CN101945060A - Channel estimation method based on pilot frequency signal in 3GPP LTE downlink system - Google Patents
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Abstract
The invention provides a channel estimation method based on the pilot frequency signal in a 3GPP LTE downlink system aiming at the 3GPP LTE downlink system, which is the channel estimation method using the frequency two dimension combining Weiner iterative filtering and based on the Linear Minimum Mean-square Error (LMMSE) algorithm. In the invention, the Weiner iterative filtering is performed in the frequency domain based on the LMMSE algorithm firstly, then the value estimated by the frequency domain is used to perform the first Weiner iterative filtering in the frequency domain. The experimental test indicates that the channel estimation method can effectively enhance the property of Bit Error Rate (BER), and is closer to the ideal channel estimation compared with the traditional methods. In addition, the method of the invention reduces the operation complexity of the algorithm through the Singular Value Decomposition (SVD) of the LMMSE algorithm, therefore the channel estimation method provided by the invention can not increase too much operation complexity.
Description
Technical Field
The present invention belongs to the technical field of channel estimation in a communication system, and more particularly, to a channel estimation method based on pilot signals in a 3GPP LTE downlink system.
Background
Lte (long Term evolution) is a "quasi 4G" technology developed on the basis of the technical reserve of the Beyond 3G (B3G) research over ten years in order to combat the market challenge of mobile broadband Access technologies such as World Interoperability for Microwave Access (WiMAX) under the trend of "mobile communication broadband" by the 3rd Generation partnership Project (3 GPP). In the air interface aspect, LTE uses Frequency Division Multiple Access (FDMA) instead of Code Division Multiple Access (CDMA) used by 3GPP for a long time as a Multiple Access technology, and largely adopts Multiple-input Multiple-output (MIMO) technology and adaptive technology to improve data rate and system performance, so that the transmission capability of the air interface reaches over 100 Mbit/s. The 3GPP LTE is favored by most operators in the world, and has been recognized as a mobile communication system capable of supporting the world telecommunication industry in 2010-2020.
Since the Orthogonal Frequency Division Multiplexing (OFDM) technology has high data transmission rate and spectral efficiency, and it can effectively resist multipath delay spread, the physical layer of the 3GPP LTE downlink system adopts the OFDM technology. Since a wireless channel in a broadband mobile communication system has frequency domain selectivity and time variability, it is necessary for a receiver to dynamically estimate and track a multipath fading channel before coherent demodulation of a high-speed OFDM signal. In OFDM technology, the most common channel estimation is based on pilot-assisted channel estimation methods.
In the pilot-aided channel estimation algorithm, a transmitting end inserts pilot signals known to both the transmitting end and the receiving end at fixed positions, and then the receiving end estimates a channel response of each OFDM symbol by processing the received signals at the positions. The channel estimation method based on pilot signals in the 3GPP LTE downlink system can be roughly divided into two types of methods, namely Least-Squares (LS) and Minimum Mean-Square Error (MMSE). The two methods are channel estimation on each subcarrier in a frequency domain, wherein the LS channel estimation method does not need channel information and is the simplest to realize; the MMSE channel estimation method utilizes the correlation among subcarriers, the signal-to-noise ratio and other channel statistical information, has better estimation performance, and is widely applied to the channel estimation of OFDM.
Disclosure of Invention
The invention aims to overcome the defects of the existing channel estimation method and provides a channel estimation method based on pilot signals in a 3GPP LTE downlink system with low error rate and low operation complexity.
In order to achieve the above object, the method for estimating a channel based on a pilot signal in a 3GPP LTE downlink system according to the present invention includes the following steps:
(1) frequency domain wiener iterative filtering
Firstly, obtaining a channel frequency domain response estimated value at a reference signal position on the same OFDM symbol:
in the formula (1), the reaction mixture is,
representing the autocorrelation matrix of the subcarriers at the reference signal within the same OFDM symbol, beta being a constant determined by the signal constellation, SNR being the average signal-to-noise ratio, IPIs a unit array;
for the channel frequency domain response H at the reference signal locationPLS estimate of (1), subscript (.)PIndicating the location of the transmitted reference signal.
