CN1463104A - Method for eliminating serial interference utilized in synchronous sode division multiple access (CDMA) system basd on reliability - Google Patents
Method for eliminating serial interference utilized in synchronous sode division multiple access (CDMA) system basd on reliability Download PDFInfo
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Abstract
Interference elimination is carried out according to serial mode in the order determined by the likelihood ratio based on the output of combining maximum ratios. The user with the largest absolute value of likelihood ratio is detected first and the signal generator generates the interference signal of the user. With the said signal being removed from the output of the matched filter, the procedures of de-spread and combining maximum ratios are carried out again and the likelihood ratio is recalculated. The error code platform and the error code rate of the invention are low in small spread spectrum ratio N=4 and large ratio N=128 in CDMA system.
Description
Technical field
The present invention relates to the counteracting serial interference method of code division multiple access system under the synchronous multipath channel.Present invention is specifically related to and reduce in the code division multiple access system under synchronous multipath channel because the interference cancellation method of the interference that multiple access causes and raising performance and power system capacity.
Background technology
The performance of code division multiple access system mainly is subject to interference, disturbs the signal by other user in the system to cause, reason is because non-orthogonal between the subscriber signal waveform of the non-orthogonal or channel of user's spreading code transmission back in the code division multiple access system.
A non-orthogonal example of spreading code be the Walsh transformation scrambled code that adopts in the code division multiple access system spectrum spreading method, in fact be equivalent to spread spectrum at random, non-orthogonal usually between the random sequence that each user's spread spectrum is used in this code division multiple access method, therefore introduced the interference between a plurality of users.Have again, also may be non-orthogonal between the subscriber signal waveform of channel transmission back.Consider two users' example, their spreading code or spread spectrum random sequence are respectively that 1 ,-1,1 ,-1 and 1,1 ,-1 ,-1, two code book body is a quadrature.If two users' the channel that signal experienced does not all have phase deviation, also be quadrature at receiving terminal user waveform so.But if equivalent spreading code had become 1 ,-1,1 ,-1 after the user experienced channel, and equivalent spreading code of the user 2 has become 1,1 ,-1,1.Obvious two equivalent spreading codes become non-orthogonal, because the dot product of two equivalent spreading codes is-2.2 couples of users' 1 of user despreading meeting causes interference, and 1 couple of user's 2 of user despreading also can cause interference, and this interference is corresponding to the dot product of two equivalent spreading codes.
Fortunately, the interference between a plurality of users can adopt the method for Interference Cancellation to eliminate, and this belongs to the method that multiple user signals detects.Patel has just adopted the method based on the counteracting serial interference of user power or correlation ordering in " Analysis of a simple successive interference cancellation scheme in a DS/CDMAsystem; IEEE Journal on Selected Areas in Communications; Vol.12; No 5, June 1994 " literary composition.The basic principle of its Interference Cancellation is in order to detect all users from the total received signal that comprises all user profile, at first detect first user by simple despreading method, simulate this user's interference signal then, and this signal offset from receive resultant signal, repeat the process of this detection, simulation and counteracting, all be detected up to all users.In this method, the order that the user detects can be different, can select the order that detects according to the size of user power or correlation.If with the user power ordering, so prominent user can be at first detected; If adopt the correlation ordering, the user of correlation maximum will at first be detected.
But the performance of the counteracting serial interference under above-mentioned two kinds of ordering rules is not best, because the user's who makes number one detecting reliability is not necessarily the highest, this is obvious, and the power maximum can not illustrate the moment reliability maximum of detection; And the correlation maximum also may be because other user's interference causes.The performance of counteracting serial interference method can also improve.In order to improve the performance of interference cancellation method, in the process of Interference Cancellation, be the most accurately with regard to the input of wishing each user, because user's detection is accurate more, this user's signal imitation produces also accurate more certainly, so just can remove this user's interference effectively from received signal.On the contrary, if user's detection is incorrect, the generation again of this user's signal is also just incorrect so, if deduct this incorrect signal from received signal, may strengthen interference in addition, has reduced the reliability that other user detects on the contrary.In the present invention, by taking to make each user's input accomplish that all reliability is the highest, farthest guarantee the performance of Interference Cancellation by the rule of reliability ordering.The rule of reliability ordering means that the user of reliability maximum is at first detected, rather than as by the user power sort method, prominent user at first obtains detecting, or as by the correlation sort method, the user of correlation maximum at first obtains detecting.
