CN1365210A - Multipath searching and channel evaluating method based on digital matched filter - Google Patents

Abstract

The present invention utilizes a digital matched filter to complete the digital matched filtering of one or more spread spectrum code channels, continuously provides the correlation values of the corresponding spread spectrum code channels, performs non-coherent accumulation and judgment on the correlation values of the spread spectrum code channels to be captured. The spread spectrum code acquisition of the code channel can be realized; the correlation value output by the digital matched filter can also be used for coherent or non-coherent accumulation and comparison and judgment, and the multipath signal can be searched with a high gain and the reliable information of each path can be obtained. Channel estimation results; for the spread spectrum code channel that needs chip tracking, the chip tracking error signal can be obtained by calculating the leading and lagging correlation values of the correlation peaks of one or more paths output by the digital matched filter, and then Chip tracking can be completed.

Description

Multipath Search and Channel Estimation Method Based on Digital Matched Filter

(1) Technical field:

The invention belongs to the field of direct sequence spread spectrum (DS/SS) communication, including code division multiple access (CDMA) communication, cellular mobile communication and wireless access using direct sequence spread spectrum technology.

The present invention relates to the realization method of capture, path search, channel estimation and coherent chip tracking based on digital matched filter, and is a brand-new realization scheme of receiver of CDMA communication system. Adopting the present invention can reduce the complexity of the receiver and improve the receiver performance.

(two) background technology:

The receiver of any DSSS wireless communication system must have functions such as spread spectrum code acquisition, multi-path search, channel estimation and chip tracking. The purpose of the present invention is to provide a new realization scheme based on digital matched filter, which can effectively realize functions such as capture, multi-pass search, channel estimation and chip tracking of spreading code with relatively low complexity, and improve receiver performance.

(3) Contents of the invention:

The basic principle of the present invention is: utilize digital matched filter to finish the digital matched filtering of one or more spread spectrum code channels, provide the correlation value of corresponding spread spectrum code channel continuously, carry out the correlation value of the spread spectrum code channel to be captured Non-coherent accumulation and judgment can realize the capture of the spreading code of the code channel; also use the correlation value output by the digital matched filter to carry out coherent or non-coherent accumulation and compare and judge, and the multipath signal can be searched with higher gain and Obtain reliable channel estimation results for each path; chip tracking can be obtained by performing calculations on the leading and lagging correlation values of the correlation peaks of one or more paths output by the digital matched filter for the spreading code channel that requires chip tracking The error signal, according to which the chip tracking can be completed.

Beneficial effects of the present invention: the present invention can flexibly realize high-performance spreading code capture, multi-path search, channel estimation and chip tracking with low implementation complexity.

(4) Description of drawings:

Fig. 1 is a block diagram of the method of the present invention;

Fig. 2 is the realization circuit diagram of chip tracking and channel estimation based on matched filter.

(5) Specific implementation methods:

Below in conjunction with Fig. 1, Fig. 2 further describe the specific implementation mode in detail:

The digital matched filter is the central component of this invention. The baseband analog signal enters the digital matched filter after A/D conversion, and the digital matched filter completes the code filtering of the input digital baseband signal, and continuously provides the correlation value corresponding to the spread spectrum code channel. Here, the number of stages of the digital matched filter can be flexibly adjusted according to the needs, the number of spread spectrum code channels can be adjusted according to the specific situation, and different code channels can also use the same matched filter circuit in time division, such as assuming that the signal sampling rate is 2 Double the chip rate, with 8 times the chip clock, you can use a digital matched filter circuit to realize the matched filtering of 4 code channels. Consider the case of a code channel, due to the autocorrelation characteristics of the spreading code, when the phase of the local spreading code is consistent with the input signal, a higher correlation peak appears, and when the phase is inconsistent, a lower correlation value appears. These correlation values carry the delay phase information of the input signal, multipath diffusion information, and autocorrelation/cross-correlation information of the spreading code. Acquisition, path search, channel estimation, and chip tracking can all output correlation values based on digital matched filters The information carried is completed, and they are extracted according to their respective needs.

