## 3 Proposed Joint Resource Optimization for NOMA-Enabled Vehicular Networks
### Proposed Optimal Resource Optimization
#### 3.2.1 Formulation (Phase A): Association, Channel Assignment and NOMA-OMA Switching  
Phase A involves three key operations: RSU-VU association, channel assignment, and NOMA-OMA switching. These steps collectively define preliminary resource allocation for the network, determining which VUs are associated with each RSU, how channels  
Figure 2: Flowchart of the proposed approach that consists of two phases to jointly optimize association, channel assignment, NOMA-OMA switching and transmit power allocation. are allocated, and whether a channel operates in OMA or NOMA mode based on user clustering and channel conditions. When NOMA is activated on a particular channel, two-user clustering will be applied by finding a pair of VUs to be assigned to the channel. Depending on the number of associated VUs and their channel condition, a channel may be assigned with a single VU or two, resulting in OMA or NOMA decision, respectively. Although the first phase does not determine the actual transmit power under QoS constraint, the decisions therein cannot be made without considering the power consumption. Thus, Phase A also carries out total power minimization, but the related constraint is not QoS satisfaction, but signal to noise threshold. In the proposed formulation, the binary decision variable \(x_{bcu}\) plays a major role in determining the association, channel assignment and NOMA-OMA switching. As aforementioned, when NOMA is enabled on a particular channel, two-user clustering will be applied. Thus, each of the two users assigned to the same channel is classified as either a high-power user (\(x_{bcu}^{h}\)) or low-power user (\(x_{bcu}^{l}\)), where the high-power user receives higher power from RSU for transmission. To be specific, the binary variable \(x_{bcu}^{h}\in\mathbf{X}^{h}\) determines whether VU \(u\) is associated with RSU \(b\) on channel \(c\) as a high-power user or not. In a similar manner, the binary variable \(x_{bcu}^{l}\in\mathbf{X}^{l}\) indicates whether the corresponding user is a low-power user or not. The following constraint, Equation (1) limits the number of high-power users to be assigned to RSU \(b\) on channel \(c\) up to one, and the same limit is also applied to the low-power user. The last constraint in Equation (1) will be activated when the channel \(c\) is operating at OMA mode so that the OMA user will be treated as a low power user. \[\forall b,c:\quad\sum_{u}x_{bcu}^{h}\leq 1,\quad\sum_{u}x_{bcu}^{l}\leq 1,\quad \sum_{u}x_{bcu}^{l}\geq\sum_{u}x_{bcu}^{h} \tag{1}\]  
It is worth noting that for each RSU \(b\) and channel \(c\), \(x_{bcu}^{h}+x_{bcu}^{l}\) is the number of VUs occupying the channel, and it determines the NOMA-OMA switching. In the proposed approach, when a channel is occupied by two users, the RSU configures the channel to operate as a NOMA channel. On the other hand, if there is only a single device mapped to a channel, the RSU triggers an OMA mode on the channel. According to the following constraint, Equation (2), each RSU \(b\) can associate with up to \(\kappa_{b}\) number of VUs for each time instance where \(\kappa_{b}\) is set to twice the number of available channels, \(N_{ch}\). \[\forall b:\quad\sum_{c}\sum_{u}\Bigl{(}x_{bcu}^{h}+x_{bcu}^{l}\Bigr{)}\leq \kappa_{b} \tag{2}\]  
Due to the single-antenna assumption for each user and the QoS requirement, a user must associate with a single RSU for connectivity as shown below, Equation (3). \[\forall u:\quad\sum_{b}\sum_{c}\Bigl{(}x_{bcu}^{h}+x_{bcu}^{l}\Bigr{)}=1 \tag{3}\]  
The RSUs periodically broadcast beacon signals to notify their readiness to nearby VUs. Each VU collects those signals from possibly multiple RSUs, and then uploads the list of accessible RSUs to the central controller. Collection of such lists is referred as accessibility matrix, \(\mathbf{A}=[a_{bu}]\) with \(a_{bu}\in\{0,1\}\), where \(a_{bu}=1\) if VU \(u\) can access or associate with RSU \(b\). The following constraint Equation (4) allows VU \(u\) to associate with RSU \(b\) only when \(a_{bu}=1\).