Radio frequency control for communications systems

• US 6,463,266 B1
• Filed: 01/05/2000
• Issued: 10/08/2002
• Est. Priority Date: 08/10/1999
• Status: Expired due to Fees

• Current Assignee: Avago Technologies General IP PTE Limited (Broadcom, Inc.)
• Original Assignee: Broadcom Corporation (Broadcom, Inc.)
• Inventors: Shohara, Aki
• Primary Examiner(s): Trost, William
• Assistant Examiner(s): Tran, Congvan
• Application Number:
  US09/477,720
• Time in Patent Office: 1,007 Days
• Field of Search: 455/196.1, 455/95, 455/219, 455/20, 455/21, 455/22, 455/24, 455/550, 455/91, 455/575, 455/130, 455/131, 455/138, 455/139, 455/182.1, 455/182.2, 455/192.1, 455/192.2, 455/197.1, 375/279, 375/219, 375/296, 375/303, 375/334, 375/208, 375/341, 375/345, 714/100, 714/48, 714/746
• US Class Current: 455/196.1
• CPC Class Codes: H03J 7/04 where the frequency control is accomplished by varying the electrical characteristics of a non-mechanically adjustable element or where the nature of the frequency controlling element is not significant
• CPC Class Codes: H04B 1/26 for superheterodyne receivers multiple frequency-changing H03D7/16
• CPC Class Codes: H04L 2027/0022 using the carrier of the associated receiver of a transceiver
• CPC Class Codes: H04L 2027/0028 at passband only

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.

Claims

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.

2. The system of claim 1 wherein said first inducing means comprises a downlink phase rotator means 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.

3. The system of claim 1 wherein said second inducing means comprises an uplink phase rotator means 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.

4. The system of claim 1 wherein

(a) said first inducing means comprises a downlink phase rotator means 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, and

(b) said second inducing means comprises an uplink phase rotator means 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.

5. The system of claim 1 wherein

(a) said second inducing means comprises an uplink phase rotator means 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, and

(b) means for frequency upconversion of said phase-rotated complex uplink baseband digital samples to produce the radio transmission signal.

6. 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:

(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 maintain ed in a track mode, and frequency quantization error is substantially reduced relative to conventional AFC loops employing VCXO frequency control.

7. The method of claim 6 wherein said step of inducing a downlink frequency offset comprises rotating a complex phase of said received digital baseband complex samples to induce a downlink phase rotator frequency offset command.

8. The method of claim 6 wherein said step of inducing an uplink frequency offset comprises rotating a 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.

9. The method of claim 6 wherein

(a) said step of inducing a downlink frequency offset comprises rotating a complex phase of said received digital baseband complex samples to induce a downlink phase rotator frequency offset command, and

(b) said step of inducing an uplink frequency offset comprises rotating a 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.

10. The method of claim 6 wherein

(a) said step of inducing an uplink frequency offset comprises rotating a 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 and

(b) said step of converting said uplink frequency offset digital data modulation samples comprises rotating said complex uplink baseband modulation samples to analog form and upconverting frequency to the radio transmission frequency of the local unit.

11. 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.

12. The system of claim 11 wherein said reference oscillator is a VCXO with associated digital to analog converter (DAC) to support digital control of said VCXO wherein the VCXO frequency is linearly responsive to digital VCXO command input to said DAC, further comprising:

(a) an AFC acquisition mode, said AFC acquisition mode including initialization means for a VCXO command to be set to a nominal frequency command, first AFC means to produce a VCXO command update for reduction of transmitter and frequency error in response to said frequency error estimate, and second AFC means for successive repetition of said VCXO updates for successive reduction of initial receiver and transmitter frequency error to a magnitude that is small enough for accurate data demodulation and transition to a track mode, and wherein downlink and uplink phase rotator commands are fixed at zero value during said acquisition mode,

(b) a track mode, said track mode including third AFC means for production of the uplink and downlink phase rotator commands to null receiver and transmitter frequency error in response to estimation of frequency error in accordance with linear feedback control loop principles, and

