Precoding method and device for maximizing weighted sum rate in wireless communication system using multi-user multi-input multi-output

Abstract

The present disclosure provides a precoding method and device for maximizing a weighted sum rate for a plurality of UEs in a wireless communication system using MU-MIMO, and a precoder design method therefor. According to an embodiment of the present disclosure, a base station in a wireless communication system using MIMO may comprise: a transmitter; one or more processors including a processing circuit; and a memory for storing instructions, wherein the instructions cause, when executed individually or collectively by the one or more processors, the base station to: acquire a first linear combiner for a virtual uplink at each iteration for precoder determination; acquire a first weight matrix for the virtual uplink at each iteration; and on the basis of the first linear combiner and the first weight matrix for the virtual uplink, which are acquired at each iteration, determine a downlink precoder satisfying an SPC indicating a total power limit of a plurality of transmission antennas of the base station, wherein the downlink precoder is applied to downlink transmission through the transmitter.

Description

Precoding method and device for maximizing the sum of weighted transmission rates in a wireless communication system using multi-user multi-input multi-output

[0001] The present disclosure relates to a precoding method and device in a wireless communication system using MIMO (multiple-input multiple-output).

[0002] To meet the increasing demand for wireless data traffic following the 4G system (i.e., the LTE (long-term evolution) system), the 5G system has been developed and commercialized. The 5G system can be implemented in ultra-high frequency (mmWave and / or tera-hertz) bands. To mitigate the path loss of radio waves in ultra-high frequency bands and increase the transmission distance of radio waves, beamforming, massive MIMO (massive MIMO), full-dimensional MIMO (FD-MIMO), array antenna, analog beamforming, and large-scale antenna technologies are being discussed in 5G systems.

[0003] Furthermore, how to efficiently support multiple user equipment (UEs) in wireless communication systems has always been a key issue in the field of wireless communications, and various related studies have been conducted in industry and academia for decades. Furthermore, utilizing multiple antennas can increase spectral efficiency (SE), so MIMO systems that support multiple antennas are widely utilized in wireless communication systems. Recently, extreme large-scale MIMO (XL-MIMO), which uses even more antennas than the massive MIMO of the fifth generation new radio (5G NR), which uses tens or more than 100 antennas, has been attracting attention as a core technology for sixth generation wireless communications. Therefore, multi-user multiple-input multiple-output (MU-MIMO) technology is a very important technology for current and future wireless communication systems.

[0004] The present disclosure provides a precoding method and device for maximizing a weighted sum rate for multiple terminals (UEs) in a wireless communication system using MU-MIMO, a precoder design method therefor, and a storage medium therefor.

[0005] In addition, the present disclosure provides a method and device for determining a precoder that satisfies the sum power constraint (SPC) of a base station transmission antenna in a wireless communication system using MU-MIMO, and a precoder design method therefor.

[0006] In addition, the present disclosure provides a method and device for determining a precoder that satisfies a per-antenna power constraint (PAPC) in a wireless communication system using MU-MIMO, and a precoder design method therefor.

[0007] In accordance with an embodiment of the present disclosure, a base station in a wireless communication system utilizing MIMO includes a transmitter, one or more processors including processing circuitry, and a memory storing instructions, wherein the instructions, when individually or collectively executed by the one or more processors, cause the base station to obtain a first linear combiner for a virtual uplink at each iteration for precoder determination. In one embodiment, the instructions, when individually or collectively executed by the one or more processors, cause the base station to obtain a first weight matrix for the virtual uplink at each iteration. In one embodiment, the instructions, when individually or collectively executed by the one or more processors, cause the base station to determine a downlink precoder that satisfies an SPC representing a total power limitation of a plurality of transmit antennas of the base station based on the first linear combiner and the first weight matrix for the virtual uplink obtained at each iteration. In one embodiment, the downlink precoder can be applied to downlink transmission via the transmitter.

[0008] In one embodiment, the instructions, when individually or collectively executed by the one or more processors, may further cause the base station to obtain a second linear combiner for the downlink and a second weight matrix for the downlink, obtain a precoder for the virtual uplink using the second linear combiner and the second weight matrix, and obtain the first linear combiner and the first weight matrix based on the precoder for the virtual uplink.

[0009] In one embodiment, the instructions, when individually or collectively executed by the one or more processors, may cause the base station to determine the downlink precoder to maximize a weighted sum of transmission rates representing the sum of transmission rates of a plurality of terminals in the downlink considering terminal-specific transmission rate weights in an MU-MIMO environment.

[0010] In one embodiment, the downlink precoder can be applied to the baseband precoder in a hybrid beamforming structure including a baseband precoder and an RF precoder.

[0011] As an example, the linear combiner may use a linear minimum mean square error (LMMSE) combiner, and the weight matrix may use a mean square error (MSE) weight matrix.

[0012] In accordance with an embodiment of the present disclosure, in a wireless communication system utilizing MIMO, a base station includes a transmitter, one or more processors including a processing circuit, and a memory storing instructions, which when individually or collectively executed by the one or more processors cause the base station to obtain a first linear combiner for a virtual uplink at each iteration for precoder determination. In one embodiment, the instructions, when individually or collectively executed by the one or more processors, cause the base station to obtain a first weight matrix for the virtual uplink at each iteration. In one embodiment, the instructions, when individually or collectively executed by the one or more processors, cause the base station to determine a first downlink precoder satisfying an SPC representing a total power limitation of a plurality of transmit antennas of the base station based on the first linear combiner and the first weight matrix for the virtual uplink obtained at each iteration. In one embodiment, the instructions, when individually or collectively executed by the one or more processors, may cause the base station to determine, based on the first downlink precoder satisfying the SPC obtained for each iteration, a second downlink precoder satisfying a per-antenna power limitation (PAPC) of the plurality of transmit antennas of the base station. In one embodiment, the second downlink precoder may be applied to downlink transmission via the transmitter.

[0013] In one embodiment, the instructions, when individually or collectively executed by the one or more processors, may further cause the base station to obtain a second linear combiner for the downlink and a second weight matrix for the downlink, obtain a precoder for the virtual uplink using the second linear combiner and the second weight matrix, and obtain the first linear combiner and the first weight matrix based on the precoder for the virtual uplink.

[0014] In one embodiment, the instructions, when individually or collectively executed by the one or more processors, may cause the base station to determine the second downlink precoder to maximize a weighted sum of transmission rates representing the sum of transmission rates of a plurality of terminals in the downlink considering terminal-specific transmission rate weights in an MU-MIMO environment.

[0015] In one embodiment, the instructions, when individually or collectively executed by the one or more processors, may further cause the base station to perform normalization satisfying the PAPC at each iteration and to apply values ​​used in the normalization at each iteration to the determination of the first downlink precoder satisfying the SPC at the next iteration.

[0016] In one embodiment, the linear combiner may use an LMMSE combiner, and the weight matrix may use an MSE weight matrix.

[0017] In accordance with an embodiment of the present disclosure, a computer-readable storage medium storing at least one instruction, wherein the at least one instruction, when individually or collectively executed by one or more processors including a processing circuit of a base station, causes the base station to perform at least one operation, wherein the at least one operation may include obtaining a first linear combiner for a virtual uplink at each iteration for precoder determination. In one embodiment, the at least one operation may include obtaining a first weight matrix for the virtual uplink at each iteration. In one embodiment, the at least one operation may include determining a downlink precoder, based on the first linear combiner and the first weight matrix for the virtual uplink obtained at each iteration, that satisfies an SPC representing a total power limit of a plurality of transmit antennas of the base station. In one embodiment, the downlink precoder may be applied to downlink transmission.

[0018] Figure 1 is a diagram showing the time-frequency domain transmission structure in a 5G system.

[0019] Figure 2 is a diagram showing the frame, subframe, and slot structure in a 5G system.

[0020] FIG. 3 is a drawing for explaining a hybrid beamforming structure in a wireless communication system to which the present disclosure is applied.

[0021] FIG. 4 is a diagram for explaining a transmission operation of a base station supporting multiple UEs in a wireless communication system using MU-MIMO to which an embodiment of the present disclosure is applied.

[0022] FIG. 5 is a diagram for explaining the reception operation of a UE in a wireless communication system using MU-MIMO to which an embodiment of the present disclosure is applied.

[0023] FIG. 6 is a flowchart conceptually illustrating a linear precoder design method satisfying SPC in a wireless communication system using MU-MIMO according to an embodiment of the present disclosure;

[0024] FIG. 7 is a flowchart conceptually illustrating a method for designing a linear precoder that satisfies PAPC in a wireless communication system using MU-MIMO according to an embodiment of the present disclosure.

[0025] FIG. 8 is a flowchart specifically illustrating a linear precoder design method that satisfies SPC in a wireless communication system using MU-MIMO according to an embodiment of the present disclosure.

[0026] FIG. 9a and FIG. 9b are flowcharts specifically illustrating a method for designing a linear precoder that satisfies PAPC in a wireless communication system using MU-MIMO according to an embodiment of the present disclosure.

[0027] Figure 10 is a diagram showing the communication environment assumed in the performance simulation of the present disclosure.

