CN107231326B - Cell search system in NB-IoT system downlink - Google Patents
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Abstract
The invention discloses a cell searching method in an NB-IoT system downlink, which uses the following technical means, npss signal detection adopts a method of combining segment correlation and module value correlation, adopts the segment correlation to carry out timing coarse synchronization, and then adopts the module value correlation to carry out timing coarse synchronization correction and timing fine synchronization. When the local npss signal is stored, only the npss time domain down-sampled signal and the frequency domain signal of one symbol are needed to be stored, and the space complexity is reduced. When frequency offset estimation is carried out, the frequency offset of a certain step length multiplied by a local npss signal is subjected to correlation operation with a received master synchronization signal, so that rough frequency offset estimation is obtained and compensated, and the frequency offset of the signal is controlled within a certain step length; when the fine frequency offset estimation is carried out, symbol is crossed, the fine frequency offset estimation range can be reduced to a certain range, and the frequency offset estimation precision is improved. When performing nsss signal detection, before using the received nsss frequency domain signal to correlate with the local nsss frequency domain signal, determining nfAnd the candidate value of q, and then the received nss frequency domain signal is correlated with the local nss frequency domain signal in the candidate value, so that the time complexity is reduced.
Description
Technical Field
The invention relates to a cell search system in NB-IoT system downlink, mainly relating to the components of a transmission system of the patent classification number H04 electric communication technology H04B transmission H04B1/00 which is not contained in a single group of H04B 3/00 to H04B 13/00; the component of the transmission system which is not distinguished by the transmission medium used, H04B1/69 spreading technique H04B1/707, uses direct sequence modulated H04B1/7073 synchronization aspect H04B1/7083 cell search, for example using a three-stage approach.
Background
In the NB-IoT system, the initial cell search process is a precondition for establishing a downlink communication link between the ue and the base station, and mainly aims to achieve synchronization of a downlink in time and frequency and obtain a physical layer cell identity (cellID). The NB-IoT system adopts an Orthogonal Frequency Division Multiplexing (OFDM) technique in the downlink, and the basic principle of the technique is to convert a high-speed serial data stream into multiple parallel low-speed data streams after serial-parallel conversion, and modulate the multiple parallel low-speed data streams onto multiple orthogonal subcarriers for transmission. The frequency synchronization is as accurate as possible because of frequency offset between the carrier frequency of the transmitter and the local oscillator of the receiver, or because of doppler effects, the orthogonality between the subcarriers of the OFDM system is destroyed, resulting in inter-subcarrier interference. The time synchronization is to determine the starting position of the system frame and the starting position of the OFDM symbol, and in the multipath channel, inaccurate time synchronization may cause inter-symbol interference, which greatly affects the system performance, and thus the time synchronization is required to be as accurate as possible.
Typical NB-IoT System sample time T in the prior art s1/30720000 seconds, the time domain structure thereof is shown in fig. 1, each system frame time is 10 milliseconds (ms), and comprises 10 subframes, each subframe time is 1ms, and comprises 2 slots, each slot time is 0.5ms, and comprises 7 orthogonal frequency division multiplexing symbols (symbols), each symbol length is 2048 samples and the length of CP is added, wherein the CP length of the 0 th symbol is 160 samples, and the CP length of the 1 st to 6 th symbols is 144 samples, so each subframe comprises 14 symbols, and the length of 30720 samples. The npss signal is located in the 5 th subframe of the system frame and the nsss signal is located in the 9 th subframe of the even bit system frame.
Disclosure of Invention
Aiming at the problem that the time synchronization is inaccurate under the condition of large frequency offset of the existing npss signal detection technology, the invention provides an npss signal detection system capable of resisting large frequency offset.
When the local npss signal is stored, the invention only needs to store the npss time domain down-sampled signal and the frequency domain signal of one symbol, and the npss time domain down-sampled signal and the frequency domain signal of the whole subframe do not need to be stored to the local, so that the space complexity is reduced.
When frequency offset estimation is carried out, firstly, a local npss signal is multiplied by frequency offset with a certain step length to carry out correlation operation with a received master synchronization signal, so as to obtain coarse frequency offset estimation, and the frequency offset of the signal can be ensured to be controlled within the certain step length after coarse frequency offset compensation; when the fine frequency offset estimation is carried out, the npss signal in one symbol is not subjected to conjugate multiplication with the local signal and then is divided into two sections for correlation, but the correlation is carried out across the symbol, so that the fine frequency offset estimation range can be reduced to a certain range, the range requirement of the frequency offset estimation is met, and the precision of the fine frequency offset estimation is improved to the maximum extent.
