Adaptive modem for varying communication channel
First Claim
1. An adaptive modem system for a communication channel exhibiting multipath dispersion, which has a channel response characterized by an anticipated multipath delay time length, comprising:
- transmitting means for transmitting signals over the communication channel representing a packet of data made up of a plurality of frames in series, wherein each frame is composed of two adjacent training blocks in series with a data sequence of unknown symbols, which symbols are unknown as containing data to be received and which symbols become known only when received and decoded with respect to an accurate estimate of said channel response and wherein a second of the two training blocks is made up of M known symbols, and a first of the two training blocks has a time length longer than the multipath delay time length and separates the second training block from the data sequence; and
receiving means for receiving the transmitted signals, demodulating the data packet therefrom, and processing each frame thereof, including;
(a) source means for storing and generating the sequence of M known symbols;
(b) channel estimate means for correlating the sequence of M known symbols from said source means with the received M known symbols of the second training block and obtaining an estimate of said channel response as would be associated with an anticipated multipath delay time length in each frame of the data packet, wherein said second training block is isolated by the time length of said first training block so as to be substantially unaffected by any multipath dispersion from the data sequence; and
(c) decision means for decoding the unknown symbols of the data sequence in each frame using the estimate of said channel response as obtained.
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Accused Products
Abstract
An improved modem system for HF data transmission over a varying communication channel transmits data in packets formatted into successive frames, each having two identical training sequences following a sequence of data. Each training sequence has a time length longer than the expected time length of the multipath dispersion in the communication channel, in order to isolate one training sequence in each frame from multipath interference from the preceding data sequence. The "clean" training sequence allows the receiving modem system to derive very accurate channel estimates for each frame by autocorrelation to the known symbol sequence stored at the receiver. By interpolating across the channel estimates for the current and previous frames, the modem system can track the communication channel by interpolated estimates for any part or any point in time of the data sequence. An improved data recovery algorithm divides the received data sequence in each frame into a series of data blocks, each of which is processed through a respective decision loop using the interpolated channel estimates corresponding to the time position of the data block in the data sequence. The interpolated channel estimate for each data block is used to set the tap values for the filter functions of the corresponding decision loop.
181 Citations
22 Claims
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1. An adaptive modem system for a communication channel exhibiting multipath dispersion, which has a channel response characterized by an anticipated multipath delay time length, comprising:
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transmitting means for transmitting signals over the communication channel representing a packet of data made up of a plurality of frames in series, wherein each frame is composed of two adjacent training blocks in series with a data sequence of unknown symbols, which symbols are unknown as containing data to be received and which symbols become known only when received and decoded with respect to an accurate estimate of said channel response and wherein a second of the two training blocks is made up of M known symbols, and a first of the two training blocks has a time length longer than the multipath delay time length and separates the second training block from the data sequence; and receiving means for receiving the transmitted signals, demodulating the data packet therefrom, and processing each frame thereof, including; (a) source means for storing and generating the sequence of M known symbols; (b) channel estimate means for correlating the sequence of M known symbols from said source means with the received M known symbols of the second training block and obtaining an estimate of said channel response as would be associated with an anticipated multipath delay time length in each frame of the data packet, wherein said second training block is isolated by the time length of said first training block so as to be substantially unaffected by any multipath dispersion from the data sequence; and (c) decision means for decoding the unknown symbols of the data sequence in each frame using the estimate of said channel response as obtained. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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8. An adaptive modem system for receiving signals transmitted over a communication channel in a packet made up of a plurality of frames in series, wherein each frame includes a training sequence of M known symbols in series with a data sequence of unknown symbols which symbols are unknown as containing data to be received and which symbols become known only when received and decoded with respect to an accurate estimate of said channel response, said channel having a channel response characterized by an anticipated multipath delay time length, comprising:
