CN107204953B - Blind frequency offset estimation method in CP-FBMC communication system - Google Patents
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- H04L27/00—Modulated-carrier systems
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Abstract
The invention discloses a blind frequency offset estimation method in a CP-FBMC communication system, aiming at improving the estimation performance while ensuring the spectrum efficiency by designing a reasonable virtual subcarrier distribution mode, and adopting the technical scheme that: and distributing three or more continuous virtual subcarriers at the transmitting end of the CP-FBMC system, and calculating a cost function at the receiving end of the system by using the virtual subcarriers which are not adjacent to the data subcarriers, wherein the frequency deviation trial value corresponding to the minimum value of the cost function is the estimated value of the frequency deviation.
Description
Technical Field
The invention relates to a frequency offset estimation method of an FBMC (fiber Bragg mobile communication) system in the field of wireless communication, in particular to a blind frequency offset estimation method in a CP-FBMC communication system.
Background
The multi-carrier modulation system not only can provide high-speed data transmission, multiply improve the system capacity, but also can effectively resist the frequency selective fading of the channel, so the multi-carrier modulation system has received extensive attention and research in academia and industry. However, the out-of-band leakage is always a big disadvantage to carrier modulation systems. The FBMC (cyclic filter bank multi-carrier) technique can effectively reduce the out-of-band leakage of signals by using a prototype filter having good time-frequency focusing characteristics. In addition, the FBMC technology introduces operations such as a polyphase filter, fast Fourier transform and the like, greatly reduces the complexity and the calculation amount of the FBMC technology, and has wide application prospect. In order to improve the spectrum efficiency of the FBMC, a Cyclic filter bank multi-carrier Cyclic-FBMC is proposed, and meanwhile, in order to resist frequency selective fading, CP is added on the basis of the Cyclic-FBMC to form a CP-FBMC system, and a better quick implementation structure is deduced.
Like other multi-carrier systems, FBMC is also very sensitive to synchronization distortion, and time-frequency synchronization has been a research hotspot of FBMC. The current FBMC time-frequency synchronization method mainly comprises an estimation algorithm and a blind estimation algorithm based on data assistance. The data-aided estimation method utilizes correlation operation of repeated symbols to carry out synchronous calculation, needs a plurality of synchronous symbols and protection symbols to counteract the overlapping property of the FBMC system, and has low frequency spectrum efficiency. This type of method was first proposed in "Data-aided system timing and CFOsynchonization for filter bank multicarrier systems" published by Tilde Fusco, Angelo Petrella, Mario Tanda in IEEE Transactions on Wireless Communications,2009,8(5): 2705-2715. In the chinese patent 201110215577.0 "an OFDM/OQAM system and its time-frequency synchronization method", the estimation performance in the above method is improved by the selection of the correlation operation and the maximum posterior probability criterion, but the spectrum efficiency is still very low. The early blind estimation algorithm mainly utilizes the second-order cyclostationarity of the FBMC multi-carrier system and has high complexity. Davide Mattera, Mariotanda in IEEE Transactions on Wireless Communications, 2013, 12 (1): 268-277 published "band Symbol Timing and CFO Estimation for OFDM/OQAM Systems" proposed to utilize the conjugate symmetry of FBMC, greatly reducing the complexity but reducing the performance. Therefore, the trade-off between the spectrum efficiency and the estimation performance in FBMC is currently in need of solution. Meanwhile, considering that CP needs to be added based on a Cyclic-FBMC system under the condition of a channel with serious frequency selectivity, the frequency offset estimation and compensation problems under the CP-FBMC system are also in need of solution.
Disclosure of Invention
In order to solve the problems in the prior art, the invention provides a blind frequency offset estimation method in a CP-FBMC communication system, which improves the estimation performance while ensuring the spectrum efficiency by designing a reasonable virtual subcarrier distribution mode.
