# Blockchain-Based Licensed Spectrum Fair Distribution Method towards 6G-Envisioned Communications
## 3 BEAST System Model
### System Process  
At first, in order to better understand the working process, we made a variant definition table as shown in Table 2.  
The interactions among MNOs, PBS and PUs can be described as \"transactions\" that are recorded with the blockchain nodes in networks. The nodes with strong computing power are responsible to collect spectrum distribution records from the MNOs. The strong nodes are also responsible to generate and publish new blocks. Meanwhile, the consensus process is reached among these strong nodes. The nodes without sufficient computational power can check transactions on the blockchain, but they have no right to participate in the consensus process. A general expression of a spectrum distribution transaction can be denoted as \(SD_{tx}:\{MNO_{add}||BW||R_{add}||UR\}\), where \(MNO_{add}\) and \(R_{add}\), respectively, represent an MNO address and spectrum receiver address, and \(BW\) represents a distributed bandwidth. \(UR\) is using regulations about the spectrum access, such as power control, occupation span and transmitting frequency. The main steps involved in a spectrum distribution workflow include the following six steps. The algorithm flow of BEAST is also given in Algorithm 1. And to enhance clarity, a block diagram illustrating the main components and process of BEAST is shown in Figure 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Parameter** & **Description** & **Parameter** & **Description** \\ \hline \(MNO_{add}\) & MNO address & \(R_{add}\) & Spectrum receiver address \\ \(BW\) & Distributed bandwidth & \(UR\) & Spectrum using regulations \\ \(TR_{i}\) & Trust value & \(SR_{A}\) & Available resources set \\ \(SR_{D}\) & Spectrum demands set & \(t_{d}\) & Arrival timestamp \\ \(TX_{dis,t}\) & Spectrum distribution transaction & \(TV\) & Trust value \\ \(\Pr_{i}\) & Priority index & \(Tr_{th}\) & Trust value threshold \\ \(n_{miner}\) & Registered miners & \(Count_{reward}\) & Blocks generated within the reward \\ \(E_{block\_min}\) & Expected minimum number of & & cycle \\  & generated blocks & \(C_{duration}\) & Competition cycle \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Parameters Used in the System Process. Figure 2: Block diagram of spectrum distribution algorithm process. Step 1. System initialization. The PBS and PUs with spectrum access demand in a certain coverage area become legitimate entities after registering on the consortium blockchain. A pair of keys including public key \(PK\) and private key \(SK\) are sent to them, together with an initial trust value, \(TR_{i}\). The PBS and PUs generate several wallet accounts with \(PK\) to conduct a transaction with others. Step 2. Uploading demand and available spectrum resources. The available spectrum resources owned by \(m\) MNOs in a certain service area, \(|SA|\), form a set, \(SR_{A}=\{S_{1},\ldots,S_{m}\},S_{i}\geq 0\), \(1\leq i\leq m\). In \(|SA|\), \(n\) PBS and PU spectrum accessing demands form another set, \(SR_{D}=\{P_{1},\ldots,P_{n}\},P_{i}\geq 0,\ 1\leq i\leq n\). Both sets are uploaded to the blockchain. The sending messages are respectively packed as transactions, which can trigger the spectrum distribution smart contracts. In this step, the MNOs need to pay a deposit proportional to their claimed available spectrum resources to prevent MNOs from claiming idle spectrum resources arbitrarily. Step 3. Executing spectrum distribution smart contract. Upon receiving the message, a smart contract completes the distribution process according to the supply and demand as well as the trust value of each PBS and PU. During this process, we first define a timestamp array, \(t_{d}=\{t_{d1},t_{d2},\ldots,t_{dn}\}\), to represent the successive sorting of the arrival moment of users' spectrum demand. The corresponding trust value of each user is \(TV=\{TV_{1},TV_{2},\ldots,TV_{n}\}\). As described earlier, to encourage the regulated use of the spectrum and realize fair distribution, \(t_{d}\) and \(TV\) are combined to decide the distribution order of priority.