CN102611656A - Enhanced channel estimation method and enhanced channel estimation device suitable for uplink of LTE (long term evolution) system - Google Patents
Enhanced channel estimation method and enhanced channel estimation device suitable for uplink of LTE (long term evolution) system Download PDFInfo
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Abstract
Disclosed are an enhanced channel estimation method and an enhanced channel estimation device suitable for uplink of an LTE (long term evolution) system. The method includes: extracting information on an uplink pilot frequency subcarrier, and computing a frequency domain channel coefficient at the position of the uplink pilot frequency subcarrier; obtaining a corresponding virtual subcarrier number according to the length of the subcarrier of the frequency domain channel coefficient; expanding the channel coefficient in a frequency domain according to the obtained virtual subcarrier number, and transforming the windowed channel coefficient to a time domain; filtering noise beyond the maximum effective diameter by means of primary filtering; windowing the time domain to obtain a threshold value by means of estimation on filtered noise; performing secondary filtering by filtering noise within the effective diameter according to the threshold value so as to obtain a time domain channel coefficient after secondary filtering; and transforming the time domain channel coefficient after secondary filtering to the frequency domain, and obtaining a channel frequency domain estimation value by means of window removal. As noise estimation and the effective diameter of a channel have a great influence on channel estimation, using the enhanced channel estimation method can precisely estimate the noise and the effective diameter of the channel, and accuracy of channel estimation is enhanced.
Description
Technical Field
The present invention relates to a wireless communication system, and more particularly, to a channel estimation method and apparatus for uplink enhancement in an LTE system.
Background
The LTE (Long Term Evolution) project is an Evolution of 3G, starting from 3 GPP's toronto conference in 2004. LTE is not a universally misleading 4G technology, but a transition between 3G and 4G technologies, a global standard for 3.9G that improves and enhances 3G over-the-air access technologies, using OFDM and MIMO as the only standards for their wireless network evolution. The peak rates of 326Mbit/s at the downlink and 86Mbit/s at the uplink can be provided under the 20MHz spectrum bandwidth. The performance of cell edge users is improved, the cell capacity is improved and the system delay is reduced.
With the third generation mobile communication system being worldwide, LTE has become a focus of attention in the mobile communication industry. Channel estimation is an important link in wireless communication, and a receiver can accurately complete reception only by accurately estimating a wireless channel. The conventional DFT (discrete fourier transform) channel estimation method can be divided into the following steps:
firstly, receiving channel information on an uplink pilot frequency subcarrier to obtain frequency domain response of the position of the uplink pilot frequency subcarrier;
secondly, transforming the frequency domain response of the pilot frequency subcarrier to a time domain through inverse discrete Fourier transform;
then according to the maximum effective path length L of the channeltapIn time domain filtering, all coefficients except the effective diameter length are set to be zero;
and finally, transforming the time domain channel coefficient to a frequency domain through discrete Fourier transform to obtain a channel estimation value.
In practical transmission systems it is often avoided to introduce virtual subcarriers at the receiving end due to spectral aliasing. But the virtual sub-carriers can go through the discrete fourier transform process again to introduce the energy leakage problem to the DFT channel estimation. In addition, in the time-frequency filtering process, the effective path length of the channel is selected, so that the noise filtering effect is directly influenced, and the channel estimation performance is reduced. When the length of the effective path of the channel is too large, the noise elimination is not thorough; when the effective path length of the channel is too small, a part of channel coefficients are considered as noise to be filtered, so that the energy of the channel is reduced, and information is lost. In the time domain noise filtering process, the sampling point concentrated at the front of the time domain coefficient is considered as the effective path of the channel, and the noise in the effective path is not filtered, which is similar to adding a rectangular window in the frequency domain. When transforming to the time domain, the channel coefficients at the edge of the effective path are spread over the entire frequency band due to truncation, which has a certain effect on the noise estimation. When the signal-to-noise ratio is low, this effect is negligible; when the signal-to-noise ratio is high, a part of the non-negligible channel energy is spread in the noise region, and the calculated noise has a certain deviation.
