Contents I. Introduction
Dynamic spectrum sharing (DSS) or dynamic spectrum access (DSA) have been considered for many years as the promising solution for better utilization of these limited resources [1], [2]. Static assignment of frequency bands to specific services or even stakeholders minimizes significantly - but still only to some extent - the impact of interference between the users. However, the static frequency plan had lead to the problem of high spectrum scarcity and its underutilization, even if it assumed spectrum sharing among various wireless services. Various measurement campaigns conducted in various places have shown that the frequency spectrum may be utilized (at certain locations and time) only in dozen or so percent [3]–[5]. One of the prospective remedies to the problem of high spectrum underutilization was the invention of the dynamic approach, where the frequency bands are shared flexibly. Various approaches to this aspect have been investigated in the last two decades [6]–[9]. Very recently, the vehicular dynamic spectrum access (VDSA) has been proposed as a viable solution for spectrum congestion in autonomous driving [10]. As with all dynamic schemes, it offers great efficiency (in terms of better utilization of resources) at the expense of much more complicated management, orchestration, and steering [11]. The DSA and DSS are also the basis of the cognitive radio technology, introduced more than two decades ago, where it is assumed that the radio terminals or radio networks are equipped with some source of intelligence which allows for the flexible and highly efficient functioning of the communication systems. In parallel, in the domain of networking (wired or wireless), significant progress has been observed towards software-defined networking and network virtualization [12]–[15]. These two pillars, i.e., virtualization of core and radio access part of the communication network, paved the way for offering new services, often defined only in the software way. In that context, recent achievements towards the creation of spectrum management applications [16]–[18] proved the potential of flexible, software-based allocation of spectrum resources among the stakeholders or users. Various spectrum sharing and pricing strategies have been considered, including Licensed Shared Access (LSA) or Citizen Broadband Radio Service (CBRS). Having in mind the above-mentioned trend toward higher flexibility achieved by means of softwarization, in this work we propose the elastic, fully programmable scheme for spectrum sharing, licensing, and pricing which gains from the popular concept of freemium access. We propose the hierarchical scheme, where all users (stakeholders) may access a specific spectrum band for free (as popular in wireless local area networks, WLAN) suffering from the interference phenomena. However, if they are unsatisfied they may order the premium services. In this approach, the rules on how the premium schemes are defined are easily changeable. The rest of this paper is organized as follows, first, in the next section we overview the most popular spectrum sharing schemes and identify key challenges associated with them. Next, we propose our freemium spectrum sharing, licensing, and pricing scheme. Finally, we describe the simulation setup and achieved results.