In order to achieve the above purpose, the technical scheme adopted by the invention is as follows: the method comprises the following steps:
1) the transmitting terminal allocates virtual subcarriers in a random continuous mode, and allocates three or more continuous virtual subcarriers;
2) modulating the information bit stream of the subcarriers according to the subcarrier allocation mode in the step 1);
3) partitioning the subcarrier data in the step 2) by adopting a Cyclic-FBMC system to obtain a plurality of data blocks;
4) modulating each data block in the step 3), namely multiplying the data symbols on different frequency time points by corresponding basis functions, so that the transmitting end sends a signal sl：
Wherein,denotes normalized LcDFT conversion of points;representing the frequency domain transmission process, Λm＝diag(λm)， In the form of a generalized DFT,
5) the receiving end adds CP to form a CP-FBMC system based on Cyclic-FBMC, and a signal s is sent by the sending endlThrough the multipath channel, the frequency deviation occurs at the receiving end to obtain the received signal rlThe frequency offset signal received by the receiving end is expressed as:
where phi is the actual value of the frequency offset, nlrepresenting a mean of 0 and a variance of σ2Additive white gaussian noise of (1);
6) assuming CFO heuristic valuesFor received signal rlAnd (3) performing frequency offset trial compensation:
demodulation results in:
7) calculating a cost function using virtual subcarriers in which the continuous virtual subcarriers are not adjacent to the data subcarriers:
to pairTraversing and respectively calculating cost functions, and then solving the following minimization problem to obtain the estimated value of the frequency deviation:
corresponding to a frequency offset heuristic valueI.e. an estimate of the frequency offsetAnd finishing blind frequency offset estimation.
Selecting the number P of virtual subcarrier groups in the step 1), wherein the number Q of continuous virtual subcarriers in each group is more than or equal to 3, and the serial number of the first virtual subcarrier in each group is recorded as mi,1，i＝1,2,…P，mi,1∈[0,M-1]The serial number of the rest virtual subcarriers in each group is mi,1+j＝((mi,1+j))M，((mi,1+j))MRepresents mi,1The result of M cycling for j, i ═ 1,2, … P, j ═ 0,1, … Q-1, Mi,1+j∈[0,M-1]The rest of M subcarriers except the virtual subcarrier are data subcarriers, and the number of the virtual subcarriers which are not adjacent to the data subcarriers is P (Q-2), and the virtual subcarriers are set as Mused-null。
The modulation in the step 2) comprises the following steps: and (2) allocating QPSK symbols to the subcarriers according to the subcarrier allocation mode in the step 1), wherein the virtual subcarriers do not carry symbol information, performing serial-parallel conversion, then performing OQAM preprocessing, and converting complex data symbols into real data symbols.
The step 3) of blocking the sub-carrier data comprises the following steps: on subcarrier m, a data symbol sequence am[n]Is divided to contain NcA plurality of symbols of size 2NcOf length Lc＝M·NcLet am,l[n]Representing the ith block of data symbols, a, on subcarrier mm,l[n]＝am[2lNc+n],n＝0,1,…,2Nc-1。
The modulation process of the step 4) is as follows:
4.1) the modulation signal to be sent by the transmitting terminal is:
wherein, gm,n[k]Is am,l[n]The corresponding basis functions:
wherein,is an initial phase added to the real data symbols to be transmitted,to representWith LcAs a result of the cycle, pc[k']Is a prototype filter adopted in a Cyclic-FBMC system and a prototype filter p [ k 'in the FBMC system']The following relationships exist:
4.2) the l modulation signal in Cyclic-FBMC is expressed as:
wherein,
is the center frequency on the mth subcarrier;
make variable substitutionWherein delta0≤m0<Nc+δ0Is an integer when NcWhen it is even, delta00.5, otherwise, δ00, and orderThen sl[k]And pc,m[k']Writing into:
4.3) reacting sl[k]Matrixing, sl＝[sl[0] sl[1] … sl[Lc-1]]TIs s isl[k]Is expressed in matrix form by respectively making am,l＝[am,l[0] am,l[1] … am,l[2Nc-1]]T，pc,m＝[pc,m[0] pc,m[1] … pc,m[Lc-1]]T；
After derivation, send outTransmitting end transmitting signal slThe final write is in the form:
for each m in said step 6)0＝m，The main diagonal elements are 1+0j, and the other elements are pure imaginary numbers or infinite small real parts, then
For each m0＝m， Each element of (a) is affected by a time-domain neighbor element;
for each m0≠m，|m-m0When 1, thenCannot be ignored; i m-m0|>1, only adjacent subcarrier pairs are consideredThe influence of (c).