The initial acquisition is the first step to complete the reception of the spread spectrum signal, and the spread spectrum receiver obtains the coarse synchronization of the spread spectrum code through the initial acquisition. The initial acquisition based on a digital matched filter is a classic acquisition method, which can usually be completed by making one or more decisions through a non-coherent detector. The specific acquisition circuit and search strategy can be flexibly considered according to the performance requirements of the acquisition and the quality of the input signal. Specifically, capture can be divided into time-limited and unlimited-time, and its performance indicators are embodied in several parameters such as average capture time, capture time variance, capture probability, and time-limited capture probability. The specific capture circuit should be able to meet the requirements of these indicators. performance requirements. The characteristic of capturing based on digital matched filter is that it can realize fast capture, because the digital matched filter can continuously provide the correlation value of each phase of the spread spectrum code. In addition, the capture based on the digital matched filter can improve the detection performance of the capture circuit by coherently or non-coherently accumulating the output correlation value of the matched filter. It is worth noting that this is not the characteristic of the present invention. The initial capture is only one of the functions realized by the invention. Matched filter-based capture, path search, chip tracking and channel estimation are a mutually restrictive and closely related whole.

After the initial acquisition is completed, it enters the tracking state, and can start the path search and channel estimation circuits at the same time. The tracking circuit is implemented with a delay-locked loop (DLL). Usually, the delay phase-locked loop is implemented with two late/early correlators, and the phase detection error is obtained by comparing the correlation values output by the late/early two correlators. The phase detection error is filtered by the phase-locked loop to control the ADC sampling clock phase or The phase of the local spreading code is adjusted to realize fine tracking of the spreading code.

Figure 2 shows the implementation circuit of chip tracking and channel estimation based on matched filters. The present invention adopts the mode of adjusting the sampling ADC clock to realize the accurate tracking of the spreading code. As shown in Figure 2, the phase detection error of the DLL based on the matched filter is directly extracted from the correlation value output by the matched filter, without additional late/early correlators. In the figure, D represents the delay device, and 2D represents the delay difference between the late/early correlation value, and the delay can be determined according to the input correlation value rate and the requirements of phase discrimination. The phase detection error extraction can be flexibly determined whether coherent or non-coherent is used. If coherent error extraction is used, the channel estimation result can be directly used for implementation. The correlation length of the phase detection error can be flexibly changed by adjusting the accumulation times of the correlation value, and the frequency of the DLL adjustment clock can also be flexibly adjusted. In addition, due to the difference in wireless environment and code rate, the received signal can be divided into single path and multipath. For the reception of single path signal, DLL error extraction does not need to consider the influence of multipath. The implementation mode of the DLL under the multipath signal can be flexibly determined. Since there is only one ADC sampling in this structure, that is, there is only one clock adjusted by the DLL, the phase detection error extraction in a multipath environment adopts the form of multipath diversity combination, and an error signal is synthesized to adjust the ADC clock after loop filtering. The diversity of the phase detection error can adopt selective diversity, and can also adopt the diversity method of equal gain combination or maximum ratio combination, which method can be flexibly selected according to requirements. The DLL adjusts the local clock sampled by the ADC. As a result, the local clock and the transmitted signal clock achieve frequency locking, and at the same time fine-tune the chip timing to achieve accurate tracking of the spread spectrum code. It is worth noting that the precision of the DLL adjustment clock can be adjusted flexibly. The ADC sampling clock can be directly generated by the DLL after adjustment, or it can be a low-rate clock generated by its frequency division, and it will not affect the tracking accuracy of the DLL. .

The performance and tracking accuracy of DLL will affect the performance of path search and channel estimation, while the performance of path search and channel estimation will in turn affect the performance and accuracy of DLL tracking. Specifically, the performance of the DLL depends on the implementation method adopted in a certain signal environment (as mentioned above), the DLL has high tracking accuracy and good performance, the detection probability of the path search will be high, and the false alarm probability will be reduced. At the same time the channel estimation will be more reliable. Conversely, path search provides timing reference for DLL and channel estimation, and channel estimation provides channel response parameters for coherent DLL, so the accuracy of path search and channel estimation affects the tracking performance of DLL.

The path search based on digital matched filtering is to search for the corresponding match in the interval after determining the uncertainty interval of signal arrival (the range of possible arrival time of the signal determined by the wireless frame timing of the signal and the delay extension of the wireless channel). The output correlation values are filtered to determine the arrival instants of separable paths. Unlike initial acquisition, initial acquisition captures the signal without determining the uncertainty interval in which the signal arrives. When the wireless environment changes slowly, such as satellite channel, line-of-sight transmission and other constant parameter or slow-changing channel environment, the signal arrival path is relatively stable or changes slowly. At this time, the tracking circuit can keep up with the slow change of the signal, so only the initial capture and The tracking circuit is enough, and the path searching circuit can be omitted. When the wireless environment changes rapidly, such as in a fast-moving environment, the path to the receiver changes drastically with the rapid change of the receiver's position (including the fading of the signal level, the disappearance of the old path, and the appearance of the new arrival path) At this time, the tracking circuit can no longer keep up with the drastic change of the path. The path search circuit searches the arrival time of the signal in real time, and decides whether to update the path address (the position of the signal arrival path) of tracking despreading and channel estimation, so that The receiver keeps working without having to restart the acquisition circuit when tracking fails.

