CN1124715C - Multi-path search method and device of CDMA communication system - Google Patents

Abstract

The invention discloses a multipath search method and device of a code division multiple access communication system. The method performs α filtering on the multipath search energy window output by a matching correlator, and then adopts a double-gate noise threshold and an optimal search energy window for the search energy window. The strong multipath threshold jointly judges the multipath. The search device is composed of matching correlator, α filter of search energy window, double-threshold multipath judgment, search window sliding and multipath assignment. The search device has a strong multipath acquisition capability, and can save the hardware overhead of the matching correlator, and can adapt to different multipath channel environments, whether it is an indoor channel or a vehicle channel, it has good multipath acquisition performance.

Description

Multipath search method and device for code division multiple access communication system

The present invention relates to a code division multiple access (CDMA) mobile communication system, more specifically, a multipath search method and device for a code division multiple access communication system.

The main characteristic of land mobile communication channels is multipath propagation. Since the code division multiple access mobile communication system adopts wideband spread spectrum signals, the symbol period is much smaller than the multipath delay expansion of the mobile channel, so there are multiple distinguishable multipath signals at the receiving end. Since these multipath signals arrive at the receiver through different propagation paths, they are fading signals independent of each other. The RAKE receiver using multipath signals can significantly improve the receiving performance under multipath channels, and accurately capture and lock multiple The multipath search of the path signal is a key part of the RAKE receiver.

In code division multiple access (CDMA) spread spectrum communication, the existing well-recognized multipath search method is as follows: use a matching correlator to perform correlation operations on the received signal and the local PN code, and obtain each corresponding to the search energy window A set of correlation energy values of the search phase constitutes the search energy window; then, in the multipath update period, the search energy window obtained by the current correlation operation is accumulated in phase with the last search energy window (non-coherent accumulation), to obtain The search energy window for multipath judgment; the multipath judgment unit estimates the average noise level of the search energy window, and further obtains the noise threshold, and then uses the noise threshold to make a judgment on the multipath of the search energy window to obtain a set of candidate Multipath; the search window sliding unit obtains the search window sliding signal according to the distribution of the candidate multipath; the multipath allocation unit assigns the candidate multipath to each branch received by the RAKE;

In the land mobile communication channel, not only the intensity of multipath changes drastically, but also the position of multipath will change due to the movement of the mobile station relative to the base station. The drastic change of multipath intensity will significantly reduce the multipath resolution ability of the search energy window. Although the existing non-coherent accumulation method can overcome the impact of the drastic change of multipath intensity on the resolution ability of the search energy window to a certain extent, there are the following Defects: First, since non-coherent accumulation must be completed within a multipath update cycle, the number of non-coherent accumulation is limited, thereby limiting the improvement of the method for multipath resolution; second, the method of non-coherent accumulation requires a multi-path Multiple coherent accumulation operations are completed within the path update period, and the frequency of correlation operations is high, which consumes a lot of hardware resources. In the multipath decision unit, the existing noise estimation method is rough, the reason is: because the noise estimation is carried out before the threshold decision, the candidate multipath information to be excluded in the noise estimation is inaccurate, so the noise estimation Accuracy will be affected. The existing multipath judgment unit uses a single noise threshold for multipath judgment. When the multipath energy distribution in the channel is relatively dispersed, this method is suitable, but when the multipath energy distribution in the channel is concentrated in a few multipaths, A single noise threshold decision will use noise as a candidate multipath output, thereby reducing the performance of RAKE reception, so the channel adaptability of a single noise threshold decision is poor.

For this reason, the purpose of the present invention is aimed at the above-mentioned problem that the multipath search method of existing code division multiple access communication system exists, proposes a kind of novel multipath search method of code division multiple access communication system, simultaneously provides a A multipath search device used for implementing the method,

In order to achieve the above purpose,

The multipath search method adopted in the present invention is as follows: first perform α filtering on the search energy window output by the matching correlator; then perform multipath judgment on the search energy window obtained by the α filter with a double-threshold judgment method; One path obtains the multipath output through multipath distribution, and the other path obtains the search window sliding signal through the search window sliding; finally, the signal is fed back to the matching correlator and α filter unit to adjust the search window position.

