CN115277334A - MRC iterative equalization method of OTSM system under high-speed mobile environment - Google Patents

MRC iterative equalization method of OTSM system under high-speed mobile environment Download PDF

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CN115277334A
CN115277334A CN202210859911.4A CN202210859911A CN115277334A CN 115277334 A CN115277334 A CN 115277334A CN 202210859911 A CN202210859911 A CN 202210859911A CN 115277334 A CN115277334 A CN 115277334A
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李国军
龙锟
叶昌荣
谢文希
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Chongqing University of Post and Telecommunications
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
    • H04L27/2649—Demodulators
    • H04L27/26532—Demodulators using other transforms, e.g. discrete cosine transforms, Orthogonal Time Frequency and Space [OTFS] or hermetic transforms
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/01—Equalisers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/26—Systems using multi-frequency codes
    • H04L27/2601—Multicarrier modulation systems
    • H04L27/2647—Arrangements specific to the receiver only
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Abstract

The invention belongs to the field of communication, and particularly relates to an MRC iterative equalization method of an OTSM system under a high-speed mobile environment, which comprises the steps of constructing an OTSM system model, wherein the OTSM system model comprises a sending end, a time domain channel and a receiving end; processing the data output by the time domain channel by adopting a single-tap time frequency equalizer to obtain a time delay-time domain information symbol estimated value; taking the time delay-time domain information symbol estimated value as an initial value of an MRC iterative equalization algorithm, carrying out iterative detection judgment through the MRC iterative equalization algorithm, and outputting a time delay-sequence domain information symbol estimated value; the MRC iterative equalizer provided by the invention can obtain good performance in high Doppler frequency shift.

Description

MRC iterative equalization method of OTSM system under high-speed mobile environment
Technical Field
The invention belongs to the field of communication, and particularly relates to an MRC iterative equalization method of an OTSM system in a high-speed mobile environment.
Background
Future wireless communication systems require reliable data transmission in high-mobility environments such as high-speed railways, unmanned planes, autopilots and the like. Conventional Orthogonal Frequency Division Multiplexing (OFDM) modulation can achieve high spectral efficiency and multipath interference resistance when facing frequency selective channels. However, in a high-speed mobile environment, OFDM has significantly degraded performance due to inter-carrier interference (ICI). In order to improve wireless transmission performance in a high-speed mobile environment, hadani et al propose Orthogonal Time Frequency and Space (OTFS) modulation. OTFS can obtain more complete channel diversity than OFDM by multiplexing information symbols in the delay-doppler domain, resulting in superior transmission performance.
Although OTFS can produce excellent transmission performance in a high-speed mobile environment, two-dimensional precoding in the time-frequency domain greatly increases modulation complexity. Recently, thaj et al proposed an Orthogonal Time Series Multiplexing (OTSM) modulation technique with lower modulation complexity. OTSM multiplexes transmission information symbols in the delay-sequence domain, allowing the delay spread and doppler spread of the channel to introduce inter-symbol interference (ISI) along the delay and sequence dimensions, respectively, and separating at the receiver, thereby converting the fast time-varying channel in the time-frequency domain into an approximately constant non-fading channel in the delay-sequence domain.
The WHT of OTSM along the sequence domain involves only addition and subtraction operations compared to IFFT of OTFS along the doppler domain, and therefore OTSM has lower modulation complexity. Meanwhile, the OTSM provides performance similar to that of the OTFS, and provides a low-complexity modulation scheme for realizing reliable communication of a high-mobility wireless channel. However, the existing OTSM (optical time series modulation) equalization method has the problems of poor noise immunity and high calculation complexity.
Disclosure of Invention
Aiming at the problems of poor noise resistance and high computational complexity of the existing Orthogonal Time Sequence Multiplexing (OTSM) equalization method, the invention provides a Maximum Ratio Combining (MRC) iterative equalization method of an OTSM system under a high-speed mobile environment.
The MRC iterative equalization method of the OTSM system under the high-speed mobile environment comprises the following steps:
s1, constructing an OTSM system model, wherein the OTSM system model comprises a sending end, a time domain channel and a receiving end;
s2, processing data output by a time domain channel by adopting a single-tap time frequency equalizer to obtain a time delay-time domain information symbol pre-estimated value;
and S3, taking the time delay-time domain information symbol estimated value obtained in the step S2 as an initial value of an MRC iterative equalization algorithm, and performing iterative detection judgment through the MRC iterative equalization algorithm to obtain a time delay-sequence domain information symbol estimated value from a sending end to a receiving end through a time domain channel.
