Wireless communications systems and methods
View Patent ↗Methods and systems that enhance interference cancellation in communication channels are described. Specialized time domain training sequences and simple cross correlation are used to obtain a channel estimate for use in stacked carrier beamforming and OFDM based spatial beamforming. In certain embodiments, a time domain preamble sequence is provided as an alternative to the conventional frequency domain preamble sequence. The use of a time domain preamble can increase channel estimation performance, facilitating cancellation of co-channel interference. Embodiments include generating a training sequence preamble in the time domain, computing a frequency domain response of the channel using an estimate of its time domain impulse response, and using the frequency domain response of the channel to calculate channel estimation from a cross correlation of data received from the channel against the training sequence.
1. A wireless communications system for improving performance of a communications channel, comprising:
a training sequence generator configured to produce a training sequence preamble in time domain;
a whitening filter employed to obtain frequency domain response of the channel and its interference process at a desired frequency subcarrier to calculate channel estimation; and
a processor configured to compute beamforming weights over all frequencies of the channel based on the channel estimation, wherein the computing includes interpolating the beamforming weights as a function of frequency, performing Fast Fourier Transform (FFT) on the training sequence preamble, time domain impulse response of the channel and data received over the channel having co-channel interference respectively.
2. The system of claim 1 , wherein the training sequence preamble in the time domain is provided as an alternative to a conventional frequency domain preamble sequence to increase performance of channel estimation and cancellation of the co-channel interference.
3. The system of claim 1 , wherein channel estimation in a time domain is used to obtain a model of the time domain impulse response of the channel over a set of discrete known delays.
4. The system of claim 1 , wherein the training sequence preamble is uncorrelated with other training sequences and delayed versions of training sequences, and is further uncorrelated with the interference process.
5. The system of claim 1 , wherein the processor is further adapted to compute the beamforming weights for a plurality of subcarriers in the channel to reduce effects of frequency dispersion.
6. The system of claim 5 , wherein the beamforming weights are computed for the plurality of subcarriers using one or more of a data process and an interference process autoregressive (AR) model.
7. The system of claim 6 , wherein the data process and interference AR model are used to compute inverse covariance matrices to obtain the beamforming weights.
8. The system of claim 1 , wherein neighboring cells transmit a unique training sequence preamble such that neighboring channels are estimated.
9. The system of claim 1 , wherein the processor is further configured to obtain an inverse covariance matrix for all frequencies of the channel by performing only one matrix inversion.
10. A method for improving performance of a wireless communications channel of a communication system comprising:
generating a training sequence preamble in time domain;
calculating channel estimates by using a whitening filter to obtain frequency domain response of the channel and its interference process at a desired frequency subcarrier; and
computing beamforming weights over all frequencies of the channel based on the channel estimates, wherein the computing includes interpolating the beamforming weights as a function of frequency, and performing Fast Fourier Transform (FFT) on the training sequence preamble, time domain impulse response of the channel and data received over the communications channel having co-channel interference respectively.
11. The method of claim 10 , further comprising providing the training sequence preamble in the time domain as an alternative to a conventional frequency domain preamble sequence to increase performance of channel estimation and cancellation of the co-channel interference.
12. The method of claim 10 , further comprising using channel estimation in a time domain to obtain a model of the time domain impulse response of the channel over a set of discrete known delays.
13. The method of claim 10 , wherein the training sequence preamble is uncorrelated with other training sequences and delayed versions of the training sequences, and is further uncorrelated with the interference process.
14. The method of claim 10 , wherein the computing further includes computing the beamforming weights for a plurality of subcarriers in the channel to reduce effects of frequency dispersion.
15. The method of claim 14 , further comprising computing the beamforming weights of the channel using second order cross correlation statistics from the frequency domain response of the channel.
16. The method of claim 14 , further comprising computing the beamforming weights for the plurality of subcarriers using one or more of a data process and an interference process autoregressive (AR) model.
17. The method of claim 16 , further comprising computing inverse covariance matrices to obtain the beamforming weights using the data process and interference AR model.
18. The method of claim 14 , further comprising modeling selected emitter channels using a linear weight computation, wherein unmodeled interference is treated as white noise, and wherein the beamforming weights are linear and maintained as a function of time or frequency.
