Abstract:
A system and method for dynamically transferring satellite data packets through GPRS (General Packet Radio Service) in an AGPS(Assisted Global Positioning System) is disclosed. The satellite data are transmitted in dynamic size of segments so as to improve the transmission efficiency. When the network communication is stalled or broken, the segments being successfully received are not necessary to be resent. But only the failed segments are re-transmitted so that the time and bandwidth for data transmission are reduced, and the positioning process is prevented from delay by the transmission failure.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This non-provisional application claims priority under 35 U.S.C. § 119( a ) on Patent Application No(s). 094111745 filed in Taiwan, R.O.C. on Apr. 13, 2005, the entire contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of Invention  
         [0003]     The invention generally relates to a system and method for improving data packet transmission, and in particular relates to a system and method for dynamically transferring data packets in an AGPS (Assisted Global Positioning System).  
         [0004]     2. Related Art  
         [0005]      FIG. 1  shows a schematic diagram of an AGPS (Assisted Global Positioning System). The major difference between AGPS and stand-alone GPS (Global Positioning System) is that the GPS must search for satellite  140  signals and decode the satellite navigation messages before computing its position. The tasks require strong signals and additional processing time. The AGPS applies cellular telephone  110  through BSS of GSM (Basic Station System of Global System for Mobile communications;)  130  to get the micro-cell  150  where the user belongs to and to provide an initial approximate position of a GPS receiver. By obtaining the navigation messages of satellites covering the micro-cell  150  through a server  120 , the receiver can therefore utilize weaker signals and also more quickly determine its position. Mainly, the required time for a first time positioning is decreased form original 5 or 10 minutes to 10 second. The time for further positioning is decreased from 40 seconds to 1 or 2 seconds. The drawbacks of weak signal and failed communication in conventional GPS when user moving in a building are also overcame.  
         [0006]     At a first time positioning of a global positioning system, the GPS receiver has to search satellite orbit and clock data in an outdoor open space. There are at least 24 satellites moving around the earth in a 12-hour cycle and providing specific coded signals. The GPS receiver has to obtain at least data of three satellites for calculating the current position or speed of the user from the satellite data. In accompany with technology developments, cellar phones are being developed with multiple functions, such as including personal digital assistant; or even a navigation system utilizing global satellite positioning. The satellite data acquiring and transmission are via GPRS (General Packet Radio Service) for the cellar phone.  
         [0007]     The GPRS is a new (2.5) generation GSM communication standard based on packet switch (instead of general circuit switch) transmission so as to improve data transmission rate and achieve high-speed wideband communication.  
         [0008]     The data transmission through GPRS is to deliver a serial of packets, instead of a full line transmission, so as to share the network resources with others and to fully utilize the limited bandwidth. In the AGPS, the packet size is 100 kilobytes. The GPRS also applies Internet protocol so as to interconnect with Internet.  
         [0009]     However, since radio signals are easy to be influenced by geography and environments, the quality of radio communication goes down by interferences of multi-path, path fading, shielding effect and so on. For example, a micro-cell encounters co-channel interface with another micro-cell of the same carrier frequency; the cellar phones in a micro-cell encounter adjacent channel interference caused by carrier frequencies of adjacent micro-cells. All these interferences influence the radio signal quality of cellar phones in the micro-cells.  
         [0010]     As users of cellar phones increase, the bandwidths allocated for each phone decrease. Since the total phone users in a micro-cell vary anytime, the bandwidth for each cellar phone varies accordingly. For example, supposing a bus carrying many cellar phone users into a micro-cell, the bandwidth allocated to each phone decreases promptly since the users increase. The variations of bandwidth of communication make the delay of each data packet transmission from the source to the target varying a lot.  
         [0011]     When cellar phone users getting into tunnels, trains or some dead angles of communication, the cellar phones temporarily lose signals. The radio signals are unstable and the packet transmissions fail easily. Each time when data transmission fails, the data packet has to be re-transmitted that lowers the communication efficiency. Further, since the data transmission fee is paid according to the data quantity regardless of transmission success or not, the transmission failure wastes more time and cost. Therefore, it is an important issue to improve the efficiency of data transmission.  
