CN109802910B - Synchronous reference signal sending and frequency offset estimation method suitable for UFMC waveform - Google Patents
Synchronous reference signal sending and frequency offset estimation method suitable for UFMC waveform Download PDFInfo
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Abstract
The invention provides a synchronous reference signal sending and frequency offset estimation method suitable for UFMC waveforms, wherein a synchronous reference signal with a circulating structure is designed for sending, and integer frequency offset can be estimated at a receiving end by using a simple correlation method, so that the calculation complexity is greatly reduced, and the estimation performance is ensured to be unchanged. The invention is suitable for UFMC waveform technology which is possibly adopted in the next generation communication system, and is also suitable for other multi-carrier systems, such as filter bank multi-carrier (FBMC), generalized frequency division multiplexing (GDMC) and the like. The frequency offset estimation method of the present invention is also applicable to these new waveform techniques with filters. Furthermore, the present invention is equally applicable to conventional waveforms, such as OFDM, etc.
Description
Technical Field
The invention belongs to the technical field of mobile communication, and provides a synchronous reference signal sending and frequency offset estimation method suitable for universal filtering multi-carrier (UFMC) waveforms.
Background
In mobile communication systems, waveforms have been one of the main concerns in air interface technology. Currently commercially available fourth generation mobile communication systems (4G) employ a cyclic prefix based orthogonal frequency division multiple access (CP-OFDM) technique as their over-the-air transmission waveform. The CP-OFDM technology has the advantages of high transmission efficiency, simple realization and easy combination with Multiple Input Multiple Output (MIMO). However, since CP-OFDM employs rectangular window truncation in the time domain processing, there is a higher out-of-band leakage, which is more disadvantageous to support asynchronous transmission of adjacent sub-bands. The universal filtering multi-carrier (UFMC) technology inherits the advantages of CP-OFDM, greatly reduces out-of-band leakage through the filtering technology, and can effectively support asynchronous transmission of adjacent sub-bands. Meanwhile, the UFMC can dynamically select and configure on a unified physical layer platform according to different requirements of different services on waveform parameters, and can meet the requirement that a fifth generation mobile communication system (5G) supports different scene differentiation technical schemes on the basis of a unified technical framework.
In the UFMC system, adjacent Sub-carriers constitute Sub-bands (Sub-bands), one user may occupy one or more Sub-bands, and different users occupy different Sub-bands without interference between each other. However, due to the difference between the crystal oscillators at the transmitting end and the receiving end, a frequency offset (CFO) of the carrier occurs, thereby destroying orthogonality between sub-carriers within a sub-band and between different sub-bands. Frequency synchronization is a prerequisite for a wireless communication system to function properly. To estimate the frequency offset, the transmitting end needs to transmit a synchronization reference signal, and the receiving end uses the signal to estimate. In general, the frequency offset is normalized by the subcarrier bandwidth to include an integer part and a fractional part, and the integer part is difficult to estimate. Among the existing integer frequency offset estimation methods, the most preferable method is a least mean square (LS) method. However, the implementation complexity is too high to be applied to practical systems.
Disclosure of Invention
To solve the above problems, the present invention proposes a low complexity transmission and frequency offset estimation method suitable for UFMC waveforms. The method designs a synchronous reference signal with a cycle structure for sending, and can estimate the integer frequency offset at a receiving end by using a simple correlation method.
In order to achieve the purpose, the invention provides the following technical scheme:
the synchronous reference signal sending and frequency offset estimation method suitable for the UFMC waveform comprises the following steps:
Further, in step 3, the UFMC signals are represented by a matrix as follows:
wherein D ismIs an NxNmComplex matrix of dimensions, DmSmRepresenting a frequency-domain symbol vector S for the mth subbandmPerforming inverse Fourier transform of N points, and transforming the N points to a time domain; fmIs a complex toplitz matrix.
Further, if the normalized frequency offset also contains a fractional part, the method comprises the step 6: the first and second UFMC signals are correlated at the receiver, and the averaged phase information is extracted to estimate the fractional part of the frequency offset.
