Cell-free massive MIMO system and communication control method

Abstract

A cell-free massive MIMO system according to the present disclosure comprises a plurality of APs, a plurality of RISs, and a CPU-equipped macro base station connected to the APs and the RISs. A CPU: detects a plurality of UEs by using radio waves from a macro base station; determines, for each UE, among the plurality of APs, one or a plurality of related APs to be respectively related with the plurality of UEs; gives user information pertaining to the detected UE to the one or the plurality of related APs; and determines, for each related AP, among the plurality of RISs, one or a plurality of related RISs to be related with the one or the plurality of related APs on the basis of mapping information between the plurality of APs and the plurality of RISs. The CPU: determines allocation of resources of each of the one or the plurality of related RISs to the plurality of UEs such that a minimum required quality of service is obtained in each of the plurality of UEs; gives RIS information pertaining to the allocation to the one or the plurality of related APs; and controls the one or the plurality of related RISs according to the allocation.

Description

Cell-free massive MIMO system and communication control method

[0001] The present disclosure relates to a cell-free massive MIMO system capable of providing services to a plurality of user terminals simultaneously, and a communication control method for providing services to a plurality of users simultaneously in the system.

[0002] As disclosed in Non-Patent Document 1, cell-free massive MIMO is known in which multiple low-cost distributed antennas called access points (APs) are connected to a central processing unit (CPU) via a fronthaul. The number of APs is greater than the number of user equipments (UEs) in the system. In cell-free massive MIMO, all APs attempt to coherently provide service to all UEs in the system.

[0003] In the cell-free massive MIMO system disclosed in Non-Patent Document 1, the integration of a reconfigurable intelligent surface (RIS) is attempted to improve coverage quality, spectral efficiency, and energy efficiency. Until now, the RIS has been considered to control the propagation channel by appropriately adjusting the phase and amplitude of the electromagnetic signal using its elements. However, with the recent widespread introduction of RIS, more effective utilization of RIS should also be considered. One of the main ways to improve its efficiency is to consider the characteristics of RIS that can be shared.

[0004] The association of base stations (BSs) with RISs has already been established. For example, in Non-Patent Document 2, beamforming optimization is considered assuming that multiple RISs are associated with only one BS. In addition, in Non-Patent Document 3, a RIS sharing mechanism is proposed in which one BS serves a single UE while reshaping interference signals from other BSs.

[0005] Y. Zhang, W. Xia, H. Zhao, G. Zheng, S. Lambotharan and L. Yang, "Performance Analysis of RIS-Assisted Cell-Free Massive MIMO Systems With Transceiver Hardware Impairments," in IEEE Transactions on Communications, vol. 71, no. 12, pp. 7258-7272, Dec. 2023, doi: 10.1109 / TCOMM.2023.3306890Sifan Liu, Pengfei Ni, Rang Liu, Yang Liu, Ming Li, Qian Liu, "BS-RIS-User Association and Beamforming Designs for RIS-aided Cellular Networks," arXiv, 2021, https: / / doi.org / 10.48550 / arXiv.2106.14197B. Di, “Sharing the Surface: RIS-aided Distributed Mechanism Design for Hybrid Beamforming in Multi-cell Multi-user Networks," IEEE INFOCOM 2021 - IEEE Conference on Computer Communications Workshops (INFOCOM WKSHPS), 2021, pp. 1-2, doi: 10.1109 / INFOCOMWKSHPS51825.2021.9484464.

[0006] The present disclosure has been made in light of the above circumstances, and aims to provide a technology that can effectively utilize RIS in a cell-free massive MIMO system in which multiple APs and multiple RISs are connected to a CPU.

[0007] To achieve the above object, a cell-free massive MIMO system according to an embodiment of the present disclosure is a system capable of simultaneously providing services to multiple UEs, and includes multiple APs, multiple RISs, and a macro base station with a CPU connected thereto. The CPU is configured to perform the following processes: a first process is detecting multiple UEs using radio waves from the macro base station; a second process is determining, for each UE, one or more associated APs to associate with the detected multiple UEs from among multiple APs; a third process is providing user information regarding the multiple UEs to the one or more associated APs; a fourth process is determining, for each associated AP, one or more associated RISs to associate with the one or more associated APs from among multiple RISs based on mapping information between the multiple APs and the multiple RISs; a fifth process is determining an allocation of resources of the one or more associated RISs to multiple UEs so that a minimum required service quality is obtained for each of the multiple UEs; a sixth process is providing RIS information regarding the determined allocation to the one or more associated APs; and a seventh process is controlling the one or more associated RISs according to the determined allocation.