When nsss signal detection is carried out, according to the central symmetry of a ZC sequence, n can be determined before a received nsss frequency domain signal is related to a local nsss frequency domain signalfAnd q, then using the received nss frequency domain signal to correlate with local nss frequency domain signals in the candidate value without correlating with all local nss frequency domain signals, thereby reducing time complexity.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions of the prior art, the drawings needed to be used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
FIG. 1 is a schematic diagram of the NB-IoT time domain structure of the present invention
FIG. 2 is a block diagram of a cell search function of the NB-IoT system of the present invention
FIG. 3 is a block diagram of npss signal detection function according to the present invention
FIG. 4 is a block diagram of the frequency synchronization function of the present invention
FIG. 5 is a block diagram of the nsss signal detection function of the present invention
FIG. 6 shows n according to the present inventionfAnd cellID detection flow chart
FIG. 7 is a comparison graph of time offset estimation according to the present invention
FIG. 8 is a comparison of frequency offset estimation of the present invention
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the following describes the technical solutions of the embodiments of the present invention clearly and completely with reference to the accompanying drawings in the embodiments of the present invention:
the technical abbreviations used in the present invention are as follows:
NB-IoT Narrow Band Internet of Things (IoT) Narrow Band Internet of Things (TINS)
npss Narrowband primary synchronization signal
nss Narrowband secondary synchronization signal
CP Cyclic Prefix
IFFT Inverse Fast Fourier Transformation Inverse Fast Fourier transform
OFDM Orthogonal Frequency Division Multiplexing
cell ID physical layer cell identification
nfSystem frame number
symbol orthogonal frequency division multiplexing
ε normalized frequency offset, i.e. the ratio of frequency offset to subcarrier spacing (15kHz)
As shown in fig. 1-6, a cell search system in an NB-IoT system downlink mainly includes:
npss signal detection unit
As shown in fig. 3, the npss signal detection functional block diagram firstly generates a local npss frequency domain signal, converts the signal into a time domain, performs down-sampling processing, then performs down-sampling processing on the received 21ms data, performs timing coarse synchronization by using the down-sampled data, and performs timing fine synchronization by using the non-down-sampled data after timing coarse synchronization correction, thereby completing detection of the npss signal.
The npss signal is a Zadoff-Chu sequence in the frequency domain, and the generation expression is as follows:
wherein, the root sequence index u is 5, l is symbol number, and the values of s (l) are shown in table 1:
TABLE 1S (l) value-taking table
It can be seen that each npss signal of symbol differs by only one sign bit s (l), and in order to reduce the spatial complexity, only one frequency domain npss signal of symbol needs to be generated, where the generation expression is:
then, npss _ loc _ fre (n) is transformed into the time domain to generate a time domain npss signal of symbol:
in order to reduce the amount of calculation, 16 times of downsampling is performed on the signal npss _ loc _ ti (n), as shown in equation (4), the obtained signal is denoted as npss _ loc _ ti _ sam (n), and the length of the obtained signal is 128 sampling points.
npss_loc_ti_sam(n)＝npss_loc_ti(16n),n＝0,1,...,127 (4)
The npss signal is located in the 5 th subframe of the system frame, and the npss signal of two subframes must be included in the received 21ms data.
The npss signal detection is performed on the data 11ms before actual use and is recorded as re (n), the length of the npss signal is 337920 sampling points, 16 times of down-sampling processing is performed on the signal re (n), the obtained signal is recorded as re _ sam (n), and the length of the obtained signal is 21120 sampling points.
Using npss _ loc _ ti _ sam (n) and re _ sam (n) as sliding correlation, taking out data of one subframe from re _ sam (n) every time sliding one point, as re _ sam _ sf (M), M is 0,1, …, M-1, M is 1920, re _ sam _ sf (M) is re _ sam (n + M), n is 0,1, …,19200, re _ sam _ sf (M) is divided into 14 symbols, and data of the l symbol (not including CP) is as re _ saml(n), n is 0,1, …,127, then re _ saml(n)＝re_sam_sf(137l+10+(l≥7)+n)，l＝0,1, …,13, wherein (l ≧ 7) denotes a value when l<When l is greater than or equal to 7, 0 is set, and when l is greater than or equal to 7, 1 is set. For npss signals, the first 3 symbols are 0, and the last 11 symbols are re _ saml(n) are piecewise correlated with the local signal npss _ loc _ ti _ sam (n) (2 for the number of segments, 64 for each segment), and summed, as shown in equation (5):
where n is 0,1, …,19200, i.e. the length of a frame. The maximum value of corr (n) is obtained, and the mark n corresponding to the maximum value is the timing coarse synchronization position, namely
Because of the influence of frequency offset, the coarse timing synchronization obtained in the above process has a certain deviation and needs to be corrected, 20 points are respectively taken at two sides of the coarse synchronization position coarse _ start _ time, and npss _ loc _ ti _ sam (n) and re _ sam (n) are used for sliding correlation.
