KR20020037103A - Multi-user detection system for mobile communications - Google Patents

Abstract

PURPOSE: A device for removing interferences of many users in a mobile communication system is provided to remove other user's interference received in a different direction from a receiving direction of a user signal by using a space filtering method, and to remove an interference, which is received in a similar method in the receiving direction of the user signal and is not removed by the space filtering method, by using a method of removing a detected signal interference of many users, so as to improve a system capacity. CONSTITUTION: A beam shaper(10) shapes a beam to make signals of M antenna elements N signals having directions. Parallel searchers(105) are composed of N searchers for searching in which direction a path signal having a spread code same as a spread code of a demanded user is largely received. A controller(106) outputs various system control signals by using directional information of the parallel searchers(105). The first despreader(20) performs a despreading by using beam direction signals, and couples the signals by a ratio of a received path signal power, then reproduces the signals by using a wireless channel parameter for estimating each path signal. An interference remover(30) inputs the reproduced path signals, and removes all path signals except a self path signal by using baseband signals corresponding to each direction and the reproduced path signals. The second despreader(40) inputs each path signal whose interference is removed, and performs a despreading for the signals to output the despread signals.

Description

MULTI-USER DETECTION SYSTEM FOR MOBILE COMMUNICATIONS}

The present invention relates to an apparatus and method for eliminating multi-user interference in a mobile communication system. More particularly, the present invention relates to a method for receiving a signal using array antenna elements in a base station, and performing spatial filtering using signals received from each antenna element. A mobile communication system that minimizes interference transmitted from a direction different from a desired user's path signal and removes multi-access interference that is not removed even by spatial filtering by using multi-user signal detection interference cancellation method using direction information by a parallel searcher. A multi-user interference cancellation apparatus and method of the present invention.

Currently commercial CDMA systems employ a single-user signal detection scheme, so that signals from other users in the base station are treated as interference noise. Therefore, as the number of users sharing the same frequency at one base station increases, the multiple access interference noise increases, thereby increasing the bit error rate (BER) of the received signal and degrading the performance of the system.

Therefore, in the current CDMA system, the performance of the system has been maintained above a certain level by limiting the number of users belonging to the base station. In addition, in the future IMT-2000 system, it is necessary to provide high-speed multimedia service with low BER, and therefore, interference noise must be further reduced, and in order to satisfy this, limiting the number of users serviced at one base station or processing gain ) Or increase or decrease the size of the cell.

However, as the processing gain increases, the bandwidth of the transmitted signal increases, and as the size of the cell decreases, more base stations need to be added (base station installation cost increases) and handoff occurs too frequently. There is a problem.

Therefore, in order to provide high quality multimedia communication service to a large number of users at one base station without reducing the current cell size or increasing the processing gain, a study on the multi-user signal detection interference cancellation technology linked to the smart antenna system has recently been performed. Is actively underway.

The article "Combination of an Adaptive Array Antenna and a Canceller of Interference for Direct-Sequence Spread-Spectrum Multiple-Access System" (R. Kohno, H. Imai, M. Hatori and S. Pasupathy, IEEE Journal on selected areas in communications, May, 1990) proposed an interference cancellation system in which an adaptive beam antenna system and a serial interference canceller or a parallel interference canceller are linked.

In this method, the path signal direction of the user is estimated and spatial filtering is performed to receive a signal only from the direction, and then the remaining interference is removed by using a serial or parallel interference cancellation method.

However, in this scheme, when the multi-user signal detection interference cancellation is performed, since there is no information on the reception direction of each user path signal, it is not possible to know which user's signal affects the user's signal with great interference.

Therefore, in this scheme, after performing spatial filtering using the adaptive beam, the decorrelation is performed to remove all user signals that have not been removed even after spatial filtering. Therefore, there is a problem in that the complexity of the hardware and the amount of calculations are greatly increased in order to perform the decoding for all users.

For example, if the number of users belonging to a base station is K, and each user's signal is received in L multipaths, (K-1) × L correlators and signal regenerators are required for interference cancellation in each correlator. The hardware complexity is greatly increased.

