Cancellation of interference in a communication system with application to S-CDMA
View Patent ↗Cancellation of interference in a communication system with application to S-CDMA. A relatively straight-forward implemented and computationally efficient approach of selecting a predetermined number of unused codes is used to perform weighted linear combination selectively with each of the input spread signals in a multiple access communication system. If desired, the predetermined number of unused codes is always the same in each implementation. Alternatively, the predetermined number of unused codes is selected from within a reordered code matrix using knowledge that is shared between the two ends of a communication system, such as between the CMs and a CMTS. While the context of an S-CDMA communication system having CMs and a CMTS is used, the solution is generally applicable to any communication system that seeks to cancel narrowband interference. Several embodiments are also described that show the generic applicability of the solution across a wide variety of systems.
1. An apparatus, comprising:
an equalizer that is operable to:
receive a signal from a communication channel, wherein the signal includes additive white noise and a frequency response of the communication channel includes a null at a frequency; and
equalize the signal using an inverse, zero forcing response that includes a peak at approximately the frequency of the null within the frequency response of the communication channel thereby transforming white noise within the signal to become colored noise located at approximately the frequency; and
a canceler that is operable to perform interference cancellation on the colored noise located at approximately the frequency.
2. The apparatus of claim 1 , wherein:
the signal received from the communication channel is a spread signal;
the canceler includes a vector de-spreader, a weight computation block, and a linear combiner;
the vector de-spreader is operable to employ a first plurality of codes and a second plurality of codes to de-spread the spread signal that is received from the equalizer;
the weight computation block is operable to determine a plurality of complex-valued combining weights using at least one portion of the de-spread signal and the colored noise located at approximately the frequency; and
the linear combiner is operable to:
scale a plurality of de-spread signal portions, that is de-spread using the first plurality of codes, using the plurality of complex-valued combining weights; and
combine the scaled, plurality of de-spread signal portions with one signal portion that is de-spread using one code of the second plurality of codes thereby performing the interference cancellation on the colored noise located at approximately the frequency.
3. The apparatus of claim 2 , wherein:
the weight computation block is operable to calculate one complex-valued combining weight of the plurality of complex-valued combining weights using the one signal portion that is de-spread using one code of the second plurality of codes and adjacent signal portions that are de-spread using codes adjacent to the one code of the second plurality of codes.
4. The apparatus of claim 2 , wherein:
the weight computation block is operable to calculate one complex-valued combining weight of the plurality of complex-valued combining weights using the plurality of de-spread signal portions, that is de-spread using the first plurality of codes, and at least one additional plurality of de-spread signal portions, that is de-spread using the second plurality of codes.
5. The apparatus of claim 2 , wherein:
at least one complex-valued combining weight of the plurality of complex-valued combining weights is a predetermined complex-valued combining weight.
6. The apparatus of claim 1 , wherein:
the signal received from the communication channel is a spread signal;
the canceler includes a vector de-spreader;
the vector de-spreader is operable to employ a first plurality of codes and a second plurality of codes to de-spread the spread signal that is received from the equalizer; and
of first de-spread signal portions that are de-spread using the first plurality of codes and second de-spread signal portions that are de-spread using the second plurality of codes, only the first de-spread signal portions that are de-spread using the first plurality of codes are employed by the canceler to perform interference cancellation on the colored noise located at approximately the frequency.
7. The apparatus of claim 1 , wherein:
the canceler includes a vector de-spreader and a plurality of feed-forward equalizers;
the vector de-spreader is operable to employ a first plurality of codes and a second plurality of codes to de-spread the spread signal that is received from the equalizer;
each feed-forward equalizer of the plurality of feed-forward equalizers includes a tapped delay line and a plurality of combining weights;
each feed-forward equalizer of the plurality of feed-forward equalizers is operable to sum its corresponding de-spread signal portion with delayed and scaled versions of its corresponding de-spread signal portion using the tapped delay line and the plurality of combining weights;
the linear combiner is operable to sum outputs from each feed-forward equalizer of the plurality of feed-forward equalizers and the one signal portion that is de-spread using one code of the second plurality of codes;
the plurality of feed-forward equalizers includes a first number of feed-forward equalizers;
the first plurality of codes and the second plurality of codes together include a second number of codes; and
the first number is less than the second number.
8. The apparatus of claim 1 , wherein:
the canceler includes a plurality of feed-forward equalizers;
each feed-forward equalizer of the plurality of feed-forward equalizers includes a corresponding plurality of equalizer taps; and
each equalizer tap within each corresponding plurality of equalizer taps has a frequency selective response.
