Next-generation wireless communication systems will incorporate millimeter wave (mmWave) as one of the key technologies to increase data rates. These forthcoming mmWave systems will integrate multiple-input multiple-output (MIMO) technology to ensure sufficient link margin and are expected to be deployed in conjunction with sub-6 GHz systems. Configuring a MIMO communication link usually relies on estimating channel state information (CSI), which is difficult to acquire at mmWave frequencies due to the low pre-beamforming signal-to-noise ratio (SNR). In this paper, we propose a novel approach to estimate mmWave MIMO channels by leveraging out-of-band information from a sub-6GHz band. Utilizing this approach, we develop three channel estimation methods and compare these methods with one using only in-band information. We investigate the influence of theK-factor, the SNR and the number of antennas on the performance of the proposed channel estimation methods through simulations. Additionally, we compare these methods in terms of their computational complexity. Finally, we validate the performance of the proposed methods through channel measurements conducted in an outdoor environment. The results demonstrate that the proposed methods outperform in-band mmWave channel estimation in terms of spectral efficiency, particularly in scenarios of low SNR and high K-factor.

In this paper, we consider an access point clustering-based pilot assignment scheme for user-centric cell-free massive MIMO systems. In such a system, the complexity is reduced by performing the pilot assignment for each cluster independently. However, users at the edge of the cluster will experience high interference from users in neighboring clusters. In this paper, we propose two schemes to reduce pilot contamination for cluster-edge users. The proposed schemes can perform per-group pilot assignment with zero or limited inter-cluster cooperation. Numerical results verify that, compared with conventional pilot assignment schemes, the proposed pilot assignment schemes achieve similar performance while having lower complexity and allowing for decentralized processing.

One of the key research directions to increase the capacity of new radio (NR) vehicle-to-everything (V2X) communication systems is extension of employed frequency bands from sub-6 GHz to millimeter wave (mmWave) range. To investigate different propagation effects between sub-6 GHz and mmWave bands in high-mobility scenarios, one needs to conduct channel measurements in both frequency bands. Using a suitable testbed setup to compare these two bands in a fair manner, we perform channel measurements at center frequencies of 2.55 GHz and 25.5 GHz, velocities of 50 km/h and 100 km/h, and at 126 different spatial positions. Furthermore, we conduct a comparative study of the multi-band propagation based on measurement results. We estimate the power delay profile (PDP) and the Doppler power spectral density (DSD) from a large set of measurements collected in a measurement campaign. Finally, we compare measured wireless channels at the two employed frequency bands in terms of root-mean-square (RMS) delay spread and RMS Doppler spread.

We consider a user-centric cell-free massive MIMO system in which many distributed access points are connected to a central processing unit to serve multiple users. In practice, linear precoding schemes can achieve high spectral efficiency in massive MIMO systems. However, for cell-free massive MIMO, these methods suffer from a large training overhead and high computational complexity. We reduce the complexity of the zero-forcing precoder and improve the performance of the maximum ratio transmission precoder by dividing the users into groups and assigning different frequency resources to each group. Further, we reduce the training overhead by assigning highly interfering users to different groups. We propose a graph coloringbased user grouping scheme with an adjustable group size. Numerical results show that the proposed scheme can achieve about 96% spectral efficiency of zero-forcing while requiring only 32% processing complexity of zero-forcing. Further, the proposed scheme improves the performance of maximum ratio transmission by 24%. Finally, we investigate the impact of the coherence block length.

The deployment of large antenna arrays at the base station is a prerequisite for operating at the millimeter wave bands. The large antenna arrays are required to achieve high beamforming gains and thus to guarantee sufficient received signal power. In massive multiple input multiple output systems hybrid precoding is utilized to reduce power consumption and cost. Conventional precoding schemes require full channel knowledge at the transmitter, which is hard to obtain, especially in frequency division duplex systems. In this paper, we propose a novel hybrid precoding scheme, which requires only the information acquired during the initial access phase. Furthermore, we derive an analytical expression for characterizing the beam detection capabilities of the system.