The paper "Low Computational Complexity Multiuser Detection Using Multibeam Antennas" (X. Yu and J. Litva, IEEE, 1998) uses a multi-beam antenna system and a multi-user signal detection interference canceller.

In this paper, by using the multi-beam antenna system, only the interference of the users belonging to each beam is eliminated, which greatly reduces the amount of computations caused by the multi-user signal detection interference cancellation.

In order to estimate the other user's signal received from the same beam direction as the desired user's signal, all demodulators receive the signal in the desired user's beam direction and perform despreading, and the power of the despread signal is performed. It is determined whether the signal of another user is received from the same beam direction as the desired user. For example, if the signal power after despreading is large, it is assumed to be a signal in the same beam direction, and if the signal power is small, it is regarded as a signal in another beam direction.

Therefore, in order to estimate whether another user's signal, such as a desired user's signal direction, is received in each beam direction, a demodulator corresponding to all users must be present for each beam direction. For example, when a user A signal is received in beam 1 direction, all user demodulators are needed to estimate whether another user's signal is received in beam 1 direction, and user B's signal is also in beam 2 direction. Also needed is a demodulator of all users to estimate whether another user's signal is received in beam direction 2 when received at.

Therefore, the interference cancellation technique proposed in this paper also increases the complexity of the hardware. Furthermore, the interference cancellation technique proposed in this paper does not take into account when a multipath signal of a user is received from different directions, which makes it difficult to apply it to a real wireless channel environment.

In the present invention, a signal that most significantly affects a signal of a desired user through interference in a wireless channel environment is a signal received from the same beam direction, and a signal received from a beam direction different from a desired signal reception direction of a desired user is large when forming a beam. Note the removal.

The present invention has been proposed to solve the problems of the prior art in view of the above, the purpose is to perform the spatial filtering using the signals received from each array antenna element of the switching beam antenna system to the desired user's path signal Minimize interference transmitted from the other direction, and remove the multi-user interference that is not eliminated by spatial filtering by multi-user signal detection interference cancellation method using direction information by parallel searcher To provide a device.

The multi-user interference cancellation apparatus of the mobile communication system according to the present invention for achieving the above object comprises a beam forming unit for beam-forming the signals received from the M antenna elements with N (M≤N) signals having a directionality; The output signal of the beam forming unit includes a parallel searcher composed of N searchers for searching in which beam direction a path signal having a spreading code such as a user's spreading code for each beam direction is largely received and transmitted from the parallel searcher. The controller outputs various system control signals using the direction information, and despreads using the beam direction signal inputted by the controller, and combines the received path signal power ratios to estimate each path signal. A first despreader that reproduces using one radio channel parameter, and a signal of each reproduced path is controlled by a controller And an interference canceling unit for removing all path signals except for the path signal of the self using a reproduced path signal in the corresponding beam direction received from the same beam direction and a baseband signal corresponding to each direction, and performing interference cancellation. And a second despreader which receives the received path signals under the control of the controller and performs despreading and outputs the despreading signals.

1A is a structural diagram of a linear array antenna of a mobile communication system;

1B is a structural diagram of a circular array antenna of a mobile communication system;

2 is a beam pattern diagram of an output signal by an analog beam former when dividing a sector having a range of 120 ° into eight uniform directions by 15 °;

3 is a block diagram of a base station of a mobile communication system employing a multi-user interference cancellation apparatus using an analog beam former according to a first embodiment of the present invention;

4 is a block diagram of a base station of a mobile communication system employing a multi-user interference cancellation apparatus using a digital beam former according to a second embodiment of the present invention;

5 is a beam pattern diagram of an output signal by the beam former when the traffic load in the 5th beam direction increases and the traffic load in the 2nd beam direction decreases while receiving a signal with the beam pattern shown in FIG.

