Selecting an access point to service user equipment in a d

Abstract

Techniques are provided for selecting a cluster of APs to serve UEs in a D-MIMO network. The method is performed by a centralized node in the D-MIMO network. The method includes obtaining a list of candidate APs from a UE via one of the APs in the D-MIMO network. The UE has received, from all APs in the list of candidate APs, downlink reference signals having a power higher than a power threshold. The method includes selecting a cluster of APs in the D-MIMO network to serve the UE. An AP is selected from the list of candidate APs based on a cluster selection criterion that operates on the list of candidate APs. The method includes providing the list of selected APs to the UE by transmitting the list of selected APs to the UE via one of the selected APs.

Description

Technical Field

[0001] The embodiments presented herein relate to a method, a centralized node, a computer program, and a computer program product for selecting an access point cluster in a distributed multiple-input multiple-output network to provide services to a user equipment. The embodiments presented herein also relate to a method, a user equipment, a computer program, and a computer program product for allocating an access point cluster to a UE in a distributed multiple-input multiple-output network to provide services to the user equipment. Background Art

[0002] Multiple antenna technology can significantly improve the data rate and reliability of wireless communication systems. The performance is particularly improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and / or related technologies are generally referred to as MIMO systems, or simply MIMO.

[0003] Distributed MIMO (D-MIMO, also known as Cell-free Massive MIMO, RadioStripes, RadioWeaves, and Ubiquitous MIMO) is a candidate for the physical layer of the sixth generation (6G) telecommunication systems. D-MIMO is based on geographically dispersing the antennas of the network and configuring them to operate together in a phase-coherent manner. Deployment of D-MIMO networks can be used to provide good coverage and high capacity for areas with high traffic requirements, such as factory buildings, stadiums, office spaces, and airports, to name just a few examples.

[0004] In a typical architecture, multiple access points (APs) are interconnected and configured to enable two or more APs to collaborate in coherent decoding of data from a given user equipment (UE) served by the network, and to enable two or more APs to collaborate in coherent transmission of data to the UE. Thus, these APs may collectively define the access portion of a D-MIMO network. Each AP has one or more antenna panels. Each antenna panel may include multiple antenna elements that are configured to operate together in a phase-coherent manner.

[0005] There are several options for deploying multi-user and massive MIMO networks. Some examples include deploying a large number of co-located antenna units at a single site (base station or access point (AP) or transmit and receive point (TRP), collectively referred to as AP in the following without loss of generality), which is sometimes referred to as centralized massive MIMO. Other examples include deploying antenna units (or APs, where each AP may include one or more antenna units) in a decentralized manner. In a decentralized manner, antenna units are distributed over a geographic area (in a carefully planned or random manner), and one or more centralized units or gNBs (collectively referred to as centralized units in the following without loss of generality) control all antenna units and connect these APs to the core network. Combinations of the above examples are also possible.

[0006] Decentralized distribution is often referred to as distributed massive MIMO or cell-free MIMO, where the distributed antenna units (or combinations thereof) may be referred to as APs. In distributed massive MIMO, high-capacity backhaul links (e.g., fiber optic cables or mmWave-based links) are used to connect the antenna units (or APs) to a centralized unit.

[0007] In the canonical form of the cell-free massive MIMO architecture, the network deployment follows a star topology where many distributed APs have independent fronthaul connections to a single centralized unit. However, in geographically large networks, actual deployments of cell-free systems may not rely on a single centralized unit. Furthermore, the assumption that signals sent from all APs will be processed in a coherent manner by the UEs served in the network is also not scalable. Therefore, in realistic large cell-free scenarios, the network deployment may include multiple centralized units, where each centralized unit controls a disjoint set of APs.

[0008] Achieving sufficiently precise relative timing and phase synchronization to enable all APs to jointly utilize coherent signal processing can be challenging, both in canonical deployments and in deployments with multiple centralized units. In fact, the communication theory behind coherent transmission in cell-free systems assumes perfect timing synchronization, which is physically impossible in large networks, even if the clocks from the APs are synchronized. In other words, due to the geographical dispersion of APs, it will be impossible for a UE to receive signals from all APs within a cyclic prefix (CP). In addition to this, joint coherent transmission requires that the signal to be sent to a UE be available at multiple APs simultaneously. Such stringent requirements can be difficult to achieve even in centralized deployments.

[0009] Therefore, a possible cell-free MIMO system with multiple centralized units may include multiple centralized units, each of which is configured to control its own disjoint AP cluster, without assuming that the multiple centralized units are synchronized with each other. Therefore, APs controlled by different centralized units may not be able to serve the same UE using coherent transmissions. Therefore, perfect synchronization and simultaneous signal transmission and data availability are only assumed for APs within each AP cluster (controlled by each centralized unit). In this case, a general transmission scheme can be used, in which a subset of APs controlled by a certain centralized unit can form a coherent group to serve a given UE in a coherent manner. In the case where a second coherent group controlled by a different centralized unit also serves the UE, the UE regards transmissions from two different AP coherent groups as incoherent transmissions. The fact that transmissions from APs controlled by different centralized units are incoherent transmissions occurs because it is difficult to achieve sufficient synchronization between centralized units and to achieve simultaneous data availability at all APs of all centralized units.

[0010] Furthermore, having all APs serve all UEs simultaneously is disadvantageous in terms of energy efficiency. On the other hand, in a user-centric approach, for each UE, only a subset of APs serve it simultaneously. The APs that serve each UE are usually the APs closest to the UE, as these APs usually provide the best channel conditions to the UE. Therefore, different UEs can be assigned to different numbers of APs, depending on the geographical locations of the UEs and APs and the local propagation conditions.

[0011] In order to decide which APs will serve which UE, a cluster-forming algorithm may be performed. One possible approach for developing a clustering algorithm is to design a network-wide algorithm, where clusters of all UEs served by the network are considered jointly. One problem with this approach is scalability, as its complexity increases with the number of UEs (and APs) in the system. A scalable alternative to a network-wide algorithm is to consider a user-centric clustering algorithm, where the cluster-forming algorithm therefore considers only one UE at a time. However, due to the large number of UEs that may be served by the network, such an algorithm may be cumbersome.

[0012] Therefore, the selection of which APs should serve which UE in a MIMO network (eg, in a D-MIMO network or a cell-less MIMO network with multiple centralized units) needs to be improved. Summary of the invention

[0013] The embodiments herein aim to solve the above-mentioned problems and enable efficient selection of which APs should provide services for which UEs in a D-MIMO network.

[0014] According to a first aspect, a method for selecting an AP cluster in a D-MIMO network to provide services to a UE is proposed. The method is performed by a centralized node in the D-MIMO network. The method includes obtaining a list of candidate APs from a UE via one of the APs in the D-MIMO network. The UE has received downlink reference signals with power higher than a power threshold from all APs in the list of candidate APs. The method includes selecting an AP cluster in the D-MIMO network to provide services to the UE. An AP is selected from the list of candidate APs based on a cluster selection criterion that operates on the list of candidate APs. The method includes providing a list of selected APs to the UE by sending the list of selected APs to the UE via one of the selected APs.

[0015] According to a second aspect, a centralized node for selecting an AP cluster in a D-MIMO network to provide services to a UE is proposed. The centralized node includes a processing circuit. The processing circuit is configured to enable the centralized node to obtain a list of candidate APs from the UE via one of the APs in the D-MIMO network. The UE has received downlink reference signals with power higher than a power threshold from all APs in the list of candidate APs. The processing circuit is configured to enable the centralized node to select an AP cluster in the D-MIMO network to provide services to the UE. An AP is selected from the list of candidate APs based on a cluster selection criterion that operates on the list of candidate APs. The processing circuit is configured to enable the centralized node to provide the list of selected APs to the UE by sending the list of selected APs to the UE via one of the selected APs.

