Enabling UAV cellular with millimeter-wave communication: potentials and approaches

Autonomous vehicles require vehicle-to-vehicle (V2V) and vehicle-to-infrastructure (V2I) communication. Fifth-generation (5G) and beyond wireless networks enable such infrastructure through beamforming techniques at millimetre-wave (mm-wave) frequencies. This paper characterises the impact of antenna beamwidth on the Doppler shift, cross-polarisation ratio (XPR) and the channel attenuation for the 26GHz mm-wave V2I line-of-sight (LoS) channel. It is proven that the received Doppler power spectrum is proportional to the probability density function (PDF) of the Doppler Shift. This indicates that Doppler power reduces with the Doppler shift. It has also been established that a wider antenna half power beamwidth (HPBW) of 20° instead of 17° reduces the XPR by 20dB whilst increasing the channel attenuation by 10dB in a mm-wave LoS channel with dynamic blockage. Therefore, a trade-off between the XPR and channel attenuation can be introduced by employing different antenna HPBWs.

In this paper, we study hierarchical codebook design for channel estimation in millimeter-wave (mmWave) communications with a hybrid precoding structure. Due to the limited saturation power of the mmWave power amplifier, we consider the per-antenna power constraint (PAPC). We first propose a metric, termed generalized detection probability (GDP), to evaluate the quality of an arbitrary codeword. This metric not only enables an optimization approach for mmWave codebook design, but also can be used to compare the performance of two different codewords/codebooks. To the best of our knowledge, GDP is the first such metric, particularly for mmWave codebook design. We then propose a heuristic approach to design a hierarchical codebook exploiting beam widening with the multi-RF-chain sub-array (BMW-MS) technique. To obtain crucial parameters of BMW-MS, we provide two solutions, namely, a low-complexity search (LCS) solution to optimize the GDP metric and a closed-form (CF) solution to pursue a flat beam pattern. Performance comparisons show that BMW-MS/LCS and BMW-MS/CF achieve very close performances, and they outperform the existing alternatives under the PAPC.

Due to its potential to support high data rates at low latency with reasonable interference isolation because of signal blockage at these frequencies, millimeter-wave (mmWave) communications has emerged as a promising solution for next-generation wireless networks. MmWave systems are characterized by the use of highly directional antennas and susceptibility to signal blockage by buildings and other obstructions, which significantly alter the propagation environment. The received power of each transmission depends on the direction the corresponding antennas point and whether the signal’s path is line-of-sight (LOS), non-LOS (i.e., partially blocked), or completely blocked. A key challenge in modeling blocking in mmWave networks is that, in actual networks, the blocking might be correlated. Such correlation arises, for example, when single transmitter tries to broadcast to pair of receivers that are close to each other, or more generally when they have a similar angle to the transmitter. In this situation, if the first receiver is blocked, it is likely that the second one is blocked, too. This dissertation explores four related but distinct issues associated with mmWave networks: 1) Analytical modeling of networks consisting of user devices and blockages with fixed or random, but independent, locations, 2) The careful characterization of correlated blocking and analysis of its impact on the performance of mmWave networks, 3) The proposed use of macrodiversity as an important strategy to mitigating correlated blocking in mmWave networks and the corresponding analysis, and 4) The proposed use of networks of unmanned aerial vehicles (UAVs) to provide connectivity in urban deployments. This work provides insight into the performance of variety of applications of mmWave communications, ranging from wireless personal area networks (WPAN), device-to-device networks, traditional terrestrial, cellular networks, and the UAV-based networks where the UAVs act as the cellular base stations. A common thread throughout this dissertation is the development of new tools based on stochastic geometry and their application to modeling and analysis. The analysis presented in this dissertation is general enough to find application beyond mmWave networks, for instance the results may also be applicable to systems that use free-space optical (FSO) signaling technologies.

We derive non-random and Bayesian Cramer-Rao lower bound (CRLB) for pilot-aided joint estimation of angle-of-arrival (AoA), angle-of-departure (AoD), and small scale channel gain in millimeter-wave (mmW) communication networks, in which transmitter and receiver are equipped with uniform planar antenna (UPA) arrays. Numerical results reveal that Bayesian CRLB decreases with an increase in Rice factor. Furthermore, using the derived Bayesian CRLB, we also observe that uniform design of beamforming and beamcombining codebooks, in which the angles are uniformly quantized, yield better performance than other types of codebook design.