       SUMMARY OF THE INVENTION  
       [0012]     The object of the invention is to provide a system and method for dynamically transferring data packets in an AGPS (Assisted Global Positioning System). The data packets are transmitted and received in segments. When the network communication is stalled or broken, the segments being successfully received are not necessary to be resent. But only the failed segments are re-transmitted so that the time and bandwidth for data transmission are reduced, and the positioning process is prevented from delay by the transmission failure.  
         [0013]     In order to achieve the aforesaid object, a system for dynamically transferring data packets in an AGPS according to the invention includes a server, a BSS (Basic Station System) and a cellar phone with AGPS.  
         [0014]     The Basic Station System is used to receive the positioning request of the cellar phone, transfer the request to the server, and transfer the satellite data to the cellar phone.  
         [0015]     The server searches correspondent satellite data of the micro-cell where the AGPS cellar phone locates according to the request of the cellar phone, and transmits the satellite data in segments. The server includes a satellite database for storing data of each satellite in a coverage range of the Basic Station System based on every time period; and a central processing unit for dynamically determining the segmentation of satellite data transmission.  
         [0016]     The AGPS cellar phone is used to get data of the satellites that cover the current position of the cellar phone and to determine the position. The cellar phone includes: a communication module for transmitting the request of acquiring satellite data and afterwards receiving the satellite data segments; a control module for monitoring a reception status of each data segment and integrating the satellite data after receiving all the data segments; and a memory module for registering the received satellite data segments.  
         [0017]     A method for dynamically transferring data packets in an AGPS according to the invention includes the following steps. First, a cellar phone requests satellite data through a basic station system. A server determines the position of the cellar phone according to the requests from the basic station system; and transmits data segments of the satellites corresponding to the cellar phone position. The cellar phone stores the data segments in a register; and finally integrates the satellite data segments into complete data for the positioning function.  
         [0018]     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description. On the other hand, some well-known methods, processes, components and circuits are not described in details in order to prevent from confusing the merit of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]     The present invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein:  
         [0020]      FIG. 1  is a compositional view of an AGPS cellar phone in the prior art;  
         [0021]      FIG. 2  is a schematic diagram of the invention;  
         [0022]      FIG. 3  is a flowchart of the invention at the cellar phone side; and  
         [0023]      FIG. 4  is a flowchart of the invention at the server side. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0024]     As shown in  FIG. 1  and  FIG. 2 , a Basic Station System  130  serves a plurality of honeycomb micro-cells  150  through radio signal communications. A cellar phone  110  with AGPS (Assisted Global Positioning System) in any a micro-cell  150  sends a request to the server  120  for acquiring satellite data through interconnected stations in the Basic Station System  130 . The data of satellites  140  are managed by the server  120  and transmitted through the Basic Station System  130  to the cellar phone  110 .  
         [0025]     The server  120  includes a satellite database  210  and a central processing unit  200  for locating the position of the cellar phone  110  according to the micro-cell  150  where the satellite data request from the cellar phone  110  being sent out. Then, the corresponding satellite data are divided into segments and transmitted through GPRS (General Packet Radio Service). The GPRS transmission is not quite fast, therefore, the segmentation prevents the time waste of repeated whole packet transmission when encountering network stall or communication break.  
         [0026]     The satellite database  210  stores the coverage positions of each satellite  140  at every time periods. Generally, a GPS receiver requires several minutes to find out the satellites  140  that cover the current position by continuously receiving the satellite data for a while. However, since the atmosphere conditions, building shielding and signal diffusion are all influencing on the accuracy of positioning, the GPS receiver usually spends a lot of time searching the satellites  140  when the signals are unstable. On the contrary, since the satellites are moving above the earth in a fixed speed like on-time trains, using the database  210  to locate the satellites  140  can help the AGPS cellar phone  110  quickly achieve the cold start procedure.  
