Device to Device (D2D) communication was first considered in out-band to manage energy issues in the wireless sensor networks. The primary target was to secure information about system topology for successive communication. Now the D2D communication has been legitimated in in-band by the 3rd Generation Partnership Project (3GPP). To initiate D2D communication, Device Discovery (DD) is a primary task and every D2D application benefits from DD as an end to end link maintenance and data relay when the direct path is obstructed. The DD is facing new difficulties because of the mobility of the devices over static systems, and the mobility makes it more challenging for D2D communication. For in-band D2D, DD in a single cell and multi-cell, and dense area is not legitimated properly, causing latency, inaccuracy, and energy consumption. Among extensive studies on limiting energy consumption and latency, DD is one of the essential parts concentrating on access and communication. In this paper, a comprehensive survey on DD challenges, for example single cell/multi-cell and dense area DD, energy consumption during discovery, discovery delay, and discovery security, etc., has been presented to accomplish an effective paradigm of D2D networks. In order to undertake the device (user) needs, an architecture has been projected, which promises to overwhelm the various implementation challenges of DD. The paper mainly focuses on DD taxonomy and classification with an emphasis on discovery procedures and algorithms, a summary of advances and issues, and ways for potential enhancements. For ensuring a secure DD and D2D, auspicious research directions have been proposed, based on taxonomy.

Device-to-Device (D2D) communications have recently attracted researchers, attention because of their numerous applications in industry verticals. It enables communications among devices without or with the partial involvement of a central system. To initiate a D2D communications device discovery and radio resource allocation is a critical task when devices have high mobility. Maintaining the quality-of-service and continuous connectivity requires a signaling burden. An efficient mobility management procedure is necessary to discover the neighboring devices in D2D communications systems. The Discovery of a massive number of devices requires an effective radio resource management procedure that causes signaling overhead. In 5G and beyond communication system, two mobility management methods exist; device discovery and beaconing. Since device density and traffic increases exponentially with high mobility, hence device discovery and beaconing increase the signaling overhead and energy consumption in power-limited devices. Thus, signaling overhead research needs much attention in 5G and beyond systems to meet the service requirements like accuracy, latency, and battery life. Therefore, the challenges and the techniques related to signaling overhead in D2D communications are presented.

Device to Device (D2D) communications is a candidate technology for the fifth-generation (5G) and beyond mobile networks and certainly that results in high throughput, less energy consumption, reduce delay, and data traffic offload. Proximity services are the key enablers of D2D communications. A D2D technology boosts the performance and capacity of a conventional cellular system through the proximity services. To initiate the proximity services, Device Discovery (DD) is one of the the primary tasks. A DD makes the decision for effective D2D communications in terms of accuracy, speed, and minimum energy consumption. To discover the neighbor devices, the discovery signal is transmitted directly or through some access points. The discovery signal is affected by invaders during transmission which causes inaccuracy, energy consumption, and latency. Therefore, security and privacy issues must be addressed, especially in discovery signal transmission. In this paper, security and privacy issues in DD are highlighted. It is comprehensive and proves that in-band is much better than out-band with practical and technological reasons. To enhance the scope of the research, network level, and system level Security and Privacy (S&P) issues in the distributed and centralized systems environment with or without central management are surveyed. Along with an extensive survey is provided for the most recent work on DD concerning security and privacy issues, and comparison among in-band and out-band DD is performed. In the end, open issues are identified as future work on DD security and privacy in D2D communications. It is a novel survey in terms of security and privacy aspects of DD with possible suggested solutions for readers' motivation.

In the fifth generation (5G), it is anticipated that device-to-device (D2D) operation will be locally incorporated as a part without any bounds. In D2D network, multiple devices coexisting is a challenging subject of device discovery. The device discovery is performed under a visually impaired situation such as channel information, location, and the number of devices. In this paper, centralized device discovery is chosen due to power consumption and signaling overhead of the distributed system. A distinctive approach for device discovery in an in-band cellular network, based on the device’s power, is suggested with an efficient technique which enhances the implementation of D2D communication and improves the accomplishment by alleviating the discovery issues. The group of devices forms a lattice structure, and it is positioned in the coverage area. The hypersphere is constructed based on the power knowledge of a discoverer device which helps for accurate and fast device discovery in a lattice structure. Besides, sphere decoder like (SDL) algorithm is applied for quick and precise discovery in the lattice structure. Simulation results present the performance of the proposed QR factorized lattice structure scheme regarding device power, enhanced in the number of discovered devices and controlled signaling overhead.

The narrow bandwidth and low gain performances of a reflectarray are generally improved at the cost of high design complexity, which is not a good sign for high-frequency operation. A dual resonance asymmetric patch reflectarray antenna with a single layer is proposed in this work for 5G communication at 26 GHz. The asymmetric patch is developed from a square patch by tilting its one vertical side by a carefully optimized inclination angle. A progressive phase range of 650° is acquired by embedding a circular ring slot in the ground plane of the proposed element for gain improvement. A 332-element, center feed reflectarray is designed and tested, where its high cross polarization is suppressed by mirroring the orientation of asymmetric patches on its surface. The asymmetric patch reflectarray offers a 3 dB gain bandwidth of 3 GHz, which is 4.6% wider than the square patch reflectarray. A maximum measured gain of 24.4 dB has been achieved with an additional feature of dual linear polarization. Simple design with wide bandwidth and high-gain of asymmetric patch reflectarray make it suitable to be used in 5G communications at high frequencies.