The estimation value according to the frequency deviation in the step 7)Compensating the frequency offset of the received signal, equalizing each block of data point by point at the receiving end, and demodulating to obtainWill be provided withOQAM post-processing is performed, a real symbol is changed back to a complex symbol, parallel-serial conversion is performed, QPSK demodulation is performed, and finally the transmitted bit data stream is recovered.
Compared with the prior art, the invention distributes three or more continuous virtual subcarriers at the transmitting end of the CP-FBMC system, calculates the cost function at the receiving end of the system by utilizing the virtual subcarriers which are not adjacent to the data subcarriers, and the frequency deviation trial value corresponding to the minimum value of the cost function is the estimated value of the frequency deviation.
Drawings
Fig. 1 is a schematic diagram of a virtual subcarrier allocation method at a transmitting end according to the present invention;
FIG. 2 is a flow chart of a method of the present invention;
FIG. 3 is a graph comparing the SNR with the RMS error in the case of blind frequency offset estimation using the comparative example and the present invention, where the abscissa represents the SNR Eb/N0Unit decibel (dB), range 0-30 dB, ordinate represents the corresponding estimated root mean square error.
Detailed Description
The invention is further explained below with reference to specific embodiments and the drawing of the description.
Referring to fig. 2, the present invention comprises the steps of:
1. a transmitting end:
(1) referring to fig. 1, M subcarriers at the transmitting end, where the subcarrier sequence number M is 0,1, …, and M-1, and the subcarrier sequence number difference is 1, indicate that the subcarriers are adjacent, a random continuous virtual subcarrier allocation scheme is adopted: selecting the number P of virtual subcarrier groups, the number Q of continuous virtual subcarriers in each group, wherein Q is more than or equal to 3, and recording the serial number of the first virtual subcarrier in each group as mi,1，i＝1,2,…P，mi,1∈[0,M-1](ii) a Considering CP-FBMC additionThe cyclicity of the Cyclic-FBMC system before the CP is added, and the serial numbers of the rest virtual sub-carriers of each group are mi,1+j＝((mi,1+j))M，((mi,1+j))MRepresents mi,1The result of M cycling for j, i ═ 1,2, … P, j ═ 0,1, … Q-1, Mi,1+j∈[0,M-1]The rest of M subcarriers except the virtual subcarrier are data subcarriers, and the number of virtual subcarriers P (Q-2) which are not adjacent to the data subcarrier is recorded as a set Mused-null；
(2) QPSK modulation is carried out on the information bit stream, QPSK symbols are distributed according to the subcarrier distribution mode in the step (1), symbol information is not carried on virtual subcarriers, serial-parallel conversion is carried out, then OQAM preprocessing is carried out, and complex data symbols are converted into real data symbols;
(3) the data is sent in blocks by adopting a Cyclic-FBMC system, and a data symbol sequence a is arranged on a subcarrier mm[n]Is divided to contain NcA plurality of symbols of size 2NcOf length Lc＝M·NcLet am,l[n]Representing the ith block of data symbols, a, on subcarrier mm,l[n]＝am[2lNc+n],n＝0,1,…,2Nc-1；
(4) Modulating each data block, that is, multiplying data symbols at different frequency time points by corresponding basis functions, wherein a modulation signal to be transmitted at a transmitting end is:
wherein, gm,n[k]Is am,l[n]The corresponding basis functions:
here, ,is an initial phase added to the real data symbols to be transmitted,to representWith LcAs a result of the cycle, pc[k']Is a prototype filter adopted in a Cyclic-FBMC system and a prototype filter p [ k 'in the FBMC system']The following relationships exist:
2. the quick realization form is as follows:
(1) the l-th modulation signal in Cyclic-FBMC can be further represented as:
wherein,
is the center frequency on the mth subcarrier;
make variable substitutionWherein delta0≤m0<Nc+δ0Is an integer when NcWhen it is even, delta00.5, otherwise, δ00, and ordersl[k]And pc,m[k]Can be rewritten as:
(2) will sl[k]Matrixing, sl＝[sl[0] sl[1] … sl[Lc-1]]TIs s isl[k]Is expressed in matrix form by respectively making am,l＝[am,l[0] am,l[1] … am,l[2Nc-1]]T，pc,m＝[pc,m[0] pc,m[1] … pc,m[Lc-1]]T；
Through derivation, slThe following can be written eventually:
wherein,denotes normalized LcDFT conversion of points;denotes Frequency Domain Transmit Processing (FDTP), Λm＝diag(λm)， Referred to as generalized DFT (generalized DFT, GDFT for short),
3. receiving end:
(1) CP-FBMC system formed by adding CP based on Cyclic-FBMC, and transmitting end signal slThrough the multi-path channel, the channel is,the frequency deviation occurs at the receiving end to obtain a received signal rlDue to the cyclic prefix, the frequency offset signal received by the receiving end can be expressed as:
where phi is the actual value of the frequency offset, nlrepresenting a mean of 0 and a variance of σ2Additive white gaussian noise of (1);
(2) assuming CFO heuristic valuesFor received signal rlAnd (3) performing frequency offset trial compensation:
demodulation results in:
for each m0＝m，The main diagonal elements are 1+0j, and the remaining elements are either pure imaginary numbers or very small real parts, so
For each m0＝m，Each element in the set is mainly affected by time domain neighboring elements;
for each m0≠m，|m-m0When the value is 1, the process is carried out,cannot be ignored. I m-m0|>When the pressure of the mixture is 1, the pressure is lower,is very small, so only adjacent subcarrier pairs need to be consideredThe influence of (a);
(3) calculating a cost function using virtual subcarriers in which consecutive virtual subcarriers are not adjacent to data subcarriers:
to pairTraversing and respectively calculating the cost function, the estimated value of the frequency offset can be obtained by solving the following minimization problem:
corresponding to a frequency offset heuristic valueI.e. an estimate of the frequency offset
(4) Performing frequency offset compensation on the received signal according to the estimated frequency offset value, performing point-by-point equalization on each block of data at the receiving end, and demodulating to obtainWill be provided withAnd performing OQAM post-processing, changing a real symbol into a complex symbol, performing parallel-serial conversion, performing QPSK demodulation, and finally recovering the transmitted bit data stream, and ending the algorithm.
The CP-FBMC system used in this embodiment adopts QPSK constellation modulation, and the system bandwidth B is 1/Ts11.2MHz, the number of subcarriers M is 1024, and each symbol block includes a complex symbol NcThe prototype filter is PHYDYAS, the overlap factor K is 4, the length of the cyclic prefix CP is G M/8, the number of virtual subcarrier groups P is 1, the length of the continuous virtual subcarrier Q is 3, the multi-path fading channel model adopts an ITU Vehicular A channel model, and the channel time delay is [ 00.310.711.091.732.51 ]](in μ s) and a path gain of [ 0-1-9-10-15-20](in dB). Since each data block of CP-FBMC is composed of NcSmall data blocks, so the estimation range of the frequency offset in the invention is
Fig. 3 shows a relationship curve of the bit snr and the estimated root mean square error when estimating the blind frequency offset by using the method 1 and the present invention, and the ratio of Davide Mattera, Mario Tanda in IEEE Transactions on wireless communications, 2013, 12 (1): 268-277 published "bland Symbol Timing and CFOEsimulation for OFDM/OQAM Systems" methods were compared as comparative examples and are labeled as method 1 in the comparative examples. In the method diagram of the invention, the length Q of the continuous virtual subcarrier is 3, P represents the number of virtual subcarrier groups, and two conditions of P being 1 and P being 30 are selected, wherein the larger P represents the more virtual subcarriers, and the larger the information amount for calculating the cost function is. It can be seen from the figure that the estimation performance of the method of the present invention is better than that of the method 1, and as the number of virtual subcarriers increases, the amount of available information increases, and the estimation performance is improved. The method of the invention obviously improves the estimation performance by inserting a small amount of virtual subcarriers.
