Why Is My Uber Driver Waiting for Me Two Blocks Away?

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Intermodulation Distortion (IMD) is an issue that 5G operators, device manufacturers, and consumers must not ignore when it comes to accurate positioning.

5G is a game changer. By every metric - including higher bandwidth, lower latency, and greater flexibility – 5G is a vast improvement over earlier generations. However, there are a couple of issues. The first is well-known as a characteristic of using higher frequencies - signals don’t travel as far or penetrate materials as well. This means that operators will need to deploy more transmitters or supplement existing transmitters with additional smaller cell sites. However, a less well-known issue is interference that impacts modern positioning, navigation, and timing (PNT) systems.

In simple terms, the new 5G generation networks are largely deployed in Non-Standalone (NSA) mode, which makes it easier for network operators to deploy a 5G network as it can reuse current 4G facilities. However, because 5G smartphones are required to use LTE for the control plane and 5G for the data plane, this simultaneous broadcast can lead to interference known as Intermodulation Distortion (IMD).

If you think of a modern smartphone, within that sleek body packed with integrated circuits (ICs), there is also the global navigation satellite system (GNSS) receiver. This chipset has much better performance than GNSS chipsets from a decade ago. It can decode a satellite signal even in dense urban locations (which was not possible before). However, at the same time, it is incredibly sensitive to interference and in certain instances, IMD can cause it to generate wildly inaccurate positioning data.

Smaller and noisier

Since the first radios were developed by Marconi in the late 1800s, we have always had to design products to mitigate potential interference. Yet, as devices have become smaller and denser, this has become more complex. Lower quality components, shared antennas, or even microscopic impurities in the semiconductor substrate layer can all lead to IMD.

The issue of IMD is real and in both lab and real-world tests we have seen some striking examples of just how bad this interference can impact location-based applications. In one case, a handset that displayed a 10-meter positional accuracy in the lab – when moved to a dense city and connected to a 5G NSA operator network - delivered positioning that was off by several kilometres. Yet, put the same device on a different operator’s 5G network using different frequency bands and it worked without issue! There are several caveats to this and a lot of potential variables including how each operator’s 5G network is designed, the make and model of handsets, geographic location, and accessibility to GNSS signals.

I have intentionally simplified this incredibly complex topic to make it digestible for a broader audience. However, the problem can result in several negative outcomes for the uptake of 5G. The major concern is a poorer user experience for new 5G customers. Imagine the situation where a customer on a brand new 5G smartphone summons an Uber and the driver arrives two blocks away because the GNSS data is wildly inaccurate due to IMD. Who would the customer blame? The Handset? Uber? The 5G network? Or, if it’s an emergency service like 911 and first responders receive an inaccurate position and are delayed, even for just a minute, it could make the difference between life or death. These are just a few examples, but it could impact millions of customers.

Working with industry

Spirent is currently working with several operators to identify and manage this issue. Fixing it is not simple because the cause is not one thing but a combination of multiple factors including hardware, network, band, and location. 5G NSA will be with us for the foreseeable future so we can’t just ignore it.

The first phase for many operators is to test its own branded or network-locked phones to ensure they are not impacted by this type of interference. Next, operators are testing the most popular handsets to create lists of “good” and “bad” phones so at least customer helpdesks can – you know – help customers.

We are also working with handset manufacturers that are using Spirent equipment within their own labs to simulate different operator network scenarios to ensure their handsets are working as expected. This data can aid the ongoing design process and potentially help with software-based over-the-air fixes where applicable. This approach should be applauded, as simply ignoring the issue will lead to more headaches for customer services teams while potentially damaging the brand reputation of handsets, operators, and even the wider 5G transition.

C-Band woes

On a positive note, there are several premium handsets that are well designed and sufficiently shielded for most operator environments. However, the closeness of the C-band, which ranges from 3.7 GHz to 4.2 GHz, has seen the most problems when it comes to this type of interference impacting PNT. As more C-band spectrum is sold off, this situation could get worse, so simply ignoring it is a bad option.

We know that two or three major handset brands are testing their new 5G devices to ensure they are resistant to IMD, and we would urge more to start. It is also wise for operators to insist that the handsets they resell have been tested by the manufacturers.

I’ll say it again, 5G is a game changer and IMD is not the fault of any one entity – it is an unfortunate by-product of our desire to innovate and move technology forward. Yet, if we are to make the 5G migration seamless and beneficial for all users, we need to make people aware of this problem and collectively make it manageable.

Learn how to ensure your devices can meet the needs of 5G location requirements with Spirent’s 8100 solutions.




William Chan

Senior Product Manager

William Chan is a senior product manager for Spirent Communications focused on cellular-assisted positioning solutions for automated device testing. William is actively involved in the 3GPP, CTIA, PTCRB, and GCF organizations where he is a key contributor to test specifications, test plans and validation processes for location testing, covering both GNSS and cellular location technologies such as OTDOA, E-CID, etc. He has concentrated on location technologies for over 18 years, from E-OTD in the GSM era to recent 5G NSA/SA location testing. He frequently works alongside subject matter experts at major US carriers to help define test specifications and plans to satisfy evolving FCC E911 requirements.