I. Introduction
In 5G mmWave systems, in addition to time-of-arrival (TOA) measurements, the ability to estimate angles-of-arrival (AOAs) and angles-of-departure (AODs) has been introduced [1], which provides additional geometric information, increases multipath resolvability and enables the exploitation of reflected propagation paths to improve positioning, in contrast to sub-6 GHz systems. Based on the combination of TOA, AOA and AOD measurements, joint positioning and synchronization with a single BS can be realized under multipath conditions [2]. In contrast to positioning signals in time and frequency [3, Section 7.4.1.7], spatial positioning signals are inherently directional and thus mainly meaningful under a-priori information about the location of the user equipment (UE), which can be obtained in a tracking scenario [2], [4]. Conventional spatial signal design at the BS involves a set of directional beams (e.g., from a DFT codebook), where the reported received power at the UE reveals a coarse AOD information [5].