<Explanation of symbols for the main parts of the drawings>

10, 15: beam forming portion 20: first despreading portion

30: interference canceling unit 40: second despreading unit

101: analog beam former 102: RF receiver

103: A / D converter 104: buffer

105: Parallel Explorer 106: Controller

107: first switching mattress 108: first demodulator group

109: maximum ratio combiner 110: player

111: second switching mattress 112: interference canceller

113: third switching mattress 114: second demodulator group

201: digital beam former

There may be a plurality of embodiments of the present invention, hereinafter with reference to the accompanying drawings will be described in detail a preferred embodiment. Through this embodiment, it is possible to better understand the objects, features and advantages of the present invention.

The base station antenna in the present invention is composed of a plurality of antenna elements. The antenna elements may be arranged in a linear structure as shown in FIG. 1A, or may be arranged in a circular structure as shown in FIG. 1B.

When the antenna elements A ₁ , A ₂ ,... A _M cover the entire cell, a circular structure is used, and when the cell is divided into a plurality of sectors, a linear structure is used.

Figure 3 is a base station block diagram of a mobile communication system employing a multi-user interference cancellation unit with the analog beam former according to the first embodiment of the present invention, the antenna elements _{(A 1, A 2, ...} A M) to a linear This is the case.

The beam forming unit 10 of the present invention includes M antenna elements A ₁ , A ₂ ,... A _M , analog beam former 101, and N (N ≧ _M ) RF receivers 102/1 to 102 /. N), N A / D converters 103/1 to 103 / N, and N buffers 104/1 to 104 / N.

The analog beam former 101 divides a cell or a sector into N directions so that the signal received from the M antenna elements becomes a signal received only from the divided beam direction, thereby beamforming into N signals having directionality.

The RF receivers 102/1 to 102 / N convert the output signal of the analog beamformer 101 into baseband signals of the I channel and Q channel signals.

The A / D converters 103/1 to 103 / N convert the signals converted by the RF receivers 102/1 to 102 / N into digital signals having a data rate higher than that of the system spreading code, respectively. To the buffers 104/1 to 104 / N.

Parallel searcher 105 is composed of N searchers, each searcher receives the output signal of the A / D converter (103/1 to 103 / N) through the buffer (104/1 to 104 / N) in each beam direction In the beam direction, a path signal having a spreading code such as a spreading code of a desired user is received.

The controller 106 outputs various system control signals using the direction information transmitted from the parallel searcher 105.

The first despreader 20 includes first demodulator groups 108/1 to 108 / K consisting of K demodulators, K maximum ratio combiners 109/1 to 109 / K, and K regenerators 110 /. 1 to 110 / K).

Each demodulator constituting the first demodulator group 108/1 to 108 / K performs despreading using the signal in the beam direction input by the switching of the first switching matrix 107 by the control of the controller 106. We estimate the radio channel parameters for each path signal.

The maximum ratio combiner 109/1 to 109 / K combines and outputs the despread signal in the first demodulator group 108/1 to 108 / K by the ratio of the received path signal power.

The regenerators 110/1 to 110 / K estimate the output signal of the maximum ratio combiner 109/1 to 109 / K in the corresponding demodulator of the first demodulator group 108/1 to 108 / K. Play with.

The interference canceller 30 is composed of N interference cancellers 112/1 to 112 / N, and each interference canceller is regenerator 110 through a second switching matrix 111 that is switched by the control of the controller 106. / 1 to 110 / K) receives the signal of each path reproduced from the same beam direction received from the same beam direction and transmitted from the butter (104/1 to 104 / N) corresponding to each direction The baseband signal is used to remove all path signals except its own path signal from the signals received from the corresponding buffers 104/1 to 104 / N.

The second despreader 40 is composed of K demodulators constituting the second demodulator groups 114/1 to 114 / K, and each demodulator controls each path signal from which interference cancellation is performed to the control of the controller 106. It is input through the third switching matrix 113 for switching by performing despreading and output.

The operation and operation of the multi-user interference cancellation apparatus of the mobile communication system according to the present invention configured as described above will be described in detail as follows.

