# Blockchain-Based Licensed Spectrum Fair Distribution Method towards 6G-Envisioned Communications
## 1 Introduction  
### Research Background and Starting Point  
The contradiction between limited spectrum resources and the increasing bandwidth demand facilitates the evolution of the next generation of a mobile communication paradigm. While 5G is being put into widespread commercial use, studies on 6G have been carried out. As we all know, licensed spectrum resources account for a considerable proportion of mobile communication service. Licensed spectrum access (LSA) can guarantee the licensed users' quality of service (QoS) at a high level. Different from 4G and 5G licensed spectrum distribution, 6G licensed spectrum distribution faces more challenges, including more connections, more decentralized locations and more security risks. The striking two distinguishing features from 6G to 5G are the introduction of a terahertz band [1] and Space-Ground Integrated Network (SGIN) architecture [2]. Although terahertz communication technology can significantly improve data transmission rates, it also brings greater path transmission damage and smaller cellular coverage. That is to say more micro base stations are needed to realize ubiquitous and wide-area wireless communication coverage. The wider spatial distribution is exactly one of the important characteristics of SGIN. Hence, it is inevitable for Mobile Network Operators (MNOs) to change their current centralized business model to a more flexible and decentralized one. This irreversible evolution is driven by emerging technologies, such as network virtualization, dynamic spectrum sharing, blockchain and so on. To address the unfair problem in the current licensed spectrum accessing mechanism, utilizing the blockchain technology is the study aim of this paper. Usually, in 4G and 5G mobile networks, MNOs distribute licensed spectrum resources according to a user's service protocols agreed upon in advance. A licensed user's periodic demand will be satisfied in a certain coverage region according to current geographic location. These service protocols are regulated through binding Service-Level Agreements (SLAs). Therefore, the present LSA spectrum access framework is called the distribution on demand model. Under this model, MNOs distribute the spectrum resources to different Primary Users (PUs) or a Primary Base Station (PBS) according to their demand. Some dishonest users would exaggerate their spectrum demand or violate the spectrum using regulations, obtaining extra interest. The common misconducts include transmitting with a bigger power than permitted, using a different carrier frequency than allocated and using the spectrum for more time than permitted [3]. However, there lacks an effective supervision and punishment measures for the violations. As a result, the dishonest users can obtain extra illegal interest compared to the honest users. Obviously, this is unfair for the honest users. On the other hand, the existing research results usually assume that operators and MNOs are honest participants in the spectrum distribution process. This means that users believe the obtained bandwidth resources are the same as the nominal value. Nevertheless, MNOs are actually rational participants, and the provided services may be discounted in order to obtain more benefits. For occasional and negligible service downgrades, users may not perceive without professional detection tools' help. But if it is the other way around, the MNO will be complained about, or the users will even switch to another telecom service provider. Furthermore, for the above two kinds of bad behaviors of users and MNOs, although the detection means have been rather available, the supervision and audit means are still not rich. To sum up the application status and related research results on 5G licensed spectrum distribution, the shortcomings of the present distribution model are mainly reflected in the following three aspects:  
1. Unfairness between honest and dishonest users.