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Compensation method for I/Q channel imbalance errors
| Details |
Inventors: Alelyunas, Carl H.; Norrell, Andrew L.; Parizhsky, Vladimir;
Assignee: U.S. Robotics Access Corp. (Lincolnshire, IL)
Primary Examiner: Mis; David
Assistant Examiner:
Attorney, Agent or Firm: McDonnell Boehnen Hulbert & Berghoff
A method in which correctable I/Q imbalance errors in a complex receiver can be detected and compensated for digitally without the use of special calibration signals. Differential D.C. offset errors are compensated by averaging the incoming I.sub.d and Q.sub.d digital signals and subtracting from them an expected value of differential D.C. offset, for example, computed from the long term average of the I and Q signals to create I' and Q' signals. Differential gain imbalance errors are corrected by calculating a root means square average of the I' and Q' digital signals and applies to them compensation coefficients K.sub.x and K.sub.y determined from either the RMS average or from a Stochastic Gradient Algorithm. The DSP compensates for the quadrature phase errors by calculating a compensation matrix which is independent of the frequency of the carrier and applies the compensation matrix to the I' and Q' digital signals. The compensation matrix for quadrature phase errors is completely independent of the frequency of the input carrier signal supplied to the complex receiver, and is not dependent on the use of a calibration signal. The compensation may be performed as a step in calibration of the complex receiver, or continuously. |
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Generally, the invention contemplates a method by which correctable I/Q imbalance errors in a complex receiver can be detected and compensated for without the use of special calibration signals.
A preferred implementation of the invention is shown in the FIGURE.
The system 10 includes a complex receiver 12 which receives an analog radio frequency (RF) carrier input signal represented by the function S(t), and generates I and Q analog output signal in a manner well known in the art.
The I and Q output signals from the complex receiver are converted to digital signals by an A/D converter 14, and are then supplied to a digital signal processor (DSP) referred to herein as a "compensator" 16.
After compensation within the compensator 16, and referring to the far right portion of the illustration, the compensator 16 outputs digital I'" and Q'" signals on output busses 18A and 18B for subsequent use and processing by the DSP 16, or other circuitry in a manner well known in the art.
In the FIGURE, in one possible embodiment of the invention the complex receiver 12 and compensator 16 are incorporated into a cell phone, or, alternatively, a cell-phone modem.
The system 10 uses complex (I/Q) modulation and demodulation techniques for generating and receiving frequency modulated or FM signals.
Preferably, the modulating and demodulating is done mathematically, using the DSP compensator 16, as opposed to analog methods.
There are three primary sources of I and Q imbalances in the I and Q analog signals from the complex receiver 12: differential DC offset, differential gain imbalance and quadrature phase errors.
Preferably, a capability of correction of all three types of errors is provided for in the compensator 16.
Whether, at any one instant, all three types of imbalances are being corrected for (instead of just one or two types of imbalances) depends upon their magnitude, and on the ultimate impairment in the received signal that the system 10 can tolerate
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