# Microcontroller Unit-Based Wireless Sensor Network Nodes: A Review
## 4 Technical Discussion
### Challenges and Future Trends  
The IoT technology utilizes WSNs for providing an efficient sensing and communication infrastructure. Despite numerous research efforts in WSNs, energy consumption remains a challenge that limits the lifetime of WSNs. Some of the critical features and characteristics that should be considered by designers of wireless sensor nodes are ultra-low power consumption, affordability, small physical size and software and hardware configurability. Microcontroller unit-based sensor motes have the advantages of low-power consumption, affordability, programming flexibility and fast time-to-market [9; 38]. The nodes are mostly utilized for simple embedded systems with low budget requirements.  
The general trend is to design and develop ultra-low power MCUs that consume ultra-low power when in the sleep state, since in most applications the sensor nodes are always in the sleep state. Also, the wakeup time of the motes must be reduced as much as possible. Therefore, advances in microprocessors and ultra-low power management techniques are critical to minimize power consumption and achieve the unlimited lifetime of WSNs.  
Low power MCUs are best suited for less computation-intensive applications. It can be observed that the recent wireless sensor nodes have started utilizing the ARM Cortex-based processors for applications requiring an increased computational power. Energy-harvesting techniques should be utilized for powering the wireless sensor node and extending the network lifetime. In order to achieve better energy efficiency and long-range coverage, the emerging LPWAN [39] technologies such as LoRa [40; 41; 42; 43; 44; 45; 46; 47], NarrowBand IoT (NB-IoT) [48; 49; 50; 51; 52] and Sigfox [39] can be utilized. A combination of the advantages of these emerging and promising communication technologies also needs to be explored.  
This study has revealed that most of the recent sensor nodes tend to follow an open source approach, while the older platforms follow a closed source approach. This is probably because using compatible open platforms and off-the-shelf components would allow researchers to rapidly develop improved sensor nodes in terms of power consumption, communication and processing capabilities as compared to the built-for-purpose platforms. Besides, software upgrades and changes on the hardware are made easier. However, relying on an open-source hardware setup does not potentially result in the reduction of the sensor node's size.  
It is worth noting that ignoring the sensor node's security issues and vulnerabilities during design compromises the functionality of the overall WSN. The dynamic or evolving nature of security threats calls for the need to continuously develop different mitigation technologies and countermeasures to guarantee end-to-end security in WSNs. Hence, there is a need for secure protocols that are computationally efficient and energy effective to match the limited resources of the motes. Reliable motes that give reliable measurements and do not often fail should be developed. These motes have to be robust against environmental changes by operating under different temperature, pressure, moisture and weather conditions without their accuracy being affected. The cost-effectiveness and portability of the motes are also crucial to facilitate easy deployment and utilization in many IoT applications.  
Exploring different means for energy harvesting, including wireless charging techniques that are both efficient and cost-effective, is still an open research area. How to manage the IoT motes remotely and update their firmware using technologies such as over-the-air programming (OTA) is an ongoing research. The energy efficiency issue affecting the implementation of UAVs or drones in applications requiring mobility also needs further research.