The path search circuit based on digital matched filter has a simple structure and fast capture speed, because the matched filter can continuously provide the relevant value of the signal reaching the uncertainty interval, but the specific strategy of the search can be flexibly determined.

Channel estimation refers to the estimation of the amplitude and phase of the arriving signal, which is generally completed with the aid of pilot signals after the precise tracking of the spreading code. The pilot signal can be a certain signal carried on other channels, such as data, in time division, or it can be a separate wireless channel parallel to other channels. If it is time-divisionally carried on other channels, the output result of the matched filter can be taken from a fixed position to complete the channel estimation; if it is a separate wireless channel, the relevant results of the matched filter output at the corresponding path address on the channel can be used as the channel estimate.

As shown in Figure 2, the channel estimation based on the digital matched filter is carried out for each signal arrival path after completing the path search and entering the chip tracking state. Indicated by the instruction generated by the address), the correlation value is realized by smooth accumulation or other algorithms, without additional use of correlators, thus simplifying the circuit. Specific algorithms and parameters can be selected flexibly, such as the number of smoothing accumulations can be flexibly selected according to different environments, and some auxiliary algorithms such as adaptive algorithms can be used to improve the estimation performance.

The receiver structure realized based on digital matched filter uses a simple circuit to comprehensively select various algorithms, so that the performance of each component can be optimized.

In summary, the basic features of the present invention are: spread spectrum code acquisition in DSSS communication, multi-search, channel estimation and chip tracking are all realized based on digital matched filters, and higher complexity is obtained with lower implementation complexity. Receiving performance, its outstanding advantages are its flexibility, low complexity and high performance.

The protection of the present invention is not limited to the features described in the embodiment of Fig. 1, but also includes other variants and changes within the scope of the claims.

Claims (10)

1. Based on digital matched filter multipath search and channel estimation method, it is characterized in that: utilize digital matched filter to finish the digital matched filtering of one or more spread spectrum code channels, provide the correlation value of corresponding spread spectrum code channel continuously, to The correlation value of the spread spectrum code channel to be captured is non-coherently accumulated and judged to realize the spread code capture of the code channel; similarly, the correlation value output by the digital matched filter is used for coherent or non-coherent accumulation and comparison judgment, which can be Search for multipath signals with a higher gain and obtain reliable channel estimation results for each path; for the spread spectrum code channel that needs chip tracking, advance the correlation peak of one or more paths output by the digital matched filter The chip tracking error signal can be obtained by operating with the lag correlation value, and the chip tracking can be completed accordingly. 2. The multipath search and channel estimation method based on digital matched filters according to claim 1, characterized in that: there can be one or more digital matched filters. 3. The multipath search and channel estimation method based on digital matched filter according to claim 1, characterized in that: the number of stages of digital matched filter can be arbitrary. 4. The multipath search and channel estimation method based on digital matched filter according to claim 1, characterized in that: the search strategy of spreading code acquisition can be arbitrary. 5. The multipath search and channel estimation method based on digital matched filter according to claim 1, characterized in that: the non-coherent accumulation mode and the number of times of accumulation when the spreading code is captured can be arbitrary. 6. The multipath search and channel estimation method based on digital matched filter according to claim 1, characterized in that: the non-coherent or coherent accumulation mode and the number of accumulations during multipath search can be arbitrary. 7. The multipath search and channel estimation method based on digital matched filter according to claim 1, characterized in that: the strategy of multipath search can be arbitrary. 8. The multipath search and channel estimation method based on digital matched filter according to claim 1, characterized in that: the algorithm and implementation method of channel estimation can be arbitrary. 9. The method for multipath search and channel estimation based on digital matched filter according to claim 1, characterized in that: chip tracking can adopt coherent or non-coherent mode. 10. The multipath search and channel estimation method based on digital matched filter according to claim 1, characterized in that: the implementation method of chip tracking can be arbitrary.

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