Summary of the invention
The present invention be under a kind of synchronous multipath channel code division multiple access system based on the counteracting serial interference multi-user test method of reliability.Reliability is weighed by the likelihood ratio (or log-likelihood ratio) that the user detects, and in this article, reliability, likelihood ratio, logarithm ratio have identical implication.In the method for the invention, at first calculate the likelihood ratio of input of each user's matched filter sampling output, from all users, select the user of the absolute value maximum of a likelihood ratio then, and carry out despreading frequently and the process of high specific merging.Afterwards, produce this user's signal, and this signal is removed from the sampling output of matched filter according to detected information, spreading code and channel information.This is to take turns Interference Cancellation.One take turns Interference Cancellation after, recomputate other user's detection likelihood ratio, and the process above repeating, finish up to all users are detected.One takes turns bit or the symbol that Interference Cancellation only detects a user.
The invention is characterized in:
1. it is to be that user's sequence number that the rule of likelihood ratio ordering of each user's multipath combined signal is selected to have the maximum likelihood ratio absolute value makes each user's input all accomplish the highest Interference Cancellation performance that farthest guarantees of reliability by taking rule by reliability ordering.
2. it contains successively and has the following steps: (1) carries out chip matched filtering sampling to received signal; (2) each signal is carried out the chip bit synchronous; But estimate the discrimination footpath number of each user's signal; (3) estimate each footpath parameter of multipath, comprise amplitude and phase place; The estimating noise variance; (4) calculate each user's signal reliability, i.e. log-likelihood ratio; (5) size according to the absolute value of log-likelihood ratio sorts to the user; (6) user who makes number one is carried out input, it contains following three steps successively:
(6.1) phase place in each footpath of compensation;
(6.2) despreading is handled;
(6.3) each footpath signal weighting summation, i.e. the Rake (RAKE) of high specific merging receives;
(6.4) result according to RAKE estimates transmitted bit; (7) rebuild user's after testing interference signal, and carry out Interference Cancellation; (8) going on foot for (7) step if the number of users that does not have to detect, repeats (4) more than one, is 0 up to the number of users that does not have to detect.
3. in described step (4), when having only a user, directly change step (6) over to.
4. described likelihood ratio L
mRepresent with following formula:
Wherein, m=1,2 ..., K, K represents number of users;
Spreading code is the vector C of N for length
m, N is a spreading ratio; Matrix H
kExpression L-1 rank multipath channel;
h
k(l) (l=0,1 ..., the L-1) sampling with spreading rate of the unit impulse response of the channel of expression user k experience.w
mBe the high specific of the L dimension weight vector when merging, w
m=[α
1α
2α
L]
T,
Z
0, Z
1..., Z
L-1Be the output of L correlator (being despreader).D
mBe the despreading matrix of (L) * (N+L-1) dimension, can be expressed as
Wherein, 0
lExpression l 0 column vector of forming, θ
M, l, l=0,1 ..., L-1 represents the channel phase skew in each footpath.N
0/ 2: the bilateral power spectral density of noise, i.e. variance; y
mOutput for user m despreading and RAKE reception.
The counteracting serial interference multi-user test method based on the reliability ordering that the present invention describes has solved the problem of multi-user interference restriction system performance in the code division multiple access system well.
Description of drawings
Fig. 1 is a user's a signal process of transmitting.
Fig. 2 is at the theory diagram of receiver end based on the Interference Cancellation of reliability.
