## 5 Progress of Data Transmission Protocols for Wireless Communication in Solar PV Monitoring Systems
### LoRa-Based Module  
Semtech [120] introduced a spread spectrum modulation technique known as LoRa (long-range) derived from Chirp Spread Spectrum (CSS). LoRa is a low-power wide-area network (LPWAN) technology that is specifically designed for IoT [121; 122]. In recent years, LoRa has gained significant attention amongst both industrial and research communities [123]. This method aims at being usable in long-lived battery-powered devices, where energy consumption is of paramount importance [124]. A typical LoRa network is a star-of-stars topology, which includes three different levels of devices, as illustrated in Figure 14 [116]. Figure 14: Star topology based on LoRa Network Architecture. [PERSON] et al. [125] proposed a LoRa-based solar PV monitoring system. A LoRa module was selected for its long-range and low power characteristics. Different parameters such as module voltage, current, backside temperature, ambient temperature, and irradiance were measured. The test was conducted using a 250 Wp monocrystalline PV module and measurements were recorded accordingly. The results demonstrated that a range of 9.27 km was achieved with a Spreading Factor (SF) of 12 and a bandwidth of 125 KHz. The concept of SF dominates in LoRa based technology as it works on chirp (bits per second) protocol requiring a fixed amplitude and modulation for the transmission of data. Lower SF relates to the transmission of more chirp per second and vice versa. A graph was plotted showing the elevation profile between the transmitter node A and the receiver node B as displayed in Figure 15[125]. The elevation profile suggests that the successful transmission and measurement of data packets is achievable at 10 km from the sensor node to the receiver node even without the line of sight. Although an extended range was achieved in transmitting the data from one point to another, the work could be extended for a large solar PV system. In addition, SF should be carefully selected to transmit the data with a high transmission rate. [PERSON] et al. [126] designed a monitoring system for renewable energy (solar and wind) using LoRa technology. The LoRa network used a sub 1 GHz frequency for long-distance data transmission. A low-powered LoRa network was implemented by applying an end-to-end modem without using a base station. The MongoDB database was employed to store a large amount of data received from the LoRa modem and a web server was designed using the JavaScript, PHP, apache, and CSS languages. The proposed system could be evaluated based on the efficiency of the solar PV plant and optimization could also be performed. [PERSON] et al. [106] proposed a low-cost LoRa-based solar PV monitoring system that communicated with solar photovoltaics plants located in remote locations. The proposed topology was designed using a 5 kW solar panel. The recorded data were stored in a packet size of 38 bytes with a transmission power of 14 dBm and SF metric ranging from 10 to 12. The presented work could be extended by observing the fault in the panel, array, or string by the utilization of a fault detection mechanism. LoRa is regarded as having low power and a long-range transmission of data packets. Nevertheless, it has some operational restrictions, such as network size, which is limited by duty cycle [127]. Although the performance of Low Range Wide Area Network (Lora WAN) is determined by the Physical Layer (PHY)/ Media access Control (MAC) layer which identifies the devices connected in the network, the duty-cycle regulations in the industrial, scientific, and medical (ISM) frequency bands turn out to be key limiting factors [128]. Figure 15: Elevation profile for data transmission for a LORA-based module. There is an increase in packet loss when the number of end nodes in the network is increased [129]. Moreover, large SFs are required for longer communication but time on air and off period duration also increase in proportion with SFs.