CN115865575A - Reconfigurable intelligent surface-assisted MIMO system separation channel reconstruction method - Google Patents
Reconfigurable intelligent surface-assisted MIMO system separation channel reconstruction method Download PDFInfo
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Abstract
The invention discloses a reconstruction method of a reconfigurable intelligent surface auxiliary MIMO system separation channel, which comprises the following steps: the user sends the pilot frequency of two time quantum, adopt the pilot frequency of single moment, occupying all subcarriers in the first time quantum, adopt the pilot frequency of continuous moment, only occupying some subcarriers in the second time quantum, and set up the reflection coefficient of random RIS; through RIS reflected signal, the base station extracts direction angle, time delay and gain information of all propagation paths in the separation channel of the target user; the extraction process of the separation channel parameters is divided into two stages, the base station finishes the extraction of the angle and the time delay of the UE-RIS section according to the known LoS channel of the RIS-BS section, finishes the extraction of the angle and the time delay of the rest NLoS paths of the RIS-BS section according to the previous result, estimates the gain respectively and finally finishes the estimation of all the channel parameters; the base station uses the parameters to complete the separate channel reconstruction. The invention realizes the data transmission of the RIS cascade channel under the condition of low signal-to-noise ratio.
Description
Technical Field
The invention belongs to the technical field of wireless communication, and particularly relates to a method for reconstructing a separate channel of an auxiliary MIMO-OFDM (multiple input multiple output orthogonal frequency division multiplexing) system based on a RIS (reconfigurable intelligent surface).
Background
The reconfigurable intelligent super surface is recently considered as a potential solution for meeting the requirements of a future wireless network due to the characteristics of rapid configuration and low power consumption. RIS can be used to improve the coverage and spectral efficiency of wireless communications by programmably configuring the wireless propagation environment by deploying a large number of approximately passive reflective elements, has been used in existing wireless communication systems, and has been intensively studied and applied in indoor positioning, environmental awareness, transceivers, and the like. In addition, companies that currently have the wireless industry start to perform RIS-based prototype system testing in 5G networks with base stations and commercial user equipment.
However, channel estimation in RIS assisted wireless systems is more difficult than channel estimation in conventional systems because the reconfigured channel model actually increases the number of concatenated channels around the RIS with a large number of cells. And the RIS usually works in an approximately passive state without signal processing capability, which greatly increases the difficulty of channel estimation for obtaining perfect Channel State Information (CSI). The focus of the research is directed to a more specific solution to the pilot overhead problem. Since the overhead of training pilots is closely related to the number of RIS elements, an RIS reflector with a large number of elements necessarily results in a large increase in the pilot overhead for a multi-user system, which is unacceptable in existing communication systems.
Based on sparse channel estimation, the channel parameter estimation method of the RIS system means that not only a channel matrix needs to be obtained, but also a large number of channel parameters related to each user, such as fading coefficients, direction of arrival (DOA), delay and the like, need to be obtained according to the channel matrix, and estimation of separated channel state information can be realized, thereby providing important guidance for performing work such as precoding, channel reconstruction, coherent detection, environment sensing, user positioning and the like. However, in the current research, the parameter estimation for the RIS end is currently less, mainly because the design difficulty and the calculation difficulty of the channel estimation are higher. But the importance of the method is not negligible, such as the arrival angle and the emission angle of the accurately acquired RIS channel, and the method has important significance for RIS reflection control and user positioning. Furthermore, due to the limitation of RIS complexity, research for the multi-carrier case is still less.
In summary, how to obtain RIS cascade channel CSI with high accuracy with small pilot overhead and how to achieve effective estimation of a separation channel in the RIS system become difficult problems faced by the RIS-assisted MIMO-OFDM system.
Disclosure of Invention
The technical problem is as follows: in order to solve the above problems, the present invention provides a method for reconstructing a split channel based on an RIS-assisted MIMO-OFDM system, which aims to achieve more accurate channel estimation with lower pilot overhead and achieve split channel reconstruction with higher accuracy.