The multipath search device adopted in the present invention includes a matching correlator unit, an alpha filter unit for searching an energy window, a double-threshold multipath judgment unit, a multipath allocation unit, and a search window sliding unit, wherein:

The matching correlator unit performs correlation operation on the input signal and the local PN code to obtain a group of search energy windows corresponding to each search phase of the search window;

The α filtering unit of the search energy window performs α filtering on the input search energy window to obtain the search energy window after α filtering;

The dual-threshold multipath decision unit screens out candidate multipaths from the search energy window after α filtering;

The multipath allocation unit allocates the candidate multipath to each branch received by RAKE;

The search window sliding unit calculates and obtains the search window sliding signal according to the input candidate multipath.

Method of the present invention and the device that adopts for carrying out this method have following advantage:

First, the present invention adopts the method of carrying out alpha filtering to the multipath search energy window to improve the multipath resolution capability of the search energy window. Therefore, a search energy window with better multipath resolution capability can be obtained. On the other hand, this method can also reduce the frequency of correlation operations, thereby saving correlation computing resources.

Second, the backtracking method is adopted in the noise estimation of the multipath search energy window. The result of the multipath decision is used for noise estimation, so the candidate multipath can be eliminated more accurately, and a more accurate noise estimation can be obtained.

Third, the adoption of the double-threshold judgment method. The strongest path threshold with a fixed decibel lower than the strongest multipath is set, and then the strongest path threshold is compared with the noise threshold obtained in noise estimation, and the larger one is selected from the two thresholds as the decision threshold for multipath Path judgment, which not only retains the advantages of the existing single noise threshold judgment, but also makes up for the shortcomings of the single noise threshold when the multipath energy is concentrated on a few multipaths.

Below in conjunction with accompanying drawing and embodiment, method and device of the present invention are described in further detail:

Fig. 1 is a schematic block diagram of the principle of the multipath search device of the present invention.

FIG. 2 is a schematic diagram of the workflow of the present invention using the average noise of the backtracking search energy window.

Fig. 3 is a schematic diagram of the principle of double-threshold judgment adopted in the present invention.

In this method, α filtering is first performed on the search energy window output by the matched correlator; then the multipath judgment is performed on the search energy window obtained by the α filter using a double-threshold decision method; The other path is output through the search window sliding to obtain the search window sliding signal; finally, the signal is fed back to the matched correlator and α filter unit to adjust the position of the search window.

The method further comprises the steps of:

a. Correlate the input signal and the local PN code with a matched correlator to obtain a set of correlation energy values corresponding to each search phase of the search window, which is the search energy window;

b. Perform alpha filtering on the search energy window output by the matched correlator to obtain the search energy window after alpha filtering;

c. When the multipath is updated, search for the strongest multipath with the largest correlation energy from the latest α-filtered search energy window to obtain the strongest multipath threshold;

d. Find N multipaths with the strongest energy from the search energy window after α filtering as a group of candidate multipaths, and the value of N should not be less than the number of RAKE receiving branches;

e. Estimate the average noise of the search energy window after α filtering to obtain the noise threshold;

f. Utilize two thresholds that step c and step e obtain, carry out double-threshold multipath decision to the N multipaths that step d obtains, to obtain a group of new candidate multipaths;

g. Use a group of new candidate multipaths obtained in step f for the average noise estimation of step e, repeat step e and step f for M times of backtracking, to obtain the final average noise estimation of the search energy window and a set of candidate multipaths path, and the final average noise estimate is fed back to the α filter unit of the search energy window, and then the final set of candidate multipaths is sent to the multipath allocation and search window sliding unit;

h. assign the candidate multipath that step f obtains to each branch that RAKE receives;

i. Finally, the search window sliding unit feeds back the sliding signal of the search window obtained from the candidate multipath obtained in step f to the matching correlator and the α filter unit of the search energy window, so as to control the sliding of the search window.

When performing α filtering on the search energy window output by the matching correlator in the step b, follow the formula (1-α) × A + α × B, first the energy of each phase of the search energy window output by the historical α filter The values are all set to 0;

Then perform alpha filtering on all phases of the search energy window output by each matching correlator;

On the one hand, the obtained α-filtered search energy window is sent to the multipath decision, and on the other hand, the search energy window is stored as the historical α-filter output of the next α-filter for use in the next α-filter.

If the correlation energy of the i-th phase of the search energy window input by the matching correlator is A, and the correlation energy of the corresponding phase of the search energy window output by the historical α filter is B, B is determined by the following method: Assume The current search energy window slides x phases relative to the historical search energy window, then when the historical search energy window contains the (x+i)th phase, B is the correlation energy on the (x+i)th phase of the historical search energy window value, otherwise B takes the value of the average noise of the current input search energy window.

