite communication system according to Embodiment 1 of the invention. 
         FIG. 3  is a block diagram showing an arrangement of a hub station in the satellite communication system according to Embodiment 1 of the invention. 
         FIG. 4  is a block diagram showing an arrangement of a remote station in the satellite communication system according to Embodiment 1 of the invention.  FIG. 5  is a schematic view showing an arrangement of a satellite communication system according to a prior art. 
         FIG. 6  is a schematic diagram showing an arrangement of frequency and time slots of a CSC line in the satellite communication system according to the prior art. 
         FIG. 7  is a block diagram showing an arrangement of a hub station in the satellite communication system according to the prior art. 
         FIG. 8  is a block diagram showing an arrangement of a remote station in the satellite communication system according to the prior art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiment 1 
       FIG. 1  is a schematic view showing an arrangement of a satellite communication system according to Embodiment 1 of the present invention. In the drawing, reference numeral  1  is a hub station engaged in line assignment, supervision and control of a satellite communication system. Numeral  2  is a plurality of remote stations forming a network. Numeral  3  is a communication satellite launched into space for relaying satellite communication lines. Numeral  4  is an forward CSC line from the remote stations  2  to the hub station  1  through the communication satellite  3 . Numeral  5  is a backward CSC line from the hub station  1  to the remote stations  2  through the communication satellite  3 . Numeral  6  is a DAMA communication line through the communication satellite  3 . 
       FIG. 2  is a schematic view showing an arrangement of frequency and time slots of the forward and backward CSC lines in a satellite communication system according to Embodiment 1 of the invention. In the drawing, numerals  4   a,    4   b  and  4   c  show an arrangement of frequency of a plurality of forward CSC lines each different in bandwidth for transmission from the remote stations to the hub station. Numeral  5  shows an arrangement of frequency of the backward CSC line for transmission from the hub station to the remote stations. In case of the mentioned prior art, each of remote stations demands line assignment through a single forward CSC line. On the other hand, this Embodiment 1 is characterized in that the remote stations utilize a plurality of forward CSC lines each different in bandwidth as actual communication line, when the data quantity to be transmitted is small. In addition, a different frequency band is allocated to the satellite communication line  6  for actual communication, though not shown in  FIG. 2 . 
       FIG. 3  is a block diagram showing an arrangement of a hub station  1  in the satellite communication system according to Embodiment 1 of the invention. In the drawing, numeral  10  is a terminal unit. Numeral  11  is a line connection control device. This line connection control device  11  consists of: a DAMA control section  11   a  for detecting and assigning idle lines in response to demands for line assignment from the remote stations  2 ; supervision and control section  11   b  for transmitting supervisory signals to check operation of the remote station  2  and for receiving reply signals from the remote stations  2  to check whether or not the operation is normal; and data line control section  11   c  for identifying a destined earth station (including the hub station itself) of data transmitted through the CSC line of the invention, and for transmitting such data to the destined earth station (including the hub station itself). 
     Numeral  12  is a CSC modulator section for modulating instruction of line assignment from line connection control device  11  into a frequency band of the backward CSC line. Numeral  13   a  is a CSC demodulator section for demodulating signals of a frequency corresponding to the CSC line  4   a  in  FIG. 2 . Numerals  13   b  and  13   c  correspond to numerals  4   b  and  4   c  respectively. Numeral  14   a  is a communication line modulator/demodulator (modem) section for establishing a frequency on an idle communication line in accordance with instructions from the hub station, and for transmitting/receiving radio signals at the established frequency. Numeral  15  is a transmitter/receiver for amplifying radio signals outputted from the CSC modulator section  12  and transmitting the radio signals through an antenna  16  at the time of transmission, and for converting the radio signals received through the antenna  16  into a signal of intermediate frequency and outputting such signals to the CSC demodulator  13   a,    13   b  and  13   c  at the time of reception. 
       FIG. 4  is a block diagram showing an arrangement of the remote station  2  in the satellite communication system according to Embodiment 1 of the invention. In the drawing, numeral  20  is a terminal unit. Numeral  21  is an IDU consisting of a CSC modulator/demodulator (modem) section  21   a,  a communication line modulator/demodulator (modem) section  14   b  and a control section  21   c.    
     The mentioned control section  21   c  is provided with frequency establishment means  21   d  performing a conventional function of controlling the communication line modulator/demodulator (modem) section  14   b  so as to establish a frequency on an idle line in accordance with an instruction of line assignment from the hub station. Moreover, the mentioned control section  21   c  is provided with transmission band selection means  21   e  and time slot selection means  21   f  an additional performing a function of judging which of the forward CSC line or communication line is more suitable for transmission in view of the data quantity to be transmitted, and a function of selecting and establishing a frequency bandwidth, information rate and single or plural TDMA (Time division Multiple Access) time slots suitable for the data quantity, in case that the forward CSC line is chosen to be used. 
