TY - JOUR AU - Wang,, Lei AB - Abstract Building Energy Internet of Things could collect and analyse various types of building energy consumption data in real time by means of low-energy consumption and high-precision sensing technology. In this paper, a low-energy consumption data transmission and fusion algorithm SMART-RR (Slice Mix Agg RegaTe-Repeatablibity Reduction) is proposed. Taking advantage of the periodic repeatability and data redundancy of building energy consumption data, a data fusion strategy with unequal long time intervals and adding repeatability reduction factor is proposed. The simulation results show that SMART-RR algorithm is a low-energy data transmission and fusion algorithm with small data traffic, high privacy protection and high accuracy. 1 INTRODUCTION Building energy consumption monitoring is one of the research hotspots in the field of modern urban energy management. Building energy consumption monitoring system is used for real-time monitoring of building energy consumption, including electricity consumption, heat consumption, water consumption and other modes of energy consumption [1]. It can effectively reflect the building energy consumption, and provide data basis for urban energy management and energy control [2]. The traditional energy consumption monitoring data acquisition and transmission mode relying on industrial bus and industrial Ethernet is limited by wired network, and is restricted in wiring complexity, system scalability and maintainability [3]. In recent years, the concept of Building Energy Internet of Things (BEIoT) has been proposed and attracted the attention of relevant scholars [4]. BEIoT relies on the theory and technology of wireless sensor network (WSN) and building energy management to lay the wireless sensors in or around the target buildings that need to be monitored and form a multi-layer architecture including data acquisition layer, data transmission layer and data analysis and control layer by means of sensor autonomous networking [5]. BEIoT collects energy consumption data of buildings and surrounding environment through various types of sensors, transmits them through wireless network and collects them to the central control node for data analysis, which provides a basis for energy consumption analysis and energy equipment control. Building energy consumption data acquisition system based on BEIoT can collect multi-modal building energy consumption data in real time, which provides a means for fast and accurate energy consumption analysis and energy control. Researchers have conducted in-depth research on BEIoT data fusion algorithm [6–8]. The building energy consumption data in BEIoT are uploaded and fused step by step along the wireless communication network. These data contain a lot of privacy information related to building attributes and energy consumption. Once the nodes in the sensor network are maliciously invaded, the sensitive privacy information will be monitored, which will lead to abnormal building energy consumption data fusion and result in data security disaster of building energy management and user trust in building energy consumption data monitoring based on BEIoT. In order to improve the security of WSN data transmission and fusion, researchers have proposed a variety of encryption algorithms, such as key distribution algorithm, homomorphic encryption algorithm, etc. [9, 10]. Key encryption algorithm protects sensor nodes in WSN by pre-distributing key to prevent sensitive information from being leaked during network transmission. However, due to frequent key distribution and decryption operations in data fusion process, data fusion efficiency is often reduced and energy consumption of sensor nodes is increased. In order to improve the efficiency of data fusion, a WSN-oriented homomorphic encryption algorithm is proposed, which performs fusion computation based on encrypted ciphertext [11]. However, the privacy protection of this method is weakened. If any nodes in sensor networks are maliciously intruded, the privacy data in adjacent nodes may not be effectively protected. Madden et al. [12] presented the Tiny AGgregation (TAG) service for aggregation in low-power, distributed, wireless environments, which allowed users to express simple, declarative queries and have them distributed and executed efficiently in networks of low-power, wireless sensors. Cluster-based Private Data Aggregation and Slice Mix Agg RegaTe (SMART) are proposed based on the key distribution algorithm [13, 14]. After collecting data, sensor nodes first perform fragmentation operations and transmit them to different nodes. In this way, the privacy protection of sensitive data is enhanced. Even if some nodes in the network are intruded, they could also be clearly resolved to complete