# CSNOMA: Carrier Sense Non-Orthogonal Multiple Access
## 1 Introduction  
With growing demands for wireless connectivity, a multiple access control (MAC) scheme supporting a large number of networked devices becomes increasingly crucial. Cisco predicts that the number of wireless networked devices will be 13.1 billion in 2023 [1]. In line with this trend, it is expected that new kinds of networks will arise. For example, Tactile Internet requires ultra-low delays and extremely high availability and reliability to enable transmission of tactile information [2]. The stringent requirements of Tactile Internet will drive to innovation of MAC design. The radio resources can be used by centralized (scheduling) or distributed MAC (random access). Because of its complexity, the latter is more suitable for an ultra-dense environment. There are two representative random access schemes: ALOHA and carrier sense multiple access (CSMA). To measure the performance of MAC with emphasis on the number of supporting users, an area spectral efficiency (ASE) is widely adopted [3]. ASE is the product of the successfully transmitting user density and the data rate per node. In the ALOHA system, to maximize ASE, the transmission probability is optimized [4]. On the other hand, the carrier sensing threshold is optimized in the CSMA system [5]. A multiuser interference is an avoiding factor for both schemes. \"When a station is hidden from either the transmitting or the receiving station, by detecting just one frame among the RTS and CTS frames, it can suitably delay further transmission, and thus avoid collision.\" The quote is in the seminal work by [PERSON] [6]. For the multiple access control (MAC) protocol, designers and researchers have widely accepted the idea of avoiding interference. Breaking away from the notion of a traditional thinking, we propose a new random access protocol, Carrier Sense Non-Orthogonal Multiple Access (CSNOMA). It is inspired by Non-Orthogonal Multiple Access (NOMA) [7], which has recently been in the spotlight in cellular communication [8, 9]. We sketch out the operation of CSNOMA briefly. Let us assume that a transmitter/receiver pair wants to communicate with each other in the wireless network. The transmitter sends a signal before the data transmission to reserve a radio channel. If the signal is correctly received, the corresponding receiver sends a response to the transmitter. We apply the hierarchical modulation on the response. If a node, among the neighbor nodes who overhear the signal handshaking of the communication pair,can decode the hierarchical modulation, additional information can be obtained. This node decides whether or not to transmit based on the additional information. Otherwise, the node should shutdown if the node just decodes the original response. In this way, the receiver gives a transmission chance to the neighbor node who shows the best channel gain. After this preprocessing, the transmitter and the selected neighbor node transmit their data simultaneously. The receiver carries out interference cancellation and detection. If the transmission succeeds, the receiver sends ACK to the transmitter. The wireless sensor network (WSN) is a representative field where distributed MAC is used. Because WSN uses devices with small transmission power, the transmission distance is often short. In order to increase the distance of information transmission, many routing protocols have been studied, and the MAC researchers have mainly focused on the combination of the MAC and the network layer, and thus more energy saving techniques have been studied [10; 11]. The distributed MAC protocol can be designed under the idea of utilizing interference to improve ASE. There are reception/transmission techniques utilizing interference in the physical (PHY) layer, such as interference cancellation (IC) [12], interference alignment (IA) [13], dirty paper coding (DPC) [14], analog network coding (ANC) [15], physical network coding (PNC) [16], full-duplex radio [17; 18; 19], etc.