[0008] To achieve the above object, a communication control method according to an embodiment of the present disclosure is a method for simultaneously providing service to multiple UEs using a macro base station with a CPU connected to multiple APs and multiple RISs, and includes causing the CPU to execute the following processes: a first process is detecting multiple UEs through radio waves from the macro base station; a second process is determining, for each UE, one or more associated APs to associate with the detected multiple UEs from among multiple APs; a third process is providing user information regarding the multiple UEs to the one or more associated APs; a fourth process is determining, for each associated AP, one or more associated RISs to associate with the one or more associated APs from among multiple RISs based on mapping information between the multiple APs and the multiple RISs; a fifth process is determining an allocation of resources of the one or more associated RISs to multiple UEs so that a minimum required service quality is obtained for each of the multiple UEs; a sixth process is providing RIS information regarding the determined allocation to the one or more associated APs; and a seventh process is controlling the one or more associated RISs according to the determined allocation.

[0009] To achieve the above object, a communication control program according to one embodiment of the present disclosure is configured to cause a CPU of a macro base station connected to multiple APs and multiple RISs in a cell-free massive MIMO system to execute the above communication control method. Specifically, the communication control program includes multiple instructions for causing the CPU to execute the above-described first to seventh processes. The communication control program may be stored in a computer-readable storage medium or provided via a communication network.

[0010] According to the above technology of the present disclosure, in a cell-free massive MIMO system in which multiple APs and multiple RISs are connected to a macro base station with a CPU, RIS resources are allocated so that each of multiple UEs can obtain the minimum required service quality, thereby enabling the multiple APs to effectively share the RIS.

[0011] Fig. 1 is a diagram showing a configuration of a cell-free massive MIMO system according to an embodiment of the present disclosure. Fig. 2 is a diagram showing a configuration of a macro base station provided in the cell-free massive MIMO system according to an embodiment of the present disclosure. Fig. 3 is a sequence diagram showing information transmission between objects in a chronological order in the cell-free massive MIMO system according to an embodiment of the present disclosure. Fig. 4 is a flowchart showing a communication control method executed by the cell-free massive MIMO system according to an embodiment of the present disclosure.

[0012] 1. Cell-Free Massive MIMO System Figure 1 is a diagram illustrating the configuration of a cell-free massive MIMO system (CFM-MIMO system) according to an embodiment of the present disclosure. As shown in Figure 1, the CFM-MIMO system 100 includes multiple access points (APs). The APs include AP1, AP2, and AP3, which are identified in the figure. APs are also referred to as base stations (BSs). All of the multiple APs included in the CFM-MIMO system 100 may have the same configuration, or APs with different configurations may be included.

[0013] The CFM-MIMO system 100 includes multiple reconfigurable intelligent surfaces (RISs). The RISs include RIS1, RIS2, and RIS3, as identified in the figure. The multiple RISs included in the CFM-MIMO system 100 may all have the same configuration, or the RISs may have different configurations. The RISs may be freestanding structures or may be attached to other structures, such as the walls of buildings. The RISs have multiple elements, and by appropriately modifying the reflective characteristics of each element, they can create wireless links that bypass obstacles. In the example shown in FIG. 1, the RISs are used to create wireless links between an AP and a user equipment (UE).

[0014] The CFM-MIMO system 100 is a system that can simultaneously provide services to multiple UEs within a service area. The service area is formed by multiple APs and RISs included in the CFM-MIMO system 100. In FIG. 1, the area surrounded by a dotted line is illustrated as the service area of ​​the CFM-MIMO system 100. Within the service area, regardless of the location of the UE, a wireless link can be generated that connects the UE to the AP directly or via the RIS.

[0015] The CFM-MIMO system 100 includes a macro base station (MBS). The service area of ​​the CFM-MIMO system 100 is also the communication area of ​​the MBS. The MBS is connected to all APs and RISs constituting the CFM-MIMO system 100 by wire or wirelessly. For example, the MBS and APs may be connected by a backhaul line. Furthermore, the MBS and RISs, and the APs themselves, may be connected by a backhaul line (not shown). The MBS provides services simultaneously to multiple UEs within its service area by cooperatively controlling all APs and RISs.