Channel estimation and tracking with nested sampling for fast-moving users in millimeter-wave communication

Millimeter wave (MMW) communication is a promising technology to cope up with the ever increasing demand for multi gigabit wireless applications and to alleviate the problems of spectrum scarcity for Fifth generation (5G) mobile communication applications. The coverage distance of the MMW communication system is severely affected due to the various losses such as free space loss, atmospheric loss, and foliage loss. However MMW communication allows the use of smaller antenna size which can facilitate the fabrication of large antenna arrays over small area like a postage stamp which can be used at both transmitter and receiver. This helps in efficient use of the Multi-Input Multi-Output (MIMO) technology in MMW Communication systems which can improve the coverage distance as well as capacity of the system. In this paper, we perform the Mathematical analysis of the coverage distance of MMW communication link and study the effects of different parameters on its performance. A simulation study to examine the effect of the MIMO antenna system for improving the coverage distance of MMW communication links for specific data rates for Line of Sight (LOS) and Non-Line of Sight (NLOS) cases in a highly dense mobile network scenario has been done and results are presented in this paper.

In this paper, we study hierarchical codebook design for channel estimation in millimeter-wave (mmWave) communications with a hybrid precoding structure. We propose an approach to design a hierarchical codebook exploiting BeaM Widening with Multi-RF-chain Sub-array technique (BMW-MS). To obtain crucial parameters of BMW-MS, we provide a closed-form (CF) solution to pursue a flat beam pattern. Performance comparisons show that BMW-MS/CF outperforms the existing alternatives under the per-antenna power constraint, which is adopted due to the limited saturation power of mmWave power amplifier.

Precise localization and tracking in intelligent transportation systems has aroused great interest since it is required in a large variety of applications. The positioning accuracy of global navigation satellite systems is unreliable and insufficient enough for many use cases. In urban canyons or tunnels, the positioning performance degrades due to a low received signal power, multipath propagation, or signal blocking. Instead we exploit the ubiquitous access to cellular mobile radio networks. Cellular networks are designed to cover the access to the network in an area by a single link to reduce the risk of interference from neighboring base stations. The idea of Channel-SLAM is to exploit the numerous multipath components (MPCs) of a radio signal arriving at the receiver for positioning. Each MPC can be regarded as being sent from a virtual transmitter in a pure line-of-sight condition. Within this paper, we show how to apply multipath assisted positioning in an urban scenario. Therefore, we analyze how a road user equipped with a circular antenna array is tracked in an urban scenario in the presence of only one physical transmitter. We further jointly estimate the positions of the physical and the virtual transmitters to enrich maps.

To use millimeter wave bands in future cellular and wireless networks, characterization of the radio propagation channel in outdoor urban small cell or pico cell environment is vital. In this paper we present results from analysis of wideband full polarimetric directional channel measurements in an urban pico-cell environment using the double directional millimeter wave channel sounding system we developed. Multipath clusters with excess gain ranging from −9.6 dB to −22 dB in both line of sight (LOS) and obstructed LOS cases are seen. The most significant clusters were due to first and second order reflection, and diffraction. The clusters ranged from being highly specular in nature with very small (nearly 0°) angular spread to having angular spreads of up to 5.8° in azimuth and 6.3° in elevation. Similarly the delay spread of the clusters ranged up to 2.1 ns.

Millimeter-wave (mmWave) communication has re-cently attracted substantial interest owing to its abundant bandwidth availability and low-latency capability that facilitates meeting the requirements of vehicle-to-everything (V2X) services. However, performance degradation due to signal blockage has been raised as one of the critical technical challenges. This paper investigates the signal blockage effect of a road bridge in mmWave vehicular communication by performing a ray-tracing simulation for highway scenarios. The main purpose of this study is to help the researchers understand the signal blockage effect and present considerations for cell planning of mm Wave vehicular communications in the presence of a road bridge.

In millimeter-wave communication, large antenna arrays are required to\nachieve high power gain by steering towards each other with narrow beams, which\nposes the problem to efficiently search the best beam direction in the angle\ndomain at both Tx and Rx sides. As the exhaustive search is time consuming,\nhierarchical search has been widely accepted to reduce the complexity, and its\nperformance is highly dependent on the codebook design. In this paper, we\npropose two basic criteria for the hierarchical codebook design, and devise an\nefficient hierarchical codebook by jointly exploiting sub-array and\ndeactivation (turning-off) antenna processing techniques, where closed-form\nexpressions are provided to generate the codebook. Performance evaluations are\nconducted under different system and channel models. Results show superiority\nof the proposed codebook over the existing alternatives.\n

Propagation speed and direction of nonlinear internal waves (NLIWs) are important parameters for understanding the generation and propagation of waves, and ultimately clarifying regional ocean circulation. However, these parameters cannot be directly measured from in-situ instruments, but can only be estimated from post-processing in situ data. Herein, we present two methods and an optimal approach to estimate the propagation speed and direction of waves using underway and moored observations. The Doppler shift method estimates these parameters from apparent observations concerning a moving ship using the Doppler shift induced by the changing relative distance of the NLIWs from the moving ship. The time lag method estimates the parameters using the distance between two locations of the NLIW observed at different times and the time lag. To optimize the speed and direction of NLIWs, the difference in the propagation direction independently estimated by the two methods needs to be minimized concerning the optimal propagation speed to yield the optimal propagation direction. The methods were applied to two cases observed in the northern East China Sea in May 2015 and August 2018. This study has practical significance for better estimating the propagation speed and direction of NILWs particularly over a broad continental shelf.

