CN108173611B - EVM test optimization method based on OFDM system satellite transponder - Google Patents

EVM test optimization method based on OFDM system satellite transponder Download PDF

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CN108173611B
CN108173611B CN201711375607.8A CN201711375607A CN108173611B CN 108173611 B CN108173611 B CN 108173611B CN 201711375607 A CN201711375607 A CN 201711375607A CN 108173611 B CN108173611 B CN 108173611B
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CN108173611A (en
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杨博
杨冬雪
周慧
解索非
满梓峰
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China Academy of Space Technology CAST
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B17/00—Monitoring; Testing
    • H04B17/30—Monitoring; Testing of propagation channels
    • H04B17/391—Modelling the propagation channel
    • H04B17/3912—Simulation models, e.g. distribution of spectral power density or received signal strength indicator [RSSI] for a given geographic region
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B17/00—Monitoring; Testing
    • H04B17/10—Monitoring; Testing of transmitters
    • H04B17/15—Performance testing
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B17/00—Monitoring; Testing
    • H04B17/30—Monitoring; Testing of propagation channels
    • H04B17/391—Modelling the propagation channel
    • H04B17/3911—Fading models or fading generators
    • 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
    • H04L27/2602—Signal structure
    • 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
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2657—Carrier synchronisation
    • 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
    • H04L27/2655—Synchronisation arrangements
    • H04L27/2662—Symbol synchronisation

Abstract

An EVM test optimization method based on an OFDM system satellite transponder achieves test requirements of an EVM value of the transponder by modeling a test signal, estimating amplitude-phase value change conditions of subcarriers, performing source-end predistortion processing on the test signal, and repeatedly verifying multiple measurements, solves the problem of low EVM test precision of the transponder due to introduction of a nonlinear device into a test system, and achieves the purpose of improving the EVM test precision of a system level to the maximum extent after the nonlinear device is introduced into the system.