Then the correlation energy of the corresponding i-th phase of the search energy window after α filtering is (1-α)×A+α×B, where α is the α filter coefficient, and the value of α is between 0.8 and 1.

The acquisition of the strongest multipath threshold in the described step c is to select the maximum value of the correlation energy from the search energy window after α filtering, and use the correlation energy value lower by R decibels than the correlation energy value as the strongest multipath threshold , the value of R is between 8 and 12.

In the described step d, the candidate multipath is determined according to the following steps:

First set all phases in the search energy window after α filtering to be effective phases;

Then find the phase with the largest energy from the effective phases of the search energy window after α filtering, and use this phase as a candidate multipath, and then set this phase and the phases within r chips on the left and right as invalid phases, and the r The value is between 0.75 and 1;

Finally, repeat the previous step until N candidate multipaths are found;

The determination of the estimation of the average noise in the described step e is to remove the phases of the candidate multipaths in the search energy window after α filtering and the phases in each of the left and right chips of each candidate multipath, and then the search energy window The correlation energy values of the other phases in the average, that is, the average noise is obtained.

In the step e, the noise threshold is higher than the average noise estimate by a fixed decibel, and the value of the decibel is between 2 and 4.

In the step f, the double-threshold multipath judgment is to first select the larger one of the strongest multipath threshold and the noise threshold as the judgment threshold, and then compare the N candidate multipaths obtained in step d with the judgment threshold respectively , if it is higher than the decision threshold, it is regarded as a new candidate multipath, otherwise it is not regarded as a new candidate multipath.

In the step g, the number of backtracking repetitions is M, and the value of M is between 1 and 3 times. The purpose of backtracking is to obtain a more accurate average noise estimate, which is the backtracking method used in average noise estimation. On the one hand, a larger value of M can obtain a more accurate average noise estimate, thereby obtaining a more accurate noise threshold, and on the other hand, it will increase computing resources. Considering from two aspects,

The search window sliding signal in the step i is to select two candidate multipaths with the largest phase difference from the candidate multipaths, then calculate the average phase of these two candidate multipaths, and subtract the center position of the search window from the average phase phase value.

Referring to Fig. 1, the multipath search device of the present invention includes a matching correlator unit, an alpha filter unit for searching an energy window, a double-threshold multipath decision unit, a multipath allocation unit, and a search window sliding unit, wherein:

The matching correlator unit performs matching filter correlation operation on the antenna input signal and the local PN code within the range of the search window, and outputs a search energy window composed of correlation energy values on different search phases.

The α filter unit of the search energy window performs α filter on the search energy window, and outputs the search energy window after the α filter.

The double-threshold multipath judgment unit obtains the latest search energy window output by alpha filtering from the alpha filter unit of the search energy window every other multipath update period, and outputs candidate multipath after judgment.

The search window sliding unit accepts the input of the candidate multipath, and obtains the search window sliding signal after processing, and the signal is fed back to the matching correlator unit and the alpha filter unit of the search energy window for sliding the search energy window to track the multipath drift.

The multipath allocation unit allocates candidate multipaths to each multipath branch received by RAKE.

The working process of the α filter unit searching the energy window is:

First, the historical search energy window is defined as the search energy window output by the previous α filter. Since there is no α filter output at the time of initialization, the energy values of each phase of the historical search energy window at this time are set to 0. α filtering is performed on the phase of the currently input search energy window, record the correlation energy on a certain phase of the current input energy window as A, and record the correlation energy on the corresponding phase of the historical search energy window as B, then output The correlation energy of this phase in the search energy window of is (1-α)×A+α×B, where α is the α filter coefficient, and its value is between 0.8 and 1. Perform α filtering on all phases in the input search energy window, and use the obtained search energy window as the historical search energy window for the next α filter.

Since the currently input search energy window may slide relative to the historical search energy window, it is necessary to align the historical search energy window with the current input search energy window before performing α filtering. The specific method is: Assume that the current input search energy The window slides x phases relative to the historical search energy window, then the phase of the historical search energy window corresponding to the phase i of the current search energy window is (x+i), when the historical search energy window contains phase (x+i) , the energy on this phase will be used in the α filtering of phase i of the current input search energy window, when the historical search energy window does not contain phase (x+i), the value of the energy on this phase is the current input The average noise of the search energy window of .