[0016] According to a third aspect, a centralized node for selecting an AP cluster in a D-MIMO network to provide services to a UE is proposed. The centralized node includes an acquisition module, which is configured to acquire a list of candidate APs from a UE via one of the APs in the D-MIMO network. The UE has received downlink reference signals with power higher than a power threshold from all APs in the list of candidate APs. The centralized node includes a selection module, which is configured to select an AP cluster in the D-MIMO network to provide services to the UE. An AP is selected from the list of candidate APs based on a cluster selection criterion that operates on the list of candidate APs. The centralized node includes a provision module, which is configured to provide a list of selected APs to the UE by sending the list of selected APs to the UE via one of the selected APs.

[0017] According to a fourth aspect, a computer program for selecting an AP cluster in a D-MIMO network to provide services for a UE is proposed, the computer program comprising a computer program code which, when executed on a processing circuit of a centralized node, causes the centralized node to perform the method according to the first aspect.

[0018] According to a fifth aspect, a method is proposed in which a UE is assigned an AP cluster that provides services for the UE in a D-MIMO network. The method is performed by the UE. The method includes obtaining a list of candidate APs by detecting the radio environment of the UE to receive downlink reference signals from APs in the D-MIMO network, and selecting APs for which the power of all downlink reference signals received is higher than a power threshold to be included in the list of candidate APs. The method includes providing a list of candidate APs to a centralized node in the D-MIMO network by sending the list of candidate APs to the centralized node via one of the APs in the list of candidate APs. The method includes obtaining a list of selected APs from the centralized node via one of the APs in the list of selected APs, thereby assigning its AP cluster to the UE. The method includes: in the D-MIMO network, performing a random access process with an AP in the list of selected APs so that the AP in the list of selected APs starts to provide services for the UE.

[0019] According to a sixth aspect, a UE is proposed, which is assigned an AP cluster that provides services for the UE in a D-MIMO network. The UE includes a processing circuit. The processing circuit is configured to: enable the UE to obtain a list of candidate APs in the following manner: detect the radio environment of the UE to receive downlink reference signals from APs in the D-MIMO network, and select APs for which the power of all downlink reference signals received by the UE is higher than a power threshold and include them in the list of candidate APs. The processing circuit is configured to: enable the UE to provide a list of candidate APs to a centralized node in the D-MIMO network by sending the list of candidate APs to the centralized node via one of the APs in the list of candidate APs. The processing circuit is configured to: enable the UE to obtain a list of selected APs from the centralized node via one of the APs in the list of selected APs, thereby assigning its AP cluster to the UE. The processing circuit is configured to: in the D-MIMO network, enable the UE to perform a random access process with an AP in the list of selected APs so that the AP in the list of selected APs starts to provide services for the UE.

[0020] According to the seventh aspect, a UE is proposed, which is assigned an AP cluster that provides services for the UE in a D-MIMO network. The UE includes an acquisition module, which is configured to acquire a list of candidate APs by detecting the radio environment of the UE to receive downlink reference signals from APs in the D-MIMO network, and selecting APs for which the power of all downlink reference signals received is higher than a power threshold and including them in the list of candidate APs. The UE includes a provision module, which is configured to provide a list of candidate APs to a centralized node in the D-MIMO network by sending the list of candidate APs to the centralized node via one of the APs in the list of candidate APs. The UE includes an acquisition module, which is configured to acquire a list of selected APs from a centralized node via one of the APs in the list of selected APs, thereby assigning its AP cluster to the UE. The UE includes a random access module, which is configured to perform a random access process with an AP in the list of selected APs in the D-MIMO network so that the AP in the list of selected APs starts to provide services for the UE.

[0021] According to an eighth aspect, a computer program for a UE is proposed. The UE is assigned an AP cluster serving the UE in a D-MIMO network 100. The computer program comprises a computer program code. When the computer program code is executed on a processing circuit of the UE, the UE executes the method according to the fifth aspect.

[0022] According to a ninth aspect, a computer program product is provided, the computer program product comprising a computer program according to at least one of the fourth aspect and the eighth aspect and a computer-readable storage medium on which the computer program is stored. The computer-readable storage medium may be a non-transitory computer-readable storage medium.

[0023] Advantageously, these aspects provide for efficient selection of which APs should serve which UEs in a D-MIMO network without suffering from the problems described above.

[0024] Advantageously, these aspects are applicable to a cell-less MIMO architecture with multiple centralized units.

[0025] Advantageously, these aspects enhance system key performance indicators (KPIs) utilizing a cell-free MIMO architecture with multiple centralized units.

[0026] Advantageously, through these aspects, it may be possible to avoid including APs in some clusters in situations where the APs are already overloaded due to high traffic load or have many UEs connected to them.

[0027] Advantageously, these aspects allow several KPIs or metrics to be used in deciding whether a UE should be served by an AP from one or more centralized units.

[0028] Other objectives, features and advantages of the attached embodiments will become apparent from the following detailed disclosure, the attached dependent claims as well as the accompanying drawings.

[0029] Generally, unless otherwise explicitly stated herein, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field. Unless otherwise explicitly stated, all references to "a / an / the unit, device, component, device, module, step, etc." should be openly interpreted as referring to at least one instance of a unit, device, component, device, module, step, etc. Unless explicitly stated otherwise, the steps of any method disclosed herein do not have to be strictly performed in the exact order disclosed. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The inventive concept will now be described by way of example with reference to the accompanying drawings, in which:

[0031] Figure 1 is a schematic diagram showing a communication network according to an embodiment;

[0032] Figure 2 , Figure 3 , Figure 4 and Figure 5 is a flow chart of a method according to an embodiment;

[0033] Figure 6 is a signaling diagram according to an embodiment;

[0034] Figure 7 is a schematic diagram showing functional units of a centralized node according to an embodiment;

[0035] Figure 8 is a schematic diagram showing functional modules of a centralized node according to an embodiment;

[0036] Fig. 9 is a schematic diagram showing functional units of a UE according to an embodiment;

[0037] Fig.10 is a schematic diagram showing functional modules of a UE according to an embodiment; and

[0038] Fig.11 An example of a computer program product comprising computer readable means according to an embodiment is shown. DETAILED DESCRIPTION

[0039] The inventive concept will now be described more fully below with reference to the accompanying drawings showing certain embodiments of the inventive concept. However, the inventive concept can be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that the disclosure will be comprehensive and complete and the scope of the inventive concept will be fully conveyed to those skilled in the art. Throughout this specification, similar references refer to similar units. Any steps or features shown by dashed lines should be considered optional.

