Communication systems and methods for asynchronous joint transmission using a cluster-based distributed cyclic delay diversity scheme.

Abstract

A communication system is provided, comprising a Multi-Cluster Distributed Antenna System (MC-DAS) network and a controller. Each cluster in the MC-DAS includes a cluster master (CM) and a remote radio unit (RRU) within the controller's coverage area. The controller and the DAS clusters are synchronized using a Hierarchical Precise Time Protocol (HPTP). Each DAS cluster is configured to transmit messages independently of other DAS clusters using a distributed cyclic delay diversity (CDD) scheme to determine a cyclic prefix of length. The controller also includes a controller configured to transmit messages from the controller to a receiver via one or more of the multiple DAS clusters.

Description

Technical Field

[0001] This disclosure relates to wireless communication networks, and more specifically, to a communication system and method for asynchronous joint transmission in a dedicated network using a cluster-based distributed cyclic delay diversity (CDD) scheme. Background Technology

[0002] For businesses and individuals, private networks represent a promising new connectivity model. Owners of private networks can optimize services at their facilities by planning and installing their own networks and establishing reliable communications within a specific area through the dedicated use of available resources. Because owners of private networks have complete control over every aspect of the network, they can determine how resources are used, how business is prioritized, how specific security standards are deployed, and so on. Furthermore, many potential applications related to industry, commerce, utilities, and the public sector are moving towards 5G wireless networks due to increasing stringent performance requirements in terms of availability, reliability, latency, device density, and throughput. Deployment of private networks in shared or unlicensed spectrum is feasible. However, due to the nature of the available radio spectrum for private networks, signals may require low transmission power, necessitating dense deployments of small cells such as femtocells and picocells.

[0003] To increase spectral efficiency and coverage, and to achieve flexible spatial freedom, distributed antenna systems (DAS), which involve installing antennas in a distributed manner over the coverage area of ​​a base station (BS), are considered a promising approach for dedicated networks. However, when deploying DAS in a dedicated network, it is affected by various problems such as multipath richness propagation. Furthermore, since a DAS comprises multiple antennas (transmitters), the distances of each antenna relative to the receiver are different. Therefore, the signal received at the receiver (e.g., user equipment) suffers from path-dependent propagation delay.

[0004] Therefore, there is a need for a communication system that increases spectral efficiency, reduces fading, and minimizes propagation delay in order to effectively implement DAS in dedicated networks. Summary of the Invention

[0005] One objective of some implementations is to provide a communication system implemented with a Distributed Antenna System (DAS) for transmitting messages using minimal propagation delay. A Distributed Antenna System (DAS) is a spatially separated network of antenna nodes connected to a public source via a transmission medium that provides wireless service within a geographical area or structure. Distributed Antenna Systems can be deployed indoors (iDAS) or outdoors (oDAS).

[0006] Due to their low cost and ease of deployment, DAS can be extended to provide dedicated indoor wireless coverage. Such an extended DAS may include: a centralized wireless power source, such as a base station and / or a dedicated network server (PNS) (also known as a central network server CNS) or a central controller (also known as a controller); and multiple remote radio transceivers, referred to as remote radio units (RRUs). Multiple RRUs are connected to the centralized wireless power source via a packet-based network (e.g., a local area network).

[0007] However, joint processing (JP) of transmissions via multiple RRUs in fading environments of indoor wireless communication presents a challenging problem. On the one hand, it is desirable to increase the number of RRUs to cover a larger area and increase channel diversity. On the other hand, this increase will lead to additional interference, difficulties in time synchronization, and unwanted feedback overhead.

[0008] Some implementations are based on the understanding that the negative impact of increasing the number of RRUs forming a DAS can be mitigated by using transmissions via a distributed cyclic delay diversity (CDD) scheme. Distributed CDD can achieve diversity gain by transmitting common symbol blocks without requiring full channel state information (CSI) at the transmitter. Furthermore, distributed CDD demonstrates that, under certain requirements, full diversity gain can be achieved without forward error correction (FEC) coding by employing cyclic prefix single-carrier (CP-SC) transmission with CDD. Additionally, using distributed CDD or cooperative CDD provides more reliable communication networks, etc.

[0009] However, due to the increased size of the cyclic prefix, distributed CDD only allows a limited number of RRUs to participate in message transmission. Specifically, to account for variations in synchronization errors and tap delays across different communication channels connecting the controller and receiver, some implementations determine the minimum length of the cyclic prefix in the distributed CDD scheme as a function of the sum of the synchronization error limits and maximum tap delays of the communication channels. The length of the cyclic prefix and the length of the message to be transmitted determine the maximum number of RRUs that can participate in distributed CDD transmission.

[0010] To overcome the aforementioned limitations, some implementations are based on the understanding that if the RRUs in the DAS are clustered so that each cluster operates independently of the others, this clustering transforms the CDD transfer size constraint from a constraint on all RRUs in the entire DAS to a constraint on the RRUs of each individual cluster, the latter being easier to satisfy. However, such clustering is impossible, or at least impractical. This is because in distributed CDD, time synchronization is determined as a common source, and tap latency is determined from this common source. Therefore, the controller should be part of each cluster, which would make the operation of the clusters interdependent.

[0011] Some implementations are based on the understanding that if the RRUs of a DAS arranged to send messages from the controller to the receiver are divided into multiple DAS clusters, and if each DAS cluster includes a cluster master controller (CM) that acts as a transceiver in communication with the controller and multiple RRUs that communicate with the CM transmitter, then each of these hierarchically arranged DAS clusters can operate independently based on the distributed CDD if the communication links connecting the CM transmitter and each of the controllers are not considered in the distributed CDD of each DAS cluster.

[0012] Some implementations are based on the understanding that when using a Hierarchical Precision Time Protocol (HPTP) to synchronize the controller and DAS cluster, where the controller and CM transmitter are the master clock and boundary clock respectively, and the RRU is a common clock synchronized to the CM transmitter of its corresponding cluster, the communication link in the distributed CDD scheme can be ignored. HPTP is a protocol for synchronizing clocks across computer networks, and its principles can be extended to dedicated network communication systems. On a local area network, HPTP achieves sub-microsecond clock accuracy, making it suitable for measurement and control systems. To adapt dedicated network communication systems to HPTP synchronization, some implementations configure the controller and CM as master clocks and boundary clocks with multiple network connections, while the RRU is configured to synchronize to a common clock of the CM in its corresponding cluster.

[0013] Thus, some implementations disclose a multi-cluster DAS with a distributed CDD on top of a multi-level hierarchical PTP, wherein the first level is formed by a master-slave arrangement of the master clock and boundary clocks, and the second level is formed by a master-slave arrangement of the boundary clocks and ordinary clocks. This multi-level hierarchical PTP allows breaking the dependency between different DAS clusters and a common controller, thereby allowing each DAS cluster to utilize the distributed CDD independently of another CDD cluster.

[0014] Some implementations are based on the understanding that when the antenna is replaced by a BS, DAS can be identified as Coordinated Multipoint (CoMP), which is widely used and supported by the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE). The core concept of CoMP is to provide simultaneous communication to a single or multiple users through multiple BSs, thereby improving rates across the entire communication range. Coordinated Beamforming (CB) and Joint JP, including Joint Transmission (JT), are available as primary methods of CoMP.

[0015] Some implementations are based on the understanding that geographically placed Base Stations (BSs) enable communication systems to adequately offset path loss and obstruction. However, collecting complete CSITs from a distributed system is challenging. While UEs can obtain very reliable channel estimates, the feedback overhead becomes enormous as the number of Receiving Runners (RRUs) increases. Traditional codebook-based feedback may not be feasible for CoMP due to significant differences in received signal strength. In 3GPP LTE, UEs feed back Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI), and Rank Indicator (RI) as CSITs to the serving BS for CoMP operation. Cooperating BSs can obtain the corresponding CSITs from the serving BS via the X2 interface. Furthermore, tight clock synchronization between BSs is beneficial. Lack of clock synchronization between BSs can lead to interference at UEs due to differences in signal arrival times. When GNSS signals are unavailable in dedicated areas, desired clock synchronization can be achieved via Precise Time Protocol (PTP) or via Over-the-Air Synchronization (OAS). The presence of interference is also an inherent problem when multiple BSs transmit simultaneously.

[0016] Therefore, one embodiment discloses a communication system comprising: a controller including one or more processors configured to transmit messages to a receiver; and multiple clusters of a distributed antenna system (DAS), wherein each DAS cluster includes a cluster master (CM) communicating with the controller and multiple remote radio units (RRUs) communicating with CM transmitters. The controller and DAS clusters are synchronized using a hierarchical precise time protocol (HPTP), where the controller is the master clock, the CM transmitters are boundary clocks, and the RRUs are ordinary clocks synchronized to the CM transmitters of their respective clusters. Each DAS cluster is configured to transmit messages independently of other DAS clusters using a distributed cyclic delay diversity (CDD) scheme, wherein the minimum length of the cyclic prefix is ​​determined as a function of the synchronization error bounds within the DAS cluster and the sum of the maximum tap delays of the communication channels originating from the CM transmitters of the DAS cluster.

[0017] Therefore, another embodiment discloses a method in a communication system, the communication system comprising: a controller that transmits messages to a receiver; multiple clusters of a distributed antenna system (DAS), wherein each DAS cluster includes a cluster master (CM) communicating with the controller and multiple remote radio units (RRUs) communicating with the CM. Furthermore, the controller and the DAS clusters are synchronized using a hierarchical precise time protocol (HPTP), wherein the controller is the master clock, the CM is the boundary clock, and the RRU is a normal clock synchronized to the CM of its corresponding cluster; and wherein each DAS cluster transmits messages independently of other DAS clusters using a distributed cyclic delay diversity (CDD) scheme, wherein the minimum length of the cyclic prefix is ​​determined as a function of the synchronization error bound within the DAS cluster and the sum of the maximum tap delays of the communication channel originating from the CM transmitter of the DAS cluster. Attached Figure Description

[0018] Figure 1A An example of a distributed antenna system (DAS) based on a multi-cluster (MC) configuration is illustrated.

