# Smart Inverters for Microgrid Applications: A Review
## 2 Smart Inverters
### Plug and Play
#### 2.1.2 Wireless Communication Technologies  
In wireless technologies, one initial and obvious choice can be using the existing cellular network, as it is already available and globally dispersed. However, there are some issues regarding the different nature of M2M communication and H2H communication cellular networks are designed for the latter. Even the latest technology of cellular communication, named Long Term Evolution (LTE), is designed to cope with large data range requirements for only a small number of devices. By contrast, when investigating the characteristics of M2M communication, it can be found out that here the situation is quite different. There is a vast number of devices generating sporadic transitions of short packets, and this can overload or even shut down such a network. Furthermore, the signals and commands in M2M communication may be very sensitive to delays. Therefore, if cellular networks are to be considered as an optimal communication protocol for smart inverters they still need further improvements. As a matter of fact, the 3 GPP became concerned about these issues and is currently aiming to incorporate the requirements of M2M communications in the development of the next generation of LTE or 5G [49; 50; 51; 52]. In addition to cellular networks, there are other wireless communication technologies available. Extremely short-range technologies such as Near-field Communication (NFC) are mostly useless for inverters. Nevertheless, other short-range active radio frequency systems, such as Bluetooth or the IEEE 802.15.4 standard-based family like ZigBee and 6 LoWPAN can be an alternative in some cases [49; 53]. Moreover, low-power wide area networks (LPWANs) offer many of the desired features for M2M communication applications. Among the examples in this category, LoRa technology can be considered for the application to microgrids due to its low implementation and operational cost and relatively long range [49; 54; 55; 56]. A comparison of these different methods is summarized in Table 2.  
For M2M communication applications, the literature review highlighted ZigBee and LPWAN as the standards of choice. Comparing their specifications in Table 2, it can be observed that ZigBee provides a sufficient data rate but at the same time it has a very short range [49]. In contrast, LPWAN technologies demonstrate the opposite behavior. One of the reasons for this is the different network topologies. ZigBee systems are usually implemented following a mesh topology that provides the benefit of fault tolerance but makes the routing process more complex and not energy efficient. Furthurmore, because of the multi-hop nature of mesh network topology the actual data rate might decrease dramatically to the point that it may no longer be suitable for the application of M2M communication [49]. On the other hand, the LPWANs, or specifically LoRa and SIGFOX, networks have star topology with central nodes connecting to the internet. This topology can help with the overall simplicity of the system and less power consumption of each node. However, if a failure happens to one of the principal nodes then other nodes cannot compensate for and keep the network active [53]. Figure 2 is an illustration of the mesh (a) and star (b) topologies described before. In some countries, LPWANs technologies use cellular network bandwidths, and according to the availability and robustness of those, it can be very beneficial. Nevertheless, both short and long-range technologies have the ability to perform in unlicensed bandwidths. This is mostly an economic consideration