Fig. 3 is an algorithm routine flow chart of the present invention.
Embodiment
Fig. 1 has shown a user's baseband signal process of transmitting.Bit (or being called symbol) stream is sent to forming filter through behind the spread spectrum, is sent to channel again.Each user's spreading code is all different, and not mutually orthogonal.Suppose that code division multiple access system guarantees that the bit between the user is synchronous, promptly all user's experience is synchronizing channel (even in up link, the user can guarantee also that in certain locking range therefore synchronous hypothesis is rational).And under the environment that moves, channel shows the form with multipath usually.The present invention relates to as if synchronous multipath channel under the multi-user interference cancellation signal detecting method.
In interference cancellation method, user's the input or the order of Interference Cancellation are very important.If select the at first low user of detecting reliability mistakenly, not only do not offset interference, also may introduce more serious disturbance for other user.The present invention proposes a kind of interference cancellation method that is fit to the code division multiple access system under the synchronous multipath channel, it has adopted the reliability ranking criteria.The reliability of input defines with likelihood ratio (or log-likelihood ratio).Just Interference Cancellation is that log-likelihood according to signal recently sorts.
List among the present invention the method step of the Interference Cancellation of code division multiple access system under the multipath channel synchronously below.Step 4) and 5) be fresh content of the present invention, other step is identical with general interference cancellation method in the document, but for complete, still lists all steps here.
1) carries out chip matched filtering sampling to received signal;
2) each user is carried out the chip bit synchronous; But estimate the discrimination footpath number of each user's channel;
3) estimate each footpath parameter of multipath, comprise amplitude and phase place; The estimating noise variance;
4) calculate each user's signal reliability, i.e. log-likelihood ratio; (if having only a user, just do not need to have calculated likelihood ratio, leap to the 6th) step;
5) size according to the absolute value of log-likelihood ratio sorts to the user;
6) user who makes number one is carried out input;
A) phase place in the every footpath of compensation
B) despreading is handled
C) each footpath signal weighting summation, i.e. the Rake (RAKE) of high specific merging receives
D) result according to RAKE estimates transmitted bit
7) rebuild user's after testing interference signal, and carry out Interference Cancellation;
8) if the number of users that does not have to detect more than 1, repeats the 4th) go on foot the 7th) step, be 0 up to the number of users that does not have to detect;
Below above-mentioned step is described in detail.
At first the signal that receives is carried out matched filtering.Then filtered signal is sampled, sample rate can adopt 16 times of spreading rate.The sync bit of chip bit is found out in chip bit synchronous unit, with decision user's the original position of bit.Under synchronous multipath channel, all users' bit original position is all alignd.Synchronous and the multipath discrimination of chip is carried out simultaneously.Because multipath may just not be to be positioned on the border of chip, so sample rate before will be brought up to more than 16 times.This multiple can be made adjustment to multipath resolving accuracy at interval according to receiver.Suppose that the multipath bar number of telling reaches L.L is generally between 2 to 6, and is or bigger.
Because the characteristic of multipath itself, the channel distortions factor in each footpath of telling is all different, and they all are change at random.The 3rd) go on foot, will estimate the amplitude and the phase place of these channel factors exactly.Phase place is used for carrying out the coherent detection of signal, and is used from high specific merging and signal calculated likelihood ratio with amplitude one.The method of estimation of channel parameter has pilot tone to assist and two kinds of blind estimating methods.The former relies on training sequence, and the latter carries out the estimation of channel parameter according to the statistical property of data.Any method all can not be accomplished hundred-percent accurate, but does not influence use of the present invention.According to these channel parameters that estimates, can carry out calculating and user's ordering of likelihood ratio equally.In addition, the noise variance of estimation will be used for the calculating of bit-detection reliability.