The technical scheme is as follows: the invention discloses a reconstruction method of a reconfigurable intelligent surface auxiliary MIMO system separation channel, which comprises the following steps:
step 1: a user sends pilot frequencies of two time periods, wherein the pilot frequency of a single moment occupying all subcarriers is adopted in the first time period, and a reconfigurable intelligent surface RIS reflection coefficient is designed to ensure the performance; in the second time period, pilot frequency which is continuous in time and only occupies part of subcarriers is adopted, and a random RIS reflection coefficient is set;
and 2, step: the method comprises the steps that a RIS reflection signal reaches a base station, and the base station extracts direction angles, time delays and gain information of all propagation paths in a target user channel; the extraction process of the channel parameters is divided into two stages, and the base station completes the extraction of the angle and the time delay of the user-reconfigurable intelligent surface UE-RIS section according to the known sight distance LoS channel of the reconfigurable intelligent surface-base station RIS-BS section;
and step 3: the base station completes the extraction of non-line-of-sight (NLoS) path angles and time delays of the RIS-BS section according to the result in the step 2, respectively estimates gains and finally completes the estimation of all channel parameters;
and 4, step 4: the base station completes the reconstruction of the separation channel by using the parameters and is used for the user data transmission of the RIS cascade channel.
Wherein:
the step 1 specifically comprises the following steps:
step 1.1, a user divides two time periods to send pilot signals, and different RIS reflection coefficient configurations are adopted;
step 1.2, for the first time slot, the pilot signal sent by the user has a single moment but occupies all subcarriers, and simultaneously, the RIS reflection coefficient is set to ensure reliable channel estimation, specifically, the design of DFT matrix similar to discrete Fourier transform is adopted, wherein the reflection coefficient of the nth RIS unit is
Wherein N is the number of RIS units;
step 1.3, for the second slot, the pilot signal sent by the user has a setting of consecutive multiple instants but occupying part of the subcarriers, while a random RIS reflection coefficient is set.
The step 2 specifically comprises the following steps:
step 2.1, the base station reads the received pilot frequency signal of the first time quantum, designs a dictionary according to the known LoS channel time delay and angle parameters of the RIS-BS segment, completes the time delay estimation of each path of the UE-RIS segment by using a Newton orthogonal matching pursuit algorithm NOMP, and updates the residual signal; the base station circularly executes the step until the path energy is less than the set threshold value to obtain the estimated path number and enters the next step;
step 2.2, the base station reads the received pilot signal of the second time quantum, calculates to obtain a target matrix according to the known LoS channel time delay and angle of the RIS-BS segment and the estimated time delay of each path of the UE-RIS segment, designs a dictionary, and completes the angle estimation of the UE-RIS segment by using NOMP algorithm;
and 2.3, the base station constructs a measurement matrix according to the time delay parameter and the angle parameter of each path of the UE-RIS segment obtained in the step 2.1 and the step 2.2, re-estimates the path gain of the UE-RIS segment and updates the residual pilot signal of the second time period.
The step 3 specifically comprises the following steps:
step 3.1, the base station acquires the residual receiving pilot signals of the first time period, sequences the paths of the UE-RIS segment according to the gain intensity, selects the path parameters with the strongest gain, designs a dictionary, utilizes NOMP algorithm to complete the estimation of the receiving angle and the time delay of the RIS-BS segment, updates the residual signals and calculates the path energy of the RIS-BS segment;
step 3.2, the base station reads the residual receiving pilot signals of the second time period, calculates to obtain a target matrix according to the parameters of the UE-RIS segment strongest gain path obtained by sequencing and the receiving angle and time delay parameters of each path of the RIS-BS segment obtained by estimation, designs a dictionary, and utilizes NOMP algorithm to complete the estimation of the transmitting angle of each path of the RIS-BS segment;
step 3.3) the base station constructs a measurement matrix according to the transmitting angle, the receiving angle and the time delay parameter of the RIS-BS section obtained in the step 3.1 and the step 3.2, re-estimates the path gain of each path of the RIS-BS section and updates the residual signal of the second time period;
step 3.4) the base station circularly executes the step 3.1 to the step 3.3 until the path energy of the RIS-BS section obtained in the step 3.1 is less than the set threshold value gamma, and the estimation is finished.
Has the advantages that: by adopting the technical scheme, the invention can produce the following technical effects:
1. the scheme provided by the invention is based on sparse channels, develops a parameter estimation algorithm of a separation channel of an RIS-assisted MIMO OFDM system, can accurately acquire the angle, time delay and gain information of each path of the separation channel, and effectively recovers the separation channels at two sides of the RIS.
