CN115175378A - Sky-ground fusion 5G terminal simulator based on srSRAN and implementation method - Google Patents
Sky-ground fusion 5G terminal simulator based on srSRAN and implementation method Download PDFInfo
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Abstract
The invention discloses a method for realizing a space-ground fusion 5G terminal simulator based on srSRRAN, which is characterized in that on the basis of a PSS time-frequency estimation algorithm, cyclic prefixes of a plurality of orthogonal frequency division multiplexing symbols are used as auxiliary information to improve the synchronization performance, a CP is used for estimating decimal frequency offset, and then the PSS is used for estimating complete carrier frequency offset; and matching with the existing 5G NSA network architecture embedded in the srSRAN open source project to obtain the sky-ground fusion 5G terminal simulator based on the srSRAN. According to the invention, under the condition of not changing the existing NR ground mobile communication standard, a time-frequency synchronization technology suitable for a satellite-ground link is designed by using an NR synchronization sequence, and a terminal simulator facing a 5G fusion low-orbit satellite is set up, so that a ground 5G system and a low-orbit satellite communication system can be seamlessly switched.
Description
Technical Field
The invention belongs to the field of 5G fusion low-orbit satellite terminal simulators, and particularly relates to a sky-ground fusion 5G terminal simulator based on srSRAN and an implementation method thereof.
Background
With the explosive growth of mobile traffic, the massive connection of devices, and the emergence of various new scenarios, a fifth generation mobile communication system has been brought forward. The satellite communication system and the terrestrial communication system can form a good complementary relationship: the satellite communication has the characteristics of wide coverage range, small influence of factors such as terrain, natural disasters and the like, so that on one hand, the satellite communication can be utilized to realize communication in areas which are difficult to be covered by ground networks such as remote areas and the like; on the other hand, the communication system can be ensured not to be damaged when the ground network is damaged due to the influence of factors such as natural disasters; and the integration of the ground 5G system and the low-orbit satellite communication system can give full play to the advantages of the two communication systems and realize the global seamless coverage and seamless switching network. However, since the two communication systems have great differences in network architecture, communication standard, etc., at present, the two systems only reach the degree of interconnection.
Disclosure of Invention
In order to enable seamless switching between a ground 5G system and a low-orbit satellite communication system, the invention provides a sky-ground fusion 5G terminal simulator based on srSRAN and an implementation method thereof.
Based on the background, the applicability problem of a 5G air interface signal in a satellite-ground link is researched, a time-frequency synchronization technology under a large Doppler frequency shift scene is mainly explored, the time-frequency synchronization problem under a large frequency shift environment is solved, and a terminal simulator based on an srRAN NR protocol stack is designed.
The invention provides a method for realizing a world fusion 5G terminal simulator based on srSRAN,
on the basis of a PSS time frequency estimation algorithm, cyclic prefixes of a plurality of orthogonal frequency division multiplexing symbols are used as auxiliary information to improve synchronization performance, a CP is used for estimating decimal frequency offset, and then a PSS is used for estimating complete carrier frequency offset; and matching with the existing 5G NSA network architecture embedded in the srSRRAN open source project to obtain the sky-ground fusion 5G terminal simulator based on the srSRRAN.
Further, the specific method for estimating the decimal frequency offset by using the CP and then estimating the complete carrier frequency offset by using the PSS includes:
s1, receiving a baseband signal from a radio frequency cache, and performing anti-aliasing filtering processing;
s2, calculating the cross-correlation result of the received signal processed by the S1 and K groups of local synchronous sequences to obtain K time-frequency estimation results;
s3, performing two-dimensional search on the K time-frequency estimation results in the S2, and updating time-frequency estimation peak values;
s4, repeating S1-S3 until a whole wireless frame is traversed, and obtaining a plurality of time-frequency estimation two-dimensional search results;
s5, one-dimensional search is carried out on the plurality of time-frequency estimation two-dimensional search results obtained in the step S4, and a time offset estimation value corresponding to the maximum peak value is searched
And integer frequency offset estimation
S6, calculating the signal-to-noise ratio corresponding to the maximum peak value searched in the S5, if the signal-to-noise ratio is larger than a threshold value, finishing time synchronization and frequency coarse synchronization, and if the signal-to-noise ratio is smaller than the threshold value, repeating the steps S1-S6;
s7, carrying out decimal frequency offset on the received signals processed by the S1 by utilizing a plurality of OFDM cyclic prefixes
Carrying out estimation; incorporating integer frequency offsets
Frequency synchronization is completed.
