Following the Shadow: Agile 3-D Beam-Steering for 60 GHz Wireless Networks Anfu Zhou∗, Leilei Wu∗, Shaoqing Xu∗, Huadong Ma∗, Teng Weiy, Xinyu Zhangz ∗ Beijing Key Lab of Intell. Telecomm. Software and Multimedia, Beijing University of Posts and Telecomm. y Department of Electrical and Computer Engineering, University of Wisconsin-Madison z Department of Electrical and Computer Engineering, University of California San Diego Email: fzhouanfu,layla,donggua,[email protected], [email protected], [email protected] Abstract—60 GHz networks, with multi-Gbps bitrate, are Much effort has been devoted to design agile beam-steering, considered as the enabling technology for emerging applications from the hierarchical beam scanning defined in standards [1], such as wireless Virtual Reality (VR) and 4K/8K real-time [9], to heuristic-based shortcuts [10], [11] or sensing-inspired Miracast. However, user motion, and even orientation change, can cause mis-alignment between 60 GHz transceivers’ directional solutions [12], [13]. However, these methods mainly focus on beams, thus causing severe link outage. Within the practical 3D two dimensional (2D) beam-steering, e.g., assuming a phased- spaces, the combination of location and orientation dynamics array that can steer the main beam among different angles leads to exponential growth of beam searching complexity, which within a 2D plane. In practice, the users and radios move in 3D substantially exacerbates the outage and hinders fast recovery. space; and a 60 GHz array can comprise a planar “matrix” of In this paper, we first conduct an extensive measurement to analyze the impact of 3D motion on 60 GHz link performance, antenna elements, steering the beams towards different angles in the context of VR and Miracast applications. We find that in the 3D space. Under such cases, we find that the beam 3D motion exhibits inherent non-predictability, so conventional steering becomes a far more challenging problem that cannot be beam steering solutions, which targets 2D scenarios with lower addressed by straightforward extension of 2D counterparts. In search space and short-term motion coherence, fail in practical particular, 3D movement involves not only location translation, 3D setup. Motivated by these observations, we propose a model- driven 3D beam-steering mechanism called Orthogonal Scanner but also orientation changes (e.g., VR players quickly move, (OScan), which can maintain high performance for mobile jump and crouch in 3D space), which are fundamentally 60 GHz links in 3D space. OScan discovers and leverages a different from the linear/circular device movement commonly hidden interaction between 3D beams and the spatial channel assumed in 2D beam steering solutions. Moreover, the main- profile of 60 GHz radios, and strategically scans the 3D space lobe and side-lobes inside 3D beams exhibits a more intriguing so as to reduce the search latency by more than one order of magnitude. Experiment results based on a custom-built 60 spatial relation than 2D beams (Sec. III). Due to the compound GHz platform along with a trace-driven emulator demonstrate effect, legacy 2D beam steering solutions can lead to even worse OScan’s remarkable throughput gain, up to 5×, compared with performance than brute-force search, which already entails the state-of-the-art. formidable overhead considering the exponentially-increased search space due to dimension increase (Sec. IV). I. INTRODUCTION In this paper, we conduct a systematic study on 3D beam 60 GHz technology represents a new paradigm that can steering. First, we begin with a comprehensive measurement boost wireless systems to wire-speed. The recently-ratified 60 of 3D movement in practical 60 GHz applications, and analyze GHz MAC/PHY standard, IEEE 802.11ad [1], supports up to the corresponding impacts on 60 GHz link dynamics. Using 6.7 Gbps, and the coming IEEE 802.11ay [2] will deliver 20 Google Tango [14], we collect accurate and fine-grained motion Gbps rate. It is believed that 60 GHz radios will bring wireless time-series, including location and orientation, when users are access into the multi-Gbps era, for both the out-door cellular running two typical 60 GHz applications: wireless VR and picocell [3], [4] and indoor WiFi networks [1]. Moreover, 60 Miracast. We then examine the 60 GHz link performance by GHz radios will spur ultra-high speed wireless applications feeding the traces into a custom-built software radio testbed. We such as wireless virtual reality (VR) [5], [6] and 4K real-time find that 3D rotation, in contrast to the translation movement Miracast streaming [7]. commonly assumed in previous studies, has a much greater However, despite the huge potential, some key bottleneck impact on channel dynamics and can lead to abrupt link technologies still hinder wide adoption of 60 GHz, the first performance degradation in a very short-time scale (i.e., less and foremost being efficient beam-steering. In particular, 60 than 100ms). More importantly, 3D movement under practical