Using the signal npss _ loc _ ti _ sam (n) and the signal re _ saml(n) when correlating, instead of using piecewise correlation, npss _ loc _ ti _ sam (n) and re _ sam are usedlThe (n) modes are correlated to counter the effect of the frequency offset on the time offset, as shown in equation (6).
Where n is coarse _ start _ time-20, coarse _ start _ time-19, …, coarse _ start _ time + 20. The maximum value of corr _ rev (n) is obtained, and the mark n corresponding to the maximum value is the revised value of the timing coarse synchronization, namely
The above processes of obtaining the time offset are all 16 times of down-sampled signals, and in order to obtain fine timing synchronization, after the start _ time _ rev is obtained,
first multiplying it by 16 transformEstimating the time offset of the non-downsampled data, then taking 64 points on two sides of the non-downsampled data, sliding on the non-downsampled data re (N), taking data of one subframe from re (N) every time one point is slid, wherein N is 0,1, …, N-1, N is 30720, then carrying out 16-time downsampling, obtaining a signal, wherein the signal is re _ sam _ sf '(N), N is 0,1, …, N' -1, N '-1920, and the data of the l symbol is re _ sam'l(n) of length 128, for signals npss _ loc _ ti _ sam (n) and re _ sam'lThe modulus of (n) is correlated, as shown in equation (7):
wherein n is start _ time _ rev 16-64, start _ time _ rev 16-15, …, and start _ time _ rev 16+ 64. The maximum value of corr _ acc (n) is obtained, and the mark n corresponding to the maximum value is the fine timing synchronization, i.e. the fine timing synchronization
This value is where the npss signal starts.
Frequency synchronization unit
As shown in fig. 4, the frequency synchronization functional block diagram can obtain a received npss signal according to start _ time _ acc, perform 16-fold down-sampling on the signal, then calculate coarse frequency offset estimation according to the down-sampled npss signal, perform coarse frequency offset compensation on the signal, and then perform fine frequency offset estimation.
Performing 16-fold down-sampling on the received npss signal, and recording as npss _ re _ ti _ sam (n), then:
npss_re_ti_sam(n)＝re(start_time_acc+16n),n＝0,1,...,N-1 (8)
where N is 1920, npss _ re _ ti _ sam (N) is data of one subframe, and the data of the first symbol (not including CP) is denoted as npss _ re _ ti _ saml(n), n is 0,1, …,128-1, then npss _ re _ ti _ saml(n)＝npss_re_ti_sam(137l+10+(l≥7)+n)，l＝0,1,…,13。
The local npss down-sampled signal is npss _ loc _ ti _ sam (n), and if the influence of noise is neglected when frequency offset occurs, the npss signal received by the l symbol is:
where ε is the normalized frequency offset, △l137l +10+ (l ≧ 7). Conjugate multiplying the npss down-sampled signal of the l symbol received with the local npss down-sampled signal:
yl(n)＝S(l)npss_loc_ti_sam*(n)npss_re_ti_saml(n),n＝0,1,...,127 (10)
for yl(n) compensating for the frequency offset and summing, and then adding the results of the latter 11 symbols as shown in equation (11).
Where epsilon is the normalized frequency offset estimate. The maximum value of y _ sum (epsilon) is obtained, the label epsilon corresponding to the maximum value is the rough frequency offset estimation, namely the obtained rough frequency offset estimation is
Use of the received npss down-sampled signal npss _ re _ ti _ sam (n)
And compensating to obtain an npss signal after compensation as follows:
wherein N is 1920.
