Abstract:
A collision-based multi-access system is described which is capable of recovering collisions by means of jointly detecting the mutually interfering signals, followed by re-transmission of functions of portions of some of the colliding signals. This system reduces the amount of required re-transmission of all of the colliding signals, decreasing the amount of re-transmission in the system, increasing the system throughput. The system is also applicable to non-colliding communications systems to correct interference due to noise.

Description:
RELATED APPLICATION 
     The present is application is based upon U.S. Provisional Application Ser. No. 60/083,905, entitled “Method and Apparatus for Multi-Access Collision-Based Communications System” filed May 1, 1998. The present application claims the benefit of the earlier priority date under 35 U.S.C. §119(e). 
    
    
     COPYRIGHT NOTICE/PERMISSION 
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. The following notice applies to the software and data as described below and in the drawing hereto: Copyright ©1999, Libit Signal Processing, Ltd., All Rights Reserved. 
     TECHNICAL FIELD 
     The present invention is a multi-access collision based communications system that has a higher capacity than the existing collision-based communications systems. 
     BACKGROUND OF THE INVENTION 
     Collision-based, multi-access protocols are used in a variety of multi-user communications systems, including VSAT systems, 10 Mbbits/sec Ethernet local area networks (LAN), 100 Mbits/sec Ethernet LAN, and wireless networking systems. In such protocols, two or more transmitters may simultaneously transmit in overlapping frequency bands. In such a case, which is termed “signal collision”, the receiver is incapable of detecting the transmitted data, and all the colliding signals are then retransmitted. The capacity of such systems may be degraded by a factor of 2.7 or more due to re-transmissions. There is a need in the art, therefore, to increase throughput in collision-based communications system. 
     SUMMARY OF THE INVENTION 
     The present invention solves many of the problems described above and many other problems in the communications field, which solutions will become known to those skilled in the art upon reading and understanding the present invention. The present invention is a collision-based multi-access system which recovers informnation involved in collisions by means of receivers jointly detecting the mutually interfering signals, followed by re-transmission of functions of portions of some of the colliding signals. In stead of re-transmission of all the colliding signals, the present invention enables decreasing the amount of re-transmission in the system, and therefore increasing the system throughput. The present invention is also applicable to non-colliding communications systems to correct interference due to noise. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings, in which like reference numbers represent like components throughout the several views, 
     FIG. 1 is a block diagram of a satellite communications network used with an illustrative embodiment of the present invention; 
     FIG. 2 is a state-machine flow chart of operation of a transmitter of the present invention; 
     FIG. 3 is a flow chart of the operation of a corresponding receiver of the present invention; 
     FIG. 4 depicts the content of a typical message transmitted in a communications system of the present invention; 
     FIG. 5 is a timing comparison of a sequence of messages in a communications system of the present invention; and 
     FIG. 6 illustrates the structure of a recovery message encoder in the transmitters of the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the inventions may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention, and it is to be understood that other embodiments may be utilized and that process or mechanical changes may be made without departing from the scope of the present invention. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims. 
     FIG. 1 is a block diagram of a satellite communications network used with an illustrative embodiment of the present invention which is used to illustrate the principles and the structure of a satellite communications network with which the present invention may be practiced. Those skilled in the art will readily recognize, however, that the present invention may be practiced in a wide variety of collision-based communications systems which share a communications medium and in which transmitters asynchronously transmit messages which collide. Such systems include, by way of example and not by way of limitation, 10 Mbits/sec Ethernet local area networks (LAN), 100 Mbits/sec Ethernet LAN, and wireless networking systems. 
     The structure of a satellite communication system depicted in FIG. 1 consists of a receiver  11  which is integrated in a base-station  12 , a satellite  13 , and a plurality of transmitters  14   a ,  14   b  and  14   c , collectively referenced as  14 . Many more transmitters are possible but not illustrated here. The purpose of the satellite communication system of FIG. 1 is to reliably transmit signals from the transmitters  14   a ,  14   b  and  14   c  to the receiver  11 , and to pass the information from users  15   a ,  15   b  and  15   c , respectively, to user  16 . 