[0028] Figures 11a and 11b are diagrams illustrating simulation results of a precoder design method satisfying SPC conditions according to an embodiment of the present disclosure;

[0029] Figures 12a and 12b are diagrams illustrating simulation results of a precoder design method satisfying PAPC conditions according to an embodiment of the present disclosure, and

[0030] FIG. 13 is a diagram illustrating an example configuration of a network entity in a wireless communication system according to an embodiment of the present disclosure.

[0031] The operating principles of the present disclosure are described in detail below with reference to the attached drawings. In the following description of the present disclosure, detailed descriptions of related known functions or configurations will be omitted if they are deemed to unnecessarily obscure the gist of the present disclosure. Furthermore, the terms described below are defined based on the functions of the present disclosure and may vary depending on the intent or custom of the user or operator. Therefore, their definitions should be based on the overall content of this specification.

[0032] The advantages and features of the present disclosure, and methods for achieving them, will become clearer with reference to the embodiments described in detail below together with the accompanying drawings. However, the present disclosure is not limited to the embodiments disclosed below and may be implemented in various different forms. These embodiments are provided solely to ensure that the disclosure of the present disclosure is complete and to fully inform those skilled in the art of the scope of the invention, and the present disclosure is defined only by the scope of the claims. Like reference numerals designate like elements throughout the specification.

[0033] At this time, it will be understood that each block of the processing flow diagrams and combinations of the flow diagrams can be performed by computer program instructions.

[0034] Additionally, each block may represent a module, segment, or portion of code that contains one or more executable instructions for performing a specific logical function(s). It should also be noted that in some alternative implementation examples, the functions described in the blocks may occur out of order. For example, two blocks depicted in succession may actually be executed substantially concurrently, or the blocks may sometimes be executed in reverse order, depending on their respective functions.

[0035] Here, the term '~ part' used in this embodiment means software or hardware components such as FPGA (field programmable gate array) or ASIC (application specific integrated circuit), and the '~ part' performs certain roles. However, the '~ part' is not limited to software or hardware. The '~ part' may be configured to be on an addressable storage medium and may be configured to play one or more processors. Therefore, as an example, the '~ part' includes components such as software components, object-oriented software components, class components, and task components, processes, functions, properties, procedures, subroutines, segments of program code, drivers, firmware, microcode, circuits, data, databases, data structures, tables, arrays, and variables. The functions provided within the components and '~ parts' may be combined into a smaller number of components and '~ parts' or further separated into additional components and '~ parts'. Additionally, the components and '~parts' may be implemented to activate one or more CPUs within a device or secure multimedia card. In addition, in an embodiment, the '~parts' may include one or more processors.

[0036] In this disclosure, phrases such as "A / B", "A or B", "A and / or B", "at least one of A and B", "at least one of A or B", "A, B, or C", "at least one of A, B, and C", and "at least one of A, B, or C" can each include any one of the items listed together in that phrase, or all possible combinations thereof. Terms such as "first", "second", or "first" or "second" may be used merely to distinguish the corresponding component from other corresponding components and do not limit the corresponding components in any other respect (e.g., importance or order).

[0037] The terms used in the following description to identify connection nodes, terms referring to network entities, terms referring to messages, terms referring to interfaces between network entities, and terms referring to various identification information are provided for convenience of explanation. Therefore, the present disclosure is not limited to the terms described below, and other terms referring to objects with equivalent technical meanings may be used.

[0038] In the present disclosure, a base station (BS) is a network entity that performs resource allocation of a terminal and can communicate with the terminal through a wireless network, and may be at least one of an eNode B, a Node B, a gNB, a RAN (radio access network), an AN (access network), a RAN node, an IAB (integrated access / backhaul) node, a radio access unit, a base station controller, a node on a network, or a TRP (transmission reception point). A user equipment (UE) may be at least one of a terminal, an MS (mobile station), a cellular phone, a smartphone, a computer, or a multimedia system capable of performing a communication function.

[0039] For convenience of explanation, some terms and names defined in the 3GPP NR standard may be used. However, the present disclosure is not limited to these terms and names, and can be equally applied to systems conforming to other standards.

[0040] Figure 1 is a diagram illustrating an example of the basic structure of time-frequency resources of a 5G system.

[0041] Referring to Figure 1, the horizontal axis represents the time domain and the vertical axis represents the frequency domain. In the time and frequency domains, the basic unit of resources is a resource element (RE, 101), which can be defined as 1 OFDM (orthogonal frequency division multiplexing) symbol (102) in the time axis and 1 subcarrier (103) in the frequency axis. In the frequency domain (For example, 12) consecutive REs can form one resource block (RB, 104). In one embodiment, multiple OFDM symbols can form one subframe (110). In FIG. 1 is the number of OFDM symbols per subframe (110) for setting the subcarrier spacing (μ), and a more specific description of the resource structure in a 5G system can be referred to the TS 38.211 section 4 specification.

[0042] Figure 2 is a diagram showing an example of the frame, subframe, and slot structure of a 5G system.

[0043] Referring to FIG. 2, one frame (Frame, 200) may be composed of one or more subframes (subframes, 201), and one subframe may be composed of one or more slots (slots, 202). For example, one frame (200) may be defined as 10 ms. One subframe (201) may be defined as 1 ms, in which case one frame (2-00) may be composed of a total of 10 subframes (201). One slot (202, 203) may be defined as 14 OFDM symbols (i.e., the number of symbols per slot . 1 subframe (201) may be composed of one or more slots (2-02, 2-03), and the number of slots (202, 203) per subframe (201) may vary depending on the setting value μ (204, 205) for the subcarrier spacing. In the example of Fig. 2, the cases where μ = 0 (204) and μ = 1 (205) as the subcarrier spacing setting value are illustrated. For example, when μ = 0 (204), 1 subframe (201) may be composed of one slot (202), and when μ = 1 (205), 1 subframe (201) may be composed of two slots (203). That is, the number of slots per subframe ( ) may vary, and accordingly the number of slots per frame ( ) may vary.

[0044] FIG. 3 is a diagram for explaining a hybrid beamforming structure in a wireless communication system to which the present disclosure is applied.

[0045] Beamforming can be applied to MIMO (multiple-input and multiple-output) technology that can support enhanced transmission capacity for a large number of users, thereby forming multiple beams. Referring to FIG. 3, in a 5G system, so-called hybrid beamforming (hybrid BF) (300) can be used, which combines digital beamforming (digital BF) (310), which changes the phase and / or amplitude of a signal through digital signal processing, and analog beamforming (analog BF) (320), which forms analog beams with various beam directions and beam widths by changing the phase and / or amplitude of an analog signal. The hybrid beamforming technology can provide a large antenna gain while reducing the complexity of implementation. As in the example of FIG. 3, a block for processing digital beamforming (digital BF) (310) may include a baseband digital precoder (hereinafter simply referred to as a baseband precoder or BB precoder) (311), an inverse fast Fourier transform (IFFT) unit, and a parallel / serial (P / S) conversion unit. The IFFT unit and the P / S conversion unit may be included in a number corresponding to the number of radio frequency (RF) chains, and each RF chain may be implemented by including a digital analog converter (DAC) and a mixer. In the example of FIG. 3, a block for processing analog beamforming (analog BF) (320) may include a plurality of analog phase shifters (321), power amplifiers (PAs), and antenna arrays connected to each RF chain.The above-described plurality of analog phase shifters (321) enable beam sweeping over a wide range of angles. The antenna array includes a plurality of antenna elements, and the number of RF chains increases corresponding to the number of antenna arrays. In a 5G system, a transmitter that forms a transmission beam may be implemented by including the above-described digital beamforming (digital BF) block, a plurality of RF chains, and a plurality of analog beamforming (analog BF) blocks, and a receiver that forms a reception beam and receives a signal from the transmitter may be implemented by including a plurality of analog beamforming (analog BF) blocks, a plurality of RF chains, and a digital beamforming (digital BF) block corresponding to the configuration of the transmitter. The precoder to be described in the embodiments of the present disclosure may be implemented in the baseband digital precoder (311). In the example of FIG. 3, the transmitter may be a transmitter of a base station performing precoding, and the receiver may be a receiver of a terminal. As another example, the transmitter may be a transmitter of a terminal performing precoding, and the receiver may be a receiver of a base station. The precoding may be applied to downlink transmission on a downlink channel, such as a physical downlink shared channel (PDSCH) or a physical downlink control channel (PDCCH). In addition, the precoding may be applied to uplink transmission on an uplink channel, such as a physical uplink shared channel (PUSCH) or a physical uplink control channel (PUCCH).The reference signal transmitted from the transmitter based on the beamforming may be, for example, at least one of a demodulation-reference signal (DM-RS), a channel state information-reference signal (CSI-RS), and a sounding reference signal (SRS).

[0046] In wireless communication systems utilizing MIMO (hereinafter referred to as MIMO systems), the potential spectral efficiency enhancement of MU-MIMO is well known through capacity analysis, which is the upper limit of transmission rate. Furthermore, theoretically, MU-MIMO capacity can be achieved through the well-known dirty paper coding (DPC). However, DPC is a non-linear precoding technique with high design complexity, making implementation difficult.

