Communication Required for Platooning

SoftBank is actively working towards the practical implementation of Vehicle-to-Vehicle direct communication technology (V2V) utilizing 5G through research and development on truck platooning on highways and validation experiments on autonomous driving and platooning Bus Rapid Transit (BRT) systems.
Reference:「The Future of Logistics: SoftBank Working to Evolve Truck Platooning with 5G」
Reference:「JR西日本とソフトバンクの「自動運転・隊列走行BRT」開発プロジェクト、専用テストコースでの実証実験を完了し公道での実証実験を開始」

Platooning requires two types of communication: one for vehicle control with low latency and another for video monitoring with low latency and high capacity. In the vehicle control communication, vehicles share position, speed, steering information, and other relevant data with each other for platooning. In the video communication, multiple video feeds, including the surroundings of following vehicles (FV) and the interior of the vehicles, are transmitted to the lead vehicle (LV) for safety confirmation purposes.

Types of Vehicle-to-Vehicle Communication

There are two types of Vehicle-to-Vehicle communication technology: V2N2V (Vehicle to Network to Vehicle), where in-vehicle devices communicate through networks such as 5G or LTE, and direct V2V communication, which does not rely on network connectivity. In considering the frequency for V2V communication, the utilization of millimeter waves (mmW) is being explored due to their wide bandwidth*1, which allows for low-latency and high-capacity communication*2. Given the critical impact on safety, redundancy through multiple communication means is crucial to ensure uninterrupted communication in Vehicle-to-Vehicle communication.
In this article, we will introduce the challenges associated with V2V communication and the antenna diversity technology that addresses these issues.

*1: n257 (29 GHz band) of 5G is allocated with a 400 MHz allocation.
*2: In the "Working Party 5A Draft new Report ITU-R M.[CAV] Connected Automated Vehicles (21 Sep 2023)" document, the use of millimeter waves is being discussed.

Issues with Vehicle-to-Vehicle Direct Communication

Interference caused by reflections from the road surface, and other similar sources

In urban areas, communication between conventional base stations and vehicles often occurs in non-line-of-sight (NLOS) environments where the antennas of base stations and devices such as smartphones or mobile routers cannot have direct visual contact with each other. In these environments, multiple waves undergo reflections, diffractions, and other phenomena at various locations. Additionally, the propagation environment is constantly changing due to the movement of objects such as devices and obstacles along the propagation path.
On the other hand, V2V communication takes place in a line-of-sight (LOS) environment, where the antennas of the communicating vehicles are within direct visual contact. In this case, in addition to the direct waves that arrive without reflection, there are strong reflected waves that have reflected once or a few times from the road surface, road structures, surrounding vehicles, and other objects. This leads to the issue of radio interference.
As the inter-vehicle distance increases, the arrival angle difference and received power difference between the direct wave and the reflected wave decrease. This leads to significant power reduction in the received signal from the combined waveform due to phase differences based on the relative positioning of antennas. As a result, a phenomenon occurs where the received power of the combined waveform periodically fluctuates in a direction perpendicular to the reflection surface at the rear of the lead vehicle.

Optimization of antenna configuration by antenna diversity

To reduce the degradation of wireless communication quality caused by interference and provide a stable V2V communication environment, SoftBank has focused on antenna diversity technology. Antenna diversity involves using multiple antennas to transmit and receive radio waves, thereby improving communication quality. Optimizing the antenna configuration is essential.
By solving the approximation equation of a composite wave, it has been revealed that the variation period of composite wave's received power in the vertical direction with respect to the reflecting surface is determined by factors such as the frequency used, inter-vehicle distance, and distance between the transmitting antenna and the reflecting surface. These findings have been confirmed through experiments. The following graph shows an example of road surface reflection, but the same results were obtained with lateral reflections from side walls or neighboring vehicles.

“Distance between vehicles” vs “Received power change period” “Distance between RX antenna and reflective surface” vs “Received power change period”

As a result, it has become possible to determine the optimal spacing distance between the antennas used in antenna diversity. For example, in the case of a fixed inter-vehicle distance, setting the spacing distance as an odd multiple of half the period of fluctuation can compensate for wireless quality degradation at one antenna with the other antenna. However, if the inter-vehicle distance varies, separate optimization*3 is required within that range.

*3: SoftBank’s patent technologies

Example of Received Power of Composite Wave with Constant Inter-Vehicle Distance

Assessment of Effectiveness in Demonstration Experiment

V2V Experimental Configuration

We took into account the road surface reflection wave and lateral reflection waves caused by road structures in the experiment. We employed a diagonal configuration with two antennas for diversity. Since the inter-vehicle distance varied, we optimized* the antenna spacing within a range of 15±5 m to ensure a negative correlation between the phase difference of the direct waves and reflected waves for each antenna.
In the lead vehicle, there is a radio unit and millimeter-wave antennas with a two antenna diversity configuration, installed inside the vehicle. In the following vehicles, there is a radio unit and millimeter-wave antennas with a two antenna diversity configuration, installed externally.
Regarding the millimeter-wave antennas, the lead vehicle utilizes an antenna that can select the optimal beam from multiple beams. Meanwhile, the following vehicles use a conventional directional antenna without beam selection capability.
Two vehicles were driven manually by a human operator, maintaining a measured inter-vehicle distance of 15±5 meters using laser rangefinders. The vehicles traveled approximately 1.1 kilometers, including curves, at a speed of 20 kilometers per hour.

Two Antenna Diversity Configuration

Experimental Configuration Overview

Traveled Route Overview

Experimental Results

Comparing the One-Way Latency using the CCDF (Complementary Cumulative Distribution Function) 1% value, it was observed that without antenna diversity (red legend), the latency increased significantly to approximately 2.5 seconds due to retransmissions caused by transmission errors. However, when antenna diversity was employed (blue legend), the latency reduced to approximately 6.3 milliseconds. This result, combined with the latency data prior to statistical processing, confirms the provision of a stable and low-latency V2V communication environment.
Regarding throughput, comparing the CDF (Cumulative Distribution Function) 1% value, antenna diversity again proved advantageous. The throughput achieved was approximately 52.5 Mbps. In the experiment, the MCS (Modulation and Channel Coding Scheme) was set low due to output constraints, and only one CC (Component Carrier, 100MHz) was used. However, with higher received power and the use of higher MCS and multiple CCs, it is possible to achieve transmission rates in the range of several hundred Mbps to Gbps, enabling the transmission of multiple high-definition videos and sensor raw data.

Comparison of One-Way Latency

Comparison of E2E Throughput

It has been demonstrated that by utilizing antenna diversity technology optimized with antenna configuration, a stable V2V communication environment can be achieved. This technology is expected to enable efficient operation of autonomous driving trucks and buses, which are receiving attention as a solution to the 2024 problem in the logistics and transportation industry.

SoftBank will continue to promote research and development of various technologies, including Vehicle-to-Vehicle communication, in order to realize the next-generation mobility society.