Further, in the above-mentioned case,
in S2, the generation mode of the multiple groups of local synchronization sequences is
Wherein s is l V primary synchronization sequence for multipath channel of length L i For modulating DPSS matrix A k The feature vector corresponding to the ith feature value of (1); modulating DPSS matrix A k Can be represented as
Wherein
Searching range [ epsilon ] of frequency offset min ,ε max ]Dividing the data into K segments to obtain K modulation DPSS matrixes A k And corresponding K sets of local synchronization sequences.
A space-ground fusion 5G terminal simulator based on srSRAN is realized according to the method.
A computer readable storage medium having stored therein at least one instruction, at least one program, a set of codes, or a set of instructions that is loaded and executed by a processor to implement a srsrran-based space-earth convergence 5G terminal simulator implementation method as described above.
The invention has the beneficial effects that:
(1) The design of the terminal simulator is realized in a mode of software radio and a USRP semi-physical communication simulation platform, compared with a hardware communication simulation system, the design method has the advantages of low cost and easiness in optimization, and the requirement on special communication hardware is avoided; compared with a software simulation communication system, the software simulation communication system is difficult to restore a complex satellite-ground link communication environment in a mathematical modeling mode, and the terminal simulator can adopt a real channel for experiment;
(2) Compared with the prior art, the invention realizes coarse time offset estimation and integral frequency offset estimation through the cyclic prefix of the OFDM signal used by the 5G NR; a sliding correlation algorithm based on a modulation DPSS matrix is provided, and the complexity of time-frequency synchronization under a large frequency offset environment is effectively reduced through eigenvalue decomposition and approximate calculation so as to optimize the time-frequency synchronization algorithm;
(3) The time-frequency synchronization technology of the 5G fusion low-orbit satellite system considered by the invention can utilize the NR synchronization sequence to design a time-frequency synchronization technology suitable for a satellite-ground link under the condition of not changing the existing NR ground mobile communication standard, and a terminal simulator facing the 5G fusion low-orbit satellite is built.
Drawings
FIG. 1 is a schematic flow diagram of the process of the present invention;
FIG. 2 is a functional architecture of a world fusion 5G terminal simulator in the present invention;
fig. 3 is a flow chart of a downlink synchronization algorithm in the present invention;
fig. 4 is a baseband IQ signal reception synchronization process for srsrsan.
Detailed Description
The technical scheme of the invention is further explained in detail by combining the attached drawings:
the invention provides a 5G terminal simulator based on satellite-to-ground links, which specifically considers a time-frequency synchronization technology in a large Doppler frequency shift scene so as to solve the time-frequency synchronization problem in a large frequency shift environment. There are many significant differences between satellite and terrestrial communication systems, which also present many challenges for the convergence of air interface technologies. The most important problem is the large doppler shift caused by the high-speed motion of the low-orbit satellite, so that the 5G transmission method designed for the ground communication system cannot be directly applied to the low-orbit satellite communication system. In a converged communication network, in order to realize high-speed data transmission, orthogonal frequency division multiplexing is used for a downlink, so that symbol timing synchronization and carrier frequency synchronization are very important to guarantee the performance of the converged communication network. Through receiving the cell synchronization signal, the user equipment obtains the symbol timing, the frame timing, the frequency offset estimation, the cell ID and the like of the system. Considering that a 5G transmission method designed by a ground communication system cannot be directly applied to a low earth orbit satellite communication system, but the ground mobile communication flow is increased explosively, in order to realize global seamless coverage and seamless switching of a communication network, a synchronization sequence specified by a 5G novel wireless air interface needs to be designed under the condition of not changing the existing NR ground mobile communication standard to realize more accurate time-frequency synchronization in a satellite-ground link.
The m-sequence is a binary digital pseudorandom sequence generated by a linear feedback shift register, is easy to generate, has good autocorrelation and cross-correlation properties, and is insensitive to frequency offset. And the NR selects the m sequence as a generation sequence of a downlink main synchronization sequence, and the defined main synchronization signal is mainly used for acquiring the ID of a physical layer in a cell group and realizing time-frequency synchronization. Next, a brief introduction of the PSS signal generation process of 5G NR will be described
The 5G NR physical layer protocol specifies the PSS generation sequence d pss (k) Occupies continuous 127 sub-carriers d on the frequency domain pss (k) Can be expressed as
Wherein
And the ID number of the physical layer cell in the group with the value range of 0-2 is represented, and the parameter is used for distinguishing the primary synchronization sequence of the adjacent cell. The sequence x (p) is generated in an iterative manner by the formula
x(p+7)＝(x(p+4)+x(p))mod2 (2)
The initial value of the sequence x (p) is defined as
The frequency domain OFDM symbol containing the primary synchronization signal can be represented as
Wherein k is 0 Indicating the sequence number of the starting subcarrier of the frequency domain mapping position. The fast Fourier inverse transformation of N points is carried out on the sequence d (k) to obtain the synchronous signal s of the time domain pss (n)
By using the PSS synchronization sequence specified by NR, the invention adopts the following technical scheme for solving the technical problems:
the system uses N =1536 subcarriers, and adds a length of N to its start position for each OFDM time domain transmission symbol g To prevent ISI and to preserve orthogonality between subcarriers. Suppose the transmitted mth OFDM symbol is s m (N), wherein N =0,1 g -1, corresponding received signal r m (n) can be represented as
Where ε is the normalized frequency offset for the subcarrier spacing, θ is the normalized time offset for the sampling interval, h (L) is the impulse response of the L-th path of the multipath channel with length L, and z (n) is the additive white Gaussian noise at the receiving end.
