Device-to-device (D2D) communication is one of the most promising technologies in wireless cellular networks that can be employed to improve spectral and energy efficiency, increase data rates, and reduce links latency. This paper investigates fifth generation and beyond (5GB) networks, the basics of D2D communication, applications, and classification. Herein, D2D in in-band (IBD) and out-band (OBD) modes are discussed. This paper also presents the integration of D2D communication with other prominent technologies and demonstrates the importance of integration with possible solutions in improving network performance. We further investigate the challenges of D2D communication, opportunities, and future research directions of D2D in 5GB networks. In addition, D2D communication in 6G network challenges and open research areas are introduced.

The recent advancement in the wireless technology has led to the advent of wearable antennas. These antennas are utilized for Wireless Body Area Networks (WBANs) purposes such as health-care, military sportive activities and identification systems. Compared to conventional antennas, wearable antennas operate in an environment which is in highly near proximity to the curved human body. Therefore, the wearable/flexible antenna performance parameters, including reflection coefficient, bandwidth, directivity, gain, radiation characteristic, Specific Absorption Rate (SAR), and efficiency are anticipated to be influenced by the coupling and absorption by the human body tissues. In addition, an electromagnetic band-gap structure is introduced in the wearable/flexible antenna designs to enable and give a high degree of isolation from the human body which also decreases the SAR dramatically. This paper reviews the state-of-the-art wearable/flexible antennas integrated with the electromagnetic band-gap structure on flexible materials concentrating on single and dual-band designs. Besides, it also highlights the challenges and considerations for an appropriate wearable/flexible antenna.

A compact wearable symmetrical e-slots antenna operated at 2.4 GHz was proposed for Medical Body Area Network applications. The design was printed onto a highly flexible fabric material. The final design topology was achieved by the integration of symmetrical e-slots antenna with an Electromagnetic Band-Gap (EBG) and Defected Ground Structure (DGS). The use of EBG was to isolate the body and antenna from each other whereas the DGS widened the bandwidth. This combination forms a novel and compact structure that broadens bandwidth. This broadened bandwidth makes the structure robust to deformation and loading in the human body. The design achieved a measured impedance bandwidth of 32.08 %, a gain of 6.45 dBi, a Front to Back Ration (FBR) of 15.8 dB, an efficiency of 72.3% and a SAR reduction of more than 90%. Hence, the integration of symmetrical e-slots antenna with EBG and etched DGS is a promising candidate for body-worn devices.

A compact and robust fabric antenna incorporated with metamaterials (MTM) at 2.4 GHz is introduced for wearable devices application where the MTM behaves as EBG/AMC. The benefit of introducing MTM in a wearable antenna is to diminish the influence of frequency detuning and reduces the backward radiation specifically when loaded on the human body. The overall size of the presented antenna incorporated with MTM is 60 × 60 × 2.4 mm3. Furthermore, the integrated design has the capability of controlling Specific absorption rate (SAR) and improved the bandwidth, front‐to‐back ratio (FBR), and gain up to 14.5%, 13.7dB, and 7.5dBi, respectively. The operations under different bending diameters on real and modelled human body are studied. Compared with conventional antennas, MTM‐inspired antennas reduce the SAR to safe levels of more than 90%. The presented integrated design can be a good candidate for incorporation into a variety of flexible systems for medical application.

The compact and robust high-impedance surface (HIS) integrated with the antenna is designed to operate at a frequency of 2.45 GHz for wearable applications. They are made of highly flexible fabric material. The overall size is $45\times \,\,45\times 2.4$ mm ³ which equivalent to $0.37\lambda \text{o}\times 0.37\lambda \text{o}\times 0.02$ mm ³ . The value of using HIS lies in protecting the human body from harmful radiation and maintaining the performance of the antenna, which may be affected by the high conductivity of the human body. Besides, setting the antenna on the human body by itself detunes the frequency, but by adding HIS, it becomes robust and efficient for body loading and deformation. Integrated antenna with HIS demonstrates excellent performance, such as a gain of 7.47 dBi, efficiency of 71.8% and FBR of 10.8 dB. It also reduces the SAR below safety limits. The reduction is more than 95%. Therefore, the presented design was considered suitable for wearable applications. Further study was also performed to show the useful of placing antenna over HIS compared to the use of perfect electric conductor (PEC). The integrated design was also investigated with the worst case of varying the permittivity of body equivalent model which shows excellent performance in term of reflection coefficient and SAR levels. Hence, the integrated antenna with HIS is mechanically robust to human body tissue loading, and it is highly appropriate for body-worn applications.

Recent advancements in wireless technology have prompted the development of wearable antennas, particularly for Wireless Body Area Networks (WBANs) used in sports, military activities, healthcare, and identification systems. Unlike traditional antennas, wearable antennas operate very close to the curved human body. Consequently, their performance metrics such as reflection coefficient, bandwidth, radiation characteristics, efficiency, gain, directivity, and Specific Absorption Rate (SAR) can be significantly affected by interactions with human body tissues. To address these challenges, periodic structures are integrated into wearable antenna designs to mitigate the effects of coupling and absorption by the human body. To date, a limited number of works have surveyed, compared, and categorized the proposed wearable antenna designs integrated with periodic structure based on their operating frequency bands and their interactions with the human body. To this end, this paper contributes to the literature by detailing the selection of materials, highlighting the limitations of wearable antennas, characterizing the periodic structures, elaborate on dielectric properties data of human body models, discussing the influence of periodic structures on antenna performance and their significance in isolating them from the human body. Furthermore, this study provides an in-depth review of 81 papers evaluating the performance of antennas integrated with periodic structures, both in free space and on the human body, across single-band, dual-band, and triple-band designs. The selected papers were chosen based on their publication in reputable journals, the quality of their research, and the comprehensiveness of their data analysis. In addition, it provides a summary of several common flexible materials used in wearable antennas with their electrical properties as well as the dielectric properties of the human body for different frequencies. Moreover, it discusses the challenges and considerations involved in designing appropriate wearable antennas.