Disclosure of Invention
In order to solve the problem of inaccurate channel estimation in the prior art, the invention provides an enhanced channel estimation method suitable for an LTE system, which can effectively improve the noise estimation precision and the channel estimation accuracy and reduce the error rate of a receiver.
The technical scheme of the invention is an enhanced channel estimation method suitable for uplink of an LTE system, which comprises the following steps:
step 1, extracting information on uplink pilot subcarriers, and calculating frequency domain channel coefficients at the uplink pilot subcarriers;
step 2, obtaining the corresponding virtual subcarrier number according to the length of the subcarrier of the frequency domain channel coefficient obtained in the step 1;
step 3, expanding the channel coefficient in the frequency domain according to the number of the virtual subcarriers obtained in the step 2, windowing the expanded virtual channel coefficient, and then performing inverse discrete Fourier transform to the time domain;
step 4, carrying out first filtering to filter noise outside the effective diameter to obtain a time domain channel coefficient after the first filtering;
step 5, windowing in the time domain, and obtaining a threshold value through the noise estimation filtered in the step 4;
step 6, performing second filtering, including filtering the noise in the effective diameter of the time domain channel coefficient obtained in the step 4 after the first filtering according to the threshold value obtained in the step 5 to obtain a time domain channel coefficient after the second filtering;
and 7, transforming the time domain channel coefficient obtained in the step 6 after the second filtering to a frequency domain, and performing windowing to obtain a frequency domain estimated value of the channel.
In step 3, the number of virtual subcarriers V is usedscAdding virtual subcarriers across the channel coefficients, including V to the left of the frequency domain CFRscAdd/2 coefficients to the back-end, V to the right of the frequency-domain CFRscThe/2 coefficients are added to the front end; and windowing the virtual subcarriers added at the two ends to inhibit energy leakage, wherein the length of the virtual channel coefficient obtained after expansion after windowing meets the exponential power of 2, 3 and 5.
And in the step 5, a Hamming window or a Hanning window or a Blackman window is adopted when the time domain is windowed.
In step 4, the maximum energy is used
Finding out the position of channel tap coefficient as the time bias point I of channel, shifting the windowing position on the time bias point I, and dividing the time domain noise into (M)sc-Vsc)/2+I，(Msc-Vsc) /2-I), wherein MscFor the number of sub-carriers allocated to a user, VscIs the number of virtual sub-carriers,
is the time domain channel coefficient before the first filtering and is the frequency domain channel coefficient
The inverse fourier transform of (a).
Moreover, the threshold is selected as follows:
wherein λ represents a threshold value, and λ ≧ 0, k is in the range of (1.5, 2.5),
representing the standard deviation of the noise.
The invention also correspondingly provides an enhanced channel estimation device suitable for the LTE system, which comprises a frequency domain channel coefficient acquisition module, a virtual subcarrier number acquisition module, an expansion windowing module, a first filtering module, a noise estimation module, a second filtering module and a transform windowing module;
the frequency domain channel coefficient acquisition module is used for extracting information on the uplink pilot frequency subcarrier, calculating the frequency domain channel coefficient at the uplink pilot frequency subcarrier and inputting the frequency domain channel coefficient into the virtual subcarrier number acquisition module;
the virtual subcarrier number acquisition module is used for acquiring the corresponding virtual subcarrier number according to the length of the subcarrier of the frequency domain channel coefficient and inputting the virtual subcarrier number into the expansion windowing module;
the system comprises an expansion windowing module, a first filtering module and a second filtering module, wherein the expansion windowing module is used for expanding channel coefficients in a frequency domain according to the number of virtual subcarriers, windowing the virtual channel coefficients obtained after expansion, performing inverse discrete Fourier transform to a time domain and inputting the time domain to the first filtering module;
the first filtering module is used for carrying out first filtering to filter noise outside the effective diameter to obtain a time domain channel coefficient after the first filtering and inputting the time domain channel coefficient into the second filtering module, and the filtered noise is input into the noise estimation module;
the noise estimation module is used for windowing in a time domain, obtaining a threshold value through noise estimation filtered by the first filtering module and inputting the threshold value into the second filtering module;
the second filtering module is used for carrying out second filtering, and comprises the steps of filtering the noise in the effective diameter of the time domain channel coefficient subjected to the first filtering according to the threshold value obtained by the noise estimation module to obtain the time domain channel coefficient subjected to the second filtering and inputting the time domain channel coefficient into the transform windowing module;
and the transformation and de-windowing module is used for transforming the time domain channel coefficient subjected to the second filtering to a frequency domain, and de-windowing to obtain a frequency domain estimated value of the channel.