Further, a single-tap time-frequency equalizer is adopted to obtain an estimated value of the time delay-time domain information symbol, and the specific process is as follows:
dividing a time domain vector output by a time domain channel into N time domain blocks, and respectively performing M-point FFT on the time domain blocks to obtain N frequency domain blocks, wherein the N frequency domain blocks are expressed as follows:
Figure BDA0003757874860000021
MMSE equalization is carried out on each frequency domain block to obtain information symbols of the frequency domain blocks, and an MMSE equalization formula is represented as follows:
Figure BDA0003757874860000022
performing M-point IFFT on the information symbols of each frequency domain block to obtain a time delay-time domain pre-estimation matrix vector consisting of time delay-time domain information symbol pre-estimation values, wherein the time delay-time domain pre-estimation matrix vector is expressed as
Figure BDA0003757874860000023
Wherein M = 0., M-1, N = 0., N-1,
Figure BDA0003757874860000024
representing the nth frequency-domain block vector, FMRepresenting an M-point FFT transform, rnRepresents the nth time-domain block vector and,
Figure BDA0003757874860000025
an mth information symbol representing an equalized nth frequency-domain block vector,
Figure BDA0003757874860000031
an mth information symbol representing an nth frequency-domain block vector,
Figure BDA0003757874860000032
representing the frequency domain channel coefficients, representing the conjugate transpose,
Figure BDA0003757874860000033
which represents the variance of the noise, is,
Figure BDA0003757874860000034
representing a time delay-time domain matrix vector,
Figure BDA0003757874860000035
representing an M-point IFFT transformation.
Further, the process of the MRC iterative equalization algorithm includes:
s11, obtaining a time delay-time domain information symbol pre-estimated value output by the single-tap time frequency equalizer as an initial value of an MRC iterative equalization algorithm, and setting the maximum iteration times;
s12, calculating the damaged signal component through the time delay-time domain information symbol estimated value, wherein the calculation is represented as:
Figure BDA0003757874860000036
s13, judging and estimating maximum ratio combination according to a maximum likelihood criterion, outputting an updated time delay-time domain information symbol estimated value, judging whether the maximum iteration times is reached, if so, executing a step S14, otherwise, returning to the step S12 and substituting the current time delay-time domain information symbol estimated value after iteration updating, wherein the estimated value is represented as:
Figure BDA0003757874860000037
s14, taking the current output as a time delay-time domain information symbol estimation value, and converting the time delay-time domain information symbol estimation value into a time delay-sequence domain information symbol estimation value through WHT;
wherein ,lmaxAn index is extended for a maximum discrete channel delay, and
Figure BDA0003757874860000038
Figure BDA0003757874860000039
represents the mth delay-time domain symbol vector received by the receiving end,
Figure BDA00037578748600000310
representing a time delay-time domain matrix
Figure BDA00037578748600000311
Row m + l and column l' of (1),
Figure BDA00037578748600000312
representing the m + l-l' th delay-time domain symbol vector estimated value,
Figure BDA00037578748600000313
representing the corrupted signal component at index m + l,
Figure BDA00037578748600000314
the decision value of the transmission symbol of the mth row and nth column of the sending end after each iteration is represented, namely the updated time delay-time domain information symbol estimated value, ajRepresenting the corresponding value of the constellation table, cmRepresents a time-delay-time-domain output vector of a maximal-ratio combiner, and
Figure BDA00037578748600000315
further, the sending end places NM information symbols in a time delay-sequence domain grid to obtain a first time delay-sequence domain matrix
Figure BDA00037578748600000316
Wherein M is the row number of the time delay-sequence domain grid, and N is the column number of the time delay-sequence domain grid and is the nth power of 2; walsh-Hadamard transform is carried out on the first time delay-sequence domain matrix line by line to obtain a first time delay-time domain matrix
Figure BDA0003757874860000041
The first time delay-time domain matrix obtains a first time domain vector matrix through serial-parallel conversion
Figure BDA0003757874860000042
The receiving end receives the second time domain vector matrix
Figure BDA0003757874860000043
Performing parallel-to-serial conversion to obtain a second time delay-time domain matrix
Figure BDA0003757874860000044
Walsh-Hadamard transform is carried out on the second time delay-time domain matrix line by line to obtain a second time delay-sequence domain matrix
Figure BDA0003757874860000045