[0047] Therefore, in order to actually utilize the potential of MIMO, an efficient, low-complexity linear precoding design method is needed instead of a complex nonlinear precoding technique. Unlike the optimal linear combiner in a MIMO system, the transmitter precoder must satisfy the transmission power constraint, making the precoder design problem more difficult than the linear combiner design problem. The transmission power constraint can be understood, for example, as the maximum transmission power of the base station (or the maximum available transmission power) in the downlink or the total radiated power constraint according to regulations including mobile communication standards. In wireless communication systems, there are two commonly considered power transmission constraint methods. The first method is the sum power constraint (SPC) method of the transmitter's transmission antenna elements, and the second is the per-antenna power constraint (PAPC) method of the transmitter.

[0048] In single-user MIMO (SU-MIMO) systems, it is known that capacity can be achieved through linear precoder design based on singular value decomposition (SVD) and power allocation based on water-filling. However, in MU-MIMO, there is a coupling problem between precoders for different users from a precoding design perspective, so the precoder design problem is more difficult than in SU-MIMO systems. In addition, the precoder design problem for maximizing the weighted sum rate (WSR) in MU-MIMO systems is a mathematically non-convex problem, making it very difficult to obtain a globally optimal solution. The weighted sum rate refers to the sum of the transmission rates of multiple UEs in the downlink considering the transmission rate weights for each UE, for example. The transmission rate weights for each UE can be, for example, rate weights according to the priority of each UE in downlink transmission. Considering the fairness of UEs, if there is a difference in cumulative transmission rates between UEs at any point, the UE with the lower cumulative transmission rate can be given a higher weight, thereby reducing the difference. Another example is that if the difference in downlink data required by each UE is large, the UE with the higher data rate can be given a higher weight, thereby improving overall system efficiency.

[0049] Among the precoder design methods for maximizing the weighted sum rate in MU-MIMO systems, the precoder design methods known to have excellent performance to date are mainly based on the identity of the weighted sum rate maximization problem and the weighted mean square error (MSE) minimization problem from the precoder design perspective.

[0050] Since this problem is fundamentally non-convex, most of the excellent existing precoder design methods are for finding local optimal solutions (non-convex), making it very difficult to find a global optimal solution. One approach is to iteratively find a locally optimal solution based on the block coordinate descent (BCD) method, which alternately updates the receiver's linear minimum mean square error (LMMSE) combiner, weight matrix, and precoder at each iteration.

[0051] In one example of a previous study on precoder design methods, it was revealed that the weighted sum rate maximization problem and the weighted MSE minimization problem can be identical in terms of designing a precoder that satisfies the SPC in MU-MIMO systems. Furthermore, a precoder design method based on the BCD method, which alternately updates the receiver's LMMSE combiner, weight matrix, and precoder for each iteration of the precoder design, was introduced. The precoder designed using this precoding technique is well known as a weighted-MMSE (WMMSE) precoder for WSR maximization.

[0052] In another example of the existing research on the precoder design method, a precoder design method to maximize the weighted sum rate in a single-cell environment was discussed. The precoder design method proposed in another example of the existing research suggests a method to design a precoder that satisfies SPC and a precoder that satisfies PAPC. The precoder design method proposed in another example of the existing research is that when the downlink channel is H, the non-trivial precoding matrix F is H, which is the conjugate transpose (or Hermitian transpose) matrix of H. H By using the fact that the matrix X can be expressed as a product of a matrix and a matrix X, a precoder design algorithm that can compute a weighted-MMSE precoder with low complexity in a massive MIMO environment is proposed. (The dimension of the matrix X is the total number of antennas of UEs × the total number of data layers of UEs.) In addition, an algorithm for designing a precoder that satisfies PAPC is proposed in a way that the gradient of the objective function is obtained as a value for each row vector of F in the precoder design that satisfies SPC and the corresponding row vectors are updated with the local optimum solution.

[0053] Existing precoding (or precoding design) methods have the problem that the number of design iterations required to obtain the local optimum solution is very large, and the weighted transmission rate sum performance of the final designed precoder varies greatly depending on the initial precoder.

[0054] The present disclosure relates to a precoder design method (i.e., a precoding method) for maximizing the sum of weighted transmission rates in an MU-MIMO system, and the precoding design methods proposed in the embodiments of the present disclosure below can design a precoder relatively faster than existing precoder design methods, and can enable the design of a precoder having excellent performance while being insensitive to initial precoder settings.

[0055] Additionally, embodiments of the present disclosure can be utilized as a method for designing a low-complexity baseband (BB) precoder in a MIMO system utilizing analog-digital hybrid beamforming (HBF), such as the example of FIG. 3.

[0056] The precoder design method (i.e., precoding method) proposed in this disclosure is a precoder design method for maximizing the sum of weighted transmission rates. It is applicable to communication systems using MU-MIMO systems, and thus has a very wide range of applications. Furthermore, when the weights of all transmission rates are equal, the sum of the weighted transmission rates becomes the sum transmission rate, and thus, it can be applied to a transmitter precoder design method for maximizing the total transmission rate of a communication system with limited transmission power.

[0057] The precoder design method proposed in this disclosure has implementation advantages as it enables precoder design relatively quickly compared to existing precoder design methods and enables the design of a precoder with excellent performance while being insensitive to initial precoder settings. Furthermore, the precoder design method proposed in this disclosure can be used as a BB precoder design in a transmitter with a hybrid beamforming structure, such as the example of FIG. 3, making it applicable to 5G and subsequent generations of mobile communications.

[0058] The precoder design method satisfying SPC proposed in the present disclosure can design a downlink precoder based on an LMMSE combiner obtained / determined / calculated for a virtual uplink channel and an uplink mean square error (MSE)-weight matrix obtained / determined / calculated according to the LMMSE combiner for each iteration, which is different from existing precoder design methods.

[0059] In addition, the precoder design method that satisfies SPC proposed in the present disclosure is distinguished from the existing precoder design method in that it defines a virtual uplink in the aforementioned weighted sum rate (WSR) maximization problem and obtains / determines / calculates an LMMSE combiner for the virtual uplink and an uplink MSE-weight matrix according to the LMMSE combiner. In addition, the precoder design method that satisfies SPC proposed in the present disclosure can obtain / determine / calculate a precoder of UE(s) for a virtual uplink based on an LMMSE combiner for a downlink channel and a downlink MSE-weight matrix according to the LMMSE combiner in a precoding method for downlink.

[0060] In the precoder design method that satisfies PAPC proposed in this disclosure, first, the sum of power limitations for each antenna is assumed to be SPC, a downlink precoder is designed using the precoder design method that satisfies SPC proposed in this disclosure, and a precoder that satisfies PAPC can be designed by multiplying a downlink precoding matrix that satisfies SPC by a diagonal matrix for satisfying PAPC for each antenna.

[0061] In addition, similarly to the method for designing a precoder satisfying SPC proposed in the present disclosure, a precoder for a virtual uplink can be first designed based on the LMMSE combiner for the downlink channel and the downlink MSE-weight matrix according to the LMMSE combiner for the UEs for the virtual uplink. In addition, in the present disclosure, in the process of designing a precoder satisfying SPC, a channel obtained by multiplying the channel matrix by the diagonal matrix previously used in designing a precoder satisfying PAPC instead of the original wireless channel matrix is ​​assumed as an effective channel, and the LMMSE combiner and MSE-weight matrix of the virtual uplink can be obtained / determined / calculated.

[0062] The precoder design method proposed in this disclosure has an implementation advantage in that it can design a precoder faster than the existing precoder design method(s) for maximizing the weighted sum rate in an MU-MIMO downlink system.

[0063] In addition, the precoder design technique proposed in this disclosure is not sensitive to the initial precoder settings compared to the existing precoder design method(s) described above, so the precoder design algorithm is stable.

[0064] In addition, the applicant's simulation results, which will be described later, confirm that a precoder with excellent performance can be designed even with a very small number of iterations in a massive MIMO environment.

[0065] Embodiments of the present disclosure propose a precoder design method for maximizing a weighted sum rate in an MU-MIMO downlink system, and a sum power constraint (SPC) method and / or a per-antenna power constraint (PAPC) method of base station transmit antennas may be considered in the precoder design.

[0066] Hereinafter, embodiments of the present disclosure will be described in detail. First, the definitions of functions / variables / operators included in the mathematical formulas used in the description of embodiments of the present disclosure are as shown in the example in [Table 1] below.

[0067] [Table 1]

[0068]

[0069]

[0070]

[0071]

[0072] In the embodiments of the present disclosure below, in a communication environment where downlink data is transmitted from a transmitter (transmission device) of a base station to a receiver (reception device) of a terminal in an MU-MIMO system, precoder design methods for determining a downlink precoder (i.e., a precoding matrix) that satisfies SPC and / or PAPC, applied when transmitting the downlink data, will be specifically described.

[0073] The precoder design methods of the present disclosure can be performed, for example, when a precoder suitable for a channel condition (i.e., downlink precoder change) is required depending on the channel condition during downlink transmission at a base station.

[0074] For convenience, the embodiments of the present disclosure assume a single-cell downlink system, where the base station utilizes M antenna elements (or simply referred to as antennas) and supports U UEs. The embodiments of the present disclosure can also be applied to a multi-cell downlink system, provided that interference from other cells is treated as part of the noise experienced by the UE.

