Low Overhead Codebook Design for mmWave Roadside Units Placed at Smart Intersections

In order to meet the high data rate requirements of emerging roadway use cases, mmWave vehicular communications will be needed. This work studies the ability of vehicles to communicate with a Roadside Unit (RSU) placed at an intersection. Practical mmWave radios utilize a codebook, a discrete set of analog beams, that is periodically searched during runtime to find the optimal beam to use for each receiver. This search creates overhead as the wireless channel is not used for communication while this beam search is happening. This work focuses on reducing the overhead of beam training by optimizing the site-specific codebook design of a RSU. Owing to the sparsity of the mmWave channel and the user distribution for vehicles, it is found that 85% of beams can be removed from the codebook with zero-impact. By carefully selecting the usage of wide beams the codebook size can be further reduced to just 64 beams while still providing omni-directional coverage for an intersection. Other research thrusts have focused on attempting to augment or remove beam training entirely; however, this necessitates a change to the PHY layer. Codebook optimization achieves approximately 80% of the communications performance that would be achieved if beam training overhead could be completely removed while only requiring a radio configuration update. Thus, this work finds that today’s commercial mmWave radios are sufficient for deployments in RSUs. To validate the proposed codebook optimization algorithm, a detailed mmWave ray tracing framework that encompasses 3D environmental information and material properties of reflectors is developed.