The present invention relates to a transmission control method in a radio communication system. Particularly, the represent invention relates to a transmission control method in a communication system that establishes packet communications where a radio communication medium is shared between plural terminals and a base station and that uses a multiple access system in which possible collision occurs.
Conventionally, in packet communications, plural terminals often share the communication medium because of burst due to packet occurrence. In such a case, the shared communication medium provides a point-to-multipoint transmission line. If the communication medium is wireless, the forward link which transmits signals from a radio base station to terminals is used for broadcasting so that all terminals become a receivable state. The reverse link which transmits signals from a terminal to the radio station is generally used as a multi-access in which plural terminals use a common communication medium. To provide an effective multi-access mode to the reverse link means to increase the transmission capacity per shared communication medium. As a result, this allows the subscriber capacity of the system to be greatly increased.
To solve such a problem, various multi-access systems have been conventionally proposed. The basic system which can provide the simplest control is the ALOHA system (refer to Telecommunication Networks (ISBNO-201-16423), pp. 407-408). According to the basic concept of the system, packet signals are transmitted immediately when they are newly generated. This allows a very small delay to be achieved although collision may occur. This access system in which possible collision occurs is hereinafter called a collision-based access system.
Moreover, there is the CSMA (Carrier Sense Multiple Access) system (refer to Telecommunication Networks (ISBNO-201-16423), pp. 437) which detects whether or not other terminal currently is in a transmission mode by means of a receiving section and then transmits packet signals immediately when the idle period detection. This system allows collision of transmitted packets to be relieved because packet signals are not transmitted while other terminal is in a transmission mode.
However, when the case where a small number of terminals in mobile communications exits in a line-of-sight path and the effect of a hidden terminal is large, the terminal cannot completely sense the carrier. In such a case, the ICMA system (Idle-signal Casting Multiple Access) system is used to reduce collision of a packet signal at a multi-access time, thus improving the throughput.
Moreover, in consideration of survival at a packet signal collision time due to a capture effect, the ICMA-PE (Idle-signal Casting Multiple Access With Partial Echo) system that uses a portion of an up signal, or a partial echo, as an access control signal (Umeda and Onoe, xe2x80x9cIdle-signal Casting Multiple Access With Partial Echoxe2x80x9d Communication Technical Reports RCS91-30) has been proposed. FIG. 4 is a sequence diagram showing the ICMA-PE system.
Referring to FIG. 4, when the radio base station 40 receives an reverse link message Dlu(0) from the terminal 41, it sets up a busy flag as access control information and adds a portion of the received data Dlu(0) as a partial echo PE1(0). This operation can determine whether or not which terminal has transmitted a survival signal due to packet signal collision by a capture effect, thus enabling a relief process. Moreover, as a simple ICMA-PE system, a multiple access system called the ICMA-CD (Idle-signal Casting Multiple Access With Collision Detection) has been proposed.
However, in the collision-based access system, since collision cannot be avoided at the beginning of transmission, a serious problem occurs in the case where many terminals want transmission. Moreover, since the radio station cannot judge that a certain transmission error has occurred due to collision or degradation in the quality of a radio line, it is difficult to deal with such a problem.
There is an access system in which collision does not occur different from the above-mentioned collision-based access system. The multiple access system is hereinafter called the non-collision-based access system. This access system is generally realized by circulating the transmission right of a terminal. There is a multiple access system called polling system as one of non-collision-based access systems.
In the polling system, a radio base station asks each terminal whether there is a signal to be transmitted. If there is a signal to be transmitted to a terminal, the terminal transmits a signal so that polling (transmission invitation) is sequentially performed. In this polling system, since the radio station can perform centralized management, the signal collision at a multiple access time do not occur between plural terminals. However, even if a signal to be transmitted occurs at a terminal, the signal cannot be transmitted until polling invests it with a transmission right. Hence, there is the problem that the transmission delay occurs.
As one approach to solve such a problem, JP-A-274788/1996 discloses a multiple access method that uses adaptively and selectively the collision-based access system and the non-collision-based access system. That is, according to the communication traffic between a terminal and a radio base station, the connection delay is decreased by using the collision multiple access system at the time of low traffic and using the non-collision multiple access system at the time of high traffic. Moreover, high throughput can be realized even at a high traffic time.
According to the invention disclosed in JP-A-274788/1996, the collision-based access system employs such a methodology in which the terminal that judges that a packet signal transmitted from itself to avoid re-collision after the occurrence of collision has collided is transferred to a back-off state and then re-transmits the packet signal after a period of a random time or transmits the next signal. On the other hand, in the non-collision-based access system, since a terminal can perform transmission only by the polling from a radio base station, collision does not occur, so that the terminal does not change to a back-off state.
As described above, in the multiple access method adaptively using the collision-based access system and the non-collision-based access system, when the mode changes from the collision-based access system to the non-collision-based access system due to collision of a signal transmitted from a certain terminal, the terminal is changed to a back-off state. In this case, the problem arises that the terminal cannot perform transmission operation because of its back-off state even if polling is performed from the radio base station to the terminal.
The objective of the present invention is to solve the above-mentioned tasks.
Moreover, the objective of the invention is to provide a transmission control method that can improve a decrease in throughput due to a transmission disabled state when a terminal is changed to a back-off state due to collision happened during the use of a collision access system in a communication system using at least a collision-system multiple access system, thus becoming a transmission wait state.
