Design and Experimental Evaluation of a 2.4 GHz-AoA-Enhanced Beamsteering Algorithm for IEEE 802.11ad mm-Wave WLANs Avishek Patra, Ljiljana Simi´ c and Marina Petrova Institute for Networked Systems, RWTH Aachen University Kackertstrasse 9, D-52072 Aachen, Germany Email: {avp, lsi, mpe}@inets.rwth-aachen.de Abstract—IEEE 802.11ad millimeter-wave (mm-wave) WLAN nodes achieve high throughput at the cost of the frequent re-steering requirement of highly directional antenna beams to establish and maintain links disrupted by beam misalignment, node mobility, or link blockage. Consequently, rapid and robust beamsteering algorithms are essential to enable seamless commu- nication in mm-wave WLANs. In this paper we propose AoASteer, a beamsteering algorithm that speeds up the link establishment process in IEEE 802.11ad mm-wave WLANs by preferentially searching over a subset of mm-wave antenna sectors predicted by 2.4 GHz angle of arrival (AoA) estimation at the access point (AP). We experimentally evaluate the performance of AoASteer through extensive measurements in several indoor and outdoor locations, using 60 GHz USRP-SiversIMA packet radio transceivers to gather real mm-wave link information, and a 2.4 GHz USRP-based receiver with an 8-element uniform linear antenna array for AoA estimation, both implemented using GNU Radio. Our evaluation results, obtained for APs with different numbers of beams per sector, show that AoASteer typically selects a near-optimal LOS or NLOS link to establish communication, while significantly reducing the link establishment latency. For example, for 4-sector mm-wave antennas, AoASteer reduces the latency by 46 μs for 73% of our measurement cases compared to the IEEE 802.11ad beamsteering algorithm, while achieving the highest data rate of 6.7 Gbps for 92% of the cases. I. I NTRODUCTION Directional antennas in mm-wave networks mitigate the high attenuation inherent at such high frequencies. However this makes the mm-wave link establishment process com- plex as the antennas must be precisely steered to specific orientations to successfully establish a link. The process of aligning the antenna orientations of two nodes to establish a link between them is called beamsteering. In addition to initial link establishment, beamsteering algorithms are responsible for maintaining connectivity through rapid re-establishment mechanisms. Considering that mm-wave links are easily dis- rupted by slight beam misalignment [1], node mobility [2] or link blockage [3], robust and fast algorithms are essential for seamless communication. The state-of-the-art beamsteering algorithm in the IEEE 802.11ad standard [4], adopted for mm-wave WLANs operating at 60 GHz, is essentially a breadth-first search algorithm. In IEEE 802.11ad, the antenna 978-1-5386-2723-5/17/$31.00 c 2017 IEEE radiation pattern is divided into wide beamwidth sectors, each comprising of multiple narrow, high gain beams. For beamsteering, the algorithm first sequentially scans over all sectors to obtain the best sector of a node, followed by scanning through the best sector beams to select the best beam for link establishment. Such exhaustive scanning induces high latency that significantly reduces QoS in mm-wave WLANs. In this paper, we present AoASteer, a fast beamsteering algorithm for IEEE 802.11ad mm-wave WLANs which utilizes 2.4 GHz angle-of-arrival (AoA) estimation to quickly establish a link between 60 GHz nodes. Using AoASteer, an access point (AP) equipped with an antenna array estimates the AoA of the 2.4 GHz signal from a user equipment (UE), based on which a subset of feasible 60 GHz AP sectors is predicted. We use this estimated AP sector subset to then determine the best sectors and beams of the AP–UE pair for link establishment. We de- fine the messaging protocol for AoASteer in detail, and present the results of an extensive experimental evaluation in several indoor and outdoor locations. For our measurements, we em- ploy 60 GHz USRP-SiversIMA packet-radio transceivers and a 2.4 GHz USRP-based receiver with an 8-element uniform linear array (ULA) antenna. Our results show that AoASteer significantly reduces the link establishment latency compared to the IEEE 802.11ad beamsteering algorithm while typically achieving the optimal data rate over the link. A number of mm-wave beamsteering algorithms have been proposed in the literature [1], [5], [6], [7], [8], [9]. The solution in [5] utilizes the correlation between the mm-wave links of two nodes to predict their quality when one link experiences degradation. The proposal nevertheless depends on extensive scanning to determine the links between the nodes. In [6], optimal AP-UE re-deployment positions are chosen by predicting the links at unobserved spots to pre- vent link blockage. However, the underlying assumption of re-deployment of nodes, especially APs, is not practically feasible. In the algorithm proposed in [7], the orientation re- ceiving the highest signal energy is chosen as the beamsteering direction. While promising, the work proposes no mechanism for node coordination. Additionally, energy detection along multiple directions may lead to increased latency. Solutions