[0040] Figure 1 is a schematic diagram showing a communication network 100 to which the embodiments presented herein can be applied. The communication network 100 includes APs 400a:400h, which are also identified as AP-A0, AP-A1, ..., AP-C2. In this regard, the embodiments disclosed herein are not limited to any specific number of APs. Each AP 400a:400h can be a (radio) access network node, a radio base station, a base transceiver station, a Node B (NB), an evolved Node B (eNB), a gNB, an integrated access and backhaul (IAB) node, one or more distributed antennas, etc. The AP 400a:400h is operably connected to one or more centralized nodes 200a, 200b, 200c, which are also identified as CPU-A, CPU-B and CPU-C via an interface. The centralized node can represent an interface to the core network. Each centralized node 200a:200c can be a (radio) base station, etc. Each centralized node 200a:200c controls its own non-overlapping set of APs. In Figure 1In an illustrative example of , CPU-X controls AP-XY, where X=A, B, C, and where Y=0, 1, 2. APs 400a:400h are configured to provide network access to UEs 300a, 300b, 300c, 300d (also identified as UE-#0...UE-#3). Each such UE 300a:300d may be any of a portable wireless device, a mobile station, a mobile phone, a cell phone, a wireless local loop phone, a smart phone, a laptop computer, a tablet computer, a wireless modem, a wireless sensor device, an Internet of Things (IoT) device, a network-equipped vehicle, and the like. Each such UE 300a:300d is configured to wirelessly communicate with one or more of APs 400a:400h. In some aspects, the communication network 100 is a D-MIMO network. Thus, in some examples, APs 400a:400h are part of a D-MIMO network. UE-#0 is served by only a single AP, while UE-#1 is served by four APs. Furthermore, AP-B1 provides services to both UE-#1 and UE-#2. Both UE-#1 and UE-#2 are served by APs in different disjoint sets (i.e., they are controlled by different centralized nodes). It may be challenging to determine which one or more APs should provide services to one or more UEs.

[0041] The first procedure performed by each UE when entering any mobile network is the initial access. The initial access procedure is described in detail in 3GPPTS 38.213 V16.7.0, “3GPP Technical Specification Group Radio Access Network; NR; Physical layer procedures for control (Release 16)”, September 2021. In a New Radio (NR) type of communication system, the initial access procedure starts with the sending of a synchronization signal block (SSB) burst or message 0 (MSG0) from the AP to the UE. In practice, the AP sends the SSB for each beam, and the UE probes its environment, tries to detect the beam with the highest received power, reference signal received power (RSRP), and decodes the SSB associated with it.

[0042] After decoding the SSB and aligning the downlink-uplink beam pair, the UE sends a random access request or message 1 (MSG1). Since the network does not yet know the identity of the UE, the AP is expected to receive any MSG1 at a specific time-frequency opportunity, which is denoted as a random access channel (RACH) opportunity. Therefore, by detecting the signal (random access request or MSG1) at a specific RACH opportunity, the network associates the random access identity to the UE. In addition, the network knows the decoded SSB from the received signal and thus the corresponding downlink beam. In the next step, a random access response (RAR) or message 2 (MSG2) scrambled with the UE's random access identity is sent from the AP. This message indicates to the UE the authorized uplink resources to be used in the next message and its new identity (i.e., the UE's temporary network identity).

[0043] By sending message 3 (MSG3), the UE indicates the establishment cause and the identity of this UE. If two or more UEs have decoded the same SSB and have sent the same MSG1 at the same RACH opportunity, these UEs are considered to be in conflict with each other, i.e., have the same temporary identity (from the network point of view). If at most one uplink signal is correctly decoded by the AP (or gNB), the identity of this UE is used to resolve this conflict. The initial access procedure ends with the sending of message 4 (MSG4) from the AP, in which the conflict (if any) is resolved, and the radio bearer configuration is informed and the primary cell group information is provided to the UE. Furthermore, at this stage, the UE's temporary identity is discarded and replaced by the UE's cell identity.

[0044] As disclosed above, the selection of which APs should serve which UE in a MIMO system (eg, in a distributed MIMO system or a cell-less MIMO system with multiple centralized units) needs to be improved.

[0045] In more detail, some issues involve forming an AP cluster to serve UEs when the APs are controlled by different centralized units, since the APs controlled by different centralized units may not be able to send data to some UEs in a coherent manner.

[0046] In more detail, some issues relate to the type of information that a centralized unit needs to access, or the type of information that needs to be exchanged between centralized units, when forming an AP cluster to provide services to a UE.

[0047] In more detail, some issues involve one or more criteria to apply when deciding whether a given UE is served by an AP controlled by a single centralized unit or by APs controlled by different centralized units. This may affect how clusters are formed and the information exchanged between centralized units.

[0048]

[0013] In further aspects, some issues relate to how to enhance system performance when allowing UEs to establish connections with APs controlled by one or more centralized units.

[0049] Specifically, the embodiments disclosed herein relate to a technique for selecting an AP 400a:400h cluster in a D-MIMO network 100 to provide services for a UE 300a:300d and a UE 300a:300d, which is assigned an AP 400a:400h cluster that provides services for the UE 300a:300d in the D-MIMO network 100. To achieve this technique, a centralized node 200a:200c, a method performed by the centralized node 200a:200c, and a computer program product including code (e.g., in the form of a computer program) are provided, which, when executed on a processing circuit of the centralized node 200a:200c, causes the centralized node 200a:200c to perform the method. To achieve this technology, a UE 300a:300d, a method performed by the UE 300a:300d, and a computer program product including code (eg, in the form of a computer program) that, when executed on a processing circuit of the UE 300a:300d, causes the UE 300a:300d to perform the method are also provided.

[0050] According to at least some of the embodiments disclosed herein, a centralized unit is able to form an AP cluster that provides services for each UE by exchanging a limited amount of information. Based on some information received by the centralized unit from a given UE, the centralized unit communicates information with other centralized units and decides whether the given UE will be served by an AP controlled by one or more centralized units. The cluster formation process performed at the centralized unit takes into account the inherent benefits of a distributed MIMO system with multiple centralized nodes, the KPIs to be optimized in the system, and the overload conditions at each AP. The decision made by the centralized unit is then notified to the UE, which triggers the initial process with the selected AP.

[0051] Thus, in some aspects, it is assumed that the centralized unit has knowledge of some metrics related to the link quality between each UE and at least some APs. It is assumed that the link quality varies very slowly over time, for example, with large-scale fading values ​​of the link. This knowledge can be acquired through a conventional channel estimation process, where, for example, the UE periodically or aperiodically sends an uplink reference signal to the AP, either in conjunction with a data transmission or upon request by the centralized unit or the AP.

[0052] Furthermore, there is no requirement that APs controlled by different centralized units be able to serve a certain UE using coherent transmissions, and thus the embodiments disclosed herein do not rely on or require any synchronization criteria to be met between APs controlled by different centralized units. Thus, a general transmission scheme may be employed in which a subset of APs controlled by a certain centralized unit may form a coherent group to serve a given UE in a coherent manner. Then, in the event that a second coherent AP group controlled by a different centralized unit also serves the same given UE, the UE treats the transmissions from the two different coherent AP groups as incoherent transmissions.

[0053] Reference now Figure 2 , Figure 2 A method for selecting AP clusters 400a:400h in the D-MIMO network 100 to provide services for UEs 300a:300d is shown, which is performed by a centralized node 200a in the D-MIMO network 100 according to an embodiment.

[0054] S102: The centralized node 200a acquires a list of candidate APs 400a:400h from the UE 300a:300d via one of the APs 400a:400h in the D-MIMO network 100. The UE 300a:300d has received downlink reference signals with power higher than a power threshold from all APs 400a:400h in the list of candidate APs 400a:400h.

[0055] S106: The centralized node 200a selects a cluster of APs 400a:400h to serve the UEs 300a:300d in the D-MIMO network 100. The AP 400a:400h is selected from the list of candidate APs 400a:400h based on a cluster selection criterion operating on the list of candidate APs 400a:400h.

[0056] S110: The centralized node 200a provides the list of selected APs 400a:400h to the UE 300a:300d by sending the list of selected APs 400a:400h to the UE 300a:300d via one of the selected APs 400a:400h (or via the AP that obtained the list of candidate APs 400a:400h from the UE 300a:300d).

