When it comes to embedded cellular modems, designers have options.
We set out to compare performance between two 4G LTE CAT1 Verizon embedded cellular modems, specifically NimbeLink’s Skywire vs. Digi International’s XBee®* Cellular. The intent was to capture empirical data to help customers choose which product is the right fit for their application. Not all applications care about power, but this set of experiments was set up to mimic a real life battery-powered application so we could capture data useful for designers for battery capacity planning.
The typical remote monitoring application periodically reads a sensor and sends the data to a web server using the cellular modem to provide internet access to the device. Oftentimes, the cellular modem is the largest power consumer in the design, so to save power, designers shut down the modem between reporting periods. To properly size the system battery, designers want to know how much energy is consumed to complete one full transmission cycle. The software must: turn on the modem, register on the cellular network, get an IP address, open a socket to the web server, send the sensor data, close the socket, shutdown the connection, and turn the modem off.
Measuring the power consumed by a cellular modem through these state transitions is difficult. Cellular modems demand power in high-current pulses which must be measured with specialized equipment that is capable of capturing the extreme current changes at a fast rate. In our setup, we used a Keithley 2280S-36-6 Precision Measurement DC Supply. To complete the test environment, an NL-SWDK development kit was modified to bypass the onboard regulators so the DC Supply provided power directly to the VCC pin of the modem under test at 3.8V DC. A Windows PC was used and a script was written in Python to control both the DC Supply and send the proper AT commands to the modem under test, then capture the results. Two Taoglas TG.30.8113 antennas were attached to each modem and the tests were performed in the same location to minimize any differences in setup.
The image below is the data captured for the Digi International XBC-V1-UT-001. The image is color coded based on what the modem is doing as time progresses. The blueish color that takes up the majority of the time is waiting for network registration and connecting to the web server.
The image below is the data captured for the NimbeLink Skywire NL-SW-LTE-GELS3.
The image below combines both plots onto a single chart for comparison. The NL-SW-LTE-GELS3 in green and the XBC-V1-UT-001 in red.
The image below uses an averaging function to filter out the high speed spikes to make the plot clearer.
The important information in all of these graphs is the total area under each curve which represents the total amount of energy used to accomplish the power on, register, connect, send data, close socket, and shutdown procedure.
The averaged graph makes it pretty clear that the NimbeLink Skywire is significantly lower power than the Digi International XBee Cellular.
The total energy to complete the sequence is the voltage(3.8V) times the area under the curves for each modem. The total energy required for the complete sequence on each is:
NimbeLink Skywire NL-SW-LTE-GELS3
3.8V x 2418.47uAh = 0.00919Wh
Digi International XBC-V1-UT-001
3.8V x 3896.31uAh = 0.01481Wh
What does this mean in the real world? For a simple case, assume you have a single AA battery (1.5V, 2500mAH capacity = 3.75Wh) to power the modem. Not accounting for DC-DC switching losses, shelf life, internal resistance losses, power required by the external MCU, sensor, etc. The NimbeLink Skywire modem could connect and transmit data 155 more times more over its life than the Digi International XBee Cellular. (3.75Wh/9.19mWh = 408 connections) vs. (3.75Wh/14.8mWh = 253 connections).
Another way to put these power numbers into perspective, if the example product reports sensor readings to the web server four times per day, the NimbeLink Skywire would run for 3.3 months, while the Digi International XBee Cellular would run for only 2.0 months.
Ultimately there are many factors that designers consider when evaluating products, but when it comes to power constrained applications, hopefully this information helps designers make an informed choice.
*XBEE® is a registered trade mark of Digi International