[0019] Figure 1B A schematic diagram illustrating a method for determining the length of a cyclic prefix in a communication system according to an exemplary embodiment is shown.

[0020] Figure 1C A flowchart illustrating the determination of the minimum length of the cyclic prefix used in a message to be sent from the controller to the receiver (RX) according to an example implementation is provided.

[0021] Figure 1D A block diagram of a controller for a control communication system according to an example embodiment is shown.

[0022] Figure 2A A flowchart illustrating another method for a communication network according to an example implementation is shown, which includes determining the length of a cyclic prefix and the number of transmitters in the communication system.

[0023] Figure 2B An example of a receiver according to an example implementation is shown measuring the RRU tap delay for each RRU in a set of remote radio units (RRUs).

[0024] Figure 3A A block diagram illustrating a flowchart of implementing a minimum cycle prefix into a message according to an example implementation is shown.

[0025] Figure 3B This illustrates an example implementation. Figure 3A A schematic diagram of the steps.

[0026] Figure 3C This illustrates an example implementation. Figure 3A A schematic diagram of the steps.

[0027] Figure 3D This illustrates an embodiment according to the present disclosure. Figure 3A A schematic diagram of the steps.

[0028] Figure 3E This illustrates an embodiment according to the present disclosure. Figure 3A A schematic diagram of the steps.

[0029] Figure 4 (A) illustrates an RRU with applied cyclic delay diversity (CDD) delay according to an example implementation.

[0030] Figure 4 (B) illustrates the right-shifted portion of the original transport block symbol according to the example implementation.

[0031] Figure 4 (C) illustrates the transport block structure after applying CDD delay according to the example implementation.

[0032] Figure 4 (D) shows the actual transport block symbol from the RRU according to the example implementation.

[0033] Figure 5 (A) shows another RRU with applied CDD delay according to an example implementation.

[0034] Figure 5 (B) illustrates a portion of the original transport block symbol with a right-shifted CCD delay according to an example implementation.

[0035] Figure 5 (C) illustrates the transport block structure after applying CDD delay according to the example implementation.

[0036] Figure 5 (D) illustrates an actual transport block symbol from an RRU according to an example implementation.

[0037] Figure 6 A schematic diagram illustrating a receiver 103 receiving messages from different RRUs according to an example implementation is shown.

[0038] Figure 7 A bidirectional packet switching synchronization process implemented using the Hierarchical Precision Time Protocol (HPTP) according to an example implementation is illustrated.

[0039] Figure 8 A block diagram illustrating some components that can be used in various configurations to implement a controller for controlling a communication system, according to an example implementation.

[0040] Figure 9 The spectral efficiency of various system and channel parameters according to the example implementation is illustrated.

[0041] Figure 10 The spectral efficiency of various oversaturated DASs in scenario X3 is illustrated according to the example implementation.

[0042] Figure 11 Various values ​​of spectral efficiency for a remote radio unit with different numbers of multipath components according to an example embodiment are illustrated.

[0043] Figure 12 A method for sending messages in an MC-DAS communication system according to an example implementation is illustrated. Detailed Implementation

[0044] In the following description, numerous specific details are set forth for illustrative purposes in order to provide a thorough understanding of this disclosure. However, it will be apparent to those skilled in the art that this disclosure may be practiced without these specific details. In other instances, apparatuses and methods are shown only as block diagrams to avoid obscuring this disclosure.

[0045] As used in this specification and claims, the terms "for example" and "such as," as well as the verbs "comprising," "having," "including," and other verb forms thereof, when used in conjunction with a list of one or more components or other items, shall each be interpreted as open-ended, meaning that the list should not be considered as excluding other additional components or items. The term "based on" means at least partially based on. Furthermore, it will be understood that the wording and terminology used herein are for descriptive purposes and should not be considered limiting. Any headings used within this description are for convenience only and have no legal or limiting effect.

[0046] Cyclic Delayed Diversity (CDD) and Distributed CDD (DCDD):

[0047] Traditionally, communication systems have a controller connected to a set of transmitters. This controller can form messages using symbols including data symbols and identifier symbols. The controller can control the set of transmitters to transmit messages using a cyclic delay diversity (CDD) scheme, such that each transmitter cyclically rotates the symbols of the message with a unique shift and copies the last symbol of the rotated message to a first position in the message. For example, by applying different cyclic rotations of unique words (UWs), a receiver can detect whether a particular device is using CDD operation. Therefore, the receiver can easily detect a fixed number of devices applying CDD operation. After detecting CDD transmitters, the receiver can combine only the signals from those CDD transmitters. At least one aspect of the above implementation is that the controller can form multiple different messages, such that for at least some of the different messages, the data symbols can vary, and for all the different messages, the identifier symbols can be fixed, wherein the identifier symbols are also referred to as UWs in this disclosure.

[0048] Some implementations are based on the understanding that diversity schemes improve message reliability by transmitting via multiple communication channels with different characteristics. Cyclic Delay Diversity (CDD) is a diversity scheme used in OFDM-based and single-carrier-based telecommunications systems to achieve diversity gain from only a single transmitter with multiple antennas. The length of the cyclic prefix can be greater than the channel tap delay. OFDM CDD does not require feedback and synchronization.

[0049] Some implementations are based on the understanding that CDD schemes designed for a single transmitter can be extended to concurrent transmissions from multiple transmitters, where extended CDD transmissions can be used to achieve cooperative diversity gain, etc., by coordinating multiple transmitters to send delayed copies of information to the receiver. This is referred to as extended CDD transmissions from multiple transmitters, as distributed CDD.

[0050] Therefore, some embodiments of this disclosure are based on the understanding that cooperative communication systems can have distributed CDD schemes, which include a collection of spatially distributed cooperative transmitters, rather than a single transmitter as in the construction of conventional CDD schemes. However, unlike conventional existing operations / schemes using a single transmitter, embodiments of this disclosure include system setups employing a CDD scheme between antennas mounted at the same transmitter. Typically, considering the hardware complexity and power of RRU-type transmitters, a single antenna may be used depending on the specific application. For this and other reasons, the teachings of existing conventional constructions are not applicable to embodiments of the system setup of this disclosure. At least one aspect of this disclosure is that some embodiments provide CDD schemes for spatially distributed transmitters, etc., equipped with a single antenna.

[0051] Since many different types of transmissions can coexist in the same frequency band, it is necessary to find the correct set of transmitters for CDD operation. To address this problem, this disclosure provides at least one method for selecting appropriate transmitters for CDD operation in the presence of multiple different types of transmissions. In fact, some embodiments of this disclosure may involve transmitters equipped with a single antenna, which can be distributed in a space forming a distributed antenna system (DAS) to support wireless access for users.

[0052] Some implementations are based on the understanding that the distributed CDD transmission phase presents numerous challenges, including how to overcome the conventional construction teachings for CDD transmission using a single transmitter with multiple antennas. For example, while distributed CDD schemes can achieve higher throughput, identifying or clarifying which transmitters are actually used in the CDD scheme and addressing the inherent problems of inter-symbol interference (ISI) from other transmitters while having multiple transmitters involved in simultaneous transmission all need to be overcome. Therefore, this disclosure finds it necessary to develop ISI-free CDD schemes. Another challenge is that, since CDD operations can be performed in chunks, determining how many transmitters should be assigned as CDD transmitters without causing any ISI also needs to be overcome. Furthermore, since a specific CDD delay can be assigned at any CDD transmitter, determining how to assign a CDD delay to a specific CDD transmitter must be solved.

[0053] Some implementations are based on the understanding that joint transmission processing via multiple RRUs in fading indoor wireless communication environments is a challenging problem. On the one hand, it is desirable to increase the number of RRUs to cover a larger area and increase channel diversity. On the other hand, this increase will lead to additional interference, time synchronization difficulties, and unwanted feedback overhead.

[0054] To address the aforementioned issues, the communication system is implemented as a distributed antenna system (DAS) that transmits messages using minimal propagation delay. A DAS is a network of spatially separated transmitters (or antenna nodes) connected to a common source via a transmission medium providing wireless service within a geographical area or structure. Furthermore, the spatially separated transmitters in the communication system are arranged into multiple clusters. Each cluster is a distributed antenna system (DAS), where each transmitter within the cluster is called a remote radio unit (RRU). Each cluster includes a cluster master (CM) that communicates directly with the RRUs of its cluster. Additionally, the CMs of each group communicate directly with the controller. Thus, the communication system is arranged in a hierarchical order, with the controller transmitting messages to the receiver via multiple clusters.

[0055] For a communication system to be implemented using DAS, spatially distributed transmitters should be synchronized. This disclosure includes implementations based on the understanding that the controller can use a Hierarchical Precise Time Protocol (HPTP) to synchronize different components of the communication system (controller, CM, and RRU). HPTP includes a master clock for the controller, boundary clocks for the CM, and a general clock for the RRU. Furthermore, HPTP can be used to provide the controller with an opportunity to synchronize the set of transmitters within the CDD scheme to correct for propagation delays.

