Radio frequency control for communication systems
First Claim
1. A system for reducing transmitter and receiver radio frequency error in a local unit, wherein the local unit transmits and receives radio signals bearing digital data modulation to and from a remote unit, said local unit comprising (a) a frequency downconversion means and an analog-to-digital conversion means for converting said received radio signals into received complex baseband digital signal samples, (b) first inducing means for inducing a first frequency offset in the baseband digital signal samples, (c) a digital data demodulator to extract the received data from rotated received digital samples, (e) a digital data modulator means for generation of complex uplink baseband digital samples containing data modulation, (f) second inducing means for inducing a second frequency offset in the uplink baseband digital samples, (g) means for digital-to-analog conversion of said frequency offset uplink baseband digital samples, (h) means for frequency upconversion to produce a radio transmission signal, and (i) whereby accurate transmitter and receiver frequencies are maintained in a track mode, and frequency quantization errors are substantially reduced relative to AFC loops that employ conventional VCXO frequency control means.
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Accused Products
Abstract
The present invention provides for a system and method for improvement of radio transmitter and receiver frequency accuracy for a local radio communication unit that communicates digital data with a remote communication unit. In the local unit the received radio signal is down-converted, and converted to complex baseband digital samples by an analog-to-digital converter. A downlink digital phase rotator applies a fine frequency shift to the samples in accordance with a receiver frequency offset command. The resultant baseband signal is used by the data demodulator and by a receiver frequency error estimator to obtain receiver frequency errors. A data modulator generates baseband complex samples which are shifted in carrier frequency by an integrated uplink digital phase rotator in accordance with a transmitter frequency offset command. The modulated samples are then converted by a digital-to-analog converter and upconverted in frequency for radio transmission to the remote unit. The local oscillator signals for both upconverter and downconverter are phase locked to a reference frequency generated by a VCXO. An automatic frequency control (AFC) function nulls the transmitter and receiver frequency error by the frequency adjustment commands to the uplink and downlink phase rotators or to the VCXO digital-to-analog converter (VCXO DAC) by feedback control principals based on measured receiver frequency error. During frequency track mode when communications between local and remote units are possible, the AFC only adjusts radio frequency via phase rotator commands and the VCXO command remains fixed, thereby avoiding communications performance degradation by VCXO frequency quantization error due to the VCXO DAC. The AFC adjusts VCXO frequency only during a preliminary acquisition mode prior to data communications, or to back out excessively large frequency offsets accumulated in the downlink and uplink phase rotators during track mode. When a VCXO adjustment is made in track mode, phase rotator adjustments are simultaneously applied to cancel the errors in transmitter and receiver radio frequencies caused by the step change due to VCXO frequency quantization thereby mitigating VCXO frequency quantization noise.
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Citations
25 Claims
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1. A system for reducing transmitter and receiver radio frequency error in a local unit, wherein the local unit transmits and receives radio signals bearing digital data modulation to and from a remote unit, said local unit comprising
(a) a frequency downconversion means and an analog-to-digital conversion means for converting said received radio signals into received complex baseband digital signal samples, (b) first inducing means for inducing a first frequency offset in the baseband digital signal samples, (c) a digital data demodulator to extract the received data from rotated received digital samples, (e) a digital data modulator means for generation of complex uplink baseband digital samples containing data modulation, (f) second inducing means for inducing a second frequency offset in the uplink baseband digital samples, (g) means for digital-to-analog conversion of said frequency offset uplink baseband digital samples, (h) means for frequency upconversion to produce a radio transmission signal, and (i) whereby accurate transmitter and receiver frequencies are maintained in a track mode, and frequency quantization errors are substantially reduced relative to AFC loops that employ conventional VCXO frequency control means.
