# CSNOMA: Carrier Sense Non-Orthogonal Multiple Access
## 4 Carrier Sense Non-Orthogonal Multiple Access (CSNOMA)  
### Proposed Protocol  
Under the CSMA regime, a node in the network can transmit its data when the channel is sensed as idle. The conventional CSMA-like random access scheme prohibits the transmitters in the proximity from transmitting concurrently to increase the transmission success probability. As the node density increases, more nodes struggle to seize the transmission opportunity due to the conservative behavior of CSMA. To resolve this situation, we take a radical approach; even though the channel is sensed as busy, the transmission is allowed and incurred interference is canceled by SIC. Figure 3 shows the allowed/restricted region. When the tx\({}_{1}\) has a chance to transmit, the another transmitter (tx\({}_{2}\)) within the circular region of cancelable distance \(r_{c}\) (region c, inside of the inner circle) is also allowed to transmit. The transmitters in the annulus region between \(r_{c}\) and \(r_{s}\) (region b, shaded region in the figure) are restricted to transmit, and the transmitters farther than \(r_{s}\) (region a, outside of the outer circle) can transmit if the channel is sensed as idle. Classification of allowed/restricted transmission areas according to the distance from the receiver is depicted in Table 3.  
Figure 2: Example of cancelable scenario. There are one communication pair with communication distance \(r_{t}\) and the target SIR is \(\beta\). There are three interferers. The interfering signal within the distance \(r_{c}=\beta^{-\frac{1}{\pi}}r_{t}\) would be cancelable by the rx. On the other hand, the other two interfering signals cannot be canceled. We explain the operation of CSNOMA. Let us assume that a typical transmitter/receiver pair wants to communicate with each other in the wireless network of a number of transmitter/receiver pairs. If the wireless channel is sensed as idle during the specific time, such as DIFS in IEEE 802.11 DCF, the typical transmitter goes in the contention period. Then, the typical transmitter takes random backoff and defer its transmission until the backoff counter is expired. When the typical transmitter finally reaches the time to send, the transmitter sends a request-to-send (RTS) signal before the data transmission to reserve a radio channel. If RTS is correctly received, the corresponding receiver sends clear-to-send (CTS) to the transmitter. To enable CSNOMA maintaining the coherent signaling structure of IEEE 802.11, we apply the hierarchical modulation ([42; 43]) on the CTS control message. The neighboring nodes can overhear the RTS/CTS handshaking and may obtain hierarchically modulated additional information or only basic information according to each channel state. If the node obtained additional information it can be judged that the channel gain between the typical receiver is relatively fine because the distance from the typical receiver is close. If the node does not obtain additional information, the CTS frame is decoded, as the usual IEEE 802.11 protocol works and the node will sleep according to the Network Allocation Vector (NAV). Thus, the typical receiver provides a transmission opportunity to the adjacent node that can perform IC well. After this preprocessing, the typical transmitter and the selected adjacent node transmit their data at the same time. The typical receiver extracts the signal of the adjacent node from the aggregate interference through IC, and try  
Figure 3: Transmission allowed/restricted region. When the \(\mathfrak{v}_{1}\) has a chance to transmit, the another transmitter (\(\mathfrak{v}_{2}\)) within the circular region of radius \(r_{c}\) (region \(c\), inside of the inner circle) is also allowed to transmit. The transmitters in the annulus region between \(r_{c}\) and \(r_{s}\) (region b, shaded region in the figure) are still restricted to transmit, and the transmitters farther than \(r_{s}\) (region a, outside of the outer circle) can transmit if the channel is sensed as idle.