The received signal with the cyclic prefix removed can be written as
r m ＝D(ε)S m h+z (7)
And performing frequency offset compensation on the received signal at the frequency point within the specified frequency offset range by considering the cross-correlation characteristic of the m sequence and different frequency offsets of the m sequence. Based on the method, the traditional time-frequency two-dimensional optimization problem is converted into two one-dimensional search problems, the optimization problem is converted into an approximate objective function maximization problem, and an approximate expression of a time bias estimation objective function is given:
after the above formula is expanded, the following form can be obtained:
wherein
Parameters omega,
Are respectively defined as
It can be found that the Matrix a can be divided into a long sphere Matrix (Matrix) multiplied by a modulation coefficient, so that the Matrix a can be subjected to feature decomposition by using a modulation DPSS Matrix to reduce the complexity of an estimation algorithm. Finally, the following time-frequency estimation expression is obtained:
wherein v is i To modulate the ith column vector of the DPSS matrix,
is the ith local synchronization sequence.
Furthermore, when the frequency offset search range is large, the performance of a DPSS-assisted time-frequency synchronization algorithm (DPSS algorithm for short) will be reduced, so that the large frequency offset search range is decomposed into several relatively small frequency offset search ranges, and the maximum peak position is found in these several ranges, thereby improving the timing performance without increasing the complexity. I.e., the frequency offset search range [ epsilon ] min ,ε max ]Dividing the data into K segments to obtain K modulation DPSS matrixes A k And corresponding K sets of local synchronization sequences.
On the basis of a PSS time frequency estimation algorithm, cyclic prefixes of a plurality of orthogonal frequency division multiplexing symbols are used as auxiliary information to improve synchronization performance, and the estimation complexity is effectively reduced by firstly using a CP to estimate a decimal frequency offset and then using the PSS to estimate a complete carrier frequency offset. Based on the downlink time-frequency synchronization algorithm suitable for the satellite-ground link and the existing 5G NSA network architecture embedded in the srSRRAN open source project, a terminal simulator facing a 5G fusion low-orbit satellite network can be designed and is responsible for realizing terminal high-level protocol processing and physical layer baseband signal processing.
The terminal simulator oriented to the 5G fusion low-orbit satellite network is responsible for realizing terminal high-level protocol processing and physical-level baseband signal processing. When the terminal is just started, because the terminal does not have any cell information, the terminal firstly carries out blind detection on the downlink synchronous signals sent by the base station. The terminal simulator receives the intermediate frequency signal at the downlink, obtains a digital baseband IQ signal through the ADC, and then completes the peak value search of the downlink synchronous signal, completes the frame timing and obtains a physical cell identifier 2. And then OFDM symbol demodulation, physical resource demapping and the like are carried out to receive physical broadcast channel, physical downlink control channel and physical downlink shared channel signals. After the downlink channel signal receiving and processing process is completed, the terminal simulator can send the processed signal to the high-level protocol processing module. After finishing downlink time-frequency synchronization, if the terminal has data to be sent in the high layer, the terminal physical layer will receive the TB sent by the high layer protocol processing module, and respectively perform uplink sending of the physical uplink control channel and the physical uplink shared channel signal. And secondly, the terminal physical layer performs resource mapping on the transmitted data to obtain frequency domain information, performs OFDM modulation on the frequency domain information, simultaneously transmits PRACH signals according to the downlink frame timing information and the clock signals through an uplink random access process, and converts the PRACH signals into intermediate frequency signals through a DAC (digital-to-analog converter) for transmission.