     The transmitters  14   a ,  14   b  and  14   c  transmit data messages to the receiver  11  in a common upstream channel  17 , which is a multi-access, collision-based channel. Two or more transmitters  14  may simultaneously use the same frequency band, and create mutual interference. Each message typically contains a synchronization (SYNC) word, a source identification (ID), type, a user data area, and an error detection code (such as circular redundant code or CRC code). The structure of this typical message is shown in FIG.  4  and may take the form of a packet of data. 
     There are typically two types of messages: regular data messages, and recovery messages. In one embodiment, the content of the recovery messages is one or more functions of portions of data messages that were sent earlier. The base station  12  uses a transmitter  19  on a downstream channel  18  to send control messages to the transmitters  14 . Some of the control messages send an acknowledge code to the transmitters  14  about the reliable reception of data messages, while other control messages request a certain transmitter(s)  14  to transmit a function(s) of portions of data messages that it has sent before. 
     The flow chart of the operation of a transmitter  14  that works according to the teaching of the present invention is shown in FIG.  2 . Upon a request ( 200 ) from a user  15  which is connected to a transmitter  14 , a data message is built which is subsequently transmitted in the upstream channel  17  according to state  201 . The data message is also stored in memory at the transmitter  14  in state  201  of FIG.  2 . If the transmitter  14  receives, from the downstream channel  18 , a request to transmit functions of portions of a message, these functions are evaluated at state  203  and transmitted in the upstream channel  17  at state  204 . If, however, the transmitter  14  receives from the downstream channel  18  an acknowledge message, the user&#39;s message or data is acknowledged for successful data transmission and the corresponding data message is removed from the transmitter&#39;s memory at state  202 , so that the memory can be allocated for storing new messages. 
     Referring to FIG. 6, a system for generating recovery messages in the transmitter  14  is shown. The generation of a recovery message is performed by taking every Nth bit (where N is a positive integer) of the data  602  that is stored in the message memory  601  of transmitter  14 , feeding that bit into a systematic error-correction-code (ECC) encoder  603 , and transmitting the parity bits of the ECC code which are generated by the encoder  603 . The ECC being used may be, for example, Reed-Solomon ECC, or any systematic convolutional code. A particular case of ECC is a repetition code, in which case the parity bits are equal to the encoder&#39;s input bits, so that the recovery message consists of every Nth bit of portions of the data message. 
     The flow chart of the operation of a receiver that works according to the present invention is shown in FIG.  3 . The receiver iterates in a loop. The first step in this loop is the search for synchronization words at  301 . This is done by applying a correlator, which is matched to the synchronization signal, and searching for peaks at the output of the correlator. The receiver estimates the number of simultaneous overlapping transmissions by detecting multiple peaks that appear at the output of the synchronization word correlator at  302 . The receiver then detects the content of the incoming messages by applying an algorithm for joint detection of M mutually interfering signals, the general type of which can be found in U.S. Pat. No. 5,710,797 issued Jan. 20, 1998, entitled “Method and Apparatus for Digital Communication in the Presence of Closely Spaced Adjacent Channels”, assigned to Libit Signal Processing Ltd., the same assignee of the present invention, and which is hereby incorporated by reference. One skilled in the art will readily recognize that the general considerations described in U.S. Pat. No. 5,710,797 can be also applied to fully overlapped signals. 
     Referring once again to FIG. 3, if the estimated number of overlapping signals equals one, that is, there are no collisions, then a standard detection algorithm at  303  may be applied by the receiver. The receiver may decide to avoid applying a detector on the data, in case the number of mutually interfering signals or their power levels indicate low probability of acceptable detection. Those skilled in the art will also recognize that all subsequent operations shown in FIG. 3 may be also applied in the case where there is no mutual interference, but the single signal could not be detected due to, for example, random noise, in, for example, a non-collision-based system. For all newly received messages that were identified as recovery messages at  304 , the receiver applies a detector on the corresponding previous received signal whose samples are stored in the receiver&#39;s memory, making a use of the new information contained in the recovery message at  305 . For example, if the information contained in the recovery message is every Nth bit of a portion of a data message, and the signal is encoded using a convolutional-type code, then the auxiliary information may be inserted into a Viterbi-like decoding algorithm, where inconsistent states and branches on the trellis diagram are eliminated. 
