CN1627649A

CN1627649A - Method for eliminating interence of fixed threshold value

Info

Publication number: CN1627649A
Application number: CN 200310119346
Authority: CN
Inventors: 魏立梅
Original assignee: Huawei Technologies Co Ltd
Current assignee: Huawei Technologies Co Ltd
Priority date: 2003-12-10
Filing date: 2003-12-10
Publication date: 2005-06-15
Anticipated expiration: 2023-12-10
Also published as: CN100345386C

Abstract

The disclosed method makes decision method more accurate based on fixed threshold value so as to improve performance of method for eliminating interference and performance of CDMA. The disclosed method includes following steps: (1) receiving demodulated output signal; (2) determining prior probability value of symbol in this demodulation based on information of demodulated result before this demodulation; (3) calculating first decision threshold and second decision threshold based on prior probability value of symbol; (4) determining result of decision based on demodulated output signal as well as relation of relative magnitudes between first decision threshold and second decision threshold.

Description

Fixed threshold interference cancellation method

Technical Field

The present invention relates to an interference cancellation method in a code division multiple access mobile communication system, particularly to a fixed threshold value interference cancellation method in the code division multiple access system.

Background

The third generation Mobile communication system is a new generation Mobile communication system that can meet International Mobile telecommunications (IMT-2000) standards proposed by the International telecommunications union/Future Public Land Mobile Telephone Systems (FPLMTS), and is required to have good network compatibility, to enable roaming among a plurality of different Systems in the global domain, and to provide not only voice and low-rate data services but also wide multimedia services for Mobile subscribers. According to this standard, a third-generation mobile communication system scheme has been proposed in the world, such as Wideband Code Division Multiple Access (WCDMA), Code Division Multiple Access (CDMA) 2000, Time Division Code Division Multiple Access (TD-CDMA), and Time Division Synchronous Code Division Multiple Access (TD-SCDMA). Although these schemes are not very similar, there is a general consensus that CDMA technology is adopted in third generation mobile communication systems worldwide.

The CDMA mobile communication system has the advantages of high capacity, high service quality, good security, and the like. However, there are many disadvantages, such as Multiple Access Interference (MAI). In a practical CDMA communication system, there is a certain correlation between the signals of the users, which is the root cause of the existence of multiple access interference. The MAI generated by individual users is inherently small, but as the number of users increases or the signal power increases, the MAI becomes a major interference for CDMA communication systems, directly limiting the improvement of the capacity, coverage and performance of CDMA systems.

Multi-user Detection (MUD) is a key technology for overcoming interference in CDMA systems, and is an enhanced technology for improving the capacity, coverage and performance of CDMA systems. The traditional detection technology respectively carries out spread spectrum code matching processing on the signal of each user completely according to the classical direct sequence spread spectrum theory, so that the MAI interference resistance is poor; on the basis of the traditional detection technology, the multi-user detection technology fully utilizes all user signal information causing MAI interference to detect signals of a single user, thereby having excellent anti-interference performance, solving the problem of near-far effect and reducing the requirement of the system on power control precision, thereby more effectively utilizing uplink spectrum resources and obviously improving the system capacity.

Verdu proposed in 1986 to implement maximum Likelihood Sequence Detection (MLS Detection) by using a matched filter-viterbi algorithm, and is applicable to channels affected by Inter Symbol Interference (ISI). However, the complexity of the viterbi algorithm is still an exponential power of the number of users, i.e. the k power of 2, and the MLS detector needs to know the amplitude and phase of the received signal, which is estimated. MLS detection is too complex and impractical, and people are looking for sub-optimal multi-user detection techniques that are easy to implement. Sub-optimal multi-user detection techniques fall into two categories, linear multi-user detection and non-linear multi-user detection. The former carries out decorrelation or other linear transformation on the output of the traditional detector so as to be beneficial to receiving judgment, and comprises methods of decorrelation detection, minimum mean square error detection, subspace oblique projection detection, polynomial extension detection and the like; the latter method is different from the former method and includes an Interference Cancellation (IC) detection method, in which the signal of the desired user is regarded as a useful signal, the signals of other users are regarded as Interference signals, the Interference of other users is first eliminated from the received signal to obtain the signal of the desired user, and then the signal of the desired user is detected, thereby improving the performance of the system.

