DC cancellation and restoration in receiving apparatus
First Claim
1. A method of processing a received signal comprising a carrier wave modulated with digital data signals, said method comprising the steps of:
- (a) demodulating the received signal to produce a set of digital baseband signal-value pairs, each pair being comprised of an inphase signal value and a corresponding quadrature signal value such that, when plotted on an I-Q diagram with the inphase signal value of each pair denoting distance of a plotted point from a Q-axis, and the corresponding quadrature signal value of that pair denoting distance of said plotted point from an I-axis that is orthogonal to said Q-axis, said plotted points would lie substantially on a first circle whose centre is offset from the origin of the diagram, said origin being at the intersection of said I- and Q-axes;
(b) averaging said inphase signal values of said pairs to produce an I-direction mean value, and averaging said quadrature signal values of said pairs to produce a Q-direction mean value;
(c) subtracting said I-direction mean value and said Q-direction mean value respectively from the inphase signal value and the quadrature signal value of each of said signal-value pairs so as to produce a set of adjusted signal-value pairs which, when plotted on said I-Q diagram, would produce a new set of plotted points lying substantially on a second circle such that said origin lies within the second circle;
(d) defining first, second, third and fourth regions of the I-Q diagram, the first and third regions being symmetrical to one another with respect to said Q-axis, and the second and fourth regions being symmetrical to one another with respect to said I-axis;
(e) determining for each of the said first and third regions, an average distance between said Q-axis and the plotted points of the new set that are located within the region concerned, and using a difference between the average distances of the first and third regions to produce an I-direction shift which, when subtracted from the inphase signal values of the points of the new set, causes the centre of the second circle to move closer to said Q-axis;
(f) determinings for each of the said second and fourth regions, the average distance between said I-axis and the plotted points of the new set that are located within the region concerned, and using a difference between the average distances of the second and fourth regions, to produce a Q-direction shift value which, when subtracted from the quadrature signal values of the plotted points of the new set, would cause the centre of the second circle to move closer to said I-axis; and
(g) subtracting said I-direction shift value from each of said inphase signal values of the plotted points of said new set, and subtracting said quadrature signal values of said plotted points, and delivering the resulting inphase and quadrature values as output signals.
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Abstract
In receiving apparatus of a digital communication system, a received signal burst is demodulated to produce inphase (I) and quadrature (Q) baseband signals which are digitized to provide a set of signal-value pairs for the burst. Each pair consists of an I-value and a corresponding Q-value. The modulation is such that the pairs, if plotted on a complex signal space (I-Q) diagram, would lie substantially on a common circle. To cancel DC offsets in the I and Q signal paths, the average I-value and average Q-value over the burst are found and subtracted respectively from the I- and Q-values of each signal-value pair, to shift the circle so the origin of the I-Q diagram lies within the circle. Then, to restore the DC content of the I and Q signals, the distances Ii1, Qi2, Ii3 and Qi4 of signal-value pairs from the I- or Q-axis are averaged in four regions of the I-Q diagram. The difference between the respective average distances for mutually-opposed regions is used to derive I- and Q-direction shift values which, if subtracted respectively from the I- and Q-values of each signal-value pair, would bring the centre of the circle closer to the origin. This can remove DC offsets accurately, whilst preserving the variable DC content of the received signal, in time periods consistent with the burst repetition rate in communication systems such as Global System for Mobile Communications (GSM), thereby permitting correct operation of an equalizer used to extract data from the I- and Q-values.
