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 connected thereto. The CPU acquires user information relating to a plurality of UEs from the plurality of APs, and determines one or more related APs to be associated with the plurality of UEs from among the plurality of APs for each UE on the basis of the user information. The CPU determines one or more related RISs to be associated with the one or more related APs from among the plurality of RISs for each related AP 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 more related RISs to the plurality of UEs such that the minimum required quality of service is obtained in each of the plurality of UEs, provides RIS information regarding the allocation to the one or more related APs, and controls the one or more 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 CPU connected thereto. The CPU is configured to perform the following processes: a first process is to acquire user information related to multiple UEs from the multiple APs; a second process is to determine, for each UE, one or more associated APs to be associated with the multiple UEs from among the multiple APs based on the user information; a third process is to determine, for each associated AP, one or more associated RISs to be associated with the one or more associated APs from among the multiple RISs based on mapping information between the multiple APs and the multiple RISs; a fourth process is to determine 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 fifth process is to provide RIS information related to the determined allocation to the one or more associated APs; and a sixth process is to control 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 services to multiple UEs using a CPU connected to multiple APs and multiple RISs, and includes causing the CPU to perform the following processes: a first process is to acquire user information related to the multiple UEs from the multiple APs; a second process is to determine, for each UE, one or more associated APs to be associated with the multiple UEs from among the multiple APs based on the user information; a third process is to determine, for each associated AP, one or more associated RISs to be associated with the one or more associated APs from among the multiple RISs based on mapping information between the multiple APs and the multiple RISs; a fourth process is to determine an allocation of resources 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; a fifth process is to provide RIS information related to the determined allocation to the one or more associated APs; and a sixth process is to control the one or more associated RISs according to the determined allocation.

[0009] To achieve the above object, a communication control program according to an embodiment of the present disclosure is configured to cause a CPU 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 first to sixth processes described above. The communication control program may be stored in a computer-readable storage medium or provided via a communication network.

[0010] According to the above-described technology of the present disclosure, in a cell-free massive MIMO system in which multiple APs and multiple RISs are connected to a CPU, RIS resources are allocated so that each of multiple UEs can obtain a 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 central controller 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 cell-free massive MIMO system according to an embodiment of the present disclosure in a chronological order; 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 central controller (CCR). The CCR is connected to all APs and RISs constituting the CFM-MIMO system 100 via wired or wireless connections. For example, the CCR and APs may be connected via a backhaul line. The CCR and RISs, and the APs themselves, may also be connected via backhaul lines (not shown). The CCR 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] In the CFM-MIMO system 100, the RIS and AP are associated by the CCR. The CCR determines one or more APs that will communicate with the UE according to predetermined criteria such as distance. When the RIS is used for data transmission from the UE to the AP, the RIS and AP are associated by referring to the RIS-AP mapping table. 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 CCR.

[0021] 2. Hardware Configuration of the Central Controller An example of the hardware configuration of a central controller (CCR) will be described with reference to Figure 2. In the example shown in Figure 2, the CCR includes a central processing unit (CPU) 11, memory 12, a drive 15, a wired communication module 17, and a wireless communication module 18. These elements constituting the CCR 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 external devices.

[0022] 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.

[0023] 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 CPU are the target objects. The CPU is a CPU provided in the CCR.

[0024] According to this sequence diagram, first, in process P01, each AP transmits user information of associated UEs to the CPU, which then acquires it. The user information includes the UE's channel state information (CSI). Prior to process P01, each UE selects the best nearby AP as the master AP. The master AP then communicates with the CPU and APs nearby the master AP and transmits the user information of the UEs it serves to them. AP1 is the master AP for UE2 and transmits user information about UE2 to the CPU and nearby APs. AP2 is the master AP for UE1 and transmits user information about UE1 to the CPU and nearby APs. AP3 is the master AP for UE3 and transmits user information about UE3 to the CPU and nearby APs.

[0025] Next, the CPU performs process P02. Process P02 is a process for associating 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.

