On this page, we provide a variety of information on OFDM,
including a brief introduction to the technology; an interview with the chairman and CEO of Wi-LAN, the inventor of
W-OFDM technology; information on the proposed FCC Rulemaking to permit certification of W-OFDM
devices, and links to reference books and other information sources.
OFDM stands for Orthogonal Frequency Division Multiplexing and is a modulation
technique for transmitting large amounts of digital data over a radio wave. W-OFDM
stands for Wideband OFDM. The main proponent and inventor of W-OFDM is
Wi-LAN of Calgary, Alberta.
OFDM is conceptually simple, but the devil is in the details! The implementation relies on
very high speed digital signal processing and this has only in the last several years become available at a
price that makes OFDM a competitive technology in the marketplace.
OK, so what is the simple concept behind OFDM? Take one carrier and modulate it using
Quadrature Phase Shift Keying (QPSK) where each symbol encodes 2 bits. This modulation
is at a certain symbol rate. For the purposes of this discussion let's say 1000 symbols
per second.
Modulation theory tells us that the spectrum of such a modulated signal will have a
sin(x)/x shape with the first null at 1000 Hz. Now if we have a second carrier that
has a frequency exactly 1 KHz higher than the first, and modulate it with the same
symbol rate, it turns out that both signals can be recovered without mutual interference.
To make the whole exercise worth while, take the numbers in the preceeding paragraph and multiply them
by a factor of, maybe, 256 or even more. And while you are at it, instead of using a
2-bit symbol(QPSK), use a 6-bit symbol (64-QAM). This can cram an amazing amount of
data into a relatively small bandwidth.
The problem with the simple-minded approach is that it takes lots of local oscillators
each locked to the others so that the frequencies are the exact multiples that they should be.
This is difficult and expensive. DSP to the rescue! Each of the oscillators can be a
digital representation of the sine carrier wave that can be modulated in the numerical
domain. This can happen simultaneously for all of the carriers. The resulting output
of each channel is added and then blocked. Since we have a representation of the signal
in the frequency domain but need to modulate an actual carrier in the time domain, we just
perform an Inverse Fast Fourier Transform (IFFT) to convert the block of frequency data
to a block of time data that modulates the carrier.
The receiver acquires the signal, digitizes it, and performs an FFT on it to get back to
the frequency domain. From there, it is relatively easy to recover the modulation on each
of the carriers.
In practice, some of the carriers are used for channel estimation and there are extra bits
added for error detection and correction. Doing this is called Coded Orthogonal Frequency
Division Multiplexing (COFDM). Coding is now so common that many people drop the "C",
as unnecessary, assuming that coding is used.
Q. Dr. Zaghoul, it's a pleasure speaking with you today.
A. Thank you.
Q. What did you work on before inventing W-OFDM?
A. I was a senior researcher at Telus R&D working on digital communications projects
like helping in planning the transition from analog cellular to digital cellular. I also did a lot of
propagation channel measurements and analysis, and had contributed to a number of other inventions
like a novel equalizer and speech compression.
Q. What led to the development of W-OFDM?
A. When we invented what we thought was the best channel estimator for an adaptive equalizer, we
immediately applied it to the IS54, which was a TDMA digital cellular standard. The improvements
gained were much less than theoretically expected. The reason was that the design parameters
(like clock inaccuracy and drifts) caused more errors than the channel, at times. We decided to
figure out which communications system would not suffer as much from design issues and would best
suit the channel (this last criterion was novel), and the answer was W-OFDM.
Q. You hold some of the major patents on OFDM. Who are the other
major players in OFDM patents?
A. Philips holds a number of key patents in digital video broadcasting which is one way OFDM
is used. Philips represents the patent pool for DVB. TI holds a patent on discrete multitone,
which could be viewed as a variation of OFDM. CSIRO from Australia holds a patent on specific
indoor wireless LAN implementations. I have not reviewed the CSIRO patent in any detail yet.
Q. Wi-LAN calls its OFDM W-OFDM. There is a modulation technique for
digital television called COFDM. What are the similarities and differences between W-OFDM and
COFDM?
A. "C" stands for "coded". All OFDM nowadays is coded, so the "C"
is redundant. The "W" stands for wideband, or what is commonly called broadband.
We suggested different mechanisms to minimize channel and
system design effects to make two-way W-OFDM a reality. COFDM was chosen for digital TV
broadcasting in Europe; this is a one-way transmission where the cost of the transmitter
could have been in the $250k range and higher. We introduced tricks to bring this cost down
appreciably. I have not reviewed this in detail, but I personally am inclined to think that any
use of COFDM for two-way broadband wireless communications would infringe on our patent.
Q. What were the technological breakthroughs that made OFDM
practical?
A. The introduction of channel estimation as a rule, introduction of design criteria that made
broadband OFDM possible, introduction of phase whitening to reduce the peak-to-average ratio and
hence to reduce the requirement for linear amplifiers, and ASIC developments -- all these made
W-OFDM practical.
