CN1741411A - Information channel estimating method and system in radio communication - Google Patents
Information channel estimating method and system in radio communication Download PDFInfo
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Abstract
A method for estimating channel in radio communication includes carrying out fading factor spectrum estimation according to fading factor initial estimation and calculating out relevant Doppler frequency deviation estimating result according to obtained spectrum estimating result , carrying out spectrum move for fading factor initial estimation according to Doppler frequency deviation estimating result . Carrying out low pass filtering on spectrum - moved signal and carrying out reverse move for signal being filtered with low pass to obtain final result of channel estimation .
Description
Technical Field
The present invention relates to the field of wireless communication technologies, and in particular, to a method and a system for channel estimation in wireless communication.
Background
As is well known, a wireless communication channel has a multipath propagation phenomenon, and when a receiver moves in space, the degree of coherent cancellation of multipath signals varies when the multipath signals are superimposed at the receiver due to the variation of different multipath transmission phases, thereby causing a so-called fading phenomenon of the received signals. Fading not only causes a wide range of sharp fluctuations in the amplitude of the received signal, but also causes random changes in the phase of the received signal. For phase modulation techniques such as Binary Phase Shift Keying (BPSK) and Quadrature Phase Shift Keying (QPSK), the receiver is very sensitive to phase change when receiving signals, so that the receiver must accurately estimate the phase distortion caused by the propagation of the radio channel in order to obtain the phase of the actual transmitted signal.
Currently, in order to improve spectrum utilization, the third generation personal communication system (3G) generally adopts Wideband Code Division Multiple Access (WCDMA) technology. WCDMA technology has a wider system bandwidth relative to 2G systems. For example, the system bandwidth of WCDMA reaches 3.84MHz, which means that most wireless channels in 3G systems are frequency selective, i.e. 3G systems can resolve more multipaths and combine them by using Rake reception technology.
For a frequency selective radio channel, it can be defined by the following impulse response model:
the model is that the wireless channel propagation is assumed to contain L different time delay multipath that can be analyzed by the system, and each multipath has independent fading factor sequence ai(t)ejφi(t)。
For the Rake reception technique, the structure of the Rake receiver is shown in fig. 1. In the structure, a time delay estimation module estimates and tracks the multipath time delay of an input signal; the separation module separates out multipath signals by using the time delay information estimated by the time delay estimation module; each multipath signal is accurately estimated by the channel estimation module for its respective amplitude and phase deflection, and the channel estimation module also multiplies the original signal by the conjugate of the estimation result and accumulates to obtain the maximum ratio coherent combination of the multipath signals. Wherein, the accuracy of the channel estimation result directly affects the demodulation performance of the system.
For channel estimation, a pilot sequence, i.e. a training sequence assisted method is usually adopted, that is, a part of known signals is included in a received signal, and fading information of a channel can be obtained by comparing an actual received signal with a known transmitted signal. Assuming the modulus of the pilot sequence is 1, taking multipath 1 as an example, an initial estimate is given by:
wherein, w (t) x p*(t) interference noise signals introduced by the wireless channel itself and other multipaths. It is usually necessary to perform low-pass filtering on the above initial estimation result to improve the signal-to-noise ratio of the channel estimation result. The fading factor low pass filter is shown in fig. 2. In a digital receiver, in order to make the receiving system as simple as possible, the filter is usually implemented with a simple cumulative average and a low order Infinite Impulse Response (IIR) filter.
In the low-pass filter, the selection of the filter bandwidth has a decisive influence on the signal-to-noise ratio of the estimation result. The bandwidth of the filter should be as narrow as possible from the standpoint of noise rejection as possible. However, since the fading factor itself also has a certain bandwidth, the bandwidth of the filter cannot be too narrow, otherwise the useful signal is also filtered out, and the signal-to-noise ratio is reduced.
Based on the above considerations, it is generally assumed that the fading factor power spectrum of a Rayleigh fading channel has the characteristics shown in fig. 3. Wherein, <math> <mrow> <msub> <mi>f</mi> <mi>d</mi> </msub> <mo>=</mo> <mfrac> <mi>ν</mi> <mi>λ</mi> </mfrac> <mo>,</mo> </mrow> </math> is the doppler frequency, λ is the carrier wavelength, and ν is the velocity of the receiver moving relative to the transmitter. In order to maximize the signal-to-noise ratio of the filtering result, which requires the frequency response of the low-pass filter to include the spectrum of the fading factor with a bandwidth as narrow as possible, f is set in fig. 3cThe cut-off frequency of the low-pass filter is adopted, so that the filtering result can remove noise as much as possible, and most signal energy is reserved.
