CN111555782A - Mixed precoding design method based on multi-user millimeter wave MIMO-OFDM system - Google Patents

Mixed precoding design method based on multi-user millimeter wave MIMO-OFDM system Download PDF

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CN111555782A
CN111555782A CN202010154661.5A CN202010154661A CN111555782A CN 111555782 A CN111555782 A CN 111555782A CN 202010154661 A CN202010154661 A CN 202010154661A CN 111555782 A CN111555782 A CN 111555782A
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user
precoding
millimeter wave
mimo
ofdm system
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孙钢灿
谢金坤
郝万明
赵飞
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Zhengzhou University Industrial Research Institute Co ltd
Zhengzhou University
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Zhengzhou University
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413—MIMO systems
    • H04B7/0452—Multi-user MIMO systems
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B7/00—Radio transmission systems, i.e. using radiation field
    • H04B7/02—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas
    • H04B7/04—Diversity systems; Multi-antenna system, i.e. transmission or reception using multiple antennas using two or more spaced independent antennas
    • H04B7/0413—MIMO systems
    • H04B7/0456—Selection of precoding matrices or codebooks, e.g. using matrices antenna weighting

Abstract

The invention discloses a hybrid precoding design method based on a multi-user millimeter wave MIMO-OFDM system, which comprises the following steps: the first step is as follows: establishing a MIMO-OFDM system model; each user shares all M sub-carriers, and one base station has NTXAn antenna and NRXA radio frequency chain having a total of K single antenna users, where NRF≤NTX、K≤NRF(ii) a The second step is that: establishing a broadband millimeter wave channel model; in consideration of the channel characteristics of millimeter waves, a geometric channel model with L scattering paths is adopted, and one scattering path is arranged between each user and the base station; the third step: hybrid precoding for multi-user millimeter wave MIMO-OFDM systems. The full-connection and partial-connection hybrid precoding based on the codebook adopted in the invention well realizes the approach to the full-digital precoding performance in terms of performance, and gives play to the communication advantage and reduction of millimeter waves to a great extentLow system hardware cost and energy consumption.