The interference cancellation multi-user detection method is divided into: serial Interference Cancellation (SIC) and Parallel Interference Cancellation (PIC). The SIC is composed of multiple stages, the signal sequences of all users are judged, reproduced and eliminated in sequence at each stage so as to lighten MAI for the following stages, and the operation sequence of each user is determined according to the descending sequence of the signal power. Taking the first processing of the first stage as an example, the output is the received signal after the data decision of the user with the strongest signal and the MAI caused by the user is removed. The same applies to the subsequent first treatments. The following stages are the same. The net result is that the weaker the signal the more beneficial. Compared with the traditional detector, the SIC has the advantages that the performance is greatly improved, the change on hardware is small, the implementation is easy, but the SIC has large time delay, needs power sequencing, has large calculation amount and is sensitive to initial signal estimation. PIC has a multi-stage structure, and is distinguished from SIC in that each stage estimates and removes MAI interference caused by each user in parallel, and then performs data decision. The design idea of PIC is basically the same as that of SIC, but because PIC is parallel processing, the defect of SIC large delay is overcome, and reordering is not needed when the situation changes, so that the PIC has the advantages of small delay and small calculation complexity, has higher practical value in various MUDs, and is the most possible method to realize at present.

As described above, in the interference cancellation method, after symbol decision is performed on the demodulated signal of any user, the user signal needs to be regenerated by the decision result of the symbol of the user, and the interference of the user is cancelled from the received signal, that is, the regenerated signal of the user is subtracted, so as to cancel the influence of the user signal on the detection of the signals of other users.

The current interference cancellation method adopts the following methods for symbol decision of the demodulated signal of the user, for example: a hard decision method, which directly makes decision according to the symbol of the demodulation signal of the receiving end user; the following steps are repeated: fixed threshold based decision methods and soft decision methods proposed in US patent specification US 5418814; for another example: at present, there is also a decision method based on error probability threshold, which calculates two threshold values, and if the decision result value is between the two threshold values, it indicates that the probability of error occurrence in the decision result is high, then it does not participate in cancellation processing.

The purpose of these methods is: the increase in noise power caused by erroneous decision results is minimized. The interference cancellation method based on the fixed threshold assumes the prior probability uniform distribution of the symbols, and carries out judgment according to the calculated fixed threshold.

The interference cancellation method based on the fixed threshold comprises the following steps:

receiving the demodulated user signal;

respectively regenerating each user signal by a decision method based on a fixed threshold value;

and performing cancellation processing on the regenerated user signal in the multi-user signal.

In practical applications, the above scheme has the following problems: due to the fact that the prior probabilities of the symbols are not uniformly distributed in the actual situation, the judgment threshold obtained by calculation is not reasonable enough, the performance of the method is not ideal enough, and the improvement of the system performance is influenced. Under the condition that the decision result of a certain symbol of a user is wrong, the hard decision method can cause that the symbol power of the user in a signal after interference cancellation is not cancelled but is increased by four times, which is very unfavorable for the detection of other user signals.

The main reason for this is that the existing decision methods based on fixed thresholds assume a uniform prior probability distribution of symbols in advance, without estimating the actual distribution.

Disclosure of Invention

The technical problem to be solved by the invention is to provide a fixed threshold value interference cancellation method, so that the judgment method based on the fixed threshold value is more accurate, and the performance of the interference cancellation method and the performance of a code division multiple access system are improved.

In order to solve the above technical problem, the present invention provides a fixed threshold interference cancellation method, which comprises the following steps:

a receiving a demodulated output signal;

b, determining the prior probability value of the demodulated symbol according to the demodulation result information before the demodulation;

c, calculating a first decision threshold and a second decision threshold according to the symbol prior probability value;

d, determining a judgment result according to the relative size relation between the demodulation output signal and the first judgment threshold and the second judgment threshold.

