## 3 Challenges and Trends in Wireless Sensor Nodes
### Heterogeneous Sensor Networks: Supporting Several Physical Layers  
As [44] noted, in environmental monitoring applications, sensors and data sources usually do not share the same nature of connectivity. For instance, the System for the Vigilance of the Amazon (SIVAN) [45] is based upon sensors of different types such as radars, environmental sensors and imaging systems. Therefore, such networks, composed of miscellaneous sensors and data sources, are generally called heterogeneous sensor networks [46]. These networks consist of high-speed links like imaging systems and low-speed measurements such as ground sensors. Thus, in order to reduce the network bandwidth and consequently reduce the energy consumption, nodes (particularly sensor gateways) are equipped with several network interfaces. Providing a sensor gateway with a reconfigurable Software Defined Radio (SDR) physical layer makes it possible to switch, for instance, from 802.15.4 to 802.11 links, according to the required speed and parameters of the sensor network. That is the case of [47], where the authors proposed a 2.4 GHz receiver implemented in a low-cost Spartan-3 in order to demodulate 802.11 and 802.15.4 frames. On the other hand, other works such as [48] have studied the support of several communication protocols in the same platform and the implementation of the RF network interfaces via commercial front-ends. In this respect, [PERSON] _et al_. presented a WSN-to-WLAN bridge, implemented in an FPGA (Stratix II, EP2S60) and based on a multiprocessor SoC. This platform utilizes four Nios-II soft-processors to implement TDMA scheduling, MAC layer disassembling and assembling processes and the CRC-8 algorithm as well as custom logic in order to accelerate the AES encryption and CRC-32 algorithms. At the time of writing, however, providing multiple communication standards via SDR and FPGA using partial reconfiguration in sensor networks remains an open problem. IEEE 802.15.4 Physical Layer Implementations on FPGAs by Passive Power Conservation and Signal Processing  
[PERSON] _et al_. described how passive power conservation [49] could be applied on WSNs in order to reduce the power consumption of a sensor node, based upon the principle of turning the radio transceiver off when it is not transmitting [8]. This idea can be utilized in the implementation of the physical layer of the IEEE 802.15.4 standard, _i.e_., Physical Layer Power Conservation (FLPC). Doing this on FPGAs has several advantages since independent units such as FIR structures, symbol mappers or synchronization modules can be turned off when not active. This technique, applied to other sensor node components with different granularity such as ADCs or sensors, is called Dynamic Power Management (DPM). Moreover, Dynamic Voltage Scaling (DVS) can also be applied. Such technique is based upon changing the core's operation frequency and its voltage in order to reduce its energy consumption. Several implementations of DVS on FPGAs have been proposed [50; 51]. In this respect, [PERSON] _et al_. [9] proposed the design of a sensor node (PowWow) based on the MSP430 microcontroller and the Actel IGLOO FPGA (AGL125) where Dynamic Voltage and Frequency Scaling (DVFS) is employed. A ZigBee transceiver (CC2420) is directly coupled to the FPGA, which dynamically manages the transmission power according to the channel status and performs the packet processing tasks. If a valid frame is received, the FPGA wakes up the microcontroller. According to the authors, the main tasks are carried out by the FPGA since its power consumption is reduced in comparison to the MSP430. Finally, a DC/DC converter is used to perform Dynamic Voltage Scaling (DVS) on both the FPGA and the microcontroller. Other power minimization technique consists of turning the radio transceiver off when the device is not processing incoming/outgoing packets. In this respect, [PERSON] _et al_. presented a wake-up receiver for WSN that wakes the transceiver up (based on FPGA) when a wake-up message is detected [10].