PARAMETER ESTIMATION FOR G-RAKE RECEIVERS

BACKGROUND

The present invention relates generally to G-Rake receivers, and more particularly to the estimation of combining weights for a G-Rake receiver.

Conventional Rake receivers for Code Division Multiple Access (CDMA) systems exploit multipath reception for improved signal-to-noise ratios. In operation, each of two or more Rake fingers obtains despread values from a received CDMA signal by correlating the received signal against a known spreading sequence. By aligning the processing delay of each Rake finger with a different path delay of the multipath signal, the Rake receiver effectively obtains a different copy of the desired signal for each Rake finger. Maximum ratio combining of the despread values from each Rake finger yields, at least in theory, a combined signal having an improved signal-to-noise ratio as compared to the signal from any one Rake finger. The "standard" Rake receiver works well in white noise environments, where the signal impairments are uncorrelated across the Rake fingers. Standard Rake receiver performance becomes sub-optimum in colored noise environments, where at least some components of the overall received signal impairments may be strongly correlated across the Rake fingers. In other words, the standard Rake receiver does not perform well in terms of suppressing colored interference, where the received signal impairments across Rake fingers may exhibit significant correlations. The "Generalized Rake" (G-Rake) receiver was developed to better suppress interference by taking into account impairment correlations across Rake fingers. The basic operation of a G-Rake receiver is described in Bottomley, et al., Generalized Rake Reception for Cancelling Interference from Multiple Base Stations," IEEE Vehicular Technology Conference (2000) and U.S. Patent No. 6,363,104 B1 to Bottomley et al.

The best interference suppression is achieved if up-to-date covariance information (the "instantaneous color" of the impairment) is used when determining the combining weights. U.S. Patent No. 6,714,585 B1 to Wang et al. describes a method for computing G-Rake combining weights based on received signal impairment correlations. Rather than directly calculating received signal impairment correlations, it is known to represent received signal impairments according to a parametric model that is dynamically "fitted" to ongoing observations of impairment, which may be short-term, somewhat "noisy" snapshots of received signal impairment. In parametric G-Rake receivers, an overall received signal covariance matrix is constructed based on available channel information and is expressed as the combination of various constituent components of the impairment. The relative weights (fitting parameters) of these components are determined dynamically, such as by fitting the model terms to ongoing impairment correlation measurements. U.S. Published Application No. 2005/0201447 A1 to Cairns et al. describes an exemplary parametric G-Rake receiver. One problem with the parametric G-Rake receiver is the estimation of the scaling parameters. In parametric G-Rake receivers, the performance of the receiver in terms of interference suppression depends on the accuracy of the fitting parameters. The introduction of higher-order modulation and multiple-input, multiple-output in Release 7 of the WCDMA standard will increase the signal-to-interference ratio (SINR) at the receiver and will make parameter estimation less reliable. Therefore, there is considerable interest in finding and developing new techniques for obtaining reliable estimates of scaling parameters in a parametric G-Rake receiver.

SUMMARY The present invention provides a method and apparatus for computing combining weights in a parametric G-Rake receiver. A G-Rake processor computes initial estimates

of one or more scaling parameters and initial combining weights for G-Rake combining. The G-Rake processor separately estimates the SINR of a received signal based on a mean pilot symbol estimate and the initial combining weights. Revised estimates for one or more scaling parameters are then computed by the G-Rake processor based on the estimated SINR and the initial combining weight estimates. The revised scaling parameter estimates are used to compute revised combining weights for G-Rake combining.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 illustrates an exemplary parametric G-Rake receiver.

Fig. 2 illustrates an exemplary method for computing combining weights for a parametric

G-Rake receiver.

Fig. 3 illustrates an exemplary G-Rake processor for computing combining weights for a parametric G-Rake receiver.

DETAILED DESCRIPTION

Referring now to the drawings, Fig. 1 illustrates an exemplary G-Rake receiver 100 for a

Code Division Multiple Access (CDMA) system. The G-Rake receiver 100 comprises a plurality of G-Rake fingers 102, a G-Rake combiner 104, and a G-Rake processor 106. The G-Rake fingers 102 despread a received signal r to generate despread values at each finger for a symbol of interest, represented in vector form as y . Similarly, despread values are produced for pilot symbols represented in vector form as y _p . The G-Rake combiner 104 combines the despread values y from the G-Rake fingers 102 to generate a G-Rake combined value z . G-Rake processor 106 determine finger delays for the G-Rake fingers 102 and computes a combining weight vector w used by the G-

Rake combiner 104 to combine the despread symbol values y . In computing the combining weight vector w , the G-Rake processor 106 takes the impairment correlations across the G-Rake fingers 102 into account to reduce interference in the G- Rake combined values z . In one exemplary embodiment, G-Rake processor 106 uses a parametric model of the impairment covariance to compute the G-Rake combining weight vector w . An impairment covariance matrix R is modeled as a weighted sum of an interference covariance matrix R ₁ and a noise covariance matrix R _n as shown below:

R = aRi +βR _n . Eq. (1 ) In the above model, a is a scaling parameter for a modeled interference covariance matrix R^ , and β is a scaling parameter for a modeled thermal noise covariance matrix

R _n . The combining weight vector w for a G-Rake receiver 100 may be computed according to:

W = R ^-1 Ii , Eq. (2) where h is the channel response vector corresponding to the pilot channel. Substituting the parametric model given by Eq. 1 into Eq. 2 then gives:

w = (αR _f + ^R _n ) ^"1 h . Eq. (3)

The interference covariance matrix R ₁ and noise covariance matrix R _n may be calculated based on knowledge of the channel delays, channel coefficients, pulse shape, and finger placement as known in the art. The scaling parameters a and β may be estimated by measuring the impairment covariance R on the common pilot channel and adjusting the scaling parameters a and β to fit the parametric model given by Eq. 1 to the measured impairment covariance R . A least squares (LS) fitting process may be used to estimate the scaling parameters as described in Bottomley.