[0075] In this disclosure, UE u is a UE index representing any UE among U UEs. UE u is It has 1 receiving antennas, and independent and identically distributed (iid) additive white Gaussian noise (AWGN) is added to each receiving antenna, each noise having zero mean and variance A complex Gaussian distribution (i.e., ) is assumed to be a random variable. Also, the total number of antennas of UEs is can be expressed as

[0076] The number of data layers for UE u is and the number of data layers in the entire system is is. The number of radio frequency (RF) chains of UE u and the number of RF chains of the base station are respectively and If we define it as and It must be satisfied. Note that in the fully digital (FD) beamforming structure, the number of antennas and the number of RF chains can be assumed to be the same.

[0077] Linear precoder at base station Supports UEs through UE u, where UE u is a linear combiner can recover data for itself. If the base station has an analog-digital hybrid beamforming structure, the linear precoder F is an RF precoder. and baseband (BB) precoder can be expressed as a product of (i.e., ) Similarly, if UE u has a hybrid beamforming structure, a linear combiner is an RF combiner Wow BB combiner can be expressed as a product of (i.e., )

[0078] The precoder design method proposed in the embodiments of the present disclosure can be applied to the design of a BB precoder, and in the case of a base station using an FD beamforming structure, can be applied to the overall design of the precoder, and in the case of a base station using a hybrid beamforming structure, can be applied to the design of the BB precoder of the base station when the RF precoder of the base station and the RF combiners of UEs are given.

[0079] For convenience, the present disclosure assumes a frequency flat channel environment for explanation. In a frequency selective channel, the present disclosure can be applied as a precoder design technique for each subcarrier (SC) or resource element (RE) in an orthogonal frequency division multiplexing (OFDM) system where there is a guard (e.g., cyclic prefix) sufficient to sufficiently remove inter-symbol-interference (ISI). When the present disclosure is applied to an OFDM system, the wireless channel and transmission power conditions can be applied to the channel and transmission power conditions for the SC (subcarrier) or RE (resource element) to be designed.

[0080] FIG. 4 is a diagram for explaining a transmission operation of a base station supporting multiple (egU) UEs in a wireless communication system using MU-MIMO to which an embodiment of the present disclosure is applied.

[0081] Referring to Figure 4, the number of data layers for UE u is and the transmission data for the corresponding UE is and can be modeled as is filled with all elements as 0 is a vector of dimensions Is It is a unit matrix of dimension. If all or part of the data (410) transmitted by the base station is expressed as a single vector, am.

[0082] It is assumed that the base station knows the wireless channel information between the base station and all or some UEs, and the wireless channel information can be expressed in matrix form as shown in the following [Mathematical Formula 1].

[0083]

[0084] In the above mathematical formula 1 is a downlink wireless channel between the base station and UE u. In order to efficiently transmit data, the base station may use a linear precoder (420) in the form of a matrix (i.e., a precoding matrix) as shown in the following [Mathematical Formula 2].

[0085]

[0086] In the above mathematical formula 2 is a precoder for UE u is the m-th row of matrix F and is physically the precoding vector for the m-th antenna of the base station.

[0087] Data for UE u with precoding applied is And the signal (430) that the base station precodes and transmits is as follows [Mathematical Formula 3].

[0088]

[0089] From the above mathematical expression 3, when SPC is P, the precoding matrix F that satisfies SPC satisfies the following [Mathematical expression 4].

[0090]

[0091] And the PAPC of the mth transmitting antenna of the base station is A precoder that satisfies PAPC when satisfies the following [Mathematical Formula 5].

[0092]

[0093] FIG. 5 is a diagram for explaining a receiving operation of a UE in a wireless communication system using MU-MIMO to which an embodiment of the present disclosure is applied.

[0094] Referring to Fig. 5, the downlink signal transmitted by the base station is transmitted to each UE through a wireless channel, and AWGN is added to the receiving antenna (510) of each UE. The number of receiving antennas of UE u is and the AWGN added to the receiving antenna is can be modeled as

[0095] Since a base station supports multiple UEs, the received signal of a UE contains interference including data from other UEs along with noise. To effectively suppress AWGN and interference while efficiently detecting data for itself, a linear combiner (520) is used, as shown in Fig. 5. can process the received signal.

[0096] Therefore, data (530) detected by UE u can be expressed as in the following [Mathematical Formula 6].

[0097]

[0098] In addition, the optimal linear combiner (520) of UE u is an LMMSE combiner and is expressed as follows [Mathematical Formula 7].

[0099]

[0100] Since the optimal linear combiner (520) of the UE is known in the form of the above [Mathematical Formula 7], it is assumed that the linear combiner of the UEs in the embodiments of the present disclosure uses the LMMSE combiner.

[0101] When UE u designs the linear combiner (520) as an LMMSE combiner as in [Mathematical Formula 7], the transmission rate that UE u can achieve is as in [Mathematical Formula 8].

[0102]

[0103] In the above [Equation 8] am.

[0104] The transmission rate weight of UE u is The weighted sum rate for U UEs in the downlink is as follows [Mathematical Formula 9].

[0105]

[0106] In the above [Equation 9] If, is the sum rate of the downlink system.

[0107] The linear precoder proposed in the present disclosure can be designed to maximize the sum of the weighted transmission rates.

[0108] In the present disclosure, when the transmission power limit of the base station is in the form of SPC and the corresponding value is P, a linear precoder for maximizing the sum of the weighted transmission rates can be designed as in the following [Mathematical Formula 10].

[0109]

[0110] In the present disclosure, the transmission power limit of the base station is in the form of PAPC, and the transmission power of the mth antenna of the base station is If it must be less than or equal to, a linear precoder to maximize the sum of weighted transmission rates can be designed as shown in [Mathematical Formula 11].

[0111]

[0112] FIG. 6 is a flowchart conceptually illustrating a linear precoder design method satisfying SPC in a wireless communication system utilizing MU-MIMO according to an embodiment of the present disclosure. The method of FIG. 6 can be performed in a transmission device of a base station in the downlink.

[0113] The method for designing a downlink linear precoder satisfying SPC proposed in the present disclosure is a method for iteratively designing / generating / determining a downlink precoder and combiner and a virtual uplink precoder and combiner alternately. In the present disclosure, when designing a linear precoder, the downlink and the virtual uplink are alternately considered, and the precoder of a specific link among the downlink or uplink can be designed / generated / determined based on the linear combiner and MSE-weight matrix of the opposite link.

[0114] Referring to FIG. 6, in step 601, the base station can obtain an LMMSE combiner for a virtual uplink for each iteration for linear precoder determination in the downlink. In step 602, the base station can obtain an MSE weight matrix for the virtual uplink for each iteration for linear precoder determination in the downlink. And in step 603, the base station can determine a downlink precoder that satisfies SPC based on the LMMSE combiner and MSE weight matrix for the virtual uplink obtained for each iteration for linear precoder determination in the downlink.

[0115] FIG. 7 is a flowchart conceptually illustrating a method for designing a linear precoder that satisfies PAPC in a wireless communication system utilizing MU-MIMO according to an embodiment of the present disclosure. The method of FIG. 7 can be performed in a transmission device of a base station in the downlink.

[0116] In the present disclosure, a precoder design method satisfying PAPC assumes that the sum of power limitations for each antenna is SPC, designs a downlink precoder using the precoder design method satisfying SPC, and multiplies the downlink precoding matrix satisfying SPC by a diagonal matrix for satisfying PAPC for each antenna, thereby designing a precoder satisfying PAPC.

[0117] Referring to FIG. 7, in step 701, the base station can obtain an LMMSE combiner for a virtual uplink for precoder determination at each iteration for linear precoder determination in the downlink. In step 702, the base station can obtain an MSE weight matrix for the virtual uplink at each iteration for linear precoder determination in the downlink. In step 703, the base station can determine a downlink precoder satisfying SPC based on the LMMSE combiner and MSE weight matrix for the virtual uplink obtained at each iteration for linear precoder determination in the downlink. And in step 704, the base station can determine a downlink precoder satisfying PAPC based on the downlink precoder satisfying SPC obtained at each iteration for linear precoder determination in the downlink.

[0118] Although the embodiments of the present disclosure exemplify the use of the LMMSE combiner and the MSE weight matrix, the precoder was designed by assuming the LMMSE combiner and assuming the best performance of each receiver, and each UE is not required to unconditionally use the LMMSE combiner.

[0119] FIG. 8 is a flowchart specifically illustrating a linear precoder design method that satisfies SPC in a wireless communication system utilizing MU-MIMO according to an embodiment of the present disclosure. The method of FIG. 8 specifically exemplifies the method of FIG. 6. In the example of FIG. 8, the combiner used in the receiver of each link of the uplink or downlink assumes an optimal LMMSE combiner, and the detailed operations are as shown in the example below.

[0120] Referring to FIG. 8, the base station initially sets up a linear precoder in step 801, and counts the number of iterations for designing the linear precoder in step 802.

[0121] The first downlink precoder design process is It starts with the design of the LMMSE combiner of UEs corresponding to the downlink precoder obtained through the second design.

