CN1287424A - Interference eliminator in CDMA receiver - Google Patents

Abstract

An interference eliminator used by a receiver in a code division multiple access (CDMA) communication system, by multiplying the interference replica by an adjustment coefficient and then removing the multiplied interference replica from the received signal, so as to remove the interference from the received signal to eliminate interference. The interference canceller includes an adjustment factor control unit that adjusts the adjustment factor based on the estimate of the interference canceling characteristic to improve the interference canceling characteristic.

Description

Interference Eliminator in CDMA Receiver

The present invention generally relates to an interference canceller, and more particularly to a receiver for eliminating interference caused by other channels in a receiver used in a radio base station in a cellular direct sequence code division multiple access (DS/CDMA) mobile communication system interference canceller.

In cellular direct-sequence code division multiple access (DS/CDMA) mobile communication systems, the interference generated by the cross-correlation of channel spreading codes leads to the decline of system channel capacity and transmission quality. Therefore, it is best to remove as much of this interference as possible from the received signal.

FIG. 1 illustrates the configuration of a direct sequence code division multiple access mode (DS/CDMA) radio base station receiver including a conventional interference canceller 24. As shown in FIG. The receiver uses digital modulation techniques such as quadrature phase shift keying (QPSK) technology.

As shown in FIG. 1, CDMA waves are received by an antenna 20 and transmitted to a receiving unit 21, where they are demodulated into baseband signals. Then the baseband signal is converted into a digital signal by an analog-to-digital (A/D) converter 22 and sent to a channel finding circuit 23 . Thereafter, this digital signal is input to a general interference canceller 24 as a received signal "r".

The interference canceller 24 generates a plurality of interference replicas, one for each channel, from the received signal "r", and then removes these interference replicas from the received signal "r" to eliminate the interference caused by the asynchrony of the mobile station. The interference between the spread spectrum codes, that is, the interference from other channels. The interference-canceled received signal "r" is input to each receiving unit (demodulation unit) 5 ₁ to 5 _k of users (channels 1 to k).

FIG. 2 shows the construction of a conventional interference canceller 24, a so-called parallel interference canceller.

As shown in FIG. 2, a reception signal "r" is input to each interference canceller unit (ICU) 1 ₁ to 1 _k corresponding to a mobile station (user) k. Interference canceller units (ICU) ₁₁ to _1k generate symbol replica signals _S1 to _Sk and interference replica signals _d1 to _dk corresponding to mobile station k. The symbol replica signals S ₁ to S _k are respectively sent to the receiving units 5 ₁ to 5 _k corresponding to the mobile station k, while the interference replica signals d ₁ to d _k are sent to the adder 2 .

The adder 2 accumulates the interference replica signals d ₁ to d _k generated by the corresponding interference canceller units (ICU) 1 ₁ to 1 _k and sends the accumulated value to the interference removal unit (subtractor) 4, from which the received signal "r" This accumulated value is removed from , thereby obtaining a residual signal "e".

Here, since the reliability of the interference replica signals d ₁ to d _k is low, when the low reliability interference replica signals d ₁ to d _k are removed from the received signal "r", the interference canceling characteristic of the interference canceller is degraded. Therefore, the adjustment coefficient β of the response reliability is determined based on the information on the received power, the number of mobile stations, etc., in order to improve the interference canceling characteristic by multiplying the interference replica signals _d1 to _dk by the adjustment coefficient β.

Fig. 2 shows an example, its adder 2 accumulates the interference replica signal d ₁ to d _k and sends the accumulated value to the multiplier 3; the multiplier 3 multiplies the accumulated value by the adjustment coefficient β and sends the multiplied result to Subtractor 4; subtractor 4 removes the result of this multiplication from the received signal "r" to obtain a residual signal "e". This improves the interference canceling characteristic.

Thereafter, the remaining signal "e" and the mobile station's symbol replica signals _S1 to _Sk are input to respective receiving units ₅₁ to _5k corresponding to the mobile stations. The receiving units ₅₁ to _5k decode the residual signal "e" and the replica signals _S1 to _Sk for the corresponding mobile stations.

Fig. 3 illustrates an i-th interference canceller unit ICU _i among 1 ₁ to 1 _k interference canceller units (ICU), where i represents the serial number of a corresponding certain mobile station (a user).