Measurements using the SRI Wide‐Aperture Research Facility (WARF) HF sky wave radar show that the radar's azimuthal beam width and integration time play important roles in determining the quality of sky wave (ionospherically propagated) sea echo Doppler spectra. Using as a quality index the equivalent width of a portion of the Doppler spectrum in the vicinity of the stronger Bragg line, we find a reduction of ionospheric multipath spectral contamination as beamwidth and integration time decrease. For nominal (3 dB) beam widths of 1/2°, 2°, and 4° and 256‐s averaging, the mean equivalent widths of 104 spectra were 0.090, 0.099, and 0. 105 Hz, respectively. Experiments using 51‐s integration time gave average widths of 0.061, 0.075, and 0.079 Hz for the same three beam widths. The additional contamination observed with the larger beamwidths and the longer averaging time is often sufficient to preclude the extraction of ocean wave height from the second‐order spectral structure. A geometrical optics, rough‐surface model of the ionospheric reflection process explains this beamwidth dependence by relating the number and width of multipath spectral lines to the size, shape, and number of ‘reflective glints’ in the ionospheric area illuminated by the radar. The lateral dimension of this area increases with antenna beamwidth. The model also predicts a greater dependence of contamination on beamwidth as the integration time is reduced. We use ionospheric measurements made with CW Doppler sounders to estimate the statistical parameters of the rough surface model and the amount of contamination the model predicts for finite beamwidth and integration time. Our main conclusion is that the best way to avoid spectral contamination caused by short‐term ionospheric motions is to involve the smallest possible spatial and temporal samples of the ionosphere in the radar measurement. Specifically, a successful sea state radar should have a 3‐dB beamwidth of less than 2° and preferably as small as 1/2° and should use coherent integration times shorter than about 100s.

Terahertz (THz) communication systems are promising solutions for meeting the rapidly growing data traffic demand driven by an increasing number of Internet of Things (IoT) devices. Despite the potential of THz communication systems, they have the disadvantage of extremely high beam training overhead. Hierarchical beam training has been proposed as a technique to reduce this overhead. To perform beam alignment successfully while reducing overhead, it is important to design a wide beam, i.e., a low-resolution beam identical to the ideal beam, which has the shape of a rectangular function. While several conventional wide beams, obtained by solving optimization problems, closely approximate the ideal beam, it remains unclear whether designing the beam to closely resemble the ideal beam, without achieving an identical match, is truly effective in reducing beam misalignment. Motivated by this, we explore the feasibility of designing a beam identical to the ideal beam pattern and evaluate whether closely resembling the ideal beam pattern is effective in mitigating beam misalignment. Furthermore, we propose a novel codebook design scheme based on second-order cone programming (SOCP) for THz ultra-massive multiple input multiple output (UM-MIMO) systems to reduce beam misalignment in hierarchical beam training. This scheme introduces two additional criteria to the conventional codebook design problem, specifically addressing the considerations of worst-case detection gain and coverage-edge gain, which are directly aimed at mitigating beam misalignment. Additionally, we present a low-complexity codebook design approach for real-time implementation. Numerical results demonstrate that the proposed codebook reduces beam misalignment effectively.

Due to the projected increased demand for high data rate mobile services, interest has grown in new cellular network architectures such as a tiered networks. This paper proposes a beam searching algorithm and a codebook design for multilevel beamforming in outdoor millimeter wave communication systems. In an outdoor millimeter wave scenario, the received signal suffers from severe path loss and various attenuation factors. Beamforming is an essential component to establish the wireless link in 60GHz up to 80GHz. A millimeter wave beamforming method based on joint transmitter-receiver searching of an adaptive codebook is described. To reduce search complexity, an adaptive beam width beamforming technique is developed based on a sub-array method using squinting angles. A multilevel codebook having a hierarchical tree-structure on different beam width is adaptively designed. The performance of the proposed codebook design is analyzed, and the joint search algorithm is compared with other search methods. The proposed search method and the codebook design show performance improvement with small complexity.