[0122] For convenience, the downlink precoder (precoding matrix) obtained through the second design Assuming that, the base station can calculate / obtain the LMMSE combiner of UE u corresponding to the downlink precoder in step 803 as in the following [Mathematical Formula 12] by the above [Mathematical Formula 7].

[0123]

[0124] The base station assumes that UE u uses the LMMSE combiner in the form of [Equation 12] above, and in step 804, the base station calculates the downlink mean square error (MSE) matrix of UE u can be calculated / obtained as in the following [Mathematical Formula 13].

[0125]

[0126] In addition, in the above step 804, the base station can obtain the downlink MSE-weight matrix of UE u by updating it as in the following [Mathematical Formula 14] using the above [Mathematical Formula 13].

[0127]

[0128] At step 805, the base station can obtain a precoder for the virtual uplink using [Mathematical Formula 15] to [Mathematical Formula 17] below.

[0129] Specifically, a matrix containing all the receive couplers of the downlink and a matrix containing downlink weight matrices is defined in the form of a block diagonal matrix as follows [Mathematical Formula 15].

[0130]

[0131] As explained above, the proposed precoder design method can design a precoder based on the linear combiner and MSE-weight matrix of the opposite link. Therefore, the base station can design a downlink linear combiner W and a downlink MSE-weight matrix Based on this, the precoder of UEs for the virtual overhead link can be designed / obtained as in [Mathematical Formula 16].

[0132]

[0133] In the above [Equation 16] and is. Also, in the above [Mathematical Formula 16] is as follows [Mathematical Formula 17].

[0134]

[0135] The virtual uplink considered in this disclosure is a wireless channel between UE u and a base station. and UE u is the above [Mathematical Formula 17]. AWGN is used as a precoder and added to the base station antenna. and represents the variance of AWGN per antenna of the base station. is as follows [Mathematical Formula 18].

[0136]

[0137] In the above [Equation 18] is. The transmission data of UE u in the virtual uplink and in that link A linear coupler at the base station to detect Then, the virtual uplink can be modeled as shown in [Mathematical Formula 19].

[0138]

[0139] In the virtual uplink, the optimal receiver combiner is the LMMSE combiner, just like in the downlink. Therefore, the combiner for the virtual uplink considered in this disclosure is as follows [Mathematical Formula 20].

[0140]

[0141] Just as the LMMSE combiner and weight matrix of the downlink were considered in order to design a precoder for a virtual uplink in step 805, in steps 806 to 808, the base station can calculate / obtain an updated downlink precoder based on the LMMSE combiner of the uplink expressed by [Mathematical Formula 20] and the uplink weight matrix according to the LMMSE combiner.

[0142] If steps 806 to 808 above are described in detail, the above [Mathematical Formula 20] can be expressed as the following [Mathematical Formula 21].

[0143]

[0144] If the above [Equation 21] is transformed into the following [Equation 22], all UEs request as many data layers as the number of their receiving antennas and the matrix At each iteration when the inverse matrix of exists can be obtained by low-dimensional inverse matrix operation.

[0145] Each UE requests as many data layers as its number of receive antennas. In this case, [Equation 22] can be expressed as follows.

[0146]

[0147] In the above [Equation 22] is the Woodbury matrix identity It can be obtained using the following algorithm [Table 2].

[0148] [Table 2]

[0149]

[0150] The virtual uplink MSE matrix of UE u according to the LMMSE combiner expressed by the above [Mathematical Formula 20] is as follows [Mathematical Formula 23].

[0151]

[0152] And the MSE-weight matrix of the virtual uplink is calculated as follows [Mathematical Formula 24].

[0153]

[0154] And a matrix containing virtual uplink MSE-weight matrices is defined in the form of a block diagonal matrix. That is, is. The base station is W and Just as we designed the precoder of UEs for virtual port links based on and Based on this, the downlink precoder F can be updated as in [Mathematical Formula 25].

[0155]

[0156] In the above [Equation 25] and am.

[0157] At step 809, the base station uses the weighted sum rate The operations of steps 802 to 808 are repeated until convergence or the number of design iterations reaches the maximum number of iterations. The pseudo-code of the precoder design method that satisfies SPC proposed in the present disclosure is as shown in the example in [Table 3] below.

[0158] [Table 3]

[0159]

[0160] In the present disclosure, a precoder satisfying SPC is according to the above [Mathematical Formula 25]. Therefore, in step 808, the base station uses a downlink precoder that satisfies SPC. It can be expressed in the form of . Therefore, it is possible to design a low-complexity precoder by updating a relatively low-dimensional matrix X instead of F in each iteration process of Fig. 8. In addition, according to the present disclosure, when designing a precoder, It can achieve very low complexity in massive MIMO or XL-MIMO systems.

[0161] From It is also and is a matrix determined by the wireless channel, so it is a value that does not change for each iteration. Designing X that satisfies satisfies SPC.

[0162] , and If we re-express the mathematical equations above, they can be expressed as follows. First, the above [Mathematical Equation 12] is as follows [Mathematical Equation 26].

[0163]

[0164] In the above [Equation 26] am.

[0165] The above [Mathematical Formula 14] can be expressed as the following [Mathematical Formula 27].

[0166]

[0167] In the above [Equation 20] do If expressed as [Mathematical Formula 20], The formula to update can be replaced with the following [Equation 28] to update .

[0168]

[0169] For example, each UE requires as many data layers as the number of receiving antennas, and the matrix At each iteration when the inverse matrix of exists can be obtained by low-dimensional inverse matrix operation.

[0170] For example, each UE requires as many data layers as the number of receiving antennas. If, can be expressed as follows [Mathematical Formula 29].

[0171]

[0172] Woodbury matrix identity Woodbury matrix identity It can be obtained through the same process as [Table 2] above.

[0173] The above [Mathematical Formula 24] representing the MSE-weight matrix of the virtual uplink can be expressed as the following [Mathematical Formula 30].

[0174]

[0175] And the above [Mathematical Expression 25] for updating the precoding matrix F can be separated into [Mathematical Expression 31] below for updating X and [Mathematical Expression 32] below for obtaining F from X. In the design process of the precoder, [Mathematical Expression 31] below is required for each iteration, but [Mathematical Expression 32] below is required only when returning F at the end.

[0176]

[0177]

[0178] Using the above [Equation 26] to [Equation 33] The pseudo-code of the low-complexity algorithm of the precoder design technique that can be applied in the human communication system is as shown in the example in [Table 4] below. According to the above process, the base station can determine the downlink precoder F that satisfies the SPC in step 808.

[0179] [Table 4]

[0180]

[0181] Hereinafter, an example of a BB BF design method of hybrid beamforming as a linear precoder design method satisfying SPC proposed in the present disclosure will be described.

[0182] This embodiment describes a method for applying a linear precoder design method to a hybrid beamforming design.

[0183] RF precoder and BB precoder of the base station and Define the RF combiner and BB combiner of UE u and If we define the definition of La, the downlink system can be exemplified as follows [Mathematical Formula 33].

[0184]

[0185] The precoding technique proposed in this embodiment is a method for designing a BB precoder when RF precoder and RF combiners are given. SPC In this hybrid beamforming system, SPC is as follows [Mathematical Formula 34].

[0186]

[0187] In the above [Equation 34] cast and It is expressed as a product of . Similarly cast and When expressed as a product of [Equation 33], the above equation is as follows [Equation 35].

[0188]

[0189] In the above [Equation 35] and am.

[0190] For base stations using FD (Fully-digital) precoder, ( Since it is an M×Midentity matrix, and if UE u uses FD combiner, since If the base station uses an FD precoder and the UE uses an FD combiner, am.

[0191] also am.

[0192] The above [Mathematical Formula 6] shows the data detected from UE u. and Each of them and If replaced by , the above [Equation 6] is identical to the above [Equation 35]. Therefore, the BB precoder of the base station for UE u using the precoding technique proposed above is can be designed, and the designed From can be designed. Also, the optimal BB combiner of UE u is and is as follows [Mathematical Formula 36].

[0193]

[0194] also am.

[0195] The equation corresponding to the above [Equation 14], which represents the downlink MSE-weight matrix of UE u, is as follows [Equation 37].

[0196]

[0197] also and And the equation corresponding to the above [Equation 16], which represents the precoder of UEs for virtual uplink, is as follows [Equation 38].

[0198]

[0199] Therefore, the equation corresponding to the above [Equation 17] is as follows [Equation 39].

[0200]

[0201] The equation corresponding to the above [Mathematical Equation 18], which shows the variance of AWGN per antenna of the base station, is It is a formula that introduces [Mathematical Formula 40].

[0202]

[0203] And the equation corresponding to the above [Mathematical Formula 20] showing a coupler for a virtual uplink is as follows [Mathematical Formula 41].

[0204]

[0205] In this case, the Woodbury matrix identity based inversion algorithm is applied to the following [Equation 42]. can be obtained.

[0206]

[0207] And the equation corresponding to the above [Equation 24], which represents the MSE-weight matrix of the virtual uplink, is as follows [Equation 43].

[0208]

[0209] Based on the above [Mathematical Formula 43] and is determined. In addition, the equation corresponding to [Equation 25] representing the downlink precoder is as follows [Equation 44].