Further, the input-output relationship between the time delay and the time domain in the time domain channel is represented as:
Figure BDA0003757874860000046
zero padding which can serve as an interleaving guard band in a time domain is placed in a time delay-sequence domain grid, so that the time delay-time domain input-output relation in a time domain channel can be independently processed, and an equivalent input-output relation is obtained, and is expressed as:
Figure BDA0003757874860000047
wherein ,
Figure BDA0003757874860000048
representing the mth received delay-time domain symbol vector received by the receiving end
Figure BDA0003757874860000049
Representing a time delay-time domain matrix
Figure BDA00037578748600000410
The m + l th row/column of (c),
Figure BDA00037578748600000411
representing a vector x of transmission symbolsmThe delay-time domain symbol vector after WHT,
Figure BDA00037578748600000412
representing time-delay-time-domain gaussian noise,/maxFor maximum discrete channel delay spread index and
Figure BDA00037578748600000413
further, after obtaining the time delay-time domain information symbol, performing turbo iteration, where the one-time turbo iteration process includes:
performing OTSM demodulation and QAM soft demodulation on the output time delay-time domain information symbol;
de-interleaving the data after QAM soft demodulation and transmitting the data to an LDPC decoder;
the LDPC decoder processes the transmitted de-interleaving data output bit information and interleaves the bit information;
and carrying out QAM modulation and OTSM modulation on the interleaved data to obtain an improved time delay-time domain information symbol.
The invention has the beneficial effects that:
the invention provides a low-complexity iterative rake decision feedback equalizer based on MRC (maximum likelihood ratio) aiming at the problem of high computation complexity of a traditional equalizer in a high-speed moving scene. The equalizer utilizes MRC to extract and coherently combine the received multipath components of the transmitted symbols in an equivalent delay-time domain grid to improve the signal-to-noise ratio after signal combination. Simulation results show that the MRC iterative equalizer proposed herein can achieve good performance in high Doppler shift. Meanwhile, compared with the GS iterative equalizer widely used at present, the performance and the computational complexity are greatly improved, and a low-complexity equalization scheme is provided for a future high-mobility communication system.
In order to accelerate the convergence of the MRC iterative algorithm, a single-tap time-frequency equalizer is designed to provide initial estimation for the MRC iterative algorithm, and then the error code performance is further improved by combining an external error correcting code.
Drawings
FIG. 1 is a diagram of an OTSM system model according to an embodiment of the present invention;
FIG. 2 is a diagram illustrating a process of generating time domain vectors by using a time delay-time domain matrix according to an embodiment of the present invention;
FIG. 3 is a block diagram of the steps of the MRC iterative equalization algorithm according to the embodiment of the present invention;
FIG. 4 is a flow chart of the operation of an OTSM system based MRC-turbo receiver according to an embodiment of the present invention;
fig. 5 is a simulation diagram of error performance of different algorithms at 270km/h (fd =1000 Hz) for speed according to an embodiment of the present invention;
FIG. 6 is a simulation diagram of error performance of different algorithms at a speed of 540km/h (fd =2000 Hz) according to an embodiment of the present invention;
fig. 7 is a simulation diagram of the error performance of the MRC iterative equalizer at different N times according to the embodiment of the present invention;
fig. 8 is a simulation diagram of the error performance of the MRC iterative equalizer with different fd;
fig. 9 is a simulation diagram of the error performance of the MRC-turbo receiver under different coding lengths according to the embodiment of the present invention.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the drawings in the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.
The invention can be used as Zero Padding (ZP) of an interleaving protection band in a time domain by placing the Zero Padding (ZP) in a time delay-sequence domain of an OTSM system, thereby obtaining a simplified channel input-output relationship; the simplified channel input-output relation is utilized to provide an MRC iterative equalization method of the OTSM system in a high-speed mobile environment. The MRC iterative equalization method has the main idea that multipath components of a transmitting symbol are extracted iteratively and combined coherently in a time delay-time domain by utilizing a maximum ratio combining algorithm so as to improve the signal-to-noise ratio of a combined signal.
Example 1
In the present embodiment, an OTSM system transmission model is provided. For convenience, the OTSM system transmission model will be represented herein in matrix form.