According to the technical scheme provided by the invention, after the receiving end receives the uplink reference signal, the number of the virtual subcarriers is calculated according to the obtained frequency domain channel coefficient with noise, the frequency domain channel coefficient with noise is expanded by an edge value repetition method to obtain the expanded channel coefficient, then energy leakage is inhibited by windowing, and the time domain channel coefficient is obtained by IDFT. And obtaining a threshold value for noise filtering through noise estimation, filtering through the first step and the second step, then transforming to a frequency domain channel coefficient through Discrete Fourier Transform (DFT), and finally performing windowing to obtain a frequency domain channel estimation value. By applying the technical scheme of the invention to estimate the channel coefficient, the noise interference in wireless transmission can be effectively eliminated, and the noise estimation accuracy under high SNR is particularly improved; and the accuracy of channel estimation is improved by two-step noise filtering. Meanwhile, the method for obtaining the virtual channel system in the technical scheme of the invention is very simple, so that the hardware processing efficiency can be improved, and the method is suitable for being applied to an LTE system.
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FIG. 1 is a flow chart of an embodiment of the present invention;
FIG. 2 is a diagram of an expanded channel coefficient spectrum according to an embodiment of the present invention;
fig. 3 is a block diagram of an embodiment of the present invention.
Detailed Description
In the following embodiments of the present invention, an embodiment provides an enhanced channel estimation method suitable for LTE system uplink, and an embodiment provides an enhanced channel estimation apparatus suitable for LTE system uplink. The method mainly comprises the steps of spectrum expansion, windowing and de-windowing, first-step noise filtering and second-step noise filtering when the channel coefficient is estimated.
In order to make the technical solutions in the embodiments of the present invention better understood and make the above objects, features and advantages of the embodiments of the present invention more obvious and understandable to those skilled in the art, the technical solutions in the embodiments of the present invention are further described in more detail below with reference to the accompanying drawings.
Referring to fig. 1, the embodiment is configured to uplink a system bandwidth of 20MHz, and the resource allocated to a single user is 10 RBs (i.e., 10 resource blocks), and the detailed channel estimation steps are as follows:
step 1, extracting information on uplink pilot subcarriers, calculating frequency domain channel coefficients at the uplink pilot subcarriers, which can be recorded as
In the embodiment, a pilot signal is obtained as an uplink reference signal according to demapping when uplink data is received; and the conjugate multiplication operation is carried out on the uplink reference signal and the local reference signal generated locally to obtain a frequency domain channel coefficient with noise, which is defined asWherein k is 0.1.2sc-1(MscNumber of subcarriers allocated to user), M in the present embodimentsc10 × 12-120. According to the 3GPP 36.211 protocol, the uplink reference signal is a set of ZC sequences (sequences with strong autocorrelation), so that the frequency domain channel coefficients with noise can be obtained by conjugate multiplication of the received reference signal and the locally generated reference signal.
Step 2, according to the length of the sub-carrier of the frequency domain channel coefficient obtained in the step 1, obtaining the corresponding virtual sub-carrier number, and recording the number as Vsc。
The size of the uplink resource allocation RB specified by the LTE protocol needs to meet certain requirements, namely the size of the current allocated RB
α, β and γ are all non-negative integers. Under the system bandwidth of 20MHz, the RB allocation set satisfying the condition has only 34 elements, that is, only 34 uplink RB resource allocations are available. For this reason, the number of frequency domain subcarriers after the virtual subcarrier is complemented should satisfy the subcarrier length of the next RB size. Suppose the number of virtual subcarriers required is VscAccording to said method there are
For the number of sub-carriers on each RB,
for the size of the RB allocated this time,
the size of the RB is allocated for the next. A table of the subcarrier association under one RB number and the subcarrier association under the next RB number can be established in advance, and the virtual subcarrier number V can be obtained by looking up the tablesc. In this embodiment, the spectrum length after the spreading is 12 × 12-144, and the number of virtual subcarriers is 144-120-24.
