Abstract:
A method for controlling data transmission rate in order to save waste of resources due to Non-Line Of Sight (NLOS) region when a mobile terminal passes through the NLOS region in an interactive satellite communication system is disclosed. The method includes the steps of setting at least one threshold; at a Network Control Center (NCC) adjusting allocated resources based on the Constant Resources Allocation (CRA) after detecting whether Satellite Access Control is lost or not; compensating transmission delay by reallocating if the mobile terminal returns from the NLOS region within a first threshold duration, wherein the mobile terminal maintains the fine sync state during the first threshold duration; and deallocating all resources allocated based on the CRA if the mobile terminal does not return from the NLOS region within the first threshold duration.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an interactive satellite communication system; and more particularly, to a method for controlling data transmission rate in order to save waste of resources due to Non-Line Of Sight (NLOS) region in an interactive satellite communication system. 
   DESCRIPTION OF RELATED ART 
   In a mobile interactive satellite communication system, plural mobile terminals request a service to a Network Control Center (NCC) through a satellite, and, in response to the request, the NCC provides the service through the satellite to the mobile terminals. Because value of frequency in the mobile satellite communication system is much higher than the value of frequency in a terrestrial mobile communication system, it is very important to minimize shadowing outages occurred when the mobile terminal instantly passes through the NLOS region. 
   The interactive satellite communication system provides a multimedia service to mobile terminals on the move. However, temporary fading of signal is usually occurred when a mobile terminal passes through the NLOS region. In other words, mobile terminals pass through the NLOS region generally in the mobile interactive satellite network, at that time, data transmission is usually failed. Such a signal cut-off decreases a Quality of Service (QoS) of the mobile interactive satellite communication system and wastes precious radio resources. 
   In detail, to the mobile interactive satellite communication network, Continuous Rate Assignment (CRA), which allocates resources to maintain a certain transmission rate for the certain period of time and a resource allocation method based on requests from mobile terminals are applied in order to increase the utilization of radio resources and to make target value of the QoS according to service kinds. In CRA, because the scheduler continuously allocates resources to mobile terminals as long as the scheduler doesn&#39;t receive a request of resource release, the resources allocated to the mobile terminals are wasted when the mobile terminals have the instant signal cut-off in the NLOS region. Also, if the NCC collects all resources allocated to the mobile terminals having instant signal cut-off for saving resources, set-up time for a new session is needed when the mobile terminals return from the NLOS region. Therefore, the communication cut-off time takes longer, and QoS is decreased. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a data transmission rate control method for saving waste of radio resources due to the NLOS region when a mobile terminal passes through the NLOS region in the interactive satellite communication system. 
   It is another object of the present invention to guarantee the quality of service by controlling the data transmission rate in the satellite communication system having communication blockage for minimizing the damage of the instant signal cut-off occurred when the mobile terminal instantly enter and/or depart from the NLOS region. 
   In accordance with an aspect of the present invention, there is provided a method for controlling a data transmission rate in an interactive satellite communication system, the method including the steps of: setting at least one threshold, which is a statistical time duration maintaining a fine sync state while a mobile terminal enters and departs from a Non-Line Of Sight (NLOS) region; at a Network Control Center (NCC) adjusting allocated resources based on the Constant Resources Allocation (CRA) after detecting whether Satellite Access Control (SAC) is lost or not; compensating transmission delay by reallocating if the mobile terminal returns from the NLOS region within a first threshold duration, wherein the mobile terminal maintains the fine sync state during the first threshold duration; and deallocating all resources allocated based on the CRA if the mobile terminal does not return from the NLOS region within the first threshold duration. 
   In accordance with an aspect of the present invention, there is provided a method for controlling a data transmission rate in an interactive satellite communication system, the method further including the steps of: converting the fine sync state of the mobile terminal into a coarse sync, if the mobile terminal returns from the NLOS region within a second threshold duration; and converting the coarse sync state of the mobile terminal into a log-on initial state, if the mobile terminal does not return from the NLOS region within the second threshold duration. 
   In accordance with an aspect of the present invention, there is provided a method for controlling a data transmission rate in an interactive satellite communication system, the method further including the steps of: converting the coarse sync state of the mobile terminal into the log-on initial state, if the mobile terminal returns from the NLOS region within third threshold duration; and logging off by terminating the log-on state, if the mobile terminal does not return from the NLOS region within the third threshold duration. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a diagram showing an interactive satellite communication system to which the present invention is applied; 
