Optimizing MRC coefficients for RAKE receiver for increasing SNR

Abstract

There is described a method of determining an MRC coefficient vector for a RAKE receiver. The method comprises (a) estimating a channel impulse response vector, (b) estimating a noise variance vector, (c) calculating a multiplication factor vector based on the estimated channel impulse response vector and the estimated noise variance vector, (d) calculating a modified channel impulse response vector by multiplying each element in the estimated channel response vector with a corresponding element in the multiplication factor vector, and (e) calculating the MRC coefficient vector as the complex conjugate of the modified channel impulse response vector. There is also described a corresponding device, an UWB receiver, a computer program and a computer program product.

Claims

1. A method of determining an Maximal Ratio Combiner (MRC) coefficient vector for a RAKE receiver, the method comprising estimating a channel impulse response vector, estimating a noise variance vector, calculating a multiplication factor vector based on the estimated channel impulse response vector and the estimated noise variance vector, calculating a squared modulus of each element in the estimated channel impulse response vector; calculating a modified channel impulse response vector by multiplying each element in the estimated channel response vector with a corresponding element in the multiplication factor vector only if the squared modulus of that element in the estimated channel response vector exceeds a predetermined threshold value, and calculating the MRC coefficient vector as the complex conjugate of the modified channel impulse response vector.

2. The method according to claim 1, wherein an i-th element k.sub.i of the multiplication factor vector is calculated as $k_i=\sqrt{K\left[1+\frac{{\hat{\sigma }}_i^2}{{.Math.{\hat{h}}_i.Math.}^2}\right]},$ where δ.sub.i.sup.2 is the i-th element of the estimated noise variance vector, ĥ.sub.i is the i-th element of the estimated channel impulse response vector, and K is a predetermined scalar value.

3. The method according to claim 2, wherein the predetermined scalar value K is between 0.3 and 0.5.

4. The method of claim 1, wherein a computer program stored in a non-transitory computer readable medium comprising computer executable instructions which, when executed by a computer, causes the computer to perform the steps of the method.

5. An integrated circuit device for determining an Maximal Ratio Combiner (MRC) coefficient vector for a RAKE receiver, the device comprising a first input for receiving an estimated channel impulse response vector, a second input for receiving an estimated noise variance vector, and a calculation circuit adapted to calculate a multiplication factor vector based on an estimated channel impulse response vector received at the first input and an estimated noise variance vector received at the second input, calculate a squared modulus of each element in the estimated channel impulse response vector; calculate a modified channel impulse response vector by multiplying each element in the estimated channel response vector with a corresponding element in the multiplication factor vector only if the squared modulus of that element in the estimated channel response vector exceeds a predetermined threshold value, and calculate the MRC coefficient vector as the complex conjugate of the modified channel impulse response vector.

6. The integrated circuit device of claim 5 further comprising a channel estimation circuit for estimating a channel impulse response vector, a noise variance estimating circuit for estimating a noise variance vector, and wherein the first input of the device is coupled to the channel estimation circuit, wherein the second input of the device is coupled to the noise variance estimating circuit, and wherein the device is coupled and adapted to provide a MRC coefficient vector to the RAKE receiver.

Description

BRIEF DESCRIPTION OF THE DRAWING

(1) FIG. 1 shows an UWB receiver in accordance with an embodiment.

(2) FIG. 2 shows a comparison of bit error rate performance in a conventional receiver and a receiver according to an embodiment.

DETAILED DESCRIPTION

(3) The illustration in the drawing is schematic. It is noted that in different figures, similar or identical elements are provided with the same reference signs or with reference signs, which differ only within the first digit.