Then toWiener iterative filtering based on LMMSE algorithm is carried out to reduce the influence of noise and interference on the channel frequency response estimation value on the reference signal position, the bit error rate performance is improved, and the channel frequency response estimation value on the reference signal position after the wiener iterative filtering
Finally, channel frequency domain response estimated value at the position of the reference signal after wiener iterative filtering is utilized
Estimating the channel frequency domain response value of all subcarriers on the same OFDM symbol
In the formula (3), the reaction mixture is,
a cross-correlation matrix representing all subcarriers within the same OFDM symbol and subcarriers at the reference signal;
(2) time-domain wiener iterative filtering
Channel frequency domain response estimation value on nth sub-carrier
And (2) performing wiener iterative filtering based on an LMMSE algorithm to reduce related residual noise and improve the bit error rate performance:
in the formula (4), the reaction mixture is,
is the autocorrelation matrix between all OFDM symbols containing reference signals on the nth subcarrier in a subframe, ILIs an identity matrix, L represents the number of OFDM symbols containing reference signals in each subframe;
channel frequency response estimation after iterative filtering using wiener
Estimating the channel frequency response value of the nth subcarrier of all OFDM symbols in each subframe
Wherein,
is the cross correlation matrix between all OFDM symbols on the nth subcarrier and the OFDM symbol containing the reference signal;
channel frequency domain response estimation on all subcarriers
And performing the calculation, and finally obtaining channel transmission function values at all resource particles through wiener iterative filtering of a frequency domain and a time domain.
In the present invention, in the above algorithm description, there are pairs in wiener iterative filtering, whether in the frequency domain or the time domain:
if the number of reference signals is large, the inversion operation becomes complicated.
To reduce the computation complexity of the channel estimation method proposed by the present invention, the autocorrelation matrix is processed
Singular value decomposition is performed and then the inverse matrix is calculated
The inverse matrix is decomposed into:
where U is an orthogonal matrix column and Λ is a diagonal matrix.
Due to calculationThe matrix inversion is not needed, the calculation can be conveniently carried out, and meanwhile, in a 3GPP LTE downlink system, each subframe only has 14 OFDM symbols, and the OFDM symbols occupied by the reference signals only have 4, so that the autocorrelation matrix
The singular value decomposition is relatively easy, so that the operation complexity of the algorithm is greatly simplified.
The invention provides a channel estimation method based on pilot signals in a 3GPP LTE downlink system, which aims at the 3GPP LTE downlink system, adopts a time-frequency two-dimensional joint wiener iterative filtering channel estimation method, and is based on a Linear Minimum Mean Square Error (LMMSE) algorithm. In the invention, firstly wiener iterative filtering based on an LMMSE algorithm is carried out in a frequency domain, and then wiener iterative filtering is carried out once in a time domain by utilizing a value estimated from the frequency domain. Experimental tests show that the signal estimation method can effectively improve Bit Error Rate (BER) performance, and is closer to ideal channel estimation compared with the traditional method. In addition, the invention reduces the operation complexity of the algorithm by performing Singular Value Decomposition (SVD) on the LMMSE algorithm, so that the channel estimation method provided by the invention does not increase too much operation complexity.
Drawings
Fig. 1 is a diagram of a 3GPP LTE downlink system model;
fig. 2 is a schematic diagram of distribution of reference signals in a resource block;
fig. 3 is a graph comparing the error rate performance of different channel estimation methods.
Detailed Description
The following description of the embodiments of the present invention is provided in order to better understand the present invention for those skilled in the art with reference to the accompanying drawings. It is to be expressly noted that in the following description, a detailed description of known functions and designs will be omitted when it may obscure the subject matter of the present invention.
Fig. 1 is a 3GPP LTE downlink system model diagram, as shown in fig. 1, at a transmitting end, input bits first enter a channel coding module, then the coded bits are scrambled and then modulated into complex modulation symbols, the complex modulation symbols are mapped onto transmission layers of one or more antenna ports, each layer is mapped onto resource particles of each antenna port after precoding, a reference signal is inserted at the same time, and finally, the symbols of each antenna port, including data and reference signals, are modulated into complex time domain OFDM signals and transmitted in a wireless multipath channel.
