Cellular Design Considerations for an Optimal IoT Strategy

Josh Jaeger Josh Jaeger, Director of Sales, WDS, Digi International
May 13, 2020

There has never been a better time to dive into the Internet of Things. Today, IoT developers have access to a wide variety of cloud platforms, embedded devices, sensors, and connectivity options – enough to cause analysis paralysis. Whether you are beginning your "digital transformation" to improve operational efficiency and lower operational costs, or creating whole new business models around service, a cornerstone to this effort will be developing your IoT strategy. A key ingredient in your success will be the connectivity from the edge. One of the most popular methods today is integrating cellular connectivity into your IoT application.
To provide some background, Digi Wireless Design Services has helped hundreds of companies successfully implement cellular connectivity in their IoT solutions over the past 15 years. In addition to architecture definition, design and build services, we offer educational workshops to help organizations jump-start their development process. These services help organizations focus on their core competency and solve their business problems while WDS eliminates inefficiencies and design challenges, which helps the organizaiton accelerate time-to-market.
In this article, we specifically address some tips for integrating cellular into your application.

Why Integrate Cellular into Your IoT Design?

 Here are some of the key reasons to consider cellular:

  • Cellular devices can be installed without the need to be set-up on the customer’s Wi-Fi network.
  • No setup time for your installers or your customer’s installers. You don't have to worry about changing passwords and signal strength. It "just works" out of the box.
  • Cellular options abound. Connections are an order of magnitude less expensive than they were even just a couple of years ago.
  • Cellular travels miles versus feet for typical Wi-Fi networks. The world is blanketed in cellular.

Digi has several excellent options that make getting to market fast and easy, including cellular-enabled and pre-certified Digi XBee modules. Even so, designing products with cellular presents new design challenges that must be considered. There are some things that developers must pay attention to that are unique to cellular, especially the newer technologies that have greater receiver sensitivity.  Failure to account for this in your design will mean you risk developing devices that drop connectivity or, worse yet, fail to connect at all. This can cause your device to fail certifications, delay your product launch, and disrupt your business plans.
Your customers demand a seamless and easy setup experience. Profits can be destroyed by sending out service trucks or sending replacements by express shipment to fix or replace problematic devices and parts. Among its many benefits, cellular enables you to integrate simple setup as well as remote troubleshooting and remediation features that support quality of service.

Lessons Learned from Cellular Integration

The following are some hard-earned lessons that the Digi WDS team members have learned and recommend considering when undertaking a new cellular-enabled device effort.

1. Beware of EMI and mitigate in your schematic and layout

The decisions you make in this regard affect your application’s radio implementation and noise filtering, processor configuration/strapping, clocks/oscillators, memory configuration/connections, peripheral configuration/connections, and power system configuration.
Some “do’s” related to schematic design:

  • Isolate the antenna from power, processors and switching regulators.
  • Consider the use of LDOs instead of switchers when possible.
  • Carefully plan PCB construction, stack-up, and routing strategy. Contiguous ground or power planes adjacent to every signal layer.
  • For every cable or wire connection, be sure to filter all entry and exit points on the PCB.
  • Use a single solid ground plane. Do not create separate analog/digital grounds unless absolutely necessary.
  • High-speed processor decoupling is especially important.
  • Provision for shields over processors, regulators, and switching devices as a precaution. A shield can always be omitted from the final bill of materials if testing indicates it is not required.
  • Consider a design review of your schematic before layout and a layout review before building boards.

Engaging outside expertise to look over your design can save time and money by finding issues early and mitigating them before expending precious labor and material dollars. Digi has performed hundreds of these both as an internal development process and also as an aid to clients who are developing their own designs.

2. Carefully consider antenna design

It’s all about the antenna. Nearly everything in a small IoT device is part of the antenna system pre-planning for location and allowing for mechanical considerations is crucial. Metal and other materials will detune an antenna, affect pattern, degrade efficiency, or couple and radiate, which will cause your application to fail certification.
Some “do’s” related to antenna design:

  • Review antenna selection and integration in the mechanical assembly. Having adequate space for a proper antenna vs. fitting it into the available space can have severe impact on the system performance. As size decreases, efficiency decreases.
  • If possible, perform an antenna performance simulation. Simulations can help to guide the mechanical design process as well as evaluate potential antenna candidates prior to construction.
  • Keep antennas away from noise sources such as switching regulators and microprocessors that can cause receiver de-sensitizing.
  • Keep antenna and RF transmission away from power nets.
  • When using ceramic antennas, provide adequate ground plane. A good reference is the antenna’s evaluation board ground plane.
  • Try to keep cable runs as straight as possible. Avoid kinks and sharp bends that can cause reflections.
  • Perform in-situ tuning measurements to verify performance at each stage of prototyping and especially in the final configuration. Tune the antenna to favor transmit frequencies instead of receive.

For questions or support on antenna design, reach out to Digi WDS. We support customers all the time in resolving antenna-related issues and ensuring that their designs will pass certifications.

3. Add receiver sensitivity testing to your development cycle

Perform receiver sensitivity testing (including intermediate-channel) to characterize interference and compare to CTIA and carrier requirements. As mentioned before, excessive noise can desensitize the receiver and severely degrade its performance. This is especially true with Cat-M and NB-IoT radios that have increased sensitivity over other LTE radios. Consider stationary devices, for example, that are deployed inside of buildings or in rural areas where cell signals may be weaker. A poorly performing receiver may cause connection failures.
Some “do’s” related to sensitivity-related:

  • Design with an eye to EMI as outlined above.
  • Perform pre-scan testing at each prototyping stage to gauge your performance using a cellular callbox. Perform your testing within an anechoic chamber.
  • Perform in-situ spectral analysis to identify interference emissions sources as needed.

Digi has helped many clients in the past to identify and correct issues related to receiver sensitivity. Having done hundreds of cellular designs ourselves, we can be your trusted advisor and help to guide you through the resolution process.

4. Design for optimal transmit power

Transmit power directly translates to your device’s ability to be heard by the network. Being able to answer back or initiate a connection can be compromised with low power output or may be indecipherable in the presence of high noise levels when transmitting. As with receiver sensitivity, each carrier has requirements that your device must meet in order to be approved for use on their network.
Some “do’s” related to transmit power:

  • As mentioned, tune your antenna to favor transmit frequencies. While proper reception is important, the ability of your device to be heard is directly impacted by inadequate transmit performance.
  • Perform transmit power testing to verify the design meets specifications.
  • Again, perform pre-scan testing at each prototyping phase to ferret out issues early.

In Conclusion

Several considerations must be factored in when designing cellular into your IoT device. Digi understands that your business may not be exclusive to engineering cellular devices. That's why Digi Wireless Design Services is here to help you accelerate your time-to-market while reducing risk. Contact us today to get your risk assessment and support your successful IoT deployment!

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