Claims (7)
1. A blind frequency offset estimation method in a CP-FBMC communication system is characterized by comprising the following steps:
1) the transmitting terminal allocates virtual subcarriers in a random continuous mode, and allocates three or more continuous virtual subcarriers;
2) modulating the information bit stream of the subcarriers according to the subcarrier allocation mode in the step 1);
3) partitioning the subcarrier data in the step 2) by adopting a Cyclic-FBMC system to obtain a plurality of data blocks;
4) modulating each data block in the step 3), namely multiplying the data symbols on different frequency time points by corresponding basis functions, so that the transmitting end sends a signal sl：
Wherein,denotes normalized LcDFT conversion of points;representing the frequency domain transmission process, Λm＝diag(λm)， In the form of a generalized DFT,Ncrepresenting the complex symbols contained in each symbol block, am,lIndicating the l-th block of data symbols, p, on the transmit terminal carrier mc,mThe representation forms part of a frequency-domain transmission processing matrix, and K represents the weight of the prototype filterThe superposition factor m represents the subcarrier number m'0Represents a variable;
5) the receiving end adds CP to form a CP-FBMC system based on Cyclic-FBMC, and a signal s is sent by the sending endlThrough the multipath channel, the frequency deviation occurs at the receiving end to obtain the received signal rlThe frequency offset signal received by the receiving end is expressed as:
where phi is the actual value of the frequency offset, nlrepresenting a mean of 0 and a variance of σ2Is a white additive gaussian noise of (1),representing normalized 2NcPoint DFT conversion, M represents the number of sub-carriers at the transmitting end, G represents the length of the cyclic prefix,a diagonal element representing H;
6) assuming CFO heuristic valuesFor received signal rlAnd (3) performing frequency offset trial compensation:
wherein,represents the l number of the subcarrier m after frequency shiftDemodulating according to the symbol block to obtain:
7) calculating a cost function using virtual subcarriers in which the continuous virtual subcarriers are not adjacent to the data subcarriers:
wherein M isused-nullRepresenting a set of virtual sub-carriers that are not adjacent to the data sub-carriers,indicating a frequency offsetTime, subcarrier m0N of the l-th data symbol block of (1)0A plurality of demodulated symbols;
to pairTraversing and respectively calculating cost functions, and then solving the following minimization problem to obtain the estimated value of the frequency deviation:
corresponding to a frequency offset heuristic valueI.e. an estimate of the frequency offsetComplete blind frequencyAnd (6) estimating deviation.
2. The blind frequency offset estimation method in CP-FBMC communication system as claimed in claim 1, wherein the number P of virtual subcarrier groups, the number Q of continuous virtual subcarriers in each group, is selected in step 1), wherein Q is not less than 3, and the serial number of the first virtual subcarrier in each group is recorded as mi,1，i＝1,2,…P，mi,1∈[0,M-1]The serial number of the rest virtual subcarriers in each group is mi,1+j＝((mi,1+j))M，((mi,1+j))MRepresents mi,1The result of M cycling for j, i ═ 1,2, … P, j ═ 0,1, … Q-1, Mi,1+j∈[0,M-1]The rest of M subcarriers except the virtual subcarrier are data subcarriers, and the number of the virtual subcarriers which are not adjacent to the data subcarriers is P (Q-2), and the virtual subcarriers are set as Mused-null。
3. The method of claim 1, wherein the modulation in step 2) comprises: and (2) allocating QPSK symbols to the subcarriers according to the subcarrier allocation mode in the step 1), wherein the virtual subcarriers do not carry symbol information, performing serial-parallel conversion, then performing OQAM preprocessing, and converting complex data symbols into real data symbols.