[0026] In process P02A, the CPU determines whether to permit each UE to access the AP based on predetermined criteria, which is based on the user information acquired in process P01. This means that the AP to associate with each UE is determined from among multiple APs in the vicinity of the UE. Hereinafter, an AP associated with a UE may be referred to as an associated AP. One or more associated APs may be associated with one UE. Furthermore, the CPU determines a RIS to associate with the associated AP based on a RIS-AP mapping table. Hereinafter, a RIS associated with an associated AP may be referred to as an associated RIS. RIS information regarding the associated RIS is transmitted from the CPU to the associated APs AP1, AP2, and AP3.

[0027] Furthermore, in process P02A, the CPU determines RIS configuration information for the associated RIS. The RIS configuration information includes information regarding the associated RIS resources to be allocated to each UE. The information regarding the resources to be allocated to each UE includes, for example, the number of elements to be allocated to each UE and the reflection phase and amplitude of the elements allocated to each UE. The CPU transmits the RIS configuration information to the associated RISs, RIS1, RIS2, and RIS3, and controls the configurations of RIS1, RIS2, and RIS3. The CPU determines the allocation of the associated RIS resources to the UEs so that the minimum required quality of service (QOS) is obtained for all UEs. The method for allocating RIS resources will be described later in the description of the algorithm for RIS sharing.

[0028] In process P02B, each associated AP establishes an association link with an associated RIS based on the RIS information provided by the CPU. 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 AP3, resources of RIS2 are allocated to both UE1 associated with AP2 and UE3 associated with AP3.

[0029] 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 to the UE via the RIS occurs in AP1, AP2, and AP3. Hereinafter, a UE receiving data from an AP via a RIS may be referred to as a RIS user. UE2 is a RIS user of RIS1, and UE1 and UE3 are RIS users of RIS2.

[0030] 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 each UE. Each UE discovers a communication network via a nearby AP and establishes an initial connection to the communication network.

[0031] Step S102 is executed by the CPU and the AP. In step S102, the CPU acquires user information about the UE from the UE's master AP. The user information includes CSI. In step S102, the master AP also transmits the user information to other nearby APs.

[0032] Step S103 is executed by the CPU. In step S103, the CPU processes the information acquired in step S102 and makes a centralized decision. The decisions made by the CPU are to associate the RIS with the AP and allocate RIS resources to the AP, and to associate the AP with the UE and allocate AP resources to the UE. The CPU allocates one or more RIS 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.

[0033] Step S104 is executed by the CPU. In step S104, the CPU communicates the information determined in step S103 to the AP. The AP establishes an association link with the RIS based on the information communicated from the CPU, and transmits data to the UE via the RIS. The information communicated from the CPU includes RIS information.

[0034] Step S105 is executed by the AP. In step S105, the AP receives feedback from the UE and forwards it to the CPU. The feedback from the UE to the AP is received via the RIS. The feedback received by the AP is then forwarded to the CPU via the backhaul line. The feedback from the UE is used to improve the performance of the CFM-MIMO system.

[0035] 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 the program executed by the CPU of the CCR. The following table lists the definitions of typical parameters used in the algorithm.

[0036] The algorithm for sharing the RIS used in the CFM-MIMO system is as follows.

[0037] According to this algorithm, the CPU qos For all RIS users, min,u Then, the CPU 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.

[0038] For the second and subsequent users, the CPU 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 CPU 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.

[0039] 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 CPU 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.

[0040] The CPU 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 CPU 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.

[0041] 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.

[0042] 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.

[0043] 100 Cell-free Massive MIMO System CCR Central Controller 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 multiple user equipments (UEs), comprising: multiple access points (APs); multiple reconfigurable intelligent surfaces (RISs); and a central processing unit (CPU) connected to the multiple APs and the multiple RISs, wherein the CPU is configured to: acquire user information regarding the multiple UEs from the multiple APs; determine one or more associated APs from the multiple APs for each UE based on the user information; determine one or more associated RISs from the multiple RISs for each associated AP to associate with the one or more associated APs based on mapping information between the multiple APs and the multiple RISs; determine 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; provide RIS information regarding the allocation to the one or more associated APs; and control 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 CPU connected to multiple access points (APs) and multiple reconfigurable intelligent surfaces (RISs), comprising causing the CPU to perform the following operations: acquiring user information about the multiple UEs from the multiple APs; determining one or more associated APs from the multiple APs for each UE to be associated with the multiple UEs based on the user information; determining one or more associated RISs from the multiple RISs for each associated AP to be associated with the one or more associated APs 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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