Further, in step 4, the second UFMC signal is represented as follows:
R＝[r(0),r(1),...,r(N+LF-2)]T
complement N-L at the tail of the signalFThe column vector obtained after +1 zeros is as follows:
wherein L isFIs the filter length.
Further, the received signal in step 5 is assumed as follows:
Y＝[y(0),y(1),...,y(LC-1)]
the correlation operation adopts the following formula:
wherein ((n-k)) means a cyclic shift operation, i.e., ((n-k)) ((L) when n-k < 0)C+ n-k), and when n-k is not less than LCThen, ((n-k)) ═ n-k-LC)。
Compared with the prior art, the invention has the following advantages and beneficial effects:
1. the invention designs a synchronous reference signal with a circulating structure, adopts a simple correlation method when estimating the integral frequency offset, greatly reduces the calculation complexity while ensuring the estimation performance, and is quick and reliable.
2. The invention is suitable for UFMC waveform technology which is possibly adopted in the next generation communication system, and is also suitable for other multi-carrier systems, such as filter bank multi-carrier (FBMC), generalized frequency division multiplexing (GDMC) and the like. The frequency offset estimation method of the present invention is also applicable to these new waveform techniques with filters. Furthermore, the present invention is equally applicable to conventional waveforms, such as OFDM, etc.
Drawings
Fig. 1 is a schematic diagram of the UFMC system transmission principle.
Fig. 2 is a schematic diagram of a proposed structure of a synchronization reference signal.
Figure 3 is a schematic diagram of a UFMC time-domain signal.
Fig. 4 is a simulation result of the present invention at a normalized frequency offset of-3.
Fig. 5 is a simulation result of the present invention at a normalized frequency offset of 2.
Detailed Description
The technical solutions provided by the present invention will be described in detail below with reference to specific examples, and it should be understood that the following specific embodiments are only illustrative of the present invention and are not intended to limit the scope of the present invention.
The signal structure and existing estimation methods of UFMC systems are as follows:
step one, a system configures a sub-band.
Suppose a UFMC system includes N subcarriers in the frequency domain, where the bandwidth of each subcarrier is Δ f and the total bandwidth is B — N Δ f. These sub-carriers are configured according to the systemThe division into M Sub-bands (Sub-bands) is performed by NmAnd each adjacent subcarrier. With U ═ 0,1]Representing a set of sub-carrier position components, then
Denotes the sub-carrier position contained in the m-th sub-band, and N0+N1+…NM-1N. The M subbands may be assigned to the same user or to different users, depending on system settings. In the following, it is assumed that the same user is allocated, the number of subcarriers included in each sub-band is the same, and so on for different users and different numbers of subcarriers.
Assume that after signal modulation, the symbol vector generated by the user is S ═ S (0), S (1), …, S (N-1)]TWhere s (i) denotes a symbol modulated by a method such as Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM), and i is 0, 1. The sending end divides S into M non-overlapping sub-symbol vectors which are respectively configured on M sub-bands for transmission. At this time, the sub-symbol vector transmitted on the m-th sub-band is
Let the filter of the mth subband be fm＝[fm(0),fm(1),...,fm(LF,m-1)]Wherein L isF,mThe filter length of the mth subband is indicated. In general, the length and tap coefficients of different subband filters may be different. However, for the sake of analysis, we assume here that the same filter is chosen and all lengths are LF。
Step two, the sending end carries out Fourier inverse transformation on the sub-band to transform the sub-band into a time domain, and forms a UFMC signal
Define one (N + L)F-1) x N-dimensional complex Toeplitz (Toeplitz) matrix FmThe first column of which is
First behavior [ fm(0),01×(N-1)]T. As shown in fig. 1, the UFMC signaling may be in the form of a matrixExpressed as:
wherein D ismIs an NxNmA complex matrix of dimensions. Suppose D is an N x N dimensional inverse energy normalized Fourier transform (IDFT) matrix with the i row and N column elements of
Then D ismFrom D
Is listed to
Column element composition. DmSmRepresenting a frequency-domain symbol vector S for the mth subbandmAn inverse fourier transform (IDFT) of the N points is performed to transform it to the time domain. The resulting time domain signal is then passed through a complex Toeplitz matrix FmThe multiplication completes the filtering process. And finally, adding all the M filtered time domain sub-band signals to obtain the UFMC baseband signal. After filtering by the sub-band filter, the length of the UFMC baseband signal at this moment is changed from N samples to Nt＝N+L F1 sample.
