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Millimeter-Wave Vehicular Communication to Support Massive Automotive Sensing

As driving becomes more automated, vehicles are being equipped with more sensors generating even higher data rates. Radars are used for object detection, visual cameras as virtual mirrors, and LIDARs for generating high resolution depth associated range maps, all to enhance the safety and efficiency of driving. Connected vehicles can use wireless communication to exchange sensor data, allowing them to enlarge their sensing range and improve automated driving functions. Unfortunately, conventional technologies, such as DSRC and 4G cellular communication, do not support the gigabit-per-second data rates that would be required for raw sensor data exchange between vehicles. This article makes the case that mmWave communication is the only viable approach for high bandwidth connected vehicles. The motivations and challenges associated with using mmWave for vehicle-to-vehicle and vehicle-to-infrastructure applications are highlighted. A high-level solution to one key challenge - the overhead of mmWave beam training - is proposed. The critical feature of this solution is to leverage information derived from the sensors or DSRC as side information for the mmWave communication link configuration. Examples and simulation results show that the beam alignment overhead can be reduced by using position information obtained from DSRC.

Millimeter Wave Vehicular Communications: A Survey

Future vehicles will require massive sensing capability. Leveraging only onboard sensors, though, is challenging in crowded environments where the sensing field-of-view is obstructed. One potential solution is to share sensor data among the vehicles and infrastructure. This has the benefits of providing vehicles with an enhanced field-of-view and also additional redundancy to provide more reliability in the sensor data. A main challenge in sharing sensor data is providing the high data rates required to exchange raw sensor data. The large spectral channels at millimeter wave mmWave frequencies provide a means of achieving much higher data rates. This monograph provides an overview of mmWave vehicular communication with an emphasis on results on channel measurements, the physical PHY layer, and the medium access control MAC layer. The main objective is to summarize key findings in each area, with special attention paid to identifying important topics of future research. In addition to surveying existing work, some new simulation results are also presented to give insights on the effect of directionality and blockage, which are the two distinguishing features of mmWave vehicular channels. A main conclusion of this monograph is that given the renewed interest in high rate vehicle connectivity, many challenges remain in the design of a mmWave vehicular network.

The Impact of Beamwidth on Temporal Channel Variation in Vehicular Channels and its Implications

Millimeter wave (mmWave) has great potential in realizing high data rates, thanks to the large spectral channels. It is considered as a key technology for fifth-generation (5G) wireless networks and is already used in wireless LAN (e.g., IEEE 802.11ad). Using mmWave for vehicular communications, however, is often viewed with some skepticism due to a misconception that the Doppler spread would become too large at these high frequencies. This is not necessarily true when directional beams are employed. In this paper, closed-form expressions relating the channel coherence time and beamwidth are derived. Unlike prior work that assumed perfect beam pointing, the pointing error due to the receiver motion is incorporated to show that there exists a nonzero optimal beamwidth that maximizes the coherence time. We define a novel concept of beam coherence time, which is an effective measure of beam alignment frequency. Using the derived correlation function, the channel coherence time, and the beam coherence time, an overall performance metric considering both the channel time variation and the beam alignment overhead is derived. Using this metric, it is shown that beam realignment in every beam coherence time performs better than beam realignment in every channel coherence time.

Beam Switching for Millimeter Wave Communication to Support High Speed Trains

This paper considers a millimeter wave (mmWave) system that enables multi-Gbps wireless service for high speed trains. MmWave systems require proper beam alignment to achieve good performance making it difficult to apply to high speed trains due to the need for frequent realignment. In this paper, using the IEEE 802.11ad system parameters, we first show that the channel coherence time will not be enough for communication with wide beams, and thus conventional approaches based on beam sweeping become inefficient if not impossible. Then we consider a beam switching approach that leverages the position information from the train control system for efficient beam alignment. Using this network architecture, we investigate the optimal choice of beamwidth and show that a properly optimized system can achieve multi-Gbps throughput.

Beam design for beam switching based millimeter wave vehicle-to-infrastructure communications

Beam alignment is a source of overhead in mobile millimeter wave communication systems due to the need for frequent repointing. Beam switching architectures can reduce the amount of repointing required by leveraging position prediction. This paper presents an optimization of beam design in terms of rate. We consider a non-congested two-lane highway scenario where road side units are installed on lighting poles. Under this scenario, line-of-sight to the road side unit is very likely and vehicle speed does not vary much. We formulate and solve numerically using a gradient descent method for an optimal beam design to maximize the data rate for non-overlap beams. The result shows close performance to the equal coverage beam design. We study the effect of the overlap on the average rate and outage and compare the equal coverage with the equal beamwidth design. Numerical examples show that the equal coverage design can achieve up to 1.5× the rate of the equal beamwidth design confirming the importance of the choice of beam design.