[0057] Thus, the centralized node 200a is able to select an optimized AP cluster to serve each UE in a D-MIMO network having one or more centralized nodes 200a:200c controlling their own AP groups.

[0058] Embodiments will now be disclosed relating to further details of the process performed by the centralized node 200a for selecting a cluster of APs 400a:400h in the D-MIMO network 100 to serve the UEs 300a:300d.

[0059] like Figure 1 As shown, the D-MIMO network 100 may include more than one centralized node 200a: 200c. Therefore, the centralized node 200a may be considered as a first centralized node 200a, wherein the D-MIMO network 100 also includes (at least) a second centralized node 200b: 200c. In some embodiments, the list of candidate APs 400a: 400h includes at least a first AP 400a controlled by the first centralized node 200a and at least a second AP 400d controlled by the second centralized node 200b: 200c.

[0060] In some embodiments, the AP 400a:400h from which the list of candidate APs 400a:400h is obtained is controlled by the first centralized node 200a. As will be described below, the AP 400a:400h from which the list of candidate APs 400a:400h is obtained is represented as the master AP of the UE, and the centralized node 200a of the master AP is represented as the master centralized node.

[0061] In some embodiments, the selected list of APs 400a:400h includes at least a first AP 400a controlled by a first centralized node 200a and at least a second AP 400d controlled by a second centralized node 200b. Thus, the selected list of APs 400a:400h may contain some APs controlled by a primary centralized node and some APs controlled by another centralized node.

[0062] There may be different cluster selection criteria, and embodiments related thereto will now be disclosed.

[0063] In some aspects, cluster selection is driven by some optimization criteria. Specifically, in some embodiments, the cluster selection criteria is an optimization criteria.

[0064] These criteria may be different examples of optimization criteria.

[0065] Some optimization criteria involve maximizing the total system rate and / or maximizing the minimum UE data rate. Therefore, in some non-limiting examples, the optimization criteria involve at least one of the following: maximizing the system data rate of the D-MIMO network 100, maximizing the minimum data rate of each of the UEs 300a:300d.

[0066] In further aspects, the selected cluster is based on historical link quality between the UE 300a:300d and the candidate AP 400a:400h. Specifically, in some embodiments, the cluster selection criteria is based on historical link quality values ​​of previous links between the UE 300a:300d and the AP 400a:400h in the D-MIMO network 100.

[0067] In further aspects, the selected cluster is based on one or more channel-related metrics, such as combined path loss and shadowing of a channel between a given AP and a UE. Specifically, in some embodiments, the historical link quality values ​​relate to path loss and / or shadowing.

[0068] In still other aspects, the AP is randomly selected among the candidate APs 400a:400h. Specifically, in some embodiments, the cluster selection criteria include randomly selecting a cluster of APs 400a:400h from a list of candidate APs 400a:400h to serve the UE 300a:300d.

[0069] In addition, in some aspects, UE 300a:300d is served only by a group of APs 400a:400h that are all controlled by the same centralized node. That is, in some embodiments, the cluster selection criteria include selecting only APs 400a:400h that are controlled only by the first centralized node 200a or only by the second centralized node 200b to provide services for UE 300a:300d. In the case where only APs from non-master centralized nodes are selected to form an AP cluster for a target UE, the centralized node becomes the master centralized node for the UE, and one of the selected APs becomes the master AP. This method forms a single coherent group that provides services to the UE, which generally provides a high system data rate.

[0070] Another option is to select APs controlled by two or more different centralized nodes. This method forms multiple coherent AP groups that provide services to the UE, and the multiple coherent AP groups will transmit to the UE in an incoherent manner. This method can help increase the minimum data rate of the UE without excessively compromising the overall system data rate. In addition, the multiple coherent AP groups can also transmit to the UE in a coherent manner.

[0071] In some examples, the AP cluster serving a given UE is selected based on the following rules:

[0072] in, yes and A measure of channel correlation between channels (such as combined path loss and shadowing), where It will be A set of APs providing services, where: is used for A set of APs in a list of candidate APs providing service, and wherein, Is indicated by The first step of the algorithm is to sort the expected value of the total power received in descending order. Then, add APs to the set one by one. , calculate the value of that part, and check if it is greater than or equal to If yes, no more APs are added and a set is formed. Through this program, The channel with the strongest channel and whose contribution to the total power received by the UE exceeds AP.

[0073] In some aspects, the centralized node 200a is configured to inform the surrounding centralized nodes 200b, 200c of the received list of candidate APs 400a:400h. This may be the case when the received list of APs 400a:400h contains identifiers (e.g., physical cell identifiers (PCIDs)) of APs 400a:400h that are not controlled by the centralized node 200a itself. Therefore, in some embodiments, the centralized node 200a is configured to perform (optional) step S104.

[0074] S104: The centralized node 200a informs the second centralized nodes 200b, 200c of the list of candidate APs 400a:400h.

[0075] In some aspects, the centralized node 200a is configured to inform the surrounding centralized nodes 200b, 200c of the selected candidate AP 400a: 400h list. This may be the case when the selected AP list includes APs 400a: 400h controlled by different centralized nodes. Therefore, in some embodiments, the centralized node 200a is configured to perform (optional) step S108.

[0076] S108 : The centralized nodes 200a : 200c inform the second centralized node 200b that the at least one second AP 400d will provide services for the UEs 300a : 300d in the D-MIMO network 100 .

[0077] In some aspects, the centralized node 200a is configured to inform the surrounding centralized nodes 200b, 200c of the availability of its AP 400a: 400h for forming an AP 400a: 400h cluster. This may be the case when the centralized node 200a receives a list of candidate APs 400a: 400h from another centralized node 200b, 200c. This may be the case when the centralized node 200a is not the master centralized node. Therefore, in some embodiments, the centralized node 200a is configured to perform (optional) steps S112 and S114.

[0078] S112: The centralized nodes 200a:200c receive a list of additional candidate APs 400a:400h for serving additional UEs 300a:300d in the D-MIMO network 100 from the second centralized node 200b.

[0079] S114: The centralized nodes 200a:200c inform the second centralized node 200b of the capabilities of the other candidate APs 400a:400h to provide services for the other UEs 300a:300d.

[0080] Reference now Figure 3 , Figure 3 A method performed by a UE 300a:300d according to an embodiment is shown, wherein the UE 300a:300d is assigned a cluster of APs 400a:400h in the D-MIMO network 100 that provides service to the UE 300a:300d.

[0081] S204: UE 300a:300d acquires a list of candidate APs 400a:400h. To this end, UE 300a:300d detects the radio environment of UE 300a:300d to receive downlink reference signals from APs 400a:400h in D-MIMO network 100. UE 300a:300d selects to include in the list of candidate APs 400a:400h all APs 400a:400h for which the power of the received downlink reference signals is higher than the power threshold.

[0082] S206: The UE 300a:300d provides the list of candidate APs 400a:400h to the centralized node 200a:200c in the D-MIMO network 100. The list of candidate APs 400a:400h is sent to the centralized node 200a:200c via one of the APs 400a:400h in the list of candidate APs 400a:400h.

[0083] S208: The UE 300a:300d acquires the list of selected APs 400a:400h from the centralized node 200a:200c via one of the APs 400a:400h in the list of selected APs 400a:400h. Thus, the UE 300a:300d is assigned its cluster of APs 400a:400h.