(4) FIG. 1 shows an UWB receiver 1 in accordance with an embodiment. More specifically, the receiver 1 comprises a data signal input 5, a channel estimation unit 10, a correlator 12, a noise variance estimating unit 20, a device 30 for determining MRC coefficients, and a RAKE receiver 40 having an MRC input 42 and a receiver output 44. The device 30 comprises a processing unit 31 coupled to receive an estimated channel impulse response vector 32 from the channel estimation unit 10, an estimated noise variance vector 33 from the noise variance estimating unit 20, and a predetermined scalar value K at input 34. The processing unit 31 is adapted to calculate multiplication factors (or a multiplication factor vector) based on the estimated channel impulse response vector 32, the estimated noise variance vector 33, and the predetermined scalar value K as follows:

(5) $k_i=\sqrt{K\left[1+\frac{{\hat{\sigma }}_i^2}{{.Math.{\hat{h}}_i.Math.}^2}\right]},$

(6) where k.sub.i is the i-th element of the multiplication factor vector, δ.sub.i.sup.2 is the i-th element of the estimated noise variance vector 33, and ĥ.sub.i is the i-th element of the estimated channel impulse response vector 32. The scalar value K is between 0.3 and 0.5, preferably equal to 0.4

(7) The multiplication factors 35 are supplied to an input of a multiplexer 36. The multiplexer is controlled by an output from comparator unit 37. The comparator unit 37 is adapted to compare the squared modulus of each element ĥ.sub.i in the estimated channel impulse response vector 32 with a predetermined threshold value. If for a given element (i.e. a given value of i), the squared modulus exceeds the threshold value, the comparator unit 37 controls the multiplexer 36 to let the corresponding multiplication factor through to multiplier 38. If the squared modulus is below or equal to the threshold value, the multiplexer 36 is instead controlled to let a factor 1 through to the multiplier 38. Thereby, each element in the estimated channel impulse response vector 32 is multiplied with either a corresponding multiplication factor as calculated by the processing unit 31 or by a factor 1. This results in a modified channel impulse response vector which is supplied to conjugation unit 39. The conjugation unit 39 calculates the MRC coefficients as the complex conjugate of the modified channel impulse response vector and supplies them (as a vector) to the MRC input 42 of RAKE receiver 40.

(8) As shown in FIG. 1 and discussed above, the present invention relies on a multiplication of the traditional MRC coefficients with a certain factor. Each MRC coefficient is multiplied with an individual factor. The variation in factor evaluation comes from the fact that each factor depends on individual channel tap and the estimated noise variance. It also requires another adjustable constant K, which is derived experimentally. It has been found that K can lie in [0.3 0.5] across any channel model. Further, it has also been found that a K value of 0.4 works very well for most cases while any other value within the range [0.3 0.5] will not degrade the performance much.

(9) The present invention, in particular the embodiment shown in FIG. 1, has been derived by maximizing the received SINR (signal to interference plus noise ratio). The interference arises due to the channel estimation noise, which is propagated to the MRC coefficients. The solution becomes closed form if BPSK (binary phase shift key) modulation is adopted as signal constellation, i.e. all real, which is the case in the IEEE 802.15.4a standard.

(10) FIG. 2 shows a comparison of bit error rate (BER) performance in a conventional receiver and a receiver according to an embodiment. More specifically, the plot 50 shows the dependency between BER and Eb/N0 (normalized SNR per bit) for a conventional RAKE receiver (curve 52), where the MRC coefficients are obtained directly from the estimated CIR, and for a RAKE receiver (curve 54) utilizing the method of determining the MRC coefficients of the present invention. As can be seen, the latter provides a lower BER for any value of Eb/N0.

(11) More specifically, FIG. 2 shows a simulation on a UWB modem MATLAB model based on the IEEE 802.15.4a standard, which deploys BPM-BPSK (burst per modulation-BPSK) signal sets. The chosen system is a single-input single-output (SISO) system. The channel is chosen as multi-path with all its complex coefficients having Rayleigh or Rician (Only NLOS component) distribution. More precisely, a channel model of Residential NLOS type was chosen. The total number of frames is 2000. As can be seen from the curves 52 and 54, the method according to the present invention provides an SNR gain of almost 1 dB for a BER of 1.0e-3.

(12) It is noted that, unless otherwise indicated, the use of terms such as “upper”, “lower”, “left”, and “right” refers solely to the orientation of the corresponding drawing.

(13) It is noted that the term “comprising” does not exclude other elements or steps and that the use of the articles “a” or “an” does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.