Multiplex communications – Communication over free space – Combining or distributing information via time channels
Reexamination Certificate
1998-06-23
2001-10-30
Cangialosi, Salvatore (Department: 2661)
Multiplex communications
Communication over free space
Combining or distributing information via time channels
C455S422100
Reexamination Certificate
active
06310872
ABSTRACT:
TECHNICAL FIELD
The present invention relates to packet switching in a cellular radio communication system, in particular to a device and a method for transmission of packet data in each cell, so that the system is stabilized relating to co-channel interference.
PRIOR ART
In a cellular radio communication system with packet data switching there is no dedicated connection for each packet switching. When a transmitting station wishes to transmit a packet, it should be able to use as much capacity as is needed depending on what the system can handle. One or more channels can for instance be dedicated for packet data switching in the entire radio communication system. Then, all transmitting radio stations can use said channel or channels when they need to transmit packet data. Then, the remaining channels can for instance be used for transfer of circuit-switched speech. The packet data stream within a cell is not continuous but in the form of bursts. It may happen that the same channel is used simultaneously within more than one cell for transfer of packet data. This implies that co-channel interference is introduced. If a packet contains a too high noise level relating to the interference, the reception of the packet will fail.
If the reception of a packet fail, the packet must be retransmitted at a later moment. There are different known strategies for how retransmission of failed packets shall be performed, so that the risk of a new collision is decreased. There are also strategies for how the transmission shall be controlled, so that collisions are avoided or the risk of collision decreases.
One known retransmission strategy is the so-called ALOHA protocol. According to the ALOHA protocol packets, which earlier failed after a randomly chosen time period are retransmitted. If the transmission does not succeed this time either, the packet is retransmitted once again according to the same strategy.
One drawback with this retransmission strategy is that the packets can be retransmitted infinitely many times. Each time a packet is retransmitted it means that co-channel interference for other packets, which are transmitted simultaneously on the same channel in the system will increase. In a system with many users, only using ALOHA protocols, the same packets can be transmitted again and again from different transmitters. In principle all transmission stations can transmit packets simultaneously without any packet being received correctly. Therefore, such a system becomes unstable relating to co-channel interference. To improve the stability, some further control scheme must be used.
Another protocol to be used in a packet switching system to avoid collision is the so-called CSMA protocol (Carrier Sense Multiple Access). According to this protocol, a station having a packet to transmit, listens on the channel to be used for transmitting said packet. If the channel is idle, the packet is transmitted with a probability s. If the channel is not idle, the station waits until the channel is idle and then transmits the packet with a transmission probability s.
In EP-A-0432 315 a system and a method of controlling the access rate in stations in a packet transferring network are disclosed. The system comprises a number of transmitting and receiving stations, which are able to communicate mutually according to a directing scheme. Access rate represents the rate at which a transmitting station transmits packets in an ALOHA network, or the rate at which a transmitting station senses the status of the channel in a CSMA network. In each station a first parameter is calculated relating to the number of successfully transmitted packets per unit of time. Further a second parameter is calculated relating to the input flow, depending on the output flow from adjacent stations. A third parameter is calculated relating to the load of the station. The calculated parameters are transmitted to all adjacent stations. In each station the parameters are compared and the access rate is changed depending on the parameters. By controlling the transmission rate in each station, the throughput in the system is increased.
In US-A-4979168 a packet switching system according to the CSMA protocol is disclosed. Before a packet is transmitted, the system controls if the channel which the packet should be transmitted on is idle. If the channel is busy, a new point in time is determined for sensing the channel, which point in time is randomly selected within a dynamically determined time interval. The dynamically determined time interval is depending on the mean time the channel is idle. If the channel is idle, the packet is transmitted. According to this invention, the transmission in question is controlled relating to avoiding collision.
A drawback with this method is that no consideration is taken to, if the station that wishes to transmit packets has failed or succeeded to transfer the packets to the receiver at previously attempts. Thus, a station that has often failed with transmission can continue to fail.
DISCLOSURE OF THE INVENTION
The present invention relates to a problem how the packet transmission from a transmitting station shall be controlled depending on the present knowledge of the transmitting station, so that the transmission becomes fair both to stations successful in transmitting many packets, as well as stations not successful in transmitting an equal amount of packets.
Another problem is how to control transmission of packet data in a cellular radio communication system, so that the co-channel interference in the system is stabilized.
Thus, an object of the present invention is to control the transmission of packets, so that the transmission of packet data from different transmitting stations become fair. Fair relates to that transmitting stations, in which transmitted packets have low fraction of successful receptions not entirely ought to stop transmitting packets for the benefit of transmitting stations, in which transmitted packets have a higher fraction of successful receptions. Another object is to control the transmission in a packet switching mobile radio communication system, so that the co-channel interference in the system is stabilized.
The problems are solved by each transmitting station transmitting with a certain transmission probability, being updated after each transmission depending on how large fraction of the transmitted packets have been received correctly. The updating occurs according to a functional dependability having values between zero and one and whose derivative is less than one.
In particular the problems are solved by initially determining a transmission probability for each radio station, transmitting packet data in a mobile radio communication system. Thereafter each radio station transmit packet data according to said determined transmission probability. Thereafter, a value of the fraction of transmitted packets, correctly received in relation to the number of transmitted packets, for each transmitting station, is determined. This fraction is in the following related to as throughput fraction.
The transmission probability is updated for each transmission station after each transmitted packet. The updated value is determined depending on the value of the throughput fraction according to a predetermined function. This function hereinafter is referred to as mapping function. The mapping function has values higher than zero and less than one and its derivative is less than 1 in all points.
By choice of a mapping function having that configuration, the values of the throughput fraction will after a while reach an equilibrium in the system, whereby the system is stabilized relating to co-channel interference. When the system is stable all radio stations will be allowed to transmit packet data. The radio stations, which have low values of the throughput fraction will have lower transmission probability than those radio stations, which have had higher values on the throughput fraction. No radio station will achieve transmission probabilities reaching the values zero or one
Almgren Magnus
Larsson Per
Burns Doane Swecker & Mathis L.L.P.
Cangialosi Salvatore
Telefonaktiebolaget LM Ericsson (publ)
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