Wherein the step D comprises the following substeps

Judging whether the demodulation output signal is less than or equal to the first judgment threshold, if so, setting the judgment result to be-1;

judging whether the demodulation output signal is greater than or equal to the second judgment threshold, and if so, setting the judgment result to be 1;

and judging whether the demodulation output signal is larger than the first judgment threshold and smaller than the second judgment threshold, and if so, setting the judgment result to be 0.

The first decision threshold and the second decision threshold are formulated as

Wherein, T₁The first decision threshold is set; t is₂Determining a threshold for the second decision; sigma_m，k ²Is the variance of the noise in the demodulated signal; a (m, k) is the channel fading pair when the mth symbol is transmittedDistortion coefficients caused by the symbols; p₁、P_-1The prior probability values of +1 and-1 are respectively taken as the symbol values in the k-th demodulation; ln (·) is taken from a natural logarithm operation.

The method for determining the symbol prior probability value comprises the following steps: the symbol prior probability value of the current demodulation is equal to the symbol posterior probability value of the previous demodulation result, the symbol posterior probability value is calculated by assuming the uniform distribution of the symbol prior probability of the previous demodulation, and the ratio of the symbol prior probability values in the k-th demodulation is represented as follows:

wherein, P₁、P_-1The prior probability values of +1 and-1 are respectively taken as the symbol values in the k-th demodulation, A (m, k-1) is a multiplicative distortion coefficient of a channel in the k-1 demodulation of the mth symbol, Y (m, k-1) is a k-1 demodulation output signal value of the mth symbol, and sigma (m, k-1) is a signal value of the kt-1 demodulation of the mth symbol_m，k-1Ln (-) represents the variance of noise in the k-1 demodulated signal of the mth symbol, which is taken from the natural logarithm operation.

The interference cancellation method based on the fixed threshold value, when the symbol prior probability is determined, the ratio of the symbol prior probability value required by the current demodulation is taken as the ratio of the symbol posterior probability value of the previous demodulation result, and the posterior probability value is obtained by a plurality of times of iterative calculation and is expressed as:

wherein P is₁、P_-1Respectively the prior probability values of +1 and-1 of the symbol in the k demodulation₁(m，k-2)、P_-1(m, k-2) are posterior probability values of +1 and-1 of the symbol values in the k-2 demodulation, A (m, k-1) is multiplicative distortion coefficient of the channel in the k-1 demodulation of the mth symbol, and Y (m, k-1) is the k-1 demodulation output signal of the mth symbolNumber value, σ_m，k-1Ln (-) represents the variance of noise in the k-1 demodulated signal of the mth symbol, which is taken from the natural logarithm operation.

The difference between the technical scheme of the present invention and the prior art can be found by comparing, based on the interference cancellation method disclosed in US patent No. 5418814, the present invention determines the prior probability of the symbol of the current demodulation according to the demodulation result information before the current demodulation in the multiple demodulation, and performs the decision threshold calculation according to the prior probability value for symbol decision.

The difference of the technical scheme brings obvious beneficial effects, namely, the precision of the threshold value for judgment and the reliability of the judgment method are improved through the accurate estimation of the symbol prior probability, and the effectiveness of the interference cancellation method is further improved, thereby improving the performance of the whole CDMA mobile communication system.

Drawings

Fig. 1 is a flow chart of a decision method of a fixed threshold interference cancellation method according to an embodiment of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention will be described in further detail with reference to the accompanying drawings.

A fixed threshold-based decision method of an embodiment of the present invention will be described.