Description

EVM test optimization method based on OFDM system satellite transponder
Technical Field
The invention relates to an EVM test optimization method based on an OFDM system satellite transponder, and belongs to the field of communication satellites.
Background
With the continuous development of satellite communication technology, the digitalized payload based on new communication system is in a variety, wherein the multi-carrier modulation technology is a main trend of communication load in the future.
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation technique in which subcarriers are mutually aliased, and the subcarriers are mutually orthogonal in the time domain and partially overlapped in the frequency spectrum. Future LTE system communication satellites, ultra-wideband system communication satellites and the like are pre-researched by relying on an OFDM core technology. The EVM performance is a key index for measuring the communication performance, the correctness of a communication protocol and the signal transmission quality in the communication digital load under the OFDM system. Therefore, the high-precision test of the EVM performance aiming at the satellite load under the OFDM system is very important.
The EVM performance of the communication satellite transponder is tested, and the EVM deterioration condition after the signal is forwarded is mainly tested. A test transmitter of the test system sends out test signals with good enough EVM performance, and the test signals are transmitted by a satellite and received by a test receiver. After the receiver passes through a series of receiver synchronization algorithms such as frame synchronization, bit synchronization, carrier synchronization and the like, the EVM performance of the satellite forwarding output signals is measured, and the EVM deterioration condition of the satellite transponder forwarding signals is directly evaluated according to the EVM performance. In practical applications, the frequency or power of the signal sent by the test transmitter may not meet the requirement of the satellite access signal (for example, the frequency is not in the uplink pass band of the satellite transponder, the power is lower than the sensitivity of the satellite receiver, etc.), so a non-linear device such as a power amplifier or a frequency converter needs to be added to the back end of the test transmitter. At this time, the introduction of such a nonlinear device may cause the EVM of the test signal input by the satellite transponder to be degraded, i.e. become non-ideal or higher than 3% of the engineering requirement, which may cause the EVM of the transponder output signal to be affected by the EVM of the input signal, resulting in inaccurate test result, and such a test error is completely caused by the test system itself.
Disclosure of Invention
The technical problem solved by the invention is as follows: aiming at the problem of low EVM test precision of a repeater caused by introduction of a nonlinear device into a test system, the EVM test optimization method based on the satellite repeater of the OFDM system is provided, the EVM predistortion algorithm of a test transmitter is provided, and the system-level EVM test precision is improved to the maximum extent.
The technical scheme for solving the technical problems is as follows:
an EVM test optimization method based on an OFDM system satellite transponder comprises the following steps:
(1) modeling a test signal passing through a nonlinear device, and determining a test signal training sequence frequency domain expression and related parameters;
(2) carrying out down-conversion and analog-to-digital conversion on the test signal, carrying out symbol synchronization and carrier synchronization on the test signal obtained after conversion, and carrying out FFT (fast Fourier transform) processing on the test signal to obtain a frequency domain signal;
(3) estimating the amplitude and phase values of the frequency domain signal obtained in the step (2) by using a least square method to obtain amplitude and phase change values of each subcarrier of the test signal after passing through a nonlinear device, and comparing all the amplitude and phase values to obtain amplitude and phase change conditions;
(4) pre-distorting the test signal before entering the nonlinear device by using the amplitude-phase change value of each subcarrier obtained in the step (3) to obtain a pre-distorted emission test signal;
(5) and (4) calculating and outputting the EVM value of the emission test signal obtained in the step (4) in real time, if the EVM value meets the test requirement, carrying out subsequent EVM value test, otherwise, repeating the steps (2) to (5) to carry out amplitude and phase value estimation of the subcarrier and test signal pre-distortion treatment again.
In the step (1), the training sequence of the test signal is an OFDM symbol stream, after passing through a nonlinear device, the subcarrier undergoes amplitude-phase fading, the modeling is to establish a parallel Gaussian fading channel model, and the frequency domain expression and related parameters of the training sequence of the test signal are as follows:
Y＝HX+N
in the formula, Y is a training sequence signal vector of the transponder entrance after passing through the nonlinear device, X is a known training sequence sent by the transmitter before entering the nonlinear device, H is a frequency domain characteristic vector of amplitude and phase change of each subcarrier after passing through the nonlinear device, and N is additive noise of each subcarrier.
The specific expression of the relevant parameters of the test signal training sequence is as follows:
Figure BDA0001514489030000021
Figure BDA0001514489030000031
Figure BDA0001514489030000032
Figure BDA0001514489030000033
in the step (3), the method for calculating the amplitude-phase change value after each subcarrier passes through the nonlinear device by using the least square method is as follows:
Figure BDA0001514489030000034
the partial derivative of H is obtained
Figure BDA0001514489030000035
In the formula (I), the compound is shown in the specification,
Figure BDA0001514489030000036
and the amplitude and phase change value of each subcarrier after passing through the nonlinear device.
In the step (4), the predistortion processing method for transmitting the test signal comprises the following steps:
Figure BDA0001514489030000037
in the formula (I), the compound is shown in the specification,
Figure BDA0001514489030000038
the amplitude and phase values of the transmitting test signal after predistortion.
Preferably, the EVM value test requirement of the test signal itself in the step (5) is less than 3%.
Compared with the prior art, the invention has the advantages that:
(1) the invention provides an EVM test optimization method based on an OFDM system satellite transponder, which utilizes the orthogonality among subcarriers of a communication load transmission multicarrier signal under the OFDM system and adopts a least square method to estimate the amplitude-phase change condition of a transmission source signal introduced into a nonlinear device on each subchannel;
(2) the method respectively pre-distorts the data of each sub-channel before the IFFT of the transmitter according to the algorithm estimation result, cancels or reduces the deterioration of the EVM of the source signal by utilizing the anti-compensation effect after passing through the nonlinear device, and repairs the EVM loss of the test signal after passing through the nonlinear device through the proposed EVM pre-distortion algorithm, so that the EVM performance of the test signal at the entrance of the satellite transponder is better, and the system-level test precision is further improved.
Drawings
Fig. 1 is a connection structure diagram for testing the EVM performance of a satellite transponder according to the present invention;
FIG. 2 is a diagram of an EVM test connection configuration after a non-linear device is added to the system provided by the present invention;
FIG. 3 is a functional block diagram of the interior of a transmitter in the EVM test system provided by the present invention;
FIG. 4 is a schematic block diagram of the amplitude-phase variation estimation and predistortion of a test signal provided by the present invention;
