benz

Exploring “Near-Far” Problems—and Opportunities

Blog Post created by benz on Oct 14, 2016

Originally posted Aug 24, 2016

 

Sometimes a common understanding is not common

 

It’s always an interesting experience when I find that one of my assumptions is wrong, or at least not very right. Yes, it’s a chance to learn and grow and all that, but it sometimes provokes puzzlement or a little disappointment. This is one of those puzzling and slightly disappointing cases.

I first encountered the “near-far” term and associated concepts in an internal training talk by an R&D project manager with deep experience in RF circuits and measurements. He was explaining different dynamic range specifications in terms of distortion and interference. His context was the type of real-world problems that affect spec and design tradeoffs.

In his talk, the project manager explained some of the ways design considerations and performance requirements depend on distance. After a couple of examples his meaning was clear in terms of the wide range of distances involved and the multitude of implications for RF engineering. Whether over-the-air or within a device, physical distance really matters.

At the time, I thought it was a neat umbrella concept, linking everything from intentional wireless communications and associated unintentional interference to the undesirable coupling that is an ever-present challenge in today’s multi-transceiver wireless devices. For example, even small spurious or harmonic products can cause problems with unrelated radios in a compact device where 5 cm is near and 5 km—the base station you want to reach—is far.

That talk was a formative experience for me, way back in the 1980s. I kept near-far considerations and configurations in my mind as I learned about wireless technologies, equipment design and tradeoffs, and avoidance of interference problems. The near-far concept illuminated the issues behind a wide range of schemes and implementations.

Though I didn’t hear the near-far concept too often, I assumed most RF engineers thought along those lines and presumably used those terms. A recent Web search for near-far problem let me know my assumption was faulty. The search results are relatively modest and mostly focus on the“hearability problem” in CDMA wireless schemes. This is an excellent example of a near-far situation, where transmitters at shorter distances are received at higher power, making it difficult for correlators—which see every signal but the target as noisy interference—to extract smaller signals.

Measuring power in the code domain separates received power according to individual transmitters and their codes. Demodulation is most effective when code powers are equal.

Measuring power in the code domain separates received power according to individual transmitters and their codes. Demodulation is most effective when code powers are equal. This example is from the N9073C W-CDMA/HSPA+ X-Series Measurement App.

However, I don’t think the narrowing of meaning is a matter of the era when the term came into use, since I heard the term years before CDMA, and some people were using it years before that. Perhaps it reflects the fact that CDMA was a very interesting and high profile example, and a single association with the term was thus established.

In any case, I think this shrunken use of the term is unfortunate. Careful consideration of potential near-far issues can help engineers avoid serious problems, or at least address them early on, before solutions are foreclosed or too much money is spent.

One cautionary example is the c.2010 effort by LightSquared (now Ligado Networks) to expand mobile 4G-LTE coverage using terrestrial base stations in a band originally intended for satellites. The band was adjacent to some GPS frequencies, and the switch from satellite distances (far) to terrestrial ones (near) dramatically increased the likelihood and severity of interference problems. The large reduction in distance upset earlier assumptions about relative signal strength—assumptions that drove the design, performance, and cost of many GPS receivers.

The potential interference problems prevented approval of the original LightSquared plan, and the fate of its portion of the L-Band is not yet determined. Whatever it is, I expect it will more fully account for the near-far issues, along with the cost and performance requirements related to both new and existing equipment.

The near-far concept also has a probability dimension. As you’d expect, some sins of RF interference are more likely to be a critical issue as the density of radios in our environment continues its dramatic increase. Some problems that were once far away are getting nearer all the time.

 

To appease my own curiosity, I’ll leave you with two questions: Have you encountered the near-far concept? Or do you rely on a touchstone idea, learned from an experienced hand, that isn’t as widely known as you once thought?

Outcomes