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OFDM is Ubiquitous. Why?

Blog Post created by benz on Sep 14, 2016

Originally posted Oct 1, 2014

 

One transport scheme to rule them all. And I get to use the word ubiquitous!

In the early 1990s, working with the first vector signal analyzers, I had a front row seat as digital modulation schemes came to the fore. Digital modulation wasn’t new, but the advent of second-generation cellular standards such as GSM, NADC, CDMA/IS-95 and PDC put digital modulation in the hands of the masses.

The pace of innovation seemed never to slacken during the decade: broadcast television began to go digital, and third-generation cellular consumed vast amounts of money and brainpower.

Over a period of years, I was amazed at the proliferation of modulation types and transport schemes, and the apparently endless combinations and refinements. These required an equally constant flow of innovations to enable understanding, analysis, optimization and troubleshooting.

With mild exasperation, I asked my expert colleagues: “Are we going to continue to see this constant rollout of different modulation types and transport schemes?” The nearly universal answer was, “Yes, for quite a while.”

They were correct, but an important trend emerged late in the decade. One transport scheme grew from niche to dominance in the following decade and beyond: orthogonal frequency division multiplexing or OFDM.

I’ve mentioned various aspects of OFDM and its analysis in past posts, but haven’t explained the fundamentals and why it has become so widely used. I can only scratch the surface in this blog format, but can summarize the technological and environmental drivers.

The first word in the acronym is key: Orthogonality of a large number of RF subcarriers is the central feature of this transport scheme. As a transport scheme, rather than modulation type, it can employ multiple different modulations, typically simultaneously. The figure below illustrates this RF subcarrier orthogonality.

The spectrum of three overlapping OFDM subcarriers, in which the center of each subcarrier corresponds with spectral nulls for all of the other subcarriers. This non-interfering overlap provides the orthogonality necessary to allow independent modulation of each subcarrier.

The spectrum of three overlapping OFDM subcarriers, in which the center of each subcarrier corresponds with spectral nulls for all of the other subcarriers. This non-interfering overlap provides the orthogonality necessary to allow independent modulation of each subcarrier.

In OFDM, orthogonality and carrier independence do not mean that the subcarriers are non-overlapping. Indeed, they are heavily overlapped and the center frequencies are arranged with a specific close spacing that places the main spectral peak of every subcarrier on frequencies where all other subcarriers have nulls.

With the independence of its subcarriers, OFDM can be seen as a multiplexing or multiple-access technique, somewhat similar to CDMA. It doesn’t increase theoretical channel capacity, but it has benefits that allow systems to operate closer to their theoretical capacity in real-world environments:

  • A high degree of operational flexibility by allocating subcarriers and symbols as needed, along with signal coding schemes, to accommodate different users with different needs for data rates, latency, priority, and more.
  • Multiple access (OFDMA) to support multiple users (radios) simultaneously using flexible and efficient subcarrier allocations.
  • High symbol and data integrity by transmitting at a relatively slow symbol rate to mitigate multipath effects and reduce the impact of impulsive noise, and by spreading data streams over multiple subcarriers with symbol coding and forward error correction.
  • High data throughput by transmitting on hundreds or thousands of carriers simultaneously and using appropriate signal coding.
  • Robust operation in interference-prone environments due to its spread spectrum structure and tolerance for the loss of a subset of subcarriers.
  • High spectral efficiency by spacing many subcarriers very closely and arranging them to be independent, allowing each subcarrier to be separately modulated.
  • High spatial efficiency through compatibility with spatial multiplexing techniques such as multiple-input/multiple-output (MIMO) transmission.

Potential benefits of OFDM were anticipated for years, but the technique only became practical for wide use as signal processing power became available in high quantity at low cost. As that performance/cost ratio improved, OFDM increased its dominance, and that is a major RF wireless story of the past 15 years or so.

 

You can read more about the technique in a recent OFDM introduction application note, and I’ll discuss some of the implementation and test implications in future posts.

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