And 3, expanding the channel coefficient in the frequency domain according to the number of the virtual subcarriers obtained in the step 2, windowing the expanded virtual channel coefficient, and performing inverse discrete Fourier transform to the time domain.
After the number of virtual subcarriers is calculated in the method, the channel coefficient of the part needs to be obtained, and a method of copying edge values is adopted in this embodiment, see fig. 2. In the figure, CFR represents the original channel coefficient, and the front end and the rear end of the channel coefficient with noise in the frequency domain are respectively copied for longDegree of VscAnd/2, adding the copied channel coefficient of the front end to the tail part of the original channel coefficient, adding the copied channel coefficient of the rear end to the front end of the original channel coefficient, windowing at the added virtual subcarrier, and selecting a window function with better side lobe suppression, such as cosine windows adopted at two ends shown by arrows in fig. 2.
And performing inverse discrete Fourier transform to the time domain after expanding the virtual channel coefficients. The IDFT length is 144 in this embodiment.
And 4, carrying out first filtering to filter noise outside the effective diameter to obtain a time domain channel coefficient after the first filtering. The invention further provides that the effective path length of the channel is half M/2 of the frequency domain subcarrier length, namely the maximum effective path is selected to be half of the total sampling point. From maximum energy
Finding out the position of channel tap coefficient as the time bias point number I of channel, shifting the position of window on I, and making time domain noise part as
((Msc-Vsc)/2+I，(Msc-Vsc)/2-I)。
Ideally, the peak of the channel should be at the first of the time domain sample points, and all other sample points should be noisy. But in a real channel, the peak of the signal may be advanced or delayed. Therefore, determining the length of the effective path directly affects the accuracy of channel estimation. Example calculation of peak position yields the number of time offsets I:
wherein,F-1(. cndot.) denotes an inverse discrete fourier transform. Here, abs (.) represents modulo, max (.) represents maximum value, and index (.) represents the index of maximum value.
The effective path length of the channel is taken as half M/2 of the frequency domain subcarrier length, wherein n is the number of time domain sampling points. And setting the non-effective diameter to be zero to filter out noise. The specific first-step filtering process is formulated as
Where Q is the noise portion.
Representing the time domain channel coefficients after the first filtering.
And 5, windowing in the time domain, and obtaining a threshold value by estimating the noise filtered in the step 4.
The noise power can be estimated through the first noise filtering, the embodiment accurately estimates the noise through windowing, and window functions with large sidelobe fading, such as raised cosine functions, can be selected in the window. For example, window functions having a good side lobe suppression effect, such as hamming window, hanning window, blackman window, etc., are used, but the present invention is not limited to these window functions. The concrete formula is described as follows
Where K denotes the number of subcarriers whose frequency edge contains a large attenuation due to windowing, and is taken to beThe value K is 11. The calculated noise is more accurate at high signal-to-noise ratios.
Represents the standard deviation of noise,
Representing the noise variance resulting from the first filtering.
And calculating a threshold lambda according to the noise power estimated by the noise.
And 6, performing second filtering, including filtering the noise in the effective diameter of the time domain channel coefficient obtained in the step 4 after the first filtering according to the threshold value obtained in the step 5, so as to obtain the time domain channel coefficient after the second filtering. In the step, the effective path length of the channel is also taken as half M/2 of the frequency domain subcarrier length.
The second filtering mainly filters the noise in the effective path of the channel, and the accurate channel estimation value is obtained by further filtering on the basis of the first filtering. And setting all time domain coefficients lower than the threshold to be zero according to the threshold obtained by the noise estimation module, reserving sampling points with energy higher than the threshold in the effective path, setting the sampling points with energy lower than the threshold to be zero, and filtering out the noise, wherein the coefficients higher than the threshold are useful channel information.
And 7, transforming the time domain channel coefficient obtained in the step 6 after the second filtering to a frequency domain, and performing windowing to obtain a frequency domain estimated value of the channel.
And after the time domain channel coefficient after noise filtering is converted to a frequency domain, windowing to remove the virtual subcarrier, and directly taking out the channel coefficient at the corresponding position to obtain a frequency domain estimated value of the channel.
