## 2 WirelessHART
### The IEEE 802.15.4 Physical Layer  
As a common aspect for the aforementioned main industrial technologies, all of them adopt the IEEE 802.15.4 physical layer [43] (see Figure 5) and implement upper layers of their own. The low rate wireless personal area networks (LR-WPANs) were designed to support devices with limited physical features (low energy consumption, a communication range of a few meters, low power processing and reduced memory), enabling the development of pervasive applications in diverse areas (forest fire and landslide detection, greenhouse and industrial monitoring, passive localization and tracking, _etc._). When an LR-WPAN is designed for an industrial environment, it is called an industrial wireless sensor network.  
The standard for LR-WPANs is IEEE 802.15.4, which specifies the physical and data link layers. This standard was initially approved in 2003; however, its more recent version was approved in 2011 [6]. It is considered a _de facto_ standard for wireless sensor networks.  
Figure 4: Time division multiple access (TDMA) frame organization.  
The IEEE 802.15.4 physical layer can operate in three distinct frequency bands: the 2450-MHz ISM band (worldwide) with 16 channels, the 915-MHz band (in the U.S. only) with 10 channels and the 868-MHz band with only one channel (European and Japanese standards). All of them use the direct sequence spread spectrum (DSSS) access mode. Signaling in the 2450- MHz band is based on Orthogonal Quadrature Phase Shift Keying (O-QPSK), while the 868-/915-MHz bands rely on Binary Phase Shift Keying (BPSK). The throughputs of the bands are 250 kbps, 40 kbps and 20 kbps, respectively. Besides the management of the physical layer, services include the activation and deactivation of the radio transceiver, energy detection (ED) within the current channel, link quality indication (LQI) for received packets, clear channel assessment (CCA) for carrier sense multiple access with collision avoidance (CSMA-CA), channel frequency selection and data transmission and reception. The main features present in the IEEE 802.15.4 physical layer are summarized in Table 1.  
Although the physical layer can support three frequency bands, the 2450 MHz ISM band is by far the most frequently used. The use of this band generates a coexistence problem in the sense that other wireless technologies also employ the same frequency band (e.g., IEEE 802.11 and Bluetooth). In relation to coexistence with the IEEE 802.11 devices, it is possible to carry out a configuration, so  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Features** & **IEEE 802.15.4 Physical Layer** \\ \hline \multirow{3}{*}{Frequency bands} & 868 MHz (1 channel) \\  & 915 MHz (10 channels) \\  & 2450 MHz (16 channels) \\ \multirow{3}{*}{Channel width} & 2 MHz \\ \cline{1-1}  & 250 kbps, 40 kbps, and 20 kbps \\ \cline{1-1}  & Modulation & O-QPSK, BPSK and ASK \\ \cline{1-1}  & Communication range & 100 m to 200 m \\ \cline{1-1}  & Transmission power & 0 dBm \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main features present in the IEEE 802.15.4 physical layer. O-QPSK, quadrature phase shift keying; BPSK, binary phase shift keying; ASK, amplitude shift keying.  
Figure 5: A layer-to-layer comparison of WirelessHART, ISA100.11a and WIA-PA [7, 44].  
that there are four non-overlapping IEEE 802.15.4 channels, as described in Figure 6. Obviously, as only four channels are available in this scenario, it is expected that the transmissions have a higher probability of suffering interference than a scenario where 16 channels are available [45]. On the other hand, a minor impact is expected when coexistence with Bluetooth devices is considered, because in the Bluetooth standard, the frequency band is modified 1600 times per second. Thus, it is more likely that the interferences occur in the first transmission attempts. However, an efficient retransmission mechanism can be implemented in the IEEE 802.15.4 to mitigate this interference [46].