That's right! Our company, SSS Online, Inc., purchased the site from our friend Randy Roberts effective October 21, 2000. We've been doing our best since then to bring it up to date and add more technical content as quickly as possible. We're not done yet -- SSS-mag.com has more than 460 pages -- but we've made a good start. So far, we've done a basic update on about a third of the pages. This includes revising the contact information, repairing broken links, sprucing up the "look"of the site, improving internal navigation, and doing some minor editing. In addition, we've added a whole new segment on Pegasus Technologies (this is what we're called when we're working for money), added a new page on OFDM, and have completed major content updates on our Ultra Wideband and RF Safety pages. We've also reactivated our mailing list, added a "live" site search program, and put together this new issue of SSS Online. And there's more to come! You can keep tuned to the changes and new material by checking the blue features box on our Home Page, or our SSS Online News page.

As many of you may know, Randy is in partial retirement -- taking more time to enjoy the beautiful California scenery -- and we wish him the very best in his new "job." It's tough work, but someone's gotta do it! In the meantime, we'll try to continue the great work he did with this website. We appreciate your comments -- please drop us a line!

Pegasus Technologies is pleased to announce the development of a small, low-cost and low-power short-range transceiver system that can be tailored for use in a wide variety of electronic products. This design, which is now available under a non-exclusive license, will a add high quality and reliability RF link to your product while saving you development time.

The PTSS-2000 is a versatile short range transceiver system. It operates in the 902MHz to 928MHz ISM band and uses Frequency Shift Keying (FSK) to encode digital data. FSK has a higher interference tolerance than the more common On-Off-Keyed (OOK) or Amplitude Shift Keyed (ASK) transceivers. The PTSS-2000 incorporates a single conversion superheterodyne receiver that has an IF frequency of 10.7 MHz allowing the use of inexpensive ceramic filters to tailor the passband for a particular data rate. This is a much higher quality receiver that the super regenerative receivers that are commonly used for low cost wireless links.

The PTSS-2000 utilizes direct digital synthesis (DDS) and a phase locked loop (PLL) to generate the carrier wave. This allows very high resolution of frequency tuning for the module. The tuning steps are 375 Hz throughout the 902-928 MHz band. The degree of frequency shift that is used is programmable and is an integral part of the DDS system. This also allows tailoring of the module to meet the data rate requirements of a particular project.

The receiver incorporates a FM demodulator that in addition to decoding data can also receive analog signals such as voice communication. A single PTSS-2000 can rapidly alternate between receiving analog signals and digital data.

When mated to an external control processor the PTSS-2000 can implement a Frequency Hopping Spread Spectrum (FHSS) data link. This allows greater interference avoidance and higher allowable transmitter output power. This could yield a range of several miles in an outdoor environment.

The design of the PTSS-2000 is available for a nonexclusive license for your product. Plans with and without royalties are available. The starter kit that comes with the license includes:

2 PTSS-2000 modules

Hard copy of schematics, bill of materials, and printed circuit board

OrCAD files of the schematic and printed circuit board

Hard copy of microcontroller firmware.

Computer files of the microcontroller firmware.

20 hours of consulting with one of Pegasus Technologies RF experts.

For complete details please contact Pegasus Technologies.

The mud has really been flying recently as the FCC released its Notice of Proposed Rule Making (NPRM) on May 10, 2000. They are proposing to modify Part 15 Rules to allow license-free use of UWB transmitters. The public comment period just ended October 27. FCC must now respond to the comments and its next step may be to issue a final rule, but there is no set time frame for when this will happen. To review the FCC docket rulings and other information on UWB, stay tuned to our UWB page and we'll keep you posted!

The key issue in the public comments is the interference potential of unfettered UWB use with traditional weak signal radio services, such as GPS. Some UWB proponents, most notably Time Domain Corporation, argue that very brief UWB pulses will not cause significant interference. Others, such as Multispectral Solutions, Inc., think that some forms of UWB will cause interference and believe that UWB emissions should not be allowed in any critical weak signal frequency, mainly those below 3 GHz.

I had the pleasure of speaking recently with Dr. Robert Fontana about Ultra Wideband. Bob is the founder and President of Multispectral Solutions, Inc. (MSSI) of Germantown, MD. He earned a BSEE with honors from the Illinois Institute of Technology, receiving a fellowship to study at the Institut National des Sciences Appliquées (Lyon, France) during his junior year. He received the SMEE degree from the Massachusetts Institute of Technology, and a Ph.D. degree in Electrical Engineering from Stanford University. Bob has over twenty-eight years of experience in the areas of signal processing, high-speed digital design, microwave/RF design, and ultra wideband technology. We had a far-ranging conversation on the technical and business aspects of UWB. Our conversation is summarized below.