In order to estimate the frequency offset, the transmitting end needs to transmit a synchronization reference signal known to the receiving end. In the UFMC system, the transmitting end usually continuously transmits two identical synchronization reference signals.
Step three, the receiving end carries out estimation on the signal
After passing through a Multipath fading (Multipath) channel, the second received synchronization reference signal can be expressed as:
wherein W represents NtAn additive white gaussian noise vector of x 1 dimension; f. ofdRepresenting the frequency offset after the bandwidth normalization of the subcarriers. E is a diagonal matrix consisting of frequency offsets, i.e.:
since the two transmitted synchronization reference signals are the same, H is a cyclic shift matrix whose diagonal element is H (0) and the element in row 1 is [ H (0)0.. H (L)h-1)...h(1)]. Wherein h (L) represents the channel parameter of the L root path, and the channel has L in commonh1 path with maximum delay spread of Lh-1. By derivation, [ formula two ]]It can also be expressed as:
wherein h ═ h (0) h (1)h-1)]T；
A matrix representing the composition of the transmitted signals can be written as:
typically, the normalized frequency offset contains an integer part and a fractional part, whereas the integer part is more difficult to estimate. In the existing frequency synchronization method, the optimal integer estimation method is to utilize
To obtain a least mean square (LS) estimate, i.e.:
wherein the content of the first and second substances,
is to
Estimation of (i.e. arbitrarily chosen (-N/2, N/2)]An integer k within the interval, yielding:
when a certain k is taken to enable formula six]When the result is maximum, the k value is the integral frequency deviation fdIs estimated value of
However, the biggest problem with this approach is that the computational complexity is too high. According to [ formula five ]]，
Is a number NtLine LhA column matrix. Then it is determined that,
operation requires NtLhLhA second complex multiplication operation and NtLhLhA complex addition operation. Assuming that the UFMC system has 1024 sub-carriers, each sub-band adopts a Dolph-Chebyshev Finite Impulse Response (FIR) filter with 74 tap coefficients, and the channel length Lh＝37，
A total of 3003586 complex multiply and add operations are required. In addition, the computational complexity of matrix inversion is typically o (n)3) Magnitude, where n represents the dimension of the matrix. Then, calculate
Is o (50653), plus other matricesThe operation of multiplication is hardly realized at the receiving end.
In order to greatly reduce the computational complexity during frequency synchronization, the invention provides a synchronous reference signal transmission and frequency offset estimation method, wherein a synchronous reference signal with a cyclic structure is designed, and the transmission structure is shown in fig. 2.
Specifically, step 1 in the method is added with the following processes on the basis of the step one: the system depends on the channel coherence bandwidth BCSetting the length L of a constant-envelope zero-autocorrelation (CAZAC) sequenceCLength of sequence LCLess than the associated bandwidth B of the channelCI.e. Δ f.LC＜BC。
The synchronization reference signal is composed of 3 identical sequences, each of which is an identical constant amplitude zero auto-correlation (CAZAC) sequence having a length LCAccording to the system configuration in step 1, the sub-bands are placed in adjacent sub-bands, i.e. at sub-carrier positions of Um，Um+1And Um+2And so on.