In practical environments, the effect of the key module in channel estimation, i.e. the filter, is difficult to optimize for several reasons, including: first, the speed of movement of the receiver relative to the transmitter varies over time, so that the optimum cut-off frequency of the filter must be dynamically adjusted. Secondly, an additional frequency offset f may be caused by the difference between the local oscillator frequencies of the receiver and the transmitter, or in some wireless environments by the relative movement of the receiver and the transmitterdThe modulation causes a spectrum shift phenomenon on the spectrum. The spectrum shift phenomenon is shown in detail in fig. 4. In this case, the frequency of the fading factor is not symmetric about the zero frequency center, and a filter with the same cut-off frequency will cause a large deviation of the estimation result if the cut-off frequency of the low-pass filter is made larger than fd+foThe large bandwidth also degrades the output signal-to-noise ratio, which degrades the receiver's demodulation performance. Third, the spectral shape of the fading factor due to the actual wireless environment can be very complex and irregular, which also can make the design of the low pass filter difficult.
At present, the channel estimation method may adopt a fixed filter structure and bandwidth, and a specific technical solution is shown in fig. 5. The processing method of the channel estimation is the simplest, and the bandwidth of the low-pass filter in the scheme is usually a compromise result, and the result ensures that the demodulation performance meets the requirement in the application range of the system. For example, if the system is required to have a certain demodulation performance at the doppler frequencies of 0Hz to 400Hz, the low-pass filter is usually designed according to the doppler frequency of 200Hz, and of course, the low-pass filter should also make the performance degradation of the system at the doppler frequencies of 0Hz and 400Hz meet the design requirement.
However, the scheme shown in fig. 5 is optimal only for fading factors of a certain shape and a certain doppler frequency, and otherwise causes degradation of the demodulation performance of the system. Therefore, the channel estimation method can only be applied to the occasions with low requirements on the demodulation performance of the system.
The current channel estimation method can also add a doppler estimation element to the scheme shown in fig. 5, and the added scheme is shown in fig. 6. The added Doppler estimation link can obtain the Doppler frequency f of the fading factor according to the input initial channel estimation resultdThe bandwidth of the low pass filter is then dynamically adjusted according to the doppler frequency. Common doppler frequency estimation methods include a level passing rate (LCR) statistical method, a spectrum analysis method, an autocorrelation value estimation method, and the like, which are all capable of obtaining a fading factor spectrum width, i.e., a doppler frequency. The relationship between the doppler frequency and the filter bandwidth can be determined using some empirical formula or criteria.
For the scheme shown in fig. 6, the maximum spectrum spread point of the doppler spectrum is obtained by the doppler spectrum estimation step, and the effect of the fading factor of the ideal centrosymmetric doppler spectrum shown in fig. 3 is better. However, for the Doppler spectrum shown in FIG. 4, the Doppler frequency estimated by the Doppler spectrum estimation step is the highest spectrum spread point f of the Doppler spectrumd+foTherefore, although the spread width and shape of the doppler frequency are consistent in fig. 3 and 4, the bandwidth of the low pass filter in the scheme shown in fig. 6 is wider by f in fig. 4 than in fig. 3oThis may result in a decrease in the signal-to-noise ratio of the final estimation result.
Fig. 7 shows a third current channel estimation implementation scheme, which is mainly to add a spectrum shifting module on the basis of the scheme shown in fig. 6. The added frequency spectrum shifting module firstly estimates the Doppler frequency offset of the fading factor, then generates an oscillation signal by the frequency offset, and shifts the frequency spectrum of the fading factor by using the oscillation signal and a frequency mixing method, wherein the shifted fading factor is as shown in fig. 3. Thereafter, subsequent processing can be performed using the processing scheme shown in FIG. 6.
For spectrum shifting, assume that the fading factor signal shown in fig. 3 is ai(t)ejφi(t)The fading factor signal shown in FIG. 4 is ai(t)ejφi(t)+j2πfotThe fading factor shown in fig. 4 is approximately equal to the frequency spectrum shifted fading factor
Therefore, the fading factor after the spectrum shift can be handled by the solution shown in fig. 6. In addition, the spectrum shifting module also sends out a mixing signal e for reverse spectrum shiftingj2π fot The fading factor after low-pass filtering is reversed shifted through the signal to obtain the original fading factor estimation result ai(t)ej φi(t)+j2π fot 。
The spectrum shifting section is also called an automatic frequency correction section or an Automatic Frequency Control (AFC) section, and there are two implementation methods mainly shown in fig. 8 and 9. FIG. 8 is a diagram generally referred to as a feed-forward method, in which an input fading factor first passes through a frequency offset estimation procedure to obtain a Doppler frequency offset estimation of the fading factor
Then, the estimated value passes through a low-pass filter to obtain a more accurate frequency deviation estimated value fo(ii) a Then the signal is sent to a voltage-controlled or digital-controlled oscillator to obtain a mixing signal; and the obtained mixed signal is conjugated, and the conjugated mixed signal is multiplied by the previously input signal to be output.