[0210]

[0211] Finally A BB precoder can be designed in the form of a weighted sum rate precoder. The proposed precoder design method that satisfies SPC is The above process is repeated until convergence or the number of design iterations reaches the maximum number of iterations. The pseudo-code of the precoder design method of the hybrid beamformer satisfying SPC proposed in this disclosure is as shown in the example in [Table 5] below.

[0212] [Table 5]

[0213]

[0214] A BB precoder of a base station designed according to a precoder design method that satisfies SPC proposed in the present disclosure. Is since It can be expressed in the form of . Therefore, in each iteration process, Instead, a relatively low-dimensional matrix By updating, a low-complexity precoding design is possible. In particular, this In hybrid beamforming systems, very low complexity can be achieved.

[0215] and Similarly to and is a matrix determined by the RF precoder of the transmitter, the RF combiner of the receiver, and the wireless channel, so it is a value that does not change for each iteration. Design X that satisfies satisfies SPC.

[0216] and If we re-express the preceding equations, they can be expressed as [Equation 45] below. First, [Equation 35], which represents the optimal BB combiner of UE u, can be expressed as [Equation 45] below.

[0217]

[0218] In the above [Equation 45] And [Mathematical Formula 37] corresponding to the downlink MSE-weight matrix of UE u is as follows [Mathematical Formula 46].

[0219]

[0220] And the above [Mathematical Formula 41] corresponding to the coupler for the virtual uplink do If expressed as such, in the above [Mathematical Formula 41] The formula to update can be replaced with the following [Equation 47] to update .

[0221]

[0222] In this case, the Woodbury matrix identity based inversion algorithm is applied in the above [Equation 42]. The formula to update can be replaced with the following [Equation 48] to update .

[0223]

[0224] And the above [Mathematical Expression 43] corresponding to the MSE-weight matrix of the virtual uplink can be expressed as the following [Mathematical Expression 49].

[0225]

[0226] Corresponding to the downlink precoder The above [Equation 44] that updates [Equation 50] below From It can be separated into [Equation 51] below to obtain [Equation 50], and [Equation 51] is required at each iteration during the design process, but [Equation 51] is the last. It is only needed when returning.

[0227]

[0228]

[0229] In the above [Mathematical Formula 51] From Since we can obtain [Equation 52], which represents the baseband precoder directly from can be obtained.

[0230]

[0231] Using the above [Equation 45] to [Equation 52] The pseudo-code of the low-complexity algorithm of the proposed BB precoder design technique, which is very useful in hybrid beamforming systems, is as shown in the example in [Table 6].

[0232] [Table 6]

[0233]

[0234] FIG. 9a and FIG. 9b are flowcharts specifically illustrating a linear precoder design method that satisfies PAPC in a wireless communication system using MU-MIMO according to an embodiment of the present disclosure.

[0235] In the embodiments of FIGS. 9a and 9b, a linear precoder design method satisfying PAPC limits the transmission power of the m-th transmission antenna of the base station. When we say that, first of all, SPC A linear precoder that satisfies Design a diagonal matrix whose diagonal elements are all non-negative real numbers. By introducing The linear precoder is designed to satisfy [Mathematical Formula 53] below. refers to the downlink precoder of the base station for UE u that satisfies SPC. In addition, similar to the design of a precoder that satisfies SPC, the precoder is designed iteratively and the matrix is used to design the precoder in the next iteration.

[0236]

[0237] In the above [Equation 53] can be obtained by the following [Mathematical Formula 54].

[0238]

[0239] In the above [Equation 54] Silver matrix is the mth row vector.

[0240] Referring to Figure 9a, the base station initializes the linear precoder in step 901, and the matrix Count the number of iterations for designing the linear precoder in step 902.

[0241] In steps 903 to 905 of FIG. 9A, the base station can obtain a precoder for a virtual uplink by obtaining a downlink LMMSE combiner and a downlink MSE-weight matrix. The operations of steps 903 to 905 are similar to the operations of steps 803 to 805 of FIG. 8 in the method for designing a precoder that satisfies SPC.

[0242] In the linear precoder design method satisfying PAPC, similarly to the precoder design process for maximizing the weighted sum rate in SPC for the previously designed precoder F at each iteration, the base station can design / obtain a downlink linear combiner (e.g., downlink LMMSE combiner) using [Mathematical Formula 55] below at step 903.

[0243]

[0244] At step 904, the base station can calculate / obtain the MSE-weight matrix of the downlink through the following [Mathematical Formula 56].

[0245]

[0246] At step 905, the base station can calculate / obtain a virtual uplink precoder similar to the precoder design under SPC conditions through the following [Mathematical Formula 57].

[0247]

[0248] In the above [Equation 57] and In addition, the dispersion of base station AWGN in the virtual uplink is calculated as follows [Mathematical Formula 58].

[0249]

[0250] In the above [Equation 58] am.

[0251] In the above [Mathematical Formula 55] By substituting the linear coupler of the above downlink can be expressed as follows [Mathematical Formula 59].

[0252]

[0253] thus is an effective channel Precoder when It can be seen as an LMMSE combiner for . And )am.

[0254] The definition of variable tau (τ) in steps 906 and 907 is the same as step 913 in Fig. 9b, and the iterative process of step 907 is an iterative process for matrix G convergence and a process for the stability of normalization.

[0255] In step 908 of Fig. 9b, the base station indicates the limit of the number of iterations of the iteration process for stabilizing the matrix G for the iterative operation for normalization per antenna satisfying PAPC. , and a diagonal matrix whose diagonal elements are all non-negative real numbers. Multiplying the channel H by the effective channel in the uplink can be calculated / obtained.

[0256] In steps 909 to 911 of FIG. 9b, the base station can obtain a precoder for the downlink that satisfies SPC by obtaining a virtual uplink LMMSE combiner and a virtual uplink MSE-weight matrix. The operations of steps 909 to 911 are similar to the operations of steps 806 to 808 of FIG. 8 in the method for designing a precoder that satisfies SPC.

[0257] At step 909, the base station Considering this, the linear coupler of the virtual uplink can be designed as follows [Mathematical Formula 60].

[0258]

[0259] And at step 910, the base station can calculate / obtain the MSE-weight matrix of the virtual uplink as shown in [Mathematical Formula 61].

[0260]

[0261] Next, in step 911, the base station uses the obtained virtual uplink LMMSE combiner and the virtual uplink MSE-weight matrix to create a downlink precoder that satisfies SPC. can be updated / obtained as follows [Mathematical Formula 62].

[0262]

[0263] In the above [Equation 62] And the base station is obtained from the above [Mathematical Formula 62] at step 912. Accordingly, the diagonal matrix is updated as in [Mathematical Formula 63].

[0264]

[0265] And in the above step 912, the base station is newly updated cast The downlink precoder F satisfying PAPC is updated as shown in [Mathematical Formula 64] by multiplying it.

[0266]

[0267] In the above [Equation 60] and [Equation 61] And in the above [Equation 64] are respectively according to the above [Mathematical Formula 63] Since they are before and after the update, they are different. To design a stable precoder that satisfies PAPC (i.e., so that F can converge within the tolerance range), the base station can add an algorithm that repeats [Equations 60] to [Equations 64], as in steps 907 to 914 of FIG. 9b. According to the above operation, the base station can determine a stable downlink precoder that satisfies PAPC in step 915.

[0268] The pseudo-code of the precoder design algorithm satisfying PAPC proposed in the embodiments of FIGS. 9a and 9b described above is as shown in the example in [Table 7] below.

[0269] [Table 7]

[0270]

[0271] Hereinafter, an example of a linear precoder design method that satisfies the proposed PAPC when the UE uses an HBF structure and an RF combiner is determined in the present disclosure will be described.

[0272] UE u's combiner It has a structure of , and the base station If known, a precoder design satisfying the proposed PAPC can be illustrated as an example in [Table 8].

[0273] [Table 8]

[0274]

[0275]

[0276] Hereinafter, an example of a precoder design method satisfying PAPC will be described in the present disclosure when a base station has a hybrid beamforming structure with an RF precoder of a partially connected structure or a sub-array structure.

[0277] For example, each antenna element of a base station is connected to only one RF chain and one phase shifter, and the PAPC of all connected antenna elements for a specific RF chain is the same and the RF precoding matrix Assuming that is given, a precoder satisfying PAPC can be designed in the following manner. In the following description, the set of base station antenna element indices connected to the nth RF chain of the base station is and define the size of the set It is written as .

[0278] RF precoding matrix of RF precoder with partially connected structure or sub-array structure The (m,n)th element of has a size of , only if the mth transmitting antenna and the nth RF chain are connected by a phase shifter. It has a value of , and is 0 if the m-th transmitting antenna and the n-th RF chain are not connected to the phase shifter.

[0279] For example, when the number of RF chains in the base station is 2 and there are 6 transmitting antennas, is a (6×2) dimensional matrix. And if antennas 1 to 4 are connected to the first RF chain and the phase shifter, and the remaining antennas are connected to the second RF chain, can be expressed as follows [Mathematical Formula 65].

[0280]

[0281] In the above [Equation 65] Defined for means the phase value of the phase shifter connected to the mth antenna element.