58 Citations
13 Claims
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1. A method of processing a received signal comprising a carrier wave modulated with digital data signals, said method comprising the steps of:
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(a) demodulating the received signal to produce a set of digital baseband signal-value pairs, each pair being comprised of an inphase signal value and a corresponding quadrature signal value such that, when plotted on an I-Q diagram with the inphase signal value of each pair denoting distance of a plotted point from a Q-axis, and the corresponding quadrature signal value of that pair denoting distance of said plotted point from an I-axis that is orthogonal to said Q-axis, said plotted points would lie substantially on a first circle whose centre is offset from the origin of the diagram, said origin being at the intersection of said I- and Q-axes; (b) averaging said inphase signal values of said pairs to produce an I-direction mean value, and averaging said quadrature signal values of said pairs to produce a Q-direction mean value; (c) subtracting said I-direction mean value and said Q-direction mean value respectively from the inphase signal value and the quadrature signal value of each of said signal-value pairs so as to produce a set of adjusted signal-value pairs which, when plotted on said I-Q diagram, would produce a new set of plotted points lying substantially on a second circle such that said origin lies within the second circle; (d) defining first, second, third and fourth regions of the I-Q diagram, the first and third regions being symmetrical to one another with respect to said Q-axis, and the second and fourth regions being symmetrical to one another with respect to said I-axis; (e) determining for each of the said first and third regions, an average distance between said Q-axis and the plotted points of the new set that are located within the region concerned, and using a difference between the average distances of the first and third regions to produce an I-direction shift which, when subtracted from the inphase signal values of the points of the new set, causes the centre of the second circle to move closer to said Q-axis; (f) determinings for each of the said second and fourth regions, the average distance between said I-axis and the plotted points of the new set that are located within the region concerned, and using a difference between the average distances of the second and fourth regions, to produce a Q-direction shift value which, when subtracted from the quadrature signal values of the plotted points of the new set, would cause the centre of the second circle to move closer to said I-axis; and (g) subtracting said I-direction shift value from each of said inphase signal values of the plotted points of said new set, and subtracting said quadrature signal values of said plotted points, and delivering the resulting inphase and quadrature values as output signals. - View Dependent Claims (2, 3, 4)
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5. An apparatus for processing a received signal comprising a carrier wave modulated with digital data signals, which apparatus comprises:
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demodulation means for demodulating the received signal to produce a set of digital baseband signal-value pairs, each pair being comprised of an inphase signal value and a corresponding quadrature signal value such that, when plotted on an I-Q diagram with the inphase signal value of each pair denoting distance of a plotted point from a Q-axis, and the quadrature signal value of that pair denoting distance of said plotted point from an I-axis that is orthogonal to said Q-axis, the plotted points would lie substantially on a first circle whose centre is offset from the origin of the diagram, said origin being at the intersection of the I- and Q-axes; averaging means for averaging said inphase signal values to produce an I-direction means value, and for averaging said quadrature signal values to produce a Q-direction mean value; first adjustment means for subtracting said I-direction means value and said Q-direction mean value;
respectively, from the inphase signal value and the quadrature signal value of each of said signal-value pairs so as to produce a set of adjusted signal-value pairs which, when plotted on said I-Q diagram, would produce a new set of plotted points lying substantially on a second circle such that said origin lies within that circle;region defining means for defining first, second, third and fourth regions of the I-Q diagram, the first and third regions being symmetrical to one another with respect to said Q-axis, and the second and fourth regions being symmetrical to one another with respect to said I-axis; I-shift calculating means coupled to said region defining means and operable, for each of said first and third regions, for determining an average distance between said Q-axis and said plotted points of the new set that are located within the region concerned, and using a difference between the determined average distances of the first and third regions to produce an I-direction shift value which, when subtracted from the inphase signal values of the plotted points of the new set, would cause the centre of the second circle to move closer to said Q-axis; Q-shift calculating means coupled to said region defining means and operable, for each of said second and fourth regions, for determining an average distance between said I-axis and said plotted points of the new set that are located within the region concerned, and using a difference between the determined average distances of the second and fourth regions to produce a Q-direction shift value which, when subtracted from the quadrature signal values of the plotted points of the new set, would cause the centre of the second circle to move closer to said I-axis; and second adjustment means for subtracting said I-direction shift value from each of the said inphase signal values of the points of said new set, and for subtracting said Q-direction shift value from each of said quadrature signal values of the plotted points of the new set, and for delivering the resulting inphase and quadrature values as output signals. - View Dependent Claims (6, 7, 8, 9, 10, 11, 12, 13)
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Specification