     Numeral  22  is an ODU for amplifying radio signals outputted from the IDU  21  and transmitting the signals into space at the time of transmission, and for converting radio signals received from the satellite  3  into intermediate frequency signals and outputting such signals to the IDU  21  at the time of reception. Numeral  23  is an antenna. 
     The satellite communication system of above arrangement operates as described hereunder. First, in the case that the remote station  2   a  transmits any data to the hub station  1 , the control section  21   c  in the IDU  21  judges whether or not the data are within a quantity that can be transmitted through the forward CSC line. If the data quantity is judged transmittable, the most suitable forward CSC line is selected among the plurality of forward CSC lines  4   a,    4   b,    4   c . . .  each different in bandwidth, and the CSC modulator/demodulator (modem) section  21   a  is established on the selected channel. Then, the data is transmitted to the hub station  1  through single or a plurality of time slots (whether the time slots are continuous or discontinuous). 
     Upon receipt of the radio signals from the remote station  2  through the antenna  16 , the hub station  1  converts the radio signals into signals of an intermediate frequency at the transmitter/receiver  15 , and demodulates them at the CSC demodulator  13   a,    13   b  or  13   c  corresponding to the frequency of the respective forward CSC lines. The demodulated data is transmitted to line connection control device  11 . In this case, the mentioned DAMA control section  11   a  and the supervision and control section  11   b  are not involved, instead the data line control section  11   c  judges whether or not the data are destined to the hub station itself. If it is judged that the data are destined to the hub station itself, the data are outputted to the terminal unit  10 . 
     If the data proves to be destined to the other remote station  2   b,  the data are transmitted  2   b  from the data line control section  11   c  to this remote station through the CSC modulator  12 , transmitter/receiver  15  and the antenna  16 . 
     When, for instance, the mentioned remote station  2   b  receives the data transmitted from the hub station  1 , the control section of the IDU  21  judges whether or not the data is destined to the mentioned remote station itself. If it is judged that the data are destined to the remote station itself, the received data are transmitted to the terminal unit  20 . 
     In this respect, for the purpose of connecting the communication line under DAMA system according to the prior art, following three steps are essentially required for data transmission among remote stations, for example from a remote station  2   a  to another remote station  2   b:  
     1. Demanding line assignment from the remote station  2   a  to the hub station  1 ;   2. Assignment of a communication line by the hub station to remote stations  2   a  and  2   b;  and   3. Connection of the remote stations  2   a  and  2   b  through the assigned communication line.   

     However, in the arrangement disclosed in this Embodiment 1, the data transmission can be performed in following two steps:
     1. From the remote station  2   a  to the hub station  1 ; and   2. From the hub station  1  to the remote station  2   b.      

     As a result, by using the satellite communication system according to this Embodiment 1, the time before starting data transmission can be shortened on condition that the data quantity is below a certain level, and therefore the communication lines can be utilized more efficiently since the communication lines are not occupied for transmission of small sized data. When any data are larger than a data quantity (size) that can be transmitted by a single burst, such large data can be divided into plural bursts for transmission. In case of data excessively large to transmit even in plural bursts, it is more efficient to use ordinary DAMA system without intermediation of the hub station  1  and to use communication lines to connect the remote stations to each other directly. 
     Up to what data quantity (size) the satellite communication method according to this Embodiment 1 can be advantageously used, must be determined depending on specifications of individual systems, considering bandwidth of forward CSC line and other specifications including methods of error correction and decoding, methods of data compression and expansion, etc. 
     Embodiment 2 
     In this Embodiment 2, arrangement of time slots and devices are the same as those in the foregoing Embodiment 1. 
     The foregoing Embodiment 1 discloses a method for transmitting data from the remote station  2  to the hub station  1  in one or more bursts through the forward CSC line in case that the data quantity is below a certain level. It is, however, also possible to temporarily allocate an appropriate time slot of the forward CSC line exclusively for data transmission conducted by a specific remote station  2 , thus preventing data loss caused by collision of different data. 
     More specifically, the control section  21   c  in the IDU  21  of the remote station  2  judges whether or not the data are within a data quantity that can be transmitted through the forward CSC line. If the data quantity is judged transmittable, the most suitable forward CSC line and time slot are selected among a plurality of forward CSC lines  4   a,    4   b,    4   c . . . ,  and approval for exclusive use of the selected channel is requested to the hub station  1 . In this operation, the time slot to be selected can be either continuous or discontinuous. 
     Upon receipt of the mentioned request, the DAMA control section  11   a  of the line connection control device  11  of the hub station  1  judges whether or not exclusive use of the forward CSC line  4  and the time slot is feasible. If it is judged feasible, control signals announcing exclusive use by the remote station  2   a  are transmitted to all the remote stations  2 . 
     As has been described so far, in the satellite communication system according to the invention, data loss caused by collision of data is prevented by securing an exclusive forward CSC line for a specific earth station, and the communication lines can be efficiently utilized because of using a CSC line instead of securing a communication line by DAMA system for transmission of small data quantity.