The following is a fine frequency offset estimation of the signal. npss _ cmp (n) is data of one subframe, and is divided into 14 symbols, and data of the first symbol (not including CP) is denoted as npss _ cmpl(n), n is 0,1, …,128-1, then npss _ cmpl(n) ═ npss _ cmp (137l +10+ (l ≧ 7) + n), l ═ 0,1, …, 13. For signal npss _ cmpl(n) the same operation as in the formula (9) and the formula (10) can be carried out to obtain:
wherein,
for fine frequency offset estimation, △l137l +10+ (l ≧ 7). Let y3' (n) and y5' (n) conjugate multiplication and addition:
wherein △ - △5-△3274. In the same way, for y4' (n) and y6'(n)、y7' (n) and y9'(n)、y8' (n) and y10'(n)、y11' (n) and y13' (n) the same operation is performed, p values are obtained and summed to obtain p _ sum, and the obtained fine frequency offset estimation is as follows:
the estimation range of the fine frequency offset is (-0.234,0.234), the estimation error of the coarse frequency offset is (-0.2,0.2), so the fine frequency offset estimation can compensate the estimation error of the coarse frequency offset and improve the estimation accuracy of the frequency offset to the maximum extent. The frequency offset of the finally estimated signal is estimated as
nsss signal detection unit
The nsss signal detection functional block diagram is shown in fig. 5, and is characterized in that firstly, an npss signal after down-sampling is converted into a frequency domain after compensating frequency offset, channel estimation and a nsss signal threshold are obtained according to an npss frequency domain signal generated locally, then a received nsss signal is obtained according to start _ time _ acc, the signal is down-sampled and frequency offset compensated and then converted into the frequency domain, and the nsss frequency is converted into the frequency domainPerforming channel compensation on the domain signal, determining a frame number and a cellID candidate value by utilizing the self characteristics of the nsss signal, then generating a local nsss frequency domain signal, performing cross correlation on the local nsss frequency domain signal and the nsss frequency domain receiving signal, and finally acquiring nfThe low 3bit information and the cellID.
The received npss down-sampled signal is denoted as npss _ re _ ti _ sam (n), the frequency offset is estimated as freoffset, and the frequency offset is expressed by equation (12)
The npss signal obtained by calculating the compensated frequency offset by replacing the freoffset is denoted as npss _ cmp (n), and is divided into 14 symbols, and the data (not including CP) of the l symbol is denoted as npss _ cmpl(n), n is 0,1, …,127, then npss _ cmpl(n) ═ npss _ cmp (137l +10+ (l ≧ 7) + n), l ═ 0,1, …, 13. Will signal npss _ cmpl(n) converting the signal into the frequency domain according to the formula (17) to obtain the npss frequency domain reception number npss _ re _ fre of the first symboll(k)。
Where l is 3,4, …, 13. Let npss signal frequency domain resource grid be npss _ re _ fre _ vec (k, l), where k is 0,1, …,11, l is 0,1, …, 13. The resource grid represents the time-frequency distribution of the signal, npss _ re _ frel(k) And correspondingly filling the values l and k into the resource grid to obtain the frequency domain resource grid of the received npss signal.
From equation (2), the local npss frequency domain signal of a symbol is npss _ loc _ fre (n), and the local npss frequency domain signal of the i-th symbol is s (l) npss _ loc _ fre (n), and the npss frequency domain resource grid npss _ loc _ fre _ vec (k, l) is filled into the resource grid, so as to obtain the channel estimation as shown in equation (18):
ce′(k,l)＝npss_re_fre_vec(k,l)/npss_loc_fre_vec(k,l) (18)
where k is 0,1, …,10, l is 3,4, …, 13. The difference in the amplitudes of the first symbol of ce' (k, l) is
abs represents a modulo operation with an angular mean of
angle represents an angle calculation, and a typical interpolation algorithm is used to spread the channel estimation to obtain ce (k, l), where k is 0,1, …,10,11, l is 3,4, …,13, and then the amplitude value of ce (11, l) is ce _ mag (11, l) ═ abs (ce '(10, l)) + ce _ mag' (l), and the angle value is ce _ ang (11, l) ═ angle (ce '(10, l)) + ce _ ang' (l), and then ce (k, l) takes the value as shown in equation (19).
The npss _ re _ fre _ vec (k, l) and npss _ loc _ fre _ vec (k, l) are subjected to resource grid mapping, that is, data in each symbol are sequentially taken out according to the position of the resource grid mapping, and then the data are connected in series, so that an npss frequency domain receiving signal npss _ re _ fre (n) and a frequency domain local signal npss _ loc _ fre (n) with the length of 121 are obtained. The nsss signal threshold is derived from the frequency domain npss signal, as shown in equation (20):
the nsss signal is located in the 9 th subframe of the even-numbered systematic frame, and one subframe must exist in the 21ms data.
According to the start _ time _ acc, two received nss signals can be obtained, only one of the two received nss signals is an actually existing nss signal and is marked as nss _ re _ ti (n), 16 times of down sampling is carried out on the signal nss _ re _ ti (n) to obtain a signal nss _ re _ ti _ sam (n), frequency offset compensation is carried out on the signal nss _ re _ ti _ sam (n) according to the operation, the signal nss _ re _ fre _ vec (k, l) is converted into a frequency domain, a nss signal resource grid nss _ re _ fre _ vec (k, l) is obtained, channel compensation is carried out on the signal nss _ re _ fre _ vec _ cmp (k, l) according to a formula (21), and a compensated.
nsss_re_fre_vec_cmp(k,l)＝nsss_re_fre_vec(k,l)/ce(k,l) (21)
And (3) performing inverse resource grid mapping on the nsss _ re _ fre _ vec _ cmp (k, l) to obtain a nsss frequency domain signal nsss _ re _ fre (n) with the length of 131.