[0084] S210 : The UE 300a : 300d performs a random access procedure with the AP 400a : 400h in the list of selected APs 400a : 400h . The random access procedure is performed on the AP 400a : 400h in the list of selected APs 400a : 400h to start providing service to the UE 300a : 300d in the D-MIMO network 100 .

[0085] Embodiments will now be disclosed with further details of a UE 300a:300d that is assigned a cluster of APs 400a:400h in the D-MIMO network 100 that serves the UE 300a:300d.

[0086] In some aspects, the UE 300a:300d first connects to a single AP. Specifically, in some embodiments, the UE 300a:300d is configured to perform (optional) step S202.

[0087] S202: The UE 300a:300d establishes a connection with one of the APs 400a:400h in the D-MIMO network 100 before acquiring the list of candidate APs 400a:400h. The list of candidate APs 400a:400h is sent to the AP in the AP 400a:400h. The list of selected APs 400a:400h is received from the AP in the AP 400a:400h.

[0088] Then, the first AP 400a is referred to as a master AP, and the centralized node controlling the AP is referred to as a master centralized node.

[0089] As disclosed above, in some aspects, the list of candidate APs 400a:400h includes some APs 400a:400h controlled by the master centralized node and some APs controlled by another (non-master) centralized node. That is, in some embodiments, the list of candidate APs 400a:400h includes at least a first AP 400a controlled by a first centralized node 200a and at least a second AP 400d controlled by a second centralized node 200b. Then, the list of candidate APs 400a:400h is sent to the master centralized node via the master AP. That is, in some embodiments, one of the APs 400a:400h is controlled by the first centralized node 200a.

[0090] As disclosed above, in some aspects, the list of selected APs 400a:400h includes some APs 400a:400h controlled by the master centralized node and some APs selected by another (non-master) centralized node. That is, in some embodiments, the list of selected APs 400a:400h includes at least a first AP 400a controlled by a first centralized node 200a and at least a second AP 400d controlled by a second centralized node 200b.

[0091] Now refer to Figure 4 and Figure 5 Flowchart and Figure 6 A specific embodiment of selecting which APs 400a:400h to provide services for which UEs 300a:300d in the D-MIMO network 100 based on at least some of the embodiments disclosed above is disclosed in detail with reference to a signaling diagram of FIG.

[0092] In the current illustrative example, there are two centralized units (hereinafter respectively indicated as CPU-A and CPU-B) and four APs (hereinafter respectively indicated as AP-A0, AP-A1, AP-B0 and AP-B1). CPU-A controls AP-A0 and AP-A1, while CPU-B controls AP-B0 and AP-B1. A set of APs will be selected to provide services for a UE (hereinafter indicated as UE#0). As will be explained below, AP-A0 is selected as the master AP for UE#0, so CPU-A is the master CPU for UE#0. During the execution of the clustering algorithm, CPU-A selects AP-B0 and AP-B1 from CPU-B to form a cluster that provides services for UE#0. Figure 4 Schematic diagram showing the steps of the method executed by CPU-A. Figure 5 The steps of the method performed by UE#0 are shown. Figure 6 The steps of the method performed by CPU-A, CPU-B, AP-A0, AP-A1, AP-B0, AP-B1 and UE#0 are shown.

[0093] Next, operations performed by CPU-A and CPU-B will be disclosed.

[0094] S301: CPU-A receives a message from AP-A0. The message includes at least an identifier of UE#0. As further disclosed below, this means that AP-A0 is the master AP of UE#0. This in turn means that CPU-A is the master CPU of UE#0.

[0095] S302: CPU-A receives from AP-A0 a list of candidate APs for forming an AP cluster serving UE#0. As further disclosed below, the list includes the PCID of the best AP selected by the UE.

[0096] S303: CPU-A informs CPU-B of the received list of candidate APs for UE#0. S303 is executed only when CPU-B notices that the received list of PCIDs includes at least one PCID of an AP that is not controlled by CPU-A (as checked in step S302a).

[0097] S304: CPU-B receives the list of candidate APs from CPU-A.

[0098] S305: CPU-B informs CPU-A about the availability of AP-B0 and AP-B1 to form an AP cluster for UE#0. The availability is based on some criteria, such as the traffic load at AP-B0 and AP-B1 or the number of UEs currently served.

[0099] S306: CPU-A receives a list of available APs from CPU-B and updates the list accordingly (S306a).

[0100] S307: CPU-A selects a subset (or cluster) of APs that will provide services to UE#0 for UE#0. In this aspect, CPU-A selects a subset (or cluster) of APs that will provide services to UE#0 based on any of the cluster selection criteria disclosed above for operating on a list of candidate APs and possible constraints defined by the list of available APs received from CPU-B.

[0101] S308: CPU-A informs CPU-B that AP-B0 and AP-B1 controlled by CPU-B will provide services for UE#0.

[0102] S309: CPU-A sends the selected AP list to AP-A0.

[0103] Next, operations performed by AP-A0, AP-A1, AP-B0, and AP-B1 will be disclosed.

[0104] S401: AP-A0, AP-A1, AP-B0, and AP-B1 each periodically send SSB bursts.

[0105] S402: AP-A0 performs a RA process based on 3GPP to establish a connection for UE#0.

[0106] S403: AP-A0 sends a message to CPU-A, wherein the message includes at least the identifier of UE#0. At this stage, the centralized node is referred to as the master CPU of UE#0.

[0107] S404: AP-A0 receives from UE#0 a list of candidate APs for forming an AP cluster serving UE#0. As will be further disclosed below, the list includes the PCID of the best AP selected by UE#0.

[0108] S405: AP-A0 sends a list of candidate APs to CPU-A.

[0109] S406: AP-A0 receives the list of selected APs from CPU-A.

[0110] S407: AP-A0 sends the list of selected APs to UE#0.

[0111] S408: AP-B0 and AP-B1 perform a RA process based on 3GPP to establish a connection for UE#0.

[0112] Next, the actions performed by UE#0 will be disclosed.

[0113] S501: UE#0 detects the environment to detect the best beam from the SSB burst sent from the AP as disclosed above.

[0114] S502: UE#0 performs a RA process based on 3GPP to establish a connection with the AP (ie, AP-A0). Therefore, AP-A0 is called the primary AP.

[0115] S503: UE#0 continues to detect SSB bursts to detect N best beams, and creates a list, in which the PCID of the AP that sends the beam is included in the list.

[0116] Figure 4 The detailed process for creating the list of best PCIDs is shown in FIG.

[0117] S503a: UE#0 continues to detect SSB bursts.

[0118] S503b: Check whether UE#0 has not detected any new SSB (or PCID) and the predefined time interval has expired. If yes, proceed to step S504. If no, proceed to step S305c.

[0119] S503c: UE#0 detects an SSB whose RSRP value is higher than a threshold value, and decodes the primary synchronization signal (PSS) and the secondary synchronization signal (SSS) of the SSB to obtain the PCID.

[0120] S503d: Check whether the PCID is in the list of best PCIDs. If yes, proceed to step S503a. If no, proceed to step S503e.

[0121] S503e: Add the PCID to the list of best PCIDs.

[0122] S503f: Check whether the list of best PCIDs is full. If yes, proceed to step S504. If no, proceed to step S503a.

[0123] S504: UE#0 sends a list of best PCIDs to the primary AP.

[0124] S505: UE#0 receives a list of selected APs from the master AP, and these selected APs form an AP cluster that provides services for UE#0.

[0125] S506: UE#0 contacts the APs in the selected AP list to perform a 3GPP-based RA procedure to establish connections with these APs.