Corresponding to the embodiment of the channel estimation method suitable for the uplink of the LTE system, the invention also provides an embodiment of the channel estimation device suitable for the uplink of the LTE system.
Referring to fig. 3, a block diagram of the apparatus for channel estimation suitable for uplink of LTE system enhanced in the second embodiment includes:
a frequency domain channel coefficient obtaining module M1, configured to extract information on the uplink pilot subcarriers, calculate frequency domain channel coefficients at the uplink pilot subcarriers, and input the frequency domain channel coefficients to the virtual subcarrier number obtaining module M2;
a virtual subcarrier number obtaining module M2, configured to obtain, according to the length of the subcarrier of the frequency domain channel coefficient, the number of corresponding virtual subcarriers, and input the number of virtual subcarriers to the extended windowing module M3;
the expansion windowing module M3 is used for expanding the channel coefficient in the frequency domain according to the number of the virtual subcarriers, windowing the expanded virtual channel coefficient, performing inverse discrete Fourier transform to the time domain, and inputting the time domain to the first filtering module M4;
the first filtering module M4 performs first filtering to filter noise outside the effective diameter to obtain a time domain channel coefficient after the first filtering and inputs the time domain channel coefficient into the second filtering module M6, and the filtered noise is input into the noise estimation module M5;
a noise estimation module M5, configured to perform windowing in the time domain, obtain a threshold value by estimating the noise filtered by the first filtering module M4, and input the threshold value into the second filtering module M6;
a second filtering module M6, configured to perform second filtering, where the second filtering includes filtering noise in the effective diameter of the time domain channel coefficient after the first filtering according to the threshold obtained by the noise estimation module M5, to obtain a time domain channel coefficient after the second filtering, and inputting the time domain channel coefficient into the transform windowing module M7;
and the transform windowing module M7 is configured to transform the time domain channel coefficients subjected to the second filtering to the frequency domain, and perform windowing to obtain a frequency domain estimated value of the channel.
The frequency domain channel coefficient acquisition module M1, the virtual subcarrier number acquisition module M2, the extended windowing module M3, the first filtering module M4, the noise estimation module M5, the second filtering module M6 and the transform windowing module M7 are sequentially connected, and the first filtering module M4 is connected with the second filtering module M6.
Each module can be realized by adopting a software modular design method, and specific work refers to each step of the method. By the above device, the channel estimation working process is as follows: after receiving the uplink reference signal, obtaining a frequency domain channel coefficient with noise according to the uplink reference signal, obtaining the number of required virtual subcarriers by looking up a table, expanding the number of the required virtual subcarriers, simultaneously inhibiting energy leakage by adding a window function, and converting the expanded channel coefficient to a time domain by IDFT; filtering for the first time to filter out noise outside the effective diameter, and estimating the noise to obtain a threshold value; second filtering, using the threshold value obtained by the noise estimation to eliminate the noise in the effective path; and transforming the channel estimation value to a frequency domain through discrete Fourier transform, and performing windowing to obtain an accurate channel estimation value. By applying the embodiment of the application to the channel estimation of the LTE system uplink, the noise interference introduced by the signal in the transmission can be effectively inhibited, the accuracy of the channel estimation is improved, the realization is relatively simple, and the method is suitable for being applied to the LTE system.
It will be clear to those skilled in the art that the techniques of the embodiments of the present invention may be implemented by software plus a required general-purpose hardware platform. The scope of protection of the invention is not limited to the example described for this purpose, but is to be accorded the widest scope consistent with the innovative features of the claims.
