The Secret Life of Modern RF Signals - Part 1
For some, dealing with all those new RF technologies like antenna diversity, MIMO and beamforming (in all its incarnations) is part of the job description. But many of us who deal less directly with the radio access network can benefit with a bit of knowledge about some of the arcane but interesting aspects of modern radio technology. Since I have the pleasure of working with some of the world’s leading experts in this field, this blog is an attempt to translate their very detailed knowledge into terms that make sense to the rest of us.
The purpose of modern RF antenna technologies is to tweak what we know about the wireless link in order to squeeze more data bandwidth out of limited radio-network resources. The driver for all of this is, let’s face it, money. There’s an old saying that “time is money”. In wireless there’s an addendum: “… and spectrum is a whole lot of money.”
Traditional methods of radio multiplexing have come at the expense of resources in one domain or another. In time domain multiplexing (TDD), we re-use frequency at the expense of time-domain resources. Likewise, frequency domain multiplexing (FDD) allows us to make simultaneous connections but at the expense of frequency spectrum.
In order to get the most out of these costly resources we have to create or discover new domains in which to differentiate signals. As an example, the code domain uses scrambling codes to distinguish channels from each other even though they are sent at the same time and in the same frequency channel.
Modern antenna techniques continue down this path. We take “domains” that are available for free (or nearly for free) and learn how to make different orthogonal connections. When two signals are sent so as to not impact on each other they are “orthogonal” in some domain. So two FDD signals are orthogonal in the frequency domain and two TDD signals are orthogonal in the time domain.
Once we begin to use three-dimensional space as a wireless channel domain, we can create orthogonal channels in the spatial domain (more to come on this topic in future blog entries). To get to the point where all of this makes sense, we’ll spend some time discussing the more fundamental aspects of the RF channel, on how it’s emulated for testing, and eventually providean understanding of all the new technologies some of you touch on a daily basis.
When you don’t deal directly with the radio link, wireless RF can seem like more of an art than a science. Due to the complexity of modern digital wireless, there is some artistry involved. So this blog won’t make you an expert - that takes decades. Hopefully, though, this blog will unveil an intuitive understanding of the RF link, the fundamental foundation of all things wireless.
In the meantime, I’ll commit to doing my best to keep this blog current. None of my plans here are written in stone, so feel free to write if you think I’m moving too fast, too slowly, or that I haven’t explained something well enough.