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4. A method for reducing transmitter and receiver radio frequency error in a local unit, wherein the local unit transmits and receives radio signals bearing digital data modulation to and from a remote unit, the method comprising the steps of:
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(a) downconverting the radio frequency of a received radio signal to a complex analog baseband signal and further converting said analog baseband signal to digital baseband complex samples, (b) inducing a downlink frequency offset in the digital baseband complex samples, (c) demodulating modulated data contained in the downlink frequency offset digital samples, (d) producing complex uplink baseband digital data modulation samples for transmission to a remote unit, (e) inducing an uplink frequency offset in the uplink baseband digital data modulation samples, (f) converting said uplink frequency offset digital data modulation samples to analog form, and (g) upconverting said converted uplink frequency offset modulation samples to the radio transmission frequency of a local unit, whereby accurate transmitter and receiver frequencies are maintained in a track mode, and frequency quantization error is substantially reduced relative to conventional AFC loops employing VCXO frequency control. - View Dependent Claims (5, 6, 7, 18, 19, 20, 21, 22, 23, 24, 25)
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8. A system for reducing transmitter and receiver radio frequency error in a local unit,
wherein the local unit transmits and receives radio signals bearing digital data modulation to and from a remote unit, said local unit comprising: -
(a) a frequency downconverter and an analog-to-digital converter for converting said received radio signals into received complex baseband digital signal samples, (b) a downlink phase rotator to produce complex digital phase rotation of said received baseband digital signal samples such that a frequency offset is induced in the samples in accordance with a downlink frequency offset command, (c) a frequency estimator for estimation of the receiver frequency error by processing of said phase rotated received baseband digital samples, (d) a digital data demodulator to extract received data from the rotated received digital samples, (e) a digital data modulator for generation of complex uplink baseband digital samples containing data modulation, (f) an uplink phase rotator to produce complex digital phase rotation of said complex uplink baseband digital samples such that a frequency offset is induced in the uplink samples in accordance with an uplink frequency offset command, (g) a digital-to-analog converter for converting said phase-rotated complex uplink baseband digital samples, (h) a frequency upconverter for frequency upconversion to produce a radio transmission signal, (i) a frequency synthesizer and a voltage controlled oscillator (VCO) to produce local oscillator signals to serve as mixer signals for said frequency upconversion means and frequency downconverter, said mixer signals being phase locked to a frequency reference signal, (j) a reference oscillator to produce said frequency reference signal, and (k) an automatic frequency controller (AFC) means operating in a track mode to produce said downlink frequency offset command for the downlink phase rotator, the downlink frequency offset command forming a feedback control loop responsive to said receiver frequency error estimate, and AFC generator to generate said uplink frequency offset command by a scale factor, whereby accurate transmitter and receiver frequencies are maintained in a track mode, and frequency quantization errors are substantially reduced relative to AFC loops that employ conventional voltage controlled crystal oscillator (VCXO) frequency control means. - View Dependent Claims (9, 10, 11, 12, 13, 14, 15, 16)
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17. A method for reducing transmitter and receiver radio frequency error in a local unit, wherein the local unit transmits and receives radio signals bearing digital data modulation to and from a remote unit, the method comprising the steps of:
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(a) downconverting the radio frequency of said received radio signal to a complex analog baseband signal and further converting said analog baseband signal to digital baseband complex samples, (b) rotating the complex phase of said received digital baseband complex samples to induce a downlink frequency offset in the samples in accordance with a downlink phase rotator frequency offset command, (c) estimating receiver frequency error by processing said phase rotated received baseband digital complex samples, (d) demodulating modulated data contained in the phase rotated received digital complex samples, (e) calculating complex uplink baseband digital data modulation samples for transmission to the remote unit, (f) rotating the complex phase of said uplink baseband digital data modulation samples to induce an uplink frequency offset in the samples in accordance with an uplink phase rotator frequency offset command, (g) converting said rotated complex baseband frequency offset modulation samples to analog form and, (h) upconverting said converted frequency offset modulation samples to the radio transmission frequency of the local unit, (i) generating local oscillator signals to serve as mixer signals for respective frequency upconversion and downconversion operations by phase locking local oscillator signals to a frequency reference signal, (j) generating said frequency reference signal by a reference oscillator, and (k) generating an automatic frequency converter (AFC) feedback control signal from said receiver frequency error estimate to serve as said downlink phase rotator frequency offset command, and producing said uplink phase rotator frequency offset command with a value equal to the downlink frequency offset command multiplied by a scale factor, whereby accurate transmitter and receiver frequencies are maintained in a track mode, and frequency quantization error is substantially reduced relative to conventional AFC loops employing voltage controlled crystal oscillator (VCXO) frequency control.
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Specification