Fig. 9 is generally referred to as a feedback method, and the frequency offset estimation, low pass filter and oscillator stages are the same as the feed forward method shown in fig. 8, except that: the frequency mixing of the feedback method is performed before the frequency offset estimation step, or the previous output signal after the frequency mixing is input to the frequency offset estimation step. Specifically, there is a delay element on the side for the oscillator, low pass filter and frequency offset estimation, and in the first signal processing, the delay element provides some initial values, and the whole loop is started by the initial values, and the initial values are mixed with the initial estimation of the fading factor and then directly output. After the whole loop is started, when current signal processing is carried out, firstly, a frequency offset estimation link carries out frequency offset estimation on the initial estimation of a currently input fading factor according to a signal output last time, the obtained frequency offset estimation is the error between the last frequency offset estimation and the current signal frequency offset, and is also called residual frequency offset, the frequency offset estimation link uses the residual frequency offset to correct the last frequency offset estimation output so as to form current frequency offset estimation output, then, the current frequency offset estimation output is processed through a low-pass filter and an oscillator, and then, the obtained oscillation signal and the input signal are output after conjugate multiplication.
The frequency offset estimation links in fig. 8 and fig. 9 generally adopt the cross product frequency discrimination method shown in fig. 10, which obtains doppler frequency offset by estimating phase deflection of two fading factors before and after, and mainly includes the links of delay, conjugate multiplication, phase calculation, gain adjustment, and the like.
The solution shown in fig. 7 solves the estimation problem for the fading factor of the doppler spectrum shown in fig. 4 to some extent, but the solution also has the following problems: the frequency offset estimation and the Doppler frequency estimation are respectively and independently carried out, so that the complexity of the system is increased; in the cross product frequency discrimination, a phase solving link is arranged, the link needs division and arc tangent operation, and the two complex operations need to consume a large amount of software and hardware resources; in addition, the doppler frequency offset estimation method of cross product frequency discrimination re-filtering has better accuracy in an environment with a smaller doppler frequency, but when the doppler frequency is expanded greatly, the estimation error obtained by the method is very large, thereby affecting the overall accuracy of channel estimation.
Disclosure of Invention
In view of the above, the present invention provides a channel estimation method in a wireless channel to improve the channel estimation accuracy and the demodulation performance of the wireless communication system, and simplify the system design.
Another object of the present invention is to provide a channel estimation system in wireless communication.
The invention relates to a channel estimation method in wireless communication, which comprises the following steps:
a. performing frequency spectrum estimation of the fading factors according to the initial estimation of the fading factors, and calculating corresponding Doppler frequency offset estimation results according to the obtained frequency spectrum estimation results;
b. and carrying out frequency spectrum shifting on the initial estimation of the fading factor according to the Doppler frequency offset estimation result, carrying out low-pass filtering on the signal after the frequency spectrum shifting, and carrying out reverse shifting on the signal after the low-pass filtering to obtain a final channel estimation result.
In the step a, the frequency spectrum estimation of the fading factor is obtained by performing Fourier transform on the initial estimation of the fading factor;
and obtaining a Doppler frequency offset estimation result by calculating the power spectrum center of gravity of the fading factor.
In the step a, the first time of estimating the frequency spectrum of the fading factor according to the initial estimation of the fading factor is as follows: directly taking a preset initial value as the frequency spectrum estimation of the fading factor;
after the first frequency spectrum estimation is finished, the step a obtains a Doppler frequency offset estimation result by calculating the power spectrum gravity center of the fading factor;
the second and above second estimation of the frequency spectrum of the fading factor according to the initial estimation of the fading factor is as follows: carrying out Fourier transform on the signal after the previous frequency spectrum shifting to obtain the frequency spectrum estimation of the fading factor;
after the second and above frequency spectrum estimation is completed, the step a obtains a corresponding doppler frequency offset estimation result by calculating the power spectrum center of gravity of the fading factor, corrects the previous doppler frequency offset estimation result by using the obtained doppler frequency offset estimation result, and then takes the corrected doppler frequency offset estimation result as the current doppler frequency offset estimation result.
The step a further comprises: after conjugation processing, the pilot frequency symbol is multiplied by the input pilot frequency channel data to obtain the initial estimation of the fading factor;
the pilot frequency symbol is a continuous signal;
in the step a, calculating a Doppler frequency offset estimation result through the following formula:
wherein B (f) is an estimation of the fading factor spectrum obtained by Fourier transform, fmaxEstimating a range for the effective spectrum;
the pilot symbols are either discrete signals;
in the step a, calculating a Doppler frequency offset estimation result through the following formula:
wherein, B [ f ]]For estimation of the fading factor spectrum by Fourier transformation, fmaxThe range is estimated for the available spectrum.
In step b, the shifting the frequency spectrum of the initial estimation of the fading factor according to the doppler frequency offset estimation result includes: and c, low-pass filtering the Doppler frequency offset estimation result obtained in the step a, and then carrying out frequency spectrum shifting on the initial estimation of the fading factor according to the filtered information.
