# Communication Protocol Design for IoT-Enabled Energy Management in a Smart Microgrid
Moreover, this can lead to a trade-off between energy consumption and reliability depending on the number of retransmissions [34]. An alternative strategy could be the use of precoding methods to maximise the transmission power of the legitimate users [38]. LoRa-based transmissions have been used in [39] to design the communication between smart meters and microcontrollers for a partially grid-connected microgrid to demonstrate the suitability of LoRa technologies in microgrid applications. The use of the unlicensed spectrum in LoRa and Sigfox technologies can expose the smart grid communication system to security threats and additional security mechanisms may need to be incorporated before their application in the energy system context. The NB-IoT protocol utilises the LTE band or re-farmed GSM band, though it adopts repetition mechanism to enhance coverage with low complexity [40]. To reduce collision at the preamble, an access reservation protocol is proposed in [41] that is based on partial preamble transmission. The authors in [42] have investigated the uplink coverage for NB-IoT and obtained that coverage significantly depends on the channel estimation and coherence time. To realise the actual characteristics of NB-IoT systems, the authors have developed a simulation model for NB-IoT uplink and downlink [43]. Regarding smart grid applications, the authors in [44] have examined different IoT protocols for different smart grid applications and evaluated their latency. To improve the communication efficiency for IoT-enabled demand management applications, the authors in [45] have adopted an adaptive data compression mechanism and reduced the information flow in the smart metering communication infrastructure. Existing IoT communication technologies such as NB-IoT and LoRa have not incorporated multicast or inter-terminal communication yet. Usually, these protocols allow user equipments or IoT terminals to send messages only to the IoT base stations or gateways using separate time slots [31]. Thus, multiple IoT terminals are not allowed to transmit at the same time. However, allowing for the direct communication between IoT terminals can be useful in applications like decentralised energy management. For local energy trading operations, HEMSs can act like IoT terminals and the control centre, equipped with central EMS, can act like IoT gateways. If the generation/demand information is available only at the central EMS for optimising energy sharing among participants in a local energy market, the energy trading will be adversely affected in the case of a single point of failure at the control centre. Thus, decentralised energy management is often recommended by researchers to enhance the effectiveness and reliability of energy trading operations [46; 47; 48]. For accommodating the specific requirements for smart microgrid applications like local energy trading in a decentralised manner, a new communication protocol needs to be added to the existing IoT standards, which can support direct communication between IoT-enabled HEMSs. To address this open research problem, in this paper, we propose a communication protocol for including the feature of direct communication between IoT-enabled HEMSs. The protocol utilises the intermediate HEMSs as relay nodes which can forward the sum of the signals received from nearby HEMSs to the other HEMSs, and thus enable HEMSs to extract each other's messages without going through the central controller. The most beneficial attribute of this scheme is that it allows information exchange in a smart microgrid with a lower latency compared to the cases where decode\(-\)and\(-\)forward (DF) is employed directly or where the messages are exchanged through the central controller. To the best of our knowledge, direct communication between IoT terminals for decentralised energy management in a smart microgrid has not been addressed in the existing literature.