The OTSM system transmission model is shown in fig. 1, and includes three parts, namely a transmitting end, a time domain channel and a receiving end. In the transmitting side part, the transmitting side firstly sets NM information symbols x = [ x = [ x ]1,...,xNM]Placing the time delay-sequence domain grid in a time delay-sequence domain grid, wherein M is the row number of the time delay-sequence domain grid, and N is the column number of the time delay-sequence domain grid and is the nth power of 2; thereby obtaining a first time delay-sequence domain matrix
Figure BDA0003757874860000061
Performing Walsh-hadamard transform (WHT) on the first time delay-sequence domain matrix X line by line to obtain a first time delay-time domain matrix
Figure BDA0003757874860000062
First time delay-time domain matrix
Figure BDA0003757874860000063
Obtaining a first time domain vector matrix through serial-to-parallel conversion
Figure BDA0003757874860000064
wherein ：
Figure BDA0003757874860000065
Figure BDA0003757874860000066
in the above two formulae, WNRepresenting progressive walesThe sh-hadamard transform, vec () represents a serial-to-parallel transform.
The above-described transmit-end operation can be expressed in a simple matrix form as:
Figure BDA0003757874860000067
p denotes a row-column interleaver matrix, IMAn identity matrix of length M rows and M columns is shown,
Figure BDA0003757874860000069
representing the kronecker product.
In the time domain channel portion, since the number of propagation paths P in the delay-doppler domain channel is usually limited, the delay-doppler domain channel response can be expressed as:
Figure BDA0003757874860000068
wherein hi、τi and viPath gain, delay and doppler shift of the ith path, respectively, let li and kiNormalized integer delay offset and doppler offset of the ith path, respectively, the actual delay and doppler offset of the ith path can be expressed as
Figure BDA0003757874860000071
NT and M Δ f denote the frame duration and bandwidth, respectively, of the OTSM signal frame, assuming lmaxFor maximum discrete channel delay spread index, the delay-sequence domain matrix X last lmaxThe row symbol vectors are zeroed out to avoid inter-block interference due to channel delay spread. The continuous time-varying channel impulse response can be obtained by equation (4):
g(τ,t)＝∫h(τ,v)ej2πv(t-τ)dv (6)
thus the time domain input output relationship can be written as
Figure BDA0003757874860000072
At the receiving end, a discrete time domain signal is obtained by sampling the received waveform r (t) at sampling intervals t = qM Δ f
Figure BDA0003757874860000073
Wherein q is more than or equal to 0 and less than or equal to NM-1,
Figure BDA0003757874860000074
a set of discrete delay taps representing the delay offset at integer multiples of the sampling period 1M Δ f. Receiver sampling discretization of time-delayed time-domain channel g (tau, t)
Figure BDA0003757874860000075
Therefore, the time-domain input-output relationship is represented in a matrix form:
Figure BDA0003757874860000076
wherein ,
Figure BDA0003757874860000077
is a time-domain discrete baseband channel matrix,
Figure BDA0003757874860000078
representing a white gaussian noise in the time domain,
Figure BDA0003757874860000079
representing a second time domain vector matrix.
At the receiving end part, the receiving end receives the second time domain vector matrix r, and performs parallel-to-serial conversion on the r to obtain a second time delay-time domain matrix
Figure BDA00037578748600000710
Second time delay-time domain matrix
Figure BDA00037578748600000711
Obtaining a received second time delay-sequence domain matrix through WHT
Figure BDA00037578748600000712
wherein ：
Figure BDA00037578748600000713
Figure BDA0003757874860000081
Figure BDA0003757874860000082
representing parallel-to-serial conversion, the above-mentioned receiving-end operation can be represented in a simple matrix form as:
Figure BDA0003757874860000083
wherein ,PTRepresenting the transpose of the row and column interleaver matrix.
