Method and apparatus for synthesizing a modulated carrier to reduced interchannel interference in a digital communication system
First Claim
1. Apparatus for use in a digital communication system for synthesizing a digital-data-modulated carrier waveform for transmission via a selected medium, said digital data being a sequence of binary data bits each having one of first and second binary data values, and said carrier waveform is phase-shift-keyed by said binary data, comprising:
- read-only memory means for storing at each of a multiplicity of individually addressable memory locations a data word defining each of a sequential plurality of carrier amplitudes;
counter means having an input enabled at a clock frequency for providing an output signal sequentially and cyclically counting the input clock frequency, said count output signal coupled to said memory means to sequentially and cyclically read the carrier-amplitude-defining data words stored therein from the memory locations of said memory means;
means receiving the sequential data words cyclically read out of said read-only memory means for converting each data word to an associated amplitude of an analog voltage, said analog voltage being a step-wise representation of said carrier waveform having a desired carrier frequency and a desired envelope shape;
means for low-pass filtering said analog voltage to provide a substantially sinusoidal representation of said carrier waveform;
means for modulating the substantially sinusoidal carrier waveform in accordance with each bit of digital data to be transmitted, and including means receiving the carrier waveform from said low-pass-filtering means for inverting the phase thereof;
means receiving said binary data for selecting the output of said low-pass-filtering means for transmission of said modulated carrier waveform having said first binary data value and for selecting the output of said phase inverting means for transmission of said modulated carrier waveform having said second binary data; and
means for coupling the data modulated carrier waveform to said selected medium.
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Abstract
The modulated carrier in a frequency-multiplexed digital communications system is synthesized with each band of the modulated carrier derived from data stored in a read-only-memory. The carrier envelope shape is selected to reduce the sidebands of the digitally modulated carrier and thus reduce interchannel interference. The read-only-memory digital output is applied to a digital-to-analog converter for synthesis of an analog voltage, representing the modulated sinusoidal carrier, having reduced higher frequency content, without the necessity for use of a costly, narrow-bandpass filter.
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Citations
20 Claims
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1. Apparatus for use in a digital communication system for synthesizing a digital-data-modulated carrier waveform for transmission via a selected medium, said digital data being a sequence of binary data bits each having one of first and second binary data values, and said carrier waveform is phase-shift-keyed by said binary data, comprising:
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read-only memory means for storing at each of a multiplicity of individually addressable memory locations a data word defining each of a sequential plurality of carrier amplitudes; counter means having an input enabled at a clock frequency for providing an output signal sequentially and cyclically counting the input clock frequency, said count output signal coupled to said memory means to sequentially and cyclically read the carrier-amplitude-defining data words stored therein from the memory locations of said memory means; means receiving the sequential data words cyclically read out of said read-only memory means for converting each data word to an associated amplitude of an analog voltage, said analog voltage being a step-wise representation of said carrier waveform having a desired carrier frequency and a desired envelope shape; means for low-pass filtering said analog voltage to provide a substantially sinusoidal representation of said carrier waveform; means for modulating the substantially sinusoidal carrier waveform in accordance with each bit of digital data to be transmitted, and including means receiving the carrier waveform from said low-pass-filtering means for inverting the phase thereof; means receiving said binary data for selecting the output of said low-pass-filtering means for transmission of said modulated carrier waveform having said first binary data value and for selecting the output of said phase inverting means for transmission of said modulated carrier waveform having said second binary data; and means for coupling the data modulated carrier waveform to said selected medium. - View Dependent Claims (2, 3, 4)
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5. Apparatus for use in a digital communication system for synthesizing a digital-data-modulated character waveform for transmission via a selected medium, comprising:
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read-only memory means for storing at each of a multiplicity of individually addressable memory locations a data word defining each of a sequential plurality of carry amplitudes, the data words stored in said multiplicity of memory locations in said read-only memory means defining each of a sequential plurality of band-pass-filtered carrier amplitudes for the carrier waveform in a first half of a time interval during which a single bit of data is to be transmitted; bi-directional up/down counter means having an input enabled at a clock frequency, an output at which is provided an output signal sequentially and cyclically counting the input clock frequency, and a control input enabling the count at said output to respectively increase and decrease responsive to first and second control signals present thereat; means receiving the sequential data words cyclically read out of said read-only memory means for converting each data word to an associated amplitude of an analog voltage, said analog voltage being a step-wise representation of said carrier waveform having a desired carrier frequency and a desired envelope shape; control means receiving said clock frequency for providing said first control signal to said counter control input to enable said output count ot sequentially address the plurality of memory locations defining said carrier amplitudes during said first half of said bit time interval and providing said second control signal to said counter control input for reversing the count of said counter to cause said memory means addresses to sequentially return through all memory location addresses during the second half of said bit time interval, to provide therein carrier amplitudes which are the mirror image of the carrier amplitudes provided during said first half of said bit time interval; means for low-pass filtering said analog voltage to provide a substantially sinusoidal representation of said carrier waveform; and means for modulating the substantially sinusoidial carrier waveform in accordance for coupling data modulated carrier waveform to said selected medium. - View Dependent Claims (6, 7, 8, 9, 10)
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11. A method of synthesizing a digital data modulated carrier waveform for transmission in a digital communication system, wherein said digital data is a sequence of binary data bits each having one of first and second binary data values, comprising the steps of:
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(a) storing a multiplicity of data words each defining a sequential one of a multiplicity of amplitudes of a carrier waveform having a selected envelope shape; (b) sequentially and cyclically reading each of the data words; (c) converting each of the sequentially cyclically read data words directly to an associated amplitude of an analog voltage to provide a step-wise representation of said carrier waveform at a desired carrier frequency and having a desired envelope shape; (d) filtering the step-wise carrier waveform representation analog voltage to reduce the harmonic content thereof at frequencies above the desired carrier frequency; and (e) simultaneously providing said carrier waveform analog voltage at a reference phase and at another phase substantially shifted 180°
therefrom; and(f) selecting one of the reference phase and substantially 180°
shifted phase for transmission responsive to a data bit respectively having one of the respective first and second binary data values. - View Dependent Claims (12, 13, 14)
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15. A method for synthesizing a modulated data carrier waveform for transmission in a digital communication system, comprising the steps of:
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(a) storing a multiplicity of data words each defining a sequential one of a multiplicity of carrier amplitudes of a carrier waveform having a selected envelope shape for only a first half of a time interval in which a data bit is to be transmitted; (b) sequentially reading all the stored data words in a first direction during the first half of the data bit time interval; (c) then reading the stored data words in reverse order during a remaining half of the data bit time interval to provide a mirror image of carrier waveform amplitudes generated during the first half of the data bit time interval; (d) converting each of the sequentially cyclically read data words to an associated amplitude of an analog voltage to provide a step-wise representation of said carrier waveform at a desired carrier frequency and having a desired envelope shape; (e) filtering the step-wise carrier waveform representation analog voltage to reduce the harmonic content thereof at frequencies above the desired carrier frequency; and (f) varying the phase of the filtered carrier waveform analog voltage in accordance with the data value of each bit of the digital data to be transmitted. - View Dependent Claims (16, 17, 18, 19, 20)
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Specification