Apparatus and method for establishing frame synchronization in distributed digital data communication systems
First Claim
1. A process for aligning frame timing in a bidirectional distributed data communication system that communicates payload digital data organized into frames, each frame having an alignment gap during which transmission of payload data is forbidden, said communication system having a central unit and one or more remote units, said alignment process ensuring that frames of data transmitted from each said remote unit arrive at said central unit aligned in time with a frame timing reference established by said central unit, comprising the steps of:
- (1) broadcasting a unique trigger signal from said central unit to remote unit that is attempting to align its transmit timing to a frame timing reference established by said central unit, said frame timing reference being said alignment gap;
(2) receiving said unique trigger signal in said remote unit attempting to align to said frame timing reference, and setting a programmable delay at a first trial value, and retransmitting said unique trigger signal to said central unit at a time delayed from said time of receipt of said unique trigger signal from said central unit by the interval of said programmable delay;
(3) performing a correlation calculation in said central unit between said unique trigger signal transmitted by said central unit and any signal received during said alignment gap following the frame in which said unique trigger signal was transmitted by said central unit;
(4) if no peak is found resulting from said correlation calculation, broadcasting a message from said central unit to said remote unit attempting to align indicating that the remote unit is not properly aligned yet and retransmitting said unique trigger signal during the next frame;
(5) receiving in said remote unit attempting to align said unique trigger signal sent by said central unit during said next frame and receiving said broadcast message indicating alignment has not yet been achieved, and setting said programmable delay at a new trial value and retransmitting said unique trigger signal to said central unit at a time delayed from said time of receipt of said unique trigger signal from said central unit by the interval of said programmable delay;
(6) performing a correlation calculation between said unique trigger signal transmitted during said next frame and any signal received during said alignment gap following said next frame;
(7) repeating steps (4), (5) and (6) in successive frames until a peak is found in a step that corresponds to step (6);
(8) sending a message from the central unit to the remote unit that is attempting to align that alignment has been achieved and to hold the programmable delay value constant at the value that resulting in proper alignment for future transmissions.
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Accused Products
Abstract
A method and apparatus for carrying out synchronous codivision multiple access (SCDMA) communication of multiple channels of digital data over a shared transmission media such as a cable television system coaxial cable, a fiber optic or copper conductor telephone link, terrestial microwave, satellite link, local or wide area network, wireless including cellur network or some combination of these media using suitable interface circuitry. The system includes modems at remote units and a central unit to receive time division multiplexed digital data arranged into timeslots or channels and uses orthogonal codes to encode each channel of multiple data and spread the energy of each channel data over a frame of data transmitted in the code domain. Spreading the data this way makes the system less susceptible to impulse noise. Frames are synchronized as between remote and central units using a ranging scheme which is also useful in any other system transmitting data by frames in a distributed system where synchronizing the frames as between all units regardless of differences in propagation delays is necessary. Each frame in the SCDMA modulation scheme includes a gap or guardband containing no other data. The ranging process involves training each remote unit to impose enough delay prior to re-transmission of a barker code received from the CU such that a barker code sent by the RU arrives at the CU during the gap. The process of setting the delay in each RU is a trial and error process, and each RU starts the ranging process asynchronously. Contention resolution protocols such that only one RU is aligning to the gap at any particular time are taught.
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Citations
14 Claims
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1. A process for aligning frame timing in a bidirectional distributed data communication system that communicates payload digital data organized into frames, each frame having an alignment gap during which transmission of payload data is forbidden, said communication system having a central unit and one or more remote units, said alignment process ensuring that frames of data transmitted from each said remote unit arrive at said central unit aligned in time with a frame timing reference established by said central unit, comprising the steps of:
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(1) broadcasting a unique trigger signal from said central unit to remote unit that is attempting to align its transmit timing to a frame timing reference established by said central unit, said frame timing reference being said alignment gap; (2) receiving said unique trigger signal in said remote unit attempting to align to said frame timing reference, and setting a programmable delay at a first trial value, and retransmitting said unique trigger signal to said central unit at a time delayed from said time of receipt of said unique trigger signal from said central unit by the interval of said programmable delay; (3) performing a correlation calculation in said central unit between said unique trigger signal transmitted by said central unit and any signal received during said alignment gap following the frame in which said unique trigger signal was transmitted by said central unit; (4) if no peak is found resulting from said correlation calculation, broadcasting a message from said central unit to said remote unit attempting to align indicating that the remote unit is not properly aligned yet and retransmitting said unique trigger signal during the next frame; (5) receiving in said remote unit attempting to align said unique trigger signal sent by said central unit during said next frame and receiving said broadcast message indicating alignment has not yet been achieved, and setting said programmable delay at a new trial value and retransmitting said unique trigger signal to said central unit at a time delayed from said time of receipt of said unique trigger signal from said central unit by the interval of said programmable delay; (6) performing a correlation calculation between said unique trigger signal transmitted during said next frame and any signal received during said alignment gap following said next frame; (7) repeating steps (4), (5) and (6) in successive frames until a peak is found in a step that corresponds to step (6); (8) sending a message from the central unit to the remote unit that is attempting to align that alignment has been achieved and to hold the programmable delay value constant at the value that resulting in proper alignment for future transmissions.