The nsss local signal is a Zadoff-Chu sequence in a frequency domain, and the generation expression is as follows:
wherein
n＝0,1,...,131
n′＝nmod131
m＝nmod128
mod represents the remainder operation. bq(m) is shown in Table 2:
TABLE 2bq(m) value-taking table
For the received signal, if the noise effect is neglected:
compensating b for the signal nsss _ re _ fre (n)q(m) obtaining:
from formula (24):
when n isfWhen 0 or 4, zc (n) has central symmetry, that is, zc (n) zc (130-n). Let q equal 0,1,2,3, nfFor nsss _ re _ fre (n), 0,2,4,6, we compensate:
conjugate summation is carried out on the front part and the rear part of the material to obtain:
define corr (q, n)f) N corresponding to the maximum valuefIs nfMax, q is q _ max, then nfIs (n)f_max,(nfMax +4) mod8) and the candidate value for q is q _ max.
Let n befWhen the cellID is 0,2,4,6, 1, …,503, the local nss frequency domain signal nss _ loc _ fre (n, n) is generated as shown in equation (22)fcellID) to local. According to the method described above, two nss frequency domain compensation signals obtained from the received 21ms data according to the start _ time _ acc are respectively marked as nss _ re _ fre0(n)、nsss_re_fre1(n), n calculatedfIs set to nf0、nf1Using nsss _ re _ frei(n) and nsss _ loc _ fre (n, n)fcellID) is shown in equation (28), the detection process is shown in fig. 6, when corr is detectednsss>nsss _ thresh, n corresponding to local signalfAnd the cellID value is n obtained by calculationfThe low 3bit information and the cellID.
Example 1
To verify the effectiveness of the present invention, several tests were performed. The experimental input was 21ms data received from the receive antennas.
FIG. 7 is a comparison graph of time offset estimation, which is compared with the method of the present invention by using a segment correlation method, wherein normalized frequency offsets of-1.8 to 1.8 are added to input data, each algorithm corresponding to each frequency offset is simulated 100 times, and the Root Mean Square Error (RMSE) of the detected time offsets is shown in the figure. Fig. 8 is a comparison diagram of frequency offset estimation, which is compared with the method of the present invention by using a method (denoted as method one) of dividing an npss signal in a symbol by a local signal after conjugate multiplication into two sections, randomly adding normalized frequency offsets in the range of (-1,1) to data under different signal-to-noise ratios (SNRs), simulating 100 times for each algorithm under each SNR, calculating the normalized frequency offset of the data after compensation of the frequency offset by using a frequency synchronization tracking algorithm, and multiplying the normalized frequency offset by 15kHz as a difference value between the estimated frequency offset and the actual frequency offset, wherein the RMSE of the detected frequency offset is shown in the figure, and it can be known from the figure that the frequency offset estimation result of the present invention is closer to the actual frequency offset value and has higher accuracy. Table 3 is a nsss signal detection time comparison table, comparing with the method of the present invention using a direct correlation method, the data shows that the nsss signal detection of the present invention takes less time and the time complexity is reduced. When storing the local npss signal, the invention only needs to store the npss time domain down-sampled signal and the frequency domain signal of one symbol, and the space complexity is reduced.
TABLE 3nsss Signal detection time comparison table
The above description is only a preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered as the technical solutions and the inventive concepts of the present invention in the technical scope of the present invention.