One problem with the parametric approach is the determination of the scaling parameters a and β . The use of higher order modulation and multi-input multiple- output (MIMO) in Release 7 of the WCDMA standard has increased the required signal- to-interference ratios (SINRs) of WCDMA receivers. The G-Rake receiver 100 improves the estimation of the scaling parameters by using a separate measure of the SINR to refine the initial estimates of the scaling parameters a and β to obtain revised estimates.

Conventionally, the SINR in a parametric G-Rake receiver, denoted herein by γ , is computed according to:

To improve the estimation of the scaling parameters a and β , the G-Rake processor 106 exploits the fact that the scaling parameters a and β may be expressed as a function of γ . Solving Eq. (4) for β yields:

By obtaining a separate estimate of the SINR, the G-Rake processor 106 may compute a revised estimate of β from Eq. 5. The same process may be used to express a as a function of γ and to obtain a revised estimate of a .

One possible measurement of the SINR may be obtained based on the mean pilot symbol J received over the common pilot channel. More specifically, the Rake processor 106 may calculate the mean pilot symbol power and the noise power of the

mean pilot symbol. From these power estimates, the Rake processor 106 may compute an estimate of the SINR. One estimate of the mean pilot symbol J is given by:

where y _k is a vector of despread values from the G-Rake combiner 104 associated with

the k' ^h pilot symbol and w ₀ represents a conventional solution of the combining weights

for a G-Rake receiver 100. The noise power, denoted σ _n ² , may be estimated as the variance of the mean pilot symbol, which is given by:

The signal power of the mean pilot symbol is calculated as the square of the mean pilot

— 2 symbol s . The SINR γ may then be computed from the mean pilot symbol power and the noise power according to:

It may be noted that a small portion of the noise estimate σ _n ² may be subtracted from

— 2 the mean pilot symbol power s to remove any bias from noise power in the estimate of the pilot symbol power.

Fig. 2 illustrates an exemplary method for computing G-Rake combining weights. Initial estimates ά ₀ and β ₀ of the scaling parameters a and β based on the impairment

covariance R on the common pilot channel (block 202). The initial estimates ά ₀ and

β ₀ are then used to compute initial G-Rake combining weights w ₀ (block 204). Next, a

separate estimate γ of the SINR is obtained (block 206) that includes information not

used in the computation of the initial combining weights vector w ₀ . For example, an

independent estimate γ of SINR may be the measured SINR on the pilot channel. A

revised estimate β of the scaling parameter β is then computed based on the

measured SINR γ and the initial G-Rake combining weight vector w ₀ (block 208). As

an example, a revised estimate β _x may be computed according to Eq. 5 based on the

measured SINR γ , the initial estimate ά ₀ , and the initial combining weight vector w ₀ .

Similarly, a revised estimate ά _x of a may be computed based on the measured SINR

γ , the initial estimate β ₀ or revised estimate β of β , and the initial combining weight

vector w ₀ . When the revised estimates ά _γ and β _γ of the scaling parameters a and β

are obtained, revised G-Rake combining weights w ₂ may be recomputed according to

Eq. 3 (block 210).

Figure 3 illustrates one exemplary embodiment of the G-Rake processor 106. G-Rake processor 106 comprises an impairment covariance estimator 108, first pass scaling parameter estimator 1 10, combining weight calculator 1 12, second pass scaling parameter estimator 1 14, and SINR estimator 1 16. The impairment covariance estimator 108 estimates the impairment covariance from the common pilot symbol. The impairment covariance estimate is applied to the first pass scaling parameter estimator 1 10. The first pass scaling parameter estimator 1 10 generates initial estimates ά ₀ and

β ₀ of scaling parameters a and β . The initial estimates ά ₀ and β ₀ may be computed by a least squares fitting process to fit the parametric model of the interference to the measured impairment covariance according to Eq. 1 . The initial estimates ά ₀ and β ₀ are applied to a combining weight calculator 1 12. The combining weight calculator 1 12

generates initial G-Rake combining weight vector w ₀ according to Eq. 3. The initial combining weight vector w ₀ is applied to a second pass scaling parameter estimator

1 14. The second pass scaling parameter estimator 1 14 computes revised estimates ά _x

and P ₁ of one or more of the scaling parameters. In the exemplary embodiment described herein, the second pass scaling parameter estimator 1 14 computes a revised estimate P ₁ of /? according to Eq. 5. Those skilled in the art will appreciate that the

second pass scaling parameter estimator 1 16 may also compute a revised estimate ά _x

of a in a similar fashion. The SINR estimator 1 16 provides an estimate γ of the SINR to the second pass scaling parameter estimator 1 14 to use in computing the revised scaling parameter estimates ά _x and P ₁ . The revised scaling parameter estimates ά _x

and P ₁ are fed back to the combining weight calculator 1 12 to generate the final combining weight vector W ₁ .

Using an independent estimate γ of the SINR to generate revised (2d pass) estimates

ά _x , P ₁ of the scaling parameters a and P may significantly improve the final G-Rake combining weights, resulting in better interference suppression during Rake combining and better error rate performance.

The present invention may, of course, be carried out in other ways than those specifically set forth herein without departing from essential characteristics of the invention. The present embodiments are to be considered in all respects as illustrative and not restrictive, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.