FIG. 5 is a diagram of a parallel Gaussian channel model provided by the present invention;
FIG. 6 is a flow chart of the steps of the invention.
Detailed Description
The present invention will be described in further detail with reference to the accompanying drawings and examples.
In normal testing, as shown in fig. 1, a test signal is transmitted from a transmitter to a transponder under test, and an EVM value of an output signal is measured by a receiver. When the test signal does not meet the transponder ingress signal requirement, as shown in fig. 2, a nonlinear device, typically an amplifier, a frequency converter, etc., is added between the transmitter and the transponder under test.
The internal schematic block diagram of the test transmitter is shown in fig. 3, and the test original data is first subjected to channel coding, then constellation mapping and pilot frequency insertion are performed, IFFT is performed to form an OFDM symbol, and finally, a final test signal is formed through digital-to-analog conversion and up-conversion and is input to the tested transponder. The most important point for testing the EVM performance of the transponder signal of the satellite transponder is to require that the input test signal of the transponder to be tested has a sufficiently ideal EVM performance, so that the EVM value of the transponder output signal measured by the test receiver completely reflects the EVM performance of the transponder. Therefore, in the test system, if a nonlinear device is introduced, the EVM degradation of the test signal input by the transponder under test is inevitably caused, so that the EVM degradation condition of the test signal input by the transponder due to the introduction of the nonlinear device (that is, the amplitude-phase change condition of the signal) needs to be estimated first, then the EVM predistortion is performed on the test signal inside the transmitter according to the amplitude-phase change estimation result, and finally the EVM value of the transponder inlet signal is measured to meet the test requirement (the EVM value of the input test signal is generally required to be lower than 3% in engineering).
In the test signal generated by the transmitter, each OFDM symbol data is preceded by a training sequence, the training sequence comprises the same number of subcarriers as the number of subcarriers in the OFDM symbol of the test signal, and the amplitude and phase change experienced by the training sequence on each subcarrier is the same as the amplitude and phase change experienced by the OFDM symbol of the test signal on each subcarrier. The invention utilizes the training sequence to complete the estimation of the amplitude-phase change of the emission test signal and further complete the pre-distortion processing of the transmitter signal EVM. As shown in fig. 4, a schematic block diagram of amplitude-phase change estimation and predistortion of a test signal is that a signal input to a transponder to be tested is extracted and preprocessed, then least square estimation is performed on amplitude-phase change of the test signal, and finally, a source-end signal is subjected to predistortion processing by using an estimation result.
As shown in fig. 6, the implementation steps of the present invention are as follows:
(1) modeling test signals after passing through the nonlinear device
The training sequence of the test signal is also an OFDM symbol stream, and after passing through the nonlinear device, each subcarrier can be considered to experience different amplitude-phase fading and gaussian noise. Therefore, the training sequence of the test signal after passing through the nonlinear device can be modeled as a parallel gaussian fading channel model as shown in fig. 5. Therefore, the frequency domain expression of the test signal training sequence at the entrance of the transponder after passing through the nonlinear device is as follows:
Y＝HX+N
wherein Y is a training sequence signal vector of the transponder entrance after passing through the nonlinear device,
Figure BDA0001514489030000051
the corresponding X is the known training sequence sent by the transmitter before entering the non-linear device,
Figure BDA0001514489030000052
h is a frequency domain characteristic vector of amplitude-phase change after each subcarrier passes through the nonlinear device,
Figure BDA0001514489030000053
n is the additive noise on each subcarrier,
Figure BDA0001514489030000054
(2) processing the test signal before extraction and amplitude and phase estimation
As shown in fig. 4, to estimate the amplitude and phase change of each subcarrier of the transponder ingress signal, the test signal needs to be down-converted to an intermediate frequency first, then analog-to-digital conversion is performed, and meanwhile, to accurately extract the training sequence, symbol synchronization and carrier synchronization need to be completed, and finally the test signal is converted into a frequency domain signal through FFT processing.
The above operations are well known techniques, and a specific amplitude and phase estimation method will be described below.
(3) Transponder ingress signal amplitude and phase change estimation
The invention adopts a least square method to estimate the amplitude-phase change condition of the training sequence on each subcarrier after passing through the nonlinear device, and the estimation result is considered to be the EVM deterioration experienced by the OFDM symbol of the test signal. The frequency domain least square amplitude-phase change estimation algorithm is explained in detail below.
The frequency domain least square amplitude-phase change estimation algorithm is to estimate the amplitude-phase change estimated value so that the following formula has the minimum value:
Figure BDA0001514489030000061
the partial derivative of H is calculated and the result is zero, so that the result can be obtained
Figure BDA0001514489030000062
Wherein, X is a known training sequence vector, and Y is a received training sequence signal vector of the transponder entrance after passing through the nonlinear device.
In practical application, due to the existence of noise in the system, when frequency domain least square estimation is adopted, the noise causes large estimation error, and the estimation error can be reduced by increasing the transmission power of the training sequence.
(4) Transmitter EVM predistortion
Calculated in step 3
Figure BDA0001514489030000063
I.e. the EVM penalty of each sub-carrier after the test signal passes through the non-linear device.
Will obtain
Figure BDA0001514489030000064
The feedback is applied to each OFDM symbol before IFFT in the transmitter to obtain a transmitting test signal after predistortion.
Figure BDA0001514489030000065
Where X' is the test signal vector before the IFFT in the transmitter. At this time, EVM predistortion on each subcarrier signal in a test signal inside the transmitter is completed, then the test signal after predistortion passes through the nonlinear device, amplitude-phase fading of each subcarrier by the nonlinear device and a predistortion processing result inside the transmitter are mutually offset, so that EVM performance of the test signal (namely, an inlet signal of the tested repeater) output by the nonlinear device is improved.
(5) EVM real-time measurement on test signal of inlet of transponder
And (3) after FFT is carried out on the inlet signal of the tested transponder, two paths are respectively led out, one path is subjected to amplitude-phase change estimation in the step (2), the other path is subjected to parallel-serial conversion and constellation inverse mapping, and the EVM value of the test signal is further calculated. If the EVM value of the test signal reaches the value lower than 3% of the transponder test requirement, subsequent EVM tests can be carried out, and if the EVM value of the test signal does not reach the value lower than 3%, the steps (2), (3) and (4) are repeated until the EVM value of the test signal finally reaches the value meeting the requirement.
Those skilled in the art will appreciate that those matters not described in detail in the present specification are well known in the art.