At the receiving end, in the ideal synchronization case, the receiving end signal Y after OFDM demodulation can be expressed as:
Y＝XH+W (7)
where H and W represent a channel frequency domain response and additive white gaussian noise, respectively, and X and Y represent a transmission signal and a reception signal, respectively.
In order to recover the transmitted bits from the received signal, the channel estimation module needs to obtain an estimate of the parameter H. For this reason, the 3GPP LTE downlink system inserts reference signals at fixed time-frequency two-dimensional positions, and the receiving end signals at these positions can be represented as:
YP＝XPHP+WP (8)
wherein XP、HPAnd WPIs a subset of the correlation matrix in equation (7), subscript (.)PIndicating the location of the transmitted reference signal.
The 3GPP LTE standard defines three downlink reference signals: a cell-specific reference signal, a Multicast Broadcast Single Frequency Network (MBSFN) reference signal, and a terminal-specific reference signal. In this embodiment, a cell-specific reference signal is used for the analysis.
Fig. 2 shows a scheme for allocating downlink cell-specific reference signals transmitted by a single antenna on a resource block. Visible, reference signal (R)0) 7 OFDM symbols are spaced on the same subcarrier in the time domain, and 6 subcarriers are spaced in the same OFDM symbol in the frequency domain.
The existing channel estimation algorithm:
by data Y at the position of the received reference signalPAnd the transmitted reference signal XPFrequency domain response H of the channel at the position of the reference signal in equation (8)PThe LS estimate of (a) can be expressed as:
then, linear interpolation is performed in time domain and frequency domain on the estimated value of the channel frequency domain response at the position of the reference signal, so as to obtain the channel transmission function of all the data resource elements in fig. 2. The channel estimation algorithm based on the LS criterion is simple in structure, but the correlation characteristics of the frequency domain and the time domain of a channel are not utilized in the LS estimation, and the influence of noise is ignored in the estimation, so that the channel estimation value is sensitive to the influence of the noise.
Another channel estimation method is MMSE algorithm, which is widely used in OFDM channel estimation, and this method utilizes Signal-to-Noise Ratio (SNR) and other channel statistical characteristic information, and the performance is superior to LS algorithm. The expression of the MMSE frequency domain channel estimation algorithm is as follows:
wherein,
represents H in formula (9)PThe LS estimate of (a) is,
is a cross-correlation matrix representing all subcarriers within the same OFDM symbol and the subcarriers at the reference signal,
represents the autocorrelation matrix of the sub-carriers at the reference signal within the same OFDM symbol,
is the variance of additive white Gaussian noise, superscript (. cndot.)HRepresenting a conjugate transpose. If the transmitted symbols are mapped to the same constellation,
then in equation (10)
Can use its expectation
To show that:
wherein,
is a constant, X, determined by the signal constellationkIs the point on the constellation diagram, β -1 for QPSK, β -17/9 for 16QAM, SNR is the average signal-to-noise ratio, IPIs a unit array. Then equation (11) can be further simplified as:
and then, linear interpolation is carried out on the channel frequency domain response estimation value of the frequency domain in the time domain, and finally, the channel transmission functions of all the data resource particles in the figure 2 are obtained.
As can be seen from equation (12), the LMMSE channel estimation algorithm actually performs a filtering process on the LS channel estimation result according to the MMSE criterion, so as to eliminate the influence of partial noise. In order to further eliminate noise, improve the performance of the error rate of a system and simultaneously not increase too much operation complexity, considering that a 3GPP LTE downlink system inserts reference signals in two directions of a time domain and a frequency domain, the invention provides a time-frequency two-dimensional joint wiener iterative filtering channel estimation method suitable for the 3GPP LTE downlink system.
In this embodiment, performance of various channel estimation methods in a veha (vehicular a) channel environment of a 3GPP LTE downlink system is tested by taking a single-user single-input single-output (SUSISO) as an example. Testing environmental parameters: the bandwidth is 1.4M, QPSK modulation is performed, a cell special reference signal is adopted as a reference signal, the sampling frequency is 1.92MHz, the subcarrier interval is 15KHz, and the frequency is 1000 times.
Fig. 3 is a graph comparing the error rate performance of different channel estimation methods.