In the step b, before the low-pass filtering is performed on the signal after the frequency spectrum shifting, the method further includes: determining the extension range of the Doppler frequency spectrum, determining the Doppler frequency estimation result after the frequency spectrum is shifted through the following formula, and determining the bandwidth of a low-pass filter for low-pass filtering through the estimation result;
fd＝max(|fd min- fo|，|fd max- fo|)
wherein f isdEstimating Doppler frequency after shifting frequency spectrumAs a result, [ f ]d min，fd max]For the extended range of the Doppler spectrum, foIs the result of Doppler frequency offset estimation.
The method may further comprise: and establishing a corresponding relation between the Doppler frequency and the bandwidth of the low-pass filter, and determining the bandwidth according to the corresponding relation and the determined Doppler frequency estimation result.
The determining the bandwidth of the low-pass filter by the estimation result includes: low-pass filtering the determined Doppler frequency estimation result, and then determining the bandwidth of the low-pass filter according to the information after the low-pass filtering.
In the step b, the shifting the frequency spectrum according to the doppler frequency offset estimation result is: and obtaining an oscillating signal with specified frequency according to the Doppler frequency offset estimation result, performing conjugation processing on the oscillating signal, and multiplying the processed oscillating signal by the initial estimation of the fading factor.
A channel estimation system in wireless communication of the present invention includes:
the fading factor initial estimation module is used for obtaining the initial estimation of the fading factor according to the pilot frequency symbol and the pilot frequency channel data, and the obtained fading factor initial estimation is sent to the Doppler frequency offset estimation processing module and the conjugate product module;
the Doppler frequency offset estimation processing module is used for carrying out frequency spectrum estimation according to the initial estimation of the fading factor, calculating a Doppler frequency offset estimation result according to the frequency spectrum estimation result, converting the Doppler frequency offset estimation result into an oscillation signal, and sending the obtained oscillation signal to the conjugate product module and the low-pass filtering processing module;
the conjugate product module is used for performing conjugate processing on the received oscillation signal, performing frequency spectrum shifting on the initial estimation of the fading factor through the oscillation signal obtained by conjugate, and sending the processed signal to the low-pass filtering processing module;
and the low-pass filtering processing module is used for carrying out low-pass filtering on the signal subjected to frequency spectrum shifting and carrying out reverse shifting on the filtered signal through the oscillation signal.
The Doppler frequency offset estimation processing module carries out frequency spectrum estimation on the initial estimation of the fading factor through Fourier transform; and calculating the Doppler frequency offset estimation result by calculating the power spectrum gravity center.
The conjugate product module carries out spectrum shifting by multiplying the signal obtained by conjugation and the initial estimation of the fading factor.
The Doppler frequency shift estimation processing module further obtains the spread range of the Doppler frequency spectrum by setting a threshold, calculates the Doppler frequency spectrum spread width after the frequency spectrum is shifted according to the obtained Doppler frequency spectrum spread range and the Doppler frequency shift estimation result, and sends the obtained Doppler frequency spectrum spread width to the low-pass filtering processing module;
the low-pass filtering processing module further determines the bandwidth of the low-pass filtering processing module according to the Doppler frequency spectrum spread width.
Another channel estimation system in wireless communication of the present invention includes:
the fading factor initial estimation module is used for obtaining the initial estimation of the fading factor according to the pilot frequency symbol and the pilot frequency channel data, and sending the obtained initial estimation of the fading factor to the conjugate product module;
the Doppler frequency offset estimation processing module is used for carrying out frequency spectrum estimation according to the signal sent by the conjugate product module, calculating a Doppler frequency offset estimation result according to the frequency spectrum estimation result, converting the Doppler frequency offset estimation result into an oscillation signal, and sending the obtained oscillation signal to the conjugate product module and the low-pass filtering processing module;
the conjugate product module is used for performing conjugate processing on the oscillation signal sent by the Doppler frequency offset estimation processing module, performing frequency spectrum shifting on the initial estimation of the fading factor through the oscillation signal obtained by processing, and sending the processed signal to the low-pass filtering module and the Doppler frequency spectrum estimation processing module;
and the low-pass filtering module is used for carrying out low-pass filtering on the signal subjected to frequency spectrum shifting and carrying out reverse shifting on the filtered signal through the oscillation signal.
The Doppler frequency offset estimation processing module carries out frequency spectrum estimation on the initial estimation of the fading factor through Fourier transform; and calculating the Doppler frequency offset estimation result by calculating the power spectrum gravity center.
The conjugate product module carries out spectrum shifting by multiplying the signal obtained by conjugation and the initial estimation of the fading factor.
