High speed modem suitable for operating with a switched network
First Claim
1. A modulator-demodulator for the transmission and reception of information adapted for efficient information transmission through a switched network and for reducing the effects of distortions produced by the unknown characteristics of the transmission medium, said modulator comprising:
- means for generating representation of plurality of synchronous information sub-carriers, each of said information sub-carriers harmonically related to a fundamental f0,means for assembling information for transmission into a plurality of separate information portions, equal in number to the number of information sub-carriers,means for modulating each representation of an information sub-carrier with a different information portion,means for transmitting representation of said modulated information sub-carriers for a period TB wherein TB >
TO =1/f0,said demodulator adapted for connection to a transmission medium and responsive to signals received therefrom said demodulator comprising;
first means for generating representations of a plurality of corresponding information sub-carriers each nominally of frequency equal to the frequency of said plurality of information sub-carriers,correlating means for correlating said signal received from said transmission medium with representations of each of said corresponding information sub-carriers for a period T0 <
TB, anddecoding means responsive to said correlating means for detecting the information portion originally modulating said information sub-carriers.
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Accused Products
Abstract
A modem capable of achieving essentially 9600 bit per second information transfer rate through the switched telephone network. The high bit rate is achieved notwithstanding the variations in the characteristics of the switched telephone network by phase and amplitude modulating a plurality of sub-carriers at a relatively low baud rate. Correlation detection is employed at the receiver to extract or demodulate the information contained in the various sub-carriers. To allow correlation detection with orthogonal signals, each of the sub-carriers is harmonically related to a fundamental which fundamental is, however, slightly higher than the baud rate. This allows the receiver to delay correlation for a "gap" time, which reduces the effect of transients produced by modulation and also provides greater tolerance to inter-symbol distortion. Since the fundamental of all the sub-carriers is higher than the baud rate, the receiver "sees" orthogonal signals during the integration or correlation time which is maintained as the period of the fundamental. In addition to the information sub-carriers, a pair of unmodulated sub-carriers are also transmitted to enable the receiver to maintain the correlation interval in proper synchronization with the transmitter, and to correct for phase distortion and frequency offset.
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Citations
36 Claims
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1. A modulator-demodulator for the transmission and reception of information adapted for efficient information transmission through a switched network and for reducing the effects of distortions produced by the unknown characteristics of the transmission medium, said modulator comprising:
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means for generating representation of plurality of synchronous information sub-carriers, each of said information sub-carriers harmonically related to a fundamental f0, means for assembling information for transmission into a plurality of separate information portions, equal in number to the number of information sub-carriers, means for modulating each representation of an information sub-carrier with a different information portion, means for transmitting representation of said modulated information sub-carriers for a period TB wherein TB >
TO =1/f0,said demodulator adapted for connection to a transmission medium and responsive to signals received therefrom said demodulator comprising; first means for generating representations of a plurality of corresponding information sub-carriers each nominally of frequency equal to the frequency of said plurality of information sub-carriers, correlating means for correlating said signal received from said transmission medium with representations of each of said corresponding information sub-carriers for a period T0 <
TB, anddecoding means responsive to said correlating means for detecting the information portion originally modulating said information sub-carriers. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15)
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16. A low baud rate, high bit rate modem adapted for efficient information transfer through a communication medium with good tolerance to noise bursts and effective to time the demodulation process to compensate for phase distortion, frequency offset and amplitude distortion comprising a modulator and demodulator, said modulator comprising:
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a sub-carrier generator for generating representations of a plurality of information and timing sub-carriers, first means responsive to digital information signals for periodically modulating each of said information sub-carriers once per baud time, means for accumulating a representation of each of said sub-carriers and transmitting a sum of said representation a fixed plurality of times within each said baud time, said demodulator comprising; a timing unit, A/D converting means responsive to a received signal for periodically producing a digital representation thereof when initiated by said timing unit, a second sub-carrier generator for generating representations of a second plurality of information and timing sub-carriers, each nominally frequency coherent with a different sub-carrier of said first plurality, correlating means for correlating said first and second plurality of sub-carriers, decision means responsive to said correlating means with respect to said information sub-carriers for recreating digital signals representative of said modulating digital information signals and for compensating for phase and amplitude distortion and responsive to said correlating means with respect to said timing sub-carriers for compensating for frequency offset and for inhibiting said timing unit to enable said A/D converting means and said correlating means only for a period TO, less than said baud time, wherein said period TO is a period of a fundamental f0 of each of said sub-carriers. - View Dependent Claims (17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29)
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30. A method of data transmission suitable for the transmission of digital data over a switched network and for reducing the effects of distortion, comprising the transmitting steps of:
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(a) generating within a selected time interval Tb at least plural samples of each of a plurality of sub-carriers, each of said sub-carriers being harmonics of a fundamental f0, (b) modifying samples of different ones of said sub-carriers in accordance with intelligence to be transmitted at a rate no higher than once within said selected time interval Tb, wherein said fundamental period TO is less than Tb, (c) summing a sample from each of said sub-carriers to produce a sum, (d) transmitting a signal representative of said sum, (e) repeating steps (c) and (d) a number of times within said selected time interval Tb, and the receiving steps of; (f) sampling a received signal, (g) correlating samples of said received signal with samples of a plurality of locally generated sub-carriers throughout the interval TO less than Tb, (h) detecting said intelligence by the result of said correlation, and (i) adjusting frequency and absolute phase of said locally generated sub-carriers to achieve coherency between transmitter sub-carriers and receiver sub-carriers in response to said correlation. - View Dependent Claims (31, 32, 33, 34, 35, 36)
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Specification