In particular, a first time delay-time domain matrix
Figure BDA0003757874860000084
The specific process of the column-by-column vectorization is shown in fig. 2, so the input-output relationship between the time delay and the time domain can be further expressed as:
Figure BDA0003757874860000085
wherein ,
Figure BDA0003757874860000086
representing a time delayed-time domain received symbol,
Figure BDA0003757874860000087
Figure BDA0003757874860000088
representing a time delay-time domain channel matrix,
Figure BDA0003757874860000089
Figure BDA00037578748600000810
representing a time-delayed time-domain transmitted symbol,
Figure BDA00037578748600000811
Figure BDA00037578748600000812
representing the time delay-time domain gaussian white noise,
Figure BDA00037578748600000813
example 2
This embodiment further analyzes the delay-time domain input/output relationship on the basis of embodiment 1, and places Zero Padding (ZP) capable of acting as an interleaving guard band in the time domain in the delay-sequence domain of the OTSM system, where the ZP prevents interference between time domain blocks, so that the time domain input/output relationship in equation (10) can be handled independently, and is expressed as:
Figure BDA00037578748600000814
therefore, the equivalent input-output relationship of the time delay-time domain can be expressed as:
Figure BDA00037578748600000815
Figure BDA00037578748600000816
Figure BDA00037578748600000817
rnn-th time domain block vector, s, representing the receiving endnN-th time-domain block vector, G, representing the transmitting endnAn nth time domain block vector representing a time domain channel,
Figure BDA00037578748600000818
represents the m + l time delay-time domain symbol vector received by the receiving end,
Figure BDA00037578748600000819
representing a time delay-time domain matrix
Figure BDA00037578748600000820
The m + l th row/column of (c),
Figure BDA00037578748600000821
representing a vector x of transmission symbolsmThe delay-time domain symbol vector after WHT,
Figure BDA00037578748600000822
representing the time-delay-time-domain noise component,/maxFor maximum discrete channel delay spread index and
Figure BDA0003757874860000091
example 3
In this embodiment, on the basis of embodiment 2, a low-complexity iterative rake decision feedback equalizer based on MRC is proposed, which is referred to as MRC iterative equalizer in the present invention for short. As shown in fig. 3, the proposed MRC iterative equalizer can be viewed as being in a time delay-time domain grid
Figure BDA0003757874860000092
Impaired reception at different delay branchesMRC of the signal. The branch selected for combining in each MRC iterative equalizer cancels the estimated ISI, thereby iteratively improving the combined signal-to-noise ratio.
Order to
Figure BDA0003757874860000093
To index the corrupted signal component at m + l,
Figure BDA0003757874860000094
for a pre-estimated value of the output of the single-tap frequency domain equalizer, then
Figure BDA0003757874860000095
Can be expressed as
Figure BDA0003757874860000096
In the present embodiment, the formula (20) is used for estimation
Figure BDA0003757874860000097
Instead of estimating the transmitted symbols separately from equation (19), the maximum ratio combining (21) is followed by symbol-by-symbol QAM demapping:
Figure BDA0003757874860000098
wherein
Figure BDA0003757874860000099
Is provided with
Figure BDA00037578748600000910
Representing the estimated symbol c for each iterationmIs determined by
Figure BDA00037578748600000911
Hard decision function
Figure BDA00037578748600000912
Once the estimated delay-time domain information symbol is updated, given by the Maximum Likelihood (ML) criterion in equation (20)
Figure BDA00037578748600000913
We will increase M and repeat the same operation, then estimate all M' = M-l in the form of decision feedbackmaxTime delay-time domain information symbol estimates.
Example 4
In this embodiment, on the basis of embodiment 3, a single-tap time-frequency equalizer is designed to obtain an initial value of the MRC iterative equalizer, so that the number of iterations of the MRC iterative equalization algorithm is reduced and convergence is achieved quickly.
In static wireless channels it can be assumed that the channel matrix of each time domain block is a cyclic matrix and can be diagonalized in the frequency domain, but in high mobility channels doppler spreading introduces interference between the frequency domain signals of each block (the time domain channel matrix is no longer cyclic due to the time varying channel). However, since the duration of each time domain block is much smaller than the duration of the entire OTSM frame, it can be assumed that the channel is time-invariant within each block and different from block to block. Therefore, the single-tap frequency domain equalizer can be used separately in each block for detection, which specifically includes:
receiving the time domain block and performing M-point FFT to obtain a plurality of frequency domain blocks, which are represented as:
Figure BDA0003757874860000101
MMSE equalization is carried out on each frequency domain block to obtain information symbols of the frequency domain blocks, and an MMSE equalization formula is represented as follows:
Figure BDA0003757874860000102
wherein M = 0., M-1, N = 0., N-1,
Figure BDA0003757874860000103
for noise variance, the frequency domain channel coefficients are:
Figure BDA0003757874860000104
performing M-point IFFT on the information symbols of each frequency domain block to obtain a time delay-time domain pre-estimate composed of time delay-time domain information symbol pre-estimates, which is expressed as
Figure BDA0003757874860000105
Wherein M = 0., M-1, N = 0., N-1,
Figure BDA0003757874860000106
represents the nth frequency-domain block vector, FMRepresenting an M-point FFT transform, rnRepresents the nth time-domain block vector and,
Figure BDA0003757874860000107
an mth information symbol representing an equalized nth frequency-domain block vector,
Figure BDA0003757874860000108
an mth information symbol representing an nth frequency-domain block vector,
Figure BDA0003757874860000109
representing frequency domain channel coefficients, representing a conjugate transpose,
Figure BDA00037578748600001010
which represents the variance of the noise, is,
Figure BDA00037578748600001011
representing a delay-time domain prediction matrix vector,
Figure BDA00037578748600001012
representing an M-point IFFT transformation.