Description

Mixed precoding design method based on multi-user millimeter wave MIMO-OFDM system
Technical Field
The invention relates to the technical field of radio transmission systems, in particular to a hybrid precoding design method based on a multi-user millimeter wave MIMO-OFDM system.
Background
With the increasing demand of mobile users for data in the future, the current cellular network (LTE) based on low frequency microwave communication will not meet such huge data demand, so the industry and academia propose to adopt a high frequency millimeter wave communication technology with ultra wide bandwidth. Millimeter wave communication is considered one of the most promising wireless communication technologies in future communication due to the ability to provide wider bandwidth and higher spectral efficiency. As is well known, in conventional MIMO systems, a dedicated Radio Frequency (RF) chain is typically required for each antenna to achieve all-digital signal processing. This would require the same number of RF chains using a large number of antennas in a millimeter wave massive MIMO system, which would result in unbearable hardware costs and energy consumption. To address this problem, Hybrid Precoding (HP) significantly reduces the number of RF chains required in mmwave massive MIMO systems without significant performance loss. The core idea of HP is to decompose an all-digital precoder into a high-dimensional analog precoder (implemented by analog circuitry) to increase antenna array gain, and a low-dimensional digital precoder (implemented by a small number of radio frequency chains) to cancel interference. The MIMO system can utilize multipath components in propagation to a certain extent, that is, MIMO can resist multipath fading, but for frequency selective deep fading, the MIMO system still cannot be used, and in order to solve the problem of frequency selective fading in the MIMO system, an Orthogonal Frequency Division Multiple Access (OFDMA) technology can be introduced to form a MIMO-OFDMA system. Most of the existing work is based on the mixed precoding design of a single-user narrow-band MIMO-OFDMA system, and in order to fully exert the advantage of large millimeter wave frequency spectrum range and combine the practical use condition, the mixed precoding design of the multi-user millimeter wave MIMO-OFDM system has profound significance in research.
Disclosure of Invention
The invention aims to provide a hybrid precoding design method based on a multi-user millimeter wave MIMO-OFDM system, which solves the problems in the background technology.
In order to achieve the purpose, the invention provides the following technical scheme: a mixed precoding design method based on a multi-user millimeter wave MIMO-OFDM system comprises the following steps:
the first step is as follows: establishing a MIMO-OFDM system model;
each user shares all M sub-carriers, and one base station has NTXAn antenna and NRXA radio frequency chain having a total of K single antenna users, where NRF≤NTX、K≤NRF；
The second step is that: establishing a broadband millimeter wave channel model;
in consideration of the channel characteristics of millimeter waves, a geometric channel model with L scattering paths is adopted, and one scattering path is arranged between each user and the base station;
the third step: hybrid precoding of a multi-user millimeter wave MIMO-OFDM system;
because the phase shifters used in the analog beamforming design are digitally controlled, the selectable angles are quantized and thus can be designed by codebook design.
Preferably, in order to solve the precoding problem in the wideband case, the precoding is performed in two cases, namely, in a full connection structure and in a sub-array partial connection structure.
Preferably, in a fully connected configuration, each rf chain is connected to all antennas through a phase shifter group.
Preferably, in the sub-array partial connection structure, the antenna NTXAccording to a radio frequency chain NRXDivided into N number of antennasSB＝NTX/NRFEach radio frequency chain is connected with the antenna subarray through a phase shifter group.
Compared with the prior art, the invention has the following beneficial effects: firstly, the method comprises the following steps: the method is different from the situation mainly discussing a narrow-band system, discusses the situation of a wide-band system, and better exerts the advantage of millimeter wave spectrum width. Second, unlike the case of mainly discussing a single-user MIMO system, the case of multiple users under actual use is considered, and a multi-user MIMO-OFDMA system is discussed. Thirdly, the method comprises the following steps: the overall system is based on hybrid precoding, and Hybrid Precoding (HP) significantly reduces the number of RF chains required in a mmwave massive MIMO system, and reduces hardware cost and energy consumption without significant performance loss. A large amount of experimental data show that the fully-connected and partially-connected hybrid precoding based on the codebook adopted in the invention well realizes the approach to the full-digital precoding performance in terms of performance, thereby exerting the communication advantage of millimeter waves to a great extent and reducing the hardware cost and energy consumption of the system.
Drawings
FIG. 1 is a diagram of a hybrid precoding system for a multi-user millimeter wave MIMO-OFDM system of the present invention;
FIG. 2 is a flow chart of the design of analog precoding under the full-connectivity architecture of the present invention;
FIG. 3 is a flow chart of the design of analog precoding under part of the connection structure of the present invention;
FIG. 4 is a graph showing the comparison of precoding effects of the two structures in the invention, namely, the total-digital precoding and the full-digital precoding, with the change of the signal-to-noise ratio when the number of the transmitting antennas is constant;
fig. 5 is a graph comparing the precoding effect of the two structures in the invention and the full-digital precoding with the change of the number of the base station transmitting antennas at a certain signal-to-noise ratio.
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.
Referring to fig. 1 to 5, the present invention provides a technical solution: the design overall thought of the invention is to divide a multi-user millimeter wave MIMO-OFDMA system into a full connection structure and a sub-array partial connection structure for analysis.
The first step is as follows: establishing a MIMO-OFDM system model;
each user shares all M sub-carriers,one base station has NTXAn antenna and NRXA radio frequency chain having a total of K single antenna users, where NRF≤NTX、K≤NRF. In the full-connection structure, each radio frequency chain is connected with all the antennas through a phase shifter group, and in the sub-array partial-connection structure, the antennas NTXAccording to a radio frequency chain NRxDivided into N number of antennasSB＝NTX/NRFEach radio frequency chain is connected with the antenna subarray through a phase shifter group. The downlink signals received at m subcarriers for all K users are shown in (1),
y[m]＝H[m]FRFFBB[m]x[m]+n[m]\*MERGEFORMAT (1)
wherein
Figure RE-GDA0002579440500000041
Represents the downlink channel matrix from the base station to the K users,
Figure RE-GDA0002579440500000042
Represents a wideband analog precoding matrix,
Figure RE-GDA0002579440500000043
Representing the mth sub-carrier digital baseband precoding matrix, xm]＝[x1[m]，x2[m]，...，xK[m]]TRepresenting a vector of transmitted signals, satisfying E { x [ m [ ]]x[m]H}＝Diag{σ1[m]，...，σK[m]}，σK[m]Representing the transmission energy between the mth subcarrier and the kth user, n m]＝[n1[m]，n2[m]，...，nK[m]]TRepresenting an additive white Gaussian noise vector, nK[m]Obeying complex Gaussian distributions
Figure RE-GDA0002579440500000044
To satisfy the constraint of total power, FRFAnd FBBShould satisfy the condition
Figure RE-GDA0002579440500000045
In the full connection structure
Figure RE-GDA0002579440500000046
In sub-array partial structure FRFIn the form of a block diagonal matrix, namely:
Figure RE-GDA0002579440500000047
wherein
Figure RE-GDA00025794405000000410
Representing N associated with the r-th radio frequency chainSB× 1, the analog beamforming vectors,
With the mth subcarrier, the signal-to-interference-plus-noise ratio (SINR) received by the kth user can be expressed as formula (3):
Figure RE-GDA0002579440500000048
wherein the SNRK[m]Representing the signal-to-noise ratio,
Figure RE-GDA0002579440500000049
the achievable rate between the mth carrier and the kth user can be expressed as (4):
Rk[m]＝E{log2(1+SINRk[m])} \*MERGEFORMAT (4)
the second step is that: establishing a broadband millimeter wave channel model;
in consideration of the channel characteristics of millimeter waves, a geometric channel model with L scattering paths is adopted, and one scattering path is arranged between each user and the base station. In this channel model, the channel delay vector between the kth user and the base station can be represented by equation (5):
Figure RE-GDA0002579440500000051
wherein in the full connection structureWhere N is equal to NTXIn the partial connection structure of the sub-array, N is NSB、αk，lRepresenting the gain of the ith path relative to the kth user, subject to an independent co-distribution αk，l～CN(0，1)、θk，l∈[0，2π]Denotes the departure Angle (AOD) of the ith path with respect to the kth user, p (τ) denotes the response of the pulse-forming filter at time τ, a (θ)kL) is relative to the angle θk，lA (θ) in a Uniform Linear Array (ULA)kL) can be represented by formula (6):
Figure RE-GDA0002579440500000052
wherein λ and β represent the signal wavelength and the antenna spacing, respectively;
in the sub-array partial structure, since the radio frequency chain is connected to the antennas of different sub-arrays, the antenna direction vector a (θ)kL) is composed of a plurality of sub-antenna direction vectors, represented by formula (7):
Figure RE-GDA0002579440500000053
wherein
Figure RE-GDA0002579440500000054
Through the channel delay vector (5), the channel vector between the base station and the user at the mth subcarrier can be expressed as formula (8):
Figure RE-GDA0002579440500000061
the third step: hybrid precoding for multi-user millimeter wave MTMO-OFDM systems
Since the phase shifters used in the analog beamforming design are digitally controlled and the selectable angles are quantized, the angle design can be performed by a codebook design, and the codebook and the antenna direction vectors in equation (6) have the same structure. In this case, analog precoding may design each rf chain through a codebook, and in digital precoding, conventional zero-forcing (ZF) precoding is used to eliminate interference between multiple users.
Although various schemes exist for precoding codebooks for multi-user millimeter waves, the schemes are based on a narrowband system, a main difference exists between the design of the narrowband system and the design of a wideband system, and the weight of analog precoding is the same in the whole frequency band, so that the whole subcarrier is considered instead of only one subcarrier. In order to solve the precoding problem under the broadband condition, the following processing is carried out under two conditions of a full connection structure and a sub-array part connection structure:
in the first case: a full connection structure;
with reference to expressions (5) to (8), it can be seen that the antenna direction vector a (θ) of all subcarriers in the first cluster isk，l) Are the same, the codebook design for all subcarriers for the kth user can be expressed as
Figure RE-GDA0002579440500000063
Because the weights of all sub-carriers in the same radio frequency chain in analog precoding must be the same, each sub-carrier cannot occupy a dedicated analog precoding. Therefore, we first assign a radio frequency chain to a user and then select an analog precoding by codebook to maximize the channel gain of the downlink for all subcarriers, i.e.:
Figure RE-GDA0002579440500000062
to ensure fairness, it is assumed that the radio frequency chains allocated to each user are the same, i.e.: n is a radical ofSUB＝NRFand/K, different from each user being allocated a radio frequency chain, the phase offset in different analog precodes of the same user is different. The analog precoding of the full-link structure can be realized by algorithm 1, and the process is as follows:
the method comprises the following steps: according to the number K, F of usersk，FRFInitializing the codebook;
step two: entering a cyclic decision structure by passing the gain of the downlink of each radio frequency chain for each user according to equation (9)
Figure RE-GDA0002579440500000077
The updating is carried out, and the updating is carried out,
step three: until the channel gain of the downlink of all sub-carriers is found to be maximized, output FRF，
Then, according to a predefined codebook, finding out an optimal simulation precoding matrix FRFThen, the equivalent mth subcarrier downlink channel matrix can be obtained:
Figure RE-GDA0002579440500000072
digital precoding matrix FBB[m]The method can be obtained by eliminating interference among multiple users in a traditional zero-forcing precoding mode, namely:
Figure RE-GDA0002579440500000073
wherein
Figure RE-GDA0002579440500000074
In the second case: a sub-array partial connection structure;
unlike the fully-connected structure, there is only one steering direction in each antenna subset, and therefore, the phase steering for the kth user in the codebook is represented as:
Figure RE-GDA0002579440500000075
in this case, formula (9) is converted to formula (11):
Figure RE-GDA0002579440500000076
wherein h isk，s[m]Indicating the channel parameters of the mth antenna subset and the kth user in the mth sub-carrier. Because of the fact thatThe antenna subset associated with each radio frequency chain is allocated to only one user, and we first allocate all antenna subsets to K users, and in order to increase the total downlink transmission rate, the antenna subset with the maximum total downlink channel gain is allocated to the user by sub-carriers, that is:
Figure RE-GDA0002579440500000081
combining equation (12), obtaining the optimal analog precoding of the subarray partial connection structure by algorithm 2, the process is as follows:
the method comprises the following steps: according to the number K, F of usersk，s，FRFInitializing the codebook;
step two: entering a cyclic judgment structure, allocating the antenna subset s to the user k according to the formula (12), and searching according to the formula (11)
Figure RE-GDA0002579440500000082
Assign steering vectors to user k by
Figure RE-GDA0002579440500000083
Figure RE-GDA0002579440500000084
Updating n-n + 1;
step three: until finding the optimal simulation precoding effect of each user, outputting FRF，
Obtaining an analog precoding matrix FRFAnd then, the interference among multiple users is eliminated by adopting the traditional zero-forcing precoding.
The invention has the advantages. Firstly, the method comprises the following steps: the method is different from the situation mainly discussing a narrow-band system, discusses the situation of a wide-band system, and better exerts the advantage of millimeter wave spectrum width. Second, unlike the case of mainly discussing a single-user MIMO system, the case of multiple users under actual use is considered, and a multi-user MIMO-OFDMA system is discussed. Thirdly, the method comprises the following steps: the overall system is based on hybrid precoding, and Hybrid Precoding (HP) significantly reduces the number of RF chains required in a mmwave massive MIMO system, and reduces hardware cost and energy consumption without significant performance loss. A large amount of experimental data show that the fully-connected and partially-connected hybrid precoding based on the codebook adopted in the invention well realizes the approach to the full-digital precoding performance in terms of performance, thereby exerting the communication advantage of millimeter waves to a great extent and reducing the hardware cost and energy consumption of the system.
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 (4)

