Cell-free massive MIMO (CF-mMIMO), where each user equipment (UE) is connected to multiple access points (APs), is emerging as an important component for fifth-generation (5G) and sixth-generation (6G) cellular systems. Accurate channel models based on measurements are required to optimize the design and deployment of such systems. This paper presents an extensive measurement campaign for CF-mMIMO in an urban environment. A new “virtual AP” technique measures channels between 80 UE locations and more than 20, 000 possible micro-cellular AP locations. Measurements are done at 3.5 GHz carrier frequency with 350 MHz bandwidth (BW). The paper describes the measurement setup and data processing, shows sample results and their physical interpretation, and provides statistics for key quantities such as pathloss, shadowing, delay spread (DS), and delay window. We find pathloss coefficients of 2.9 and 10.4 for line-of-sight (LOS) and non line-of-sight (NLOS), respectively, where the high LOS coefficient is mainly because larger distance leads to more grazing angle of incidence and thus lower antenna gain in our setup. Shadowing standard deviations are 5.1/16.6 dB, and root mean squared (RMS) DSs of -80.6/-72.6 dBs, where dBs is defined as$t_{\mathrm { dBs}}=10\log _{10}(t_{\mathrm { sec}})$. The measurements can also be used for parameterizing a certain type of channel model, namely Cell-free massive MIMO for Urban Non-stationary Environment with Correlations (CUNEC), which will be reported in future work.

The design of cell-free massive MIMO (CF-mMIMO) systems requires accurate, measurement-based channel models. This paper provides the first results from the by far most extensive outdoor measurement campaign for CF-mMIMO channels in an urban environment. We measured impulse responses between over 20, 000 potential access point (AP) locations and 80 user equipments (sUEs) at 3.5 GHz with 350 MHz bandwidth (BW). Measurements use a “virtual array” approach at the AP and a hybrid switched/virtual approach at the UE. This paper describes the sounder design, measurement environment, data processing, and sample results, particularly the evolution of the power-delay profiles (sPDPs) as a function of the AP locations, and its relation to the propagation environment.

The THz band (0.1-10 THz) has attracted considerable attention for next-generation wireless communications due to the large amount of available bandwidth that may be key to meet the rapidly increasing data rate requirements. Before deploying a system in this band, a detailed wireless channel analysis is required as the basis for proper design and testing of system implementations. One of the most important deployment scenarios of this band is the outdoor microcellular environment, where the Transmitter (Tx) and the Receiver (Rx) have a significant height difference (typically ≥ 10 m). In this paper, we present double-directional (i.e., directionally resolved at both link ends) channel measurements in such a microcellular scenario encompassing street canyons and an open square. Measurements are done for a 1 GHz bandwidth between 145-146 GHz and an antenna beamwidth of 13 degree; distances between Tx and Rx are up to 85 m and the Tx is at a height of 11.5 m from the ground. The measurements are analyzed to estimate path loss, shadowing, delay spread, angular spread, and multipath component (MPC) power distribution. These results allow the development of more realistic and detailed THz system performance assessment.

Condensed channel parameters, such as Root Mean Square (RMS) delay spread and angular spread, are essential ways of representing results from the measurement of wireless propagation channels. However, real-world measurements are invariably affected by noise, which is not inherently reduced by the Fourier processing - in contrast to high-resolution parameter estimation. Even when the signal-to-noise ratio is relatively high, e.g., 20 dB, the impact of the noise on the condensed parameters can be significant and lead to erroneous estimates. This can be explained, e.g., for the computation of the RMS delay spread, where long delays (which often carry noise only) are "up-weighted" by the square of the delay. While techniques like noise thresholding are often applied, the reasons for choosing thresholds are usually not described in detail. This paper systematically analyzes noise thresholding for joint delay-angle measurements and derives equations for the impact of thresholds on the computation of delay and angle RMS spreads and window parameters under various measurement circumstances. The paper provides results based on closed-form equations, as well as MonteCarlo (MC) simulations, and application to real-world measurements in the sub-THz band.