As shown in FIG. 3, a received signal "r" input to the ICU is first input to a despreading unit 10, and the received signal "r" is despread using a spreading code corresponding to a mobile station "i" to output a received symbol R _i . Then, the received symbol R _I is input to the channel estimation circuit 11 for estimating the channel (transmission line characteristics, such as: phase rotation or amplitude variation in the transmission space, etc.), and utilizes existing information such as: derivatives contained in the transmission signal frequency symbols to output a channel estimation value ξ _i .

FIG. 5 shows a time-division pilot transmission information frame diagram. As shown in the figure, both pilot symbols and information symbols are inserted into each time slot of a transmission signal (for example: 0.625ms).

The multiplier 12 multiplies the received signal R _i generated by the despreading unit 10 by the conjugate complex number ξ _i * of the channel estimation value ξ _i generated by the channel estimation circuit 11, and removes the phase shift applied to the transmission symbol to obtain A received signal R _i ξ _i * with phase shift removed. Then, the received signal R _i ξ _i * is sent to the data determination unit 13, and the received signal R _i ξ _i * is compared with a threshold value, so as to complete the temporary judgment of the data symbol.

The multiplier 14 generates a symbol replica signal S _i which is a replica of the received symbol R _i by multiplying the temporarily judged symbol by the channel estimation value ξ _i . Afterwards, the symbol replica signal S _i is input to the respreading unit 15, which respreads the symbol replica signal S _i using the spreading code of the corresponding mobile station (ie, the i-th mobile station) to generate an interference replica signal d _i . Afterwards, the interference replica signal d _i is sent to the interference removal unit 4, and the symbol replica signal S _I is sent to the receiver 5 _i corresponding to the i mobile station.

Fig. 4 illustrates the configuration of a receiving unit _5i corresponding to the i-th mobile station among the receiving units ₅₁ to _5k .

As shown in FIG. 4 , the receiving unit 5 _i mainly includes a despreading processing unit 50 and a decoder 55 . The despreading processing unit 50 has a despreading unit 51 and a channel estimating circuit 52 which are basically the same as the despreading unit and channel estimating circuit in the above-mentioned ICU. The remaining signal "e" generated by the interference removing unit 4 is sent to the despreading unit 51 to generate a received symbol Qi by performing the same process as ICU _i . The adder 53 adds the received symbol Qi and the symbol replica signal S _i generated by the corresponding ICU _i to generate a received symbol R _i from which interference from other mobile stations has been removed. Afterwards, the multiplier 54 multiplies the received symbol R _i by the conjugate complex number ξ i * of the channel estimation value ξ _i generated by the channel estimation circuit 52 to generate a received symbol R _{i ξ i *} with the transmission line phase shift removed, and The received symbol R _i ξ _i * is sent to decoder 55 . The decoder 55 compares the received symbols R _i ξ _i * with a threshold and decodes them as data symbols.

However, in a general interference canceller, interference is eliminated by removing its internally generated interference replica from the received signal, if the generated interference replica is inaccurate (low reliability), since the interference replica will be removed from the received signal by mistake, will increase the interference power. As a result, the instability of interference replication seriously affects the interference cancellation characteristics of the interference canceller.

Therefore, multiplying the interference replicas by an adjustment coefficient β representing the reliability can effectively improve the interference canceling characteristics in order to alleviate the influence caused by the less reliable interference replicas.

Usually, based on the information about the received power or the number of mobile stations, the reliability of interference reproduction can be predicted in advance before interference removal, and an adjustment coefficient β is determined according to the prediction. However, the reliability of interference replication depends on various conditions, and it is difficult and unrealistic to compile an adjustment coefficient table containing various combinations of conditions in order to find the optimal adjustment coefficient β. Among them, the maximum Doppler frequency is a key factor in determining the reliability of the interference replica, but the radio base station has no information about it. As a result, the predetermined adjustment coefficient β does not necessarily achieve optimum interference canceling characteristics.

A basic object of the present invention is to provide an interference canceller which overcomes the disadvantages mentioned above.

Another more specific object of the present invention is to provide an interference canceller which improves the interference canceling characteristic as much as possible by properly controlling an adjustment coefficient.