The signal of a certain user demodulated and output from the receiving end is represented by Y (m, k), m represents the corresponding demodulated output of the mth symbol, k represents the output result of the kth demodulation, and Y (m, k) can be represented by the following formula:

formula-Y (m, k) ═ a (m, k) a (m) + n (m, k)

Wherein, a (m) is the mth symbol of the user, and the value is +1 or-1; a (m, k) is multiplicative distortion coefficient caused by channel fading to symbols, and A (m, k) > 0; n (m, k) is additive white Gaussian noise, obeys positive-Tailored distribution N (0, sigma)_m，k ²) Mean value of 0, σ_m，k ²Is the variance.

Here, the probability density function of n (m, k) of the normal distribution is:

wherein f is the probability density value, x is the value variable, exp (·) represents the exponential operation of the natural constant e.

Formula one can be physically interpreted as: when a symbol a (m) is transmitted from a transmitting end to a receiving end through a channel, the channel causes the symbol to generate multiplicative distortion (first term) of channel fading and additive distortion (second term) of white gaussian noise. This is a common channel model.

The first demodulation of the user signal is to demodulate the baseband signal, and each demodulation is performed after the interference cancellation processing of the previous demodulation, that is, the baseband signal after the previous interference cancellation is demodulated.

In one embodiment of the present invention, it is assumed that the prior probabilities of the symbols a (m) at the receiving end taking values of +1 and-1 are respectively denoted as P { a (m) ═ 1} ═ P₁，P{a(m)＝-1}＝P_-1Wherein P {. is } generationThe probability values of the occurrence of the situation in parentheses.

The invention adopts the demodulation result information before the demodulation to calculate the prior probability P of the symbol value used by the demodulation₁、P_-1. In a preferred embodiment of the present invention, the symbol posterior probability value calculated according to the previous demodulation result is used to determine the symbol prior probability value, i.e. P, required for calculating the decision threshold in the current demodulation₁、P_-1. After the method for calculating the decision threshold is described, a method for calculating the symbol prior probability value will be described in detail.

When the result signal of the k-th demodulation output signal Y (m, k) after hard decision is d (m, k), d (m, k) is expressed as:

formula II

Wherein T is₁(m, k) and T₂(m, k) are the left and right thresholds of the decision, respectively.

Based on Bayes criterion, the invention solves left and right thresholds, and then obtains hard decision result of Y (m, k) according to formula two. The specific method comprises the following steps:

the decision cost of symbol decision is: c is A²(m，k)(a(m)-d(m，k))²Based on the Bayesian criterion, the left and right thresholds which minimize the average E (C) of the decision cost can be obtained to satisfy the following formula:

formula III

Formula IV

Wherein P { a (m) ═ 1} ═ P₁、P{a(m)＝-1}＝P_-1

In the US patent 5418814, a threshold T is set, and T ≧ 0. In US patent 5418814, T and-T are used as left and right thresholds, respectively, for the decision. Equality of a priori probabilities according to a (m) values of +1 and-1

(P_{1} = P_{- 1} = \frac{1}{2})

And deducing the value of the threshold T, thereby obtaining the threshold in the fixed thresholding method. And the demodulation result of each symbol is judged according to a threshold value and a formula. When in use

P_{1} = P_{- 1} = \frac{1}{2}

Then, equations three and four degenerate to the fixed threshold method in US 5418814.

In the case where no a priori information is available, only the assumption can be made that the a (m) values are equal in a priori probability of +1 and-1. The decision under this assumption is actually the worst case of the decision. Therefore, the prior probabilities of a (m) values of +1 and-1 are obtained through estimation, and the corresponding left and right decision thresholds and decision formulas are obtained by utilizing the information, so that the correct probability of decision and the performance of the interference cancellation method are further improved.