Q. Are your FFTs implemented in hardware or in programmable
processors?
A. We have implementations in both. Customer Premise Equipment would have to be in ASICs for
cost reasons.
Q. I believe that you use 16-QAM as the form of modulation on each of
the subcarriers. Could you use a higher order of modulation to get even higher data
rates?
A. We now have 64 QAM and are working on higher levels.
Q. The FCC has said that OFDM is not a form of direct sequence
spread spectrum. Do you agree?
A. No, I don't agree. OFDM and multicode direct sequence spread spectrum converge
when you use all possible codes for a single transmitter.
Q. You have petitioned the FCC to allow OFDM at 2.4 GHz under 15.247
for spread spectrum. Why?
A. We believe that allowing higher data rates in the 2.4GHz band will minimize pollution of the
band. Also, some radio parts are cheaper in the 2.4GHz at the current time; this fact combined
with the longer range of 2.4GHz products makes it a more favorable band for indoor applications.
Q. So you think it's fair to say that 2.4GHz still has economic
potential, or is all the "action" moving to 5 GHz?
A. Yes to the economic potential of 2.4GHz. The current pollution is mostly outdoors, and once
OFDM chips are inexpensive, most devices would move to them and the band's order would be restored.
This may take 5 to 10 years but it's a definite possibility.
Q. OFDM must be linearly amplified. What is the impact on link
performance on nonlinearities in the power amplifier?
A. Nonlinear amplifiers cause clipping of the signal, and some data packets would not make it
through if the system is not designed appropriately. Wi-LAN introduced phase whitening, and this
reduces the linearity requirement.
Q. Do you think that there will be chips that implement OFDM on
the merchant semiconductor market?
A. Yes. I think they'll be on the market in 2001.
Q. Are there applications for OFDM outside of IEEE 802.11a?
A. Yes -- fixed wireless access, cellular in 4G applications, home multimedia, and road access for
internet into vehicles, to name a few.
Q. What products have been developed as a result of the partnership
between Wi-LAN and Philips?
A. We jointly developed an ASIC that is used in our I.WiLL(tm) System.
Q. Does Wi-LAN have other partnerships that will lead to new
products?
A. We have signed a marketing agreement with Ercisson Canada that should lead to products
in the 2.5GHz band.
Q. What technologies do you think will be the main competitors to
OFDM for delivery of wireless broadband?
A. I do not see anything that can compete with it for the next five years.
Q. Does OFDM have the capacity to go to even higher data
rates?
A. There is no theoretical upper limit on the capacity.
Q. What do you think the future of OFDM will be?
A. Hopefully, inexpensive products that provide high speed communications to individuals and
appliances around the globe.
The FCC released a
Second Report and Order on May 16, 2002,
to improve spectrum sharing by unlicensed devices operating in the 2.4 GHz band
(2400 - 2483.5 MHz), provide for introduction of new digital transmission technologies,
and eliminate unnecessary regulations for spread spectrum systems. Specifically, this
rule change would revise the rules for frequency hopping spread spectrum
systems operating in the 2.4 GHz band to reduce the amount of spectrum that must be
used with certain types of operation, and to allow new digital transmission technologies
to operate pursuant to the same rules as spread spectrum systems. The rulemaking also
eliminated the processing gain requirement for direct sequence spread spectrum systems.
Following this Report and Order, which became effective July 25, 2002, a Petition for Reconsideration was
filed by Warren C. Havens and Telesaurus Holdings GB, LLC, d/b/a LMS Wireless, requesting that FCC delay
implementation of the new rules. This petition was denied in
FCC's Memorandum Opinion and Order released May 30, 2003.
FCC's Complete Docket File on the Rulemaking contains some 377 documents. In the dialog box you reach via this link, enter
"99-231" in box 1, "Proceeding" and press the "Retrieve Document List"
button at the bottom of the screen. This will bring up the complete list of documents on this issue, with
information on the author of the document and the date. Links on this list will take you to PDF
views of the documents.
FCC Eliminates Requirement that 256 QAM-modulated carriers
be used with OFDM, 9/26/2001
Since March 1999, OFDM has been permitted in Multipoint Distribution Systems (MDS) and Instructional
Television Fixed Service (ITFS), with a minimum of 256 QAM-modulated carriers. In March 2001,
Cisco Systems, Inc. filed a petition requesting that FCC drop the condition, in order to allow
designers more flexibility in design/cost tradeoffs. Read the
Declaratory Ruling and
Order on this matter issued September 26, 2001.
WiMedia Alliance(formerly MultiBand OFDM Alliance) —: this
is the group that has put out an ISO-published radio platform standard for high-speed ultra wideband (UWB) wireless
connectivity. For more information on the standards dispute over UWB,
see our UWB News page.
Wi-LAN: Wi-LAN has gone to an icky frames setup and has
taken down most of the interesting technical material they used to feature. Now they are mainly a patent
licensing site. To see some of their patents, press the "Patents" tab on the top menu,
then do a search on WiMax or OFDM. Most of their good material on this topic has vanished.