The above and other objects of the present invention are realized by an interference eliminator of a CDMA communication system. In order to eliminate interference from a received signal, the interference replica is multiplied by an adjustment coefficient and then removed from the received signal. The interference eliminator Devices include:

An adjustment coefficient control unit for improving the interference cancellation characteristic by controlling the adjustment coefficient based on the estimation of the interference cancellation characteristic.

A better understanding of the features and advantages of the present invention may be obtained from the following detailed description of the invention, taken with reference to the accompanying drawings, which illustrate examples of application of the principles of the invention.

Figure 1 illustrates the structure of a base station receiver including a conventional interference canceller;

Fig. 2 illustrates the structure of the conventional interference canceller shown in Fig. 1;

Fig. 3 illustrates the structure of the interference canceller unit (ICU) of the conventional interference canceller shown in Fig. 2;

FIG. 4 illustrates the configuration of a receiving unit of the general interference canceller shown in FIG. 2. Referring to FIG.

Fig. 5 shows a time-division pilot transmission information frame diagram;

Fig. 6 illustrates the structure of an interference canceller according to the first aspect of the present invention;

Fig. 7 illustrates the structure of an interference canceller according to the second aspect of the present invention;

Fig. 8 illustrates the structure of an interference canceller according to the third aspect of the present invention;

Fig. 9 illustrates the structure of an interference canceller according to the fourth aspect of the present invention;

Fig. 10 illustrates the structure of an interference canceller according to the fifth aspect of the present invention;

Fig. 11 illustrates the structure of the nth interference canceller unit (ICU) of the interference canceller of the fifth scheme; With

Fig. 12 illustrates the structure of an interference canceller according to the sixth aspect of the present invention.

Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

First, the interference canceller of the first aspect of the present invention will be described with reference to FIG. 6 .

The interference canceller of the first scheme shown in FIG. 6 can be applied to the receiver used by the base station of the CDMA mobile communication system shown in FIG. 1 .

As shown in FIG. 6, the interference canceller includes interference canceller units (ICU) 1 ₁ to 1 _k , an adder 2, an adjustment coefficient multiplier 3, an interference removal unit 4, and receivers 5 ₁ to 5 _k , in "with this They have been described in "Description of Technologies Related to the Invention".

Different from the common interference canceller, the interference canceller of the first solution further includes an adjustment coefficient control circuit 6 , an interference power measurement circuit 7 and an interference power measurement circuit 8 .

The interference power measurement circuit 7 is for measuring the power of the received signal "r" and defining the power before interference removal as interference power (hereinafter referred to as: interference power before removal). The interference power measurement circuit 8 is used to measure the power of the residual signal "e" output by the interference removing unit 4 (interference-removed received signal "r") and define the power of the residual signal "e" as the interference-removed power (hereinafter referred to as: interference power after removal). The adjustment coefficient control circuit 6 determines the adjustment coefficient according to the interference power before removal and the interference power after removal.

In this scheme, the received signal "r" contains the signals of all the plurality of mobile stations. When the received signal "r" is despread using the codes of the respective mobile stations, all signals except the signal of the own station are regarded as interference. That is, the larger the power of the received signal "r" becomes, the larger the power of the interference becomes. Likewise, the power of the remaining signal "e" obtained by removing the interference replica from the received signal "r" is regarded as the removed interference power. Correspondingly, the interference measurement circuit 7 takes the power of the received signal "r" as the measurement of the interference power before removal, and the interference measurement circuit 8 takes the power of the remaining signal "e" as the measurement of the interference power after removal.

Regarding the estimation of the interference cancellation characteristic, the most direct method is to compare the interference power before removal with the interference power after removal. It should be noted that the better (more accurate) the interference cancellation characteristic becomes, the more the interference power after removal is reduced. Here, the interference power before removal and the interference power after removal measured by the corresponding interference power measurement circuits 7 and 8 are sent to the adjustment coefficient control circuit 6, and they are compared to reduce the interference power when adjusting the adjustment coefficient β. The value becomes the largest (that is, the difference between the interference power before removal and the interference power after removal becomes the largest).