Claims (4)

1. An EVM test optimization method based on an OFDM system satellite transponder is characterized by comprising the following steps:
(1) modeling a test signal passing through a nonlinear device, and determining a test signal training sequence frequency domain expression and related parameters;
(2) carrying out down-conversion and analog-to-digital conversion on the test signal, carrying out symbol synchronization and carrier synchronization on the test signal obtained after conversion, and carrying out FFT (fast Fourier transform) processing on the test signal to obtain a frequency domain signal;
(3) estimating the amplitude and phase values of the frequency domain signal obtained in the step (2) by using a least square method to obtain amplitude and phase change values of each subcarrier of the test signal after passing through a nonlinear device, and comparing all the amplitude and phase values to obtain amplitude and phase change conditions;
the amplitude-phase change value calculation method for estimating each subcarrier after passing through the nonlinear device by using the least square method is as follows:
Figure FDA0002768657180000011
the partial derivative of H is obtained
Figure FDA0002768657180000012
In the formula (I), the compound is shown in the specification,
Figure FDA0002768657180000013
the amplitude and phase change value of each subcarrier after passing through the nonlinear device is obtained;
(4) pre-distorting the test signal before entering the nonlinear device by using the amplitude-phase change value of each subcarrier obtained in the step (3) to obtain a pre-distorted emission test signal;
the predistortion processing method for transmitting the test signal comprises the following steps:
Figure FDA0002768657180000014
in the formula (I), the compound is shown in the specification,
Figure FDA0002768657180000015
the amplitude and phase values of the transmitting test signals after the predistortion;
(5) and (4) calculating and outputting the EVM value of the emission test signal obtained in the step (4) in real time, if the EVM value meets the test requirement, carrying out subsequent EVM value test, otherwise, repeating the steps (2) to (5) to carry out amplitude and phase value estimation of the subcarrier and test signal pre-distortion treatment again.
2. The EVM testing optimization method based on the OFDM system satellite transponder is characterized in that: in the step (1), the training sequence of the test signal is an OFDM symbol stream, after passing through a nonlinear device, the subcarrier undergoes amplitude-phase fading, the modeling is to establish a parallel Gaussian fading channel model, and the frequency domain expression and related parameters of the training sequence of the test signal are as follows:
Y＝HX+N
in the formula, Y is a training sequence signal vector of the transponder entrance after passing through the nonlinear device, X is a known training sequence sent by the transmitter before entering the nonlinear device, H is a frequency domain characteristic vector of amplitude and phase change of each subcarrier after passing through the nonlinear device, and N is additive noise of each subcarrier.
3. The method according to claim 2, wherein the EVM test optimization method based on the satellite transponder with the OFDM system is characterized in that: the specific expression of the relevant parameters of the test signal training sequence is as follows:
Figure FDA0002768657180000021
Figure FDA0002768657180000022
Figure FDA0002768657180000023
Figure FDA0002768657180000024
4. the EVM testing optimization method based on the OFDM system satellite transponder is characterized in that: the EVM value test requirement of the test signal per se in the step (5) is lower than 3%.
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