## 2 System Model  
We consider a COM-NOMA system, which is shown in Figure 1, constituted by a BS with coverage radius \(R\) and two MGs \(MG_{i}\), \(i=\{1,2\}\). In each MG, \(M_{i}\) users are uniformly distributed in the cellular. The probability density function of user's distance from the BS is \(f\left(\gamma\right)=\frac{2\gamma}{R^{2}}\), \(0<\gamma\leq R\). Time slot \(T\) is divided into two equal sub-slots to conduct data transmission. \begin{table}
\begin{tabular}{l l} \hline \(B\) & The bandwidth originally allocated to each MG in OFDMA \\ \(x_{i}\) & The required signal of \(MG_{i}\) \\ \(\alpha_{i}\) & The NOMA power allocation factors for \(x_{i}\) \\ \(N_{0}\) & The power of additive white Gaussian noise \\ \(\delta_{n}^{2}\) & The variance of additive white Gaussian noise \\ \(\sigma_{0}\) & The SINR threshold for UR selection \\ \(\gamma_{k_{i},u}\) & The distance from \(SU_{i,k_{i}}\) to USU \(u\) \\ \(SINR_{k_{i},u}^{x_{2}}\) & The reception SINR of \(x_{2}\) from \(SU_{i,k_{i}}\) \\ \(R_{0}\) & The radius of ETR \\ \(\Gamma_{\mathcal{U}}\) & The set of distance from BS to each USU \\ \(\gamma_{u}^{j}\) & The \(j\)-th USU in \(\Gamma_{\mathcal{U}}\) \\ \(\mathcal{A}\) & The annular range where USU locates \\ \(DI_{ex}^{\mathcal{A}}\) & The external diameter of annular area \(\mathcal{A}\) \\ \(DI_{in}^{\mathcal{A}}\) & The inner diameter of annular area \(\mathcal{A}\) \\ \(A_{n}\) & The \(n\)-th small annular area, \(1\leq n\leq N\)\(\bigcup\limits_{n=1}^{N}A_{n}=\mathcal{A}\) and \(\bigcap\limits_{n=1}^{N}A_{n}=\varnothing\) \\ \(r_{n}\) & The radius of ERSR allocated to \(A_{n}\) \\ \(\theta\) & The radius set for \(\mathcal{A}\) \\ \(\Theta\) & The available \(\theta\) set \\ \(\mathcal{E}\) & The optional radius set of ERSR \\ \(E_{t}\) & The efficient transmission distance between two users’ devices \\ \(W_{n}\) & The width of \(A_{n}\) \\ \(N^{\theta}\) & The total number of URs when the radius set for \(\mathcal{A}\) is \(\theta\) \\ \(N_{s}^{\theta}\) & The number of MG users who can successfully receive when the radius set for \(\mathcal{A}\) is \(\theta\) \\ \(C_{RDUR}^{\theta}\) & The coverage efficiency when the radius set for \(\mathcal{A}\) is \(\theta\) \\ \(C_{RDUR}\) & The coverage efficiency of RDUR system \\ \(\lambda_{MG}\) & The Poisson distribution density of MG users in the cell with radius \(R\) \\ \(\lambda_{A_{n}}\) & The MG user density in annular area \(A_{n}\) \\ \(\lambda_{s}^{A_{n}}(\gamma_{u})\) & The Poisson distribution density of SU in the ERSR of \(u\), when \(u\) is in \(A_{n}\) \\ \(M_{\text{UR}}(\gamma_{u})\) & The number of URs when USU is at \(\gamma_{u}\) \\ \(M_{\text{UR}}^{A_{n}}\) & The total number of UR in \(A_{n}\) \\ \(M_{\text{IR}}^{A_{n}}\) & The number of URs in \(A_{n}\) after expurgating repeated calculations \\ \(M_{s}^{A_{n}}\) & The number of SUs in \(A_{n}\) \\ \(M_{u}^{A_{n}}\) & The number of USUs in \(A_{n}\) \\ \(R^{A_{n}}\) & The average repetition rate \\ \(P_{s}(\gamma_{u})\) & The probability that MG user successfully receives when the user is at \(\gamma_{u}\) \\ \(P^{A_{n}}\) & The probability that MG user locates in \(A_{n}\) \\ \(R_{\text{IUR}DR}\) & The UR selection ratio of RDUR system \\ \(P_{\text{UR}_{1}}^{A_{n}}\) & The probability that USU can be served by one UR \\ \(P_{\text{UR}_{1}}\) & The sum probability that USU is served by one UR in \(\mathcal{A}\) \\ \(P_{u\to s}\) & The probability that USU can successfully receive after RDUR scheme \\ \hline \end{tabular}
\end{table} 
Table 1: \(Cont\). In order to reduce the burden of networks on keeping track of the data, especially when the number of user is large, the rateless code, such as the fountain code [14], is employed throughout the content delivery process [15]. In the first stage, two MGs are regarded as one quasi-MG. BS multicasts NOMA superposition signaling to the quasi-MG at coverage ratio \(C_{1}\).