Claims (7)
1. A cell search system in an NB-IoT system downlink, comprising:
the npss signal detection unit is used for converting a frequency domain signal of the generated local master synchronization signal npss into a time domain signal, performing down-sampling processing, then performing down-sampling processing on data received from the receiving antenna, and performing timing coarse synchronization by using the down-sampled data; correcting the result of the timing coarse synchronization to obtain a corrected value of the timing coarse synchronization, then performing timing fine synchronization by using data which is not subjected to down-sampling to finish the detection of the npss signal, and giving a timing deviation start _ time _ acc;
the frequency synchronization unit is used for obtaining a received npss signal according to the start _ time _ acc, carrying out down-sampling on the signal, and calculating to obtain a coarse frequency offset estimation and carrying out coarse frequency offset compensation according to the down-sampled npss signal;
performing fine frequency offset estimation on the npss signal after the coarse frequency offset compensation, wherein the frequency offset estimation calculated according to the npss signal can be approximate to the frequency offset estimation of the received signal, namely the frequency offset estimation of the received signal is finally obtained;
the nsss signal detection unit compensates frequency offset for the npss signal after down sampling and then transforms the npss signal to a frequency domain;
obtaining channel estimation and an auxiliary synchronization signal nsss signal threshold according to a locally generated npss frequency domain signal, then obtaining a received nsss signal according to start _ time _ acc, performing down-sampling and frequency offset compensation on the signal, converting the signal into a frequency domain, performing channel compensation on the nsss frequency domain signal, determining a frame number and a cellID candidate value by using the self characteristics of the nsss signal, then generating a local nsss frequency domain signal, performing cross-correlation on the local nsss frequency domain signal and the nsss frequency domain received signal, and finally obtaining nfLow 3bit information and cellID; wherein cellID represents a physical layer cell ID, nfIndicating the system frame number.
2. The cell search system of claim 1, wherein the npss signal detection unit generates only one local npss frequency-domain signal of symbol, transforms the local npss frequency-domain signal into a time-domain signal, and performs the down-sampling process as follows:
the npss signal is a Zadoff-Chu sequence in the frequency domain, and the generation expression is as follows:
wherein, the root sequence index u is 5, l is symbol number, S (0) is S (1) is S (2) is 0, and only one frequency domain npss signal of symbol is generated, and the generation expression is:
transform npss _ loc _ fre (n) to time domain, generating a time domain npss signal of symbol:
performing 16-time down-sampling on the signal npss _ loc _ ti (n), as shown in formula (4), wherein the obtained signal is denoted as npss _ loc _ ti _ sam (n), and the length of the obtained signal is 128 sampling points;
npss_loc_ti_sam(n)＝npss_loc_ti(16n),n＝0,1,...,127 (4)
the npss signal is located in the 5 th subframe of the system frame, and the npss signal of two subframes must be included in the received 21ms data.
3. A cell search system in NB-IoT system downlink according to claim 2, wherein the coarse timing synchronization procedure is as follows:
down-sampling 21ms data re (n) received from a receiving antenna, and obtaining a signal designated as re _ sam (n), wherein npss _ loc _ ti _ sam (n) and re _ sam (n) are used for sliding correlation, and when one point slides, data of one subframe is taken out from re _ sam (n), designated as re _ sam _ sf (M), M is 0,1, …, M-1, M is 1920, re _ sam _ sf (M) is re _ sam (n + M), n is 0,1, …,19200, re _ sam _ sf (M) is divided into 14 symbols, data of the first symbol, wherein no CP is contained, and designated as re _ sam (n) is used for sliding correlationl(n), n is 0,1, …,127, then re _ saml(n)＝re _ sam _ sf (137l +10+ (l ≧ 7) + n), l ═ 0,1, …,13, where (l ≧ 7) denotes a value when l<When l is more than or equal to 7, 0 is obtained, and when l is more than or equal to 7, 1 is obtained; for npss signals, the first 3 symbols are 0, and the last 11 symbols are re _ saml(n) are associated with the local signal npss _ loc _ ti _ sam (n) in segments, where the number of segments is 2, each segment is 64, and summed, as shown in equation (5):
where n is 0,1, …,19200, i.e. the length of a co-sliding frame; the maximum value of corr (n) is obtained, and the mark n corresponding to the maximum value is the timing coarse synchronization position, namely
Respectively taking 20 points at two sides of the obtained coarse synchronization position coarse _ start _ time, and performing sliding correlation by using npss _ loc _ ti _ sam (n) and re _ sam (n);
using npss _ loc _ ti _ sam (n) and re _ samlThe (n) modes are correlated to counter the effect of the frequency offset on the time offset, as shown in equation (6)
Wherein n is coarse _ start _ time-20, coarse _ start _ time-19, …, coarse _ start _ time + 20; the maximum value of corr _ rev (n) is obtained, and the mark n corresponding to the maximum value is the revised value of the timing coarse synchronization, namely
4. A cell search system in NB-IoT system downlink according to claim 3, wherein the fine timing synchronization procedure is as follows:
after obtaining the revision value start _ time _ rev of the timing coarse synchronization, multiplying the revision value start _ time _ rev by 16 to convert the revision value into the time offset estimation of the non-downsampled data;
taking 64 points on both sides of the sampling point, sliding on non-downsampled data re (N), taking data of one subframe from re (N) every time one point is slid, and taking data of one subframe as re _ sf (N), wherein N is 0,1, …, N-1, N is 30720, then carrying out downsampling by 16 times, and obtaining a signal as re _ sam _ sf '(N), wherein N is 0,1, …, N' -1, N 'is 1920, and data of the l symbol is as re _ sam'l(n) of length 128, for signals npss _ loc _ ti _ sam (n) and re _ sam'lThe modulus of (n) is correlated, as shown in equation (7):
wherein n is start _ time _ rev 16-64, start _ time _ rev 16-15, …, and start _ time _ rev 16+ 64; the maximum value of corr _ acc (n) is obtained, and the mark n corresponding to the maximum value is the fine timing synchronization, i.e. the fine timing synchronization
This value is where the npss signal starts.