[0016] Figure 1 shows a schematic diagram of how services are provided to UE1, UE2, and UE3. In Figure 1, the area enclosed by a dashed line is shown as the communication area of ​​UE1. Wireless links are formed between UE1 and an AP and a RIS within its communication area, and data is transmitted to UE1 via these APs. Specifically, data is transmitted directly to UE1 from AP1 and another AP within UE1's line of sight, and data is transmitted from AP2, which is outside UE1's line of sight, via RIS2. RIS2 is located within the line of sight of both UE1 and AP2.

[0017] UE2 is connected to AP1 and another AP. Data is sent directly to UE2 from the other AP, and data is sent from AP1 via RIS1. As mentioned above, AP1 is also connected to UE1. AP1 sends data to UE1 and also to UE2 at the same time. In other words, AP1 is shared between UE1 and UE2.

[0018] UE3 is connected to AP3 and another AP. Data is transmitted to UE3 from AP3 via RIS2, and data is transmitted from the other AP via RIS3. As mentioned above, RIS2 is also used to transmit data from AP2 to UE1. In other words, RIS2 is shared between AP2 and AP3.

[0019] As in the example of RIS2, in the CFM-MIMO system 100, one RIS is shared among multiple APs. Conversely, one AP can be associated with multiple RISs at the same time. In the CFM-MIMO system 100, the locations at which RISs are placed are strategically determined so as to minimize the AP placement density. The following table is an example of a RIS-AP mapping table in which mapping information between RISs and APs is tabulated. In the RIS-AP mapping table, "1" indicates a combination of a RIS and an AP that can be associated, and "1" indicates a combination of a RIS and an AP that cannot be associated.

[0020]

[0021] In the CFM-MIMO system 100, the association between the RIS and the AP is performed by a central processing unit (CPU) included in the MBS. The CPU included in the MBS determines one or more APs that will communicate with the UE based on predetermined criteria such as distance. When a RIS is used for data transmission from the UE to the AP, the association between the RIS and the AP is performed by referring to the RIS-AP mapping table described above. When multiple APs are associated with one RIS, as in RIS2, RIS resources must be allocated to the UEs with which those APs communicate. The RIS resources allocated to the UEs are the numerous elements, i.e., antenna elements, that the RIS has. In the case of RIS2, the number of elements to be allocated to UE1 and the number of elements to be allocated to UE3 must be determined separately. The allocation of RIS resources to each UE is performed by the CPU included in the MBS.

[0022] 2. Hardware Configuration of Macro Base Station An example of the hardware configuration of a macro base station (MBS) will be described with reference to Fig. 2. In the example shown in Fig. 2, the MBS includes a CPU 11, a memory 12, a drive 15, a wired communication module 17, and a wireless communication module 18. These elements that make up the MBS are wired together. The wired communication module 17 is provided for wired communication using an optical line including a backhaul line. The wireless communication module 18 is provided for wireless communication with an external device.

[0023] The memory 12 stores a program 13 and data 14. The program 13 is a computer-executable program for implementing a communication control method according to an embodiment of the present disclosure, and is composed of a plurality of instructions. The data 14 is data used when the CPU 11 executes the program 13, and includes parameters for the RIS and AP. The drive 15 includes a storage medium 16. The storage medium 16 stores various information including a RIS-AP mapping table. The storage medium 16 is also used to store the program 13 and the data 14. The program 13 may be provided via a communication network.

[0024] 3. Sequence of Information Transmission Between Objects in a Cell-Free Massive MIMO System Figure 3 is a sequence diagram showing the time series of information transmission between objects in a CFM-MIMO system. In this sequence diagram, AP1, AP2, AP3, RIS1, RIS2, and RIS3 in Figure 1, and the MCPU are the target objects. The MCPU is a CPU provided in the MBS. Hereinafter, the CPU provided in the MBS will be abbreviated as MCPU.

[0025] According to this sequence diagram, first, process P01 is performed by the MCPU. In process P01, the MCPU detects UEs using Sub6-GHz radio waves and determines, for each UE, an AP to associate with the detected UE from among the APs constituting the CFM-MIMO system. Hereinafter, an AP associated with a UE may be referred to as an associated AP. One UE may be associated with one or more associated APs. In addition, the MCPU classifies UEs associated with an AP via a RIS as RIS users. The MCPU transmits user information of the UE to each associated AP associated with the UE. The user information includes channel state information (CSI) of the UE.