       FIG. 2  is a flowchart illustrating a method for controlling data transmission rate in accordance with an embodiment of the present invention; and 
       FIGS. 3 and 4  are graphs describing an adaptive resources collection quantity as frequency occupancy state by controlling data transmission rate in accordance with an embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Other objects 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. 
     FIG. 1  is a diagram showing an interactive satellite communication system to which the present invention is applied. 
   The interactive satellite communication system is a satellite network, wherein, plural mobile terminals  11  request a service to Network Control Center (NCC)  13  through a satellite  12 , in response to the request from the mobile terminal  11 , the NCC  13  provides the service to the mobile terminal  11  through the satellite  12 . In this network, it is very important to adjust the allocated resources to the mobile terminal having trouble of the communication cut-off in the NLOS region. 
   As shown, the interactive satellite communication system in this embodiment includes a satellite  12 , a NCC  13  and plural mobile terminals  11 . It is apparent that an interactive satellite communication system including plural satellites  12 , plural NCCs  13  and plural mobile terminals  11 , could be implemented by combining or modifying the system in accordance with the present invention including the present system. 
   Each of plural mobile terminals  11  requests the required resources and uses the allowed resources, i.e., time slot, by the NCC  13  in response to the request from the mobile terminal  11 . 
   The NCC  13  collects resource requests from the mobile terminals  11 , and selects the time slot for allocating to each of mobile terminals  11 , and reports to the related mobile terminals through the satellite  12 . 
   Particularly, because the NCC  13  includes a data transmission rate control algorithm for minimizing damage of the communication cut-off generated when the mobile terminal  11  instantly enters into and/or departs from the NLOS region, the NCC  13  receives requests of frequency resources from plural mobile terminals  11 , and preserves the average transmission rate by compensating transmission rate due to instant communication cut-off generated when the mobile terminal passes through the NLOS region. 
     FIG. 2  is a flowchart illustrating a method for controlling a data transmission rate in accordance with the present invention, and showing the method for determining state of the mobile terminal  11  by the NCC  13  while the mobile terminal  11  passes through the NLOS region. 
   The transmission rate control algorithm in accordance with the present invention, for the prevention of the resource waste, determines the state of the mobile terminal  11  based on the multi-stage of the return time from the NLOS region, and collects partially the allocated resources according to the stage because the related resources are wasted when the mobile terminal  11  passes the NLOS region and have communication cut-off in the NLOS region. 
   Herein, the NCC  13  determines the state of the mobile terminal  11  based on the return time, which is time duration between the instants of entering and departing from the NLOS region. 
   Hereinafter, “fine sync” means as a state, which the time is accurately synchronized means for normal data transmission. “Coarse sync” means a state before the fine sync, which the time is roughly synchronized. “RspT” denotes response time, which is detecting time after the occurrence of the event. “rt-session” denotes a real-time session. 
   “Terminal Burst Time Plan (TBTP)” is referred to an allocation plan of time slots. t a  is a statistical time duration for maintaining the fine sync state after returning from the NLOS region. For the embodiment of the present invention, t a , t b  and t c  could be assumed based on the method of the ordinary least square technique. 
   c j , x j  and y j  denote resources allocated to the mobile terminal  11  by the NCC  13  when the mobile terminal  11  enters and/or departs from the NLOS region. 
   c, c j , x j , y j  and p are design parameters, and could be determined based on a statistical assumption or a strategy of the service provider. 
   A process for decreasing and compensating the resources allocated to the mobile terminal  11  by the CRA method will be described hereafter. 
   In the interactive satellite communication network, at step S 2   a , when the mobile terminal  11  logs on and gets ready to be the coarse sync, at step S 201 , the mobile terminal  11  acquires coarse sync and at step S 2   b , gets ready to be the fine sync. At step S 202 , when the mobile terminal  11  gets ready to be the fine sync, the mobile terminal  11  acquires the fine sync. At step S 2   c , the mobile terminal  11  communicates with the NCC  13  in the fine sync. 
   At step S 203 , if the NCC  13  couldn&#39;t receive a Satellite Access Control (SAC) field, which is a field indicating control information from the mobile terminal  11  for connecting with the satellite  12 , at step S 204 , the NCC  13  assumes that the mobile terminal  11  exists in the NLOS region, and reallocates resources allocated by the CRA, that is, optimizes the CRA capacity. 
   Thereafter, at step S 205 , if the mobile terminal  11  does not return from the NLOS region in t a , at step S 206 , the NCC  13  collects resources allocated by the CRA. At step S 2   c , if the mobile terminal  11  returns from the NLOS region in t a , the mobile terminal  11  maintains the fine sync. 