2. The scheme provided by the invention also provides a corresponding pilot frequency protocol and a RIS phase shift matrix configuration method aiming at a channel model of an RIS auxiliary MIMO OFDM system, effectively solves the problem of overlarge pilot frequency overhead in channel estimation of the RIS auxiliary system, and can further expand by fusing the situations of multiple users and the like.
Drawings
Fig. 1 is a pilot protocol diagram of an RIS-assisted MIMO-OFDM-based system according to an embodiment of the present invention;
fig. 2 is a flowchart of a channel parameter estimation algorithm based on the RIS-assisted MIMO-OFDM system according to an embodiment of the present invention.
Detailed Description
In the following description, for purposes of explanation and not limitation, specific details are set forth, such as particular pilot protocols, in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention may be practiced in other embodiments that depart from these specific details.
The invention discloses a reconstruction method of a reconfigurable intelligent surface auxiliary MIMO system separation channel, which comprises the following steps:
step 1: a user sends pilot frequencies of two time periods, wherein the pilot frequency of a single moment occupying all subcarriers is adopted in the first time period, and a reconfigurable intelligent surface RIS reflection coefficient is designed to ensure the performance; in the second time period, pilot frequency which is continuous in time and only occupies part of subcarriers is adopted, and a random RIS reflection coefficient is set;
step 2: the method comprises the steps that a RIS reflection signal reaches a base station, and the base station extracts direction angles, time delays and gain information of all propagation paths in a target user channel; the extraction process of the channel parameters is divided into two stages, and the base station completes the extraction of the angle and the time delay of the user-reconfigurable intelligent surface UE-RIS section according to the known sight distance LoS channel of the reconfigurable intelligent surface-base station RIS-BS section;
and step 3: the base station completes the extraction of non-line-of-sight (NLoS) path angles and time delays of the RIS-BS section according to the result in the step 2, respectively estimates gains and finally completes the estimation of all channel parameters;
and 4, step 4: the base station completes the reconstruction of the separation channel by using the parameters and is used for the user data transmission of the RIS cascade channel.
In order to make the objects, technical solutions and advantages of the present invention more apparent, the following detailed description is provided with reference to the accompanying drawings. In the method for reconstructing a separate channel based on an RIS-assisted MIMO-OFDM system, a Base Station (BS) and an RIS are both Uniform Linear Arrays (ULAs) and are respectively provided with M antennas and N reflecting units, and a single-antenna user access is realizedPassing N s The subcarrier transmits the signal to the base station end by taking RIS as the transfer, and totally occupies T time slots.
The received signal of the base station is represented as
Wherein->
Z＝[z 1 … z T ]Respectively a carrier coefficient matrix, an RIS coefficient matrix and a noise coefficient matrix. Wherein +>
Is a combined gain, is combined with>
In order to cascade the gains, the gain is,
is the phase shift vector->
Is the phase shift coefficient of the RIS at the t-th instant, z t Is the noise vector at the t-th time, L ur And L rb Respectively the number of UE-RIS paths and the number of RIS-BS paths, the subscripts respectively corresponding to the following l 1 And l 2 . Setting +>
Is a base station angle vector, d is a base station antenna spacing, and lambda is a carrier wavelength; setting +>
Is OFDM time delay vector, delta f is subcarrier interval; setting +>
Is the RIS angle vector, r is the RIS element spacing. />
Is the gain, receiving angle, time delay and transmitting angle of the q-th RIS-BS path,
is the gain, reception angle and delay of the p-th UE-RIS path. In one example, the base station terminal received signal at the t-th time may be represented as
1.1 For the first slot, as shown by the dotted box on the left in fig. 1, the pilot signal sent by the user has a single time instant but occupies all subcarriers, while a special RIS reflection coefficient is set to ensure reliable channel estimation. Particularly, the design of DFT-like matrix is adopted, wherein the reflection coefficient of the nth RIS unit is
And the like;
1.2 For the second time period, the pilot signal transmitted by the user has a continuous multiple time but occupies a part of the sub-carriers, as shown by the right-side hatched block in fig. 1, and a preset N is adopted in this time period u Number of subcarriers and actual path upper limit of UE-RIS subchannel
It is related. Is considered to be fullFoot-based or based on>
Is the minimum requirement that the estimation can be done, otherwise path separation errors may result, leading to estimation errors. The RIS unit is then set with random reflection coefficients and similar configurations.