The availability of large bandwidths in the terahertz (THz) band will be a crucial enabler of high data rate applications in next-generation wireless communication systems. The urban microcellular scenario is an essential deployment scenario where the base station (BS) is significantly higher than the user equipment (UE). Under practical operating conditions, moving objects (i.e., blockers) can intermittently obstruct various parts of the BS- UE link. Therefore, in the current paper, we analyze the effect of such blockers. We assume a blockage of the strongest beam pair and investigate the availability and extent of angular diversity, i.e., alternative beampairs that can sustain communication when the strongest is blocked. The analysis uses double-directional channel measurements in urban microcellular scenarios for 145– 146 GHz with BS-UE distances between 18 to 83 m. We relate the communication-system quantities of beam diversity and capacity to the wireless propagation conditions. We show that the SNR loss due to blockage depends on the blocked angular range and the specific location, and we find mean blockage loss to be on the order of 10–20 dB in line-of-sight (LOS) and 5–12 dB in NLOS (non-LOS). This analysis can contribute to the design of intelligent algorithms or devices (e.g., beamforming, intelligent reflective surfaces) to overcome the impact of the blockage.

Device-to-Device (D2D) communication systems have recently received great interest especially for use by Public Safety Organizations (PSOs); in these scenarios, communications involving infrastructure nodes (base stations) might not always be possible, or sufficiently reliable. LTE Sidelink, the 3GPP standard for D2D communication at sub-GHz frequencies, has been mandated by many PSOs for such situations. Of particular interest are Indoor-to-Outdoor (I2O) communications, where an emergency responder inside a building communicates with a command post at street level. In this paper, we present simulation results for LTE Sidelink performance over measured I2O channels, and provide comparison between these measurement results and the 3GPP channel models. We also provide an "applique" on top of the standard (i.e., requiring no changes in the standard) that uses multiple-antenna combining to provide SNR gains compared to the standard SISO D2D Sidelink channel. We observe on average around 10 dB SNR gain for applying Maximum Ratio Combining (MRC) compared to a best-case SISO channel.

THz band is envisioned to be used in 6G systems to meet the ever-increasing demand for data rate. However, before an eventual system design and deployment can proceed, detailed channel sounding measurements are required to understand key channel characteristics. In this paper, we present a first extensive set of channel measurements for urban outdoor environments that are ultra-wideband, and double-directional where both transmitter and receiver are at the same height. In all, we present measurements at 38 locations, consisting of nearly 50,000 impulse responses, representing both line-of-sight (LoS) and non-line-of-sight (NLoS) cases in the 1-100 m range between 145-146 GHz. We provide modeling for path loss, shadowing, delay spread, angular spread, multipath component (MPC) power distribution and Qtapnumber. We find, among other things, that outdoor communication over tens of meters is feasible in this frequency range even in NLoS scenarios, that omni-directional delay spreads of up to 100 ns, and directional delay spreads of up to 10 ns are observed, while angular spreads are also quite significant, and a surprisingly large number of MPCs are observed for 1 GHz bandwidth and 13° beamwidth. These results constitute an important first step towards better understanding the wireless channel in the THz band.

THz communication has attracted a great deal of attention due to the large bandwidth available in this band. However, system development and deployment requires detailed knowledge of the wireless channel, which must be provided by measurements (channel sounding) and subsequent modeling in key scenarios of interest. One important scenario in this regard is the urban outdoor microcellular scenario where the Transmitter (Tx) and the Receiver (Rx) have a significant height difference. In this paper, we present ultra-wideband double-directional channel measurements in a microcellular scenario for the band between 145-146 GHz in a street canyon with Tx-Rx of distances up to 67 m. The results are analyzed in the delay and angular domain, and the impact of the street canyon on the power delay profiles and angular power spectra is detailed. These measurements thus contribute towards the creation of a more realistic and detailed THz channel model in these scenarios.

Measurements of the propagation channels in real-world environments form the basis of all realistic system performance evaluations, as foundation of statistical channel models or to verify ray tracing. This is also true for the analysis of cell-free massive multi-input multi-output (CF-mMIMO) systems. However, such experimental data are difficult to obtain, due to the complexity and expense of deploying tens or hundreds of channel sounder nodes across the wide area a CF-mMIMO system is expected to cover, especially when different configurations and number of antennas are to be explored. In this paper, we provide a novel method to obtain channel data for CF-mMIMO systems using a channel sounder based on a drone, also known as a small unmanned aerial vehicle (UAV). Such a method is efficient, flexible, simple, and low-cost, capturing channel data from thousands of different access point (AP) locations within minutes. In addition, we provide sample 3.5 GHz measurement results analyzing deployment strategies for APs and make the data open source, so they may be used for various other studies. To our knowledge, our data are the first large-scale, real-world CF-mMIMO channel data.

