WO2008014631A1

WO2008014631A1 - A demodulation method for downlink received signal in cdma

Info

Publication number: WO2008014631A1
Application number: PCT/CN2006/001493
Authority: WO
Inventors: Guan Chen
Original assignee: Zte Corporation
Priority date: 2006-06-29
Filing date: 2006-06-29
Publication date: 2008-02-07
Also published as: CN101390319A; CN101390319B

Abstract

A demodulation method for downlink received signal in CDMA, including the following steps: multipath searching the received baseband digital I/Q signal, channel estimating the searched L pieces of effective multipath to obtain the column vector of the channel estimation value (110); the 3 pieces of paths respectively called a strong-path, a week-path, a dummy-path which the weak-path is related to the strong-path are assigned (120), if L is equal to 2 and the Signal-To-Noise ratio is larger than the threshold predetermined; multipath de-scrambling and despreading the paths assigned to obtain a despreading column vector for a target symbol (130); one of the interfering item of the strong-path is replaced by the dispreading result multiplied by a coefficient, so that the despreading result of the strong-path updated is obtained (140); the old despreading result is replaced by the strong-path despreading result updated for maximal ratio combining to obtain demodulated target symbol (150). The present method operates simply and can achieve better performance than Rake receiver, when the number of paths is 2 and the Signal-To-Noise ratio is relatively large, and the interference in cellular presents on main situation.

Description

Demodulation method for CDMA downlink received signal

Technical field

The present invention relates to a mobile station receiving technique in a CDMA wireless communication system, and more particularly to a Rake receiving method of a CDMA downlink Rake receiver. Background technique

In CDMA mobile communication systems, the receiver usually uses the Rake scheme. In this receiving method, the receiver uses the estimated multipath information to perform channel estimation for each path, and then performs channel correction for each path, and finally corrects the deviation of the respective paths for maximum ratio combining.

The principle can be expressed by the formula, the descrambling and despreading output of each path:

y = hS ₀ + u (1)

In the above formula,

, L is the effective multipath number of multipath search. . Is the target symbol to be detected, A is the product of channel distortion and the transmitted power of the symbol, and y is a column vector, _3⁄4 is thermal noise and inter-cell interference (modeled as white noise) descrambling the target symbol The total interference and noise of the despread output is the column vector;

The maximum ratio is combined as:

z = h ^H y (2)

In the above formula, /; is the channel estimation result of each path, which is a column vector;

z is an estimate of the symbol S ₀ .

This reception method is optimal in the case where the interference of each path (i.e., u) is independent. However, in the CDMA downlink, different users spread the spectrum with different channelization codes, and in the case of synchronous transmission, they are orthogonal to each other. Since multipath breaks this orthogonality, inter-path interference is introduced. This inter-path interference can be colored as long as some improvements are made to the multipath distribution.

Literature [1] proposed a method for whitening colored interference, called generalized

Rake (g-Rake, or generalized Rake) achieves better performance than conventional Rake receivers.

[1]: GE Bottomley, T. Ottosson, Y.-PE Wang, "A generalized Rake Receiver for interference suppression", IEEE Journal on Selected Areas in Communication, Volume 18, Number 8, August/2000.

However, g-Rake needs to calculate the interference correlation matrix and its inverse, requiring additional processing. The inverse of the matrix can cause anomalies, and the matrix is not reversible under any circumstances.

Summary of the invention

The technical problem to be solved by the present invention is to provide a demodulation method for a CDMA downlink received signal. When the path-to-noise ratio of the path number is 2 and the path difference is large, and the interference in the small area is dominant, the operation is simple and does not need to be performed. The matrix inverse is obtained, and better performance than Rake can be obtained.

In order to solve the above technical problem, the present invention provides a method for demodulating a CDMA downlink received signal, which includes the following steps:

(a) performing a multipath search on the received baseband digital signal, performing channel estimation on the searched L effective multipaths, and obtaining a channel estimation value column vector/

(b) If L=2 and the ratio of the signal-to-noise ratio of the strong path ^ and the weak path ₂ in the effective path is greater than the set threshold, then three paths are assigned: the strong path weak path r ₂ , and the weak path ₂ relative to the strong diameter η virtual path _3;

(c) performing multipath descrambling and despreading on the assigned path to obtain a despreading vector for the target symbol; ^^^,;^^^refers to the strong path ^, the weak path r ₂ and the virtual path r _{3 respectively} Despreading result;

(d) Substituting an interference term of the strong path η by a despreading result of the virtual path r ₃ by a coefficient to obtain a despreading result of the updated strong path η;

(e) Replace the original despreading result with the despreading result of the updated strong path; i perform the maximum ratio combining to obtain the demodulated target symbol z.

