Information transmitting method and information transmitting system

Abstract

A master station surely and quickly transmits identical information to plural terminals while controlling consumption of a transmission capacity. First, the master station determines the number of times of multicast transmission from the master station to slave stations. The master station notifies the slave stations of the number of times of multicast transmission determined. Subsequently, the master station transmits information to the slave stations in multicast the determined number of times of multicast transmission. After receiving the information from the master station the number of times of multicast transmission notified, a slave station identifies lacking information in information to be received. The slave station transmits an identifier of the lacking information to the master station. The master station receives the identifier of the lacking information from the slave station and retransmits information corresponding to the identifier of the lacking information to the slave station in unicast.

1 . An information transmitting method including: 
 a step of determining, on the basis of a packet error rate from a master station to slave stations, a number of times of multicast transmission in accordance with an expected value of a number of slave stations that transmit a retransmission request, an expected value of a number of retransmitted packets responding to the retransmission request, or a probability that the slave stations receive all packets forming information;    a step of notifying the determined number of times of multicast transmission to the slave stations;    a step of transmitting, to plural slave stations in multicast, same information formed of plural packets the determined times of multicast transmission;    a step of connecting unicast connection to a slave station that identifies a lacking packet after receiving information the notified times of multicast transmission;    a step of receiving, from the slave station that receives information in multicast, a retransmission request including an identifier of the lacking packet among packets forming the information by the unicast connection; and    a step of retransmitting a packet corresponding to the identifier of the packet to the slave station by the unicast connection.    
     
     
         2 . An information transmitting method according to  claim 1 , wherein 
 in the determining step, the number of times of multicast transmission is determined for plural areas of base stations communicating with the slave stations, and    in the step of transmitting to the plural slave stations, the same information is transmitted to the slave stations in the respective areas, the determined times of multicast transmission in the respective areas.    
     
     
         3 . An information transmitting method according to  claim 1 , wherein, 
 in the determining step, the number of times of multicast transmission is determined such that an expected value of the number of slave stations that transmit retransmission requests is equal to or smaller than a predetermined ratio of a total number of slave stations managed by the base stations.    
     
     
         4 . An information transmitting method according to  claim 3 , wherein, 
 in the determining step, a minimum integer of a value x obtained in accordance with Expression below is set as the number of times of multicast transmission:        x ≧log P (1− N √{square root over (1 −C   1 )})  Expression 1    where x is the number of times of multicast transmission, P is a packet error rate, N is a number of packets of transmitted information, and C 1  is a coefficient set in advance.    
     
     
         5 . An information transmitting method according to  claim 1 , wherein, 
 in the determining step, the number of times of multicast transmission is determined such that an expected value of the number of retransmitted packets responding to a retransmission request from one slave station is equal to or smaller than a predetermined ratio of the number of packets of transmitted information.    
     
     
         6 . An information transmitting method according to  claim 5 , wherein, 
 in the determining step, a minimum integer of a value x obtained in accordance with Expression below is set as the number of times of multicast transmission:      x≧log P  C 2   Expression 2    where x is the number of times of multicast transmission, P is a packet error rate, and C 2  is a coefficient set in advance.    
     
     
         7 . An information transmitting method according to  claim 1 , wherein, 
 in the determining step, the number of times of multicast transmission is determined such that a probability that slave stations, which receive information the notified times of multicast transmission, receive all packets of information to be received is equal to or larger than a probability set in advance.    
     
     
         8 . An information transmitting method according to  claim 7 , wherein, 
 in the determining step, the number of times of multicast transmission is determined in accordance with Expression below:        x =log p (1− NM √{square root over (C 3 )})  Expression 3    where x is the number of times of multicast transmission, P is a packet error rate, N is a number of packets of transmitted information, M is a number of slave stations, and C 3  is a target probability value set in advance.    
     
     
         9 . A master station comprising: 
 a transmitting unit for transmitting information to slave stations;    a receiving unit for receiving information from the slave stations; and    a control unit that controls information transmission to the slave stations,    wherein the control unit:    determines, on the basis of a packet error rate from the master station to the slave stations, a number of times of multicast transmission in accordance with an expected value of a number of slave stations that transmit retransmission requests, an expected value of a number of retransmitted packets responding to the retransmission requests, or a probability that the slave stations receive all packets forming information,    notifies the determined number of times of multicast transmission to the slave stations via the transmitting unit,    transmits same information formed of plural packets to plural slave stations via the transmitting unit in multicast, the determined times of multicast transmission,    connects unicast connection to a slave station that identifies a lacking packet after receiving information, the notified number of times of multicast transmission,    receives, from the slave station that receives information in multicast, a retransmission request including an identifier of the lacking packet among packets forming the information by the unicast connection via the receiving unit; and    retransmits a packet corresponding to the identifier of the packet to the slave station by the unicast connection.    
     
     
         10 . The master station according to  claim 9 , wherein 
 the master station is a packet data serving node that transmits information to radio base stations communicating with radio terminals and/or to the radio terminals via the radio base stations in multicast and, transmits information to the radio terminals in unicast via the radio base stations in response to retransmission requests from the radio terminals.