And the time delay-time domain information symbol estimated value is judged and then used as the initial estimation of the MRC iterative detection equalizer to carry out iterative detection.
Example 5
In this embodiment, on the basis of embodiment 3, in order to further implement potential full channel diversity and reduce the system error rate, a turbo technique is applied to the MRC iterative equalizer, and an MRC-turbo receiver based on the OTSM system is proposed.
At the transmitter, the information bits are randomly interleaved before QAM modulation, then OTSM modulated and transmitted into the channel. At the receiver, the time-domain information symbols are obtained using the low-complexity MRC iterative equalizer proposed in embodiment 3, followed by turbo iteration. Wherein each turbo iteration process at least comprises one MRC iteration equalizer and one LDPC decoder, and the turbo iteration number can be set according to the required bit error rate and complexity requirement.
As shown in fig. 4, the MRC-turbo receiver operates on the principle that the time-domain information symbols output by the MRC iterative equalizer are first soft-demodulated to obtain LLRs for each bit of information, which are then deinterleaved and passed to the LDPC decoder. The LDPC decoder outputs the coded bit information, and then performs interleaving, QAM modulation and OTSM modulation on the coded bit information to obtain an improved time delay-time domain estimation symbol.
Example 6
This example summarizes the total computational complexity of the MRC iterative equalization algorithm, as shown in Table 1
TABLE 1 Total computation complexity of MRC iterative equalization algorithm
Figure BDA0003757874860000111
Wherein, the step (1) is the calculation complexity required by one MRC iteration, the step (2) is the calculation complexity of an initial value required by the MRC iteration, and the step (3) is the calculation complexity of the single-tap equalizer.
Example 7
This example investigated the case of coding versus uncodingThe parameter settings of the error performance of the MRC iterative equalization algorithm of (1) are shown in table 2. Demodulation performance in the uncoded and coded cases is represented by BER and FER plots, respectively, and 5 × 10 is transmitted for each point in the BER plot4OTSM frame, each point in FER map sends 104And in the OTSM frame, standard LDPC codes in a 5G new air interface (5G NR,5G new radio) scheme are adopted for external coding. To avoid the detection performance loss caused by channel estimation error, we assume that the channel response is completely known at the receiving end, and the doppler shift of the channel is given by Jakes' formula vi＝vmaxcos(θi) Generation vmaxAt the maximum moving speed, θiIn the range of [ - π, π]Are uniformly distributed. In addition, the LMMSE linear equalizer and GS iterative equalizer, which are currently widely used, are compared in terms of BER performance and implementation complexity.
TABLE 2 System simulation parameters
Figure BDA0003757874860000121
Fig. 5 and fig. 6 compare the error performance of different modulation modes at 270km/h and 540km/h respectively, wherein the maximum iteration times of QPSK, 16-QAM and 64-QAM for MRC and GS iterative equalization are set to 5, 15 and 35 respectively. Simulation results show that the performance of the MRC iterative equalizer is significantly better than that of the LMMSE equalizer, as shown in fig. 5, at BER =10-4There is a performance gain of 2.34dB for QPSK modulation. Furthermore, the performance gains under QPSK, 16-QAM modulation are 0.62dB and 0.8dB, respectively, compared to the GS iterative equalizer. Similarly, as shown in fig. 6, at BER =10-4The performance gains under QPSK and 16-QAM modulation were 0.63dB and 1.02dB, respectively, compared to the GS iterative equalizer.