[0056] Furthermore, due to the potentially significant spatial separation between transmitters (e.g., RRUs), different transmitters may have different tap delays for their corresponding communication channels with the receiver. For example, signal power propagates in space in an isotopic manner and degrades inversely proportional to the square of the propagation distance. Therefore, significant spatial separation results in different tap delays depending on the random distance from a particular RRU to the receiver. One aspect of why this is important is that paths leading to larger delay spreads (or larger tap delays) are more prone to ISI, necessitating attention to different delay spreads on the path from the RRU to the receiver. This disclosure provides a method for removing ISI, etc., by selecting a cyclic prefix larger than the maximum tap delay. Therefore, for cooperative transmission via distributed CDD, different delays need to be considered. One of the many reasons for this consideration is that in the distributed CDD scheme of this disclosure, different delay spreads on the path can also be considered when designing CDD delays and selecting CDD transmitters among coexisting transmitters.

[0057] Furthermore, another challenge with spatially distributed cooperative transmitter assemblies is that, due to the joint use of bandwidth allocated for transmissions, receivers can receive different types of signals comprising multiple data symbols. Because of the coexistence of different types of transmissions, the received signals are a mixture of different transmission powers, signal structures, and cooperative schemes. For example, one transmission may use a CDD scheme, while others may not. Therefore, some signals may be transmitted from different transmitters using different cooperative transmission schemes and / or independently of each other. For this reason, it is necessary to distinguish between transmissions via distributed CDD and other types of transmissions.

[0058] For example, in part, it can be useful to distinguish a particular transmission from other types of transmissions when the CDD operation is unique for that transmission. Signals received through transmissions of different types will interfere with the desired received signal. Therefore, it is necessary to distinguish transmissions that apply the CDD operation from other transmissions that do not. For this purpose, a unique preamble or signal structure can be used first. By applying cross-correlation to the unique preamble, the receiver can find the desired set of RRUs for the desired transmission.

[0059] Since the number of CDD transmitters can be limited to a fixed number, it is also necessary to distinguish between CDD transmitters and non-CDD transmitters. Because some embodiments of this disclosure use a unique identifier as a cyclic prefix, and its position is predetermined for all CDD RRUs, the receiver searches for its reception processing within the desired set of CDD RRUs. At least one advantage may include providing a way to find CDD RRUs among coexisting RRUs using different types of transmissions, thereby providing a way to increase throughput by removing ISI and other interference during its reception processing.

[0060] Embodiments of this disclosure may use HPTP to provide timing to a controller. The controller can synchronize sets of RRUs and CMs within a CDD scheme to correct propagation delays and establish a cooperative communication system, resulting in a reduction in the need for tight synchronization within the system. HPTP can provide synchronization that enables and maintains coordination between local clocks in independent sets of RRUs, providing a common time concept across sets of RRUs and CMs in the cooperative communication system. Based on this configuration, the average time error generated by the HPTP receiver can be reduced. Therefore, by knowing the components of the cooperative communication system (i.e., CDD scheme, RRUs, CMs, receiver, and HPTP timing, etc.), the synchronization delay or synchronization error of the specific cooperative communication system of this disclosure can be determined.

[0061] Some implementations are based on the understanding that synchronization errors and tap delay differences between different communication channels in distributed CDD can be addressed by selecting an appropriate length for the cyclic prefix. Some implementations of this disclosure use a signal structure with a unique identifier symbol used as the cyclic prefix. The length of this identifier symbol needs to be greater than the maximum tap delay on the channel from the desired set of transmitters to the receiver. With this signal structure, ISI occurs in receiver operation when we apply cyclic shift to the remainder of the data packet excluding the unique word. Therefore, the desired throughput cannot be achieved.

[0062] To overcome this throughput loss problem for this novel signal structure, some implementations provide a CDD scheme that applies cyclic shift to the entire data symbol, including the identifier. For example, considering this synchronization error, the systems and methods of this disclosure are also based on another understanding: combining the synchronization error with the maximum tap delay of the cooperative transmitter set to obtain a minimum CP length. Furthermore, tap delays can be determined for the communication channel between the receiver and each transmitter in the transmitter set to generate a set of tap delays for the cooperative communication system, from which the maximum tap delay can be obtained. This is achieved by combining the synchronization error with the maximum tap delay to obtain the minimum CP length that can be used to transmit messages via the transmitter set.

[0063] In addition, refer to the following Figure 1ADescribe a detailed analysis of the proposed communication system.

[0064] Figure 1A An example of a multi-cluster (MC) based distributed antenna system (DAS) 100 according to an example embodiment is illustrated. The MC-DAS forms a communication system usable in a dedicated network. In some embodiments, the MC-DAS 100 may be deployed indoors, such as in a shopping mall, home, factory interior, ship, etc. In some other embodiments, the MC-DAS 100 may be deployed outdoors. The MC-DAS 100 includes a controller 101 configured to coordinate or control multiple clusters C1 and C2 to transmit messages from the controller 101 to a receiver RX 103 via the multiple clusters C1 and C2. The multiple clusters C1 and C2 may be arranged in a non-overlapping manner. Furthermore, each cluster in the multiple clusters C1 and C2 is identified as a DAS because each cluster includes multiple remote radio units (RRUs) and a cluster master (CM).

[0065] The MC-DAS 100 in a dedicated network can be combined with existing telecommunications networks such as 2G, 3G, 4G, and 5G. Therefore, the controller 101 utilizes a wired connection to connect to a base station associated with the existing telecommunications network to act as a coordinating multipoint transmitter and receiver.

[0066] exist Figure 1A In the i-th cluster, the k-th RRU is deployed by the RRU. i,k This is represented as follows. For example, the first RRU (k=1) in the first cluster C1 (i=1) is represented by RRU. 1,1 Therefore, the first cluster C1 (i=1) includes RRUs. 1,1 RRU 1,2 RRU 1,3 RRU 1,4 and RRU 1,5 Furthermore, C1 includes a first cluster master controller (CM1), wherein all RRUs in C1 communicate with the cluster's CM1, and wherein CM1 communicates with controller 101. Similarly, the second cluster C2 (i=2) includes RRUs 2,1 RRU 2,2 RRU 2,3 and RRU 2,4 Similarly, C2 includes a second cluster master controller (CM2), in which all RRUs in C2 communicate with the cluster's CM2, and where CM1 communicates with controller 101. RRUs are wirelessly connected to the CMs of their corresponding DAS clusters, and controller 101 is wirelessly connected to the CMs.

[0067] The MC-DAS 100 also includes a receiver RX 103, which shares a location with two trunks, C1 and C2, such that receiver RX 103, trunks C1 and C2 are all within the coverage area of ​​controller 101. Furthermore, backhaul links {b1,b2} are configured to provide wideband backhaul access to trunks C1 and C2 via a coordinator or control residing at controller 101. Other backhaul links {b i,j Broadband backhaul access to Remote Radio Units (RRUs) is provided via CM, CM1, and CM2, where i = 1, 2; j = 1, ..., K. Each CM controls all RRUs within a trunk group and their responses to transmitted signals. For example, CM1 controls all RRUs within trunk group C1, while CM2 controls all RRUs within trunk group C2.

[0068] In addition, the MC-DAS 100 uses a Hierarchical Precision Time Protocol (HPTP) to synchronize the controller 101 with multiple clusters C1 and C2. For this purpose, HPTP synchronizes the clocks of different components, such as the controller 101, CMs, and RRUs. HPTP is used to configure the controller 101 as the master clock, the CMs (e.g., CM1 and CM2) as boundary clocks, and the RRUs as normal clocks.

[0069] In another implementation, HPTP is used to configure each CM as the master clock and the controller as the boundary clock, so that the CM can calculate the propagation delay of a message reaching the RX 103 via a specific RRU. For deploying RRUs, each node in the cluster can be equipped with a single antenna.

[0070] Furthermore, controller 101 may include one or more processors, wherein controller 101 is arranged using a multi-level hierarchical tree structure including a master clock, boundary clocks, and normal clocks, so that each DAS cluster in multiple DAS clusters can operate independently of each other. Controller 101 may also be configured to send messages from controller 101 to receiver RX 103 through one or more of multiple independent DAS clusters (i.e., multiple clusters C1 and C2). For example, in Figure 1A In this configuration, controller 101 is configured to send messages from controller 101 to receiver RX 103 via multiple clusters C1 and C2. Since receiver RX 103 receives messages (or signals) from multiple clusters C1 and C2, receiver RX 103 utilizes a single-user multiple-input single-output (SU-MISO) operating mode, thus increasing throughput.

[0071] When a message from controller 101 arrives at receiver RX 103 via multiple paths associated with multiple clusters C1 and C2, the message may suffer frequency-selective fading. To address frequency-selective fading, cyclic prefix single-carrier (CP-SC) transmission can be employed in the MC-DAS 100. For this purpose, frequency selectivity can be utilized as frequency diversity based on the available information of the maximum number of multipath components on the MC-DAS 100. Furthermore, by utilizing cooperative CP-SC transmission, the MS-DAS 100 can achieve increased transmit diversity in multipath-rich environments.

[0072] exist Figure 1A In the i-th cluster, the k-th RRU (deployed by RRU) is... i,k (Indicates) The frequency-selective fading channel to receiver RX 103 is determined by h i,k It means that L(h) i,k ) = N i,k , where L(h i,k ) represents h i,k The cardinality of N. i,k It is a set of non-negative integers. Distance-dependent large-scale fading is caused by α. i,k Indicates. For RRU i,k Distance d to RX 103 i,k α i,k From α i,k =(d i,k ) -∈ Define , where ∈ represents the path loss exponent.

[0073] In some implementations, the receiver RX 103 may be positioned relative to the RRU at a specific location. Thus, independent but distinct frequency-selective fading channels are obtained from the RRU to the RX 103. In some implementations, by transmitting training sequences or adding pilots as suffixes to individual symbol blocks, the RX 103 may have information about the number of multipath components connected to its own channels.

