Pulse or digital communications – Spread spectrum – Direct sequence
Reexamination Certificate
2000-12-29
2004-08-03
Liu, Shuwang (Department: 2634)
Pulse or digital communications
Spread spectrum
Direct sequence
C375S130000, C375S148000, C375S152000, C370S335000, C370S342000
Reexamination Certificate
active
06771690
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to estimating an unknown, random parameter in a linear system in the minimum mean-square-error (MSE) sense, and adapting the estimator according to gradual changes in the system. It also relates to the application of estimation algorithms to digital radio communications, more particularly to implementation of low-complexity adaptive Linear Minimum Mean Square Error (LMMSE) wireless terminal receivers for, by example, Code Division, Multiple Access (CDMA) systems, including Wideband CDMA (WCDMA) systems.
BACKGROUND OF THE INVENTION
The capacity of WCDMA systems is inherently interference limited. Users are separated by spreading codes which are orthogonal to one other. However, this orthogonality is lost at the receiver when there is multipath propagation in the channel that results in multiple-access interference. This interference is particularly severe for high data rate users that use very short spreading codes. Moreover, neighboring cells in a WCDMA system use the same frequency band, which results in significant inter-cell interference. The currently used CDMA receiver is the well-known RAKE-type. However, the use of the RAKE receiver is optimal only when there is no multipath propagation and the interference is “white”. Unfortunately, such conditions occur only rarely, thus making the RAKE a sub-optimal receiver.
The RAKE receiver used for WCDMA terminal receivers has no capability to reduce the interference caused by multipath propagation, and furthermore it cannot utilize the structure of the interference, i.e. its spatial and time-correlation properties. In a highly loaded cell, or with strong interference from neighboring cells, RAKE receiver does not function properly. Furthermore, if multiple wireless terminal antennas are used the RAKE receiver cannot intelligently direct the beam of the antenna array so that the signal-to-interference-plus-noise ratio (SINR) would be maximized.
The most optimum linear receiver, in the minimum mean-square-error (MMSE) sense, is well-documented in the literature, but too complex to be implemented in most applications.
In the literature, almost all algorithms used for finding the LMMSE solution are either too complex to implement in a practical receiver, and/or require a suitable training sequence. However, a suitable training sequence is not present in the third generation CDMA systems.
One problem that is common to most adaptive algorithms is that they are designed to operate on the symbol level. However, due to the long scrambling codes used in the WCDMA system the symbol level algorithms do not function properly. This is due to the fact that the scrambling code makes the signal non-cyclostationary on the symbol level. In other words, scrambling randomizes the signal correlation properties, thereby making adaptation impossible.
This problem can be avoided when the filter is designed to operate on the chip level, as opposed to the symbol level. However, due to the lack of a suitable training sequence on the chip level, the adaptation algorithm must be blind. Only a few practical algorithms for this purpose have thus far been developed or proposed.
One improved adaptive algorithm known to the inventors is the so-called Griffiths' algorithm. This algorithm uses the channel impulse response to train the filter. However, training is still required, and the adaptation time may not be optimum for all applications.
OBJECTS AND ADVANTAGES OF THE INVENTION
It is a first object and advantage of this invention to provide an adaptive filter that eliminates the requirement to multiply by the inverse of the covariance matrix.
It is a further object and advantage of this invention to provide an adaptive filter that has computation requirements that are suitable for use in a wireless terminal, such as a WCDMA terminal, that employs one or more antennas.
It is another object and advantage of this invention to provide an adaptive finite impulse response (FIR) filter that does not require the use of a training sequence, and that is suitable for use in a wireless terminal, such as a WCDMA terminal, employing at least one reception antenna.
SUMMARY OF THE INVENTION
The foregoing and other problems are overcome and the foregoing objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
This invention provides an algorithm that is embodied as an adaptive linear finite impulse response (FIR) filter which can be applied to determine a linear minimum mean-square-error (LMMSE) estimate of unknown, random parameters. Whereas the direct computation of the LMMSE solution is very complex and requires inversion of a large matrix, the complexity of the adaptive algorithm of this invention is of the order of the well known and significantly less complex Least Mean Square (LMS) algorithm.
The presently preferred adaptive FIR filter is blind, i.e., it does not require any training for the adaptation, which makes it applicable in, by example, WCDMA downlink receivers with one or multiple antennas. It is shown that the performance of the adaptive WCDMA terminal receiver is superior to the currently used RAKE receiver.
In general, the invention provides an adaptive algorithm which can be used to derive a linear filter which minimizes the mean-square-error of the estimate of some unknown parameter such as, but not limited to, a transmitted data symbol. Due to the adaptive processing, the high computational complexity required by the conventional direct solution for the optimal filter is avoided. The algorithm is applicable for use in advanced third generation wireless terminal receivers, as it does not require any training for the adaptation.
The adaptive algorithm in accordance with the teachings herein can be used to find a linear MMSE (LMMSE) solution with low computational requirements. When applied to WCDMA receivers, the adaptive LMMSE algorithm in accordance with these teachings equalizes the channel so that the interference from other users is minimized. In addition, due to the inherent whitening operation performed by the adaptive filter, the interference from neighboring cells is effectively suppressed. At the same time, the adaptive receiver is able to (near-)optimally perform beam steering in the case of a multi-antenna receiver.
Performance results show that the adaptive LMMSE receiver clearly outperforms the conventional RAKE receiver. The improved performance can be used to increase the reception reliability of the wireless terminal receiver, especially in severe interference conditions, or to increase the cell coverage, or the capacity of the system.
Unlike the Griffiths' algorithm approach, in the adaptive algorithm of this invention the LMMSE estimator is divided into a blind adaptive filter and a filter matched to the channel impulse response and to the spreading code of the desired user. Thus no training is required at all.
In the case of significant multipath propagation, or very short spreading codes (high data rates), or a highly loaded home cell, or high interference from a neighboring cell, the adaptive receiver in accordance with this invention is shown to be superior to the conventional RAKE receiver.
The adaptive filter can, however, be used prior to a RAKE receiver, thereby causing the overall receiver to function as an adaptive LMMSE receiver that converges to an optimal linear receiver in the sense of minimizing the signal-to-noise-plus-interference ratio at the receiver output. If the adaptive filter portion is by-passed for some reason, for example to save power when experiencing good channel conditions, the receiver then functions as a conventional RAKE receiver.
The adaptive receiver has an inherent capability to (asymptotically) optimally perform beam forming, if the receiver has multiple antennas. Thus, if adaptive filtering is used in a multi-antenna receiver prior to a conventional multi-antenna RAKE, the overall receiver functions as an adaptive LMMSE receiver and performs beamforming in an optimal way. A RAKE re
Harrington & Smith ,LLP
Liu Shuwang
LandOfFree
Method and apparatus for providing blind adaptive estimation... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for providing blind adaptive estimation..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for providing blind adaptive estimation... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3327529