and accurate data. After analysing the shortcomings of the SMART algorithm that the accuracy of data fusion decreases due to the fragmentation of the transmitted data, a series of improved algorithms with multiple optimization factors are proposed [15]. The performance of the proposed algorithm in privacy protection, accuracy and fragmentation reception rate is verified by comparison experiments and analysis. Because the building energy consumption data collected by BEIoT often have multi-modal characteristics, data fragmentation strategy for multi-modal health data and data fusion algorithm based on unequal time interval data can be further studied on the basis of existing privacy protection algorithms, so as to improve the privacy protection of health data and reduce the network traffic of BEIoT [16]. Aiming at the security, low power consumption and accuracy requirement of data transmission and fusion for building energy consumption monitoring, this paper introduces local optimization factor based on SMART and related algorithms, and proposes a low-power data transmission and fusion algorithm, which can improve the accuracy of building energy consumption monitoring data on the premise of reducing the redundancy of sensor node data and the traffic of sensor network in BEIoT. Section 2 discusses the building energy consumption monitoring system based on WSN, and introduces the topological structure of the building energy consumption monitoring system and low-energy consumption sensor nodes. Section 3 proposes a data fusion model based on multi-hop self-organizing network and a data fusion algorithm with optimization factor from the defect analysis of the application of SMART algorithm in BEIoT. Section 4 analyses and evaluates the performance of the algorithm in terms of data traffic, data accuracy and privacy protection through simulation experiments. Finally, Section 5 proposes a brief summary and looks forward to the future work. 2 BUILDING ENERGY CONSUMPTION DATA MONITORING BEIoT locates many types of low-power and high-precision sensors in and around the monitored buildings. With the help of wireless sensor technology, it forms a WSN structure that can collect and transmit real-time information of building energy consumption, and supports building energy consumption monitoring and data analysis. This section first introduces the topology of building energy consumption monitoring system based on WSN, and then describes the low-energy consumption sensor nodes for building energy consumption data acquisition and transmission. 2.1 Topology of BEIoT The topological structure of building energy consumption data monitoring system based on WSN is shown in Figure 1, which is realized by networking of a variety of data acquisition sensor nodes, data transmission nodes and data analysis nodes inside and around buildings. Figure 1 Open in new tabDownload slide Structural sketch of building energy consumption monitoring system based on IoT. Figure 1 Open in new tabDownload slide Structural sketch of building energy consumption monitoring system based on IoT. Multi-type data acquisition sensor nodes located inside and around buildings acquire multi-modal energy consumption and environmental data such as electricity consumption, water consumption, geothermal information, temperature information, humidity information of the monitored buildings, and transmit them periodically to data transmission nodes in the cloud through wireless communication network. After collecting multi-modal building energy consumption and environmental data, the data transmission node carries out preliminary noise filtering, and carries out data fusion calculation at the first level according to the data type, and uploads the calculation results to the data fusion and analysis node. The data fusion and analysis node obtains the preliminary processed building energy consumption information, carries on the final data fusion and calculation, obtains the results of building energy consumption data analysis and environmental assessment indicators and assists municipal administrators and engineering technicians in building energy consumption monitoring, energy consumption evaluation and environmental assessment. 