In this solution, the adjustment coefficient β can be set between 0-1. In this way, even if there are unpredictable changes in the transmission conditions in the transmission channel, since the adjustment coefficient β can be adjusted to adapt to the transmission conditions, the best interference cancellation characteristics can always be obtained.

Furthermore, in this scheme, two interference measurement circuits 7 and 8 are used to measure the interference power before removal and the interference power after removal. The interference cancellation characteristic can be estimated by some interference measurement circuit, so that the interference power before removal and the interference power after removal can be measured in a time-sharing manner or estimated from the interference power after removal only.

Next, the interference canceller of the second aspect of the present invention will be described below with reference to FIG. 7. FIG.

Unlike the interference canceller of the first scheme in which the interference power before removal and the interference power after removal are compared with each other, in the interference canceller of the second scheme, the signal interference before the interference is removed from the received signal "r" The signal-to-interference-to-noise ratio (SIR) is compared with the signal-to-interference-to-noise ratio (SIR) after interference is removed from the received signal "r". In this scheme, by comparing the two ratios, the interference cancellation characteristic is estimated when adjusting the adjustment coefficient β.

Hereafter, the signal-to-interference/noise ratio (SIR) before the interference is removed from the received signal "r" and the signal-to-interference/noise ratio (SIR) after the interference is removed from the received signal "r" are referred to as the signal-to-interference before removal, respectively /noise ratio (SIR) and the signal-to-interference/noise ratio (SIR) after removal.

As shown in FIG. 7 , the interference canceller of the second scheme includes signal-to-interference/noise ratio (SIR) averaging circuits 9 and 10 compared to the interference power measurement circuits 7 and 8 included in the first scheme. Further, interference canceller _units (ICU) ₁₁ to _1k and receivers ₅₁ to _5k can measure signal-to-interference/noise ratios SIR1 to _SIRk of signals received by corresponding mobile stations. Furthermore, each interference canceller unit ICU _i (1≤i≤k) of the interference canceller units (ICU) ₁₁ to _1k obtains a diffusion component as interference, the signal-to-interference/noise ratio SIR _i is then obtained using the average power and diffuse components.

In order to obtain an average signal-to-interference/noise ratio (SIR) before removal, the signal-to-interference/noise ratios SIR 1 to SIR _k measured in the corresponding interference canceller units (ICU) 1 ₁ to 1 k are sent to the signal-to-interference/noise ratios SIR ₁ to SIR _k Noise Ratio (SIR) averaging circuit 9 and averaged. On the other hand, in order to obtain a signal-to-interference/noise ratio (SIR) after removal, it is necessary to send the signal-to-interference/noise ratios SIR ₁ to SIR _k measured in the corresponding receiving units 5 ₁ to 5 _k to the signal-to-interference/noise ratio (SIR) averaging circuit 10 and averaging. Then, the average signal-to-interference/noise ratio (SIR) before removal and the average signal-to-interference/noise ratio (SIR) after removal produced by corresponding signal-to-interference/noise ratio (SIR) averaging circuit 9 and signal-to-interference/noise ratio (SIR) averaging circuit 10 The noise ratio (SIR) is sent to the adjustment coefficient control circuit 6, and the adjustment coefficient β is adjusted according to the two average signal-to-interference/noise ratios (SIR).

As can be understood from the description of the second scheme, the interference cancellation characteristic can be estimated by comparing the average SIR before removal with the average SIR after removal. The better the interference cancellation characteristics become, the larger the signal-to-interference/noise ratios SIR ₁ to SIR _k of received symbols of the mobile station. In this scheme, when adjusting the adjustment coefficient β, the adjustment coefficient control circuit 6 compares the average signal-to-interference/noise ratio (SIR) before removal and the average signal-to-interference/noise ratio (SIR) after removal to achieve the best improvement in SIR /noise ratio SIR ₁ to SIR _k .

In this scheme, in order to obtain the average signal-to-interference/noise ratio (SIR) before removal, the signal-to-interference/noise ratio (SIR 1 to SIR _k ) of the mobile station is measured in the corresponding interference canceller unit (ICU) _{1 1} to 1 _k And carry out averaging at signal-to-interference/noise ratio (SIR) average circuit 9; In order to obtain the average signal-to-interference/noise ratio (SIR) after removing, measure the signal-to-interference/noise of mobile station at corresponding receiving unit ₅₁ to _5k SIR ₁ to SIR _k are compared and averaged in a signal-to-interference/noise ratio (SIR) averaging circuit 10 .