2.1 ) the base station reads the received signal y of the first time period 1，res ＝y 1 Based on the known LoS channel delay and angle parameters of the RIS-BS segment, at an oversampling rate beta τ Configuring delay sample vectors
And design the dictionary
And using Newton's Orthogonal Matching Pursuit (NOMP) algorithm with D 1 Is a dictionary with y 1，res For the input signal to be estimated, the l-th signal in the UE-RIS segment is obtained 1 Time delay estimation value of strip path
And combine the gain estimates->
And updates the residual signal y 1，res Wherein->
The receiving angle and the time delay of the base station of the known RB-RIS segment LoS path，y 1，res Is the first time period remaining signal set in step 1. The base station executes the step circularly until the path energy is less than the set threshold value gamma, namely the path energy meets the requirement
Ending the loop to obtain the total path number of the estimated UE-RIS
And proceeds to step 2.2.
2.2 ) the base station reads the received signal Y of the second time period 2：T Calculating the phase shift vector according to the known LoS channel delay and angle of the RIS-BS segment and the delay of the UE-RIS segment obtained in step 2.1
Wherein
To use N u A delay vector for each restricted subcarrier. Calculating to obtain a target matrix
At an oversampling rate
Configuring an angular sampling vector->
And designs a sub-dictionary>
Considering the RIS phase shift matrix C of 2 to T time 2：T Designing a dictionary
Will be provided with
As input signal to be estimated for NOMP algorithm, with D 2 For dictionary, get the l' th in UE-RIS segment 1 Acceptance angle estimate for a strip path>
And the cascade gain estimate->
Co-cycling>
The path to all UE-RIS segments is estimated.
2.3 Based on all the estimated parameters obtained from 2.1 and 2.2, the base station constructs a measurement matrix
Wherein->
Re-estimating path gain for UE-RIS segment
And updating the residual signal of the second time period
Wherein +>
Is the receive signal of the second time period->
Combining the recombined results in columns to obtain an updated residual signalY′ res After that, the operation is reversed to obtain Y 2：T，res Where res represents the residual signal.
3.1 ) the base station acquires the remaining received signal y of the first time period 1，res Extracting the parameter of the strongest path of the UE-RIS segment gain in step 2, and the oversampling ratio beta θ Configuring delay sample vectors
Design dictionary->
Using NOMP algorithm, with D 3 Is a dictionary with y 1，res For the input signal to be estimated, the l-th signal in the RIS-BS segment is obtained 2 Time delay estimation value of strip path
Angle evaluation value->
And combined gain estimate>
And updates the residual signal y 1，res 。
If the estimated gain satisfies
Steps 3.2 and 3.3 are entered to continue the estimation of other parameters, otherwise the whole step 3 is stopped and the estimation ends.
3.2 ) the base station reads the remaining received signal Y of the second time period 2：T，res Calculating the phase shift vector by using the estimated path parameters with the strongest gain of the UE-RIS segment (the path with the strongest gain is selected by default according to the ranking of the path gain, so that the path with the strongest gain is represented by the index of 1) and the parameters obtained in 3.1
Calculating to obtain a target matrix
At an oversampling rate
Configuring an angular sampling vector->
Designing RIS angle dictionary>
And constructing a dictionary according to the phase shift matrix of the RIS
Will be provided with
Input signal to be estimated as NOMP algorithm, with D 4 For dictionary, get the l' th in RIS-BS section 2 Emission angle estimate for a strip path>
And cascade gain estimate>
3.3 The base station constructs a measurement matrix W according to the parameters obtained by the 3.1 and the 3.2
Re-estimating the path gain of a RIS-BS segment
And updates the residual signal of the second time period
Wherein Y' is the received signal Y of the second period 2：T，res The recombined results are combined in line to obtain Y 'after the updated signal' res Reverse operation gets updated Y 2：T，res 。
And 4, the base station completes the reconstruction of the cascade channel by using the parameters obtained by estimation:
reconstruction of UE-RIS channel:
and reconstruction of the RIS-BS channel:
and is used for user data transmission of RIS cascade channel and other related technologies.
The above description is only for the specific embodiments of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art can easily conceive of the changes or substitutions within the technical scope of the present invention, and all the changes or substitutions should be covered within the scope of the present invention. Therefore, the protection scope of the present invention shall be subject to the protection scope of the claims.