[0074] One of the purposes of this disclosure is to determine the length of the cyclic prefix. Figure 1B and Figure 1C The description of determining the minimum length of the cyclic prefix is ​​combined. Figure 1B An exemplary cluster 100A according to an example implementation is illustrated, wherein each RRU in the cluster has received a CDD latency Δ i And a cyclic prefix to be applied to the messages to be sent to receiver 103. In addition, the cluster includes a cluster master controller (CM) 107, RRU1, RRU2, and RRU... K and RRU M And receiver 103. CM 107 and controller 101 ( Figure 1B(Not shown in the diagram) Direct communication. The CDD and cyclic prefix of each RRU can be calculated by the controller 101 and forwarded to the corresponding RRU via CM 107. In addition, the receiver 103 communicates wirelessly with RRUs 109, 111, 113 and 115 (also referred to as RRUs 109-115) via signals 109A, 111A, 113A and 115A.

[0075] The cluster master controller (CM) 107 communicates with M RRUs via channels 117A, 121A, 123A, and 125A, respectively. CM 107 communicates with controller 101 (…). Figure 1B (Not shown) Communication, wherein controller 101 uses HPTP to synchronize different components of communication system 100. HPTP enables controller 101 to operate as a master clock, CM 107 as a boundary clock, and RRUs 109-115 as boundary clocks. In another embodiment, HPTP enables controller 101 to operate as a boundary clock, CM 107 as a master clock, and RRUs 109-115 as normal clocks.

[0076] CM 107 enables synchronization of the set of RRUs 109, 111, 113, and 115 within the CDD scheme to correct for propagation delays, resulting in a reduced need for tight synchronization within the communication system. HPTP provides synchronization that implements and maintains coordination between the master clock, boundary clocks, and normal clocks, providing a common time concept for the set of CM 107 and RRUs 109-115 across the cooperative communication system. Furthermore, the cyclic prefix used by each RRU is calculated by the controller. RRUs are configured to use a minimum-length cyclic prefix. See below. Figure 1C Provide a detailed description of determining the minimum cyclic prefix.

[0077] Figure 1C A flowchart illustrating the determination of the minimum length of the cyclic prefix to be used in a message to be sent from controller 101 to receiver 103 according to an example implementation is provided. Controller 101 may perform operations including steps 127-139 to determine the minimum length of the cyclic prefix.

[0078] In step 127, for multiple clusters (e.g., Figure 1B The exemplary cluster shown collects HPTP RRU synchronization errors (ΔS1,...,ΔS) in the RRU set of each RRU. M For example, from an RRU such as C1. 1,1 ,..,RRU 1,5 and C2's RRU 2,1 ,..,RRU 2,4 Clusters C1 and C2 (shown in) Figure 1ACollect HPTP RRU synchronization errors (ΔS1,...,ΔS) M The HPTPRRU synchronization error is determined based on HPTP. Furthermore, all ΔS are assumed to be less than ΔS. bound .

[0079] In step 129, the maximum HPTP synchronization error from the HPTP RRU synchronization error set is determined. Based on the HPTP protocol, the HPTP RRU synchronization error limit ΔS can be found offline. bound To operate even in worst-case applications, the communication system (MC-DAS) needs to account for the worst-case synchronization error (maximum synchronization error) ΔS. bound This is to eliminate the possibility of it occurring as inter-symbol interference (ISI). Therefore, the synchronization error limit ΔS bound It is considered the upper bound of the error.

[0080] In step 131, receiver 103 measures the RRU tap delay of each RRU in the RRU set. In the case of multiple receivers (e.g., receiver RX 103), each receiver can measure the maximum tap delay via channel sounding techniques, where the RRUs send known pilot signals to receiver RX 103, and receiver RX 103 then measures the channel dispersion. Without knowing the exact maximum tap delay, receiver RX 103 experiences ISI. Therefore, to achieve better reliability of the received signal, some implementations are based on the understanding that the coordinator processor only knows the maximum tap delay, not the full channel state information.

[0081] In step 133, the maximum RRU tap delay from the RRU tap delay set is measured. Receiver RX 103 calculates the delay spread of each channel between itself and the RRU. Once receiver RX 103 has the channel delay set (N... f1 ,...,N fM It sorts them in ascending order and then selects the maximum delay N from the measurements. f =max(N) f1 ,...,N fM ).

[0082] In step 135, the maximum synchronization error ΔS of HPTP is determined. bound and maximum RRU tap delay N f Add to determine the minimum CDD delay length Δ i In some implementations, the additional CP length N needs to be satisfied. CPThe minimum overhead ratio to the original symbol block size Q. Generally, a smaller overhead (i.e., a shorter CP length) is preferred. For example, if the CP length is reduced, the transmission time can be increased. However, if the CP length is reduced without restriction, interference will occur at the receiver. Therefore, two objectives need to be met: reducing the CP length while removing interference at receiver 103. Since time synchronization between distributed RRUs affects receiver performance, time synchronization error needs to be considered. In this scenario, the worst-case time synchronization error is considered. To remove interference from the received signal, a CP length N is used. CP ≥N f +ΔS bound To achieve the minimum overhead ratio, use a CP length N. CP ≥N f +ΔS bound Based on the calculated N CP The CDD delay was determined to be Δ. i = (i-1)N CP Where Δ1 = 0. Some implementations are based on the understanding that the CP length needs to be extended beyond the effects of the longest channel delay spread and HPTP timing error limits.

[0083] In step 137, when applying CDD, M RRUs are determined from K (K≥M) RRUs. The number of RRUs is determined by M = floor(Q / N) for the transport block size Q. CP The effective signal-to-noise ratio (SNR) is determined by floor(.), where floor(.) denotes the floor function. To determine the RRU from the K RRUs, receiver 103 calculates the set of effective signal-to-noise ratios (SNRs) on the channel from the RRU to receiver 103. For the same pilot symbol, the effective SNR of the k-th RRU is determined by... Given, where P T This indicates the transmit power from the RRU. Represents noise power, ||h k || 2 Represents the channel vector h k The channel power. Furthermore, receiver 103 sorts the valid SNRs to obtain the corresponding RRU indices. For example, (RRU... 1,2 RRU 1,1 RRU 1,4 RRU 1,3 Instructions for RRUs among the four transmitters. 1,3 With maximum effective SNR, RRU 1,2It has the minimum effective SNR. Receiver RX 103 sends the RRU index (e.g., ((1,2),(1,1),(1,4),(1,3)))) back to controller 101. Controller 101 then selects the M RRUs indexed by the last M elements of the received RRU index vector. For example, when the CDD supports M = 2 RRUs, controller 101 selects RRUs for the CDD. 1,3 and RRU 1,4 This is because these two RRUs provide two maximum effective SNRs at receiver 103. Since receiver 103 only sends back the RRU index, the feedback overhead can be reduced.

[0084] In step 139, messages using Cyclic Delay Diversity (CDD) are sent via the RRU set with CDD delay and a cyclic prefix. A different CDD delay Δ is applied to each RRU according to the CDD operation. i Furthermore, a cyclic prefix is ​​applied, which appends the same number of last symbols as the length of the cyclic prefix to the beginning of the original transport symbol block. Figures 3A to 5 The use of CDD and cyclic prefixes in the (D) further explains the message.

[0085] Figure 1D A block diagram illustrating a controller 101 of a control communication system according to an exemplary embodiment is shown. (In conjunction with...) Figure 1C To describe Figure 1D The controller 101 includes multiple components, including a processor 141, a receiver 143, a transmitter 145, etc. The controller 101 collects the maximum time synchronization error and the measured maximum channel tap length from the receiver 143. The controller 101 further calculates the maximum allowed number of RRUs, taking into account the maximum channel tap length, the maximum time synchronization error, and the transmitted symbol block size. The controller 101 also selects multiple RRUs 109-115 to which CDD is applied. The controller 101 then assigns different delays to the selected RRUs.

[0086] Furthermore, receiver 103 calculates the maximum channel tap length and provides it to controller 101. Controller 101 considers the maximum time synchronization error to calculate the CP length and CDD delay. Additionally, controller 101 determines the RRUs to which CDD is applied. Then, controller 101 provides the selected CDD delay Δ to each RRU individually. i .

[0087] Figure 2A A flowchart illustrating another method for a communication network according to an example embodiment is shown, which includes determining the length of the cyclic prefix and the number of transmitters in the communication system. In step 201, the HPTP RRU synchronization error of each RRU in the RRU set is determined.

[0088] In step 203, the RRU set (RRU) is collected. 1,1 ,..,RRU 1,5 ), (RRU 2,1 ,..,RRU 2,4 HPTP RRU synchronization error (ΔS1,...,ΔS) for each RRU in the dataset. M In step 205, the maximum HPTP synchronization error from the HPTP RRU synchronization error set can be determined. In step 207, the RRU set (RRU...) 1,1 ,..,RRU 1,5 ), (RRU 2,1 ,..,RRU 2,4 The RRU tap delay of each RRU in the set is measured by the receiver. In step 209, the maximum RRU tap delay from the set of RRU tap delays is determined. In step 211, the minimum CDD delay is determined by adding the maximum RRU tap delay to the maximum HPTP synchronization error.

[0089] In step 213, based on the maximum channel tap length N f Synchronization error limit ΔS bound The number of transmitters M is determined by the transport block size Q. Since there may be more RRUs than CDD can support, it is necessary to determine the maximum number of transmitters required. For this, the block size needs to be considered to remove interference. The control unit randomly selects a delay Δ for a specific transmitter. i One of them uses cyclic delay diversity (CDD) to send messages with a minimum-length cyclic prefix. If a maximum likelihood detector is used for data detection in the receiver, the detector performance is independent of different delays when the equivalent channel matrix is ​​cyclic. Therefore, the control unit randomly selects one of the delays that is not selected for other RRUs for a particular RRU, and then controls that RRU to send messages using cyclic delay diversity.