In the present invention, when the k-th decision is made, the posterior probability P obtained in the (k-1) -th decision is used₁(m，k-1)＝P{a(m)＝1|Y(m，k-1)}、P_-1(m, k-1) ═ P { a (m) ═ -1| Y (m, k-1) } instead of the kth decision, the prior probabilities used in equations three and four: p₁、P_-1。

The posterior probability P is then calculated as₁(m，k-1)、P_-1(m，k-1)：

Formula five

P_{1} (m, k - 1) = P {a (m) = 1 | Y (m, k - 1) > 0}

= \frac{P_{1} f_{1} (Y (m, k - 1))}{P_{1} f_{1} (Y (m, k - 1)) + P_{- 1} f_{- 1} (Y (m, k - 1))}

Formula six

P_{- 1} (m, k - 1) = P {a (m) = - 1 | Y (m, k - 1) < 0}

= \frac{P_{- 1} f_{- 1} (Y (m, k - 1))}{P_{1} f_{1} (Y (m, k - 1)) + P_{- 1} f_{- 1} (Y (m, k - 1))}

Wherein,

the ratio of the posterior probabilities is represented by the formula five and the formula six:

formula seven

When the above formula calculates the ratio of the posterior probabilities, we can adopt two specific approaches:

first, assume:

P_{1} = \frac{1}{2},

P_{- 1} = \frac{1}{2},

then the ratio of the posterior probabilities can be found to be:

formula Ba a

\frac{P_{- 1} (m, k - 1)}{P_{1} (m, k - 1)} = \frac{f_{- 1} (Y (m, k - 1))}{f_{1} (Y (m, k - 1))},

When k > 1.

When k is 1, there is no a posteriori probability information. To conform to the formula octa when k > 1, let

Formula eight b

\frac{P_{- 1} (m, 0)}{P_{1} (m, 0)} = 1

Second, the ratio of the prior probabilities in equation seven is replaced by the ratio of the posterior probabilities of the k-2 th decision, i.e., the order

\frac{P_{- 1}}{P_{1}} = \frac{P_{- 1} (m, k - 2)}{P_{1} (m, k - 2)},

And substituting the formula into seven, a recurrence formula of the ratio of the posterior probabilities can be obtained as follows:

formula Jiua

When k > 1.

When k is 1, no a posteriori probability information is available, and the same order:

formula nine b

\frac{P_{- 1} (m, 0)}{P_{1} (m, 0)} = 1

By values of the formula eight or nine

Substituted in formula III and formula IV

The left and right thresholds for the kth decision can be obtained.

Formula ten

Formula eleven

And calculating a left threshold and a right threshold of the decision according to the formula ten and the formula eleven, and then performing symbol decision according to the formula two. The decision result is used for signal regeneration.

After determining the decision threshold and the decision method, the specific interference cancellation method is as follows:

the interference cancellation method can adopt a parallel interference cancellation structure or a serial interference cancellation structure; and when the symbol is judged, a judgment method of an equation two is adopted, and a judgment threshold value is given by an equation eight, an equation ten and an equation eleven or is given by an equation nine, an equation ten and an equation eleven.

In a preferred embodiment of the present invention, when the user completes the first demodulation and performs symbol decision from the first demodulation result, since there is no a (m) value-taking prior information, the left and right thresholds of the symbol are calculated according to the equations eight b, ten and eleven, and symbol decision is performed according to equation two to obtain the decision result. Then calculated according to the formula octa

This value is used for the calculation of the left and right thresholds in the second symbol decision. After the decision result of the user symbol is obtained, the decision result is used to perform the signal regeneration and subsequent interference cancellation of the user.

When the user finishes the second demodulation, the first judgment is obtained

And (4) carrying out formula eleven and formula eleven, calculating to obtain a left threshold and a right threshold of the symbol, and then judging according to formula II to obtain a judgment result. Then calculated according to the formula octaThis value is used for the third decision threshold calculation.

The other subsequent symbol decision processes of the user perform exactly the same processing.

In another preferred embodiment of the present invention, when the user completes the first demodulation and performs the symbol decision according to the first demodulation result, since there is no a (m) value any prior information, the left and right thresholds of decision are calculated according to the equations nine b, ten and eleven, and then the symbol decision is performed according to the equation two. And calculated according to the formula of nine aThis value is used for the calculation of the threshold in the second symbol decision. After the decision result of the user symbol is obtained, the decision result is used to perform the signal regeneration and subsequent interference cancellation of the user.