The Doppler frequency shift estimation processing module further obtains the spread range of the Doppler frequency spectrum by setting a threshold, calculates the Doppler frequency spectrum spread width after the frequency spectrum is shifted according to the obtained Doppler frequency spectrum spread range and the Doppler frequency shift estimation result, and sends the obtained Doppler frequency spectrum spread width to the low-pass filtering processing module;
the low-pass filtering processing module further determines the bandwidth of the low-pass filtering processing module according to the Doppler frequency spectrum spread width.
The scheme of the invention carries out the frequency spectrum estimation of the fading factors by the initial estimation of the fading factors and calculates the corresponding Doppler frequency offset estimation result according to the obtained frequency spectrum estimation result, so that the frequency offset estimation process is simple to realize compared with the conventional cross product frequency discrimination method, the frequency offset estimation result can be ensured to be more accurate, and the accurate Doppler frequency offset estimation can be obtained even if the Doppler frequency spectrum is in a complex shape.
The invention adopts a frequency spectrum shifting method to preprocess the fading factors and also ensures that the processed frequency spectrum is easy to realize the optimal filtering by a low-pass filter.
In addition, compared with the scheme of respectively carrying out spectrum moving and spectrum estimation in the prior art, the scheme of the invention simultaneously carries out spectrum moving and spectrum estimation, so that the system is simpler to realize, and less resources are consumed.
Under the condition of simultaneously realizing frequency spectrum shifting and frequency spectrum estimation, the scheme of the invention also realizes the self-adaptive adjustment of the bandwidth of the low-pass filter in the channel estimation according to the Doppler frequency spectrum spreading width of the fading factor, thereby ensuring that the bandwidth of the filter is always in an optimal value, improving the signal-to-noise ratio of the channel estimation result and improving the demodulation performance of the system.
Drawings
Fig. 1 is a block diagram of a current Rake receiver;
FIG. 2 is a diagram of a current fading factor filtered by a low pass filter;
FIG. 3 is a diagram of a fading factor spectrum and a low pass filter spectral response;
FIG. 4 is a diagram illustrating shifting of a frequency spectrum due to frequency offset;
FIG. 5 is a diagram illustrating a current channel estimation scheme using a fixed filter structure and bandwidth;
FIG. 6 is a schematic diagram of a channel estimation scheme for a current incremental Doppler frequency estimation procedure;
FIG. 7 is a schematic diagram of a channel estimation scheme for adding spectrum shifting and Doppler frequency estimation at present;
FIG. 8 is a schematic diagram of feed forward spectral shifting;
FIG. 9 is a schematic diagram of frequency spectrum shifting of feedback;
FIG. 10 is a diagram illustrating a cross-product frequency discrimination scheme employed in the frequency offset estimation stage of FIGS. 8 and 9;
FIG. 11 is a schematic diagram of a channel estimation system using a feed-forward method for Doppler frequency offset estimation;
FIG. 12 is a process flow diagram corresponding to FIG. 11;
FIG. 13 is a schematic diagram of a channel estimation system using a feedback method for Doppler frequency offset estimation;
FIG. 14 is a schematic diagram of a channel estimation system using a feed-forward method for Doppler frequency offset estimation and Doppler frequency estimation;
fig. 15 is a schematic structural diagram of a channel estimation system using a feedback method to perform doppler frequency offset estimation and doppler frequency estimation.
Detailed Description
The invention will be further described in detail with reference to the accompanying drawings and specific embodiments.
For some simplified channel estimation methods, the bandwidth of the fading factor filter is generally fixed, and in this case, only doppler frequency offset estimation needs to be performed on the initial estimation of the fading factor.
Specifically, the structure of the channel estimation system for performing doppler frequency offset estimation is shown in fig. 11 and 13. Fig. 11 illustrates a feedforward method, and fig. 13 illustrates a feedback method.
The corresponding process flow will be described in detail with reference to fig. 11. The corresponding processing procedure is shown in fig. 12, and corresponds to the following steps:
step 1201, after the pilot symbols pass through a conjugation link 111, multiplying the input pilot channel data by a link 112 to obtain an initial estimation of the fading factor;
Step 1203, oscillator 114 to 113 links to obtain dopplersEstimation result f of the doppler frequency offsetoProcessing to obtain an oscillating signal e with a specified frequencyj2π fot ；
Step 1204, conjugate processing is performed on the obtained mixing signal through a conjugate link 115, and the obtained mixing signal is multiplied by an output signal of a 112 link at a link 116, so that frequency spectrum shifting is realized, and a shifted signal is obtained
Step 1205, the signal after the frequency spectrum shift is low-pass filtered by the low-pass filter 117 to obtain ai(t)ej φi(t) ；
Step 1206, low pass filtered fading factor ai(t)ej φi(t) With the output signal e of the oscillator 114j2π fot And multiplying to carry out reverse shift of the frequency spectrum, thereby obtaining a final channel estimation result.
