Smart antenna receiver and signal receiving method

The invention relates to a mobile communication system, and more particularly, to a smart antenna receiver in a CDMA (Code Division Multiple Access) communication system and to a signal receiving method therefor.

A smart antenna receiver using an adaptive antenna array technology automatically adjusts an antenna to the optimal direction according to information obtained by receiving an input signal by respective elements.

As indicated in FIG. 1, a conventional smart antenna receiver is comprised of a multiplier circuit 110 including a plurality of multipliers 110-1,...,110-n formed correspondingly to an adaptive antenna array, a summer 120, and an adaptive processor 130 for adjusting adaptive weights.

In operation, array input vector signals X1,...Xn received from a plurality of antennas of an adaptive array are respectively applied to the multipliers 110-1,...,110-n where they are multiplied by weights W1,...,Wn of complex numbers adjusted adaptively by the adaptive processor 130. The output signals generated from the multipliers 110-1,...,110-n are supplied to the summer 120 where they are summed up to generate an array output y. Thus, in a receiving beam pattern, the gain is increased in the direction of a received signal which includes the desired information whereas a null is formed in the direction of a received interference signal, so that a signal can be spatially selectively received. For that reason, the circuit shown in FIG. 1 is called a spatial filter. The spatial filter increases the capacity of a system by reducing the interference between the same channels in a CDMA mobile communication system.

The adaptive processor 130 of the conventional antenna receiver adjusts the adaptive weights W1,...,Wn only using the array input vector signals X1,...,Xn and the array output y. The adaptive processor 130 carries out complicated calculation processes of searching for the direction of a signal source by calculating an optimal weight, for including those for obtaining an autocorrelation matrix of a received vector signal, and for obtaining an inverse matrix and a unique vector of the autocorrelation matrix.

Therefore, it takes much time for the adaptive processor to process the calculations, and the circuit of the adaptive processor is complicated. Consequently, it is difficult to apply the conventional smart antenna system to the CDMA mobile communication system.

It is therefore the object of the invention to provide a smart antenna receiver and a signal receiving method in a CDMA mobile communication system allowing for simplified calculation processes carried out by the adaptive processor.

This object is solved by the subject matters of independent claims 1 and 10.

The invention is advantageous in making use of the information about the differences between the desired signal and the interference signal in a more effective way than conventional receivers and methods do. Moreover, it is advantageous to use the pilot signal in the receiver and the method according to the invention, since the pilot signal is also used to coherently detect data in a reverse link of the CDMA system

Preferred embodiments are defined by the dependent claim.

The invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: <ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 is a circuit diagram of a conventional spatial filter; and</li><li>FIG. 2 is a circuit diagram of a spatial filter according to a preferred embodiment of the present invention.</li></ul>

In general, if data is coherently detected by using a pilot signal in a reverse link of a CDMA system, a pilot PN (Pseudo-Noise) code of a desired user can be extracted without particular processing. Since the pilot signal has great correlation with the desired signal and no correlation with any interference signal, the pilot signal in the reverse link can be used in an adaptive processor of a smart antenna receiver.

As illustrated in FIG. 2, a smart antenna receiver according to a preferred embodiment of the invention includes a multiplier circuit 210, a summer 220, an adaptive processor 230, multipliers 240 and 280, a data bandwidth filter 250, a PN code generator 260, a limiter 270, and a subtracter 290.

A plurality of linearly or circularly arranged antennas receive radio signals X1,...,Xn. The multiplier circuit 210 includes a plurality of multipliers 210-1,...,210-n connected respectively to the antennas and multiplies the radio signals X1,...,Xn by adaptive weights W1,...,Wn. The summer 220 sums the outputs of the multipliers 210-1,...,210-n to generate an array output signal y. The adaptive processor 230 adjusts the adaptive weights W1,...,Wn by performing an adaptive algorithm using an error signal ε generated from the subtracter 290 and using the radio signals X1,...,Xn. The PN code generator 260 generates a pilot PN code from a received pilot signal in the reverse link. The multiplier 240 multiplies the pilot PN code by the array output signal y generated from the summer 220. A reference signal generating loop including the filter 250, the limiter 270 and the multiplier 280 generates a reference signal y_d used for the adaptive algorithm of the adaptive processor 230. The subtracter 290 generates the error signal ε corresponding to the in phase or time aligned difference between the reference signal y_d and the array output signal y.

