# Full-Duplex MAC Protocol for CSMA/CA-Based Single-Hop Wireless Networks
## 1 Introduction  
Recently, with the progress in self-interference (SI) cancellation techniques, the desired reception signals of wireless nodes are no longer overwhelmed by the interference signals produced by the nodes themselves [1]. They can simultaneously transmit and receive using the same frequency band. In the physical layer, the throughput of a node working in the full-duplex (FD) mode is twice that in the half-duplex (HD) mode. It is clear that the FD mode provides new opportunities for improving the throughputs. It has been noted that the nodes generally share the communication medium and work in a distributed manner. The traditional carrier sensing multiple access/collision avoidance (CSMA/CA) protocol has proven to be an effective media access control (MAC) protocol for the nodes to contend for the channel. Intuitively, for backward compatibility, we should apply it to the FD networks. However, FD communication means that the number of senders increases from one to two within a certain range. Originally, we only need to determine the transmission timing of one node, but now, we have to arrange two nodes to send frames simultaneously. Before sending a frame, the protocol requires that each node should sense the channel first. If it is busy, then the node cannot send the frame. In other words, when a node transmits the signal, it does not allow its neighbors to access the channel (one of them may be its FD peer). Thus, the protocol cannot adapt to the FD communication directly. Based on the CSMA/CA protocol [2], for the single-hop network model, the following studies have proposed some effective FD MAC protocols. The protocols proposed in [3; 4] used the FD feature to increase their system throughputs. They extended the analytical model proposed in [2], and analyzed the collision probability of the FD node. Moreover, they fully considered the detection errors caused by the SI signal. In the HD networks, if more than one node sends frames, then the collision must occur. However, in the FD networks, when two nodes send frames simultaneously, the collision may not occur. For example, if they send the frames for each other, then both frames can be received correctly. Unfortunately, the collision probabilities solved in these protocols do not consider the actual communication mode. More importantly, they did not provide any mechanisms to coordinate the nodes to work in the FD modes. The authors in [5] extended the structures and semantics of the control frames Request-to-Send/Clear-to-Send (RTS/CTS). In addition to avoiding the transmission interferences, they can also initiate the FD communication between the nodes. However, at an idle time slot, the authors specified the transmission probability of each node and ignored that the new state transition caused by the FD communication may change its value. The protocol proposed in [6] has a similar problem. In this paper, for this network model, we proposed an FD MAC protocol for the network layer to fully benefit from the FD mode. By using the FD features of the nodes and the cut-through mechanism, we reduced the channel access time wasted by the collisions and increased the FD opportunities in the network. The main contributions of the protocol are as follows:  
First, we summarized the three scenarios where the frames are sent by the nodes in the network. We further discussed the protocol design requirements for each scenario. Second, we provided the specific working process description of the protocol. After reading the information of the received frame's header, the node can determine whether collisions exist and whether it can communicate with other nodes in the FD mode. Third, we formulated the analytical model of the protocol. With reference to [2; 7], we modeled the states (\"active\" and \"passive\" transmission, back-off) of each node and their transitions to a Markov chain. We solved the \"active\" transmission probability of the node and further solved the normalized throughput of the network.