4. The blind frequency offset estimation method in CP-FBMC communication system as claimed in claim 1, wherein said sub-carrier data blocking in step 3) comprises: on subcarrier m, a data symbol sequence am[n]Is divided to contain NcA plurality of symbols of size 2NcOf length Lc＝M·NcLet am,l[n]Representing the ith block of data symbols, a, on subcarrier mm,l[n]＝am[2lNc+n],n＝0,1,…,2Nc-1。
5. The method for blind frequency offset estimation in CP-FBMC communication system as claimed in claim 1, wherein the modulation procedure of step 4) is:
4.1) the modulation signal to be sent by the transmitting terminal is:
wherein, gm,n[k]Is am,l[n]The corresponding basis functions:
wherein,is an initial phase added to the real data symbols to be transmitted,to representWith LcAs a result of the cycle, pc[k']Is a prototype filter adopted in a Cyclic-FBMC system and a prototype filter p [ k 'in the FBMC system']The following relationships exist:
4.2) the l modulation signal in Cyclic-FBMC is expressed as:
wherein,
is the center frequency on the mth subcarrier;
make variable substitutionWherein delta0≤m0<Nc+δ0Is an integer when NcWhen it is even, delta00.5, otherwise, δ00, and orderThen sl[k]And pc,m[k']Writing into:
4.3) reacting sl[k]Matrixing, sl＝[sl[0] sl[1] … sl[Lc-1]]TIs s isl[k]Is expressed in matrix form by respectively making am,l＝[am,l[0] am,l[1] … am,l[2Nc-1]]T，pc,m＝[pc,m[0] pc,m[1] … pc,m[Lc-1]]T；
After derivation, the transmitting end sends a signal slThe final write is in the form:
6. the method of claim 1, wherein the step of blind frequency offset estimation is performed in a CP-FBMC communication system6) For each m0＝m，The main diagonal elements are 1+0j, and the other elements are pure imaginary numbers or infinite small real parts, then
For each m0＝m，Each element of (a) is affected by a time-domain neighbor element;
for each m0≠m，|m-m0When 1, thenCannot be ignored; i m-m0|>1, only adjacent subcarrier pairs are consideredThe influence of (c).
7. The blind frequency offset estimation method in CP-FBMC communication system as claimed in claim 1, wherein said step 7) is based on the estimated value of frequency offsetCompensating the frequency offset of the received signal, equalizing each block of data point by point at the receiving end, and demodulating to obtainWill be provided withPerforming OQAM post-processing, converting real symbols to complex symbols, performing parallel-to-serial conversion, performing QPSK demodulation, and recovering transmitted bit data stream。
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| US8064528B2 (en) * | 2003-05-21 | 2011-11-22 | Regents Of The University Of Minnesota | Estimating frequency-offsets and multi-antenna channels in MIMO OFDM systems |
| CN100493064C (en) * | 2006-11-09 | 2009-05-27 | 上海交通大学 | Method for estimating OFDM integer frequency shift based on virtual subcarrier and frequency domain differential sequence |
| CN100539569C (en) * | 2007-01-11 | 2009-09-09 | 西安交通大学 | A kind of blind frequency-offset estimating method of ofdm communication system carrier wave |
| CN101867548B (en) * | 2010-05-24 | 2014-01-15 | 北京科技大学 | Blind frequency tracking algorithm based on multi-carrier of filter bank |
| US8781008B2 (en) * | 2012-06-20 | 2014-07-15 | MagnaCom Ltd. | Highly-spectrally-efficient transmission using orthogonal frequency division multiplexing |
| CN105187345A (en) * | 2015-09-23 | 2015-12-23 | 浙江工业大学 | Non-iterative blind phase noise compensation method suitable for CO-OFDM system |
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2017
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