# Design and Performance of a XBee 900 MHz Acquisition System Aimed at Industrial Applications
## 1 Introduction  
Smart manufacturing, also known as Industry 4.0, involves improving production processes in terms of productivity, efficiency, safety and intelligence by means of integrating software powered devices for monitoring, predictive maintenance, supervision and control operations [1, 2]. These operations require the use of advanced communication technologies capable of carrying the data to the analysis, storage and decision devices distributed along the processes. New communication technologies are being introduced in industrial environments to facilitate these operations [3, 4]. In fact, network capabilitiesconstitute a key dimension of cyber physical production systems (CPPS) [5], which are at the core of Industry 4.0 [6; 7]. They are aimed at improving the connectivity among sensors, actuators and controllers, while providing the quality of service (QoS) requirements of the applications. In this scenario, wireless technologies are playing an increasing role in industrial applications, since they provide several benefits, when compared to wired technologies [8; 9]. The major benefits are the enabling of configuration flexibility, supporting mobility, eliminating costly cabling and allowing for easier network expansion. In general, these advantages improve productivity and efficiency [10]. Currently, they are assumed to be in a critical point of massive deployment at production applications. However, wireless technologies have been penalized by a lower reliability, integrity and security of the wireless links when compared to wired links. Most drawbacks are due to the nature of wireless communication technologies which have a negative impact in the predictability and error rate. Wireless technologies introduce new phenomena such as signal fading, interferences and propagation by different paths. These phenomena are particularly relevant in industrial environments, full of metallic surfaces and obstacles which may difficult wave propagation. In addition, a large number of industrial applications require stringent QoS requirements and cannot tolerate long communication delays or package losses. Although wireless technologies are being introduced in industrial applications for noncritical operations, its usage in critical applications is currently under scrutiny. Consequently, more research is needed in order to introduce them massively for this kind of operations [10]. Several issues must be solved, e.g., mitigating the influence of interferences, especially those caused by other communication technologies, avoiding congestions and collisions in order to guarantee the QoS requirements of the applications and adequately managing the frequency spectrum [11; 12]. Wireless technologies are starting to be accepted for some industrial operations. In [13], the typical latency values that different industrial applications require are discussed. In monitoring applications, they tend to be accepted at low sampling frequencies (lower than 10 Hz), which limits its usage for monitoring plants with faster dynamics since the evolution of the plant may be not adequately captured with such a low range of sampling frequencies. Most low-power wireless technologies available for industrial applications in the context of Industry 4.0 operate at the 2.4 GHz frequency band, e.g., ZigBee, Bluetooth, ISA 100.11a WirelessHART and WIA-PA [13]. Although these technologies employ mechanisms to improve their coexistence, they are still vulnerable to transmissions from uncoordinated stations with similar technologies or issued with higher-power technologies such as WiFi [14], which typically use a transmission power which is 100 times higher than a typical IEEE 802.15.4 node [15]. WiFi technology is frequently found in industrial scenarios, particularly at the 2.4 GHz band. Several studies [16; 17] have checked the apparition of interferences that may cause a degradation of the QoS of the applications.