19. The method of claim 10 , wherein the computing further includes performing only one matrix inversion to obtain an inverse covariance matrix as a function of frequency for all frequencies of the channel.
20. A wireless communications system for improving performance of a communications channel having co-channel interference, comprising:
a training sequence generator configured to produce a training sequence preamble in the time domain; and
a processor configured to compute beamforming weights over all frequencies of the channel, including for a plurality of subcarriers in the channel to reduce effects of frequency dispersion, wherein the computing includes: performing Fast Fourier Transform on the training sequence preamble and data received over the communications channel respectively, interpolating the beamforming weights as a function of frequency, and using second order cross correlation statistics from frequency domain response of the channel.
21. The system of claim 20 , further comprising a whitening filter employed to calculate channel estimation over time or frequency.
22. The system of claim 21 , wherein a Fast Fourier Transform (FFT) is performed on time domain impulse response of the channel and the whitening filter is employed to obtain the frequency domain response of the channel and its interference process at a desired frequency subcarrier.
23. The system of claim 22 , wherein the processor is further adapted to model selected emitter channels using a linear weight computation, wherein unmodeled interference is treated as white noise, and wherein the beamforming weights are linear and maintained as a function of time or frequency.
24. The system of claim 20 , wherein the training sequence preamble in the time domain is provided as an alternative to a conventional frequency domain preamble sequence to increase performance of channel estimation and cancellation of co-channel interference of the channel.
25. The system of claim 20 , wherein channel estimation in a time domain is used to obtain a model of the time domain impulse response of the channel over a set of discrete known delays.
26. The system of claim 22 , wherein the training sequence preamble is uncorrelated with other training sequences and delayed versions of training sequences, and is further uncorrelated with the interference process.
27. The system of claim 20 , wherein the beamforming weights are computed for the plurality of subcarriers using one or more of a data process and an interference process autoregressive (AR) model.
28. The system of claim 27 , wherein the data process and interference AR model are used to compute inverse covariance matrices to obtain the beamforming weights.
29. The system of claim 20 , wherein the processor is further adapted to model selected emitter channels using a linear weight computation, wherein unmodeled interference is treated as white noise, and wherein the beamforming weights are linear and maintained as a function of time or frequency.
30. The system of claim 20 , wherein neighboring cells transmit a unique training sequence preamble such that neighboring channels are estimated.
31. A method for improving performance of a wireless communications channel having co-channel interference of a communication system comprising:
generating a training sequence preamble in time domain;
computing beamforming weights for a plurality of subcarriers in the channel to reduce effects of frequency dispersion, wherein the computing includes interpolating the beamforming weights as a function of frequency, and performing Fast Fourier Transform on the training sequence preamble and data received over the communications channel respectively, and using second order cross correlation statistics from frequency domain response of the channel.
32. The method of claim 31 , further comprising calculating time or frequency channel estimates using a whitening filter.
33. The method of claim 32 , further comprising performing Fast Fourier Transform (FFT) on time domain impulse response of the channel and the whitening filter is employed to obtain the frequency domain response of the channel and its interference process at a desired frequency subcarrier.
34. The method of claim 21 , further comprising providing the training sequence of the time domain preamble as an alternative to a conventional frequency domain preamble sequence to increase performance of channel estimation and cancellation of co-channel interference.
35. The method of claim 31 , further comprising using channel estimation in a time domain to obtain a model of the time domain impulse response of the channel over a set of discrete known delays.
36. The method of claim 31 , wherein the training sequence preamble is uncorrelated with other training sequences and delayed versions of the training sequences, and is further uncorrelated with the interference process.
37. The method of claim 31 , further comprising computing the beamforming weights for the plurality of subcarriers using one or more of a data process and an interference process autoregressive (AR) model.
38. The method of claim 31 , further comprising modeling selected emitter channels using a linear weight computation, wherein unmodeled interference is treated as white noise, and wherein the beamforming weights are linear and maintained as a function of time or frequency.
39. The method of claim 31 , wherein the computing further includes obtaining one inverse covariance matrix as a function of frequency for all frequencies of the channel.