[0026] Next, the MCPU performs process P02. Process P02 associates the RIS with the AP. Process P02 includes process P02A for permitting the AP to access the RIS and process P02B for establishing a link between the RIS and the AP.

[0027] In process P02A, the MCPU determines the RIS to associate with the associated AP based on the RIS-AP mapping table. However, the associated AP to which the RIS is associated is the AP to which the UE classified as a RIS user is associated. AP1 is the associated AP associated with UE2, which is a RIS user, AP2 is the associated AP associated with UE1, which is a RIS user, and AP3 is the associated AP associated with UE3, which is a RIS user. The RIS associated with such an associated AP may be called an associated RIS. RIS information regarding the associated RIS is transmitted from the MCPU to AP1, AP2, and AP3, which are the associated APs to which the RIS users are associated.

[0028] Furthermore, in process P02A, the MCPU determines RIS configuration information for the associated RIS. The RIS configuration information includes information regarding the associated RIS resources to be allocated to each RIS user. The information regarding the resources to be allocated to each RIS user includes, for example, the number of elements to be allocated to each RIS user and the reflection phase and amplitude of the elements allocated to each RIS user. The MCPU transmits the RIS configuration information to the associated RISs, RIS1, RIS2, and RIS3, and controls the configurations of RIS1, RIS2, and RIS3. The MCPU determines the allocation of the associated RIS resources to the RIS users so that the minimum required quality of service (QOS) is obtained for all RIS users. The method for allocating RIS resources will be described later in the description of the algorithm for RIS sharing.

[0029] In process P02B, each associated AP establishes an association link with an associated RIS based on the RIS information provided by the MCPU. AP1 establishes an association link with RIS1. AP2 establishes an association link with RIS2, and AP3 also establishes an association link with RIS2. By sharing RIS2 between AP2 and APS3, resources of RIS2 are allocated to both UE1 associated with AP2 and UE3 associated with AP3.

[0030] Next, process P03 is performed by the associated AP. In process P03, data is transmitted from the AP to the associated UE. At this time, traffic is generated from AP1 to UE2, which is a RIS user of RIS1. Traffic is generated from AP2 to UE1, which is a RIS user of RIS2. Traffic is generated from AP3 to UE3, which is a RIS user of RIS2.

[0031] 4. Communication Control Method in Cell-Free Massive MIMO System Figure 4 is a flowchart showing a communication control method executed by a CFM-MIMO system. Step S101 is executed by the MCPU. In step S101, the MCPU detects a UE using sub6-GHz radio waves and establishes an initial connection with the detected UE.

[0032] Step S102 is performed by the MCPU. In step S102, the MCPU determines the association of the UE detected in step S101 with the AP. The MCPU also classifies the UE eligible for association with the AP via the RIS as a RIS user.

[0033] Step S103 is executed by the MCPU. In step S103, the MCPU sends user information of the UE to each AP associated with the UE. The user information includes CSI.

[0034] Step S104 is executed by the MCPU. In step S104, the MCPU makes a decision based on the classification of the UE. The decisions made by the MCPU are to associate the AP with the UE and allocate AP resources to the UE, and to associate the RIS with the AP and allocate RIS resources to the AP. The MCPU allocates one or more RISs in response to a request from the AP. A RIS-AP mapping table is used to associate the RIS with the AP. The RIS resources allocated to the AP are the RIS resources allocated by the AP to the UE with which it communicates. The association between the AP and the UE is performed according to a predetermined criterion, such as distance. The AP resources allocated to the UE include, for example, frequency, time, space, and power.

[0035] Step S105 is executed by the MCPU. In step S105, the MCPU communicates the information determined in step S104 to the AP. The AP establishes an association link with the RIS based on the information communicated from the MCPU, and transmits data to the UE via the RIS. The information communicated from the MCPU includes RIS information.

[0036] Step S106 is performed by the AP. In step S106, the AP receives feedback from the UE and forwards it to the MCPU. The feedback from the UE to the AP is received via the RIS. The feedback received by the AP is then forwarded to the MCPU via the backhaul link. The feedback from the UE is used to improve the performance of the CFM-MIMO system.

[0037] 5. Algorithm for RIS sharing in cell-free massive MIMO systems Finally, we will explain the algorithm for RIS sharing. The algorithm described below is reflected in a program executed by the MCPU. The following table lists the definitions of typical parameters used in the algorithm.

[0038]

[0039] The algorithm for sharing the RIS used in the CFM-MIMO system is as follows.