Dr. Robert J. Fontana

Q: How long has MSSI been involved with UWB?

A: We started UWB work in 1984. At first we also used impulse excited antennas like some UWB proponents are still doing now. When you use this solid state analog of a "spark gap" approach, the spectrum of the radiated RF is essentially determined by the antenna and you have very little control over it. This makes it very hard to prevent interference to other radios. For the last six years we have utilized a form of UWB where the spectral content of the pulses are determined by electronics using tapering and shaping. This lets us tailor the spectrum of the RF before it goes to the antenna.

Q.What about the concern that UWB might interfere with GPS reception? Would you comment on that?

A. Yes, interference is quite possible with impulse excited antenna UWB transmitters. Public reply comments on the Notice of Proposed Rulemaking published in May to allow UWB were due in to the FCC October 27, 2000. I think that the only way that the FCC will allow impulse excited antenna UWB transmitters is at a very, very low power level. There have been a couple of studies of the power levels at which a UWB transmitter will interfere with GPS and PCS. These have had conflicting conclusions, but one of them was commissioned by a party with a vested interest in the outcome. This same party was startled when a major PCS service provider found that impulse excited antenna UWB had a very detrimental impact on their PCS quality.

Q. What were MSSI's recommendations to the FCC on the future of UWB?

A. We believe that the FCC should allow UWB, but should push it up to above 3GHz. This way it will be above the critical frequencies for GPS and PCS and will be a win-win for both UWB and GPS users.

Q. Some companies are using a form of pulse position modulation to encode data on their UWB signals. Is this the modulation technique that MSSI uses?

A. No. They move a pulse a few picoseconds before or after it should arrive and code binary data this way. The problem with this is that multipath environments will cause delay spreads of up to several hundred nanoseconds. This will not only cause the data to be decoded improperly, but also will cause the receiver to lose sync.

Q. What modulation technique does MSSI use?

A. We use a very simple amplitude modulation on/off keying. Using our tunnel quantum detector, we find this to be a very satisfactory method of sending data. It's very similar to the way that optical fiber systems work. Of course, optical fiber also uses wavelength division multiplexing. We can also do this, since we can control the center frequency of our UWB signal.

Q. Some UWB companies use PN codes to encode the time between pulses. Do you do this?

A. No, we use the on/off keying at a fixed pulse repetition rate, and transmit data in short packets. We also use pulse dithering for some of our more specialized systems.

Q. Who are MSSI's customers for UWB?

A. For our first 10 years, the work was classified at a fairly high level. We did a lot of "spook" work. Since 1995, much of the new work was done at the unclassified level. The vast majority of our work continues to be for the U.S. government and military.

Q. What do you think are the most exciting applications for UWB in the future?

A. We think that the really exciting possibilities for UWB are where communication, radar, and position localization can all be combined in small devices -- applications like tagging vehicles or even a device that would help blind people navigate by identifying obstacles and having position tags that would let them know, for example, that they are at the east entrance of Sears.

Q. How about ID tag location?

A. I'm glad that you mentioned that. You know that the Navy had tons of stuff that they moved to Saudi Arabia during the Gulf War. There were billions of dollars of goods that they couldn't find. To avoid this problem in the future, the Navy asked us to participate in a program to develop ID tags that could locate crates on ships to sub-foot resolutions. Another company using conventional spread spectrum also participated, but they could not achieve the type of resolution achievable with our UWB system.

Q. Do you think UWB will be widely used for high speed data?

A. There are definite issues with using UWB for very high speed data. For instance, since the receive bandwidth is high, KTB (Boltzman's Constant, times temperature, times bandwidth) noise is also high. Also, channelization can be difficult with UWB since such wide instantaneous bandwidths are used. This being said, UWB does have great potential for high speed, wireless communications -- particularly in the presence of multipath -- with the proper architecture.

Q. What distinguishes MSSI from other companies in the UWB field?

A. We have been building UWB systems for more than 11 years. We are not a start-up and we are not funded by venture capital. We don't have shareholders to satisfy, so we can concentrate on satisfying our clients. We have successfully performed on dozens of projects, each of which has resulted in tangible and useful hardware.

Q: Thank you, Dr. Fontana. It's been a pleasure talking to you.

A: It's been my pleasure.