Claims (4)
1. A reconstruction method of a reconfigurable intelligent surface auxiliary MIMO system separation channel is characterized in that: the method comprises the following steps:
step 1: a user sends pilot frequencies of two time periods, wherein the pilot frequency of a single moment occupying all subcarriers is adopted in the first time period, and a reconfigurable intelligent surface RIS reflection coefficient is designed to ensure the performance; in the second time period, pilot frequency which is continuous in time and only occupies part of subcarriers is adopted, and a random RIS reflection coefficient is set;
step 2: the method comprises the steps that a RIS reflection signal reaches a base station, and the base station extracts direction angles, time delays and gain information of all propagation paths in a target user channel; the extraction process of the channel parameters is divided into two stages, and the base station completes the extraction of the angle and the time delay of the user-reconfigurable intelligent surface UE-RIS section according to the LoS channel with the known sight distance of the reconfigurable intelligent surface-base station RIS-BS section;
and step 3: the base station completes the extraction of non-line-of-sight (NLoS) path angles and time delays of the RIS-BS section according to the result in the step 2, respectively estimates gains and finally completes the estimation of all channel parameters;
and 4, step 4: the base station completes the reconstruction of the separation channel by using the parameters and is used for the user data transmission of the RIS cascade channel.
2. The method for reconstructing the separated channel of the reconfigurable intelligent surface-assisted MIMO system according to claim 1, wherein: the step 1 specifically comprises the following steps:
step 1.1, a user divides two time periods to send pilot signals, and different RIS reflection coefficient configurations are adopted;
step 1.2, for the first time slot, the pilot signal sent by the user has a single moment but occupies all subcarriers, and simultaneously, the RIS reflection coefficient is set to ensure reliable channel estimation, specifically, the design of DFT matrix similar to discrete Fourier transform is adopted, wherein the reflection coefficient of the nth RIS unit is
Wherein N is RIThe number of S units;
step 1.3, for the second slot, the pilot signal sent by the user has a setting of consecutive multiple instants but occupying part of the subcarriers, while a random RIS reflection coefficient is set.
3. The method for reconstructing the separated channel of the reconfigurable intelligent surface-assisted MIMO system according to claim 1, wherein: the step 2 specifically comprises the following steps:
step 2.1, the base station reads the received pilot frequency signal of the first time quantum, designs a dictionary according to the known LoS channel time delay and angle parameters of the RIS-BS segment, completes the time delay estimation of each path of the UE-RIS segment by using a Newton orthogonal matching pursuit algorithm NOMP, and updates the residual signal; the base station circularly executes the step until the path energy is less than the set threshold value to obtain the estimated path number and enters the next step;
step 2.2, the base station reads the received pilot signal of the second time quantum, calculates to obtain a target matrix according to the known LoS channel time delay and angle of the RIS-BS segment and the estimated time delay of each path of the UE-RIS segment, designs a dictionary, and completes the angle estimation of the UE-RIS segment by using NOMP algorithm;
and 2.3, the base station constructs a measurement matrix according to the time delay parameter and the angle parameter of each path of the UE-RIS segment obtained in the step 2.1 and the step 2.2, re-estimates the path gain of the UE-RIS segment and updates the residual pilot signal of the second time period.
4. The method for reconstructing the separated channel of the reconfigurable intelligent surface-assisted MIMO system according to claim 1, wherein: the step 3 specifically comprises the following steps:
step 3.1, the base station acquires the residual receiving pilot signals of the first time period, sequences the paths of the UE-RIS segment according to the gain intensity, selects the path parameters with the strongest gain, designs a dictionary, utilizes NOMP algorithm to complete the estimation of the receiving angle and the time delay of the RIS-BS segment, updates the residual signals and calculates the path energy of the RIS-BS segment;
step 3.2, the base station reads the residual receiving pilot signals of the second time period, calculates to obtain a target matrix according to the parameters of the UE-RIS segment strongest gain path obtained by sequencing and the receiving angle and time delay parameters of each path of the RIS-BS segment obtained by estimation, designs a dictionary, and utilizes NOMP algorithm to complete the estimation of the transmitting angle of each path of the RIS-BS segment;
step 3.3) the base station constructs a measurement matrix according to the transmitting angle, the receiving angle and the time delay parameter of the RIS-BS section obtained in the step 3.1 and the step 3.2, re-estimates the path gain of each path of the RIS-BS section and updates the residual signal of the second time period;
step 3.4) the base station circularly executes the step 3.1 to the step 3.3 until the path energy of the RIS-BS section obtained in the step 3.1 is less than the set threshold value gamma, and the estimation is finished.
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