(c) during said track mode fourth AFC means for scheduled adjustment of the VCXO command for reduction of the magnitude of downlink phase rotator offset command whenever the magnitude of said offset command exceeds a pre-determined limit L, and fifth AFC means for scheduled simultaneous adjustments to downlink phase rotator and uplink phase rotator frequency offset commands such that the step in receiver and transmitter frequency error caused by quantization error from said VCXO frequency adjustment are substantially and instantaneously canceled by said phase rotator adjustments, said scheduling of VCXO command, downlink frequency offset command, and uplink frequency offset command timed to occur during an idle interval when neither transmission nor reception by the local unit is in progress,

whereby in acquisition mode initial frequency errors of the frequency reference such as caused by crystal temperature sensitivity and aging are substantially removed,

whereby in the tracking mode local unit ratio transmission and reception frequency errors are minimized by use of uplink and downlink phase rotators having relatively fine frequency resolution as the primary means of frequency tracking adjustment, with the relatively coarse VCXO frequency adjustments utilized only in the contingency that the downlink phase rotator frequency offset magnitude exceeds a predetermined limit, and

whereby in the tracking mode a VCXO frequency adjustment occurs whenever the downlink frequency offset of the downlink phase rotator exceeds a predetermined limit, thereby providing a mechanism for maintenance of a maximum permissible offset of received and transmitted signals relative to respective receiver and transmitter filter center frequencies, and

whereby in the tracking mode whenever said VCXO frequency adjustment does occur, the resultant effects of a jump in VCXO reference frequency on signal transmission and reception frequency by the local unit are substantially mitigated by cancellation by downlink and uplink phase rotators, and

whereby in track mode scheduling of said VCXO adjustment is scheduled to occur when transmitter and receiver are idle.

13. The system of claim 12 wherein said AFC acquisition mode further comprises means for measurement of VCXO temperature and means for addition of an increment to the VCXO nominal frequency command wherein said increment is dependent on said temperature measurement and calculated to cancel the VCXO reference frequency error caused by VCXO temperature sensitivity,

whereby the receiver and transmitter frequency error present at the initiation of said acquisition mode due to VCXO temperature sensitivity is substantially removed, thereby facilitating reduction in acquisition time or the use of a lower cost VCXO.

14. The system of claim 12 wherein said AFC acquisition mode further comprises:

(a) means for storage of computational parameters in nonvolatile memory,

(b) means for storage of the VCXO command for nominal reference frequency in said non-volatile memory, and

(c) means to update said VCXO command for nominal reference frequency at the end of each successful acquisition mode,

whereby VCXO frequency error caused by VCXO crystal aging accumulated since prior local unit operation is substantially removed after each successful acquisition mode by the local unit,

thereby facilitating reduction in acquisition time or the use of a lower cost VCXO.

15. The system of claim 12 wherein said fifth AFC during track mode produces downlink phase rotator frequency offset commands in accordance with one of a linear Type 2 or higher order tracking loop controller,

whereby receiver and transmitter frequency errors having the form of ramp or higher order frequency changes caused by dynamic temperature conditions are tracked out by the downlink and uplink phase rotator frequency offset commands in accordance with linear feedback control principles,

thereby substantially reducing receiver and transmitter frequency errors in track mode caused by dynamic temperature variations of the VCXO.

16. The system of claim 12 further comprising:

(a) means during the track mode for receiving and demodulating control data received from the remote unit wherein said control data indicates local unit transmitter frequency error as measured by the remote unit, and

(b) AFC controller means in said track mode for subtracting said transmitter frequency error measurement from the uplink rotator frequency offset command,

whereby local unit transmitter error caused by Doppler shift is removed.

17. The system of claim 12 wherein:

(a) the uplink data modulation is of a class of continuous phase modulation, and

(b) the data modulator means are configured so that phase is an explicit modulator processing variable, and

(c) the uplink phase rotator means produce phase rotation by addition of incremental phase to said modulator phase variable to induce a frequency offset commensurate with the uplink frequency offset command,

whereby complexity of the uplink phase rotator is substantially reduced because of the absence of complex multiplication operations ordinarily required to produce uplink phase rotation.

18. The system of claim 11 wherein said reference oscillator is a VCXO with associated digital to analog converter (DAC) for digital control and the VCXO frequency is responsive to digital VCXO command input to said DAC, further comprising:

(a) an AFC acquisition mode, said AFC acquisition mode including sixth AFC means to produce a VCXO frequency update wherein a VCXO command is produced for reduction of transmitter and frequency error in response to said frequency error estimate, and AFC controller means for successive repetition of said VCXO updates for successive reduction of initial receiver and transmitter frequency error to a magnitude that is small enough for reliable data demodulation and transition to a track mode, wherein downlink and uplink phase rotator commands are fixed at zero value during said acquisition mode,

(b) a track mode, said track mode including seventh AFC means for production of the uplink and downlink phase rotator commands to null receiver and transmitter frequency error in response to estimation of frequency error in accordance with linear feedback control loop principles, and

(c) a fixed VCXO command in said track mode,

whereby in said acquisition mode initial frequency errors of the frequency reference such as caused by crystal temperature sensitivity and aging are sufficiently reduced for reliable data demodulation,

whereby in the track mode local unit ratio transmission and reception frequency errors are substantially reduced by use of uplink and downlink phase rotators having relatively fine frequency resolution in comparison to VCXO corrections normally used and having relatively coarse frequency resolution.

