benz

MIMO, CDMA, STC. One of these things is not like the others

Blog Post created by benz on Sep 16, 2016

Originally posted Jan 10, 2014

 

Cheating the Shannon limit

My career has, happily, been punctuated by opportunities to work with a series of amazing RF technologies. And by “amazing” I mean innovative, impressive and effective. Many of these technologies also seemed, at first glance, to be overly complicated or of doubtful practicality. The name Rube Goldberg comes to mind.

Fractional-N technology in synthesizer phase-locked loops (PLLs) is the first example I remember. This technology came of age in the late 1970s and I first encountered it in 1980. With it, a divider in a PLL could be an arbitrary fraction instead of an integer, allowing very fine frequency resolution over a wide range with a single—albeit complex—PLL. And boy, what complexity! An intricate dance of analog signals and digital controls was updated at the astonishing rate of 100,000 times per second by a proprietary ASIC. Division constants were continuously changing, even for CW signals.

To compensate for the effect of the changing division constants, there were analog phase interpolators, which were switchable current sources if I recall correctly. From there it seemed to me only a short step to something like a flux capacitor* or even an oscillation overthruster.**

However, my skepticism simply demonstrated how shortsighted I could be. I gradually learned to nurture a degree of open-mindedness as a series of new technologies drove the RF world onward. While some of these were indeed fanciful or at least impractical, many worked well and made important contributions to our art. Achieving maturity and profitability took a lot of time and work, but of course as engineers that’s what we’re here for.

Two amazing-at-first-glance technologies of recent years are CDMA and MIMO. Along with space-time coding (STC)—which is perhaps a little less amazing—they share the approach of transmitting multiple RF signals at the same frequency at the same time.

A conceptual diagram of 2×2 MIMO is shown at left and a CDMA code-domain power measurement is shown at right. Both schemes transmit multiple signals at the same frequency and at the same time but the purpose and benefits are different.

A conceptual diagram of 2×2 MIMO is shown at left and a CDMA code-domain power measurement is shown at right. Both schemes transmit multiple signals at the same frequency and at the same time but the purpose and benefits are different.

Though these techniques all transmit multiple simultaneous signals over the same RF channel, their purposes are different:

  • STC is a diversity scheme, transmitting differently-coded versions of the same signal and using processing at the receive end to increase the likelihood that the signal can be successfully demodulated.
  • CDMA is a multiplexing scheme, providing a way to share spectrum among multiple users by multiplying signals from different users by different codes at the transmit end. A receiver with knowledge of the codes can use signal-processing techniques to separate them.
  • MIMO is a capacity enhancement scheme, transmitting independent signal streams over a single RF channel by using multiple independent transmitters and receivers to create the (partial) equivalent of separate transmit channels.

Thus, two of the three techniques aim to use the available RF channel more effectively or efficiently but don’t change the fundamental carrying capacity of the channel, often referred to as the Shannon limit. MIMO techniques seek to cheat or evade Shannon (and Hartley and Nyquist too!) and effectively create more of the RF spectrum that is so valuable these days.

Of course, by using coding and multiplexing schemes, the extra capacity of MIMO can be traded for other benefits as needed.

Amazing technologies indeed, and I’m no longer skeptical about them. I just have to keep an open mind about the next new thing.

Finally, if you need a quick break from real, complex and difficult-to-tame technology you might make a two-minute time investment to learn the technology of the Retro Encabulator.

* From a 1985 science fiction movie

** From a 1984 science fiction movie

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