     If the information in a recovery message is a portion of a signal, then the changes in signal parameters (timing phase, carrier phase, amplitude, channel impulse response, etc.) occurring between the time of the first transmission and the time of the re-transmission, may be estimated and compensated to retrieve the original message. The re-transmitted signal is subtracted from the initially received signal, so that the mutual interference is reduced and other signals that collided with the above signal may be detected. If the information in a recovery message contains ECC parity bits on every Nth bit of a portion(s) of the data, then the receiver may iterate between using these parity bits to correct detector output bits, and applying the detector again with the corrected bits fed as side information. 
     The next step of the algorithm in the receiver is to test if all data messages in the newly received signal, and, if applicable, in the signal that may have been re-detected using a newly received recovery message, have been detected with a valid CRC field at  306 . If the CRC is valid, the corresponding signal is removed from the receiver&#39;s memory at  307 , so that the memory can be allocated for samples of other received signals. Then, acknowledge messages are sent in the downstream channel to confirm accurate data reception at  308 , and the received data is transferred to the user at  309 . 
     If messages were detected with an invalid CRC, then the receiver assesses the amount and type of additional auxiliary information required for reliable detection of the received signal at  310 . If the receiver decides to use additional auxiliary data at  311 , it sends requests in the downstream channel for certain transmitters to transmit functions of portions of a data message(s) that was received incorrectly at step  312 , and the receiver keeps the associated received signals in memory for later processing at  313 . 
     FIG. 5 depicts an example of some messages that are sent in the communications channel with which the present invention operates. Referring to both FIG.  1  and FIG. 5, messages  501  and  502  are sent simultaneously in the same frequency range in the upstream channel  17  by two transmitters, and since the receiver  11  does not succeed to detect these messages, it sends, in the downstream channel  18 , a request  503  for transmitting a function(s) of the data message  501 . The corresponding transmitter then generates the function of the data message  501  and transmits a recovery message  504  in the upstream channel  17 . Using message  504  as auxiliary data, the receiver jointly detects messages  501  and  502 , obtains valid CRC, and transmits acknowledge messages  505 ,  506  in the downstream channel to confirm reception of messages  501  and  502 . Of course, there my be more than two collisions in a transmission, such as three or more transmitters  14  attempting to send messages at the same time. In this case, the present invention will operate on all colliding messages and correct all overlapping signals. 
     In another embodiment of the present invention, the content of the recovery message  504  consists of the retransmission of only the overlapping portion of data message  501  and data message  502 . In this embodiment, the content of both message  501  and  502  can be reconstructed using the overlapping portion from either message  501  or  502 . In this case, the changes in signal parameters (timing phase, carrier phase, amplitude, channel impulse response, etc.) occurring between the time of the first transmission  501 ,  502  and the time of the transmission of the recovery message  504 , may be estimated and compensated to retrieve the original message. The recovery message signal in its analog form is subtracted from the initially received analog signal, so that the mutual interference is reduced and other signals that collided with the above signal may be detected. 
     Those skilled in the art will readily recognize that the present invention may be used in a non-collision-based communication system where messages are corrupted by noise, fading and other forms of interference. In such a system, the messages can be corrected with recovery messages which are smaller and hence use less bandwidth than a retransmission of the entire message. Those skilled in the art will also readily recognize that the present invention may be used in a non-feedback communication system (such as a simplex system) where recovery messages are always automatically sent under the assumption that the recovery messages are used if there is a collision, or they are discarded by the receiver if there are no collisions. 
     The present invention also enables reduced channel spacing in collision-based digital communication systems and thereby increases the system capacity (i.e. the number of users per bandwidth unit) without incurring any significant loss in system performance (e.g. power margins, BER, and channel availability). It also allows a reduced power margin that may be required to maintain a pre-specified performance level without sacrificing system capacity. 
     CONCLUSION 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. Many adaptations of the invention will be apparent to those of ordinary skill in the art. For example, the present invention may be utilized in cable TV modems, Ethernet networks and the like. The upstream and downstream channels my consist of different media such as wire and wireless. The upstream channel and downstream channels may also be the same with collisions occurring in both directions. The recovery messages may also be subject to collisions and the system may work in a full duplex environment. Accordingly, this application is intended to cover any adaptations or variations of the invention. It is manifestly intended that this invention be limited only by the following claims and equivalents thereof.