[0090] Figure 2B This example illustrates how receiver 103, according to an exemplary implementation, measures the RRU tap delay for each RRU in an RRU set. (In conjunction with...) Figure 2A Step 207 describes Figure 2B The receiver measures the tap delay (N) of the link between itself and the RRU. f1 N f2 ,...,N fM Then calculate the maximum tap delay N. f =max(N) f1 N f2 ,...,N fMReceiver 103 also measures the effective received signal-to-noise ratio (SNR) at receiver 103 via channels (109A, 111A, 113A, and 115A). Receiver 103 also sorts the effective received SNRs according to their size and selects the RRU with the largest size, such as RRU1 (109). For CDD operation, receiver 103 will, via channel 109A, represent the RRU index vectors (e.g., (2,1,4,3)) and the maximum RRU tap delay N in ascending order of effective SNR. f Send back to RRU 109.

[0091] For example, receiver 103 first measures the RRU set (RRU) 1,1 ,..,RRU 1,5 ), (RRU 2,1 ,..,RRU 2,4 The receiver 103 first calculates the RRU tap delay for each RRU in the set 109A, 111A, 113A, and 115A, and then calculates the maximum RRU tap delay. Next, it calculates the effective signal-to-noise ratio (SNR) for each channel in the set 109A, 111A, 113A, and 115A. Then, the receiver 103 sorts the effective SNRs and selects the RRU with the highest effective SNR. Finally, it sends the RRU index vector representing the effective SNRs in ascending order and the maximum RRU tap delay back to the RRU with the highest effective SNR. In this example, RRU 109 is selected because it has the highest effective SNR.

[0092] After receiving the RRU index vector representing the effective SNR in ascending order and the maximum RRU tap delay from receiver 103 via channel 109B, RRU 109 sends this information back to CM 107 via channel 117A.

[0093] Figure 3A A block diagram illustrating a flowchart of implementing a minimum cycle prefix into a message according to an example implementation is shown. Figure 3B This illustrates an example implementation. Figure 3A A schematic diagram of step 301. Figure 3B This shows that different RRUs can experience different time synchronization errors. All errors are less than one symbol interval T. s For example, RRU1 109 experiences a time synchronization error ΔS1, RRU2 111 experiences a time synchronization error ΔS2, and RRU... K 113 experiences time synchronization error ΔS K .

[0094] Figure 3C This illustrates an example implementation. Figure 3A A schematic diagram of step 303. Figure 3C This illustrates the CP operation that appends multiple final symbols to the beginning of a transport block symbol.

[0095] Figure 3D and Figure 3E This illustrates an embodiment according to the present disclosure. Figure 3A A schematic diagram of step 307. For example, since Δ1 = 0, RRU 109 does not apply CDD ( Figure 3D This causes RRU 109 to apply only the cyclic prefix, that is, a copy of the cyclic prefix, with block 313 being appended to the front of block 317.

[0096] Figure 4 (A) to Figure 4 (D) is an example of an embodiment according to this disclosure. Figure 3A A schematic diagram of step 309. Figure 4 (A) illustrates a second RRU2 111 according to an example implementation, which applies Δ2 = N to CDD delay. P .exist Figure 4 In (A), block 313 is the first loop delay. Together with this part, Figure 4 (B) illustrates the right-shifted portion of the original transport block symbol according to the example implementation. Figure 4 (C) illustrates the transport block structure after applying CDD delay according to the example implementation. Figure 4 In (C), the length of block 315 is equal to N. P .exist Figure 4 In (D), CP is applied. That is, block 315 is appended before block 313. Therefore, Figure 4 (D) shows the actual transport block symbol from RRU2 111 according to the example implementation.

[0097] Figure 5 (A) to Figure 5 (D) is an example of an embodiment according to this disclosure. Figure 3A A schematic diagram of step 311. Figure 5 (A) illustrates a third RRU according to an example implementation. K 113, which applies Δ3 = 2N to CDD delay. P .exist Figure 5 In (A), blocks 313 and 315 are cyclically delayed. Together with this part, Figure 5 (B) illustrates a right shift of the original transport block symbol by 2N according to the example implementation. P The part. Figure 5 (C) illustrates the transport block structure after applying the CDD delay according to the example implementation. Furthermore, a portion of 317 (i.e., 317A) is used as a prefix. Therefore, block 317A is appended to the front of block 315. Figure 5(D) illustrates an example implementation from an RRU K The actual transport block symbol for 113.

[0098] Figure 6 A schematic diagram illustrating a receiver 103 receiving messages from different RRUs according to an example implementation is shown. Figure 6 An example with three RRUs is shown. Each RRU (RRU1, RRU2, RRU3) K It applies its own CDD delay, and then applies the cyclic prefix (e.g., based on the calculated minimum CP length) according to the determined minimum CP length. Figure 2A (As shown in step 211). Based on the RRU index, the final transport blocks have different structures, although they are shifted versions of each other. First, since each RRU applies a cyclic prefix, receiver 103 removes the cyclic prefix interval from the received signal in block 601. Furthermore, in block 603, receiver 103 applies a data detector. The detected block symbol is then obtained as output.

[0099] Some implementations are based on the understanding that interference occurs due to clock synchronization mismatches between different components of the MC-DAS 100. This causes disturbances in the messages received at receiver 103. To reduce these disturbances, bidirectional synchronization is achieved using HPTP, which is related to... Figure 7 Let me explain further.

[0100] Figure 7 An example of a bidirectional packet switching synchronization process 700 implemented using HPTP according to an example implementation is shown. Figure 7 The diagram illustrates devices D1 and D2, where communication is established between them. Furthermore, timestamps in event messages recorded during transmission and reception are shown. It is assumed that all nodes (e.g., devices D1 and D2) spend a specific processing time, such as "p". To estimate the clock skew (θ), HPTP specifies four event messages, such as Sync, Delay-req, Pdelay-Req, and Pdelay-Resp, within which accurate hardware timestamps are generated and recorded during the transmission and reception of their respective messages. Therefore, after the exchange of bidirectional packets between devices D1 and D2 (devices D1 and D2 can correspond to any two different components of the MC-DAS 100, such as RRUs), accurate hardware timestamps are generated. 1,2 (And receiver RX 103), four hardware timestamps (t1, t2, t3, t4) are available for synchronization at device D1 via bidirectional packet switching. Based on the four available timestamps, the propagation delay (d) and clock offset (θ) are determined as follows:

[0101] and Where it is assumed that the forward propagation delay is d f Almost equal to the backpropagation delay d r That is, d f ≈d r Applying the same process, D1 can estimate the propagation delay to another PTP-enabled node D3, so that D3 can synchronize with D2.

[0102] Therefore, controller 101 has a set of propagation delay estimates {d} on cluster C1. 1,k} k＝1,…,K Similarly, controller 101 can estimate the propagation delay of other clusters (e.g., C2). Therefore, by employing HPTP, the complete propagation delay set {d} at controller 101 is obtained. i,k} i＝1,2；k＝1,…,K Available. Based on this set {d i,k} i =1,2; k=1,…,K, controller 101 calculates the propagation delay corresponding to the signal that first arrives at receiver RX 103, that is,

[0103]

[0104] And further calculations relative to d ref The relative propagation delay, that is,

[0105] For i = 1, 2; and k = 1, ..., K. (3)

[0106] Some implementations use the distributed cyclic prefix diversity (dCDD) scheme for distributed CP-SC transmission to achieve transmit diversity without requiring a full CSIT. This is based on the transmission symbol s∈C. Q×1 Block size Q and cyclic prefix (CP) length N CP equals N max The maximum number of RRUs required to achieve ISI-free reception at receiver RX 105 is determined by the following formula. in This represents the floor function. The maximum number of RRUs M is also determined based on the message length to be sent to the receiver RX 103 and the minimum length of the cyclic prefix.

[0107] Excess DAS

[0108] When the number of RRUs in the i-th DAS is greater than the maximum number of RRUs required to achieve ISI-free message reception at receiver RX 103 (i.e., K), i When >M), CM i Only M RRUs (a subset of the total RRUs in the DAS cluster) need to be selected for dCDD operations. CM iThe selected RRU is then used to send messages to receiver RX 103. Therefore, receiver RX 103 needs to feed back necessary information to controller 101. Based on the available channel estimate, receiver RX 103 rearranges them according to their strength as follows:

[0109]

[0110] According to equation (4), receiver RX 103 forms a list with a specified strength order, that is, Then send D to controller 101 i via b i Backhaul communication on CM i It can have D i CM i Select from D i The M RRUs are the indices of the last M elements, i.e., RRUs. i,<K-M+1> ,…,RRU i, <k>< / k> The remaining KM RRUs are handled by CM. i Control from communication idle. For the selected M RRUs, CM i Assigning CDD delay to RRU as follows i,<K-M+m> :

[0111] Δ m =(m-1)N CP , m=1, …, M (5)

[0112] Dissatisfaction and full DAS

[0113] When the number of RRUs in the i-th DAS is less than the maximum number of RRUs required to achieve ISI-free reception at RX 103 (i.e., K i When ≤M), CM i All K need to be used i One RRU is used to perform dCDD operations. CM i The selected RRU is then used to send messages to the receiver RX103. Therefore, the receiver RX103 does not need to feedback the channel strength order D. i However, CM i To RRU i,k Assign CDD delay as follows:

[0114] Δ k =(k-1)N CP k = 1, …, K i (6)

[0115] Therefore, for over-full DAS, under-full DAS, and fully full DAS, a complete CSIT is not required at controller 101. In summary, for dCDD operation, the CM needs to know the M and N available at each CM through the backhaul communication established via controller 101. CP and D i .