Basic Relationship between Channel Coherence Time and Beamwidth in Vehicular Channels

The availability of large bandwidth at millimeter wave (mmWave) frequencies makes mmWave an ideal candidate to realize multi-Gbps data rates. MmWave has been applied to indoor applications (e.g. IEEE 802.11ad), and it is also being considered for cellular. There is not much work, however, in applying mmWave to vehicular channels. This is likely due to the misconception that the Doppler spread will be too high at small mmWave wavelengths, which is true only for omnidirectional communication. In this work, closed form approximate expressions relating the coherence time and beamwidth are derived taking directional communication into account. The result shows that the coherence time increases at least proportional to the inverse of the beamwidth. The derivation also takes the beam misalignment due to receiver movement into account and it can be shown that the optimal beamwidth is non-zero in contrast to models that assume perfect beam pointing.

Beam tracking for mobile millimeter wave communication systems

Millimeter wave (mmWave) is an attractive option for high data rate applications. Enabling mmWave communications requires appropriate beamforming, which is conventionally realized by a lengthy beam training process. Such beam training will be a challenge for applying mmWave to mobile environments. As a solution, a beam tracking method requiring to train only one beam pair to track a path in the analog beamforming architecture is developed. Considering its low complexity which is suitable for mobile settings, the extended Kalman filter is chosen as the tracking filter. Several effects impacting the performance of the proposed tracking algorithm, such as the signal-to-noise ratio (SNR) and array size, are investigated. It is found that at the same SNR, narrower beams, which are more sensitive to angular changes, can provide more accurate estimate. Too narrow beams, however, degrade tracking performance because beam misalignment could happen during the measurement. Finally, a comparison to prior work is given where it is shown that our approach is more suitable for fast-changing environments thanks to the low measurement overhead.

Millimeter-Wave Communication with Out-of-Band Information

Configuring the antenna arrays is the main source of overhead in mmWave communication systems. In high mobility scenarios, the problem is exacerbated, as achieving the highest rates requires frequent link reconfiguration. One solution is to exploit spatial congruence between signals in different frequency bands and extract mmWave channel parameters with the aid of side information obtained in another band. In this article we propose the concept of out-of-band information aided mmWave communication. We analyze different strategies to leverage information derived from sensors or from other communication systems operating at sub-6 GHz bands to help configure the mmWave communication link. The overhead reductions that can be obtained when exploiting out-of-band information are characterized in a preliminary study. Finally, the challenges associated with using out-of-band signals as a source of side information at mmWave are analyzed in detail.

Inverse Multipath Fingerprinting for Millimeter Wave V2I Beam Alignment

Efficient beam alignment is a crucial component in millimeter wave systems with analog beamforming, especially in fast-changing vehicular settings. This paper proposes to use the vehicles position (e.g., available via GPS) to query a multipath fingerprint database, which provides prior knowledge of potential pointing directions for reliable beam alignment. The approach is the inverse of fingerprinting localization, where the measured multipath signature is compared to the fingerprint database to retrieve the most likely position. The power loss probability is introduced as a metric to quantify misalignment accuracy and is used for optimizing candidate beam selection. Two candidate beam selection methods are developed, where one is a heuristic while the other minimizes the misalignment probability. The proposed beam alignment is evaluated using realistic channels generated from a commercial ray-tracing simulator. Using the generated channels, an extensive investigation is provided, which includes the required measurement sample size to build an effective fingerprint, the impact of measurement noise, the sensitivity to changes in traffic density, and beam alignment overhead comparison with IEEE 802.11ad as the baseline. Using the concept of beam coherence time, which is the duration between two consecutive beam alignments, and parameters of IEEE 802.11ad, the overhead is compared in the mobility context. The results show that while the proposed approach provides increasing rates with larger antenna arrays, IEEE 802.11ad has decreasing rates due to the higher beam training overhead that eats up a large portion of the beam coherence time, which becomes shorter with increasing mobility.

Performance Analysis of Beam Sweeping in Millimeter Wave Assuming Noise and Imperfect Antenna Patterns

Beam alignment is an important component of millimeter wave communication systems, which use analog beam steering.We analyze the performance of beam alignment methods based on the hierarchical beam sweeping that are adopted in IEEE 802.11ad and IEEE 802.15.3c. Previous work studies misalignment probability due to thermal noise or antenna gain fluctuation alone. At the early stage of the alignment process, however, both effects are important. We incorporate both effects and derive probability of misalignment not only with respect to the best path but also to secondary ones. Our analysis can capture the misalignment severity in terms of power loss with respect to the perfect alignment case. Numerical examples show that line-of-sight channels are more constrained by the thermal noise, while the non- line-of-sight channels are more constrained by the antenna gain fluctuations.