[0126] Figure 7 The components of the centralized nodes 200a:200c according to the embodiment are schematically shown in the form of a plurality of functional units. Fig.11 The processing circuit 210 may be provided by any combination of one or more of a suitable central processing unit (CPU), a multiprocessor, a microcontroller, a digital signal processor (DSP), etc., which may be implemented in accordance with software instructions stored in a storage medium (e.g., in the form of a storage medium 230). The processing circuit 210 may also be provided as at least one application specific integrated circuit (ASIC) or a field programmable gate array (FPGA).

[0127] Specifically, the processing circuit 210 is configured to cause the centralized node 200a:200c to perform a set of operations or steps as described above. For example, the storage medium 230 can store the set of operations, and the processing circuit 210 can be configured to retrieve the set of operations from the storage medium 230 to cause the centralized node 200a:200c to perform the set of operations. The set of operations can be provided as an executable instruction set. Therefore, the processing circuit 210 is thereby arranged to perform the method as disclosed herein.

[0128] The storage medium 230 may also include a persistent storage device, which may be, for example, any single memory or any combination of magnetic memory, optical memory, solid-state memory, or even remotely mounted memory.

[0129] The centralized node 200a:200c may also include a communication interface 220 for communicating directly or indirectly with other entities, functions, nodes and devices (eg, AP 400a:400h, UE 300a:300d, and other centralized nodes 200a:200c), such as Figure 1 Thus, the communication interface 220 may include one or more transmitters and receivers including analog components and digital components.

[0130] The processing circuit 210 controls the overall operation of the centralized nodes 200a:200c, for example, by sending data and control signals to the communication interface 220 and the storage medium 230, by receiving data and reports from the communication interface 220, and by retrieving data and instructions from the storage medium 230. Other components of the centralized nodes 200a:200c and related functions are omitted so as not to obscure the concepts presented herein.

[0131] Figure 8 The components of the centralized nodes 200a : 200c according to the embodiment are schematically shown in the form of a plurality of functional modules. Figure 8 The centralized nodes 200a:200c include multiple functional modules: an acquisition module 210a configured to execute step S102, a selection module 210c configured to execute step S106, and a providing module 210e configured to execute step S110. Figure 8 The centralized nodes 200a:200c may also include multiple optional functional modules, such as any of the following modules: a notification module 210b configured to execute step S104, a notification module 210d configured to execute step S108, a receiving module 210f configured to execute step S112, and a notification module 210g configured to execute step S114.

[0132] In general, each functional module 210a:210g may be implemented in hardware or software. Preferably, one or more or all functional modules 210a:210g may be implemented by a processing circuit 210 that may cooperate with a communication interface 220 and / or a storage medium 230. The processing circuit 210 may therefore be arranged to obtain instructions provided by the functional module 210a:210g from the storage medium 230, and to execute these instructions, thereby performing any steps of the centralized node 200a:200c disclosed herein.

[0133] The centralized node 200a:200c may be provided as a stand-alone device or as part of at least one additional device. For example, the centralized node 200a:200c may be arranged in a node of a radio access network or a node of a core network. Alternatively, the functions of the centralized node 200a:200c may be distributed between at least two devices or nodes. The at least two nodes or at least two devices may be part of the same network portion (such as a radio access network or a core network), or may be spread between at least two such network portions. In general, instructions that need to be executed in real time may be executed in a device or node that is closer to the cell than instructions that do not need to be executed in real time. Therefore, a first part of the instructions executed by the centralized node 200a:200c may be executed in a first device, and a second part of the instructions executed by the centralized node 200a:200c may be executed in a second device; the embodiments disclosed herein are not limited to any specific number of devices on which the instructions executed by the centralized node 200a:200c may be executed. Therefore, the method according to the embodiment disclosed herein is suitable for being executed by the centralized node 200a:200c residing in the cloud computing environment. Figure 7 A single processing circuit 210 is shown in FIG. 1 , but the processing circuit 210 may be distributed among multiple devices or nodes. The same applies to Figure 8 Functional modules 210a:210g and Fig.11 Computer program 1120a.

[0134] Fig. 9 The components of the UE 300a:300d according to the embodiment are schematically shown in the form of a plurality of functional units. Fig.11 The processing circuit 310 may be provided by any combination of one or more of a suitable central processing unit (CPU), a multiprocessor, a microcontroller, a digital signal processor (DSP), etc., which may be implemented in accordance with software instructions stored in a storage medium (e.g., in the form of a storage medium 330). The processing circuit 310 may also be provided as at least one application specific integrated circuit (ASIC) or a field programmable gate array (FPGA).

[0135] Specifically, the processing circuit 310 is configured to cause the UE 300a:300d to perform a set of operations or steps as described above. For example, the storage medium 330 may store the set of operations, and the processing circuit 310 may be configured to retrieve the set of operations from the storage medium 330 to cause the UE 300a:300d to perform the set of operations. The set of operations may be provided as an executable instruction set. Therefore, the processing circuit 310 is thereby arranged to perform the method as disclosed herein.

[0136] The storage medium 330 may also include a persistent storage device, which may be, for example, any single memory or any combination of magnetic memory, optical memory, solid-state memory, or even remotely mounted memory.

[0137] The UE 300a:300d may also include a communication interface 320 for communicating directly or indirectly with other entities, functions, nodes and devices (eg, AP 400a:400h and centralized nodes 200a:200c), such as Figure 1 Thus, the communication interface 320 may include one or more transmitters and receivers including analog components and digital components.

[0138] The processing circuit 310 controls the overall operation of the UE 300a:300d, for example, by sending data and control signals to the communication interface 320 and the storage medium 330, by receiving data and reports from the communication interface 320, and by retrieving data and instructions from the storage medium 330. Other components of the UE 300a:300d and related functions are omitted so as not to obscure the concepts presented herein.

[0139] Fig.10 The components of the UE 300a:300d according to the embodiment are schematically shown in the form of multiple functional modules. Fig.10 The UE 300a:300d includes multiple functional modules: an acquisition module 310b configured to execute step S204, a providing module 310c configured to execute step S206, an acquisition module 310d configured to execute step S208, and a random access (RA) module 310e configured to execute step S210. Fig.10The UE 300a:300d may also include a plurality of optional functional modules, such as a setup module 310a configured to perform step S202. In general, each functional module 310a:310e may be implemented in hardware or software. Preferably, one or more or all functional modules 310a:310e may be implemented by a processing circuit 310 that may cooperate with a communication interface 320 and / or a storage medium 330. Therefore, the processing circuit 310 may be arranged to obtain instructions provided by the functional module 310a:310e from the storage medium 330 and execute these instructions, thereby performing any steps of the UE 300a:300d as disclosed herein.

[0140] Fig.11 An example of a computer program product 1110a, 1110b including a computer readable device 1130 is shown. On the computer readable device 1130, a computer program 1120a may be stored, which may cause the processing circuit 210 and entities and devices (e.g., communication interface 220 and storage medium 230) operatively coupled to the processing circuit 210 to perform methods according to the embodiments described herein. Therefore, the computer program 1120a and / or the computer program product 1110a may provide means for performing any steps of the centralized node 200a:200c disclosed herein. On the computer readable device 1130, a computer program 1120b may be stored, which may cause the processing circuit 310 and entities and devices (e.g., communication interface 320 and storage medium 330) operatively coupled to the processing circuit 310 to perform methods according to the embodiments described herein. Thus, the computer program 1120b and / or computer program product 1110b may provide means for performing any of the steps of the UE 300a:300d disclosed herein.