   Herein, though the mobile terminal  11  returned from the NLOS region, the NCC  13  does not start collecting resources until t a  so that the mobile terminal  11  could immediately restart receiving data at the speed of x j  in  FIG. 3  in the fine sync when the mobile terminal  11  returns in the fine sync. 
   At step S 207 , if the mobile terminal  11  didn&#39;t return in t b  from the NLOS region, at step S 208 , the NCC  13  converts state of the mobile terminal  11  into former state, i.e., the logon state, that is, the all allocated resources will be returned and the rt-session will be terminated. At step S 2   b , if the mobile terminal  11  returns from the NLOS region in t b , the mobile terminal  11  maintains the coarse sync, i.e., ready to be the fine sync. 
   Then, at step S 209 , if the mobile terminal  11  does not return from the NLOS region in t c , at step S 2   d , the NCC  13  logs off, i.e., terminates the logon by force. At step S 2   a , if the mobile terminal  11  returns from the NLOS region, the mobile terminal  11  maintains the logon state. 
   When the mobile terminal  11  maintains the fine sync, traffic can be transmitted received and the mobile terminal  11  responses to the NCC  13  in t a . There are some cases could be occurred. One is the case that the mobile terminal  11  departs from the NLOS region after the NCC  13  detecting the mobile terminal&#39;s  11  being in the NLOS region generates the TBTP, referred in  FIG. 3 . Another is the case that the mobile terminal  11  departs from the NLOS region before receiving the TBTP generated by the NCC  13 , referred in  FIG. 4 . 
     FIGS. 3 and 4  are graphs showing the method for controlling data transmission rate, i.e., controlling the resource allocation in accordance with the present invention. 
   The reason of having duration of t a  after entering the NLOS region by the mobile terminal  11  is to restart transmitting data at the rate of x j  immediately when the mobile terminal in the “fine sync” state returns from the NLOS region. 
   Based on the a scenario for controlling the resource allocation according to the present invention, the mobile terminal  11  enter the NLOS region at time  0 , the NCC  13  detects the entrance of the mobile terminal  11  in the NLOS region at τ d  and transmits the TBTP in order to reallocate the allocated resources from c x  to x j , and the mobile terminal receives the TBTP at time  2 τ d . The mobile terminal  11  has resources as x j  at time  2 τ d . Herein, the mobile terminal  11  could depart from the NLOS region after receiving the TBTP, referred in  FIG. 3 , and departs from the NLOS region before receiving the TBTP referred in  FIG. 4 . 
   When the mobile terminal  11  enters the NLOS region, the allocated resources are reallocated as x j , that is, the capacity of c j -x j  will be released, referred at the step S 204  in  FIG. 2 . If the mobile terminal  11  doesn&#39;t return within t a , referred at the step S 205  in  FIG. 2 , the all allocated resources are released, referred at the step S 206  in  FIG. 2 . If the mobile terminal  11  returns from the NLOS region, referred at the step S 205  in  FIG. 2 , the NCC  13  allocates resources as y j  according to probability of p, i.e., y j -c j . The NCC  13  reallocates resources after the mobile terminal  11  returns in order to decrease the risk that the mobile terminal  11  enters the NLOS region at point of the resource reallocation. 
   The effect of the present invention will be shown as the state of the frequency allocation in  FIGS. 3 and 4 . 
     FIG. 3  illustrates the effect in the case of t r &lt;t a  wherein t r  denotes a time between the mobile terminal  11  enter the NLOS region and returns from the NLOS region. 
   From time  0  to time t r +2τ d +c, an amount of the allocated the frequencies is obtained by an equation 1 according to the conventional allocation method. In the present invention, an amount of the allocated frequencies from 0 to t r +2τ d +c is calculated by equation 2. 
   It is obvious that the allocated but unused resources are decreased based on equation 3 in accordance with the present invention. 
   
     
       
         
           
             
               
                 
                   
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   The adaptive released resources could be provided to other mobile terminals in accordance with the present invention. The NCC  13  allocates resources as y j -c j  based on probability of P for transmitting data delayed by passing through the NLOS region. 
   Herein, the allocation of the frequency by probability is for decrease the risk that the mobile terminal enters the NLOS region. The NCC could minimize the capacity of the allocated but unused resources. 
   As above-mentioned, the data transmission rate control method in accordance with the present invention can be embodied as a program and stored in recording media readable by a computer, e.g., CD-ROM, RAM, floppy disk, hard disk, magneto-optical disk, etc. 
   The data transmission rate control method prevents a waste of radio resources by minimizing resources which are allocated to the terminal but unused, and guarantees the QoS in the interactive satellite communication system having a signal cut-off when the mobile terminal passes through the NLOS region. 
   The present application contains subject matter related to Korean patent application no. 2003-96830, filed in the Korean intellectual Property Office on Dec. 24, 2003, the entire contents of which being incorporated herein by reference. 
   While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.