9. The apparatus of claim 1 , wherein:
the apparatus is operable to be implemented within an S-CDMA (Synchronous Code Division Multiple Access) communication system.
10. The apparatus of claim 1 , wherein:
the apparatus is operable to be implemented within a cellular communication system that employs at least one type of S-CDMA (Synchronous Code Division Multiple Access) communications.
11. An apparatus, comprising:
an equalizer that is operable to:
receive a spread signal from a communication channel, wherein the spread signal includes additive white noise and a frequency response of the communication channel includes a null at a frequency; and
equalize the signal using an inverse, zero forcing response that includes a peak at approximately the frequency of the null within the frequency response of the communication channel thereby transforming white noise within the signal to become colored noise located at approximately the frequency; and
a canceler, that includes a vector de-spreader, a weight computation block, and a linear combiner, that is operable to perform interference cancellation on the colored noise located at approximately the frequency; and wherein:
the vector de-spreader is operable to employ a first plurality of codes and a second plurality of codes to de-spread the spread signal that is received from the equalizer thereby generating a first plurality of de-spread signal portions and a second plurality of de-spread signal portions;
the weight computation block is operable to determine a plurality of complex-valued combining weights using at least one portion of the de-spread signal and the colored noise located at approximately the frequency; and
the linear combiner is operable to:
scale the first plurality of de-spread signal portions using the plurality of complex-valued combining weights; and
combine the scaled, plurality of de-spread signal portions with one signal portion that is de-spread using one code of the second plurality of codes thereby performing the interference cancellation on the one signal portion that is de-spread using one code of the second plurality of codes that corresponds to the colored noise located at approximately the frequency; and
the apparatus is operable to be implemented within an S-CDMA (Synchronous Code Division Multiple Access) communication system.
12. The apparatus of claim 11 , wherein:
the weight computation block is operable to calculate one complex-valued combining weight of the plurality of complex-valued combining weights using the one signal portion that is de-spread using one code of the second plurality of codes and adjacent signal portions that are de-spread using codes adjacent to the one code of the second plurality of codes.
13. The apparatus of claim 11 , wherein:
at least one complex-valued combining weight of the plurality of complex-valued combining weights is a predetermined complex-valued combining weight.
14. The apparatus of claim 11 , wherein:
of the first de-spread signal portions and the second de-spread signal portions, only the first de-spread signal portions are employed by the canceler to perform interference cancellation on the colored noise located at approximately the frequency.
15. The apparatus of claim 11 , wherein:
a plurality of numerically values symbols is modulated using the first plurality of codes by at least one additional apparatus when generating the spread the signal; and
the spread signal is launched into the communication channel by the at least one additional apparatus.
16. The apparatus of claim 11 , wherein:
the linear combiner includes a plurality of feed-forward equalizers;
each feed-forward equalizer of the plurality of feed-forward equalizers includes a corresponding plurality of equalizer taps; and
each equalizer tap within each corresponding plurality of equalizer taps has a frequency selective response.
17. A method, comprising:
receiving a spread signal from an S-CDMA (Synchronous Code Division Multiple Access) communication channel, wherein the spread signal includes additive white noise and a frequency response of the communication channel includes a null at a frequency;
equalizing the signal using an inverse, zero forcing response that includes a peak at approximately the frequency of the null within the frequency response of the S-CDMA communication channel thereby transforming white noise within the signal to become colored noise located at approximately the frequency; and
performing interference cancellation on the colored noise located at approximately the frequency.
18. The method of claim 17 , further comprising:
employing a first plurality of codes and a second plurality of codes to de-spread the spread signal;
determining a plurality of complex-valued combining weights using at least one portion of the de-spread signal and the colored noise located at approximately the frequency;
scaling a plurality of de-spread signal portions, that is de-spread using the first plurality of codes, using the plurality of complex-valued combining weights; and
combining the scaled, plurality of de-spread signal portions with one signal portion that is de-spread using one code of the second plurality of codes thereby performing the interference cancellation on the one signal portion that is de-spread using one code of the second plurality of codes that corresponds to the colored noise located at approximately the frequency.
19. The method of claim 17 , further comprising:
calculating at least one complex-valued combining weight of the plurality of complex-valued combining weights; or
selecting at least one complex-valued combining weight of the plurality of complex-valued combining weights from a plurality of predetermined complex-valued combining weights.
20. The method of claim 17 , wherein:
the method is performed within an apparatus implemented within a cellular communication system that employs S-CDMA communications.