Algorithm 1 Stackelberg Game based Optimal Strategies

1. All secondary users inform the primary user to join an active cooperation. The set is S.

2. The primary user decide the initial strategies α1 and β1 according to its data length, and then inform all secondary users in S of the initial strategies.

3. All secondary users in S calculate the initial optimal payment strategies ci1 in Equation (18) according to the primary user's initial strategies, and feedback these to the primary user.

4. According to secondary users' strategies, the primary user calculates the optimal payment strategies αn and βn with Equations (22) and (27).

5. The primary user compares the current strategies αn and βn with the last strategies αn−1 and βn−1. If αn–αn−1= ε and βn–βn−1= ε (ε is a small number, for example: 0.001), the primary user will no longer update the strategies which means it will use αn−1 and βn−1 as the fixed strategies. Otherwise it has to change strategies to αn and βn. Then the primary user sends the strategies to S.

6. All secondary users in S calculate their optimal payment strategies cin in Equation (18) according to the primary user's strategies.

7. All secondary users in S compare their current strategies cin with the last strategies cin−1. If cin –cin−1= ε, SU will no longer update strategies which means they use cin−1 as the fixed strategies. Otherwise they change strategies to cin.

8. If the primary user and all secondary users in S do not change their strategies in the last round, the game reaches the Nash equilibrium and then end. Otherwise all secondary users in S send the strategies to the primary user and go to step 4.