Maybe I date myself, but it seems to me that the word “cool” best describes a wireless technology called Orthogonal Frequency Division Multiplexing (OFDM). I think it’s about the neatest thing–there’s another word from my youth–developed thus far to provide multiple access as we seek the most efficient use of spectrum within wireless cells.
OFDM elegantly overcomes the problem of fade in a multipath environment without requiring either band filtering or equalization across wide channel bandwidth. Consequently, it is especially well-suited for broadband wireless in such applications as Wi-Fi®, WiMAX and UWB. The technology provides throughput advantages over CDMA systems that will become increasingly important as demand for wireless broadband services grows. To help explain why I think OFDM is so cool, let me offer a quick tutorial.
I don’t want to get into mathematical complexity here. I’ll just touch a couple of high spots.
If you look at the history of OFDM, you'll find that the concept dates back to about the 1950s -- around the time that the word cool took on meanings that had nothing to do with temperature. It was in the 1950s that engineers realized that signal fade could be overcome by transmitting essentially the same signal over several frequencies at the same time. By linearly modulating sinusoids, they could transmit orthogonal signals in frequencies very close together without out-of-band interference.
The trouble, however, was that radios still relied on vacuum tubes and all broadcasts were analog. There was no practical way for engineers to put their theory to use.
But with the advent of digital signals, the theory of OFDM became reality. Finally, it was possible to transmit data across a large number of frequencies within a channel. With appropriate interleave schemes, no duplication of data from one frequency to another was necessary. If a small bit of the transmission was lost on one frequency, other frequencies would convey enough of the whole to permit recovery with error correcting codes.
Though OFDM has been an option, technically, for a decade or more, the wireless industry grew up with simpler approaches to multiple access. The underlying idea was to isolate each user onto a frequency by one of three techniques: assigning an extremely narrow frequency range to each user; assigning specific time slices to each; or encoding data bits to distinguish one user from another. These approaches have been refined over the years. Bluetooth® wireless technology, for example, assigns a frequency to each user but hops from one frequency to another to look for one without fade or interference problems. CDMA replaces each bit in the data stream with a code, then uses signal processing techniques to separate the desired signal from interferers at the receiver.
Recently, though, wireless operators have begun to pay greater attention to OFDM because it uses spectrum efficiently and provides greater capacity than CDMA.
Studies conducted by Texas Instruments show that the actual system throughput advantage that OFDM delivers over other widely-used technologies is in the range of 5 percent to 25 percent. The diagrams below illustrate what TI found when we measured throughput at pedestrian walking speeds. The advantage OFDM offers at vehicle driving speeds are slightly smaller.
[see graphic at link]
I point out all of this to put the benefits of OFDM into factual perspective. The throughput advantage, while not measured in orders of magnitude, is impressive nonetheless. The technology offers other important advantages, too. It is:
• Robust to multi-path
• Extremely flexible, offering choices of number of carriers, cyclic prefix length, modulation, interleave time and frequency, etc.
• Excellent for wide channel bandwidths, requiring no difficult equalization
• Well-suited to multiple antennae in MIMO systems
As I look down the road to the future of wireless, I see more and more new wireless air interface technologies adopting OFDM. IEEE802.11a/g, ultra-wideband, IEEE802.16d/e, and even some WAN technologies are making use of OFDM already. In broadcast data services and mobile television, various industry standards such as DVB-H, ISDB-T, and DMB all incorporate OFDM. At TI, our wireless OFDM activity includes WLAN production chipset solutions for 802.11 a/g and product development activity for DVB-H and ISDB-T; the standards supported by TI's "Hollywood" single-chip solution for live digital tv on cellphones. With an eye toward future technologies, TI proposed the multi-band OFDM version of UWB and is actively involved in research regarding advanced OFDM systems such as IEEE802.16.
In the brutally objective world of high-performance communications, where even minor enhancements in system efficiency can translate to bottom line profits for service providers, OFDM has become a technology of choice for a very wide range of wireless systems. There’s no other way to put it. OFDM is cool. <<
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