5. The system of claim 1, wherein the frequency synchronization unit is configured to derive the coarse frequency offset estimate and to provide the coarse frequency offset compensation by:
determining a received npss signal from received data according to the calculated timing deviation start _ time _ acc, performing 16-time down-sampling processing on the determined npss signal, and recording the result as npss _ re _ ti _ sam (n), wherein:
npss_re_ti_sam(n)＝re(start_time_acc+16n),n＝0,1,...,N-1 (8)
where N is 1920, npss _ re _ ti _ sam (N) is data of one subframe, and is divided into 14 symbols, i.e. data of the first symbol, which does not include CP, and is denoted as npss _ re _ ti _ saml(n), n is 0,1, …,128-1, then npss _ re _ ti _ saml(n)＝npss_re_ti_sam(137l+10+(l≥7)+n)，l＝0,1,…,13；
The local npss down-sampled signal is npss _ loc _ ti _ sam (n), and if the influence of noise is ignored, the npss signal received from the ith symbol is:
where ε is the normalized frequency offset, △l137l +10+ (l ≧ 7); conjugate multiplying the npss down-sampled signal of the l symbol received with the local npss down-sampled signal:
yl(n)＝S(l)npss_loc_ti_sam*(n)npss_re_ti_saml(n),n＝0,1,...,127 (10)
for yl(n) compensating for the frequency offset and summing, and then adding the results of the last 11 symbols as shown in equation (11):
wherein epsilon is the normalized frequency offset estimation;
the maximum value of y _ sum (epsilon) is obtained, the label epsilon corresponding to the maximum value is the rough frequency offset estimation, namely the obtained rough frequency offset estimation is
Use of the received npss down-sampled signal npss _ re _ ti _ sam (n)
And compensating to obtain an npss signal after compensation as follows:
wherein N is 1920.
6. The system of claim 1, wherein the fine frequency offset estimation process comprises:
npss _ cmp (n) is data of one subframe, andit is divided into 14 symbols, the first symbol data is denoted as npss _ cmpl(n), n is 0,1, …,128-1, then npss _ cmpl(n)＝npss_cmp(137l+10+(l≥7)+n)，l＝0,1,…,13；
For signal npss _ cmpl(n) the same operation as in the formula (9) and the formula (10) can be carried out to obtain:
wherein,
for fine frequency offset estimation, △l137l +10+ (l ≧ 7); let y3' (n) and y5' (n) conjugate multiplication and addition:
wherein △ - △5-△3274; in the same way, for y4' (n) and y6'(n)、y7' (n) and y9'(n)、y8' (n) and y10'(n)、y11' (n) and y13' (n) the same operation is performed, p values are obtained and summed to obtain p _ sum, and the obtained fine frequency offset estimation is as follows:
the estimation range of the fine frequency offset is (-0.234,0.234), the estimation error of the coarse frequency offset is (-0.2,0.2), the fine frequency offset estimation can compensate the estimation error of the coarse frequency offset and improve the estimation accuracy of the frequency offset to the maximum extent, and the frequency offset estimation of the finally estimated signal is
7. The cell search system in the downlink of NB-IoT system as claimed in claim 6, wherein the nsss signal detection unit specifically operates as follows:
the received npss down-sampled signal is denoted as npss _ re _ ti _ sam (n), the frequency offset is estimated as freoffset, and the frequency offset is expressed by equation (12)
The npss signal obtained by calculating the compensated frequency offset by replacing the freoffset is denoted as npss _ cmp (n), and is divided into 14 symbols, and the data (not including CP) of the l symbol is denoted as npss _ cmpl(n), n is 0,1, …,127, then npss _ cmpl(n) ═ npss _ cmp (137l +10+ (l ≧ 7) + n), l ═ 0,1, …, 13; will signal npss _ cmpl(n) converting the signal into the frequency domain according to the formula (17) to obtain the npss frequency domain reception number npss _ re _ fre of the first symboll(k)；
Wherein l is 3,4, …, 13; let npss signal frequency domain resource grid be npss _ re _ fre _ vec (k, l), where k is 0,1, …,11, l is 0,1, …, 13; the resource grid represents the time-frequency distribution of the signal, npss _ re _ frel(k) Correspondingly filling the values l and k into a resource grid to obtain a frequency domain resource grid of the received npss signal;