Next, the interference canceller of the third aspect of the present invention will be described below with reference to FIG. 8 .

In a third scheme, the mobile stations are divided into groups according to certain criteria. In each group, the common adjustment coefficient is adjusted for the mobile stations in the group, so that the desired adjustment coefficient β can be flexibly determined for the group, and the interference cancellation characteristic can be improved.

That is, in the first and second schemes, the adjustment coefficient control is performed for all mobile stations, and since the transmission conditions differ among mobile stations, the optimum value of the adjustment coefficient β varies according to the mobile stations. However, since the scale of the system will become large, it is not practical to individually perform adjustment coefficient control for each mobile station. Furthermore, if only the interference cancellation characteristics of a certain mobile station are improved, the improvement is so small that it will be canceled out by noise or changes in transmission conditions. As a result, in the first and second schemes, it is not ideal to control the adjustment coefficient based on the improvement of the characteristics.

Accordingly, in the third scheme, the mobile stations are divided into several groups, and the common adjustment coefficient is controlled in each group.

As shown in FIG. 8, in this scheme, all interference canceller units (ICUs) are divided into M groups #1 to #M, each group including a plurality of interference canceller units (ICUs). The definition of ideal grouping criteria is that mobile stations in the same group have the same level of interference reproduction reliability. For example, the criteria may be received symbol power, received symbol signal-to-interference/noise ratio (SIR), transmission rate, and the like. If the received symbol power is defined as the grouping criterion, since the received symbol power is affected by the distance between the mobile station and the base station, for example, the mobile station can be divided into a far group, where the mobile station is far away from the base station; and a near group, where The mobile station is close to the base station. This makes it possible to adjust the adjustment factor β in the far group and the near group.

Each of the mobile station groups #1 to #M includes: an adder (2 1 -2 M ) for accumulating interference replicas produced by the interference canceller unit (ICU) of each group, an adder (2 ₁ -2 _M ) for adding the interference replica A multiplier (3 _#1 -3 _#M ) that multiplies the adjustment coefficient β, an interference removal unit (4 _#1 -4 _#M ), a control adjustment coefficient (β _#1 -β _#M ), an adjustment coefficient control circuit (6 _#1 -6 _#M ), and an interference removal circuit (8 _#1 -8 _#M ) for measuring the removed interference power.

Also, in FIG. 8, "M" is a group number, and "M, KM" is the number of mobile stations belonging to group #M.

In this structure, the adjustment coefficient is individually controlled in each of the mobile station groups #1 to #M, so that the adjustment coefficient can be adjusted to a relatively optimum value in each group for the mobile stations in the group. As a result, desired adjustment coefficients can be flexibly determined and interference cancellation characteristics can be improved.

Next, the interference canceller of the fourth aspect of the present invention will be described below with reference to FIG. 9 .

In a mobile communication system, in order to obtain a good reception effect, space diversity technology is often used to combine signals received by multiple antenna branches. In each antenna branch, an interference canceller having an adjustment coefficient control unit in any one of the first to third schemes may be employed.

As shown in FIG. 9, in this scheme there are a first antenna branch and a second antenna branch respectively having adjustment coefficient control circuits 6 and 6' which are the same as adjustment coefficient control circuit 6 in the first scheme. The interference cancellers corresponding to the first and second antenna branches have the same structure. The despreading processing units ( _501-50k and _501' - _50k ') of the receivers ₅₁ to _5k are the same as those _shown in Fig. 4, and each branch includes a despreading unit for accumulating symbol replica signals An adder, a channel estimation circuit, and a multiplier for multiplying the replica signal by the channel estimation value.

In the first and second antenna branches, the two received signals "r1" and "r2" are processed by both interference cancellation and despreading. Then for each mobile station, the signals "r1" and "r2" are diversity combined by diversity combining units ₅₆₁ to _56k and output to decoders ₅₅₁ to _55k to decode the data.

Next, an interference canceller according to a fifth aspect of the present invention will be described below with reference to FIG. 10 .