2.2 Sensor nodes for building energy consumption monitoring In the practical application of wireless data acquisition and transmission in the field of building energy consumption monitoring and environmental monitoring, low-energy consumption and high-precision wireless sensor nodes are often used to locate and collect data. Firstly, wireless sensor nodes used to collect building energy consumption and environmental information are limited by energy, so energy consumption of sensor nodes must be strictly controlled to prolong the life cycle of BEIoT. The energy of wireless sensor nodes in BEIoT is mainly consumed in the process of wireless network communication, so it is necessary to minimize the data traffic and data redundancy between sensor nodes. Secondly, due to the influence of external environment, electromagnetic interference and other factors, the data collected by BEIoT sensor nodes are often mixed with a variety of noise signals, which reduces the accuracy and integrity of building energy consumption data. Therefore, wireless sensor nodes with lower energy consumption and higher accuracy are often needed for wireless acquisition and transmission in the field of building energy consumption monitoring and environmental monitoring. In addition, a more efficient and low-power data transmission and fusion method is adopted. 3 LOW-POWER DATA TRANSFER AND FUSION ALGORITHM After acquiring real-time data of building energy consumption, various sensor nodes in BEIoT upload and fuse layer by layer through tree WSN. The data transmission traffic and encryption and decryption mechanism directly affect the energy consumption of BEIoT and the privacy protection of building energy consumption and environmental monitoring data. In this section, BEIoT is abstracted into a tree network with three layers. A data fusion algorithm based on repeatability protocol factor is proposed, and the key distribution mechanism is used to improve the data privacy protection. 3.1 Data fusion model based on multi-hop network structure BEIoT can be represented by a connected digraph |$G(V,E)$|⁠, where vertex |$v(v\in V)$| represents nodes in BEIoT and directed arc |$e(e\in E)$| represents data transmission links between nodes. Typical BEIoT generally consists of three types of nodes: (1) leaf nodes, which are composed of sensors that collect and transmit building energy consumption and environmental information; (2) fusion nodes, which collect building energy consumption data and perform data fusion and calculation; and (3) Query Server (QS) nodes, which are responsible for the final fusion and analysis of building energy consumption data. Three types of nodes constitute a tree structure, among which QS node is the root node to get the data fusion results and provide the basis for further analysis of building energy consumption and environment. The fusion node is responsible for receiving data from leaf node and transferring the fusion calculation to root node. The leaf node collects various types of building energy consumption data and uploads them to the corresponding node. The data fusion process based on tree structure is one-way transmission. The data fusion function structure currently used in WSN can be expressed as formula (1): $$ f(t)=\varphi \left({d}_1(t),\textsf{K} \kern0.5em ,{d}_n(t)\right) $$ (1) Among them, |${d}_i(t),\kern0.5em (i=1,2,\textsf{K}\kern0.5em ,n)$| represents the data collected by node |$i$| at the point of time |$t$|⁠, and operator |$\varphi$| represents the fusion calculation factors, such as count, average, max, min and other sum functions. As a specific application of WSN in the field of building energy and environment monitoring, BEIoT collects and transmits information on building energy consumption and environment of various types and structures. Some types of data have obvious periodic characteristics, and the data of sudden increase or abnormal fluctuation periods are often more valuable for building energy consumption analysis. Therefore, the characteristics of the above fields can be considered in the design of BEIoT data fragmentation and transmission strategy, and a large number of periodic and redundant building energy consumption data can be compressed, focusing on the collection of occasional abnormal data, so as to reduce the data traffic of WSNs. Data fusion function based on unequal time intervals presents formula (2): $$ f\left(\hat{t}\right)=\varphi \left({d}_1\left(\varDelta{t}_1\right),\textsf{K} \kern0.5em ,{d}_n\left(\varDelta{t}_n\right)\right) $$ (2) Among them, |${d}_i(\varDelta{t}_i)\kern0.5em (i=1,2,\textsf{K} \kern0.5em ,n)$| denotes the data collected by node |$i$| within the time interval |$\varDelta{t}_i$|⁠, and |$\hat{t}=[\varDelta{t}_1,\textsf{K} \kern0.5em ,\varDelta{t}_n]$| denotes the minimum common-fold time interval for all nodes. For periodic redundant data, the time interval can be set relatively long, and some duplicate building energy consumption data can be removed, which can reduce data traffic and improve data fusion efficiency. 3.2 Data fusion algorithm based on repetitive reduction factor In this paper, a data fusion algorithm SMART-RR based on Repeatability Reduction is proposed for BEIoT applications. Data encryption and decryption adopt random key distribution strategy. First, a key pool containing |$K$| keys is generated, from which |$k$| keys (⁠|$k