Fig. 7 shows the error performance of the OTSM system under different system parameters, and it can be seen from the figure that as N increases, the performance of the MRC iterative equalizer also gradually increases. This is because increasing the size of the OTSM block can improve the sampling resolution of the doppler frequency (i.e. delay-sequence domain grid resolution), and the receiver can resolve more channel paths to improve the error rate performance.
Fig. 8 shows the error performance of the MRC iterative equalizer at 10-2000Hz (corresponding to speeds of 2.7-540 km/h) for doppler frequency shift (fd), which is suitable for radio transmission in most environments. It can be seen from the figure that BER is rather lower as fd increases, which result is surprising for conventional modulation schemes requiring quasi-static channels. In fact, modulation in the delay-sequence domain can benefit from a larger doppler shift, i.e., stronger doppler interference (IDI) does not degrade and also improves the performance of the designed equalizer. This is because the receiver can resolve more channel paths through fd to improve the error rate performance.
Fig. 9 shows the frame error rate of the MRC-turbo receiver at different code lengths, and it can be seen from the figure that the larger the code length, the better the system performance for different modulation schemes.
In the present invention, unless otherwise expressly stated or limited, the terms "mounted," "disposed," "connected," "fixed," "rotated," and the like are to be construed broadly, e.g., as meaning fixedly connected, detachably connected, or integrally formed; can be mechanically or electrically connected; the terms may be directly connected or indirectly connected through an intermediate, and may be communication between two elements or interaction relationship between two elements, unless otherwise specifically limited, and the specific meaning of the terms in the present invention will be understood by those skilled in the art according to specific situations.
Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that changes, modifications, substitutions and alterations can be made in these embodiments without departing from the principles and spirit of the invention, the scope of which is defined in the appended claims and their equivalents.

Claims (6)

1. The MRC iterative equalization method of the OTSM system under the high-speed mobile environment is characterized by comprising the following steps:
s1, constructing an OTSM system model, wherein the OTSM system model comprises a sending end, a time domain channel and a receiving end;
s2, processing data output by a time domain channel by adopting a single-tap time frequency equalizer to obtain a time delay-time domain information symbol pre-estimated value;
and S3, taking the time delay-time domain information symbol estimated value obtained in the step S2 as an initial value of an MRC iterative equalization algorithm, and carrying out iterative detection judgment through the MRC iterative equalization algorithm to obtain a time delay-sequence domain information symbol estimated value from a sending end to a receiving end through a time domain channel.
2. The MRC iterative equalization method for an OTSM system in a high-speed mobile environment according to claim 1, wherein a single-tap time-frequency equalizer is used to obtain a time delay-time domain information symbol estimated value, and the specific process is as follows:
dividing a time domain vector output by a time domain channel into N time domain blocks, and respectively performing M-point FFT on the time domain blocks to obtain N frequency domain blocks, wherein the N frequency domain blocks are expressed as follows:
Figure FDA0003757874850000011
MMSE equalization is carried out on each frequency domain block to obtain information symbols of the frequency domain blocks, and an MMSE equalization formula is represented as follows:
Figure FDA0003757874850000012
performing M-point IFFT on the information symbols of each frequency domain block to obtain a time delay-time domain pre-estimation matrix vector consisting of time delay-time domain information symbol pre-estimation values, wherein the time delay-time domain pre-estimation matrix vector is expressed as
Figure FDA0003757874850000013
Wherein M = 0., M-1, N = 0., N-1,
Figure FDA0003757874850000014
representing the nth frequency-domain block vector, FMRepresenting an M-point FFT transform, rnRepresents the nth time-domain block vector and,
Figure FDA0003757874850000015
an mth information symbol representing an equalized nth frequency-domain block vector,
Figure FDA0003757874850000016
an mth information symbol representing an nth frequency-domain block vector,
Figure FDA0003757874850000017
representing the frequency domain channel coefficients, representing the conjugate transpose,
Figure FDA0003757874850000021
which represents the variance of the noise, is,
Figure FDA0003757874850000022
representing a delay-time domain prediction matrix vector,
Figure FDA0003757874850000023
representing an M-point IFFT transformation.