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2. A process for aligning frame timing in a bidirectional distributed data communication system that communicates payload digital data organized into frames, each frame having an alignment gap during which transmission of payload data is forbidden, said communication system having a central unit and a plurality of remote units, said alignment process ensuring that frames of data transmitted from each said remote unit arrive at said central unit aligned in time with a frame timing reference established by said central unit, comprising the steps of:
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(1) broadcasting a unique trigger signal from said central unit to all remote units that are attempting to align their transmit timing to a frame timing reference established by said central unit, said frame timing reference being said alignment gap; (2) receiving said unique trigger signal in each said remote unit that is attempting to align to said frame timing reference, and setting a programmable delay at a first trial value, and retransmitting said unique trigger signal to said central unit at a time delayed from said time of receipt of said unique trigger signal from said central unit by the interval of said programmable delay; (3) performing a correlation calculation in said central unit between said unique trigger signal transmitted by said central unit and any signal received during said alignment gap following the frame in which said unique trigger signal was transmitted by said central unit; (4) if no peak is found resulting from said correlation calculation, broadcasting a message from said central unit to all said remote unit attempting to align indicating that no remote unit is properly aligned yet and retransmitting said unique trigger signal during the next frame; (5) receiving in each said remote unit attempting to align said unique trigger signal sent by said central unit during said next frame and receiving said broadcast message indicating alignment has not yet been achieved, and setting said programmable delay at a new trial value in each said remote unit attempting to align and retransmitting said unique trigger signal from each said remote unit attempting to align to said central unit at a time delayed from said time of receipt of said unique trigger signal from said central unit by the interval of said programmable delay; (6) performing a correlation calculation between said unique trigger signal transmitted during said next frame and any signal received during said alignment gap following said next frame; (7) repeating steps (4), (5) and (6) in successive frames until a peak is found in a step that corresponds to step (6); (8) broadcasting a message from the centrol unit to all remote units that are attempting to align that a peak has been found in said alignment gap; (9) transmitting from each said remote unit attempting to align a unique signature sequence over a plurality of frames comprising an even number of successive signature sequence frames, each said remote unit having a unique signature sequence comprising a permutation of transmission of said unique trigger signal during some frames of said signature sequence frames but not others with the number of frames during which said unique trigger signal is sent comprising 50% of the total number of signature sequence frames; (10) performing in the central unit a correlation calculation between the unique trigger signal of each frame and any signals received during the alignment gap of each frame in said signature sequence frames; (11) counting the number of alignment gaps in said signature sequence frames during which correlation peaks occurred and comparing that number to the total number of signature sequence frames; (12) if the number of alignment gaps in said signature sequence frames during which correlation peaks occurred is 50%, examining the exact sequence of said alignment gaps during which peaks occurred and identifying the particular remote unit that has successfully aligned from the signature sequence received; (13) sending a message to the remote unit that has successfully aligned to hold the programmable delay that caused said successful alignment constant for future transmissions; (14) if the number of alignment gaps during which correlation peaks occurred is greater than 50%, broadcasting a collision message to all remote units that more than one remote unit has aligned thereby preventing proper identification and requesting that all remote units execute collision resolution protocols; (15) receiving said collision message in each remote unit attempting to align and executing in each remote unit attempting to align a collision resolution protocol comprising a random decision whether to continue to attempt to align or cease attempting to align; (16) in each remote unit which has decided to continue to attempt to align, retransmitting the unique signature sequence; (17) performing in the central unit a correlation calculation between the unique trigger signal of each frame and any signals received during the alignment gap of each frame in said signature sequence frames; (18) counting the number of alignment gaps in said signature sequence frames during which correlation peaks occurred and comparing that number to the total number of signature sequence frames; (19) if the number of alignment gaps in said signature sequence frames during which correlation peaks occurred is 50% of the total number of frames in the signature sequence, examining the exact sequence of said alignment gaps during which peaks occurred and identifying the particular remote unit that has successfully aligned from the signature sequence received and sending that remote unit a message telling it to hold the programmable delay that resulted in the successful alignment constant for future transmissions; (20) if the number of alignment gaps in said signature sequence during which correlation peaks occurred is still