19. The system of claim 12 wherein parameter L therein set to infinity.

20. 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:

(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.

21. The method of claim 20 wherein said crystal oscillator is a VCXO with associated digital-to-analog converter (DAC) to support digital control of said VCXO wherein the VCXO frequency is linearly responsive to digital VCXO command input to said DAC, further comprising the steps of:

(a) in an acquisition mode, first initializing the VCXO command to a nominal frequency command, then producing a VCXO command update to reduce the estimated frequency error, and successively repeating said VCXO updating to successively reduce transmitter and receiver frequency errors to a magnitude that is small enough for accurate data demodulation and transition to a track mode wherein downlink and uplink phase rotator commands are fixed at zero value throughout said acquisition mode,

(b) forming in a track mode an AFC feedback control loop by using estimated frequency errors to produce uplink and downlink phase rotator frequency offset commands to null receiver and transmitter frequency errors in accordance with feedback control loop principles, and

(c) adjusting the VCXO command during said track mode, according to a schedule in order to reduce the magnitude of said downlink phase rotator offset command whenever said magnitude exceeds pre-determined limit L, and simultaneously adjusting uplink and downlink phase rotator frequency offset commands such that the step in receiver and transmitter frequency error caused by quantization error from said VCXO adjustment are substantially and instantaneously canceled by said phase rotator adjustments, and scheduling the VCXO adjustments to occur during an idle interval when neither signal transmission nor reception by the local unit occurs,

whereby in acquisition mode initial frequency errors of the frequency reference such as caused by crystal temperature sensitivity and aging are substantially removed,

whereby in tracking mode local unit ratio transmission and reception frequency errors are minimized by use of uplink and downlink phase rotators having relatively fine frequency resolution as the primary means of frequency tracking adjustment, with the relatively coarse VCXO frequency adjustments utilized only in the contingency that the downlink phase rotator frequency offset magnitude exceeds a predetermined limit, and

whereby in tracking mode a VCXO frequency adjustment occurs whenever the downlink frequency offset of the downlink phase rotator exceeds a predetermined limit, thereby providing a control mechanism for imposition of a maximum permissible offset of received and transmitted signals relative to respective receiver and transmitter filter center frequencies, and whenever said contingency VCXO frequency adjustment does occur, the resultant effects of a jump in VCXO reference frequency on signal transmission and reception frequency by the local unit are substantially mitigated by cancellation by downlink and uplink phase rotators and scheduling the adjustment to occur when transmitter and receiver are idle.

22. The method of claim 21 further comprising the steps of:

(a) measuring VCXO temperature at initiation of an acquisition mode, and

(b) adding an increment dependent on said temperature measurement to said VCXO nominal frequency command wherein the increment is calculated to cancel the VCXO frequency error caused by VCXO temperature sensitivity,

whereby the receiver and transmitter frequency errors present at the initiation of acquisition mode due to VCXO temperature sensitivity are substantially removed

thereby facilitating reduction in acquisition time or the use of a lower cost VCXO.

23. The method of claim 21 during acquisition mode further comprising the steps of:

(a) storing said VCXO nominal frequency command in non-volatile memory, and

(b) updating the VCXO nominal frequency command stored in non-volatile memory at the end of each successful acquisition mode, wherein said updating is based on VCXO adjustments accumulated during said acquisition mode,

whereby VCXO frequency error accumulated due to VCXO crystal aging since prior local unit operation is substantially removed after each successful acquisition mode operation by the local unit,

thereby facilitating reduction in acquisition time or the use of a lower cost VCXO.

24. The method of claim 22 further comprising the steps of producing the uplink and downlink phase rotator frequency offset commands in accordance with one of linear Type 2 or higher order tracking loop controllers,

whereby receiver and transmitter frequency offsets having the form of ramp input or higher order frequency change induced in the VCXO by dynamic temperature conditions are tracked out by the downlink and uplink phase rotator frequency offset commands in accordance with linear feedback control principles,

thereby substantially reducing receiver and transmitter frequency errors in track mode caused by dynamic temperature variations of the VCXO.