[0040]

[0041] According to this algorithm, the MCPU qos For all RIS users, min,u Then, the MCPU arranges the RIS users in the order of decreasing received power. RIS users with high received power have the same λ qos To achieve this, we need to use less N min,u Since it requires N min,u This is the same as arranging RIS users in order of increasing received power. min,u to the first RIS user, and N min,u The rate at is calculated as the rate of the first RIS user.

[0042] For the second and subsequent users, the MCPU checks the condition in step 9 for each RIS user. If the condition in step 9 is true, then N min,u The element of serves the u-th RIS user and qos Since elements are available to achieve N, the MCPU selects those elements as the elements to be assigned to the u-th RIS user. min,u Calculate the rate at as the rate of the u-th RIS user.

[0043] On the other hand, if the condition in step 9 is false, N rem Even if it contributes to improving the gain, λ qosTherefore, in this case, the MCPU does not allocate any elements of the r-th RIS to the u-th RIS user, and allocates all remaining elements of the r-th RIS to the previous u-1-th RIS user. min,u +N rem The rate at is recalculated as the rate of the u-1th RIS user.

[0044] The MCPU repeats the calculations from step 9 to step 16 until the total number of RIS users reaches N. When the total number of RIS users reaches N, the MCPU increments the RIS counter by one and repeats the calculations from step 5 to step 19 until the total number of RIS users reaches R. Then, when all RIS elements have been assigned to RIS users, the algorithm stops.

[0045] According to the above algorithm, RIS resources are allocated so that each of multiple UEs can obtain the minimum required QOS, thereby enabling effective sharing of the RIS by multiple APs in a CFM-MIMO system.

[0046] 6. The above embodiments can be modified in various ways without departing from the spirit and scope of the present disclosure. In other words, when the above embodiments refer to the number, quantity, amount, range, etc. of each element, the technology according to the present disclosure is not limited to the mentioned numbers unless otherwise specified or clearly specified in principle. Furthermore, the structures, etc. described in the above embodiments are not necessarily essential to the technology according to the present disclosure unless otherwise specified or clearly specified in principle.

[0047] 100 Cell-free Massive MIMO System MBS Macro Base Station UE1, UE2, UE3 User Terminal AP1, AP2, AP3 Access Point RIS1, RIS2, RIS3 Reconfigurable Intelligent Surface

Claims

1. A cell-free massive MIMO system capable of simultaneously providing services to a plurality of user equipments (UEs), comprising: a plurality of access points (APs); a plurality of reconfigurable intelligent surfaces (RISs); and a macro base station with a central processing unit (CPU) connected to the plurality of APs and the plurality of RISs, wherein the CPU: detects the plurality of UEs using radio waves from the macro base station; determines one or more associated APs from the plurality of APs to associate with the plurality of UEs for each UE; provides user information regarding the plurality of UEs to the one or more associated APs; determines one or more associated RISs from the plurality of RISs to associate with the one or more associated APs for each associated AP based on mapping information between the plurality of APs and the plurality of RISs; determines allocation of resources of each of the one or more associated RISs to the plurality of UEs so that a minimum required service quality is obtained for each of the plurality of UEs; and provides RIS information regarding the allocation to the one or more associated APs. and controlling the one or more associated RISs in accordance with the allocation.

2. A cell-free massive MIMO system according to claim 1, wherein the CPU is configured to determine the allocation based on the received power of each of the plurality of UEs.

3. A cell-free massive MIMO system according to claim 1 or 2, characterized in that determining the allocation includes determining the number of elements to be allocated to each of a plurality of UEs sharing one RIS.

4. A communications control method for simultaneously providing services to multiple user equipments (UEs) using a macro base station with a CPU connected to multiple access points (APs) and multiple reconfigurable intelligent surfaces (RISs), the communications control method comprising causing the CPU to perform the following: detecting the multiple UEs using radio waves from the macro base station; determining one or more associated APs from the multiple APs to associate with the multiple UEs for each UE; providing user information regarding the multiple UEs to the one or more associated APs; determining one or more associated RISs from the multiple RISs for each associated AP based on mapping information between the multiple APs and the multiple RISs; determining an allocation of resources of each of the one or more associated RISs to the multiple UEs so that a minimum required service quality is obtained for each of the multiple UEs; providing RIS information regarding the allocation to the one or more associated APs; and controlling the one or more associated RISs in accordance with the allocation.

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