Optimizing mmWave Wireless Backhaul Scheduling

Edgar Arribas, Antonio Fernandez Anta, Dariuz Kowalski, Vincenzo Mancuso, Miguel Mosteiro, Joerg Widmer, Prudence W.H. Wong
<span title="">2019</span> <i title="Institute of Electrical and Electronics Engineers (IEEE)"> <a target="_blank" rel="noopener" href="https://fatcat.wiki/container/m6d55yg2wncgnomdnlvb6ofsmq" style="color: black;">IEEE Transactions on Mobile Computing</a> </i> &nbsp;
Millimeter wave (mmWave) communication not only provides ultra-high speed radio access but is also ideally suited for efficient and flexible wireless backhauling. Specifically for dense deployments, a mmWave macro base station (MBS) that serves a large number of mmWave micro base stations (µBSs) is much more cost effective than legacy cellular architectures which connect µBSs to the core network through fibers. In addition, µBSs can cooperate with each other by acting as relay nodes. The
more &raquo; ... onal nature of mmWave communication allows for spatial reuse, even in the presence of interference, which can be exploited to optimize mmWave wireless backhaul performance. The optimization opportunistically prioritizes the use of good connections at the MBS and further leverages compact and concurrent transmissions between µBS. Relays and directional antennas speed up communication, but increase the complexity of the scheduling problem. In this work, we study the mmWave backhaul scheduling problem and derive an MILP formulation for it as well as upper and lower bounds. We prove that the problem is NP-hard and can be approximated, but only if interference is negligible. By means of numerical simulations, we compare theoretical results with heuristics in small system sizes. Results validate the analysis and demonstrate the high performance of our heuristics in realistic cellular settings. ! 1536-1233 (c) SUBMITTED TO IEEE TRANSACTIONS ON MOBILE COMPUTING 2 scheduling data for delivery, as one needs to choose whether to relay or not, to which µBSs, and which MBS links must be used. Therefore, understanding whether scheduling data delivery is NP-hard in this context, and if so, which approximations can be guaranteed even in the limit, is fundamental to gain insight on practical challenges such as scalability. Also, in such scenario, finding out which heuristics perform well, and for which system sizes, is crucial for practical purposes. In this work, we carry out such studies as follows. Given a collection of data to deliver to a set of µBSs, we study the mmWave relay optimization problem of minimizing the time to complete the delivery, i.e. the makespan 1 , in a network managed by an MBS. We consider both the case of interference-free links and the case of more realistic transmissions in the presence of directional cross-link interference. We call such optimization problem mmWave Backhaul Scheduling (MMWBS). Solving the problem results in a compact concurrent relaying schedule of links, which flexibly and opportunistically reuses mmWave resources over the backhaul links. However, (re-)configuring mmWave links brings with it a beam training and steering overhead that needs to be taken into account to implement a scheduling strategy that works efficiently at packet level. Plenty of work has been done in scheduling communications in related models, including different wireless and optical networks. For instance, the work in [6] applies to wireless networks of arbitrary topology, but link activation cost, interference, and concurrent communication through multiple outgoing links are not taken into account. Even if communication models differ only in minor aspects, problems may be entirely different [7], [8], [9], [10] . To the best of our knowledge none of these solutions apply to our setting. Roadmap. We first present an overview of the most relevant related work in Section 2. Then, we summarize the contribution and main findings of our work in Section 3 and present the system model in detail in Section 4. We formulate the problem in Section 5 and analyze it in Section 6. We discuss the design of heuristics in Section 7 and report on performance evaluation in Section 8 through numerical simulation. Finally, we discuss the lessons learnt in Section 9, and summarize and conclude the paper in Section 10. RELATED WORK The use of relays in cellular networks has been proposed to extend cellular and ad-hoc/WLAN coverage and improve user throughput. Many proposals focus on the use of orthogonal relay resources, to not interfere with the direct communication link. Authors of [11] and [12] discuss how to implement and optimize cellular relay with opportunistic features, using legacy 802.11 and LTE bands, while authors of [13] apply the D2D paradigm to mmWave relays in the 60 GHz band. They only use simple heuristics and model interference without considering beamforming gains due to steerable antennas used for mmWave. While relays on sub-6 GHz bands cause and suffer from significant interference due to their omnidirectional transmissions, the directionality of mmWave antennas mitigates 1536-1233 (c)
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