3. The MRC iterative equalization method for an OTSM system in a high-speed mobile environment according to claim 2, wherein the MRC iterative equalization algorithm comprises:
s11, obtaining a time delay-time domain information symbol pre-estimated value output by the single-tap time frequency equalizer as an initial value of an MRC iterative equalization algorithm, and setting the maximum iteration times;
s12, calculating the damaged signal component through the time delay-time domain information symbol estimated value, wherein the calculation is represented as:
Figure FDA0003757874850000024
s13, judging and estimating maximum ratio combination according to a maximum likelihood criterion, outputting an updated time delay-time domain information symbol estimated value, judging whether the maximum iteration times is reached, if so, executing a step S14, otherwise, returning to the step S12 and substituting the time delay-time domain information symbol estimated value updated by the current iteration into the estimated value, and expressing as follows:
Figure FDA0003757874850000025
s14, taking the current output as a time delay-time domain information symbol estimation value, and converting the time delay-time domain information symbol estimation value into a time delay-sequence domain information symbol estimation value through WHT;
wherein ,lmaxAn index is extended for a maximum discrete channel delay, and
Figure FDA0003757874850000026
Figure FDA0003757874850000027
represents the m + l time delay-time domain symbol vector received by the receiving end,
Figure FDA0003757874850000028
representing a time delay-time domain matrix
Figure FDA0003757874850000029
The m + l row/column of (c),
Figure FDA00037578748500000210
representing the m + l-l' th delay-time domain symbol vector estimated value,
Figure FDA00037578748500000211
representing the corrupted signal component at index m + l,
Figure FDA00037578748500000212
the decision value of the transmission symbol of the mth row and nth column of the transmitting end after each iteration is represented, namely the updated time delay-time domain information symbol estimated value, ajRepresenting the corresponding value of the constellation table, cmRepresents a time-delay-time-domain output vector of a maximal-ratio combiner, and
Figure FDA00037578748500000213
4. the iterative balancing method for MRC in OTSM system in high-speed mobile environment according to claim 1,
the transmitting end places NM information symbols in a time delay-sequence domain grid to obtain a first time delay-sequence domain matrix
Figure FDA0003757874850000031
Wherein M is the row number of the time delay-sequence domain grid, and N is the column number of the time delay-sequence domain grid and is the nth power of 2; walsh-Hadamard transform is carried out on the first time delay-sequence domain matrix line by line to obtain a first time delay-time domain matrix
Figure FDA0003757874850000032
The first time delay-time domain matrix obtains a first time domain vector matrix through serial-parallel conversion
Figure FDA0003757874850000033
The receiving end receives the second time domain vector matrix
Figure FDA0003757874850000034
Performing parallel-to-serial conversion to obtain a second time delay-time domain matrix
Figure FDA0003757874850000035
Walsh-Hadamard transform is carried out on the second time delay-time domain matrix line by line to obtain a second time delay-sequence domain matrix
Figure FDA0003757874850000036
5. The MRC iterative equalization method for OTSM system under high-speed mobile environment according to claim 4, characterized in that the time delay-time domain input-output relationship in the time domain channel is expressed as:
Figure FDA0003757874850000037
zero padding which can serve as an interleaving guard band in a time domain is placed in a time delay-sequence domain grid, so that the input-output relation of the time delay in a time domain channel and the time domain can be independently processed, and an equivalent input-output relation is obtained, and is expressed as follows:
Figure FDA0003757874850000038
wherein ,
Figure FDA0003757874850000039
represents the m + l received time delay-time domain symbol vector received by the receiving end,
Figure FDA00037578748500000310
representing a time delay-time domain matrix
Figure FDA00037578748500000311
Row/column (m + l),
Figure FDA00037578748500000312
representing a vector x of transmission symbolsmThe delay-time domain symbol vector after WHT,
Figure FDA00037578748500000313
representing the time-delay-time-domain noise component,/maxFor maximum discrete channel delay spread index and
Figure FDA00037578748500000314
6. the MRC iterative equalization method for an OTSM system under a high-speed mobile environment of claim 1, wherein a turbo iteration is performed after obtaining the time-delay-time-domain information symbol, and a turbo iteration process comprises:
performing OTSM demodulation and QAM soft demodulation on the output time delay-time domain information symbol;
de-interleaving the data after QAM soft demodulation and transmitting the data to an LDPC decoder;
the LDPC decoder processes the transmitted de-interleaving data output bit information and interleaves the bit information;
and carrying out QAM modulation and OTSM modulation on the interleaved data to obtain an improved time delay-time domain information symbol.
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