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receiving means for receiving the transmitted signals, demodulating the data packet therefrom, and processing each frame thereof, including; (a) source means for storing and generating the sequence of M known symbols; (b) channel estimate means for correlating the sequence of M known symbols from said source means to the received M known symbols of the training sequence in each frame of the data packet, in order to derive an estimate of the channel response for a corresponding time position of the training sequence in each frame with said estimated channel response being that which would be associated with an anticipated multipath time delay; and (c) decision means for decoding the unknown symbols of the data sequence in each frame, said decision means including the interpolating means for interpolating an estimate of the channel response at a time position corresponding to a selected part of the data sequence based upon the estimates obtained for the training sequence for said frame and an adjacent frame, and wherein said decision means uses said interpolated estimate for deciding the unknown symbols in at least said selected part of the data sequence. - View Dependent Claims (9, 10, 11, 12)
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13. A method of operating an adaptive modem system for receiving signals transmitted over a communication channel in a packet made up of a plurality of frames in series, wherein each frame includes a training sequence of M known symbols in series with a data sequence of unknown symbols which symbols are unknown as containing data to be received and which symbols become known only when received and decoded with respect to an accurate estimate of said channel response, said channel having a channel response characterized by an anticipated multipath delay time length, comprising the steps of:
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receiving the transmitted signals, demodulating the data packet therefrom, and processing each frame thereof, including; (a) storing and generating the sequence of M known symbols; (b) correlating the sequence of M known symbols to the received M known symbols of the training sequence in each frame of the data packet, in order to derive an estimate of the channel response for a time position of the training sequence in each frame with said estimated channel response being that which would be associated with an anticipated multipath time delay, and (c) interpolating an interpolated estimate of the channel response at a time position corresponding to a selected part of the data sequence intermediate to the time positions of the training sequences for a frame and an adjacent frame based upon the estimates obtained for the training sequences, and using said interpolated estimate for deciding the unknown symbols in at least said selected part of the data sequence. - View Dependent Claims (14)
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15. A method of operating an adaptive modem system for a communication channel exhibiting multipath dispersion, which has a channel response characterized by an anticipated multipath delay time length, comprising the step of:
transmitting signals over the communication channel representing a packet of data made up of a plurality of frames in series, wherein each frame is composed of two adjacent training blocks in series with a data sequence of unknown symbols which symbols are unknown as containing data to be received and which symbols become known only when received and decoded with respect to an accurate estimate of said channel response, and wherein a second of the two training blocks is made up of M known symbols, and a first of the two training blocks has a time length longer than the multipath delay time length and separates the second training block from the data sequence such that an estimate of the channel response for each frame can be obtained by correlating the received signals for the second training block of each frame with a stored version of the M known symbols without interference from the unknown symbols of the data sequence. - View Dependent Claims (16, 17, 18)
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19. A method of operating an adaptive modem system for receiving signals transmitted over a communication channel in a packet made up of a plurality of frames in series, wherein each frame includes a training sequence of M known symbols in series with a data sequence of unknown symbols which symbols are unknown as containing data to be received and which symbols become known only when received and decoded with respect to an accurate estimate of said channel response, said channel having a channel response characterized by an anticipated multipath delay time length, comprising the steps of:
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receiving the transmitted signals, demodulating the data packet therefrom, and processing each frame thereof, including; dividing the data sequence of each frame into a series of at least two data blocks of N symbols each; processing each of said at least two data blocks through a corresponding one of at least two decision loops in at least two passes, wherein each decision loop includes a matched filter for combining input signals for the symbols in each frame into enhanced signals, a feedforward filter and a feedback filter for removing intersymbol interference of adjacent symbols in the enhanced signals from said matched filter, and a decision unit for deciding the data symbols filtered by said feedforward and feedback filters, wherein on the first pass, a preceding training sequence of a prior adjacent frame is used to initialize the feedback filter of a first decision loop for a first data block of the current frame, and decisions made by the decision unit on the first data block are used to initialize the feedback filter of a second decision loop for a following, second data block, and wherein on the second pass, decisions on the first data block on the second pass are used to initialize the feedback filter for the second decision loop. - View Dependent Claims (20, 21, 22)
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Specification