Algorithm 2: User preselection algorithm.

Step 1: Orthogonal user preselection algorithm among the different multicast groups. Input: CSI. 1. Let $\mathrm{I}{\mathrm{d}}_{\left(1\right)}=Index\left\{\max \left(‖{\mathit{h}}_{k,l}‖\right),\forall k,l\right\}$, select the user with the largest channel gain, $\mathrm{I}{\mathrm{d}}_{\left(1\right)}$ is the index of the user. 2. while $l\le L,l\ne \mathrm{I}{\mathrm{d}}_{\left(1\right)}$ 3.   For all users in the $l\mathrm{th}$ multicast group, calculate $Z_{k,l}={\mathit{h}}_{k,l}\left(I_N-{\displaystyle {\sum }_{j=\mathrm{I}{\mathrm{d}}_{\left(1\right)}}^{\mathrm{I}{\mathrm{d}}_{\left(l\right)}}\frac{{\mathit{h}}_{\left(j\right)}^H{\mathit{h}}_{\left(j\right)}}{{‖{\mathit{h}}_{\left(j\right)}‖}_2^2}}\right)$ in turn. 4.  $\mathrm{I}{\mathrm{d}}_{\left(l\right)}=Index\left\{\max \left(‖Z_{k,l}‖\right),\forall k\in l\right\}$, the user with index $\mathrm{I}{\mathrm{d}}_{\left(l\right)}$ is the preselected orthogonal user of the $l\mathrm{th}$ multicast group. 5. end Output: Orthogonal users among the different multicast groups. Step 2: User preselection algorithm in each multicast group. Input: Orthogonal users among the different multicast groups, CSI. 1. For $l=1:L$ 2.   For other users in the $l\mathrm{th}$ multicast group except the orthogonal user preselected in step 1, calculate the linear correlation value between each user and the preselected orthogonal user of the multicast group in turn, i.e., $C_{k,l}={\mathit{h}}_{k,l}^H\frac{{\mathit{h}}_{\mathrm{I}{\mathrm{d}}_{\left(l\right)}}{\mathit{h}}_{\mathrm{I}{\mathrm{d}}_{\left(l\right)}}^H}{{‖{\mathit{h}}_{\mathrm{I}{\mathrm{d}}_{\left(l\right)}}^H‖}_2^2}$. 3.   Based on the $C_{k,l}$ of users in each multicast group, select top $\left(U_{l,p}-1\right)$ largest users, plus the orthogonal users in step 1 as the preselected users of each multicast group. 4. end 5. end Output: Preselected users for each multicast group.