If the desired signal and the interference signal are received by the antennas from different directions, an adaptive weight W is adjusted such that a main robe of a receiving beam pattern is toward the desired signal and a null of the receiving beam pattern is toward the interference signal. In order to adjust the adaptive weight W, the array output signal y, that is, a linear coupling of data vector (x) components of the radio signal X1,...,Xn should approximate to the desired signal y_d. In this case, <maths id="math0001" num=""><math display="inline"><mrow><msup><mrow><mtext>y = W · x</mtext></mrow><mrow><mtext>T</mtext></mrow></msup></mrow></math><img file="EP0899894A2_D0001.tif" /></maths>.

In a least mean square (LMS) algorithm given by the following equation, the adaptive weight comes to converge upon an optimal adaptive weight for minimizing a mean square error (MSE) through repeated weight updating. The operation implemented by the adaptive processor 230 for obtaining the optimal adaptive weight can be represented by:<maths id="math0002" num="(1)"><math display="block"><mrow><msup><mrow><mtext>W(k+1) = W(k) + 2µε</mtext></mrow><mrow><mtext>*</mtext></mrow></msup><mtext>(k) · x(k)</mtext></mrow></math><img file="EP0899894A2_D0002.tif" /></maths> where k is a variable for counting time in units of a discrete time such as an adaptive period, µ a scalar constant for adjusting an adaptive rate and stability, ε an error signal (y_d-y), and ∗ indicates complex conjugation.

To obtain the array output signal y used to calculate the error signal ε, the radio signals X1,...,Xn received at the respective antennas are supplied to the multipliers 210-1,...,210-n where they are multiplied by the adaptive weights W1,...,Wn provided from the adaptive processor 230. The signals produced from the multiplies 210-1,...,210-n are then applied to the summer 220 to generate the array output signal y.

To obtain the reference signal y_d needed to calculate the error signal ε, the array output signal y is applied to the subtracter 290 and the multiplier 240. The array output signal y applied to the multiplier 240 is multiplied by the pilot PN code generated from the PN code generator 260. The pilot PN code is the same as the PN code which has been multiplied by the signal of the desired user. Then the desired signal is despread by the multiplier 240 and its bandwidth is reduced to a data bandwidth. An interference component remains in a spread bandwidth. While the output of the multiplier 240 passes through the data bandwidth filter 250, the desired signal remains and the interference component except an intermediate bandwidth is eliminated. The output of the filter 250 is applied to limiter 270 for adjusting the amplitude of the reference signal y_d. The output of the limiter 270 is applied to the multiplier 280 where it is multiplied by the pilot PN code and respread. Thus the reference signal y_d is generated.

The array output signal y and the reference signal y_d generated respectively from the summer 220 and the multiplier 280 are then applied to the subtracter 290 where the error signal ε corresponding to the difference therebetween is generated.

Consequently, while the desired signal passes through a loop consisting of the two multipliers 240 and 280, the filter 250, the limiter 270 and the subtracter 290 without changing, the interference component undergoes a great change in its waveform. Therefore, there is no substantial correlation with the interference between the reference signal yd and the array output signal y.

The error signal ε is applied to the adaptive processor 230 so as to obtain the adaptive weight W by the above equation (1) together with the data vector x of the radio signals X1,...,Xn. The discrete time k such as an adaptive period and the scalar constant µ needed to obtain the adaptive weight W by the equation (1) are set constant.

As described above, since the smart antenna receiver uses the reference signal generating loop using the pilot signal and the adaptive processor which applies the LMS algorithm, the amount of calculation can be greatly reduced.