from equation (2), a local npss frequency domain signal of a symbol is npss _ loc _ fre (n), and a local npss frequency domain signal of the l-th symbol is s (l) npss _ loc _ fre (n), and the npss frequency domain signal is filled into the resource grid to obtain a local npss frequency domain resource grid npss _ loc _ fre _ vec (k, l), and the obtained channel estimation is as shown in equation (18):
ce′(k,l)＝npss_re_fre_vec(k,l)/npss_loc_fre_vec(k,l) (18)
wherein k is 0,1, …,10, l is 3,4, …, 13; the difference in the amplitudes of the first symbol of ce' (k, l) is
abs represents a modulo operation with an angular mean of
angle represents an angle calculation, and a typical interpolation algorithm is used to spread the channel estimation to obtain ce (k, l), where k is 0,1, …,10,11, l is 3,4, …,13, then the amplitude value of ce (11, l) is ce _ mag (11, l) ═ abs (ce '(10, l)) + ce _ mag' (l), the angle value is ce _ ang (11, l) ═ angle (ce '(10, l)) + ce _ ang' (l), and ce (k, l) takes the value as shown in equation (19):
inverse resource grid mapping is carried out on npss _ re _ fre _ vec (k, l) and npss _ loc _ fre _ vec (k, l), namely data in each symbol are sequentially taken out according to the position of the resource grid mapping, and then the data are connected in series, so that an npss frequency domain receiving signal npss _ re _ fre (n) and a frequency domain local signal npss _ loc _ fre (n) are obtained, wherein the length of the npss _ re _ fre _ vec (k, l) is 121; the nsss signal threshold is derived from the frequency domain npss signal, as shown in equation (20):
the nsss signal is positioned in the 9 th subframe of the even-numbered system frame, and the nsss signal of one subframe must exist in the 21ms data;
obtaining two received nss signals according to start _ time _ acc, wherein only one of the two received nss signals is an actually existing nss signal and is marked as nss _ re _ ti (n), carrying out 16-time down-sampling on the signal nss _ re _ ti (n) to obtain a signal nss _ re _ ti _ sam (n), carrying out frequency offset compensation on the signal nss _ re _ ti _ sam (n) according to the operation and converting the signal nss _ re _ fre _ vec (k, l) into a frequency domain to obtain a nss signal resource grid nss _ re _ fre _ vec _ cmp (k, l), and carrying out channel compensation on the signal nss _ re _ fre _ vec _ cmp _ p (k, l) according to a formula (21) to obtain a compensated nss frequency domain;
nsss_re_fre_vec_cmp(k,l)＝nsss_re_fre_vec(k,l)/ce(k,l) (21)
inverse resource grid mapping is carried out on nsss _ re _ fre _ vec _ cmp (k, l), so that a nsss frequency domain signal nsss _ re _ fre (n) with the length of 131 is obtained;
the nsss local signal is a Zadoff-Chu sequence in a frequency domain, and the generation expression is as follows:
wherein
n＝0,1,...,131
n′＝n mod 131
m＝n mod 128
For the received signal, if the noise effect is neglected:
compensating b for the signal nsss _ re _ fre (n)q(m) obtaining:
from formula (24):
when n isfWhen 0 or 4, zc (n) has central symmetry, that is, zc (n) zc (130-n), and q is 0,1,2,3, nfFor nsss _ re _ fre (n), 0,2,4,6, we compensate:
conjugate summation is carried out on the front part and the rear part of the material to obtain:
define corr (q, n)f) N corresponding to the maximum valuefIs nfMax, q is q _ max, then nfIs (n)f_max,(nfMax +4) mod8) with the candidate value for q being q _ max;
let n bef＝0,2,4,6，cellID＝0,1,…,503，
The local nss frequency domain signal nss _ loc _ fre (n, n) is generated as shown in equation (22)fcellID) to local; according to the operation, two nss frequency domain compensation signals obtained from the received 21ms data according to the start _ time _ acc are respectively marked as nss _ re _ fre0(n)、nsss_re_fre1(n), n calculatedfIs set to nf0、nf1，
Using nsss _ re _ frei(n) and nsss _ loc _ fre (n, n)fcellID) is calculated as shown in equation (28):
when corr is presentnsss>nsss _ thresh, n corresponding to local signalfAnd the cellID value is n obtained by calculationfThe low 3bit information and the cellID.
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