[0282] RF precoding matrix of RF precoder with partially connected structure or sub-array structure The features of . Also, the power limit of all antenna elements connected to the nth RF chain as a phase shifter is If it is equal to , the Euclidean norm of the nth row vector of the BB precoder is If the following is true, the entire precoder satisfies PAPC.

[0283] Therefore, the effective downlink channel to UE u is Considering the power constraint per RF chain instead of PAPC, it is possible to design a precoder that satisfies PAPC for a base station with a hybrid beamforming structure having a partially connected structure or a sub-array structure RF precoder as a precoder design method that satisfies PAPC. (The power constraint of the nth RF chain is )

[0284] The pseudo-code is as shown in the example in [Table 9] below. Silver BB precoder is the nth row vector.

[0285] [Table 9]

[0286]

[0287]

[0288] Referring to FIGS. 10 to 12 below, the performance simulation results of a linear precoder satisfying SPC or PAPC according to embodiments of the present disclosure will be described.

[0289] Simulations were conducted using MATLAB 2021a, a numerical analysis and programming software developed by MathWorks. All simulations assumed base stations using FD precoders and UEs using FD combiners. Channel modeling was based on the Uma NLoS channel model in 3GPP TR 38.901 V17.0.0.

[0290] Referring to Figure 10, the communication environment assumed in the simulation is as follows. The antenna of the base station (1010) is located 25 meters above the ground, and the UE is located between a minimum of 1.5 meters and a maximum of 22.5 meters above the ground. The minimum two-dimensional distance between the base station (1010) and the UE is 100 meters, and the maximum distance is 500 meters. The standard deviation of shadow fading is 6 dB.

[0291] Carrier frequency and bandwidth and the noise power spectral density (PSD) is . In addition, the number of transmitting antennas of the base station (1010) is M = 128, and it supports a total of 16 UEs. Every UE has two receiving antennas and requires two data layers. That is, It is about UE u and am.

[0292] Path loss between base station and terminal is the same. Also, the random variable that means shadow fading is Finally, the downlink channel toward UE u is generated as a matrix as shown in [Mathematical Formula 66].

[0293]

[0294] also and am.

[0295] All simulation results are based on experiments conducted on 500 randomly generated channels. In addition, for the PAPC simulation, . Also, the maximum number of iterations for all precoder design techniques is The convergence condition was set to be a sum rate change of less than 0.01% compared to the previous sum rate. Regarding the proposed PAPC design technique, and am.

[0296] Figures 11a and 11b illustrate simulation results of a precoder design method satisfying SPC conditions according to an embodiment of the present disclosure. Figure 11a illustrates a case where the base station's transmission power is approximately 40 Watts, and Figure 11b illustrates a case where the base station's transmission power is approximately 100 Watts.

[0297] The initial precoder under SPC conditions is either an MRT or ZF precoder. The simulation results show two examples of existing precoder design methods, labeled "Original" and "Rethink," respectively, while the precoder design method according to the present disclosure is labeled "Proposed." The precoder design method satisfying SPC conditions according to the embodiments of the present disclosure demonstrates that a precoder exhibiting sufficiently good performance can be designed even with a relatively small number of iterations of the initial precoder according to the algorithm.

[0298] Figures 12a and 12b illustrate simulation results of a precoder design method satisfying PAPC conditions according to an embodiment of the present disclosure. Figure 11a illustrates a case where the base station's transmission power is approximately 40 Watts, and Figure 11b illustrates a case where the base station's transmission power is approximately 100 Watts.

[0299] The initial precoder under PAPC conditions is a precoder and EGT that normalizes each row of the MRT to the PAPC for each antenna. The simulation results show two examples of existing precoder design methods, labeled "Original" and "Rethink," while the precoder design method according to the present disclosure is labeled "Proposed." It can be seen that the precoder design method that satisfies the PAPC conditions according to the embodiments of the present disclosure can design a precoder that exhibits sufficiently good performance even with a relatively small number of iterations of the initial precoder according to the algorithm.

[0300] According to the above-described precoder design method according to the present disclosure, it is possible to provide an excellent precoder design algorithm that is faster and less sensitive to initial precoder settings than existing precoder design methods, and it is possible to provide a precoder design method that can guarantee performance even with a small number of iterations. In addition, it is possible to maximize the sum of the weighted transmission rates of a communication system by utilizing limited time / frequency resources, thereby improving the efficiency of a MIMO communication system. In addition, it can be applied to a base station using multiple antennas without additional devices, and the time required for precoder design can be shortened compared to existing technologies, thereby reducing the power required for precoder calculations. Furthermore, as a precoding technique that can increase spectral efficiency, it can efficiently utilize the radio frequency band, thereby presenting economic benefits. Due to various benefits such as beamforming gain and multiplexing gain, massive MIMO is a promising technology to the extent that its expansion to extremely large-scale MIMO (XL-MIMO) is being discussed, and it is highly likely to be a core technology in future wireless communication systems. In addition, the proposed technique enables the design of a baseband beamformer in a hybrid beamforming structure considered in massive MIMO and XL-MIMO systems. Therefore, the precoder design method of the present disclosure can be efficiently applied to MU-MIMO systems utilizing multiple users.

[0301] FIG. 13 is a diagram illustrating an example configuration of a network entity in a wireless communication system according to an embodiment of the present disclosure. The configuration of FIG. 13 corresponds to the base station or terminal described in the embodiments of FIGS. 3 to 12b.

[0302] The network entity of FIG. 13 may include a processor (1301), a transceiver (1303), and a memory (1305). The processor (1301), the transceiver (1303), and the memory (1305) of the network entity of FIG. 13 may operate according to at least one of the methods proposed in the embodiments of FIGS. 3 to 9. However, the components of the network entity are not limited to the examples described above. For example, the network entity may include more or fewer components than the components described above. In addition, the processor (1301), the transceiver (1303), and the memory (1305) may be implemented in the form of a single chip. The transceiver (1303) is a general term for at least one of a receiver of the network entity and a transmitter of the network entity, and may transmit and receive signals with a terminal or another network entity. At this time, the signal to be transmitted and received may include at least one of control information and data. To this end, the transceiver (1303) may include a wired or wireless transceiver and may include various configurations for transmitting and receiving signals. The transceiver (1303) may receive a signal, output it to the processor (1301), and transmit the signal output from the processor (1301). In addition, the transceiver (1303) may receive a communication signal, output it to the processor (1301), and transmit the signal output from the processor (1301) to another network entity via a network. When the method of the present embodiments is implemented in a transmission device of a base station or a terminal, the transceiver (1303) may be referred to as a transmitter. The memory (1305) may store programs and data necessary for the operation of a network entity according to at least one of the embodiments of FIGS. 3 to 12B. Additionally, the memory (1305) can store control information or data included in a signal obtained from a network entity.The memory (1305) may be configured as a storage medium or a combination of storage media, such as a ROM, a RAM, a hard disk, a CD-ROM, and a DVD. In addition, the processor (1301) may control a series of processes so that the network entity can operate according to at least one of the embodiments of FIGS. 3 to 12B. For example, the processor (1301) may include at least one processor and may control a precoder design / determination method that satisfies SPC and / or PAPC according to the present disclosure.

[0303] In accordance with an embodiment of the present disclosure, in a wireless communication system using MIMO, a base station includes a transmitter (1303), one or more processors (1301) including a processing circuit, and a memory (1305) storing instructions, which, when individually or collectively executed by the one or more processors (1301), cause the base station to obtain (601, 806) a first linear combiner for a virtual uplink at each iteration for precoder determination, obtain (602, 807) a first weight matrix for the virtual uplink at each iteration, and determine (603, 810) a downlink precoder based on the first linear combiner and the first weight matrix for the virtual uplink obtained at each iteration, which satisfies an SPC representing a total power limitation of a plurality of transmit antennas of the base station, wherein the downlink precoder can be applied to downlink transmission via the transmitter (1303).

[0304] In one embodiment, the instructions, when individually or collectively executed by the one or more processors (1301), may further cause the base station to obtain a second linear combiner for the downlink and a second weight matrix for the downlink (803, 804), obtain a precoder for the virtual uplink using the second linear combiner and the second weight matrix (805), and obtain the first linear combiner and the first weight matrix based on the precoder for the virtual uplink.

[0305] In one embodiment, the instructions, when individually or collectively executed by the one or more processors (1301), may cause the base station to determine the downlink precoder to maximize a weighted sum of transmission rates representing the sum of transmission rates of a plurality of terminals in the downlink considering terminal-specific transmission rate weights in an MU-MIMO environment.

[0306] In one embodiment, the downlink precoder can be applied to the baseband precoder in a hybrid beamforming structure including a baseband precoder and an RF precoder.

[0307] In one embodiment, the linear combiner may use an LMMSE combiner, and the weight matrix may use an MSE weight matrix.

