  The ever-increasing demand for intelligent, automated, and connected mobility
solutions pushes for the development of an innovative sixth Generation (6G) of
cellular networks. A radical transformation on the physical layer of vehicular
communications is planned, with a paradigm shift towards beam-based millimeter
Waves or sub-Terahertz communications, which require precise beam pointing for
guaranteeing the communication link, especially in high mobility. A key design
aspect is a fast and proactive Initial Access (IA) algorithm to select the
optimal beam to be used. In this work, we investigate alternative IA techniques
to fasten the current fifth-generation (5G) standard, targeting an efficient 6G
design. First, we discuss cooperative position-based schemes that rely on the
position information. Then, motivated by the intuition of a non-uniform
distribution of the communication directions due to road topology constraints,
we design two Probabilistic Codebook (PCB) techniques of prioritized beams. In
the first one, the PCBs are built leveraging past collected traffic
information, while in the second one, we use the Hough Transform over the
digital map to extract dominant road directions. We also show that the
information coming from the angular probability distribution allows designing
non-uniform codebook quantization, reducing the degradation of the performances
compared to uniform one. Numerical simulation on realistic scenarios shows that
PCBs-based beam selection outperforms the 5G standard in terms of the number of
IA trials, with a performance comparable to position-based methods, without
requiring the signaling of sensitive information.
