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 estimating performance of a communications channel with co-channel interference, the system comprising:
a training sequence generator configured to produce a training sequence preamble in time domain; and
a processor having a cross-correlator, a channel impulse response estimator, and a Fast Fourier Transform (FFT) calculator, wherein the cross-correlator cross-correlates raw data in a received signal in the communications channel with the training sequence preamble, the channel impulse response estimator estimates time domain impulse response for the received signal cross-correlated with the training sequence preamble, and the FFT calculator computes frequency domain response from the output of the channel impulse response estimator;
wherein the processor uses second order cross-correlation statistics obtained from the output of the FFT calculator to create a channel matrix that estimates performance of the communications channel with co-channel interference.
2. The system of claim 1 , wherein the processor uses the channel matrix in computing beam-forming weights for a plurality of subcarriers for reducing co-channel interference in the communications channel.
3. The system of claim 2 , wherein the system further includes an interference whitening filter, which uses an autoregressive (AR) model and FFT to obtain an inverse covariance matrix in frequency domain that, along with the channel matrix, facilitates computation of the beam-forming weights for the plurality of subcarriers to reduce the effect of frequency dispersion.
4. The system of claim 2 , wherein characteristics of at least some of the co-channel interference is different from additive white Gaussian noise (AWGN) characteristics.
5. The system of claim 2 , where the received signal has code repetitions.
6. The system of claim 5 , wherein neighboring cells in the wireless communications system use same code repetition numbers and same data block size.
7. The system of claim 6 , wherein de-permutation is performed to extract information about number of code repetitions in the received signal.
8. The system of claim 6 , wherein the signal in the communications channel is received by a receiver having a plurality of antennas.
9. The system of claim 8 , wherein the number of antennas is factored in while exploiting code repetitions to cancel the co-channel interference.
10. The system of claim 1 , wherein the channel matrix is sparse.
11. A method wireless communications system for estimating performance of a communications channel with co-channel interference in a wireless communications system, the method comprising:
receiving a signal with raw data in a communications channel;
producing a training sequence preamble in the time domain;
cross-correlating, at a processor, the raw data in the received signal with the training sequence preamble, the processor having a cross-correlator, a channel impulse response estimator, and a Fast Fourier Transform (FFT) calculator;
estimating, at the channel imulse response estimator, time domain impulse response for the received signal cross-correlated with the training sequence preamble, and
performing, at the FFT calculator, Fast Fourier Transform (FFT) on the output of the channel impulse response estimator to calculate frequency domain response;
using second order cross-correlation statistics obtained from the FFT process to create a channel matrix that estimates performance of the communications channel with co-channel interference.
12. The method of claim 11 , wherein the method further includes:
using the channel matrix to compute beam-forming weights for a plurality of subcarriers for reducing co-channel interference in the communications channel.
13. The method of claim 12 , wherein the method further includes:
providing a whitening filter which uses an autoregressive (AR) model and FFT to obtain an inverse covariance matrix in frequency domain;
using the inverse covariance matrix in frequency domain, along with the channel matrix, to compute the beam-forming weights for the plurality of subcarriers to reduce the effect of frequency dispersion.
14. The method of claim 12 , wherein characteristics of at least some of the co-channel interference is different from additive white Gaussian noise (AWGN) characteristics.
15. The method of claim 12 , where the received signal has code repetitions.
16. The method of claim 15 , wherein neighboring cells in the wireless communications system use same code repetition numbers and same data block size.
17. The method of claim 16 , wherein de-permutation is performed to extract information about number of code repetitions in the received signal.
18. The method of claim 16 , wherein the signal in the communications channel is received by a receiver having a plurality of antennas.
19. The method of claim 18 , wherein the number of antennas is factored in while exploiting code repetitions to cancel the co-channel interference.
20. The method of claim 11 , wherein the channel matrix is sparse.