[0308] In a wireless communication system using MIMO according to an embodiment of the present disclosure, a base station includes a transmitter (1303), one or more processors (1301) including a processing circuit, and a memory (1305) storing instructions, which instructions, when individually or collectively executed by the one or more processors (1301), cause the base station to obtain a first linear combiner for a virtual uplink at each iteration for precoder determination (701, 909), obtain a first weight matrix for the virtual uplink at each iteration (702, 910), determine a first downlink precoder satisfying an SPC representing a total power limit of a plurality of transmit antennas of the base station based on the first linear combiner and the first weight matrix for the virtual uplink obtained at each iteration (703, 911), and determine the first downlink precoder satisfying the SPC obtained at each iteration for the plurality of transmit antennas of the base station based on the first downlink precoder satisfying the SPC obtained at each iteration. Causes (704, 912) a second downlink precoder that satisfies the per-antenna power constraint (PAPC) of the antennas to be determined, and the second downlink precoder can be applied to downlink transmission via the transmitter (1303).

[0309] In one embodiment, the instructions, when individually or collectively executed by the one or more processors (1301), may further cause the base station to obtain a second linear combiner for the downlink and a second weight matrix for the downlink (903, 904), obtain a precoder for the virtual uplink using the second linear combiner and the second weight matrix (905), and obtain the first linear combiner and the first weight matrix based on the precoder for the virtual uplink.

[0310] In one embodiment, the instructions, when individually or collectively executed by the one or more processors (1301), may cause the base station to determine the second downlink precoder to maximize a weighted sum of transmission rates representing the sum of transmission rates of a plurality of terminals in the downlink considering terminal-specific transmission rate weights in an MU-MIMO environment.

[0311] In one embodiment, the instructions, when individually or collectively executed by the one or more processors (1301), may further cause the base station to perform normalization satisfying the PAPC at each iteration and to apply values ​​used in the normalization at each iteration to the determination of the first downlink precoder satisfying the SPC at the next iteration.

[0312] In one embodiment, the linear combiner may use an LMMSE combiner, and the weight matrix may use an MSE weight matrix.

[0313] The methods according to the embodiments described in the claims or specification of the present disclosure may be implemented in the form of hardware, software, or a combination of hardware and software.

[0314] When implemented in software, a computer-readable storage medium storing one or more programs (software modules) may be provided. The one or more programs stored in the computer-readable storage medium are configured for execution by one or more processors within an electronic device. The one or more programs include instructions that cause the electronic device to execute methods according to the embodiments described in the claims or specification of the present disclosure.

[0315] These programs (software modules, software) may be stored in random access memory, non-volatile memory including flash memory, read only memory (ROM), electrically erasable programmable read only memory (EEPROM), magnetic disc storage devices, compact disc ROMs (CD-ROMs), digital versatile discs (DVDs) or other forms of optical storage devices, magnetic cassettes, or may be stored in memories formed by a combination of some or all of these. In addition, each configuration memory may include multiple copies.

[0316] Additionally, the program may be stored in an attachable storage device that is accessible via a communication network such as the Internet, an intranet, a local area network (LAN), a wide local area network (WLAN), a storage area network (SAN), or a combination thereof. Such a storage device may be connected to a device performing an embodiment of the present disclosure via an external port. Additionally, a separate storage device on the communication network may be connected to a device performing an embodiment of the present disclosure.

[0317] In the specific embodiments of the present disclosure described above, components included in the invention are expressed in the singular or plural form, depending on the specific embodiment presented. However, the singular or plural expressions are selected to suit the presented situation for convenience of explanation, and the present disclosure is not limited to singular or plural components. Components expressed in the plural form may be composed of singular elements, or components expressed in the singular form may be composed of plural elements.

[0318] While the detailed description of this disclosure has described specific embodiments, it should be understood that various modifications are possible without departing from the scope of this disclosure. Therefore, the scope of this disclosure should not be limited to the described embodiments, but should be defined not only by the scope of the claims described below, but also by equivalents thereof.

Claims

1. In a wireless communication system using MIMO (multiple-input multiple-output), at the base station, transmitter; One or more processors including processing circuitry; and A memory for storing instructions, wherein when the instructions are individually or collectively executed by one or more processors, the base station, At each iteration for precoder decision, obtain the first linear combiner for the virtual uplink (601, 806), Obtaining the first weight matrix for the virtual uplink at each iteration (602, 807), and In each of the above repetitions, based on the first linear combiner and the first weight matrix for the virtual uplink, a downlink precoder that satisfies a sum power constraint (SPC) representing a total power limit of a plurality of transmission antennas of the base station is determined (603, 810). The above downlink precoder is a base station applied to downlink transmission through the above transmitter.

2. In paragraph 1, The above commands, when individually or collectively executed by one or more processors, cause the base station to: Obtain a second linear combiner for the downlink and a second weight matrix for the downlink (803, 804), Obtaining a precoder for the virtual uplink using the second linear combiner and the second weight matrix (805), and A base station further causing the first linear combiner and the first weight matrix to be obtained based on the precoder for the virtual uplink.

3. In paragraph 1, The above commands, when individually or collectively executed by one or more processors, cause the base station to: A base station that causes the downlink precoder to be determined so as to maximize the sum of weighted transmission rates, which represents the sum of transmission rates of multiple terminals in the downlink, considering transmission rate weights for each terminal in an MU (multi-user)-MIMO environment.

4. In paragraph 1, The above downlink precoder is a base station applied to the baseband precoder in a hybrid beamforming structure including a baseband precoder and an RF (radio frequency) precoder.

5. In any one of paragraphs 1 to 4, A base station in which the linear combiner uses an LMMSE (linear minimum mean square error) combiner and the weight matrix uses an MSE (mean square error) weight matrix.

6. In a wireless communication system using MIMO (multiple-input multiple-output), at the base station, transmitter; One or more processors including processing circuitry; and A memory for storing instructions, wherein when the instructions are individually or collectively executed by one or more processors, the base station, At each iteration for precoder decision, obtain the first linear combiner for the virtual uplink (701, 909), Obtain the first weight matrix for the virtual uplink at each iteration (702, 910), Based on the first linear combiner and the first weight matrix for the virtual uplink obtained at each iteration, a first downlink precoder satisfying a sum power constraint (SPC) representing a total power limit of a plurality of transmission antennas of the base station is determined (703, 911), and At each repetition, based on the first downlink precoder satisfying the SPC obtained, a second downlink precoder satisfying the per-antenna power constraint (PAPC) of the plurality of transmission antennas of the base station is determined (704, 912). The above second downlink precoder is a base station applied to downlink transmission through the transmitter.

7. In paragraph 6, The above commands, when individually or collectively executed by one or more processors, cause the base station to: Obtain a second linear combiner for the downlink and a second weight matrix for the downlink (903, 904), Obtaining a precoder for the virtual uplink using the second linear combiner and the second weight matrix (905), and A base station further causing the first linear combiner and the first weight matrix to be obtained based on the precoder for the virtual uplink.

8. In paragraph 6, The above commands, when individually or collectively executed by one or more processors, cause the base station to: A base station that causes the second downlink precoder to be determined so as to maximize the sum of weighted transmission rates, which represents the sum of transmission rates of a plurality of terminals in the downlink considering transmission rate weights for each terminal in an MU (multi-user)-MIMO environment.

9. In paragraph 6, The above commands, when individually or collectively executed by one or more processors, cause the base station to: Perform regularization that satisfies the PAPC at each iteration above, A base station that causes the values ​​used for the normalization at each iteration to be applied to the determination of the first downlink precoder that satisfies the SPC at the next iteration.

10. In any one of paragraphs 6 to 9, A base station in which the linear combiner uses an LMMSE (linear minimum mean square error) combiner and the weight matrix uses an MSE (mean square error) weight matrix.

11. In a storage medium storing at least one instruction readable by a computer, wherein said at least one command, when executed individually or collectively by one or more processors comprising processing circuitry of the base station, causes said base station to perform at least one action; At least one of the above actions: An operation (601, 806) of obtaining a first linear combiner for a virtual uplink at each iteration for precoder determination; An operation (602, 807) of obtaining a first weight matrix for the virtual uplink at each iteration; and An operation (603, 810) of determining a downlink precoder that satisfies a sum power constraint (SPC) representing a total power limit of a plurality of transmission antennas of the base station, based on the first linear combiner and the first weight matrix for the virtual uplink obtained at each iteration, The above downlink precoder is a storage medium applied to downlink transmission.

12. In paragraph 11, At least one of the above actions: An operation (803, 804) of obtaining a second linear combiner for the downlink and a second weight matrix for the downlink; An operation (805) of obtaining a precoder for the virtual uplink using the second linear combiner and the second weight matrix; and A storage medium further comprising an operation of obtaining the first linear combiner and the first weight matrix based on the precoder for the virtual uplink.

13. In paragraph 11, At least one of the above actions: A storage medium including an operation for determining the downlink precoder to maximize a weighted transmission rate sum representing the sum of transmission rates of a plurality of terminals in the downlink considering transmission rate weights for each terminal in an MU (multi-user)-MIMO environment.

14. In paragraph 11, The above downlink precoder is a storage medium applied to the baseband precoder in a hybrid beamforming structure including a baseband precoder and an RF (radio frequency) precoder.

15. In any one of paragraphs 11 to 14, The linear combiner uses an LMMSE (linear minimum mean square error) combiner, and the weight matrix is ​​a storage medium that uses an MSE (mean square error) weight matrix.

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