The business case
Low Power versus Ultra low Power, these are popular statements used by sensor manufacturers, it makes sense, as from various reports it becomes clear battery lifetime is the most important aspect when considering a sensor beside functionality of course. However, the type of wireless network has an enormous impact on the sensor power consumption, making the selection of the LPWAN wireless network protocol a critical factor to consider when developing your application.
Many articles are written nowadays around the subject of successful IIoT implementations and lack of it. For one, the business case plays an important role and understanding the IIoT inter-dependability of sensors, network, gateways and IoT infrastructure and how they influence performance such as battery consumption and consequently the business case is an essential part of potential success.
Power consumption and related battery lifetime has an enormous impact on the business case, for some sensors this would be the end of life and for others it would result in battery replacements costing both time and money.
As the network impacts the battery lifetime in multiple ways it is important to really understand network pro’s and con’s and its role in the ecosystem rather than taking the network selection for granted based on characteristics that seem to be important however have a negative effect on the business case. The tradeoff between range and battery consumption is one of these aspects to review closely.
Net capacity of the battery
The most important aspects determining the net capacity of the battery are peak current level, peak duration, sleep current level, recovery time between peaks and ambient temperature. The peak level and peak duration are both influenced by the choice of network technology and the network deployment.
There is a strong relation between transmission power and the peak current height, the power amplifier of the radio consumes a lot of energy. DASH7 nodes can scale their TX power when the link budget to their prefered gateway is comfortable enough, resulting in energy savings (and less interference as well) on the network as a whole.
A higher data rate means a shorter peak duration and thus less battery consumption. DASH7 offers 3 data rates: high (166.7 kb/s), normal (55.6 kb/s) and lo-rate (9.6 kb/s). A higher data-rate implies a lower range, therefore it is a tradeoff which depends on the use case. The lowest data rate of DASH7 is still higher than the highest data rate offered by for example LoRaWAN (SF7, 5.468 kb/s) and 10 times higher than a more typical data rate of LoRaWAN (SF10, in EU).
As this is a trade-off, this implies that the maximum range of DASH7 is shorter. For these mid-range scenarios the higher data rates does result in serious energy consumption savings.
The network infrastructure deployment also influences the above, a denser gateway deployment allows the use of a faster data rate which is beneficial for the peak duration and thus again less battery consumption. Also, a denser gateway deployment allows downscaling the TX power, which in turn impacts the peak height.
In many scenarios it is more cost efficient to deploy a denser gateway network, compared to the cost of more frequent sensor device or battery replacements in the field. Especially when a large number of sensors are expected, a high density network has several advantages in both power savings and latency. In this case the lower range of DASH7 is not an issue anymore.
Other factors to consider to extend battery lifetime
Frequency of the peaks are partly determined by the application and/or environment. The impact of these peak loads result in a decrease of the net battery capacity compared to the nominal capacity, and the impact is typically not linear. This can be significant, for example 50% loss in capacity. This can be improved by integrating a supercapacitor in the design which can for a large part take the burden of the peak loads from the batteries, which allows to raise the net capacity, and thus increase the lifetime significantly.
To summarize, it is important to fully understand the pro’s and con’s of a network protocol, there are many tradeoffs to consider and it highly depends on the application what protocol is best suited. The DASH7 Alliance produced a whitepaper “considerations for low-power communication in industrial IoT applications” highlighting technical differences and can be downloaded on the DASH7 Alliance website. (Link)