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.

• SR Telecom Announces Development of WiMAX 802.16 Solution Using Analog Devices' TigerSHARC Processor, 7/13/05 - SR Telecom Inc.
  
  
• Unstrung Insider Exposes the WiMax Paradox, 7/13/05 - Unstrung Insider
  
  
• Wi-LAN Demonstrates High Byte Rate W-OFDM Technology in a Full Mobile Environment; 4G Networks Within Reach, TMCNet, August 5, 2004
  
  
• OFDM News
  
  
• OFDM: Old Technology for New Markets, By Steven J. Vaughan-Nichols for Wi-Fi Planet.com, November 2002.
  
  
• News Release 1/15/02: Philips Semiconductor Joins Flash OFDM Alliance -- interesting article with some explanation of current happenings with OFDM.
  
  
• "Alliance rallies around Flarion's flash-OFDM technology", Patrick Manion for Communications Design, 1/15/02
  
  
• "The OFDM Challenge", Jim Wight for Communications Design, 10/23/01
  
  
• "Partial-response signaling improves OFDM power ratio", Patrick Manion for EE Times, 8/29/01
  
  
• "Wireless Infrastructure: OFDM shoulders heavy RF traffic", by Hatim Zaghloul of Wi-Lan for EE Times, 2/27/01
  
  
• "Enabling Fast Wireless Networks with OFDM", Jim Geier for Communications Design, 2/01/01
  

Dr. Zaghloul is the co-founder of Wi-LAN Inc., and is recognized internationally as a leading innovator in the field of OFDM radio technology. Dr. Zaghloul holds a Bachelor of Science in Electrical Engineering from Cairo University, Egypt, and both a Master of Science and Ph.D. in Physics from the University of Calgary. Dr. Zaghloul is the co-inventor of two leading edge wireless technologies: Wide-band Orthogonal Frequency Division Multiplexing (W-OFDM) and Multi-code Direct Sequence Spread Spectrum (MC-DSSS). He has been published extensively in technical journals, and holds nine Canadian and American patents, including five pending patents - several in partnership with Dr. Michel Fattouche, the president and CEO of Cell-Loc Inc. Dr. Zaghloul is the inventor of "Network Living(tm)", allowing seamless communication through current and future technologies.

Dr. Hatim Zaghloul

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.

Q. Thank you, Dr. Zaghloul!

FCC Rulemaking on W-OFDM Completed

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