In step 1202, when the doppler frequency offset estimation is performed in the 113 link, assuming that a signal input in the 112 link is β (t), a pilot symbol corresponding to the signal is input by a continuous receiver, and a spectrum estimation obtained by the signal through fourier transform is b (f), the doppler frequency offset estimation result obtained by calculating a power spectrum center of gravity of a fading factor may be implemented by the following formula:
wherein f ismaxThe range is estimated for the available spectrum.
For the case that the pilot symbols are the discrete signals input by the discrete receiver, the signal input by the 112-element is usually subjected to Fast Fourier Transform (FFT) to obtain a spectrum estimate bf of the fading factor, and the doppler frequency offset estimation result is calculated by the following formula:
wherein f ismaxThe range is estimated for the available spectrum.
In addition, after step 1202, the obtained doppler frequency offset estimation result may be further processed, for example, a low pass filter is added, and the estimation accuracy of the estimation result is improved by the low pass filter.
The system shown in fig. 13 is similar to the above-described process, but the system employs a feedback process, and thus the two processes are different. Specifically, in fig. 13, when performing spectrum estimation, the first spectrum estimation is different from the subsequent spectrum estimation process, specifically, the first spectrum estimation is spectrum estimation directly using a preset initial value as a fading factor; correspondingly, after the first frequency spectrum estimation, a Doppler frequency offset estimation result is directly obtained by calculating the power spectrum gravity center of the fading factor. The second and subsequent spectral estimates are: carrying out Fourier transform on the signal after the previous frequency spectrum shifting to obtain the frequency spectrum estimation of the fading factor; correspondingly, after each subsequent frequency spectrum estimation, a corresponding Doppler frequency offset estimation result is obtained by calculating the power spectrum center of gravity of the fading factor, then the obtained Doppler frequency offset estimation result is used for correcting the last Doppler frequency offset estimation result, and the corrected Doppler frequency offset estimation result is used as the current Doppler frequency offset estimation result. Or, the doppler frequency estimation of the signal after frequency offset shifting is to obtain a residual frequency offset relative to the previous estimation result, and the residual frequency offset is also needed to correct the previous frequency offset estimation result, so as to obtain the current frequency offset estimation result.
In addition, when the frequency offset estimation result is corrected, different scale factors can be set so as to adjust the characteristics of the whole feedback loop.
The detailed processing of the simplified channel estimation method is described above. For the system shown in fig. 11, the 111 and 112 links can be used as an initial estimation module of the fading factor, for providing an initial estimation of the fading factor; taking the 113 and 114 links as a Doppler frequency offset estimation processing module for providing a frequency offset processed result; taking links 115 and 116 as a conjugate product module to carry out spectrum shift on the initial estimation of the fading factor; the low- pass filters 117 and 118 are used as low-pass filtering processing modules for performing low-pass filtering on the signals after the frequency spectrum shifting, and then performing reverse shifting on the obtained signals.
For the system shown in fig. 13, the 131 and 132 links can also be used as an initial estimation module of the fading factor, for providing an initial estimation of the fading factor; the 133 and 134 links are used as a Doppler frequency offset estimation processing module for carrying out frequency offset processing according to signals sent by a conjugate product module consisting of 135 and 136 links; taking the links 135 and 136 as a conjugate product module, carrying out spectrum shifting on the initial estimation of the fading factor, and sending a signal after the spectrum shifting to a Doppler frequency offset estimation processing module and a low-pass filtering module consisting of the links 137 and 138; the low- pass filters 137 and 138 are used as low-pass filtering processing modules for performing low-pass filtering on the signals after the frequency spectrum shifting, and then performing reverse shifting on the obtained signals.
For the case where the bandwidth of the fading factor filter is not fixed, the system shown in fig. 14 and 15 can be used for processing. FIG. 14 corresponds to FIG. 11 above, again using the feed forward approach; fig. 15 corresponds to fig. 13 described above, and a feedback method is used.
For fig. 14 and 15, the difference from the previous two figures is that: when the Doppler frequency offset estimation is realized, the estimation of the Doppler frequency is also realized, and the bandwidth of the low-pass filter is determined according to the estimation result of the Doppler frequency.
In fig. 14, in relation to fig. 11, specifically, the doppler frequency estimation is added in the segment 113, and the segment 113 sends the doppler frequency estimation result to the low pass filter 117, and the low pass filter 117 determines its own bandwidth according to the doppler frequency estimation result.