[0141] exist Fig.11 In the example of , the computer program products 1110a, 1110b are shown as an optical disc, such as a CD (Compact Disc) or a DVD (Digital Versatile Disc) or a Blu-ray Disc. The computer program products 1110a, 1110b may also be embodied as a memory, such as a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM), or an electrically erasable programmable read-only memory (EEPROM) and more specifically as a non-volatile storage medium of a device in an external memory, such as a USB (Universal Serial Bus) memory or a flash memory (e.g., a compact flash). Thus, although the computer programs 1120a, 1120b are schematically shown here as tracks on the depicted optical disc, the computer programs 1120a, 1120b may be stored in any manner suitable for the computer program products 1110a, 1110b.

[0142] The inventive concept has been mainly described above with reference to some embodiments. However, it is readily apparent to a person skilled in the art that other embodiments than those disclosed above are also possible within the scope of the inventive concept as defined by the attached patent claims.

Claims

1. A method for selecting a cluster of access points AP (400a:400h) in a distributed multiple-input multiple-output D-MIMO network (100) to provide services for user equipment UE (300a:300d), wherein: The method is performed by a centralized node (200a:200c) in a D-MIMO network (100), wherein the method comprises: Acquiring (S102) a list of candidate APs (400a:400h) from the UE (300a:300d) via one of the APs (400a:400h) in the D-MIMO network (100), wherein the UE (300a:300d) has received downlink reference signals having a power higher than a power threshold from all APs (400a:400h) in the list of candidate APs (400a:400h); selecting (S106) a cluster of APs (400a:400h) in the D-MIMO network (100) to serve the UE (300a:300d), wherein the APs (400a:400h) are selected from the list of candidate APs (400a:400h) based on a cluster selection criterion operating on the list of candidate APs (400a:400h); and The list of selected APs (400a:400h) is provided (S110) to the UE (300a:300d) by sending the list of selected APs (400a:400h) to the UE (300a:300d) via one of the selected APs (400a:400h).

2. The method according to claim 1, wherein: The centralized node (200a:200c) is a first centralized node (200a), wherein the D-MIMO network (100) further includes a second centralized node (200b), and wherein the list of candidate APs (400a:400h) includes at least a first AP (400a) controlled by the first centralized node (200a) and at least a second AP (400d) controlled by the second centralized node (200b).

3. The method according to claim 1 or 2, wherein: One of the APs (400a:400h) from which the list of candidate APs (400a:400h) is obtained is controlled by the first centralized node (200a).

4. A method according to any one of the preceding claims, wherein: The centralized node (200a:200c) is a first centralized node (200a), wherein the D-MIMO network (100) further includes a second centralized node (200b), and wherein the list of selected APs (400a:400h) includes at least a first AP (400a) controlled by the first centralized node (200a) and at least a second AP (400d) controlled by the second centralized node (200b).

5. A method according to any one of the preceding claims, wherein: The cluster selection criterion is an optimization criterion.

6. The method according to claim 5, wherein: The optimization criterion relates to at least one of the following: maximizing the system data rate of the D-MIMO network (100), maximizing the minimum data rate of each of the UEs (300a:300d).

7. A method according to any one of the preceding claims, wherein: The cluster selection criteria are based on historical link quality values ​​of previous links between UEs (300a: 300d) and APs (400a: 400h) in the D-MIMO network (100).

8. The method according to claim 7, wherein: The historical link quality values ​​relate to path loss and / or shadowing.

9. The method according to any one of claims 1 to 4, wherein: The cluster selection criteria includes randomly selecting the AP cluster (400a: 400h) from the list of candidate APs (400a: 400h) to serve the UE (300a: 300d).

10. The method according to claim 2, wherein: The cluster selection criteria include selecting only APs (400a: 400h) controlled only by the first centralized node (200a) or only by the second centralized node (200b) to provide services for the UE (300a: 300d).

11. The method according to claim 2, wherein: The method further comprises: The second centralized node (200b) is informed (S104) of the list of candidate APs (400a:400h).

12. The method according to claim 4, wherein: The method further comprises: The second centralized node (200b) is informed (S108) that the at least second AP (400d) will provide service for the UE (300a: 300d) in the D-MIMO network (100).

13. A method according to any one of the preceding claims, wherein: The centralized node (200a: 200c) is a first centralized node (200a), wherein the D-MIMO network (100) further comprises a second centralized node (200b), and wherein the method further comprises: receiving (S112) from the second centralized node (200b) a list of further candidate APs (400a: 400h) for serving further UEs (300a: 300d) in the D-MIMO network (100); The second centralized node (200b) is informed (S114) of the capability of the further candidate AP (400a: 400h) to provide service for the further UE (300a: 300d).

14. A method for allocating a user equipment UE (300a: 300d) to a cluster of access points AP (400a: 400h) serving the UE (300a: 300d) in a distributed multiple-input multiple-output D-MIMO network (100), wherein: The method is performed by the UE (300a: 300d), and the method includes: Acquiring (S204) a list of candidate APs (400a:400h) by: detecting a radio environment of the UE (300a:300d) to receive downlink reference signals from APs (400a:400h) in the D-MIMO network (100), and selecting to include in the list of candidate APs (400a:400h) all APs (400a:400h) for which the power of the received downlink reference signals is higher than a power threshold; providing (S206) the list of candidate APs (400a:400h) to the centralized node (200a:200c) in the D-MIMO network (100) by sending the list of candidate APs (400a:400h) to the centralized node (200a:200c) via one of the APs (400a:400h) in the list of candidate APs (400a:400h); acquiring ( S208 ) the list of selected APs ( 400a : 400h ) from the centralized node ( 200a : 200c ) via one of the APs ( 400a : 400h ) in the list of selected APs ( 400a : 400h ), whereby the UE ( 300a : 300d ) is assigned its AP cluster ( 400a : 400h ); and A random access (RA) procedure is performed (S210) with an AP (400a:400h) in the list of selected APs (400a:400h) so that the AP (400a:400h) in the list of selected APs (400a:400h) starts to provide service for the UE (300a:300d) in the D-MIMO network (100).

15. The method according to claim 14, wherein: The method further comprises: Prior to acquiring the list of candidate APs (400a:400h), establishing (S202) a connection with one of the APs (400a:400h) in the D-MIMO network (100), wherein the list of candidate APs (400a:400h) is sent to one of the APs (400a:400h), and wherein the list of selected APs (400a:400h) is received from one of the APs (400a:400h).

16. The method according to claim 14 or 15, wherein: The centralized node (200a:200c) is a first centralized node (200a), wherein the D-MIMO network (100) further includes a second centralized node (200b), and wherein the list of candidate APs (400a:400h) includes at least a first AP (400a) controlled by the first centralized node (200a) and at least a second AP (400d) controlled by the second centralized node (200b).

17. The method according to claim 16, wherein: One of the APs (400a: 400h) is controlled by the first centralized node (200a).

18. The method according to any one of claims 14 to 17, wherein: The centralized node (200a:200c) is a first centralized node (200a), wherein the D-MIMO network (100) further includes a second centralized node (200b), and wherein the list of selected APs (400a:400h) includes at least a first AP (400a) controlled by the first centralized node (200a) and at least a second AP (400d) controlled by the second centralized node (200b).

