  Platoon-based driving is an idea that vehicles follow each other at a close
distance, in order to increase road throughput and fuel savings. This requires
reliable wireless communications to adjust the speeds of vehicles. Although
there is a dedicated frequency band for vehicle-to-vehicle (V2V)
communications, studies have shown that it is too congested to provide reliable
transmission for the platoons. Additional spectrum resources, i.e., secondary
spectrum channels, can be utilized when these are not occupied by other users.
Characteristics of interference in these channels are usually
location-dependent and can be stored in the so-called Radio Environment Maps
(REMs). This paper aims to design REM, in order to support the selection of
secondary spectrum channel for intra-platoon communications. We propose to
assess the channel's quality in terms of outage probability computed, with the
use of estimated interference distributions stored in REM. A frequency
selection algorithm that minimizes the number of channel switches along the
planned platoon route is proposed. Additionally, the REM creation procedure is
shown that reduces the number of database entries using (Density-Based Spatial
Clustering of Applications with Noise) DBSCAN algorithm. The proposals are
tested using real IQ samples captured on a real road. Application of the DBSCAN
clustering to the constructed REM provided 7% reduction in its size.
Utilization of the proposed channel selection algorithm resulted in a 35 times
reduction of channel switches concerning channel assignment performed
independently in every location.