Wireless communications in the THz frequency range can allow data rates of hundreds of Gbit/s, and will thus be an important part of 6G. A first important step for system design is an understanding of the underlying propagation channel. In this paper, we present results from one of the first measurement campaigns for medium-distance (up to 35 m) outdoor channels in urban environments where both the directions and delays of multipath are measured with good resolution. The results show a surprisingly rich multipath environment, leading to significant dispersion in both delay and angular domains. We also find that metallic-covered surfaces lead to a considerable enhancement of multipath, which indicates an important impact of building materials. Overall, the results indicate that relying on pronounced sparsity for THz system design might not always be valid in this type of environment.

Estimating downlink (DL) channel state information (CSI) in frequency division duplex (FDD) massive multi-input multi-output (MIMO) systems generally requires downlink pilots and feedback overheads. Accordingly, this paper investigates the feasibility of zero-feedback FDD massive MIMO systems based on channel extrapolation. We use the high-resolution parameter estimation (HRPE), specifically the space-alternating generalized expectation-maximization (SAGE) algorithm, to extrapolate the DL CSI based on the extracted parameters of multipath components in the uplink channel. We apply the HRPE to two different channel models: the vector spatial signature (VSS) model and the direction of arrival (DOA) model. We verify these methods through real-world channel data acquired from channel measurement campaigns with two different types of channel sounders: a) a switched array-based, real-time, time-domain, outdoors setup at 3.5 GHz, and b) a virtual array-based, high-accuracy, frequency-domain, indoors setup at 2.4 and 5-7 GHz. The performance metrics of the extrapolated channels that we evaluate include the mean squared error, beamforming efficiency, and spectral efficiency in multiuser MIMO scenarios. The results show that the HRPE-based channel extrapolation performs best under the simple VSS model, which does not require array calibration, and if the BS is in an open outdoor environment having line-of-sight (LOS) paths to well-separated users.

Delay dispersion is one of the key propagation channel characteristics that impact system design and performance. In particular, for OFDM-based systems such as Wi-Fi, it determines both the amount of available frequency diversity and the minimum required cyclic prefix, which in turn impacts the spectral efficiency. For this reason, delay spread and power delay profiles have been analyzed for a long time. However, recent upgrades in Wi-Fi, in particular the addition of a new frequency range (6-7.1 GHz), and the introduction of adaptive beamforming, require a re-assessment based on new measurements. This paper presents extensive measurement results of RMS delay spread and interference quotient that take these developments into account. Results were measured in the 2.4-2.5, 5-6, and 6-7 GHz bands. Furthermore, we compare the “omni-directional” delay spread and interference quotient (i.e., when measured with omni-antennas at transmitter and receiver), to those that occur when beamformed antennas are used. We found that for both outdoor and indoor environments, beamforming typically improves the interference quotient (for a given window size) by about 3-5 dB. The 2.4 and 5-7 GHz bands show a significant difference, while we could not observe statistically significant differences between the 5-6 and 6-7 GHz bands.

This paper investigates the capability of millimeter-wave (mmWave) channel sounders with phased arrays to perform super-resolution parameter estimation, i.e., determine the parameters of multipath components (MPC), such as direction of arrival and delay, with resolution better than the Fourier resolution of the setup. We analyze the question both generally, and with respect to a particular novel multi-beam mmWave channel sounder that is capable of performing multiple-input-multiple-output (MIMO) measurements in dynamic environments. We firstly propose a novel two-step calibration procedure that provides higher-accuracy calibration data that are required for Rimax or SAGE. Secondly, we investigate the impact of center misalignment and residual phase noise on the performance of the parameter estimator. Finally we experimentally verify the calibration results and demonstrate the capability of our sounder to perform super-resolution parameter estimation.

Application of massive multiple-input multipleoutput (MIMO) systems to frequency division duplex (FDD) is challenging mainly due to the considerable overhead required for downlink training and feedback. Channel extrapolation, i.e., estimating the channel response at the downlink frequency band based on measurements in the disjoint uplink band, is a promising solution to overcome this bottleneck. This paper presents measurement campaigns obtained by using a wideband (350 MHz) channel sounder at 3.5 GHz composed of a calibrated 64 element antenna array, in both an anechoic chamber and outdoor environment. The Space Alternating Generalized Expectation- Maximization (SAGE) algorithm was used to extract the parameters (amplitude, delay, and angular information) of the multipath components from the attained channel data within the â€training†(uplink) band. The channel in the downlink band is then reconstructed based on these path parameters. The performance of the extrapolated channel is evaluated in terms of mean squared error (MSE) and reduction of beamforming gain (RBG) in comparison to the â€ground truthâ€, i.e., the measured channel at the downlink frequency. We find strong sensitivity to calibration errors and model mismatch, and also find that performance depends on propagation conditions: LOS performs significantly better than NLOS.