Claims (6)
1. An enhanced channel estimation method suitable for uplink of an LTE system is characterized by comprising the following steps:
step 1, extracting information on uplink pilot subcarriers, and calculating frequency domain channel coefficients at the uplink pilot subcarriers;
step 2, obtaining the corresponding virtual subcarrier number according to the length of the subcarrier of the frequency domain channel coefficient obtained in the step 1;
step 3, expanding the channel coefficient in the frequency domain according to the number of the virtual subcarriers obtained in the step 2, windowing the expanded virtual channel coefficient, and then performing inverse discrete Fourier transform to the time domain;
step 4, carrying out first filtering to filter noise outside the effective diameter to obtain a time domain channel coefficient after the first filtering;
step 5, windowing in the time domain, and obtaining a threshold value through the noise estimation filtered in the step 4;
step 6, performing second filtering, including filtering the noise in the effective diameter of the time domain channel coefficient obtained in the step 4 after the first filtering according to the threshold value obtained in the step 5 to obtain a time domain channel coefficient after the second filtering;
and 7, transforming the time domain channel coefficient obtained in the step 6 after the second filtering to a frequency domain, and performing windowing to obtain a frequency domain estimated value of the channel.
2. The enhanced channel estimation method suitable for uplink of LTE system according to claim 1, wherein: in step 3, according to the number V of virtual sub-carriersscAdding virtual subcarriers across the channel coefficients, including V to the left of the frequency domain CFRscAdd/2 coefficients to the back-end, V to the right of the frequency-domain CFRscThe/2 coefficients are added to the front end; and windowing the virtual subcarriers added at the two ends to inhibit energy leakage, wherein the length of the virtual channel coefficient obtained after expansion after windowing meets the exponential power of 2, 3 and 5.
3. The enhanced channel estimation method suitable for uplink of LTE system according to claim 1, wherein: and 5, adopting a Hamming window or a Hanning window or a Blackman window when windowing is carried out in the time domain.
4. The enhanced channel estimation method suitable for uplink of LTE system according to claim 1, wherein: in step 4, the maximum energy is used
Finding out the position of channel tap coefficient as the time bias point number I of channel, shifting the windowing position on the time bias point number I, and obtaining the time domainThe noise part is ((M)sc-Vsc)/2+I，(Msc-Vsc) /2-I), wherein MscFor the number of sub-carriers allocated to a user, VscIs the number of virtual sub-carriers,
is the time domain channel coefficient before the first filtering and is the frequency domain channel coefficient
The inverse fourier transform of (a).
5. The enhanced channel estimation method suitable for uplink of LTE system according to claim 1, wherein: the threshold value is selected according to the following formula:
wherein λ represents a threshold value, and λ ≧ 0, k is in the range of (1.5, 2.5),
representing the standard deviation of the noise.
6. An enhanced channel estimation device suitable for an LTE system, characterized in that: the system comprises a frequency domain channel coefficient acquisition module, a virtual subcarrier number acquisition module, an expansion windowing module, a first filtering module, a noise estimation module, a second filtering module and a transform windowing module;
the frequency domain channel coefficient acquisition module is used for extracting information on the uplink pilot frequency subcarrier, calculating the frequency domain channel coefficient at the uplink pilot frequency subcarrier and inputting the frequency domain channel coefficient into the virtual subcarrier number acquisition module;
the virtual subcarrier number acquisition module is used for acquiring the corresponding virtual subcarrier number according to the length of the subcarrier of the frequency domain channel coefficient and inputting the virtual subcarrier number into the expansion windowing module;
the system comprises an expansion windowing module, a first filtering module and a second filtering module, wherein the expansion windowing module is used for expanding channel coefficients in a frequency domain according to the number of virtual subcarriers, windowing the virtual channel coefficients obtained after expansion, performing inverse discrete Fourier transform to a time domain and inputting the time domain to the first filtering module;
the first filtering module is used for carrying out first filtering to filter noise outside the effective diameter to obtain a time domain channel coefficient after the first filtering and inputting the time domain channel coefficient into the second filtering module, and the filtered noise is input into the noise estimation module;
the noise estimation module is used for windowing in a time domain, obtaining a threshold value through noise estimation filtered by the first filtering module and inputting the threshold value into the second filtering module;
the second filtering module is used for carrying out second filtering, and comprises the steps of filtering the noise in the effective diameter of the time domain channel coefficient subjected to the first filtering according to the threshold value obtained by the noise estimation module to obtain the time domain channel coefficient subjected to the second filtering and inputting the time domain channel coefficient into the transform windowing module;
and the transformation and de-windowing module is used for transforming the time domain channel coefficient subjected to the second filtering to a frequency domain, and de-windowing to obtain a frequency domain estimated value of the channel.
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