# Trellis Tone Modulation Multiple-Access for Peer Discovery in D2D Networks
###### Abstract  
In this paper, a new non-orthogonal multiple-access scheme, trellis tone modulation multiple-access (TTMMA), is proposed for peer discovery of distributed device-to-device (D2D) communication. The range and capacity of discovery are important performance metrics in peer discovery. The proposed trellis tone modulation uses single-tone transmission and achieves a long discovery range due to its low Peak-to-Average Power Ratio (PAPR). The TTMMA also exploits non-orthogonal resource assignment to increase the discovery capacity. For the multi-user detection of superposed multiple-access signals, a message-passing algorithm with supplementary schemes are proposed. With TTMMA and its message-passing demodulation, approximately 1.5 times the number of devices are discovered compared to the conventional frequency division multiple-access (FDMA)-based discovery. t 10 April 20181  
networks (WSNs) are integrated with 5G networks [19] and can be applied to various sensor network applications [20]. D2D communication can be controlled by a BS, but it can also be operated in a distributed manner when only a resource region is allocated by a BS [21]. On certain occasions, such as critical use in natural disasters, D2D communication needs to operate out-of-coverage [22; 23]. In these cases, the D2D network becomes like a distributed network. In this paper, we discuss the distributed D2D operation rather than the BS controlled D2D operation. One of most important features of D2D communication is peer discovery. Since the main advantage of D2D communication is the proximity service, a device needs to know the presence of other peers in its proximity. Peer discovery is a process used for identifying neighbor MSs, which can directly communicate with each other and are essential for constructing D2D communication. The general procedure for peer discovery of purely distributed networks is as follows. First, each MS broadcasts discovery message necessary for discovering the network address, interest, etc., and informs neighboring MSs of its existence. At the same time, each MS receives a signal broadcasted by the other MS and identifies the discovery message of the neighbor MS. In this way, the peer discovery constitutes a mesh network, as each MS transmits a discovery signal to all other MSs and receives discovery signals from all other MSs. In addition, since all MSs share a given resource for discovery, this can be regarded as a multiple-access process. A conventional method of the peer discovery process is based on a combination of contention-based multiple-access and frequency division multiple-access [24; 25]. All MSs in a network share a given set of resources and each MS transmits a discovery signal using only part of the resources. In the orthogonal frequency division multiplexing (OFDM) physical layer environment, resources are divided into time and frequency and each MS selects one of the time-frequency resources. This is a form of frequency division multiple-access. However, in the D2D peer discovery, there is no central controller such as a BS that allocates resources exclusively to each MS. Therefore, each MS uses various contention-based multiple-access techniques such as collision avoidance and collision detection to select the resources. In the D2D communication in the distributed environment, if the density of a network increases, the orthogonal multiple-access (OMA)-based peer discovery method will not function well. In this situation, the performance can be improved by multi-user detection. The multi-user detection over a shared resource can be classified as a non-orthogonal multiple access (NOMA) technology. However, it is not straightforward to apply existing NOMA techniques to the discovery problem because those schemes assume that a distinct piece of information is allocated to each device and the receiver has the knowledge of the information.