Algorithm 1 Coalition-game-based beamwidth selection algorithm for D2D pairs during each iteration

1:Initialize$(\mathbf{\varphi },\mathit{p},{\tau }_e)$ with the output of the last iteration, and initialize $F_{ini}=\{F_1,F_2,\dots \}$ as indicated by $\mathbf{\varphi }$. 2:Set current coalition structure ${\mathcal{F}}_{cur}={\mathcal{F}}_{ini}$. Require: 3:Randomly select a player $D_n\in \mathcal{N}$, and denote its current coalition as $F_c$. 4:$D_n$ randomly chooses another coalition $F_{c^{\prime }}\in {\mathcal{F}}_{cur}$, and denote the temporary structure after $D_n$ leaves $F_c$ and switches to $F_{c^{\prime }}$ as: 5:${\mathcal{F}}_{temp}={\mathcal{F}}_{cur}\setminus (F_c,F_{c^{\prime }})\cup (F_c-D_n)\cup (F_{c^{\prime }}+D_n)$. 6:Calculate $U({\mathcal{F}}_{cur})$ and $U({\mathcal{F}}_{temp})$. 7:if$U({\mathcal{F}}_{temp})>U({\mathcal{F}}_{cur})$then 8:    $D_n$ leaves $F_c$ and joins $F_{c^{\prime }}$. 9:    Update the coalition structure as: ${\mathcal{F}}_{cur}={\mathcal{F}}_{temp}$. 10:else 11:    $D_n$ remains in its current coalition $F_c$. Ensure: The final Nash-stable coalition structure is reached. 12:Output The optimal beamwidths $\mathbf{\varphi }$ and $(\mathit{p},{\tau }_e)$ to the next block.