As shown in fig. 3, (1) the UE starts to receive downlink air interface data, and simultaneously the sliding window starts to operate to perform sliding interception and storage on the received data. Each TTI of the srSRRAN physical layer receives and processes data with the length of one subframe from the USRP, and the length of a sliding window in an initial state is set to be 14 OFDM time domain symbol data lengths;
(2) And continuously receiving half-frame length data, and performing initial synchronization DPSS correlation calculation and correlation result peak search on the intercepted data.
(3) After the detection of the field data is completed, the maximum result peak position meeting the synchronization threshold value can be obtained, and the system enters an initial synchronization state. Adjusting the initial position of the sliding window to the position of the PSS with the maximum energy detected in the initial synchronization, intercepting the CP sequence in the PSS and calculating the magnitude of frequency offset;
(4) And recording the number of synchronous frames according to the relation between the detection threshold and the initial synchronous threshold, and if the number of continuous synchronous frames reaches 3 times, entering a tracking synchronous state by the system. Shortening the length of a sliding window of tracking synchronization into 3 OFDM symbol lengths, meanwhile, calculating by using a tracking synchronization algorithm to improve timing performance, and adjusting the position of the window to enable a PSS sequence to be located at the center position of the sliding window, so that the position of the PSS sequence needs to be predicted according to the dynamic time delay characteristic of a low-orbit satellite communication system;
(5) Recording the number of out-of-step frames according to the relation between the synchronous detection threshold and the tracking synchronous threshold, and if the number of continuous out-of-step frames is less than 3, continuously searching the next half frame data according to the tracking synchronous process; if the number of continuous out-of-step frames is more than 3, returning to the initial synchronization state, and restarting the system to perform sliding search;
the above embodiments are only for illustrating the technical idea of the present invention, and the technical idea of the present invention is not limited thereto, and any modifications made on the basis of the technical solution according to the technical idea of the present invention fall within the protective scope of the present invention.
Claims (5)
1. A method for realizing a world fusion 5G terminal simulator based on srSRRAN is characterized in that,
on the basis of a PSS time frequency estimation algorithm, cyclic prefixes of a plurality of orthogonal frequency division multiplexing symbols are used as auxiliary information to improve synchronization performance, a CP is used for estimating decimal frequency offset, and then a PSS is used for estimating complete carrier frequency offset; and matching with the existing 5G NSA network architecture embedded in the srSRAN open source project to obtain the sky-ground fusion 5G terminal simulator based on the srSRAN.
2. A method for realizing a world fusion 5G terminal simulator based on srsRAN is characterized in that a CP is used for estimating decimal frequency offset, and a specific method for estimating complete carrier frequency offset by using a PSS is as follows:
s1, receiving a baseband signal from a radio frequency cache, and performing anti-aliasing filtering processing;
s2, calculating the cross-correlation result of the received signal processed by the S1 and K groups of local synchronous sequences to obtain K time-frequency estimation results;
s3, performing two-dimensional search on the K time-frequency estimation results in the S2, and updating time-frequency estimation peak values;
s4, repeating S1-S3 until a whole wireless frame is traversed, and obtaining a plurality of time-frequency estimation two-dimensional search results;
s5, one-dimensional search is carried out on the plurality of time-frequency estimation two-dimensional search results obtained in the step S4, and a time offset estimation value corresponding to the maximum peak value is searched
And integer frequency offset estimation
S6, calculating the signal-to-noise ratio corresponding to the maximum peak value searched in the S5, if the signal-to-noise ratio is larger than a threshold value, finishing time synchronization and frequency coarse synchronization, and if the signal-to-noise ratio is smaller than the threshold value, repeating the steps S1-S6;
s7, carrying out decimal frequency offset on the received signals processed by the S1 by utilizing a plurality of OFDM cyclic prefixes
Carrying out estimation; incorporating integer frequency offsets
Frequency synchronization is completed.
3. The downlink time-frequency synchronization method for the space-ground converged communication system according to claim 2, wherein in S2, the plurality of groups of local synchronization sequences are generated in a manner of
Wherein s is l For a primary synchronization sequence experiencing a multipath channel of length L, v i For modulating DPSS matrix A k The feature vector corresponding to the ith feature value of (1); modulating DPSS matrix A k May be represented as
Wherein
Searching range [ epsilon ] of frequency offset min ,ε max ]Dividing the data into K segments to obtain K modulation DPSS matrixes A k And corresponding K sets of local synchronization sequences.
4. An srsrran-based space-ground convergence 5G terminal simulator, implemented according to any of claims 1-3.
5. A computer readable storage medium having stored therein at least one instruction, at least one program, a set of codes, or a set of instructions, which is loaded and executed by a processor to implement the srsrsrran-based space-earth convergence 5G terminal simulator implementation method of any one of claims 1-3.
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