Bringing Millimeter-Wave Vehicular Communications to Life

This research presents clear guidelines to optimize millimeter-wave beamwidth, essential for millimeter-wave vehicular communication systems, by considering both vehicular speed and communication performance.

Article | Spring 2022

Vehicular environments offer fertile ground for innovative applications of wireless communications, ranging from safety to traffic efficiency to entertainment. These new applications are pushing the boundaries of what can be done with conventional wireless technologies for vehicular applications. For example, exchanging raw sensor data or high-quality multimedia could require gigabit-per-second data rates, which will require new approaches to inter-vehicular communication.

The huge spectral bandwidth available in millimeter wave (mmWave) technology has great potential for realizing gigabit-per-second data rates. While mmWave technology is already in use with fifth generation (5G) cellular networks and WLAN standards, the major concern in any application to vehicular environments is the severity of the Doppler effect, which may severely degrade communication performance due to the small mmWave frequency wavelengths. Prof. Junil Choi at the school of Electrical Engineering at KAIST and his collaborators at North Carolina State University and Samsung Research America demonstrated that the severe Doppler effect can be overcome in mmWave vehicular communication environments.

To compensate for the increased path loss due to shrinking of antenna size at mmWave frequencies, beamforming is widely accepted as a necessary component in mmWave systems. With directional reception, incoming signals are limited to a given range of angles. Each angle can be mapped to a Doppler frequency shift, and this also means that the Doppler frequency shifts are limited to a certain frequency range with directional reception. Since the average frequency shift can be corrected using standard frequency offset correction methods, Doppler effect is reduced and coherence time is increased, i.e., the time wireless communication channel is stable for transmission.

Directional transmission and reception can help slow the channel variation at the expense of beam alignment overhead, reducing loss in system spectral efficiency due to radio resources consumed to find best transmit and receive directions. Therefore, in fast changing vehicular environments, it is essential to use directional beams to achieve the potential of mmWave vehicular communications.

Prof. Choi and his collaborators derived the connection between coherence time and mmWave beamwidth and receiver motion. Their results show that there exists an optimal beamwidth that maximizes the channel coherence time. This is intuitively consistent: smaller beamwidth can better reduce Doppler effect by limiting range of angles of incoming signals, while larger beamwidth is more robust against beam misalignment due to vehicle movement. By rigorously verifying this connection, Prof. Choi and his collaborators have presented clear guidelines to optimize beamwidth for mmWave vehicular communication systems, a significant advance in this new research area.

Their work had been published in IEEE Transactions on Vehicular Technology in 2016 and received the prestigious IEEE Vehicular Technology Society Neal Shepherd Memorial Best Propagation Award in 2021.