[0116] Distributed Asynchronous CDD Joint Transport (DACDD-JT) for CP-SC Transmission

[0117] Some implementations are based on the assumption that RRU 1,1 The signal first arrives as follows Figure 1A The receiver RX 103 in the dedicated network shown comprises two non-overlapping clusters C1 and C2. Because the controller 101 has a propagation delay estimate for its entire network, it can target RRUs... 1,1 The signal is calculated to determine the distribution of relative propagation delay. As the initial interaction process between the controller 101 and the receiver RX 103, the controller 101 transmits to the receiver RX 103 via multiple clusters C1 and C2.

[0118] Furthermore, after removing the CP signal and passing through d 1,1 After post-processing, receiver RX 103 receives the composite signal from the two clusters (C1 and C2), as given by the following formula:

[0119]

[0120] In equation (7), and These represent composite signals transmitted from the first cluster C1 and the second cluster C2, respectively. In (7), P T This refers to the transmission power of a single-carrier transmission, H. 1,<K-M+m> ∈C Q×Q It is h i,<K-M+m> A defined right-circular matrix, where C represents the set of complex numbers. Additionally, ∏ 1,<K-M+m> ∈B Q×Q It is caused by δd i,<K-M+m> The right-circular orthogonal permutation matrix is ​​determined by ∈N0. Since the complete CSIT is not available in the proposed MC-DAS, the same P is assigned to all RRUs. T By making I Q×Q Shift down δd i,<K-M+m> Okay, you can get ∏ i,<K-M+m> , where I Q×Q Represents the identity matrix. Additional permutation matrix set. This will be defined later. Also, s∈C Q×1 This represents a transmitted symbol block. Additive vector noise is given by z ~ CN(0,σ). 2≈I Q () indicates. For correct operation, assume 0 ≤ d i,<K-M+m> ≤N CP So that 0≤δd can be achieved i,<K-M+m> .

[0121] dACDD for JT

[0122] Furthermore, utilizing the properties of right circular matrices, equation (7) can be rewritten as follows:

[0123]

[0124] in also, and Corresponding to respectively in and Local operations performed at this location. To achieve ISI-free reception at receiver RX 103, the following requirements are specified. and It is an orthogonal matrix and a right-circular matrix, and for It satisfies either stochastic CDD delayed assignment or linear CDD delayed assignment. Therefore, the conditions for satisfying these conditions can be easily obtained. of For operations It can be easily implemented. For Perform similar operations. Therefore, this can be achieved by cyclically shifting downwards. and Obtained from IQ and

[0125] Furthermore, ISI caused by variable propagation delay and multiple transmissions can be removed by a series of cyclic shift operations performed separately by the RRU and caused by propagation. Therefore, dACDD is an extended version of dCDD, which allows for propagation delay distribution on private networks. However, for dACDD to operate correctly, controller 101 needs to know... and However, for dACDD to operate correctly, controller 101 needs to know... However, due to the use of PTP, no additional feedback from the receiver RX 103 is required.

[0126] Some implementations are based on the understanding that multiple CP-SC transmissions do not cause any ISI when dACDD is integrated with JT, compared to multiple transmissions in general. Therefore, it is not necessary to use multiple orthogonal channels to avoid ISI. Thus, ISI is avoided by using dACDD in an efficient manner.

[0127] For example, suppose there are two clusters. For cluster 1, Q = 8, N1,1 =2, N 1,2 =3,d 1,1 =1,d 1,2 =3. Similarly, for cluster 2, Q=8, N 2,1 =4, N 2,2 =3,d 2,1 =2,d 2,2 =4. Then N CP =4,δd 1,1 =0,δd 1,2 =2,δd 2,1 =1, and δd 2,2 =3, so that δT 1,1 =0,δT 1,2 =2,δT 2,1 =7,δT 2,2 =1. Based on these calculations, and The following are given by equation (9):

[0128]

[0129] Here, h i,m (l) represents h i,m The l-th element. Furthermore, based on the properties of right-circular matrices, H is determined by the first column vector. 1,eq and H 2,eq ,Right now, and From equation (9), it can be observed that the elements in the equivalent channel matrix do not overlap, enabling dACDD to achieve new ISI-free CP-SC transmission from multiple clusters. It has been verified that the performance of a communication system with CP-SC transmission is primarily determined by the magnitude of the first column vector of the equivalent channel matrix. Therefore, the equivalent channel matrix is ​​determined by... The other system represented will have the same characteristics as having H. 2,eq The same performance as the system. For example, H 3,eq The following can be obtained by applying different CDD delays: δT 2,1 =3,δT 2,2 =5.

[0130] JT spectral efficiency through asynchronous MC-DACDD

[0131] Some implementations are based on the understanding that asynchronous signal reception at receiver RX 103 can be achieved without ISI using MC-dACDD. Therefore, the achievable signal-to-noise ratio (SNR) implemented by the JT is given by the following formula:

[0132]

[0133] in and in when and When they are independent of each other, the SNR implemented by JT in equation (10) can be realized, and H 1,eq and H 2,eq For right-loop operation, the signal power ρ achieved at receiver RX 103 is... s It is determined by the sum of the squared Euclidean norms of its first column vector. This signal power can be achieved when a maximum likelihood detector (MLD) is applied at the receiver RX 103. Therefore, the achievable SNR can be given by equation (10).

[0134] For an oversaturated DAS, the CM selects M RRUs based solely on channel strength. Therefore, sequential statistics are used in the SNR representation.

[0135] Some implementations are based on the understanding that by compensating for the different signal arrival times at receiver RX 103, MC-dACDD enables JT to provide the same benefits as the maximum ratio combination at receiver RX 103, without requiring the full CSIT at CM and controller 101.

[0136] Some implementations are based on the understanding that when the block size of s is the same, higher spectral efficiency can be achieved by combining two or more orthogonal carrier channels into an aggregated channel due to the efficient use of fragmented spectrum. Additional ISI-free JT can be achieved by using a set of one or more non-overlapping frequency bands via MC-dACDD. Therefore, a larger SNR can be obtained, which ultimately increases spectral efficiency.

[0137] Some implementations are based on the understanding that the proposed MC-DAS results in a lower SNRγ at receiver RX 103 due to the use of a dACDD-based JT. JT Its moment geometry function (MGF) is given by the following equation:

[0138]

[0139] in and

[0140] Therefore, when M or N max Upon reversal, the inverse MGF (IMGF) based on partial fractions (PF) becomes unreliable; therefore, it is necessary to... Develop more reliable expressions.

[0141] In another embodiment, a portion of the MGF A reliable expression for is given by the following equation (12):

[0142]

[0143] in N1 represents the upper limit sum, Where δ0 = 1 and

[0144] Equation (12) provides the MGF expressed by a weighted sum of N1+1 terms, each term being... Proportional. Similarly, γ JT The CDF can be expressed by a finite number of gamma distributions. Its expression is given by the following equation:

[0145]

[0146] Among them, Г(.) and Г U (.,.) represent the complete gamma and incomplete upper gamma functions, respectively.

[0147] In addition, b I G d N1 and N2 are key parameters when specifying the control distribution, representing the approximate accuracy of the distribution when represented by a finite number of gamma distributions. Equation (12) provides the MGF for both clusters. Therefore, for a general number of N1... D Cluster, γ JT MGF is expressed in proportion to the following

[0148]

[0149] in and Where δ0 = 1 and Similar to Q corresponding to two clusters k and E k It can target N D They are derived using clusters. To achieve the same level of approximate accuracy, N² ≈ N. D N1. Therefore, according to (14), the γ implemented by more than two clusters can be easily derived. JT The distribution of .

[0150] Spectral efficiency (SE)

[0151] Based on equation (10), the spectral efficiency of the proposed JT is given by the following equation.

[0152]

[0153] Using γ JT Based on the distribution, the spectral efficiency is determined as follows:

[0154] The realizable spectral efficiency of the proposed JT implemented by MC-dACDD is given by the following equation.

[0155]

[0156] in This represents the Meijer G function.

[0157] Downtime probability

[0158] Since the closed-form expression of CDF is available, the shutdown probability can be easily obtained. In the shutdown SNR o th The downtime probability is given by the following formula:

[0159] OP=F γJT (o th (17)

[0160] When DAS is not full, the JT based on MC-dACDD achieves asymptotic diversity gain in the high SNR region as follows:

[0161]

[0162] The same asymptotic diversity gain can be achieved for an overly full DAS. Therefore, when the DAS does not cause inter-cluster and intra-cluster ISI at the receiver, a larger number of non-overlapping clusters allows the JT to achieve a greater diversity gain proportional to the number of clusters. Furthermore, a greater diversity gain is achieved as the DAS becomes fuller.

[0163] Figure 8 A block diagram illustrating some components of a controller 101 for controlling a communication system, according to an example embodiment, is shown and can be used in various configurations. Processor 141 can be any type of processor configured for communication and network system operation. Controller 101 may include power supply 801. Depending on the application, power supply 801 may be located internally or externally to controller 101. Processor 141 may be configured to execute stored instructions and communicate with memory 803 storing instructions executable by processor 141. Processor 141 may be a single-core processor, multi-core processor, computing cluster, or any number of other configurations. Coordinating processor 141 is connected to one or more input and output devices 807 via bus 805. Memory 803 may include random access memory (RAM), read-only memory (ROM), flash memory, or any other suitable memory system. It is conceivable that the processor may be a coordinating processor responsible for determining and transmitting parameters such as the number of transmitters, CDD latency, and CP size.

