Massive MIMO is envisioned as a promising technology in 5G and beyond 5G communication. Channel models are of great importance for the development and performance assessment of massive MIMO systems. Since massive MIMO systems are equipped with large-aperture antenna arrays, antenna elements at different spatial positions would observe different channel multipath characteristics, which is so-called spatial non-stationarity (SnS). The SnS property of multipaths has been observed in many reported massive MIMO channel measurements. However, characterization and explanations of SnS have not been adequate in existing statistical channel modeling, and deterministic models (e.g., Ray-tracing) are difficult to implement due to the high complexity. This paper proposes a realistic yet low-complexity SnS channel modeling framework for massive MIMO systems and its validation based on both channel measurements and Ray-tracing simulations. In this work, we firstly perform an 6 GHz-bandwidth millimeter-wave (mmWave) indoor channel measurement campaign with a 0.5 m-radius virtual uniform circular array (UCA), where the SnS phenomena are clearly observed. Then, we propose the massive MIMO channel modeling framework that captures the observed SnS property from physical propagation mechanisms of dominant multipaths in mmWave channels, i.e., blockage, reflection, and diffraction. Compared to traditional stationary channel modeling, only one extra parameter accounting for SnS has been added in the proposed framework, which is desirable for its low-complexity implementation. Finally, the proposed framework is validated with site-specific Ray-tracing simulations. The SnS phenomena observed in the measurements are reproduced well in the modeling results according to the proposed framework, and high similarities between the target channels and modeling results are achieved. The proposed framework is valuable for the development of massive MIMO systems, since it is realistic, low-complexity, and accurate.

Unmanned aerial vehicles (UAVs) have been widely used in both military and civilian applications, where a stable communication link is vital for safe flight control and robust data transmission. To develop a reliable UAV communication system, it is necessary to deeply understand the UAV channel characteristics and establish accurate channel models. Channel sounding is the most effective way to obtain realistic channel characteristics and validate the theoretical channel model. However, the studies on UAV channel sounding are still insufficient in terms of system design and data processing due to the complexity of developing a UAV channel sounder. Different from the terrestrial channel sounders, the implementation of a UAV channel sounder is tortured by the limited battery life and payload capacity of the UAV platform. The sounding scheme and data post-processing also need to be specially designed for highly dynamic UAV channels. So far, most existing survey studies on UAV channels focus on modeling methodology and model presentation. To fill this gap, this paper provides a comprehensive survey on the design of the UAV channel sounder, in terms of the hardware scheme, sounding signal, time synchronization, calibration, and data post-processing. Current issues and potential research topics behind existing sounding technologies and measurement campaigns are analyzed. Moreover, future challenges and open issues are also discussed.

With the rapid development and broad applications of unmanned aerial vehicle (UAV) based wireless stations in the sky, fundamental understanding and characterization of the realistic air-to-ground (A2G) communication link properties are crucial. In this paper, a UAV-aided channel sounder with a real-time processing hardware system is developed for highly dynamic and non-stationary A2G channel measurements. In the hardware system, a global positioning system (GPS) -based triggering signal is designed, the equivalent antenna pattern affected by UAV airframe is considered, and an appropriate sounding signal is selected, in order to improve the accuracy of measured channel impulse response (CIR). Moreover, real-time hardware processing algorithms for raw channel data, i.e., CIR extraction, system response elimination (SRE), power loss recovery (PLR), and adaptive multi-path component (MPC) recognition are developed and implemented on a single field-programmable gate array (FPGA) chip. In this way, the required storage size of channel data and the processing time for one slice of CIR is greatly decreased, which can meet the requirement of non-stationary A2G channel measurement with a high sampling rate and long-time measurement. A commercial channel emulator is used to reproduce controllable channels and verify the performance of developed channel sounder. Finally, the developed channel sounder is applied to carry out A2G measurement campaigns at 3.5 GHz in a campus scenario. The channel characteristics, i.e., path loss (PL), K-factor, and path angle are analyzed. The measured channel characteristics are consistent with existing measurements under a similar scenario. The estimated path angles are also validated by the theoretical results. Thus, the channel sounder can be used to capture the non-stationary A2G channel characteristics for the system design and algorithm optimization of A2G communications.

Massive multiple-input, multiple-output (MIMO) is seen as an enabling technology to fulfill dramatic improvements in spectral efficiency for fifth-generation (5G) deployment in 2020. For massive MIMO systems, the learning loop from early-stage prototype design to final-stage performance validation is expected to be slow and ineffective. There is a strong need to evaluate massive MIMO base station (BS) performance with over-the-air (OTA) methods. Until now, such OTA solutions have not been discussed for massive MIMO BS systems.

Accurate characterization of spatial multipath channels at millimeter wave bands has gained significant interest both in industry and academia. A channel measurement was conducted at three different frequency bands, i.e., 2−4, 14−16, and 28−30 GHz in a line-of-sight (LOS) and an obstructed-LOS (O-LOS) scenarios in an empty room environment. A vector network analyzer connected to a virtual uniform circular array and to a rotational directional horn antenna was used in the measurements, respectively. Angle-of-arrivals and delay-of-arrivals of the multipath components were obtained from the measurements for the three frequency bands. Room electromagnetic properties for the three different frequencies at different propagation scenarios were investigated as well.

This paper discusses over-the-air (OTA) testing for multiple-input multiple-output (MIMO) capable terminals with emphasis on channel spatial characteristics emulation. A novel technique to obtain optimum power weights for the OTA probes based on convex optimization is proposed. The proposed technique emulates spatial correlation as well as introduces constraints on the maximum deviation between the target power azimuth spectrum (PAS) and the emulated PAS in terms of mean angle of arrival (AoA) and azimuth spread (AS). Simulation results show that the proposed emulation technique presents better performance compared with existing techniques in the literature. This improvement is further demonstrated by measurement results in a practical MIMO OTA setup.