After obtaining channel parameter, just the likelihood ratio of each subscriber signal can have been calculated.Likelihood ratio has provided the reliability that subscriber signal detects.Concrete likelihood ratio computational methods will provide in the back.After obtaining the reliability of subscriber signal, just can sort to the user, and at first detect the user make number one, and regenerate this user's signal then according to the information bit of this user's channel parameter, spreading code and detection, from the resultant signal that receives, deduct then.In fact reconstruction signal is exactly to repeat the process that signal shown in Figure 1 sends, but does not form filtering.In synchronous multipath channel, one takes turns the detection that Interference Cancellation will carry out all users' a bit.The next round Interference Cancellation just carries out the detection of all users' next bit.
A very important step is exactly the input to the user who makes number one in the Interference Cancellation.The user of likelihood ratio absolute value maximum is at first detected, and the method for detection is exactly traditional RAKE method of reseptance.If one the signal in the radio propagation path has experienced the deep fading, and may still comprise stronger signal in another relatively independent path.Therefore can select two or more signals in multipath signal, the benefit of making is it so all has raising for the instantaneous signal-to-noise ratio and the average signal-to-noise ratio of receiving terminal.RAKE receiver is exactly to utilize this principle by merging the signal to noise ratio that multipath signal improves received signal, promptly the signal after each road despreading is weighted summation.The output of supposing L correlator (being despreader) is respectively Z
0, Z
1..., Z
L-1, its weight is respectively α
0, α
1..., α
L-1The size of weight is to be determined by the power output of each branch road or signal to noise ratio.If the power output of branch road or signal to noise ratio are little, corresponding weights is just little so, and total output signal Z ' is
Weight
lThe normalization of available correlator output signal gross power, its summation are 1, that is:
The step of listing above is summarized in Fig. 2, and the switch among the figure is when pulling to dotted line one side after detection finishes with Interference Cancellation once taking turns, and it has shown the principle of code division multiple access system receiver of the present invention under the synchronous multipath channel.After sampling process despreading after the matched filter output and RAKE receive, can be used for carrying out the detection of transmitted bit.Detection is to carry out according to the order of the likelihood ratio of input.The user of likelihood ratio maximum at first obtains detecting, and produces this user's signal then again with the bit that detects, and removes the signal that produces by subtraction from received signal, has just removed this user's interference from multiple user signals.Then, remaining user's likelihood ratio will recomputate, and selects the user of a maximum likelihood ratio again, repeats top process, all detects up to all users to finish.This takes turns the detection that Interference Cancellation has just been finished all users' a bit, and the next round Interference Cancellation will be finished the detection of all users' next bit.
As can be seen, the key of counteracting serial interference is that the user detects the order with Interference Cancellation from top description.Sort according to the subscriber signal likelihood ratio, key will calculate the likelihood ratio of subscriber signal.
This part content provides the computing formula of subscriber signal likelihood ratio (or log-likelihood ratio) under the synchronous multipath channel.After supposing to carry out the counteracting of several interference users, also remain K user in the system, and the user's that balances out of hypothesis input is accurately, so do not contain user's after testing interference in the residual signal.User k (k=1,2 ..., K) the bit x of Fa Songing
kExpression, value is in { ± 1}; Spreading code is the column vector c of N (N is a spreading ratio) with length
kExpression, L-1 rank multipath channel matrix H
kExpression.They can be write as respectively:
c
k＝[c
k(1)…c
k(N)]
T，
C wherein
k(i) expression spreading code c
kI element.
H wherein
k(l) (l=0,1 ..., the L-1) sampling with spreading rate of the unit impulse response of the channel of expression user k experience.
Vector representation in a receiving terminal K user's signal summation is so
Wherein n represents that bilateral power spectral density is N
0/ 2 white Gaussian noise.User m, m=1 ..., the output that the despreading of K and RAKE receive can be write as
W wherein
mBe the high specific of the L dimension weight vector when merging, can be write as
w
m=[α
1α
2α
L]
TD
mThe despreading matrix of expression (L) * (N+L-1) dimension can be write as
Wherein, 0
lExpression l 0 column vector of forming, θ
M, l, l=0,1 ..., L-1 represents the channel phase skew in each footpath.