1. A mixed precoding design method based on a multi-user millimeter wave MIMO-OFDM system is characterized by comprising the following steps: the method comprises the following steps:
the first step is as follows: establishing a MIMO-OFDM system model;
each user shares all M sub-carriers, and one base station has NTXAn antenna and NRXA radio frequency chain having a total of K single antenna users, where NRF≤NTX、K≤NRF；
The second step is that: establishing a broadband millimeter wave channel model;
in consideration of the channel characteristics of millimeter waves, a geometric channel model with L scattering paths is adopted, and one scattering path is arranged between each user and the base station;
the third step: hybrid precoding of a multi-user millimeter wave MIMO-OFDM system;
because the phase shifters used in the analog beamforming design are digitally controlled, the selectable angles are quantized and thus can be designed by codebook design.
2. The method of claim 1 for designing a hybrid precoding based on a multi-user millimeter wave MIMO-OFDM system, wherein: in order to solve the precoding problem under the broadband condition, the precoding is carried out under two conditions of a full connection structure and a sub-array part connection structure.
3. The method of claim 2, wherein the method comprises: in a fully connected configuration, each rf chain is connected to all antennas through a phase shifter group.
4. The method of claim 3, wherein the method comprises: in the sub-array partial connection structure, an antenna NTXAccording to a radio frequency chain NRXDivided into N number of antennasSB＝NTX/NRFEach radio frequency chain is connected with the antenna subarray through a phase shifter group.
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