more than 50% of the total number of frames in the signature sequence, repeat steps (14) through (20) until the number of alignment gaps in said signature sequence during which correlation peaks occurred is 50% of the total number of frames in the signature sequence or no peaks occur during the alignment gaps of the signature sequence, and then proceed to step (21) if the number of alignment gaps in said signature sequence during which correlation peaks occurred is 50% of the total number of frames in the signature sequence, or, if no peaks occur during the alignment gaps of the signature sequence, proceed to step (22); (21) if the number of alignment gaps in said signature sequence frames during which correlation peaks occurred is 50% of the total number of frames in the signature sequence, examining the exact sequence of said alignment gaps during which peaks occurred and identifying the particular remote unit that has successfully aligned from the signature sequence received and sending that remote unit a message telling it to hold the programmable delay that resulted in the successful alignment constant for future transmissions; (22) broadcast a message from the central unit to all remote units that were attempting to align to re-execute their collision resolution protocols; (23) each remote unit that was attempting to align reexecutes its collision resolution protocol, and the remote units that decide to continue attempting to align retransmit their unique signature sequences using the same programmable delay as existed in step (14); (24) counting the number of times step (17) has been performed, and if it has been performed a number of times equal to or greater than a predetermined number, proceeding to step (25), but, if not, proceeding to step (26); (25) proceed to step (1) and repeat the entire alignment process;
repeating steps (17) through (26) until each remote unit attempting to align has successfully aligned. - View Dependent Claims (3)
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4. A ranging process for use in a distributed system comprising a central unit coupled by a shared transmission media to a plurality of remote units at physically disparate locations, comprising the steps of:
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determining the propagation time in each remote unit from said remote unit to said central unit via said shared transmission media by transmitting a unique signal which can be detected by said central unit and adjusting a transmit timing delay factor prior to each transmission until said central unit sends a message to said remote unit indicating that said unique signal has arrived at a proper time which will cause a transmission from that remote unit to arrive at said central unit at the same time as transmissions from all other remote units which have previously successfully completed a ranging process and properly set their respective transmit timing delay factors; when said message is received by said remote unit performing said ranging process from said central unit, fixing said transmit timing delay factor at the value which caused said unique signal to arrive at said central unit at said proper time so as to establish synchronization of transmission as between all said remote units, and, thereafter, using for every transmission by said remote unit to said central unit the value so fixed for said transmit timing delay until said ranging process is performed again by said remote unit. - View Dependent Claims (5, 6, 7, 8, 9)
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10. An apparatus comprising:
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a shared transmission media; a plurality of remote units physically distributed throughout a distributed system, each remote unit coupled to a central unit by said shared transmission media, said apparatus for transmitting a plurality of streams of digital data received at said remote units to said central unit organized as frames by encoding data in each frame from one of said plurality of streams of digital data with a different orthogonal code; and wherein each remote unit and said central unit each contain a transceiver, said transceiver comprising a transmitter section and a receiver section, said transmitter section comprising; a framer circuit for receiving data from different timeslots of a time division multiplexed data stream, predetermined timeslots containing data from specified ones of said plurality of streams of digital data, and for imposing a transmit frame timing delay before the transmission of each said frame so as to cause each said remote unit to transmit each frame in frame synchronization such that said frame arrives at said central unit at the same time as corresponding frames from said other remote units; an orthogonal code multiplexer for receiving data from said framer circuit from different timeslots and encoding said data with a different orthogonal code by matrix multiplication of information vectors comprised of data from said different timeslots times a code matrix containing as many different orthogonal codes as there are different timeslots; and a modulator for receiving encoded data from said orthogonal code multiplexer and using said encoded data to modulate one or more RF carriers and transmit said RF carriers on said shared transmission media; and wherein said receiver section comprises; a demodulator for demodulating said RF carrier and providing output data; an orthogonal code demultiplexer for multiplying said output data from said demodulator times a transpose of said code matrix; a detector for receiving data from said orthogonal code demultiplexer and for determining therefrom the original data which was multiplied by said code matrix; and a deframer circuit for reassembling said original time division multiplexed data stream from output data from said detector. - View Dependent Claims (11, 12, 13, 14)
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Specification