# Energy Harvesting Techniques for Wireless Sensor Networks/Radio-Frequency Identification: A Review
## 1 Introduction
### Radio Optimisation Techniques  
The radio unit is the most pronounced energy consumption unit in the WSN. The energy depletion by the radio unit is caused by two parts: (i) powering the circuit, and (ii) powering of the transmitted signal. Short distances utilize more energy in powering the circuit, while powering the transmitted signal in long range communication consumes more power. Several references [22; 48] have investigated the strategies to enhance EE by adjusting transmission power level dynamically. In addition, the authors in [21] proposed advance saving energy cooperative topology, in which sensor nodes with higher remaining energy is at liberty to increase transmitting power leading to other nodes to decrease their own transmitting power. Moreover, the proposed topology reduces interference and improves connectivity due to the decrease in transmission power. However, an increase in delay potentials is expected, because more hops will be required for packet forwarding. Nevertheless, the problem of delay can be overcome through cooperative communications among the neighbouring sensor nodes,  
Figure 2: Summarised taxonomy of energy-saving techniques for green WSNs and RFID. which creates a virtual multiple-antenna environment (spatial diversity). Virtual multiple-antenna reduces data retransmission, effectively improving the quality of the received signal by overcoming multi-path fading and shadowing phenomenon. References [49] and [50] extended the communication range among the sensor nodes as well as higher energy conservation and lower end-to-end (E-2-E) delays over certain broadcasting coverage as reported by [PERSON] et al. [23]. [PERSON] [24], compared the energy consumption of both Single Input Single Output and virtual multiple-antenna (Multiple Input and Multiple Output) systems and showed that virtual multiple-antenna systems can provide higher energy savings and minimize E-2-E delays over certain propagation range distances. On the other hand, [PERSON] et al. [25] examined the relationship among the energy consumption, transmission time, and bit error rate. The results showed that optimising the transmission time could have minimised the energy utilization needed to attain a stated bit error rate as well as delay requirement. Moreover, the authors in [26] presented a comparative study on the EE of three modulation techniques to select the optimal modulation scheme that yields the lowest energy utilization with various distances between nodes. To address the problem of joint transmission power and rate allocation in the uplink of a cellular wireless network, several works have considered transmission power and rate allocation. The two basic approaches that have been proposed in the recent literature are explained below. * Joint rate and power control are modelled as two distinct games (i.e., an uplink transmission rate allocation and an uplink transmission power allocation problem), which are based on the game theoretic perspective. Users determine first their uplink transmission rate and then given their uplink transmission rate, they apply power control to allocate their uplink transmission powers [51; 52]. The main drawback of this approach is that the optimisation problem is solved asynchronously and separately considering the two systems' resources. Thus, the combined outcome of the two distinct optimisation problems is less efficient than jointly solving the problem [53]. * The joint rate and power control problem is amended in a single-variable problem of the ratio of uplink transmission rate to the uplink transmission power [54]. However, this approach is limited in realistic cases and can only be applied in specific studies where simplified forms of utility functions are assumed (i.e., where the ratio of uplink transmission rate to power appears). As a result, the use of this approach strongly depends on problem formulation. The single variable problem is solved with respect to the substituted ratio.