# Measurement Grid Optimization for OTA Testing of 5G Smart Watches
## 1 Introduction  
In the 5G era, smart watches play a crucial role in areas such as health management, message notification, electronic payment, navigation and positioning, and social entertainment, with their convenience, versatility, and seamless connection with smartphones. For the use scenario of smart watches, the reliability, coverage, response speed, and security of wireless connections are key factors that affect user experience and system performance, which are heavily dependent on agile and high-performance antennas [1, 2]. Therefore, OTA testing, which can accurately characterize the wireless performance of devices under test (DUTs) in real-life usage environments, is of great significance for optimizing the wireless/antenna performance of smart watches and improving their quality of user experience [3, 4, 5, 6, 7, 8, 9]. OTA testing has been mandated as the standard method for wireless performance evaluation of DUTs by the Cellular Telecommunications and Internet Association (CTIA) and the 3 rd Generation Partnership Project (3 GPP), as well as network operators. For 5G Sub-6 GHz user equipment (UEs), the currently required lowest measurement grid resolutions in both CTIA and 3 GPP specifications are 15\({}^{\circ}\) and 30\({}^{\circ}\) for Total Radiated Power (TRP) and Total Isotropic Sensitivity (TIS) testing, respectively [10; 11; 12; 13; 14]. Generally, it takes around 60 and more than 120 min for cellular TRP and TIS testing, respectively. For 5G Assisted Global Navigation Satellite System (A-GNSS) TIS testing, this figure could be increased to more than 240 min. Moreover, during both TRP and TIS testing, DUTs are required to be powered by their own batteries and transmit at maximum output power. For smartphones and notebooks with relatively larger batteries, this testing method and the associated amount of test time seem acceptable. However, for smart watches with relatively low-capacity batteries due to limited space and working volume, it is impossible to complete the entire testing without recharging and repositioning due to battery depletion, which would lengthen the test time that is already considered too long to an unacceptable level. On the other hand, smart watches are designed to transmit at full power for only a short duration within each hour, rather than for long operation, and as a result, they typically have a very small heatsink for the power amplifier to dissipate the heat. As a practical matter, attempts to perform OTA testing per current CTIA and 3 GPP specifications for mobile phones will, in many cases, cause the DUTs to thermally shut down one or more times during a TIS test, and the DUTs may even be damaged due to excessive heat. Figure 1 presents an example of the influence of excessive heat associated with the cellular transmitter during a TIS test. The hot spot can be noticed in Figure 1 during the TIS testing on the screen while the watch deformed apparently after the TIS testing. As a result, the test-time reduction for OTA testing is of great significance for 5G smart watches. Unfortunately, no relevant research has been performed or published yet. The objective of this work is to optimize measurement grid configurations for OTA testing of 5G smart watches based on the measurement uncertainty (MU) analysis for grid point reduction. In this paper, passive patterns from a typical 5G commercial smart watch are measured at different bands using a very fine grid resolution as the reference patterns, representing general radiation properties of 5G commercial smart watches. Subsequently, a variety of coarse grid configurations are compared through simulations quantitatively to determine the extent to which they can accurately reconstruct those reference patterns in order to evaluate the errors and MU associated with the grid point reduction. The relevant MU terms are then analyzed and determined quantitatively based on statistical analysis.