5G millimeter-wave (mmW) systems rely on electronically steerable antenna arrays to support directional communications. Directionality complicates the initial access (IA) process, whereby a base station (BS) announces itself to nearby user equipments (UEs), giving them the opportunity to associate with this BS. Existing approaches for IA suffer from long discovery time and/or nonnegligable probability of missing UEs. In this paper, we propose FastLink, an efficient IA protocol for mmW systems, in which discovery beacons are transmitted/received using the narrowest possible beams, allowing for high beamforming gains and low misdetection rate, while maintaining low discovery time. Fastlink executes a unique algorithm, called 3-dimensional peak finding (3DPF), to find the best beam in logarithmic time. We formulate the beam-finding process as a sparse problem and use compressive sensing to determine the minimum number of measurements needed for this process. We first study FastLink for the discovery of a single UE and then extend our analysis to a multi-user scenario. Both simulations and over-the-air experiments based on a custom mmW testbed are used to evaluate FastLink. Our results verify its efficiency, and show that it can reduce the search time by 90% compared to the scanning approach used in 802.11ad systems.

中文翻译：

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毫米波网络的高效波束扫描算法和初始接入协议

5G 毫米波 (mmW) 系统依靠电子可控天线阵列来支持定向通信。方向性使初始接入 (IA) 过程变得复杂，基站 (BS) 借此向附近的用户设备 (UE) 宣布自己，让他们有机会与该 BS 关联。现有的 IA 方法存在发现时间长和/或丢失 UE 的概率不可忽视的问题。在本文中，我们提出了 FastLink，一种用于 mmW 系统的高效 IA 协议，其中发现信标使用尽可能窄的波束发送/接收，允许高波束成形增益和低误检测率，同时保持低发现时间。Fastlink 执行一种称为 3 维峰值查找 (3DPF) 的独特算法，以在对数时间内找到最佳光束。我们将波束查找过程表述为一个稀疏问题，并使用压缩传感来确定此过程所需的最小测量次数。我们首先研究用于发现单个 UE 的 FastLink，然后将我们的分析扩展到多用户场景。基于定制 mmW 测试台的模拟和无线实验都用于评估 FastLink。我们的结果验证了它的效率，并表明与 802.11ad 系统中使用的扫描方法相比，它可以将搜索时间减少 90%。