Observe the expression formula of the output of RAKE receiver, if K-1 is enough big, y so
mCan be similar to a Gaussian random variable, its mathematic expectaion is E{y
m}=w
mD
mH
mc
mx
m, variance is
The detection log-likelihood ratio of user m is designated as L
m, be defined as
Wherein p represents probability density function.By bayesian criterion, log-likelihood ratio can be write as
Can suppose x
mDistribute Deng general, have so
Following formula can be write as
Again because y
m| x
m=+1 and y
m| x
mTherefore=-1 Gaussian distributed all has
Bringing the formula of front necessity into following formula has
By following formula as seen, log-likelihood ratio need be estimated channel parameter, noise variance.They are finished by channel estimating among Fig. 2 and Noise Variance Estimation module.
Below, the counteracting serial interference multi-user test method of how using in code division multiple access system based on reliability is described with a simple example.
In this example, one has 4 users in the system, and their spreading code is respectively c
1, c
2, c
3And c
4Generally speaking, not mutually orthogonal between these 4 spreading codes, therefore can introduce multi-user interference.Multi-user interference has caused injurious effects to the performance of whole system, in order to weaken this influence, can adopt the counteracting serial interference multi-user test method of describing among the present invention based on reliability.Can according to narrate previously the 1st) to 8) individual step carries out.
By 1) to 3) step, obtain or estimate to obtain matrix H
m, D
mAnd w
m, and noise power spectral density N
0/ 2.To each user, calculate log-likelihood ratio L
m, m=1,2,3,4.Suppose L
1The absolute value maximum, so at first use y
1Detect user 1, and simulation generates user 1 interference signal, i.e. H
1c
1x
1, from receive resultant signal, deduct this signal, obtain residual signal r-H
1c
1x
1
Obtain removing the log-likelihood ratio L that will recomputate remaining users behind user 1 the residual signal
m, m=2,3,4.If suppose current L
4The absolute value maximum, this Interference Cancellation of taking turns just will detect user 4 so, and obtain the new residual signal that does not have user 4 to disturb.Calculate L according to new residual signal again
2And L
3Suppose L again
2Absolute value compare L
3Big, take turns in the Interference Cancellation at this so, user 2 obtains detecting, the residual signal of getting back and not having user 2 to disturb.Because only remaining now user 3 has not detected, and does not need to have calculated likelihood ratio, can directly detect.
Like this, through the three-wheel Interference Cancellation, 4 users all are detected.
Claims (4)
- In the synchronization code multi-address division system based on the counteracting serial interference method of reliability, it at first detects first user by simple despreading method, simulate this user's interference signal then, and this signal offset from receive resultant signal, repeat this detection, simulation and the process of offsetting, all be detected up to all users, it is characterized in that: it is to be that user's sequence number that the rule of likelihood ratio ordering of each user's multipath combined signal is selected to have the maximum likelihood ratio absolute value makes each user's input all accomplish the highest Interference Cancellation performance that farthest guarantees of reliability by taking rule by reliability ordering.
- 2. based on the counteracting serial interference method of reliability, it is characterized in that in the synchronization code multi-address division system according to claim 1 that it contains successively and has the following steps: (1) carries out chip matched filtering sampling to received signal; (2) each signal is carried out the chip bit synchronous; But estimate the discrimination footpath number of each user's signal; (3) estimate each footpath parameter of multipath, comprise amplitude and phase place; The estimating noise variance; (4) calculate each user's signal reliability, i.e. log-likelihood ratio; (5) size according to the absolute value of log-likelihood ratio sorts to the user; (6) user who makes number one is carried out input, it contains following three steps successively:(6.1) phase place in each footpath of compensation;(6.2) despreading is handled;(6.3) each footpath signal weighting summation, i.e. the Rake (RAKE) of high specific merging receives;(6.4) result according to RAKE estimates transmitted bit; (7) rebuild user's after testing interference signal, and carry out Interference Cancellation; (8) going on foot (.7) step if the number of users that does not have to detect, repeats (4) more than one, is 0 up to the number of users that does not have to detect.