The UFMC system is assumed to have 1024 subcarriers, that is, N is 1024, and 12 adjacent subcarriers constitute one subband. Each sub-band uses the same Dolph-Chebyshev filter with 74 tap coefficients, i.e., LF74. After passing through the filter, energy rising and falling regions of about 37 sampling points, called ramp/down, appear on both sides of the UMFC baseband signal, providing protection for the UFMC signal similar to the Cyclic Prefix (CP) in the OFDM system, as shown in fig. 3. Similar to CP-OFDM, the maximum delay spread of the channel is typically less than or equal to the length of the ramp region, or else will cause stronger inter-symbol interference. Assuming a channel length LhThe coherence bandwidth of the channel can be calculated to be approximately 27 subcarriers, i.e. to indicate that the frequency domain response (CFR) of the channel is statistically nearly constant within these 27 subcarriers. Assume a sequence length of L C12, is smaller than the coherence bandwidth and occupies exactly 1 subband. Then, the synchronization reference signal co-occupiesWith 3 adjacent subbands, assumed to be 5 th, 6 th and 7 th subbands. The sequence is a Chu sequence and is generated as follows:
at this time, the 48 th to 59 th subcarriers, the 60 th to 71 th subcarriers, and the 72 th to 83 th subcarriers of the UFMC are placed [ C (0), C (1),.., C (11) ].
The following steps are the processes of processing the signal and estimating the frequency offset by the receiving end. Because the transmitted synchronous reference signal has a cyclic structure, the integral multiple frequency offset can be estimated at a receiving end by using a simple correlation method, the calculation complexity is greatly reduced, and meanwhile, the estimation performance is ensured to be unchanged. The method specifically comprises the following steps:
and 4, taking out the second UFMC signal by the receiving end, performing Fourier transform (DFT) of 2N points after zero padding to obtain a transform result with the length of 2N. After taking out N numerical values at even number positions of the transformation result, the receiving signal of the frequency domain can be obtained. The method specifically comprises the following steps:
according to [ equation two ], the received signal can be expressed as:
R＝[r(0),r(1),...,r(N+LF-2)]T[ formula nine)]
The tail part of the tail part is supplemented with N-LF+1 zeros results in a 2N × 1 dimensional column vector:
to pair
DFT of 2N point is made to transform to frequency domain, and the DFT is taken outAfter N numerical values on even number positions of the result are transformed, the UFMC receiving signal of the frequency domain can be obtained.
the receiving end needs to take out the received signal from the subcarrier position placed by the second group sequence of the synchronous reference signal, and the assumption is that:
Y＝[y(0),y(1),...,y(LC-1)][ formula eleven ]]
The receiving end generates the same sequence C (n) and performs correlation operation with the received signal Y to obtain:
wherein ((n-k)) means a cyclic shift operation, i.e., ((n-k)) ((L) when n-k < 0)C+ n-k), and when n-k is not less than LCThen, ((n-k)) ═ n-k-LC)；C*(. cndot.) represents the complex conjugate of C (. cndot.). The phase rotation of the time domain can cause the displacement of the frequency domain signal, and the integral multiple frequency offset of the time domain can be estimated by detecting the displacement of the frequency domain reference sequence at the receiving end. L isCOf the correlation results, the displacement corresponding to the result with the largest modulus value gives the integer part of the frequency offset. The invention can generate the effect of cyclic shift by constructing three groups of same sequences, so that the second group of sequences in the middle can generate the effect of cyclic shift no matter the second group of sequences moves to the right or to the left. And finally, the zero autocorrelation characteristic of the CAZAC sequence is utilized, the number of the displaced sub-carriers can be conveniently estimated, and the complexity of the algorithm is greatly reduced.
Fig. 4 and 5 show two simulation results of the present invention, respectively. The simulation conditions were as follows: the UFMC system has 1024 subcarriers, that is, N is 1024, and 12 adjacent subcarriers constitute one subband. The users occupy the 5 th, 6 th and 7 th sub-bands and adopt the formula eight]The resulting sequence. Each sub-band using the same DAn olph-Chebyshev Finite Impulse Response (FIR) filter with 74 tap coefficients and a sidelobe attenuation of 40 dB. The multipath fading channel has 16 paths, i.e. L is 16, the time delay of each path is 0,2,4
Where l is 1,2, …, 16. In the simulation, we assume that the average power of the signal is 1 and do not consider the effect of noise. In fig. 4 and 5, the normalized frequency offsets are assumed to be-3 and 2 subcarriers, respectively. As can be seen from fig. 4 and 5, the method provided by the present invention accurately estimates the frequency offset through correlation operation, and greatly reduces the implementation complexity. In addition, since the channel is only statistically invariant within the correlation bandwidth, the correlation values at other shifts are not zero in fig. 4 and 5. But since its value is small, it does not affect the estimation result.