Fig. 3 shows the error rate performance of different channel estimation methods in a VehA channel of a 3GPP LTE downlink system, where "LS" represents the error rate performance of the LS channel estimation method, and "MMSE" represents the error rate performance of the MMSE channel estimation method, and the error rate performance curves of LS, MMSE, the present invention, and the ideal channel estimation method are sequentially from top to bottom. It can be seen from fig. 3 that the bit error rate performance of the time-frequency two-dimensional joint wiener iterative filtering channel estimation algorithm provided by the invention is superior to that of the LS and MMSE algorithms, and is closer to that of ideal channel estimation, and meanwhile, too much operation complexity is not increased.
The invention provides a time-frequency two-dimensional joint wiener iterative filtering channel estimation method with relatively low complexity for a 3GPP LTE downlink system. The method reduces the noise influence and improves the bit error rate performance by carrying out wiener iterative filtering in a frequency domain and a time domain. The test result shows that the channel estimation method provided by the invention has better performance than LS and MMSE channel estimation methods. Meanwhile, the method of the invention also carries out singular value decomposition on the autocorrelation matrix in the algorithm, thereby obviously reducing the operation complexity of the algorithm.
Although illustrative embodiments of the present invention have been described above to facilitate the understanding of the present invention by those skilled in the art, it should be understood that the present invention is not limited to the scope of the embodiments, and various changes may be made apparent to those skilled in the art as long as they are within the spirit and scope of the present invention as defined and defined by the appended claims, and all matters of the invention which utilize the inventive concepts are protected.
Claims (2)
1. A channel estimation method based on pilot signals in a 3GPP LTE downlink system is characterized by comprising the following steps:
(1) frequency domain wiener iterative filtering
Firstly, obtaining a channel frequency domain response estimated value at a reference signal position on the same OFDM symbol:
in the formula (1), the reaction mixture is,
representing the autocorrelation matrix of the subcarriers at the reference signal within the same OFDM symbol, beta being a constant determined by the signal constellation, SNR being the average signal-to-noise ratio, IPIs a unit array;
for the channel frequency domain response H at the reference signal locationPLS estimate of (1), subscript (.)PIndicating the location of the transmitted reference signal.
Then to
Wiener iterative filtering based on LMMSE algorithm is carried out to reduce the influence of noise and interference on the channel frequency response estimation value on the reference signal position, the bit error rate performance is improved, and the channel frequency response estimation value on the reference signal position after the wiener iterative filtering
Finally, channel frequency domain response estimated value at the position of the reference signal after wiener iterative filtering is utilized
Estimating the channel frequency domain response value of all subcarriers on the same OFDM symbol
In the formula (3), the reaction mixture is,
a cross-correlation matrix representing all subcarriers within the same OFDM symbol and subcarriers at the reference signal;
(2) time-domain wiener iterative filtering
Channel frequency domain response estimation value on nth sub-carrier
And (2) performing wiener iterative filtering based on an LMMSE algorithm to reduce related residual noise and improve the bit error rate performance:
in the formula (4), the reaction mixture is,
is the autocorrelation matrix between all OFDM symbols containing reference signals on the nth subcarrier in a subframe, ILIs an identity matrix, L represents the number of OFDM symbols containing reference signals in each subframe;
channel frequency response estimation after iterative filtering using wienerEstimating the channel frequency response value of the nth subcarrier of all OFDM symbols in each subframe
Wherein,
is the cross correlation matrix between all OFDM symbols on the nth subcarrier and the OFDM symbol containing the reference signal;
channel frequency domain response estimation on all subcarriers
And performing the calculation, and finally obtaining channel transmission function values at all resource particles through wiener iterative filtering of a frequency domain and a time domain.
2. 3GPP LTE downlink system according to claim 1 based onMethod for channel estimation of pilot signals, characterized in that the autocorrelation matrices in steps (1) and (2) are first aligned
Singular value decomposition is performed and then the inverse matrix is calculated
The inverse matrix is decomposed into:
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| CN114697178A (en) * | 2020-12-28 | 2022-07-01 | 广州慧睿思通科技股份有限公司 | Method and device for estimating pilot frequency position channel, storage medium and electronic equipment |
| CN114531326A (en) * | 2022-02-14 | 2022-05-24 | Oppo广东移动通信有限公司 | Filter coefficient determination method, filter coefficient determination device, computer equipment and storage medium |
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