When the user finishes the second demodulation, the first judgment is obtained

And (4) carrying out formula ten and formula eleven, calculating to obtain left and right threshold values, and then carrying out symbol decision according to a formula two. Then calculated according to the formula of nine a

This value is used for the third decision threshold calculation.

The following describes in detail the steps of the decision method for the kth demodulation of the interference cancellation method corresponding to a user signal according to the decision method and with reference to fig. 1.

As shown in fig. 1, the process first proceeds to step 101, where this demodulation output signal Y (m, k) is received. The expression of the demodulated output signal is expression one.

Then, step 102 is carried out, the symbol prior probability value of the current demodulation is determined according to the demodulation result information before the current demodulation, the calculation method is shown as the formula octa or the formula nona, and the difference of the calculation methods represented by the two formulas is mentioned above; here, when calculating the prior probability value, the previous demodulation result parameter is needed, and the prior probability is assumed to be uniformly distributed for the first time.

Then step 103 is entered, the threshold value T for decision is calculated according to the symbol prior probability value₁、T₂. The calculation method of the threshold value is shown in the seven formula and the eight formula.

Then, step 104 is entered to determine Y (m, k) and T₁、T₂If Y (m, k) is less than or equal to T₁Step 105 is entered and the decision result d (m, k) — 1 is set, if T is set₁＜Y(m，k)＜T₂Step 106 is entered, and the decision result d (m, k) ≧ 0 is set, if Y (m, k) ≧ T₂Step 107 is entered to set the decision result d (m, k) to 1.

The steps of the decision method can be applied to various interference cancellation methods with different structures, such as a serial interference cancellation method, a parallel interference cancellation method, and the like, and other corresponding steps included in the interference cancellation method are the same as those of the currently used technical scheme.

In the steps of the interference cancellation method, the cancellation processing method and mechanism used in the method can apply feasible schemes existing in the prior art.

While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A method for fixed threshold interference cancellation, comprising the steps of:

a receiving a demodulated output signal;

2. The fixed threshold interference cancellation method according to claim 1, wherein said step D includes the sub-steps of

3. The method of claim 1 wherein the first decision threshold and the second decision threshold are formulated as

Wherein, T₁The first decision threshold is set; t is₂Determining a threshold for the second decision; sigma_m，k ²Is the variance of the noise in the demodulated signal; a (m, k) is a distortion coefficient caused by channel fading to the mth symbol when the mth symbol is transmitted; p₁、P_-1The prior probability values of +1 and-1 are respectively taken as the symbol values in the k-th demodulation; ln (·) is taken from a natural logarithm operation.

4. The method of claim 3 wherein the method of determining the symbol prior probability value comprises: the symbol prior probability value of the current demodulation is equal to the symbol posterior probability value of the previous demodulation result, the symbol posterior probability value is calculated by assuming the uniform distribution of the symbol prior probability of the previous demodulation, and the ratio of the symbol prior probability values in the k-th demodulation is represented as follows:

5. The method for canceling interference with fixed threshold in cdma system according to claim 3, wherein the method for determining the symbol prior probability value is that the ratio of the symbol prior probability value required by the current demodulation is the ratio of the symbol posterior probability value of the previous demodulation result, and the posterior probability value is obtained by performing multiple iterative calculations, and is expressed as:

wherein P is₁、P_-1Respectively the prior probability values of +1 and-1 of the symbol in the k demodulation₁(m，k-2)、P_-1(m, k-2) are posterior probability values of +1 and-1 of the symbol values during the k-2 demodulation, A (m, k-1) is a multiplicative distortion coefficient of a channel in the k-1 demodulation of the mth symbol, Y (m, k-1) is a k-1 demodulation output signal value of the mth symbol, and sigma (m, k-1) is a symbol value of the mth symbol_m，k-1Ln (-) represents the variance of noise in the k-1 demodulated signal of the mth symbol, which is taken from the natural logarithm operation.