Specifically, in the step 113, the doppler frequency estimation is performed by first determining the extension range of the doppler spectrum, and then calculating the extension range of the doppler spectrum after the spectrum shift. When determining the extension range of the Doppler frequency spectrum, the extension range [ f ] of the Doppler frequency spectrum can be obtained by setting a certain noise threshold or a strongest path threshold and the liked min，fd max]For FIG. 4, the extended range is [ -f [ ]d+fo，fd+fo]. Determining the extended range of the Doppler spectrum and the Doppler frequency offset estimation resultThen, the doppler spectrum spread width after frequency offset, that is, the estimation result of the doppler frequency, can be calculated by the following formula:
fd＝max(|fd min- fo|，|fd max- fo|)
in addition, when the low-pass filter 117 determines its own bandwidth from the doppler frequency estimation result, that is, the doppler spread spectrum width, the overall principle is the doppler spread spectrum width fdThe larger the filter bandwidth. In particular implementations, a number of different strategies may be used, such as establishing a function or mapping table between the doppler frequency and the bandwidth of the filter in advance, so that the bandwidth of the low-pass filter is determined according to the input doppler frequency and the method of calculating or looking up the table instead. If a table mapping mode is adopted, the division precision of the table can be determined according to the actual situation, and generally, the table only needs to be divided into 3 to 5 grades to meet the requirement.
For fig. 15, compared to fig. 13, the doppler frequency estimation is added at 133, and the obtained doppler frequency estimation result is sent to the low pass filter 137, and the low pass filter 137 determines its own bandwidth according to the estimation result. The specific processing procedure is the same as the feedforward method shown in fig. 14, and therefore, the detailed description is omitted.
For the systems shown in fig. 14 and 15, in order to improve the estimation accuracy of the doppler frequency offset estimation result and/or the doppler frequency estimation result, a low pass filter may also be added, and the estimation accuracy of the estimation result is improved by the low pass filter.
The above description is only a preferred embodiment of the present invention, and is not intended to limit the scope of the present invention.
Claims (17)
1. A method for channel estimation in wireless communications, the method comprising:
a. performing frequency spectrum estimation of the fading factors according to the initial estimation of the fading factors, and calculating corresponding Doppler frequency offset estimation results according to the obtained frequency spectrum estimation results;
b. and carrying out frequency spectrum shifting on the initial estimation of the fading factor according to the Doppler frequency offset estimation result, carrying out low-pass filtering on the signal after the frequency spectrum shifting, and carrying out reverse shifting on the signal after the low-pass filtering to obtain a final channel estimation result.
2. The method according to claim 1, wherein in step a,
carrying out Fourier transform on the initial estimation of the fading factor to obtain the frequency spectrum estimation of the fading factor;
and obtaining a Doppler frequency offset estimation result by calculating the power spectrum center of gravity of the fading factor.
3. The method according to claim 1, wherein in step a, the first time performing the spectrum estimation of the fading factor according to the initial estimation of the fading factor comprises: directly taking a preset initial value as the frequency spectrum estimation of the fading factor;
after the first frequency spectrum estimation is finished, the step a obtains a Doppler frequency offset estimation result by calculating the power spectrum gravity center of the fading factor;
the second and above second estimation of the frequency spectrum of the fading factor according to the initial estimation of the fading factor is as follows: carrying out Fourier transform on the signal after the previous frequency spectrum shifting to obtain the frequency spectrum estimation of the fading factor;
after the second and above frequency spectrum estimation is completed, the step a obtains a corresponding doppler frequency offset estimation result by calculating the power spectrum center of gravity of the fading factor, corrects the previous doppler frequency offset estimation result by using the obtained doppler frequency offset estimation result, and then takes the corrected doppler frequency offset estimation result as the current doppler frequency offset estimation result.
4. The method of claim 2 or 3, wherein the step a is preceded by the further step of: after conjugation processing, the pilot frequency symbol is multiplied by the input pilot frequency channel data to obtain the initial estimation of the fading factor;
the pilot frequency symbol is a continuous signal;
in the step a, calculating a Doppler frequency offset estimation result through the following formula:
wherein B (f) is an estimation of the fading factor spectrum obtained by Fourier transform, fmaxEstimating a range for the effective spectrum;
the pilot symbols are either discrete signals;
in the step a, calculating a Doppler frequency offset estimation result through the following formula:
wherein, B [ f ]]For estimation of the fading factor spectrum by Fourier transformation, fmaxThe range is estimated for the available spectrum.
5. The method according to claim 2 or 3, wherein in the step b, the performing the spectrum shifting on the initial estimation of the fading factor according to the doppler frequency offset estimation result comprises: and c, low-pass filtering the Doppler frequency offset estimation result obtained in the step a, and then carrying out frequency spectrum shifting on the initial estimation of the fading factor according to the filtered information.
6. The method according to claim 2 or 3, wherein the step b, before the low-pass filtering the spectrum shifted signal, further comprises: determining the extension range of the Doppler frequency spectrum, determining the Doppler frequency estimation result after the frequency spectrum is shifted through the following formula, and determining the bandwidth of a low-pass filter for low-pass filtering through the estimation result;
fd＝max(|fdmin- f0|，|fdmax- f0|)
wherein f isdAs a result of the Doppler frequency estimation after the frequency spectrum shifting, [ f ]dmin，fdmax]For the extended range of the Doppler spectrum, f0Is the result of Doppler frequency offset estimation.
