Multiplex communications – Channel assignment techniques – Carrier sense multiple access
Reissue Patent
1999-01-22
2001-07-31
Hsu, Alpus H. (Department: 2662)
Multiplex communications
Channel assignment techniques
Carrier sense multiple access
C370S348000, C370S443000, C370S447000, C370S450000, C370S459000, C455S450000, C455S039000, C455S509000, C455S517000
Reissue Patent
active
RE037301
ABSTRACT:
BACKGROUND OF THE INVENTION
Many multiple access schemes have been devised to allow a group of terminals to communicate with a central station using a shared channel. One of these schemes, called random multiple access, relies on randomly transmitted messages from a group of terminals to a central station without perfect coordination between terminals. For example, in a cellular radio environment, each cell is served by a base station communicating with multiple mobile stations. Each cell has the use of a set of radio frequency channels. At least one channel is dedicated for the purpose of setting up radio phone calls in each cell. This “setup” channel is shared by all the mobile stations to communicate with the base station. The setup channel is a full duplex channel with separate frequencies in the forward (base to mobile) and reverse (mobile to base) directions. The forward setup channel is used by the base station to transmit messages to all the mobiles. This channel is a broadcast channel in which all the mobile stations can receive all the messages transmitted from the base station. The reverse channel is a random contention multiple access channel, in which mobile stations may transmit messages to the base station with relatively little coordination. The access is random in the sense that mobile stations are not assigned a fixed transmission schedule. The access is based on contention in which mobile stations with messages to transmit will try to seize channel openings as the appear. If multiple mobile stations decide to transmit at the same time, the messages may collide and mutually destroy each other. There is also a possibility that one of the messages is received with significantly higher power than the others and is correctly received at the base station. In any case the mobile stations whose transmissions are not successful have to retransmit after some random delay.
DESCRIPTION OF THE PRIOR ART
Existing multiple access protocols of this sort include the ALOHA, CSMA, ICSMA-CD access protocols, among others. Because of the random transmission nature and inevitable collisions that may occur, no random access protocols can reach 100% throughput efficiency. The random delay after a collision also introduces additional messaging delay between a terminal and a central station.
In the North American analog cellular system the base station and mobile stations transmit on separate frequencies to form a full duplex channel. The base station broadcasts the status of the multiple access setup channel to all the mobiles communicating with base periodically. A busy/idle bit inserted every 11 bits in the base to mobile transmission indicates if the mobile station (terminal) to base station multiple access channel was busy or idle in the last period. Such timely feedback enables a mobile station to make sure the channel is idle before transmitting again. A mobile station continues to monitor the busy/idle bit during transmission. If the busy/idle bit does not change from indicating idle to indicating busy within a short window at the beginning of a message transmission, the transmitting mobile station assumes a collision and the transmission is immediately aborted. However, the problem associated with this approach is that the busy/idle bit does not distinguish between a collision state and a real idle state. Without such information, the mobile station cannot make more efficient decisions on when to start or abort a transmission. Many of the multiple access protocols have a slotted variation. In a slotted protocol, the channel is divided into fixed size time slots. In the slotted ALOHA protocol, each slot is long enough to include a fixed size message. A variation of the slotted ALOHA called Reservation ALOHA allows a message of multiple slots (words) in length to be transmitted without interruption once the first word of the message is successfully transmitted. In Reservation ALOHA, if a station has successfully transmitted in a predetermined time slot, all the subsequent slots are implicitly reserved for that station until the station stops transmission. The problem with this scheme is that is expands one idle slot to indicate the channel's relinquishment. Such use of channel resources is inefficient.
There is accordingly a need for a new multiple access protocol which will enhance transmission efficiency, while reducing the access delay.
Accordingly, it is an object of the present invention to provide a new access protocol which improves transmission efficiency, while reducing access delay.
Another object of the present invention is to provide a new multiple access protocol in which a short data field is used to send feedback information from the central station to terminals providing information on the status of the access channel.
Yet another object of the present invention is to provide a new multiple access protocol in which a short data field is used in every base to mobile station burst to indicate the relinquishment of the multiple access channel and eliminates the necessary idle slot between messages.
SUMMARY OF THE INVENTION
The new access protocol of the present invention utilizes a feedback field to provide the status of the multiple access channel from the base station to the transmitting mobile station to improve performance. This feedback information identifies the outcome of the last mobile to base transmission slot as being either
idle,
containing one successful transmission, or
a collision.
In the case of a field containing one successful transmission, the feedback information also identifies the transmitting mobile station. The detailed feedback information allows all mobiles to make between decisions regarding whether to start a new message transmission or to continue transmission of the current message. This will in turn improve the throughput and delay performance of the protocol.
The current invention also uses a short field in every base to mobile station burst to indicate if the multiple access channel is open for contention in the next slot. This eliminates the necessary idle slot between messages and further enhances the throughput and delay performance of the protocol, especially at transmitting short messages.
Accordingly, it is an aspect of the present invention to provide a method of providing feedback information indicative of the status of a multiple access channel between a group of terminals and a central station, comprising the step of:
forwarding on said access channel, from said central station to said terminals, an information code indicative of the status of said channel and of the outcome of the last terminal to central station message transmission.
According to another aspect of the present invention, there is provided a transmission protocol for providing feedback information indicative of the status of a multiple access channel between a group of terminals and a central station, comprising:
an information code transmitted on said access channel between said central station and said terminals, said information code being indicative of the status of said access channel and of the outcome of the last terminal to central station message transmission.
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Datapro Research Corporation, “An Overview of Cellular Telephone Service”, Apr. 1988.
Hsu Alpus H.
Nortel Networks Limited
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