         [0027]     The central processing unit  200  dynamically determines the segmentation of the satellite data to be transmitted. Since the transmission of the GPRS is unstable that many cases, such as fast motion, mountain, tunnel or building shielding, rush communication, network stall and so on, greatly increase the loss rate of packets. Once the packet transmission fails, it has to be re-transmitted. Therefore, the invention monitors the packet transmission condition and dynamically adjusts the packet size. For example, the original size of a packet is 100 kilobytes that is hard to be finished in a time slot when the network being stalled, so the packet transmission fails. Then, the 100 kilobytes data are divided into four segments each carries 25 kilobytes in transmission. Then, if a segment fails in transmission, it is further subdivided. For example, if the third segment fails, it is subdivided into 5 kilobyte segments for transmission. After the communication condition being improved, the larger segment transmission is resumed. The AGPS cellar phone  110  receives the data of the satellites  140  that cover the position of current micro-cell  150  and fast finishes the cold start procedure of the global positioning system. The cellar phone  110  includes the following components: 
    a) a communication module  220 , including a radio frequency transceiver, for transmitting the satellite data request from the cellar phone  110  to the Basic Station System  130 ; and receiving the satellite data from the server  120  via the Basic Station System  130 ;     b) a control module  230  for monitoring the reception condition of the satellite data through the communication module  220 . The server  120  first provides a one-byte data recording the size of the segment to be transmitted. The control module  230  monitors the received data size and checks an ending tag to judge if the reception succeeds. After all the segments being fully received, the control module  230  integrates them into complete satellite data; and     c) a memory module  240  for registering the satellite data segments. To fulfill the requirements of fast and repeated reading/writing, SRAM (static random access memory) or DRAM (dynamic random access memory) are preferably used though they are relatively expensive. Fortunately, since the satellite data are less, only small memory size is needed.    
 
         [0031]      FIG. 3  and  FIG. 4  are flowcharts of an assisted global satellite positioning method of the invention at the cellar phone side and at the server side respectively. The GPS receiver in the cellar phone  110  requires a cold start procedure to find out the satellites  140  corresponding to the current position. The communication module  220  sends a request to the server  120  for acquiring satellite data (step  310 ). The request is transmitted by radio signals through the interconnected stations of the BSS (Basic Station System)  130  around the current micro-cell  150  where the cellar phone locates. The request passes through stations of the BSS  130  to the server  120 .  
         [0032]     The server  120  receives the satellite data request from the cellar phone (step  410 ). Because each station around the micro-cell  150  where the cellar phone  110  locates transfers the request to the server  120 , the server  120  checks the transfer sequence of the request to find out the location of the micro-cell  150  (step  420 ). Then, the server  120  searches with the database  210  by the current time the data of the satellites that cover the micro-cell  150  (step  430 ). The central processing unit  200  then checks the communication condition and controls the data segmentation of the satellite data transmission (step  440 ).  
         [0033]     The AGPS cellar phone  110  first receives a one-byte data that records the segment size (length) of the current satellite data to be transferred (step  320 ). The communication module  130  then receives the data segment from the server  120  (step  330 ). When the received data size conforms and an ending tag arrives (step  340 ), the data segment reception is successful. Then, the data segment is stored into the memory module  240  (step  350 ). The control module  230  checks if all the received data segments reach a packet size, for example, 100 kilobytes in the embodiment (step  360 ). If not, the communication module  220  requests for the next data segment (step  370 ). After the total data segments reach the packet size, the data segments are integrated into complete satellite data (step  380 ).  
         [0034]     After all the required satellite data being received, the AGPS cellar phone  110  can verify its position. The satellite data are transferred in dynamic segments to prevent from whole re-transmission when communication fails. The partial re-transmission reduces the transmission time and bandwidth waste, and improves the communication efficiency.  
         [0035]     The invention being thus described, it will be obvious that the same may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.