Weighing wireless options

With different technologies available, it can be a challenge to evaluate which wireless communication is best for your irrigation operation.
BY YANG WANG

Smart irrigation, including precision control and real-time monitoring, presents an interesting case study for the use of wireless communication technologies in digital agriculture. On the other hand, technical buzzwords that revolve around wireless technologies can be confusing. Farmers, ag researchers, ag DIYers and engineers often face challenges in selecting the suitable protocol(s) for their deployments or experiments, as it is difficult to navigate through a plethora of technical terms and decide what matters the most in their decision-making process.

Figure 1: This plot shows the data rates versus the power output values for wireless modules under five different protocols. Note: Technical data were downloaded from a popular electronic distributor website. A point on the figure can represent multiple modules as they share the same technical specifications.
Figure 2: This plot shows the data rates versus modules’ sensitivity values. An interesting outlier in cellular represents a module using the 4G/LTE NB-IoT protocol; a newer standard that provides sensitivity performances comparable to the LoRa standard.
Figure 3: This set of plots shows how sensitivity and link budget could potentially help to distinguish different protocols better.
Figure 4: This plot shows the power consumption for modules in RX/TX mode versus their sensitivity values. LoRa modules have obvious advantages over other types of modules in terms of their sensitivities and power consumptions.

In addition, when new technologies emerge, the tech-savvy folks tend to hop on the bandwagon to test out the new gadgets. The skeptics tend to be content with the stable but old technologies. Which crowd is right? What needs to be considered when evaluating available wireless technologies in order to pick out the best one(s)? The best way to explore this is through a set of visualizations, as shown in figures 1 through 4.

Figure 1 shows the data rate and the power output values of 1,355 wireless modules under five different protocols. A data rate represents how fast bits get transferred (throughput). A power output value is the signal power level leaving a module. Cellular modules, as the only backhaul protocol which offers internet access, span a wide range of data rates (20 kilobits per second to roughly 100 Megabits per second) with the highest power outputs, as they typically need to transmit signals far enough to communicate with distant cell towers. Wi-Fi apparently has the highest throughput centered around 100 Mbps. 

However, looking at figure 1 might raise this question: How do we distinguish among Bluetooth, Zigbee and LoRa modules, as they share overlaps in both data rates and power outputs?

Figures 2 and 3 aim to answer this question. Figure 2 shows the data rates and the sensitivities of the same modules. Apart from radio modulation schemes, a module’s reception consistency depends on its sensitivity value. A lower sensitivity value indicates a better module. LoRa modules are known for their long-range communication characteristics as reflected by a sensitivity value as low as -148 decibel-milliwatts. Zigbee and Bluetooth modules share common data rate options; however, the prior exhibits lower sensitivities on average. 

Figure 3 plots the data rates versus the link budget values; a link budget* is the difference between a power output and a sensitivity value. Radio engineers fit these values with path loss models that account for signal losses due to terrain contours, environment, propagation medium, etc. to estimate communication ranges between two devices. A higher link budget value represents a better range. Hence, throughput is not the only parameter of interest when comparing protocols; parameters like sensitivity and link budget values are critical factors as they demonstrate unique radio characteristics offered by different protocols.

Last but not least, having the right power strategy for smart irrigation devices/equipment is essential. Figure 4 plots the sensitivity versus the current consumptions of different modules. There are several observations: 1) It usually takes more power to transmit than to receive; 2) LoRa, Bluetooth and Zigbee modules normally consume less energy than Wi-Fi and cellular modules; and 3) cellular modules have the highest power consumption. Hence, if a grower intends to use a 1,000-milliampere battery to deploy a device, low-power modules (Bluetooth and LoRa) will last months with proper schedule configurations, while power-hungry Wi-Fi and cellular modules drain the battery in hours.

This brings out the core idea behind selecting suitable modules or protocols: An ideal protocol should offer highly sensitive radio (thus more consistent data reception and longer range), a low power consumption and a high throughput. However, finding one protocol that fits all use cases is not feasible. Instead, whether it is for actual deployment or evaluating a new technology, selecting the suitable wireless devices/equipment is about considering different technical parameters, finding the right balance to tailor practical needs and, finally, making informed and judicious decisions.  

* Author’s note: This definition is not rigorous. In radio planning, engineers need to account for factors like antenna characteristics, cable losses, etc. as well.
Yang Wang is a PhD candidate under James Krogmeier, PhD, and Dennis Buckmaster, PhD, at the Open Ag Technology and Systems Center of Purdue University. 
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