[0164] Reference Figure 8The controller 101 may also include a storage device 809 suitable for storing supplementary data and / or software modules used by the storage processor 141. For example, the storage device 809 may store historical data related to similar types of communication networks and systems, such as CP configuration, transmitter configuration, receiver configuration, synchronization error data, etc. The storage device 809 may include a hard disk drive, an optical disk drive, a thumb drive, a drive array, or any combination thereof.

[0165] The human-machine interface (HMI or user interface) 811 within the controller 101 can connect the controller 101 to the keyboard 813 and the display device 815. The controller 101 can be linked via a bus 805 to a display interface 817 suitable for connection to a display device 819, wherein the display device 819 may include a computer monitor, camera, television, projector, or mobile device, etc.

[0166] The network interface controller (NIC) 821 is adapted to be connected to the network 823 via the bus 805. Communication data or related communication data can be presented on a display device, imaging device, and / or printing device.

[0167] Communication data or related communication data may be transmitted via the communication channel of network 823 and / or stored in storage system 809 for storage and / or further processing. Furthermore, communication data or related communication data may be received wirelessly or wiredly from receiver 143 or transmitted wirelessly or wiredly via transmitter 145, both of which are connected to controller 101 via bus 805.

[0168] The controller 101 can be connected to an external sensor 825. For example, the external sensor 825 may include sensors for speed, direction, airflow, weather conditions, etc. The controller 101 can be connected to other external devices 827.

[0169] Analyze the verification of spectral efficiency (SE).

[0170] We consider multiple frequency-selective fading channel parameters for the two clusters based on the corresponding numbers K1 and K2 of RRUs. For notation purposes, we use H1 = {N} for C1. 1,j For C2, use H2 = {N}, j = 1, ..., K1}. 2,j ,j=1,...,K2}.

[0171] We assume the following scenario for performance verification.

[0172] Scene X1: H1 = {2,3,4,2,3} and H2 = {3,2,3,3}

[0173] Scene X2: H1 = {3, 4, 2, 3, 2} and H2 = {3, 2, 3, 3}

[0174] Scenario X3: H1 = {2, 3, 4, 2, 3, 4} and H2 = {3, 2, 3, 3, 4}

[0175] Scenario X4: H1 = {2, 3, 4} and H2 = {3, 2, 3}

[0176] Scenario X5: H1 = {3, 4, 5, 3, 4, 5} and H2 = {5, 4, 5, 5, 5}

[0177] Figure 9 Illustrates the spectral efficiency and channel parameters of various systems according to the example embodiments. Initially, verify the SE derived analytically for two overfilled systems. To this end, for the first system with Scenario X1, assume that dACDD supports two RRUs, while there are five and four RRUs in Cluster C1 and Cluster C2, respectively. For the second system with Scenario X2, dACDD supports four RRUs, while there are five and four RRUs in Cluster C1 and Cluster C2, respectively. For the two overfilled systems, Figure 9 Shows the accuracy of the SE derived analytically compared to the exact SE. Figure 9 Also shows that if N1 is not large enough, the approximation used in Equation (12) cannot provide sufficient accuracy. Therefore, later, a larger value of N1 is used without its specific description. Generally, as M increases, a larger N1 is required to obtain a very reliable analytical SE.

[0178] B. Comparing SE with respect to a single cluster

[0179] Figure 10 Illustrates the spectral efficiency of various overfilled DASs with Scenario X3 according to the example embodiments. In this simulation, assume M = 4. For the first four fading channel parameters of Scenarios X1 - X4 with Cluster C1 and M = 4, they can be classified as:

[0180] · Underfilled DAS (K1 < M): Scenario X4.

[0181] · Overfilled DAS (K1 > M): Scenarios X1, X2, and X3.

[0182] In contrast, the second DAS deployed in Cluster C2 is fully filled by Scenarios X1, X2, and X4, i.e., K2 = M. When the DAS is underfilled or fully filled, CM makes full use of the RRUs for dACDD. In Figure 10 , Scenario X3 is mainly used with various overfilled DASs. In addition, from Figure 10 , the following fact is obvious:

[0183] · As the DAS becomes more overfilled, a larger SE can be achieved.

[0184] • As the number of clusters increases, a larger SE can be achieved. However, due to the varying degrees of selective fading across clusters in a deployment, there are stricter limitations on the number of dCDD RRUs.

[0185] The number of multipath components and the effect of M on SE

[0186] Figure 11 The spectral efficiency of various values ​​of M and different numbers of multipath components according to the example implementation is illustrated. At a fixed 18dB SNR, Figure 11 The SE is shown for various system and channel parameters. For both subdued and oversaturated DAS, the effect of M on SE is investigated.

[0187] For a given K1 and K2, as M increases, DAS becomes less oversaturated. Although SE increases proportionally to M, the growth rate of SE decreases.

[0188] • As K1 or K2 increases, the growth rate of SE increases. For example, (K1=6, K2=5) vs (K1=5, K2=4).

[0189] • As the number of multipath components increases, a larger SE (Sequence Target) is achieved. For example, scene X3 vs X5.

[0190] In an example implementation, the MC-DAS communication system can be used for communication between a controller and multiple robots in a factory or warehouse. For example, robots can be used in a warehouse to move heavy objects from one location to another. To achieve this, each robot moving the goods should communicate accurately with its location within the warehouse where the goods are to be moved. For this purpose, multiple spatially distributed transceivers or RRUs can be arranged within the warehouse. Multiple RRUs can be clustered together to form multiple clusters, where each cluster forms a distributed antenna system. Furthermore, each cluster is configured to include a transceiver, which can be configured as a cluster master (CM). The cluster master (CM) of each cluster is configured to communicate directly with the controller. The CM also communicates directly with the RRUs in its corresponding cluster (e.g., ...). Figure 1A (As shown).

[0191] Furthermore, each robot in the multiple robots is configured to receive messages, including information such as location information, from the controller via multiple clusters. The controller can be configured to send messages to each robot in the multiple robots. To achieve this, the controller, CM, and RRU use HPTP for synchronization. After synchronization, the message sent by the controller is received by at least one CM in the multiple CMS and further forwarded to the RRU in its corresponding cluster, with a minimum cycle delay prefix added. Finally, the target robot in the multiple robots can receive the message from the multiple RRUs. The robot can remove the cycle prefix and obtain the original message sent by the controller without any ISI interference.

[0192] Figure 12 A method 1200 for sending messages in an MC-DAS communication system according to an example embodiment is illustrated. Method 1200 begins with step 1201.

[0193] In step 1201, the length of the message to be sent to receiver RX 103 via the DAS cluster can be determined. The message length can be determined by controller 101.

[0194] In step 1203, the maximum number of RRUs in the DAS cluster used for message transmission can be determined based on the message length and the minimum length of the cyclic prefix. The cyclic prefix is ​​determined as a function of the synchronization error bound within the DAS cluster and the sum of the maximum tap delay of the communication channel of the CM transmitter starting in the DAS cluster.

[0195] In step 1205, it can be determined whether the maximum number of RRUs required to send the message is greater than the total number of RRUs in the DAS cluster. Furthermore, for each RRU in the DAS cluster, the tap delay of the communication channel between the receiver and the CM that generates the tap delay set via the RRUs can be determined. The maximum tap delay in the tap delay set can be selected to generate the maximum tap delay of the communication channel starting with the CM in the DAS cluster.

[0196] If the maximum number of RRUs required to send a message is not greater than or equal to the total number of RRUs in the DAS cluster, then in step 1207, a subset of RRUs can be selected from the multiple RRUs in the DAS cluster.

[0197] On the other hand, if the maximum number of RRUs required to send a message is greater than or equal to the total number of RRUs in the DAS cluster, then in step 1209, all RRUs in the DAS cluster can be selected.

[0198] In step 1211, the selected RRU can be used to send a message within the DAS cluster. This message can be sent independently of other DAS clusters within the multiple DAS clusters using a distributed cyclic delay diversity (CDD) scheme with a minimum length of the cyclic prefix.

[0199] Implementation

[0200] The description provides only exemplary embodiments and is not intended to limit the scope, applicability, or configuration of this disclosure. Rather, the following description of exemplary embodiments will provide those skilled in the art with enabling descriptions for implementing one or more exemplary embodiments. Various changes to the function and arrangement of the elements will be contemplated without departing from the spirit and scope of the disclosed subject matter as set forth in the appended claims.

[0201] Specific details are set forth in the following description to provide a thorough understanding of the embodiments. However, it will be understood by those skilled in the art that embodiments may be practiced without these specific details. For example, systems, processes, and other elements in the disclosed subject matter may be shown as components in block diagram form to avoid obscuring the embodiments with unnecessary detail. In other instances, well-known processes, structures, and techniques may be shown without unnecessary detail to avoid obscuring the embodiments. Furthermore, similar reference numerals in the various figures indicate similar elements.

[0202] Furthermore, the various implementations can be described as processes depicted as flowcharts, data flow diagrams, structural diagrams, or block diagrams. Although flowcharts may describe operations as sequential processes, many operations can be executed in parallel or concurrently. Additionally, the order of operations can be rearranged. A process may terminate upon completion of its operations, but may have additional steps not discussed or included in the diagrams. Moreover, not all operations in any specifically described process may occur in all implementations. A process may correspond to a method, function, procedure, subroutine, subroutine, etc. When a process corresponds to a function, the termination of the function may correspond to the function returning to the calling function or the main function.