Further, the above demodulation method may further have the following features: a despreading result of the updated strong path η3; = ;^ - ₃ *, wherein _3⁄4 is a channel estimation result of a strong path η, which is a path of a weak path Estimated results.

Further, the above demodulation method may further have the following features: the demodulated target symbol z = + h ₂ y ₂ . It can be seen that CDMA downlink receivers usually use conventional Rake reception or chip-level Balanced. The invention provides an improved Rake receiving method, which adopts a simple method of interference cancellation, further enhances the combined signal to noise ratio based on the original maximum ratio combining, and has a small number of paths (preferably 2). When the multipath SNR is large, a better performance than the original Rake reception can be obtained. The structure is simple and the amount of calculation is not large. The present invention is more suitable for downlink reception of a CDMA system, that is, applied to a user terminal (UE:). BRIEF abstract

Figure 1 is a schematic illustration of the virtual path set forth in the present invention.

2 is a flow chart of an embodiment of the present invention. Preferred embodiment of the invention

The basic principle of multipath distribution will be described below with reference to the accompanying drawings.

Firstly, the virtual path and the assignable virtual path are introduced: the virtual path refers to the symmetrical position of one effective multipath relative to the other effective multipath; the assignable virtual path refers to the virtual path and any other effective multipath or assigned virtual path. The distance must be greater than or equal to one chip. The formula is:

+ z' l,2,~£ j = \,2,...L, i≠ j (0.1) In the above formula, L is the number of effective multipaths searched by the multipath searcher. Then ϊ is called the virtual path of the diameter _τ relative to the diameter η; where ^ and ^ are the effective multipaths searched by the multipath searcher. For example, if the multipath searcher searches for two effective multipaths η = 0 ( / φ), ₂ = 10 ( ρ), then the virtual path of 7 ₂ relative ^ should be 2 * η - T ₂ = -I0 ( Chip) , that is, the position of a virtual path f is at -lOchip; for the same reason, the virtual path of η relative to 7 ₂ should be 2*τ ₂ -

The position of another virtual path f is at 20chip; the unit of the path delay can be adjusted with the sampling rate.

In Figure 1, there are two effective multipaths P1 and P2. The horizontal direction represents the time delay of the diameter; the vertical direction represents the signal-to-noise ratio (strength) of the diameter. In the figure, the path P2 delay is larger than P1, but the signal-to-noise ratio is weaker than P1. The diameter P2—1 is indicated by a dotted line, indicating that it is a virtual path and is the virtual path of the diameter P2 relative to P1. P2-1 and P2 are symmetric about the center of P1.

Further, if f is still satisfied \ τ - τ _ι \≥ oversampleRate

I For all (0.2) - r _m \ ≥ oversampleRate, let f be the diameter r, relative to the diameter r, the virtual path of ^. In the above formula, Ζ is the number of virtual paths that have been allocated; oversampleRate is the number of oversampled points per chip. The unit of the path delay should also be the number of points after oversampling. When not sampling once per chip, but every sample is oversampleRate times, the time resolution of the equivalent path is increased; at this time, a chip is equivalent to oversampleRate samples. Since multipath search is generally oversampled, that is, one chip can produce more than one sample point, and the positional accuracy of the path is increased to the sample point, so the sample is converted into sample precision. So, what do we put? And ^{T and} so on are converted into sample points. The above can be referred to the literature [1].

In the scenario applied in this embodiment, the number of effective paths searched is L=2, and the signal-to-noise ratios of the two effective paths are quite different.

The two effective multipaths are arranged in descending order of signal to noise ratio as follows:

{ ,τ ₂ } satisfy {SNR{T _X ) » SNR{T ₂ )} (1.3)

Where η (i=l , 2) stores the position of the path. Whether the difference in signal-to-noise ratio between the two effective paths is large enough can be judged by a threshold:

SNR{ _x ) I SNR(T ₂ ) > threshold (1.4)

In the above formula, threshold is a simulation parameter and is a variable to be optimized. If the above formula is satisfied, it is considered that the difference in signal-to-noise ratio between the two paths is large enough.

In the case where there are only two effective paths, in order to enhance the cancellation effect of the interference, the virtual path of the weak path τ· ₂ with respect to the strong path ^ is preferentially assigned. In this embodiment, only one virtual path is allocated, and the allocation process is as follows:

= 2 - τ ₂ (1.5)

Improved Rake receiver principle

As mentioned above, has an virtual path been added? , then a total of three paths, the target symbol <S. The descrambling de-spreading, this step can be used in the current method, the result is:

^ = ^ ₀ + u (1.6) In the above formula, y ₃ is the despreading result of the increased virtual path f; therefore, its despreading result does not contain signal components, all of which are interference and noise, and the channel estimation value corresponding to f can be regarded as zero (ignoring reception) The effect of the shaping filter). A is a channel estimation vector, hn o], corresponding to channel estimation values of the first path, the second path, and the third path (virtual path), respectively. And have:

In the above formula, _A means a channel estimation value in the first path _([eta]) descrambling and despreading process, a second path interference (^) is formed by dividing a second diameter; _[alpha] Similarly, the first means ₂ In the two-path (r ₂ ) descrambling and despreading process, the interference formed by the first path ( _η ) is divided by the channel estimation value of the first path; and the diameter of the three paths (ie, the demagnetization result of the virtual path, p' Is the correlation result of the signal of the second path and the spreading code of the target symbol (ie, the multiplication of the channelization code and the scrambling code) divided by the channel estimation value of the second path; / _3⁄4 , « ₂ , « ₃ are the first The correlation between the thermal noise (including inter-cell interference) of the second, third, and third paths and the spreading code of the target symbol.

It can be seen that the interference of the first path contains the same component/3⁄4 as the despreading result of the third path (virtual path), and only one coefficient is difference; then one of the first paths can be subtracted by a simple elimination method. The interference term A _23⁄4 , whose formula is expressed as follows: y; a H ^p' ₊ ^~ _n3 ) _+f (1.7) Therefore, the interference term is removed / but the interference term +^^ is added. As long as this newly added interference term The power of the first path is increased by less than _{/3⁄4 Al} .

Since 'the average of them can be considered the same, the statistical characteristics are the same. From the previous preset conditions, the signal-to-noise ratio of the first path is much larger than the signal-to-noise ratio of the second path, which is equivalent to i/zj / ² , and since the intra-cell interference dominates, the thermal noise (including inter-cell interference) Power is negligible. Then you can infer:

The last equal sign in the above equation can be considered to be average.

Therefore, the signal-to-noise ratio of the first path does increase. Finally, for the diameter 2 despreading result, y ₂ and the updated path 1 despreading result, the maximum ratio combining is performed:

According to the above method, the demodulation process of the received baseband digital IQ signal in the CDMA downlink receiver of this embodiment is as shown in FIG. 2, and includes the following steps:

Step 10: After receiving the baseband digital IQ signal, the CDMA downlink receiver first performs multipath search, searches for L effective multipaths, and performs channel estimation on the L effective multipaths to obtain a channel estimation value column vector A;

Step 20: If L=2 and the ratio of the signal-to-noise ratio of the strong diameter _η and the weak path r ₂ in the effective path is greater than the set threshold threshold, three paths are allocated: the strong diameter η, the τ ₂ and the weak path r _{2 are} relative The virtual path of the strong path; step 30, performing multipath descrambling and despreading on the allocated path, and obtaining a despreading vector for the target symbol, y = [ _yi , y ₂ , y ₃ ] ^T , , , respectively Refers to the despreading result of the strong diameter η, the weak path r ₂ and the virtual path r ₃ ; Step 40, according to the formula (1.7), update the despreading result of the strongest path, ie: _{= 1} —; ₃ * _ , because it is actual Used, therefore, the channel estimation result of the strong path ^ and the channel estimation result of the weak path are used for calculation;

Step 50: Perform maximum ratio combining on the despreading result according to formula (1.9) to obtain a demodulated target symbol, that is, z = ^y; + 3⁄4j ₂ .

Industrial applicability

The invention can be applied to a CDMA downlink receiver, and further enhances the combined signal-to-noise ratio on the basis of the original maximum ratio combining, when the number of paths is small (preferably 2) and the multipath SNR is large, Can get better performance than the original Rake reception, and the structure is simple, the amount of calculation is not large.

Claims

Claim

A demodulation method for a CDMA downlink received signal, comprising the following steps:

(a) performing a multipath search on the received baseband digital signal, and performing channel estimation on the searched L effective multipaths to obtain a channel estimation value column vector A;

(b) If L=2 and the ratio of the signal-to-noise ratio of the strong diameter _η and the weak path r ₂ in the effective path is greater than the set threshold, then three paths are assigned: the strong path weak path r ₂ , and the weak path r ₂ relative The virtual path r _{3 of the} strong diameter η ;

(C) performing multipath descrambling and despreading on the assigned path to obtain a despreading vector for the target symbol; y = [ _yi , y ₂ , y ₃ f , ^ respectively refer to the strong path, the weak path ₂ and The despreading result of the virtual path r ₃ ;

(d) Substituting an interference term of the strong path η by the despreading result of the virtual path r ₃ by a coefficient to obtain a despreading result of the updated strong path η;

(E) despreading result with a strong path of the updated results of despreading instead of the original maximum ratio combining _Λ obtain demodulated target symbol ζ.

2. The demodulation method according to claim 2, wherein the despreading result of the updated strong path _η ;; = - wherein the channel estimation result of the strong path η is a channel estimation of the weak path

Κ

Count the results.

3. The demodulation method according to claim 3, wherein the demodulated target symbol