The interference eliminator of the fifth solution is a multi-stage interference eliminator, the generation and elimination of the interference replica signal is repeated multiple times, so that the interference that cannot be completely eliminated in one stage can be further eliminated in the subsequent stages. Such an interference canceller may be configured such that any adjustment coefficient control circuit in the first to fourth schemes is provided at each interference cancellation stage.

As shown in FIG. 10, in this scheme, the interference canceller includes two stages each having an adjustment coefficient control unit of the first scheme.

Note that if a multi-stage interference canceller is used, the interference canceller unit (ICU) of the cancellation stage (1) is configured as shown in Figure 3, and the cancellation stage (n) (n>1, "n" represents the number of the cancellation stage) is Figure 11 shows the configuration.

Different from the interference canceller unit (ICU) of cancellation stage (1), as shown in Fig. 11, each interference canceller unit (ICU) of cancellation stage (n) is configured to have: a The symbol replica S _i ^(n-1) input by the interference canceller unit (ICU) of 1) is added to an output of the despreading unit 10 ⁽ⁿ⁾ to generate an adder 16 ⁽ⁿ ) of a received symbol R _i ^{(n) )} , a symbol replica S _i of the elimination stage (n) obtained by multiplying the symbol replica S _i ^(n-1 ) output by the data determination unit 13 ⁽ⁿ⁾ and the channel estimation value ξ _i ^{(n) (n)} an adder 17 ⁽ⁿ⁾ for inverse addition (ie subtraction); the subtracted result of the adder 17 ⁽ⁿ⁾ is input to the respreading unit 15 ⁽ⁿ⁾ to generate an interference replica signal d _i ⁽ⁿ⁾ . In this configuration, the interference replica signal d _i ⁽ⁿ⁾ is generated from the interference that was not fully canceled in the previous cancellation stage (n-1).

Fig. 12 shows an interference canceller of the sixth aspect of the present invention. The sixth scheme is a mixture of the fourth scheme and the fifth scheme, which adopts space diversity and multi-stage interference cancellation. As shown in FIG. 12 , the interference canceller includes two antenna branches and two interference canceling stages having an adjustment coefficient control unit employed in the first scheme at each stage, and measures interference power to estimate interference canceling characteristics.

The foregoing description is provided to enable any person skilled in the art to use the invention and to describe the best mode contemplated by the inventors for carrying out the invention.

The present application is based on Japanese Priority Application No. 11-188911 filed on July 2, 1999, the entire contents of which are hereby incorporated by reference.

Obviously, various alternatives to the inventive arrangements described herein may be employed in applying the invention. It is intended that the following claims determine the scope of the invention and that structures and methods are covered within these claims and their equivalents.

Claims (6)

1. interference eliminator in cdma communication system disturbs in order to eliminate from received signal, and interference replica be multiply by an adjustment factor and removes from described received signal, and described interference eliminator comprises:

A basis is controlled the adjustment factor control section that described adjustment factor improves described interference eliminated characteristic to the estimation of interference eliminated characteristic.

2. interference eliminator according to claim 1, it is characterized in that, when regulating described adjustment factor, described adjustment factor control section compares with the interference power of eliminating after the described interference the interference power of eliminating before the described interference, so that reduce interference power as much as possible.

3. interference eliminator according to claim 1, it is characterized in that, when regulating described adjustment factor, the signal power of described adjustment factor control section before to described interference eliminated compares the ratio of interference power and the signal power after the described interference eliminated ratio to interference power, so that improve the ratio of signal power to interference power as much as possible.

4. interference eliminator according to claim 2, it is characterized in that, described adjustment factor control section is arranged in each group in a plurality of groups that have divided receive channel, so that the independent described adjustment factor of control in each group in described a plurality of groups.

5. interference eliminator according to claim 1, it is characterized in that, described adjustment factor control section is arranged in each branch in a plurality of antenna branch, so that the described adjustment factor of control separately in each branch in described a plurality of antenna branch.

6. interference eliminator according to claim 1, it is characterized in that, described adjustment factor control section is arranged in every grade of a plurality of interference cancellation stage, the generation of described interference replica and eliminate and can repeat is so that at every grade of the described interference cancellation stage described adjustment factor of control separately.

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