First, the signals received by each antenna element A ₁ , A ₂ ,... A _M are input to an analog beam former 101 (eg, butler matrix, etc.).

The analog beam former 101 divides a cell or sector into N directions and adjusts the phase of signals received from the M array antennas so that the output signals of the analog beam former 101 are received only from the divided beam direction. One signal is used to form a beam of N signals having directionality (M? N).

For example, when dividing a sector having an angle of 120 ° into eight uniform directions by 15 °, the beam pattern of the output signal of the analog beam former 101 is as shown in FIG. In this case, the input of the analog beam former 101 is a signal received from each antenna element A ₁ , A ₂ ,... A _M , and the output is the number of beams divided by sectors or cells by the same or larger number than the input.

In addition, the width of each beam may be the same or different from each other by the distribution of user signals (i.e., the traffic load in each beam direction), and the beam pattern of the output of the analog beam former 101 is partially different from the neighboring beam pattern. Causes duplicates.

The output signal of the analog beam former 101 passes through the RF receiver 102 and is then converted into baseband signals of the I and Q channel signals, which is then converted by the A / D converter 103 to the data rate of the system spreading code. It is converted into a digital signal with a higher data rate (four times the spread code data rate in current CDMA systems).

The I-channel and Q-channel signals oversampled by the A / D converter 103 are stored in the buffer 104 for each beam. For example, a signal received in the first direction is stored in the first buffer 104/1, a signal received in the second direction is stored in the second buffer 104/2, and a signal received in the N direction. Is stored in the Nth buffer 104 / N. The signal stored in each buffer 104 is input to the parallel search 105 consisting of N searchers by the number of beams formed.

Signals in the corresponding beam direction are input to each searcher of the parallel searcher 105. For example, a signal in the beam direction 1 is input to the first searcher, a signal in the beam direction 2 is input to the second searcher, and a signal in the beam direction N is input to the N-th searcher.

In the parallel searcher 105 receiving the signals from the plurality of buffers 104, the energy of the desired user signal for each beam direction is calculated, and the result is compared to the path signal having the spreading code of the spreading code of the desired user. Search in which beam direction the signal is received loudly.

The search results of the parallel searcher 105 are sent to the controller 106. Therefore, the controller 106 stores information on which beam direction each path signal of all users belonging to the base station is received from. The controller 106 controls the first switching matrix 107 by using the direction information transmitted from the parallel searcher 105, thereby connecting the parallel searcher to the correlator (eg, finger) of the corresponding demodulator (eg, RAKE receiver) 108. The signal in the beam direction selected at 105 is input.

When the number of correlators of the demodulator 108 is K, the parallel searcher 105 selects signals in K directions having different time delays, and the signals in the direction in which the signal reception power of the signal is large in the correlator of the demodulator 108 are large. Are entered one by one. In this case, if all of the K multipath signals that can be distinguished are received from one direction, all signals of the same beam direction are input to all correlators of the corresponding user demodulator 108.

The correlator of each demodulator 108 performs despreading using the signal in the beam direction input by the control of the controller 106, and estimates radio channel parameters for each path signal.

The despread signal from the demodulator 108 is combined and output as the ratio of the path signal power received by the maximum ratio combiner 109. The output signal is judged whether its digital value is 1 or 0, and is reproduced using the radio channel parameters estimated by the demodulator 108. At this time, the signals of each path are respectively reproduced and input to the plurality of interference cancellers 112 through the second switching mattress 111.

The controller 106 also controls the second switching matrix 111 so that signals received from the same beam direction are input to the same interference canceller 112. Therefore, path signals received from the same direction are input to each interference canceller 112. For example, path signals received from the first direction are input to the first interference canceller 112, path signals received from the second direction are input to the second interference canceller 112, and the N th interference canceller ( 112 / N), route signals received from the N direction are input.

Each interference canceling unit 112 uses the baseband signal transmitted from the buffer 104 corresponding to each direction and all path signals except for the path signal of the corresponding one, using the reproduced path signal in the corresponding beam direction. Perform interference cancellation to remove from the signal received from In this case, serial interference cancellation, parallel interference cancellation, and serial / parallel mixed interference cancellation may be used.