7. The method of claim 6, further comprising: and establishing a corresponding relation between the Doppler frequency and the bandwidth of the low-pass filter, and determining the bandwidth according to the corresponding relation and the determined Doppler frequency estimation result.
8. The method of claim 6, wherein determining the bandwidth of the low-pass filter from the estimation comprises: low-pass filtering the determined Doppler frequency estimation result, and then determining the bandwidth of the low-pass filter according to the information after the low-pass filtering.
9. The method according to claim 1, wherein in the step b, the shifting the frequency spectrum according to the doppler frequency offset estimation result is: and obtaining an oscillating signal with specified frequency according to the Doppler frequency offset estimation result, performing conjugation processing on the oscillating signal, and multiplying the processed oscillating signal by the initial estimation of the fading factor.
10. A channel estimation system in wireless communications, the system comprising:
the fading factor initial estimation module is used for obtaining the initial estimation of the fading factor according to the pilot frequency symbol and the pilot frequency channel data, and the obtained fading factor initial estimation is sent to the Doppler frequency offset estimation processing module and the conjugate product module;
the Doppler frequency offset estimation processing module is used for carrying out frequency spectrum estimation according to the initial estimation of the fading factor, calculating a Doppler frequency offset estimation result according to the frequency spectrum estimation result, converting the Doppler frequency offset estimation result into an oscillation signal, and sending the obtained oscillation signal to the conjugate product module and the low-pass filtering processing module;
the conjugate product module is used for performing conjugate processing on the received oscillation signal, performing frequency spectrum shifting on the initial estimation of the fading factor through the oscillation signal obtained by conjugate, and sending the processed signal to the low-pass filtering processing module;
and the low-pass filtering processing module is used for carrying out low-pass filtering on the signal subjected to frequency spectrum shifting and carrying out reverse shifting on the filtered signal through the oscillation signal.
11. The system of claim 10, wherein the doppler frequency offset estimation processing module performs a spectral estimation of the initial estimate of the fading factor by fourier transform; and calculating the Doppler frequency offset estimation result by calculating the power spectrum gravity center.
12. The system of claim 10, wherein the conjugate product module shifts the spectrum by multiplying the signal obtained by conjugation with an initial estimate of the fading factor.
13. The system according to claim 10, wherein the doppler frequency offset estimation processing module further obtains a doppler spectrum spread range by setting a threshold, and calculates a doppler spectrum spread width after the spectrum shift according to the obtained doppler spectrum spread range and the doppler frequency offset estimation result, and the obtained doppler spectrum spread width is sent to the low-pass filtering processing module;
the low-pass filtering processing module further determines the bandwidth of the low-pass filtering processing module according to the Doppler frequency spectrum spread width.
14. A channel estimation system in wireless communications, the system comprising:
the fading factor initial estimation module is used for obtaining the initial estimation of the fading factor according to the pilot frequency symbol and the pilot frequency channel data, and sending the obtained initial estimation of the fading factor to the conjugate product module;
the Doppler frequency offset estimation processing module is used for carrying out frequency spectrum estimation according to the signal sent by the conjugate product module, calculating a Doppler frequency offset estimation result according to the frequency spectrum estimation result, converting the Doppler frequency offset estimation result into an oscillation signal, and sending the obtained oscillation signal to the conjugate product module and the low-pass filtering processing module;
the conjugate product module is used for performing conjugate processing on the oscillation signal sent by the Doppler frequency offset estimation processing module, performing frequency spectrum shifting on the initial estimation of the fading factor through the oscillation signal obtained by processing, and sending the processed signal to the low-pass filtering module and the Doppler frequency spectrum estimation processing module;
and the low-pass filtering module is used for carrying out low-pass filtering on the signal subjected to frequency spectrum shifting and carrying out reverse shifting on the filtered signal through the oscillation signal.
15. The system of claim 14, wherein the doppler frequency offset estimation processing module performs a spectral estimation of the initial estimate of the fading factor by fourier transform; and calculating the Doppler frequency offset estimation result by calculating the power spectrum gravity center.
16. The system of claim 14, wherein the conjugate product module shifts the spectrum by multiplying the signal obtained by conjugation with an initial estimate of the fading factor.
17. The system according to claim 14, wherein the doppler frequency offset estimation processing module further obtains a doppler spectrum spread range by setting a threshold, and calculates a doppler spectrum spread width after the spectrum shift according to the obtained doppler spectrum spread range and the doppler frequency offset estimation result, and the obtained doppler spectrum spread width is sent to the low-pass filtering processing module;
the low-pass filtering processing module further determines the bandwidth of the low-pass filtering processing module according to the Doppler frequency spectrum spread width.
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| CN101342083B (en) * | 2007-07-12 | 2012-07-18 | 深圳迈瑞生物医疗电子股份有限公司 | Method and apparatus for calculating Doppler signal frequency spectrum |
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