The fifth-generation (5G) cellular mobile communications look promising with features that can help improving consumer experience and satisfaction. To be able to provide these features, more spectrum is required according to the Shannon–Hartley theorem. Spectrum is, however, a finite and scarce resource, and it can be allocated to a new service only when the spectral coexistence with other incumbents is ensured. New waveforms for 5G that differ from the conventional Orthogonal Frequency Division Multiplexing (OFDM) are required in order to have superior performance in terms of out-of-band emissions and to be able to utilize the fragmented spectrum in different bands. We developed analytical models for evaluating the out-of-band emissions of the conventional Cyclic Prefix (CP)-OFDM as well as its alternatives: windowed OFDM and filtered OFDM, using their signal spectral modeling. The resulting expressions for the Power Spectral Density (PSD) and the Frequency Dependent Rejection (FDR) involve simple closed-form expressions or easily computable integrals. We applied the expressions to the advanced minimum coupling loss model for assessing the feasibility of the spectral coexistence between potential 5G systems (with linearized or nonlinear power amplifier) and incumbent radar systems. The numerical simulation results indicated that both the windowed OFDM and filtered OFDM guarantee the coexistence at the low expense of the spectrum utilization and their coexistence performance can be reduced and reversed with nonlinearity distortion of the power amplifier.

In this manuscript, we propose a circularly polarized (CP) antenna design that can achieve broadband axial ratio (AR) bandwidth in the X-band. A sequentially rotated serial feeding network is applied to achieve the broadband requirement. The proposed antenna consists of two substrates, one is for feeding elements and the other is for radiating elements. The 16 radiating elements located on an upper substrate are fed through cross-shaped slots located in a ground plane between two substrates. The feed network is based on a 1:4 power divider design and consists of a serial connection of the power dividers resulting in the proper power splitting to the 16 radiating elements. The proposed antenna is designed to exhibit an optimal radiation characteristic of around 8.2 GHz with a CP antenna gain greater than 16 dB and similarly wide impedance and AR bandwidths. Experiments with the fabricated CP antenna were performed to support the proposed CP antenna design theory and are in good agreement with the simulation results.

This study presents a new S-band-receiving phased-array antenna with a phase-deviation-minimized calibration method for the ground station of a low Earth orbit (LEO) satellite. The proposed antenna consists of 16 subarrays, 16 beamforming receiving RF modules (BF-RFMs), a power/control board, and a 16-way feed network. The subarray was achieved by joining two 8 × 1 arrays with a two-way power combiner. The 16-element antenna subarrays showed a gain of 16.1 dBi and a reflection coefficient of less than −10 dB from 2.12 GHz to 2.45 GHz. The BF-RFM, which consists of three low-noise amplifiers (LNAs), a power combiner, a phase shifter, and a digital attenuator, was designed and fabricated. The BF-RFMs were provided by the power/control board and showed a gain of 30.8 ± 0.8 dB, an amplitude root-mean-square (RMS) error from 0.25 dB to 0.28 dB, and a phase RMS error from 1.8° to 2.5° over the Rx frequency range. The arrangement procedures of the 16 BF-RFMs are presented to increase beam pointing accuracy at the desired angle. A commercial 16-way feed network was employed to combine all the output ports of the 16 BF-RFMs. The assembled antenna, which has dimensions of 1.58 m × 1.58 m × 0.2 m, was measured by partial and full scans in the near-field scanning system. The back-projected algorithm was employed to calibrate the antenna’s gain patterns in the partial scan. The implemented phased-array antenna had a gain greater than 28.14 dBi, sidelobe levels less than −17.1 dB, and beam pointing errors less than 0.07° over the beam pointing angle of −20~+20°. Based on the implemented antenna system, we conducted a field test using KOMPSAT-5, which is actually operating in South Korea, in order to verify the performance of the low Earth orbit (LEO) satellite ground station system.