25. The method of claim 22 further comprising the steps of:

(a) receiving and demodulating in the track mode control data from the remote unit indicating local unit transmitter frequency error as measured by the remote unit, and

(b) subtracting in the track mode said transmitter frequency error measurement from the uplink frequency offset command,

whereby local unit transmitter frequency error caused by Doppler is removed.

26. The method of claim 22 wherein the data modulation is of a class of continuous phase data modulation and further comprising the steps of:

(a) producing the modulated baseband complex samples such that phase appears explicitly as a phase processing variable, and

(b) adding the uplink phase rotator incremental phase values directly to said phase processing variable to induce a frequency offset commensurate with the uplink frequency offset command,

whereby complexity of the uplink phase rotator is substantially reduced because of the absence of multiplication operations ordinarily required to produce uplink phase rotation.

27. The method of claim 21 wherein said reference oscillator is a VCXO with associated digital-to-analog converter (DAC) for digital control and the VCXO frequency is responsive to digital VCXO command input to said DAC, further comprising the steps of:

(a) in an acquisition mode, first initializing the VCXO command to a nominal frequency command, then producing a VCXO command update to reduce the estimated frequency error, and successively repeating said VCXO updating to successively reduce transmitter and receiver frequency errors to a magnitude that is small enough for accurate data demodulation and transition to a track mode, wherein with downlink and uplink phase rotator commands are fixed at zero value throughout said acquisition mode,

(b) using in a track mode said receiver frequency error estimate for producing the uplink and downlink phase rotator frequency offset commands in accordance with feedback control loop principles, and

(c) using a fixed value of VCXO command in said track mode,

whereby in the acquisition mode initial frequency errors of the frequency reference such as caused by crystal temperature sensitivity and aging are reduced sufficiently for accurate data demodulation and transition to said track mode,

whereby in the tracking mode local unit ratio transmission and reception frequency errors are substantially reduced by use of uplink and downlink phase rotators having relatively finer frequency resolution in comparison to VCXO corrections as normally used.

28. The method of claim 21 wherein the predetermined limit is set to infinity.

29. 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,

(d) a digital data modulator means for generation of complex uplink baseband digital samples containing data modulation,

(e) second inducing means for inducing a second frequency offset in the uplink baseband digital samples,

(f) means for digital-to-analog conversion of said frequency offset uplink baseband digital samples,

(g) means for frequency upconversion to produce a radio transmission signal, and

(h) 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, and

wherein said first inducing means comprises a downlink phase rotator means 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, and

wherein said second inducing means comprises an uplink phase rotator means 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, and

wherein means for frequency upconversion comprises means for frequency upconversion of said phase-rotated complex uplink baseband digital samples to produce the radio transmission signal.

30. The system of claim 29 further comprising:

(a) frequency estimator means for estimation of receiver frequency error by processing of said phase rotated received baseband digital samples,

(b) reference oscillator means to produce a frequency reference signal,

(c) frequency synthesizer means and voltage controlled oscillator means (VCO) to produce local oscillator signals to serve as mixer signals for said frequency upconversion means and frequency downconversion means, said mixer signals being phase locked to said frequency reference signal,

(d) AFC means operating in a track mode to produce said downlink frequency offset command for the downlink phase rotator, the offset command forming a feedback control loop responsive to said receiver frequency error estimate, and

(e) AFC means to generate said uplink frequency offset command, the value of the uplink frequency offset command being related to the downlink frequency offset command by a scale factor.

31. 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:

(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, and

wherein said step of inducing a downlink frequency offset comprises rotating a complex phase of said received digital baseband complex samples to induce a downlink phase rotator frequency offset command, and

wherein said step of inducing an uplink frequency offset comprises rotating a 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, and

wherein said step of converting said uplink frequency offset digital data modulation samples comprises rotating said complex uplink baseband modulation samples to analog form and upconverting frequency to the radio transmission frequency of the local unit.

32. The method of claim 31 further comprising the steps of:

(a) estimating receiver frequency error by processing said phase rotated received baseband digital complex samples,

(b) producing 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

(c) producing said uplink frequency offset command with a value equal to the downlink frequency offset command multiplied by a scale factor.

33. 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:

(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,

(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,

(h) producing a frequency reference signal by a reference oscillator, and

(i) producing local oscillator signals to serve as mixer signals for respective frequency upconversion and downconversion operations by phase locking local oscillator signals to said frequency reference signal.