[0203] Furthermore, implementations of the disclosed subject matter can be implemented, at least partially, manually or automatically. They can be performed, or at least assisted in, manual or automatic implementation using machines, hardware, software, firmware, middleware, microcode, hardware description languages, or any combination thereof. When implemented in software, firmware, middleware, or microcode, program code or code segments that perform the necessary tasks can be stored in a machine-readable medium. The processor can perform the necessary tasks.

[0204] Furthermore, the embodiments of this disclosure and the functional operations described in this specification can be implemented in digital electronic circuits, in tangibly implemented computer software or firmware, in computer hardware including the structures disclosed in this specification and their equivalents, or a combination thereof. Additionally, some embodiments of this disclosure can be implemented as one or more computer programs, i.e., one or more modules of computer program instructions embodied on a tangible, non-transitory program carrier for execution by a data processing device or for controlling the operation of a data processing device. Furthermore, program instructions can be encoded on artificially generated propagation signals, such as machine-generated electrical, optical, or electromagnetic signals, which are generated to encode information for transmission to a suitable receiving device for execution by the data processing device. The computer storage medium can be a machine-readable storage device, a machine-readable storage substrate, a random or serial access memory device, or a combination thereof.

[0205] According to embodiments of this disclosure, the term "data processing apparatus" can encompass all kinds of devices, apparatuses, and machines for processing data, including, for example, programmable processors, computers, or multiple processors or computers. Apparatus may include dedicated logic circuitry, such as FPGAs (Field-Programmable Gate Arrays) or ASICs (Application-Specific Integrated Circuits). In addition to hardware, apparatus may also include code that creates an execution environment for the computer program in question, such as code constituting processor firmware, protocol stacks, database management systems, operating systems, or combinations thereof.

[0206] A computer program (also referred to or described as a program, software, software application, module, software module, script, or code) may be written in any form of programming language (including compiled or interpreted languages, or declarative or procedural languages), and it may be deployed in any form, including as a standalone program or as a module, component, subroutine, or other unit suitable for a computing environment. A computer program may (but does not necessarily) correspond to a file in a file system. A program may be stored as part of a file containing other programs or data (e.g., in one or more scripts in a markup language document), in a single file dedicated to the program in question, or in multiple coordination files, such as files storing one or more modules, subroutines, or portions of code. A computer program may be deployed to execute on a single computer, at a single site, or on multiple computers distributed across multiple sites and interconnected by a communication network. As an example, computers suitable for executing computer programs include those based on general-purpose or special-purpose microprocessors or both, or any other type of central processing unit. Typically, the central processing unit receives instructions and data from read-only memory or random access memory or both. The basic elements of a computer are the central processing unit for executing instructions and one or more memory devices for storing instructions and data. Typically, a computer will also include or be operatively coupled to receive data from or transfer data to one or more mass storage devices (e.g., magneto-optical, magneto-optical, or optical discs) used for storing data, or both. However, a computer does not necessarily need to have these devices. Furthermore, a computer may be embedded in another device, such as a mobile phone, personal digital assistant (PDA), mobile audio or video player, game console, GPS receiver, or portable storage device such as a Universal Serial Bus (USB) flash drive.

[0207] The embodiments of the subject matter described in this specification can be implemented in a computing system that includes back-end components (e.g., as a data server), or middleware components (e.g., an application server), or front-end components (e.g., a client computer with a graphical user interface or web browser through which a user can interact with the implementation of the subject matter described in this specification), or any combination of one or more such back-end, middleware, or front-end components. The components of the system can be interconnected via digital data communication of any form or medium, such as a communication network. Examples of communication networks include local area networks (“LANs”) and wide area networks (“WANs”), such as the Internet.

[0208] Computing systems may include clients and servers. Clients and servers are typically geographically separated and usually interact through communication networks. The client-server relationship arises from computer programs running on individual computers that have client-server relationships with each other.

[0209] Although this disclosure has been described with reference to specific preferred embodiments, it will be understood that various other adjustments and modifications may be made within the spirit and scope of this disclosure. Therefore, aspects of the appended claims cover all such variations and modifications that fall within the true spirit and scope of this disclosure.

Claims

1. A communication system comprising: A controller, comprising one or more processors configured to send messages to a receiver; Multiple clusters of Distributed Antenna Systems (DAS) Each DAS cluster includes a cluster master controller (CM) that communicates with the controller and multiple remote radio units (RRUs) that communicate with the CM. The controller and the DAS cluster are synchronized using the Hierarchical Precision Time Protocol (HPTP). In HPTP, the controller is the master clock, the CM is the boundary clock, and the RRU is a normal clock synchronized to the CM of its corresponding cluster. Each DAS cluster is configured to send messages independently of other DAS clusters in the multiple DAS clusters using a distributed cyclic delay diversity (CDD) scheme. The minimum length of the cyclic prefix is ​​determined as a function of the synchronization error limit within the DAS cluster and the sum of the maximum tap delay of the communication channel of the CM starting in the DAS cluster.

2. The communication system according to claim 1, wherein, For each DAS cluster, the controller is configured as follows: For each RRU in the DAS cluster, the tap delay of the communication channel between the receiver and the CM via the RRU is determined to generate a tap delay set; and The maximum tap delay is selected from the set of tap delays to generate the maximum tap delay for the communication channel of the CM that begins in the DAS cluster.

3. The communication system according to claim 1, wherein, For the DAS cluster that sends the message, the controller is configured as follows: Determine the length of the message; The maximum number of RRUs in the DAS cluster used to transmit the message is determined based on the length of the message and the minimum length of the cyclic prefix. When the maximum number of RRUs is less than the total number of RRUs in the DAS cluster, a subset of RRUs is selected from the plurality of RRUs in the DAS cluster; or When the maximum number of RRUs is greater than or equal to the total number of RRUs in the DAS cluster, all RRUs in the DAS cluster are selected. and Use the selected RRU to send the message.

4. The communication system according to claim 1, wherein, The controller is connected to the base station via a wired connection to act as a coordinating multipoint CoMP transmitter and receiver.

5. The communication system according to claim 1, in, The RRU is wirelessly connected to the CM of its corresponding DAS cluster, and The controller is wirelessly connected to the CM.

6. The communication system according to claim 1, wherein, The controller is also configured to: The HPTP is used to configure the controller as the master clock, the CM as the boundary clock, and the RRU as the normal clock; and A multi-level hierarchical tree structure, including the master clock, the boundary clock, and the normal clock, is used to enable each DAS cluster in the multiple DAS clusters to be independent of each other.

7. The communication system according to claim 1, in, The HPTP includes one or more messages, wherein each of the one or more messages includes an accurate timestamp generated and recorded when the corresponding one or more messages are sent and received. The timestamps corresponding to the one or more messages are used to determine the clock offset and propagation delay associated with the messages sent to the receiver, and The propagation delay and the clock offset are used to synchronize the DAS cluster.

8. The communication system of claim 1, wherein the receiver is configured to receive the message from one or more of the plurality of DAS clusters based on a single-user-multiple-input single-output (SU-MISO) operating mode.

9. A method comprising the following steps: In the communication system, the communication system includes: The controller sends messages to the receiver; Multiple clusters of Distributed Antenna Systems (DAS) Each DAS cluster includes a cluster master controller (CM) that communicates with the controller and multiple remote radio units (RRUs) that communicate with the CM. The controller and the DAS cluster are synchronized using the Hierarchical Precision Time Protocol (HPTP). In HPTP, the controller is the master clock, the CM is the boundary clock, and the RRU is a normal clock synchronized to the CM of its corresponding cluster. In this scheme, each DAS cluster sends messages independently of other DAS clusters in the multiple DAS clusters using a distributed cyclic delay diversity (CDD) scheme. The minimum length of the cyclic prefix is ​​determined as a function of the synchronization error limit within the DAS cluster and the sum of the maximum tap delays of the communication channels of the CM starting in the DAS cluster.

10. The method according to claim 9, wherein, For each DAS cluster, the method also includes the following steps: For each RRU in the DAS cluster, determine the tap delay of the communication channel between the receiver and the CM via the RRU to generate a tap delay set; and The maximum tap delay is selected from the set of tap delays to generate the maximum tap delay for the communication channel of the CM that begins in the DAS cluster.

11. The method of claim 9, further comprising the step of: Determine the length of the message; The maximum number of RRUs in the DAS cluster used to transmit the message is determined based on the length of the message and the minimum length of the cyclic prefix. When the maximum number of RRUs is less than the total number of RRUs in the DAS cluster, a subset of RRUs is selected from the plurality of RRUs in the DAS cluster; or When the maximum number of RRUs is greater than or equal to the total number of RRUs in the DAS cluster, all RRUs in the DAS cluster are selected. as well as Use the selected RRU to send the message.

12. The method according to claim 9, wherein, The controller is connected to the base station via a wired connection to act as a coordinating multipoint CoMP transmitter and receiver.

13. The method according to claim 9, in, The RRU is wirelessly connected to the CM of its corresponding DAS cluster, and The controller is wirelessly connected to the CM.

14. The method of claim 9, further comprising the step of: The HPTP is used to make the controller the master clock, the CM the boundary clock, and the RRU the normal clock; as well as A multi-level hierarchical tree structure, including the master clock, the boundary clock, and the normal clock, is used to enable each DAS cluster in the multiple DAS clusters to work independently.

15. The method according to claim 9, in, The HPTP includes one or more messages, wherein each of the one or more messages includes an accurate timestamp generated and recorded when the corresponding one or more messages are sent and received. The timestamps corresponding to the one or more messages are used to determine the clock offset and propagation delay associated with the messages sent to the receiver, and The propagation delay and the clock offset are used to synchronize the DAS cluster.

16. The method of claim 9, further comprising the step of: The message is received from one or more of the plurality of DAS clusters based on the receiver's single-user-multiple-input single-output (SU-MISO) operating mode.

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