Each path signal from which interference cancellation is performed is input to the corresponding demodulator 114 through the third switching matrix 113. The third switching matrix 113 at this time is also controlled by the controller 106. Therefore, the path signal of another user received from the same beam direction as the corresponding user path signal is removed from the input signal of each demodulator 114. The correlator of the demodulator 114 receives a signal from which interference is received from a direction similar to a corresponding path signal of the user and despreads.

Each path signal subjected to despreading through the demodulator 114 is determined by the determiner (not shown) whether the digital value is 1 or 0, and after deinterleaving by a deinterleaver (not shown). The error is corrected and output in the Viterbi decoder (not shown).

The multi-user interference cancellation apparatus of the mobile communication system according to the present invention can be designed using a digital beam former instead of an analog beam former, and the base station system block diagram is shown in FIG. As can be easily seen through the comparison between FIG. 3 and FIG. 4, all components except for the beam forming unit 15 are the same, and thus, the same components are denoted by the same reference numerals for better understanding of the description.

The beam forming unit 15 of the multi-user interference elimination apparatus according to the second embodiment of the present invention includes M antenna elements A ₁ , A ₂ ,... A _M , and N (N ≧ _M ) RF receivers 102. / 1 to 102 / N), N A / D converters 103/1 to 103 / N, digital beam former 201, and N buffers 104/1 to 104 / N.

The signals received at the array antenna elements A ₁ , A ₂ ,... A _M are passed through the RF receiver 102 and then converted into digital signals by the A / D converter 103 and then input to the digital beam former 201. do.

The digital beam former 201 multiplies the signals received at each antenna element A ₁ , A ₂ , ... A _M and converted into digital data by a complex weight vector corresponding to different directions, whereby N output signals It is equivalent to receiving the signal in the other N directions. In addition, when there are many path signals of the user in one direction, the beamforming may be performed so that the signals of the user are evenly distributed in each beam by adjusting the dimension of the complex weight vector in the digital beam former 201.

The relationship between the input signal and the output signal of the digital beam former 201 is expressed by Equation 1 below.

In Equation 1 Is the k-th output signal of the digital beamformer 201 at time n, Is the digital signal received from the m th antenna, Is a complex weight multiplied by the digital data received from the m th antenna to receive the signal only from the k th direction.

If the distance between the antenna sensors is half of the wavelength corresponding to the maximum frequency, Is Becomes Is the kth when dividing a cell (or sector) in N directions Direction. Therefore, the output signal of the digital beam former 201 is the same as the output signal of the A / D converter 103 when the analog beam former 101 is used in FIG. 3, and the signal for which beam formation is performed is performed for each beam. It is stored in the buffer 104. The operation principle hereafter is the same as that of the system using the analog beam former 101 described above.

In addition, in order to keep the path signal power of the user belonging to each beam uniformly, when the beam width in each direction is different from each other, Equation 1 Adjust the beam width by multiplying and summ the signal received by some antennas.

For example, all signals received from M antennas correspond to the corresponding direction. ( Multiplying) decreases the beam width, and only the Q (<M) received signals from the M antennas (Dimension = Q, Multiplying Q) increases the beam width. The beam pattern of the output of the digital beam former 201 changes to FIG. 5 when the traffic load in the 5th beam direction increases and the traffic load in the 2nd beam direction decreases while receiving the signal with the beam pattern of FIG. 2. At this time, the direction and dimension of the weight vector are the average of the demodulator or correlator belonging to each beam. The total RF power received in each beam direction, the total forward power transmitted in each beam direction, and the like.

In addition, the multi-user interference cancellation system according to the present invention can be implemented using a demodulator having two input stages, such as a commercially available modem. (In a commercially available system, a modem has two inputs for using spatial diversity. Step).