GHz radios use highly directional beams, in contrast to the applications exhibits an inherent “non-predictability” property, conventional omni-directional WiFi/cellular signals below 6 i.e., the mobility pattern, especially the rotation pattern, is hard GHz. 60 GHz radios usually pack dozens or even hundreds to predict given the history information. Such non-predictability of antenna elements into a phased-array antenna, which renders the failure of heuristic based beam-steering shortcuts, concentrates the signals to form ultra-narrow “pencil beams”, which rely on accurate movement prediction. thus compensating for the attenuation loss [8]. In consequence, Second, motivated by the measurement insights, we propose a a pair of 60 GHz transceivers can establish a link only after novel 3D beam-steering mechanism called Orthogonal Scanner their beams are properly aligned. However, the alignment (OScan), which reduces steering latency by more than one may become broken due to transceiver movement or obstacle order of magnitude, in comparison to conventional approaches blockage. To realign the beams and recover from link outage, that ignore the unique issues in 3D space. Note that beam an efficient beam-steering mechanism is highly desired. steering is frequently invoked during device movement and the 8.0m 0.4 Mode Roll Pitch Yaw ) 3.0m 0.3 Plain 1.07 0.82 2.78 Z(m VR 1.49 1.12 7.40 0.2 -4 -0.9 -1.2 Miracast 3.12 1.66 10.88 -4.2 -0.6 -4.4 -4.6 -0.3 X(m) Y(m) TABLE I Fig. 1. Mobile VR setup. Fig. 2. VR playground. Fig. 3. A segment of VR movement. AVE.ROTATION(DEG./0.1S) overhead can even dominate data transmission time. By saving GHz software-radio along with a trace-driven emulator, and such overhead, OScan can maintain high performance for 60 demonstrate its multi-fold throughput gain over state-of-the-art GHz links under mobility. solutions (Sec. IV). OScan is designed based on our discovery that the dis- tribution of channel quality over all 3D beams manifests a II. UNDERSTANDING LINK DYNAMICS IN 3-D SPACE “deformed-cross” pattern. Intuitively, the channel response of A. Mobility Measurement Setup other beams is like “shadow” of the optimal beam that aligns Different from previous works where mobility trajectories are perfectly with the underlying spatial channel profile (SCP). generated by models or collected from controlled experiments not It is noteworthy that such a unique pattern is tied to [15], we investigate real-world mobility of two representative any particular beamforming code-book, but roots from the emerging applications, i.e., VR and Miracast, that rely on interaction of SCP and any 3D beam designed following general ultra-high-speed 60 GHz radios. codebook design principles (e.g., consistent with codebook in Wireless virtual reality: VR is anticipated to dominate practical planar phased-arrays). We quantitatively characterize future wireless multimedia application. However, a critical the interaction using a mathematic model. By exploiting the pain point is that a HDMI cable is required for transmitting interaction feature, OScan strategically scans the 3D space in a VR data at multi-Gbps bitrate from a rendering server to the “cross” way (i.e., following the clue of the shadow), in contrast VR headset. The cable is not only cumbersome but also a with the conventional time-consuming “blind” trail-and-error potential safety hazard of tripping down users. Both industry approach. Moreover, we also extend the basic cross searching and research communities have envisioned wireless VR over to multi-rounds, each with a deliberate starting beam. In this 60 GHz links [5], [6]. However, 60 GHz links are known to be way, OScan can avoid being trapped in any local-maximum highly unstable under mobility, and it remains unclear whether beam and keep its efficiency, even in complicated environment they can deliver seamless experience in the challenging mobile with heterogeneous reflectors. VR scenarios. We implement OScan on a custom-built 60 GHz platform, To elucidate this issue, we reproduce a typical VR setup along with a trace-driven simulator. We evaluate OScan’s (Fig. 1), and let a user play a typical VR game called “Magic performance against the default scanning in IEEE 802.11ad Sunglass” [16] using a Lenovo Phab 2 Pro [17] equipped with [1] and heuristic approach commonly used in state-of-the- a peripheral headset in an office building (Fig. 2). During the art [8], [11]. Experimental results demonstrate that OScan game, the user constantly moves, jumps and crouches in 3D converges quickly to the optimal beam with maximum channel space, and looks up/down/around as in a real gaming scenario. quality, at the cost of less than 20% of the 802.11ad overhead. Phab [17] has built-in Google Tango [14], which incorporates Overall, the beam optimality and saved scanning time translate depth cameras, motion tracking and computational vision into significant throughput gain, up to 5× for the dynamic technologies to enable mobile devices to detect their relative Miracast.