It should be noted that if the normalized frequency offset also contains a fractional part, it can be estimated by using a conventional time-domain correlation method. I.e. comprising step 6, correlating the first and second UFMC signals at the receiving end, extracting the averaged phase information to estimate the fractional part of the frequency offset.
Since the two transmitted synchronization reference signals are identical, the two signals can be correlated, and the averaged phase information is extracted to estimate the fractional part of the frequency offset, i.e., the fractional part of the frequency offset
Wherein r is0(k) And r1(k) A kth time-domain sample representing the first and second UFMC synchronization reference signals, respectively; arg [. to]Representing a phase angle taking operation. The estimation of the fractional part is independent from the estimation of the integer part, and can be performed before or after the integer estimation.
The technical means disclosed in the invention scheme are not limited to the technical means disclosed in the above embodiments, but also include the technical scheme formed by any combination of the above technical features. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principle of the present invention, and such improvements and modifications are also considered to be within the scope of the present invention.
Claims (5)
1. A synchronous reference signal sending and frequency offset estimation method suitable for UFMC waveform is characterized by comprising the following steps:
step 1, the system is according to the channel coherence bandwidth BCSetting the length L of the CAZAC sequenceCLet Δ f. LC＜BCAnd configuring a sub-band for placing the synchronous reference signal, wherein deltaf represents the bandwidth of the sub-carrier;
step 2, the sending end generates the length LCThe three same CAZAC sequences are used for forming a synchronous reference signal and are adjacently placed on a specified subband;
step 3, the sending terminal performs N-point Fourier inverse transformation on the sub-band to transform the sub-band into a time domain, and forms a UFMC signal through a filter; the sending end continuously sends the UFMC signal twice; wherein, N represents the total subcarrier number of the UFMC system;
step 4, the receiving end takes out the second UFMC signal, and after zero padding, Fourier transform of 2N points is carried out to obtain a transform result with the length of 2N; after N numerical values at even positions of the transformation result are taken out, a receiving signal of a frequency domain can be obtained;
step 5, taking out the received signal at the sub-band position of the modulation synchronous reference signal, using the locally generated reference signal to circularly shift and then to correlate the received signal to obtain LC(ii) a correlation result; wherein the displacement corresponding to the result of the maximum modulus value gives the integer part of the frequency offset.
2. The method of claim 1, wherein the UFMC signals in step 3 are represented by a matrix as follows:
wherein D ismIs an NxNmComplex matrix of dimensions, DmSmRepresenting a frequency-domain symbol vector S for the mth subbandmPerforming inverse Fourier transform of N points, and transforming the N points to a time domain; fmIs a complex toplitz matrix.
3. The method of claim 1, wherein if the normalized frequency offset further comprises a fractional part, the method comprises the steps of 6: the first and second UFMC signals are correlated at the receiver, and the averaged phase information is extracted to estimate the fractional part of the frequency offset.
4. The method of claim 1, wherein in step 4, the second UFMC signal is represented as follows:
R＝[r(0),r(1),...,r(N+LF-2)]T
complement N-L at the tail of the signalFThe column vector obtained after +1 zeros is as follows:
wherein L isFIs the filter length.
5. The method of claim 1, wherein the received signal in step 5 is assumed as follows:
Y＝[y(0),y(1),...,y(LC-1)]
the correlation operation adopts the following formula:
wherein ((n-k)) means a cyclic shift operation, i.e., ((n-k)) ((L) when n-k < 0)C+ n-k), and when n-k is not less than LCThen, ((n-k)) ═ n-k-LC)。
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