19. A centralized node (200a: 200c) for selecting a cluster of access points (AP) (400a: 400h) in a distributed multiple-input multiple-output (D-MIMO) network (100) to provide services for user equipment (UE) (300a: 300d), the centralized node (200a: 200c) comprising a processing circuit (210), the processing circuit being configured to enable the centralized node (200a: 200c): A list of candidate APs (400a:400h) is obtained from the UE (300a:300d) via one of the APs (400a:400h) in the D-MIMO network (100), wherein: The UE (300a:300d) has received downlink reference signals with power higher than a power threshold from all APs (400a:400h) in the list of candidate APs (400a:400h); selecting a cluster of APs (400a:400h) in the D-MIMO network (100) to serve the UE (300a:300d), wherein the APs (400a:400h) are selected from the list of candidate APs (400a:400h) based on a cluster selection criterion operating on the list of candidate APs (400a:400h); and The list of selected APs (400a:400h) is provided to the UE (300a:300d) by sending the list of selected APs (400a:400h) to the UE (300a:300d) via one of the selected APs (400a:400h).

20. A centralized node (200a: 200c) for selecting a cluster of access points (APs) (400a: 400h) in a distributed multiple-input multiple-output (D-MIMO) network (100) to provide services for user equipment (UEs) (300a: 300d), the centralized node (200a: 200c) comprising: an acquisition module (210a) configured to acquire (S102) a list of candidate APs (400a:400h) from the UE (300a:300d) via one of the APs (400a:400h) in the D-MIMO network (100), wherein the UE (300a:300d) has received downlink reference signals having a power higher than a power threshold from all APs (400a:400h) in the list of candidate APs (400a:400h); a selection module (210c) configured to select (210c) a cluster of APs (400a:400h) in the D-MIMO network (100) to serve the UE (300a:300d), wherein the APs (400a:400h) are selected from the list of candidate APs (400a:400h) based on a cluster selection criterion operating on the list of candidate APs (400a:400h); and A providing module (210e) is configured to provide the list of selected APs (400a:400h) to the UE (300a:300d) by sending the list of selected APs (400a:400h) to the UE (300a:300d) via one of the selected APs (400a:400h).

21. The centralized node (200a: 200c) according to claim 19 or 20, further configured to perform the method according to any one of claims 2 to 13.

22. A user equipment UE (300a: 300d), which is allocated a cluster of access points AP (400a: 400h) serving the UE (300a: 300d) in a distributed multiple-input multiple-output D-MIMO network (100), wherein the UE (300a: 300d) comprises a processing circuit (310), wherein the processing circuit is configured to enable the UE (300a: 300d): Acquiring a list of candidate APs (400a:400h) by: detecting a radio environment of the UE (300a:300d) to receive downlink reference signals from APs (400a:400h) in the D-MIMO network (100), and selecting to include in the list of candidate APs (400a:400h) all APs (400a:400h) for which the power of the received downlink reference signals is higher than a power threshold; providing the list of candidate APs (400a:400h) to the centralized node (200a:200c) in the D-MIMO network (100) by sending the list of candidate APs (400a:400h) to the centralized node (200a:200c) via one of the APs (400a:400h) in the list of candidate APs (400a:400h); acquiring the list of selected APs (400a:400h) from the centralized node (200a:200c) via one of the APs (400a:400h) in the list of selected APs (400a:400h), whereby the UE (300a:300d) is assigned its AP cluster (400a:400h); and A random access (RA) procedure is performed with an AP (400a:400h) in the list of selected APs (400a:400h) to enable the AP (400a:400h) in the list of selected APs (400a:400h) to start serving the UE (300a:300d) in the D-MIMO network (100).

23. A user equipment UE (300a: 300d), in a distributed multiple-input multiple-output D-MIMO network (100), is allocated a cluster of access points AP (400a: 400h) serving the UE (300a: 300d), the UE (300a: 300d) comprising: An acquisition module (310b) is configured to acquire a list of candidate APs (400a:400h) by: detecting a radio environment of the UE (300a:300d) to receive downlink reference signals from APs (400a:400h) in the D-MIMO network (100), and selecting to include in the list of candidate APs (400a:400h) all APs (400a:400h) for which the power of the received downlink reference signals is higher than a power threshold; a providing module (310c) configured to provide the list of candidate APs (400a:400h) to the centralized node (200a:200c) in the D-MIMO network (100) by sending the list of candidate APs (400a:400h) to the centralized node (200a:200c) via one of the APs (400a:400h) in the list of candidate APs (400a:400h); an acquisition module (310d) configured to acquire the list of selected APs (400a:400h) from the centralized node (200a:200c) via one of the APs (400a:400h) in the list of selected APs (400a:400h), whereby the UE (300a:300d) is assigned its AP cluster (400a:400h); as well as The random access module (310e) is configured to perform a random access RA procedure with an AP (400a:400h) in the list of selected APs (400a:400h) so that the AP (400a:400h) in the list of selected APs (400a:400h) starts to provide service for the UE (300a:300d) in the D-MIMO network (100).

24. The UE (300a: 300d) according to claim 22 or 23, further configured to perform the method according to any one of claims 15 to 18.

25. A computer program (1120a) for selecting a cluster of access points (AP) (400a: 400h) in a distributed multiple-input multiple-output (D-MIMO) network (100) to serve a user equipment (UE) (300a: 300d), the computer program comprising computer code, which, when executed on a processing circuit (210) of a centralized node (200a: 200c), causes the centralized node (200a: 200c) to: A list of candidate APs (400a:400h) is obtained (S102) from the UE (300a:300d) via one of the APs (400a:400h) in the D-MIMO network (100), wherein: The UE (300a:300d) has received downlink reference signals with power higher than a power threshold from all APs (400a:400h) in the list of candidate APs (400a:400h); selecting (S106) a cluster of APs (400a:400h) in the D-MIMO network (100) to serve the UE (300a:300d), wherein the APs (400a:400h) are selected from the list of candidate APs (400a:400h) based on a cluster selection criterion operating on the list of candidate APs (400a:400h); and The list of selected APs (400a:400h) is provided (S110) to the UE (300a:300d) by sending the list of selected APs (400a:400h) to the UE (300a:300d) via one of the selected APs (400a:400h).

26. A computer program (1120b) for a user equipment UE (300a:300d) which is assigned a cluster of access points AP (400a:400h) serving the UE (300a:300d) in a distributed multiple-input multiple-output (D-MIMO) network (100), the computer program comprising computer code which, when executed on a processing circuit (310) of the UE (300a:300d), causes the UE (300a:300d) to: Acquiring (S204) a list of candidate APs (400a:400h) by: detecting a radio environment of the UE (300a:300d) to receive downlink reference signals from APs (400a:400h) in the D-MIMO network (100), and selecting to include in the list of candidate APs (400a:400h) all APs (400a:400h) for which the power of the received downlink reference signals is higher than a power threshold; providing (S206) the list of candidate APs (400a:400h) to the centralized node (200a:200c) in the D-MIMO network (100) by sending the list of candidate APs (400a:400h) to the centralized node (200a:200c) via one of the APs (400a:400h) in the list of candidate APs (400a:400h); obtaining ( S208 ) the list of selected APs ( 400a : 400h ) from the centralized node ( 200a : 200c ) via one of the APs ( 400a : 400h ) in the list of selected APs ( 400a : 400h ), thereby allocating its AP cluster ( 400a : 400h ) to the UE ( 300a : 300d ); and A random access (RA) procedure is performed (S210) with an AP (400a:400h) in the list of selected APs (400a:400h) so that the AP (400a:400h) in the list of selected APs (400a:400h) starts to provide service for the UE (300a:300d) in the D-MIMO network (100).

27. A computer program product (1110a, 1110b) comprising a computer program (1120a, 1120b) according to at least one of claims 25 and 26 and a computer-readable storage medium (1130) on which the computer program is stored.

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