# Blockchain-Based Licensed Spectrum Fair Distribution Method towards 6G-Envisioned Communications
## 3 BEAST System Model
### System Process
Assuming the unit price of mobile data traffic is the same, users will certainly choose the MNOs with better service quality and better reputation. As rational participants in the 6G spectrum distribution, MNOs aim to obtain more economic income. Therefore, the MNOs also have to maintain a good trust value. We can adopt the similar trust value evaluation method as described in Section 4.1. One difference is that if the MNO is found to provide degraded services through surveillance and auditing, in addition to the loss of the trust value, the MNO will also be penalized regarding the deposit currency in the spectrum distribution smart contract. ## 5 Protocol Analysis

The properties of fairness and security of BEAST are analyzed in this section. ### Fairness

The fairness in the 6G-envisioned BEAST includes two levels. The first level is that the spectrum allocation algorithm is fair for the spectrum users and the algorithm does not favor any user operator with more resources. This absolute fairness means that all the users obtain spectrum resources on a \"first-come, first-served\" basis. To protect the interest of the honest users, BEAST will decrease a misbehaving user's trust value and further decrease the priorities in the later spectrum distribution rounds. With this way, an honest user's spectrum access rights are guaranteed first, realizing a relative fairness. The second level means that the consensus protocol is neutral, power-separated and impartial. And it is resistant to collusions among the participants in the consensus process. To achieve the second-level fairness, there are three key designs in the PoT consensus. The first design is to separate the roles of transaction validation and ledger management. The transaction validating process is accomplished in the 6G network with the validators, who vote on whether a transaction can be packed into the new block. And the nodes on the consortium blockchain can only vote on the transaction lists passed with the validator group members. That is to say, the nodes cannot add new transactions. In this way, the power separation is realized. The second design is to choose the validators based on the following two priority conditions: validators with a high trust value and validators not related to the current transactions. In this way, the neutrality and impartiality are guaranteed. The third design is the introduction of [PERSON]'s secret sharing scheme; the identity information of other participants cannot be obtained with the validators. And the transaction lists are encrypted without revealing them to other validators. ### Security

A theoretical analysis shows that BEAST performs well at defending against selfish mining attacks, overbooking attacks and repudiation attacks. #### 5.2.1 Prevention of Selfish Mining Attacks

In the blockchain network, there may exist selfish miners leveraging a special strategy in order to obtain larger revenue than what they deserve. This behavior is named selfish mining. Selfish mining can prevent honest miners from mining blocks on the latest block and waste the efforts of honest miners [28]. The selfish miners keep carrying out mining blocks secretly until the fork from the main chain is longer than the main chain. In our proposed scheme, since new generating blocks are not based on the computing power that one node or a group of nodes possess in common, in this way, a selfish mining attack can be avoided. Furthermore, a certain node can neither know the specific nodes involved in the current or the next consensus process nor learn which node will be selected as the leader. #### 5.2.2 Prevention of Overbooking Attacks and Repudiation Attacks

In an ideal scenario, our aim is to establish an environment where honest PBS and PUs can seamlessly access licensed spectrum resources based on their requests. And simultaneously, honest MNOs can be duly rewarded for delivering competent services.