In a system having a demodulator having such a structure, the parallel searcher selects two beam directions in which power of a signal having a spreading code such as a spreading code of a desired user is largely transmitted to the controller, and the controller controls the first switching matrix. The beam signals of the selected two directions are input to the demodulator. Since the controller has information on which beam direction a signal having a different time delay of each user is received, the controller uses this information to control the second and third switching matrices so that the path signals belonging to the same beam direction can be controlled. Allows multi-user signal detection interference cancellation to be performed.

According to the present invention as described above, interference of another user received from a direction different from the reception direction of the user signal is removed by using a spatial filtering method, and interference that is received similarly to the reception direction of the user signal and is not removed even by spatial filtering. The performance of the system is improved by eliminating using a multi-user signal detection interference cancellation scheme.

In addition, when the interference is removed by the multi-user interference cancellation method after spatial filtering, the complexity and the computational complexity of the hardware are greatly reduced by removing only the interference belonging to the same beam using the parallel searcher, the controller, and the switching matrix.

Claims (7)

A beam forming unit which beam-forms the signals received from the M antenna elements into N (M≤N) signals having directionality; A parallel searcher consisting of N searchers for searching in which beam direction a path signal having a spreading code, such as a spreading code of a desired user, is received for each beam direction; A controller for outputting various system control signals using direction information transmitted from the parallel searcher; A first despreading unit performing despreading using a signal in a beam direction input by the control of the controller, combining the received path signal power ratios, and reproducing each path signal using an estimated radio channel parameter; , All paths except the path signal of the self are received by using the reproduced path signal in the corresponding beam direction and the baseband signal corresponding to each direction received from the same beam direction by receiving the reproduced path signals under the control of the controller. An interference cancellation unit for removing a signal, And a second despreader configured to receive each path signal on which the interference cancellation is performed, under control of the controller, and perform despreading to output the path signal. The method of claim 1, The beam forming unit comprises: an analog beam former for dividing a cell or sector into N directions so that the signal received from the M antenna elements becomes a signal received only from the divided beam direction, and beamforming the beam into N signals having directivity; N RF receivers for converting an output signal of the analog beam former into baseband signals of I and Q channel signals; And N A / D converters converting the signal converted by the RF receiver into a digital signal having a data rate higher than that of a system spreading code and outputting the digital signal. The method of claim 1, The beam forming unit includes: N RF receivers for converting signals received from M antenna elements into baseband signals of I and Q channel signals; N A / D converters for converting the signal converted by the RF receiver into a digital signal having a data rate higher than that of a system spreading code and outputting the digital signal; A plurality of mobile communication systems including a digital beam former for outputting the signal converted into the digital data by multiplying the complex weight vector corresponding to the different directions to the N output signals as received in the different N directions User Interference Cancellation Device. The method of claim 3, wherein And the digital beam former is configured to adjust the beam width by adjusting the direction component and the dimension of the weight vector to uniformly distribute the user belonging to each beam. The method of claim 4, wherein The digital beam former is an average of the first despreader belonging to each beam. Apparatus for eliminating multi-user interference in a mobile communication system, comprising adjusting a direction and a dimension of a weight vector using any one of a value and total RF power received in each beam direction and total forward power transmitted in each beam direction. . The method of claim 1, The first despreader comprises: a first demodulator group comprising K demodulators for performing despreading using a signal in a beam direction input by the control of the controller and estimating radio channel parameters for each path signal; A K maximum ratio combiner for combining the despread signal in the first demodulator group with a ratio of the received path signal powers and outputting the combined signals; And a K player for reproducing the output signal of the maximum ratio combiner using radio channel parameters estimated by a corresponding demodulator of the first demodulator group. The method of claim 6, The demodulator has two input stages, The parallel searcher selects two beam directions in which power of a signal having a spreading code such as a spreading code of a desired user is largely transmitted to the controller, The controller may be configured to input the beam signals in the selected two directions to the two input terminals and control the interference canceling unit to perform multi-user signal detection interference cancellation between path signals belonging to the same beam direction. Multi-user interference cancellation device of communication system.