- 3. based on the counteracting serial interference method of reliability, it is characterized in that in the synchronization code multi-address division system according to claim 2: in described step (4), when having only a user, directly change step (6) over to.
- 4. based on the counteracting serial interference method of reliability, it is characterized in that described likelihood ratio L in the synchronization code multi-address division system according to claim 1 and 2 mRepresent with following formula:Wherein, m=1,2 ..., K, K represents number of users;Spreading code is the vector C of N for length m, N is a spreading ratio;Matrix H kExpression L-1 rank multipath channel;
h k(l) (l=0,1 ..., the L-1) sampling with spreading rate of the unit impulse response of the channel of expression user k experience.w mBe the high specific of the L dimension weight vector when merging,w m＝[α 1α 2…α L] T，
Wherein, 0 lExpression l 0 column vector of forming, θ M, l, l=0,1 ..., L-1 represents the channel phase skew in each footpath.N 0/ 2: the bilateral power spectral density of noise, i.e. variance;y mOutput for user m despreading and RAKE reception.
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Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN100417048C (en) * | 2005-10-18 | 2008-09-03 | 中兴通讯股份有限公司 | Method and device for increasing synchronous detection performance in synchronous CDMA system |
| CN101946416A (en) * | 2008-01-02 | 2011-01-12 | 高通股份有限公司 | Interference Detection and alleviating |
| CN102820938A (en) * | 2012-08-22 | 2012-12-12 | 电子科技大学 | Detection method for multi-antenna superimposing coding modulation system |
| CN104767547A (en) * | 2015-04-17 | 2015-07-08 | 中国电子科技集团公司第五十四研究所 | Spread spectrum communication multi-site interference elimination method |
| EP3712626A1 (en) | 2019-03-19 | 2020-09-23 | FRAUNHOFER-GESELLSCHAFT zur Förderung der angewandten Forschung e.V. | High-rate dft-based data manipulator and data manipulation method for high performance and robust signal processing |
| CN112438023A (en) * | 2018-07-20 | 2021-03-02 | 上海诺基亚贝尔股份有限公司 | Polarization encoding and decoding |
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2003
- 2003-06-18 CN CN03137555A patent/CN1463104A/en active Pending
Cited By (9)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN100417048C (en) * | 2005-10-18 | 2008-09-03 | 中兴通讯股份有限公司 | Method and device for increasing synchronous detection performance in synchronous CDMA system |
| CN101946416A (en) * | 2008-01-02 | 2011-01-12 | 高通股份有限公司 | Interference Detection and alleviating |
| CN101946416B (en) * | 2008-01-02 | 2014-10-22 | 高通股份有限公司 | Interference detection and mitigation |
| CN102820938A (en) * | 2012-08-22 | 2012-12-12 | 电子科技大学 | Detection method for multi-antenna superimposing coding modulation system |
| CN102820938B (en) * | 2012-08-22 | 2014-06-18 | 电子科技大学 | Detection method for multi-antenna superimposing coding modulation system |
| CN104767547A (en) * | 2015-04-17 | 2015-07-08 | 中国电子科技集团公司第五十四研究所 | Spread spectrum communication multi-site interference elimination method |
| CN112438023A (en) * | 2018-07-20 | 2021-03-02 | 上海诺基亚贝尔股份有限公司 | Polarization encoding and decoding |
| CN112438023B (en) * | 2018-07-20 | 2024-03-29 | 上海诺基亚贝尔股份有限公司 | Polarization encoding and decoding |
| EP3712626A1 (en) | 2019-03-19 | 2020-09-23 | FRAUNHOFER-GESELLSCHAFT zur Förderung der angewandten Forschung e.V. | High-rate dft-based data manipulator and data manipulation method for high performance and robust signal processing |
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