*Review* **On-Site and External Energy Harvesting in Underground Wireless**

#### **Usman Raza \* and Abdul Salam**

Department of Computer and Information Technology, Purdue University, West Lafayette, IN 47907, USA; salama@purdue.edu

**\*** Correspondence: uraza@purdue.edu

Received: 22 March 2020; Accepted: 16 April 2020; Published: 22 April 2020

**Abstract:** Energy efficiency is vital for uninterrupted long-term operation of wireless underground communication nodes in the field of decision agriculture. In this paper, energy harvesting and wireless power transfer techniques are discussed with applications in underground wireless communications (UWC). Various external wireless power transfer techniques are explored. Moreover, key energy harvesting technologies are presented that utilize available energy sources in the field such as vibration, solar, and wind. In this regard, the Electromagnetic (EM)- and Magnetic Induction (MI)-based approaches are explained. Furthermore, the vibration-based energy harvesting models are reviewed as well. These energy harvesting approaches lead to design of an efficient wireless underground communication system to power underground nodes for prolonged field operation in decision agriculture.

**Keywords:** soil sensing; decision agriculture; smart farming