Please refer to Figure 2. In the double-threshold multipath decision unit, the backtracking method is used to obtain a more accurate average noise estimate of the search energy window. The working process is as follows:

First, according to the input search energy window, a group of multipath with the strongest energy is obtained, and this group of multipath is used as a candidate multipath, and then the average noise estimate of the search energy window is obtained according to the candidate multipath, and the noise threshold is further obtained, and then Double-threshold multipath decision, the new candidate multipath is fed back for average noise estimation. Among them, the average noise estimation method of the search energy window is as follows: according to the candidate multipath information, remove the phase of the candidate multipath in the search energy window, and remove the left and right small ranges (between 0.75 and 1 chip) of each candidate multipath The phase in the search energy window is used as the phase of the noise, and the energy on all noise phases is averaged to obtain the average noise estimate of the search energy window.

In the average noise estimation of the search energy window, the candidate multipath information directly affects the accuracy of the noise estimation. The backtracking method adopted in the present invention uses the candidate multipath obtained by the double-threshold multipath judgment to estimate the average noise. The candidate multipath obtained by the threshold judgment is more accurate, so the average noise estimate is correspondingly more accurate, and a more accurate noise threshold can be obtained, and a more accurate noise threshold can improve the performance of the double-threshold multipath judgment, thereby obtaining a more accurate Candidate multipath information, and accurate candidate multipath information can improve the average noise estimation. According to the previous analysis, the backtracking method can not only improve the average noise estimation of the search energy window, but also improve the performance of multipath decision.

Please refer to FIG. 3 , when multipath judgment is performed, two judgment thresholds are used to jointly judge multipath. These two decision thresholds are noise threshold 1 and strongest multipath threshold 2 respectively, where noise threshold 1 is estimated from the average noise of the search energy window, which is higher than the average noise of the search energy window by a fixed decibel number, typically The value is 2 to 4 decibels, and the strongest multipath threshold 2 is determined by the strongest multipath in the search energy window, which is a fixed decibel lower than the energy value of the strongest multipath, and the typical value is 8 ~12 decibels. When judging multipath, first find a group of multipath with the strongest energy from the search energy window, which is called the primary candidate multipath set, and then according to the noise threshold 1 and the strongest multipath threshold 2, from the primary candidate multipath set Among them, the phases higher than the two thresholds are selected as candidate multipaths, while other multipaths are not selected as candidate multipaths.

Affected by the backtracking method in the average noise estimation, the double-threshold multipath decision will be made multiple times, and the noise threshold will be improved each time, resulting in better decision performance.

The method of using double thresholds in multipath judgment is based on the following considerations:

First, setting the strongest path threshold is mainly used in a channel environment with one or a few strong direct (or reflected) multipaths. In this channel environment, there are a small number of multipaths with strong energy, and the energy of most candidate multipaths obtained by using the noise threshold is much lower than the strongest path, and when the energy of the candidate multipaths is within - When it is below 10dB, it hardly contributes to the performance improvement of RAKE combination, and because the low noise threshold may introduce noise multipath, the performance of RAKE reception will decrease instead. This problem can be solved by setting the strongest path threshold.

Second, in a channel environment where there is no strong direct (or reflected) multipath, the main function is the noise threshold. The noise threshold can well eliminate noisy multipaths and obtain reasonable candidate multipaths.

Therefore, the combination of the noise threshold and the strongest path threshold can improve the ability of multipath search to adapt to different channel environments.

The multipath search method and device of the present invention have been simulated in the broadband CDMA uplink, and the simulation platform is the system simulation software Cossap of Synopsis Company. On the premise of the best performance of the search device, the multi-path search device of the present invention saves 2/3 of the matching and correlation calculation resources compared with the existing multi-path search device.

Claims (11)

