# A GNSS/5G Integrated Three-Dimensional Positioning Scheme Based on D2D Communication
## 1 Introduction  
Position information in mobile networks has become a key feature to enable various location-based services (LBSs) as well as to improve communication performance [1, 2]. Nowadays, with the increasing demand of LBSs, precise and reliable positioning is a topic of high importance and interest, but it is still a challenging task. The global navigation satellite system (GNSS) is widely utilized for positioning since it is capable of providing a continuous and global-covered position, velocity and time (PVT). Particularly, with the construction and development of existing GNSS, such as the U.S. global positioning system (GPS), the Russian GLONASS, the Chinese BeiDou navigation system (BDS) and the European Galileo, the performance of GNSS has been further enhanced in recent years [3, 4, 5, 6]. However, the accuracy of the existing GNSS can hardly satisfy the demand of some LBSs, such as the autonomous and unmanned vehicles in complex environments in the big cities with high buildings. Besides, the satellite signals may be blocked or weakened in urban environments and canyons, resulting in large positioning errors or even failed positioning. Indeed, GNSS is not enough for the positioning in these critical scenarios [7]. Therefore, many other types of positioning methods, including inertial navigation, cellular positioning, and so on, are integrated with GNSS to overcome the shortages of it [8, 9]. Actually, the fifth generation (5G) communication is featured with many new technologies, such as ultra-dense network, millimeter wave (mmWave) communication, edgecomputing and device-to-device (D2D) communication, which provide high-accuracy ranging and angle measurements with a high network density, resulting in more accurate positioning [10; 11; 12]. In practical terms, the combination of GNSS and 5G is important for seamless positioning in critical environments. Usually, 5G positioning is just a supplement to GNSS, and especially because 5G positioning may be conducted when GNSS is unavailable. Besides, GNSS is integrated into many mobile terminals due to the demand for accurate positioning [13]. In this regard, it has attracted much attention of the researchers to combine GNSS and 5G for positioning [14; 15]. [PERSON] et al. in [16] proposed a novel positioning solution, combining mmWave and GNSS to improve the positioning accuracy, which is in need of less connected satellites. The authors in [10] integrated GNSS and 5G for positioning by exploiting the satellites as the base stations (BSs) and exploring appropriate positioning methods in different scenarios. In particular, D2D communication enables two mobile users (MUs) in proximity to communicate directly without being relayed by the BS, which is regarded as an effective way to increase the signal transmission rate and to improve resource efficiency and local service flexibility [17; 18]. Accordingly, D2D communication can be expected to connect more MUs directly, as well as to obtain more accurate measurement information in positioning, especially with the densely-deployed MUs in the 5G network. In this regard, D2D communication is capable of implementing cooperative positioning. Cooperative positioning requires direct transmission and reception of the signals among the MUs, through which the position information, measurements and statistical data are jointly shared and processed [19; 20]. Based on the exchanged information among the nodes, each node can obtain the relative position information to the others and transform it into an absolute one with the reference position information. [PERSON] et al. in [19] analyzed the Cramer-Rao lower bound (CRLB) of D2D cooperative positioning, setting a benchmark of the positioning performance in an urban scenario. Inspired by this, GNSS and D2D communication can be perfectly integrated in a cooperative manner to provide the absolute and relative positions simultaneously for the MUs in the 5G network.