In order to accomplish the above mentioned objects, the objective of the present invention is achieved by the transmission control method suitable in a radio communication system which performs packet communications by sharing a radio communication medium between plural terminals and a radio station and employs a multiple access system in which collision may occur, the method comprising the steps of switching, a terminal to a transmission mode, or if the terminal is in transmission mode keeping it in transmission mode, during use of a collision-based access system when receiving a polling from said radio base station, said terminal being changed to a back-off state, thus being in a wait state, because of collision of a signal transmitted from said terminal.
In the transmission control method according to the present invention, each of the terminals judges that a packet signal transmitted from its terminal has collided when its terminal receives access control information from the radio base station after transmitting the, packet signal to the radio base station, the access control information representing no reception, and changes to a back-off state in which a different random time transmission is suspended.
In the transmission control method according to the present invention, the radio base station transmits a common polling signal to all the terminals when a packet signal from each of the terminals is not received, and sequentially transmits polling signals to other terminals after a packet is completely received from a specific terminal.
Furthermore, in the transmission control method suitable in a radio communication system which performs packet communications by sharing a radio communication medium between plural terminals and a radio station and adaptively employs a multiple access system in which collision may occur and a multiple access system in which collision does not occur, the method comprises the steps of detecting collision by means of the radio station; changing the radio communication system to a back-off state by transmitting the collision signal when the multiple access system in which collision may occur is changed to the multiple access system in which collision does not occur; and making the terminal in a transmission mode at a polling receive time when the terminal in a wait state has received polling from the base station.
In the transmission control method according to the present invention, each of the terminals judges that a packet signal transmitted from itself has collided when receiving the access control information from the radio base station, the access control information representing that the packet signal is not received, after transmission of the packet signal to the radio station, and changes to a back-off state in which a different random time transmission is sustained.
Moreover, in the transmission control method suitable in a radio communication system which performs packet communications by sharing a radio communication medium between plural terminals and a radio base station and employs a multiple access system in which collision may occur in a reverse link from the plural terminals to the radio base station and which realizes transmission confirmation regarding a forward link packet signal to be transmitted on a forward link in response to a response signal from a terminal being a destination of the forward link packet signal, the method comprises the steps of transmitting a collision signal after the radio base station detects collision; and transmitting a response signal for transmission confirmation at a polling receiving time when a terminal changed to a back-off state and in a wait state receives polling from the radio station.
According to the present invention, when the radio communication system is changed from the collision-based access system to the non-collision-based access system or a response signal is sent back to a signal transmitted on the forward link, a terminal in a back-off state or wait state can be quickly changed to a transmission mode in response to a polling signal from a radio base station. As a result, the throughput is improved and the radio line can be effectively used. Moreover, a response signal to be transmitted from a terminal for transmission confirmation can be quickly invited to a signal to be transmitted from a base station to a terminal.
According to the present invention, the transmission control method comprises the steps of transmitting a first forward link message from a radio base station to terminals; transmitting packet signals from the terminals in response to the first forward link message; transmitting a second forward link message from the radio base station when the radio base station does not receive the packet signals due to collision of the packet signals; changing the terminals to a back-off state in response to the second forward link message to avoid re-collision; retransmitting a packet signal from a terminal first recovered from the back-off state in response to a third forward link message from the radio base station; transmitting a fourth forward link message containing access control information from the radio base station first recovered terminal when the radio base station correctly receives the packet signal from the first recovered terminal; and sequentially transmitting a fifth forward link message from the radio base station when a packet signal from the first recovered terminal is transmitted.
Moreover, according to the present invention, the transmission control method comprises the steps of transmitting a first forward link message from a radio base station to terminals; transmitting packet signals from the terminals in response to the first forward link message when the radio base station is in a collision-system access mode; changing the radio base station from a collision-system access mode to a non-collision-system access mode when the radio base station does not receive the packet signals due to collision of the packet signals, and then transmitting a second forward link message from the radio base station; changing the terminals to a back-off state in response to the second forward link message to avoid re-collision; retransmitting a packet signal from a terminal first recovered from the back-off state according to transmission invitation in response to a third forward link message from the radio base station; transmitting a fourth forward link message containing polling information from the radio base station to the first recovered terminal when the radio base station correctly receives the packet signal from the first recovered terminal; and sequentially transmitting a fifth forward link message from the radio base station when a packet signal from the first recovered terminal is transmitted.
According to the present invention, the transmission control method comprises the steps of transmitting packet signals from terminals in response to a first forward link message from a radio base station; transmitting a second forward link message from the radio base station when the radio base station does not receive the packet signals due to collision of the packet signals; changing the terminals to a back-off state in response to the second forward link message to avoid re-collision; extracting forward link message for a terminal in a back-off state from a third forward link message transmitted from the radio base station retransmitting a packet signal from a terminal first recovered from the back-off state in response to the third forward link message from the radio base station; transmitting a fourth forward link message containing access control information and forward link message for first-recovered terminal from the radio base station when the radio base station correctly receives the packet signal from the first recovered terminal; extracting forward link message from the fourth forward link message; transmitting a fifth forward link message containing access control information and forward link message from the radio base station to confirm transmission of forward link message; and transmitting a response packet signal from the terminal in a back-off state in response to the fifth forward link message.