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A Cluster-Based Transmit Diversity Scheme for Asynchronous Joint Transmissions in Private Networks

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A Cluster-Based Transmit Diversity Scheme for Asynchronous Joint Transmissions in Private Networks MITSUBISHI ELECTRIC RESEARCH LABORATORIES https://www.merl.com A Cluster-Based Transmit Diversity Scheme for Asynchronous Joint Transmissions in Private Networks Kim, Kyeong Jin; Guo, Jianlin; Orlik, Philip V.; Nagai, Yukimasa; Poor, H. Vincent TR2021-075 June 12, 2021 Abstract In this paper, a multiple cluster-based transmission diversity scheme is proposed for asyn- chronous joint transmissions (JT) in private networks, in which the use of multiple clusters or small cells is preferable to increase transmission speeds, reduce latency, and bring trans- missions closer to the users. To increase the spectral efficiency and coverage, and to achieve flexible spatial degrees of freedom, a distributed remote radio unit system (dRRUS) is in- stalled in each of the clusters. When the dRRUSis disposed in the private environments, it will be associatedwith multipath-rich and asynchronous delay propagation. Taking into ac- count of this unique environment of private networks, asynchronous multiple signal reception is considered in the development of operation at the remote radio units to make an inter- symbol interference free distributed cyclic delay diversity (dCDD) scheme for JT to achieve a full transmit diversity gain without full channel state information. A spectral efficiency of the proposed dCDD-based JT is analyzed by deriving the closedform expression, and then compared with link-level simulations for non-identically distributed frequency selective fading over the entire private network. IEEE International Conference on Communications (ICC) c 2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. Mitsubishi Electric Research Laboratories, Inc. 201 Broadway, Cambridge, Massachusetts 02139 A Cluster-Based Transmit Diversity Scheme for Asynchronous Joint Transmissions in Private Networks Kyeong Jin Kim, Jianlin Guo, Philip V. Orlik, Yukimasa Nagai, and H. Vincent Poor Abstract—In this paper, a multiple cluster-based transmission transmission speeds and capacity, reduce latency, and make diversity scheme is proposed for asynchronous joint transmissions the signal closer to the users, a dense deployment of small (JT) in private networks, in which the use of multiple clusters or cells or clusters is expected [2]. small cells is preferable to increase transmission speeds, reduce latency, and bring transmissions closer to the users. To increase To increase the spectral efficiency and coverage, a dis- the spectral efficiency and coverage, and to achieve flexible tributed antenna system (DAS) [3], [4], in which antennas spatial degrees of freedom, a distributed remote radio unit system are installed in a distributed manner over a coverage area of (dRRUS) is installed in each of the clusters. When the dRRUS the base station (BS), is a promising approach for private is disposed in the private environments, it will be associated networks. When each antenna operates as a BS, the DAS with multipath-rich and asynchronous delay propagation. Taking into account of this unique environment of private networks, can be recognized as coordinated multiple point (CoMP) [5], asynchronous multiple signal reception is considered in the [6]. Its core concept is to provide simultaneous communi- development of operation at the remote radio units to make an cations by a plurality of BSs to a single or multiple users intersymbol interference free distributed cyclic delay diversity to improve the rate over a whole communication region. As (dCDD) scheme for JT to achieve a full transmit diversity gain major approaches of CoMP, coordinated beamforming and without full channel state information. A spectral efficiency of the proposed dCDD-based JT is analyzed by deriving the closed- joint processing that includes joint transmissions (JT) are form expression, and then compared with link-level simulations available. However, since we do not assume full channel state for non-identically distributed frequency selective fading over the information at the transmitter (CSIT), we will focus on JT entire private network. in this paper. Geographically placed BSs make the system Index Terms—Private networks, distributed cyclic delay diver- properly counters path loss and shadowing [7]. However, it sity, joint transmission, spectral efficiency. is challenging to collect full CSIT in the distributed system. Although a very reliable channel estimate can be obtained by the user, the feedback overhead will be overwhelming for large I. INTRODUCTION number of BSs. A conventional codebook based feeding back A private network is a promising new connectivity model may not be working for CoMP due to significant differences offering previously unavailable wireless network performance in received signal strength [8]. to businesses and individuals. The owners can optimize ser- In addition, a tight clock synchronization among BSs is vices at their specific areas by planning and installing their own required. Its mismatch will cause interference at the user due to networks, and ensure reliable communications by an exclusive the difference in signal arrival times from all the BSs. When use of available resources. Since they have complete control Global Navigation Satellite System (GNSS) signals are not over every aspect of the network, they can determine how available in the private area, it is possible to achieve a desired resources are utilized, how traffic is prioritized, how a specific clock synchronization by the precision time protocol (PTP) [9]. security standard is deployed, and so on. Potential applications When a plurality of BSs transmit simultaneously, the existence to industries, businesses, utilities, and public sectors have of interference is an intrinsic problem [3] as well. We are gravitated towards 5G wireless networks with increasingly investigating the following four problems in this paper. stringent performance requirements, in terms of availability, 1) When a DAS is installed in the private environments, it reliability, latency, device density, and throughput [1]. The will be affected by multipath-rich propagation. deployment of private networks can be feasible in the shared 2) Since the distance between each of the remote radio units spectrum or unlicensed spectrum. Furthermore, to increase (RRUs) varies with respect to the receiver (RX), a received symbol timing cannot be aligned at the RX due to a K. J. Kim, J. Guo, and P. V. Orlik are with Mitsubishi Electric Research Laboratories (MERL), Cambridge, MA 02139 USA path dependent propagation delay [10]. Note that RRU Y. Nagai is with Mitsubishi Electric Corp., Japan has only a single antenna and fixed power for simple H. V. Poor is with the Department of Electrical Engineering, Princeton processing, so that a system comprising a plurality of University, Princeton, NJ 08544 USA This work was supported in part by the U.S. National Science Foundation RRUs, called the distributed remote radio unit system under Grant CCF-1908308. (dRRUS), is recognized as the DAS [11]. 3) CSIT-dependent precoders are usually employed to mini- PNS mize interference caused by simultaneous multiple trans- Coordinator missions. Thus, it is necessary to apply an interference-free b1 b2 transmission scheme to achieve a full transmit diversity. C1 4) Feeding back overhead from the RX increases in propor- RRU 1,4 C2 tion to the number of RRUs. CM1 b1,1 RRU1,3 CM2 b2,4 b1,2 b2,1 RRU2,3 b1,3 RRU RRU2,1 Taking into four mentioned problems, we can summarize the 1,1 b2,2 RRU1,2 h1,4 b2,4 RRU2,4 h1,1 h2,1 following two key contributions comparing with existing work. h1,3 h2,3 RRU2,2 RRU1,5 h2,2 h2,4 1) A new multi-cluster-based distributed remote radio unit h1,5 system (MC-dRRUS): To provide a greater throughput for RX the private network, we propose a new MC-dRRUS, in which the private network server (PNS) provides transmis- Fig. 1. Illustration of the proposed MC-dRRUS with five RRUs and four RRUs in each of the clusters. Two nodes highlighted in blue are assigned as sion signals and synchronization to the respective cluster CMs to control the RRUs within its cluster. D1 D2 masters (CMs). Within non-overlapping clusters, each CM θ t1 Fig. 2. Two-way packet ex- Sync forms an individual dRRUS. df change for synchronization. A 2) Distributed asynchronous cyclic delay diversity-based JT Follow-up t2 (t1, t2) filled circle dot denotes a p θ (dACDD-JT) scheme: It is desired to increase the number timestamp in the event mes- t3 (t1, t2, t3) dr sage recorded at its transmis- Delay-Req of RRUs to cover larger territory and to increase channel (t1, t2, t3, t4) t4 sion and reception. The pro- p diversities. However, such an increase would result in cessing time taken at all the additional interference, difficulty in time synchronization, nodes is assumed to be p. Delay-Resp and undesirable feedback overhead. With an available (t1, t2, t3, t4) distribution for propagation delays over the whole paths, the CM is able to consider its variant in assigning the CDD clocks, which have multiple PTP ports to interact with other delay at each of its RRUs, which is necessary to remove in- clocks. The RX is represented as the ordinary clock. Very tersymbol interference (ISI) cased by asynchronous signal reliable main backhaul links, {b1,b2}, are configured to pro- reception at the RX [12]. vide backhaul access to the clusters via the coordinator that resides at the PNS2. Other very reliable secondary backhaul Notation: IN denotes an N × N identity matrix; 0 denotes an all-zero matrix of with an appropriate size; links, {bi,j,i = 1, 2, j = 1,...,K}, provide backhaul access and CN µ, σ2 denotes a complex Gaussian distribution to RRUs via the CM. The CM controls all RRUs and is with mean µ and variance σ2.
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