1, a kind of multipath search method of code division multiple access communication system, it is characterized in that: the method first carries out α filter to the search energy window that matching correlator outputs; Carry out multipath judgment; one of the candidate multipaths output by multipath judgment is multipath output through multipath distribution, and the other path is obtained by sliding the search window to obtain the sliding signal of the search window; finally, the signal is fed back to the matching correlator and α filter unit to adjust Search window position. 2. The multipath search method of CDMA communication system as claimed in claim 1, is characterized in that, the method further comprises the following steps: a. Correlate the input signal and the local PN code with a matched correlator to obtain a set of correlation energy values corresponding to each search phase of the search window, which is the search energy window; b. Perform alpha filtering on the search energy window output by the matched correlator to obtain the search energy window after alpha filtering; c. When the multipath is updated, search for the strongest multipath with the largest correlation energy from the latest α-filtered search energy window to obtain the strongest multipath threshold; d. Find N multipaths with the strongest energy from the search energy window after α filtering as a group of candidate multipaths, and the value of N should not be less than the number of RAKE receiving branches; e. Estimate the average noise of the search energy window after α filtering to obtain the noise threshold; f. Utilize two thresholds that step c and step e obtain, carry out double-threshold multipath decision to the N multipaths that step d obtains, to obtain a group of new candidate multipaths; g. Use a group of new candidate multipaths obtained in step f for the average noise estimation of step e, repeat step e and step f for M times of backtracking, to obtain the final average noise estimation of the search energy window and a set of candidate multipaths path, and the final average noise estimate is fed back to the α filter unit of the search energy window, and then the final set of candidate multipaths is sent to the multipath allocation and search window sliding unit; h. assign the candidate multipath that step f obtains to each branch that RAKE receives; i. Finally, the search window sliding unit feeds back the sliding signal of the search window obtained from the candidate multipath obtained in step f to the matching correlator and the α filter unit of the search energy window, so as to control the sliding of the search window. 3, the multipath search method of CDMA communication system as claimed in claim 2 is characterized in that, when carrying out α filter to the search energy window that matching correlator outputs in the described step b, follow formula (1- α)×A+α×B, first set the energy value of each phase of the search energy window output by the historical α filter to 0, where A is the correlation of the i-th phase of the search energy window input by the matching correlator Energy value, B is the relative energy value of the corresponding phase of the search energy window output by historical α filtering; Then perform alpha filtering on all phases of the search energy window output by each matching correlator; On the one hand, the obtained α-filtered search energy window is sent to the multipath decision, and on the other hand, the search energy window is stored as the historical α-filter output of the next α-filter for use in the next α-filter. 4. The multipath search method of CDMA communication system as claimed in claim 2, characterized in that, the acquisition of the strongest multipath threshold in the described step c is to select correlation from the search energy window after alpha filtering For the maximum value of energy, the correlation energy value that is R decibels lower than the correlation energy value is taken as the strongest multipath threshold, and the value of R is between 8 and 12. 5, the multi-path search method of code division multiple access communication system as claimed in claim 2 is characterized in that, in the described step d, candidate multi-path is determined according to the following steps: First set all phases in the search energy window after α filtering to be effective phases; Then find the phase with the largest energy from the effective phases of the search energy window after α filtering, and use this phase as a candidate multipath, and then set this phase and the phases within r chips on the left and right as invalid phases, and the r The value is between 0.8 and 1; Finally, repeat the previous step until N candidate multipaths are found. 6. The multipath search method of CDMA communication system as claimed in claim 2, characterized in that, the determination of the estimate of the average noise in the described step e is to first remove the candidate in the search energy window after alpha filtering The phase of the multipath and the phases within one chip on the left and right of each candidate multipath, and then average the correlation energy values of the other phases in the search energy window to obtain the average noise. 7. The multipath search method of the code division multiple access communication system as claimed in claim 2 or 6, characterized in that, in the described step e, the noise threshold is higher than the average noise estimate by a fixed decibel number, and the decibel number The value is between 2 and 4. 8. The multipath search method of CDMA communication system as claimed in claim 2, characterized in that, in the described step f, the double-threshold multipath judgment is to first select the stronger multipath threshold and the noise threshold. The larger one is used as the decision threshold, and then the N candidate multipaths obtained in step d are compared with the decision threshold respectively. If it is higher than the decision threshold, it is used as a new candidate multipath, otherwise it is not used as a new candidate multipath. 9. The multipath search method of a code division multiple access communication system according to claim 2, characterized in that, in said step g, the value of the number of backtracking M is between 1 and 3 times. 10. The multipath search method of CDMA communication system according to claim 2, characterized in that, in said step i, the search window sliding signal is to select two candidate multipaths with the largest phase difference from the candidate multipaths. path, and then calculate the average phase of the two candidate multipaths, and subtract the phase value at the center of the search window from the average phase. 11. A multipath search device for a code division multiple access communication system, characterized in that, The device includes a matching correlator unit, an alpha filter unit for searching an energy window, a double-threshold multipath decision unit, a multipath allocation unit, and a search window sliding unit, wherein: The matching correlator unit performs correlation operation on the input signal and the local PN code to obtain a group of search energy windows corresponding to each search phase of the search window; The α filtering unit of the search energy window performs α filtering on the input search energy window to obtain the search energy window after α filtering; The dual-threshold multipath decision unit screens out candidate multipaths from the search energy window after α filtering; The multipath allocation unit allocates the candidate multipath to each branch received by RAKE; The search window sliding unit calculates and obtains the search window sliding signal according to the input candidate multipath.

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