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Estimating the speed of aerial user equipment (UE) is critically important to provide reliable mobility management for cellular-connected unmanned aerial vehicles (UAVs) since this can enhance the quality of service. The 3GPP standard uses the number of handovers made by a UE during a predefined time period to estimate the speed and the mobility state efficiently. In this article, we introduce an approximation to the probability mass function of handover count (HOC) as a function of a cellular-connected UAV's height and velocity, HOC measurement time window, and different ground base station (GBS) densities. Afterward, we derive the Cramer-Rao lower bound (CRLB) for the speed estimate of a UAV and also provide a simple biased estimator for its speed based on the GBS density and HOC measurement period. We show that for a low handover parameter, the biased estimator turns into a minimum variance unbiased estimator (MVUE). Using this estimator, we study the problem of detecting the mobility state of a UAV as low, medium, or high mobility as per the 3GPP specifications. Using our proposed MVUE, we also characterize the accuracy improvement in speed estimation and mobility state detection as the GBS density and the HOC measurement window increase.

This paper presents a balanced antipodal Vivaldi antenna design to produce ultra-broadband radiations from microwave to millimeter-wave frequencies. In particular, this work relaxes the design constraints to optimize the geometrical parameters to broaden the operational frequency bandwidth, where 147% relative bandwidth has been achieved. Moreover, in this optimized design, the beam squint and cross-polarization (X-pol) discrepancy at high frequencies are improved by pairing two mirror-imaged antenna structures to form an integrated one. The resulted X-pol level can retain good performance in the entire band. The beam squint can be minimized by taking advantage of the 3dB gain increase provided by the array factor. HFSS full-wave simulations first examine the proposed work, further validated by measurement from a basic antenna prototype. The beam squint is less than 3 degrees in the major middle part of the achieved band, and the X-pol level is less than −30dB for the paired antennas.

This paper presents a design methodology of three-dimensional (3-D) passive metallic waveguide-array lens antenna (MWALA) by implementing the Rotman lens concept (RLC) for collinear multi-beam radiations on the same angular plane. The RLC methodology extends the original concept of two-dimensional (2-D) tri-focal Rotman lens into a 3-D one in free space. Thus, the MWALA has a planar aperture facing to the radiation boresight to resemble a planar phased array of waveguide antennas and also has a curved focal surface profile (CFSP) on the opposite side to receive the illumination by the radiations of feed antennas. The three co-planar focal points are specified to define a circular focal arc (CFA) for multiple feed placement to radiate multi-beams through this MWALA. The boundary formed by the CFSP and CFA resembles the shape of Rotman lens equivalently realized in free space. In addition, the shortage of freedoms in conventional MWALA design limited to dual-focus configurations and resulting in narrow scan angles, is relaxed by adopting the phase velocities inside the waveguides as an additional freedom to realize a tri-focal configuration and broaden the scan angles of multi-beam radiation. Theoretical foundation and design guidelines are summarized with numerical and experimental examples presented to validate the feasibility.

This study aims to investigate the impact of the presence of buildings on the radiation characteristics of MF broadcast antennas. Two different antennas are considered: a monopole operating at 1494 kHz and a two-element linear array radiating at 1008 kHz. The buildings were modeled as wire-grids and the total electric field intensity was calculated as the sum of the scattered field by the wire-grid and the field radiated from the antenna in free space. The radiation pattern of the antennas, when one or two buildings were situated in their vicinity, were the end result of the analysis, and they were compared to the corresponding patterns in free space. The results demonstrate that the radiation characteristics of antennas are mostly affected by the heights of buildings. If these heights are less than a critical value, the buildings do not significantly obstruct the operation of the antenna, despite the value of other parameters, such as the length and the width of the buildings, as well as their distance from the antenna.