Say you come up with an idea for a new product that will revolutionize our way of life. This idea creates excitement among your family, friends, and coworkers. You do a market survey checking on the marketability of this product, and it comes back overwhelmingly in your favor. If you are looking to produce this product yourself, you will need to look around for some seed money, so next you scope out some venture capital. Again you're in luck! You find that for your idea, there are plenty of investors that are calling you back begging you to take their money. You tell yourself, "This is IT. This is the product that the world has been waiting for!"

Next, you need to meet with various people to check on the reality of being able to produce such a product. You have several meetings with engineering personnel to explain your idea. After they do their research, they tell you that they can indeed come up with a design that will do what you envision. So next you meet with your manufacturing organization to see if they can produce such an item, giving them tentative start dates and the quantities they can be expected to produce. After crunching some numbers, they come back and say, "We can produce tons of these widgets for you. We'll need to work with Engineering on the details and we may need to gear up for the additional overhead, but we can do it."

Now if you are working within a corporate structure, it's time to go to upper management with your game plan. You also need to show them the information you've collected from Marketing, Engineering, and Manufacturing to back you up. If you are on your own, using various external resources to design and produce your product idea, you will need to go back to the investors with your plan. They will need to do some additional research on your idea, just as upper management would need to do within a large corporation.

You're now ready to get on with your plan after getting everyone's approval, right? Well, there is one little detail we're overlooking here. We need to verify that the components that are to be used in the production of your widget are readily available, not only for usage in the design stage, but also for future manufacturing requirements. Too many times, a great idea cannot get realized as a physical product because this step was omitted during initial planning and throughout the design process.

During the past couple of years, many electronic components have been in short supply. Component shortages started mainly in the semiconductor industry, but now have proliferated into the passive component industries as well, even for such common parts as capacitors and resistors. Even without today's unusually serious shortages, there have been uncountable instances in the past where various electronic components have gone on allocation, foundries have had serious yield problems causing semiconductor shortages, or a new hot industry popped up and caused an overall shortage for certain components There was even one famous case where a foundry in the Far East caught fire and was out of commission for a long period of time, resulting in yet another shortage.

The best medicine for this problem is a preventive antidote. During the initial design process, Engineering personnel need to verify the current and future projected availability of all components that are expected to be used in a new or updated design. This cannot be overlooked anywhere in the design stage. As a general rule, small organizations are at a greater disadvantage than large corporations in parts procurement. This is due to the buying power of the larger organizations. If a particular company uses hundreds of thousands or even millions of dollars worth of electronic components a year, it has a much better chance of acquiring enough parts for production of their products. In addition, big companies are often "savvier" about how to make the system work for them. A big company typically has an excellent working relationship with the various component manufacturers, representatives, and distributors. Their purchasing departments work closely with these various resources so they can obtain their components when they are needed. They know to advise these resources well in advance when the components need to be delivered to meet anticipated product manufacturing schedules.

This kind of savvy starts way down the line. Marketing and sales people need to let Purchasing know the projected sales figures and delivery schedules for a product, so that components that are used in the manufacturing process of this product can be procured in time for Manufacturing to meet these schedules. Sales projections can and generally do change, and Purchasing needs to stay on top of them, especially if the sales figures are exceeding initial estimates.

Now, where does all this fit in with a new design idea? It's nice to have a great idea that many people are excited over on paper, but the idea of a new product on paper usually does not put bread on the table. This design idea first needs to become a tangible working item that people can see in actual operation that still gets them excited. Now that is a work of magic! Anyone can come up with a great idea, but coming up with a working model of this idea, having overcome challenges all the while, is a totally different animal.

Many times in the past, new designs are worked on for long periods of time, either by small design teams or by several groups of teams, only to find out at the end that the new product is not manufacturable because of a shortage in even a single component. This component may only cost a few pennies, but it is essential to make the product work. As mentioned earlier, this shortage could either come from uncontrollable circumstances or it could be the result of not doing all proper research up front during the design stage. A lot of additional money is usually put into redesigning the product at this time to come up with the changes necessary to work around the shortage and get the product out to market. Not only additional engineering costs are involved here, but Purchasing, Manufacturing, and Marketing come back into the arena as well. Marketing needs to change sales forecasting, Purchasing has to change delivery schedules, and Manufacturing needs to adjust its production planning.

When selecting components for a new design, there are several guidelines to go by to minimize parts problems:

• Try to use as many components as possible that are readily available from several different manufacturers. This can take a fair amount of time up front, but can save all involved countless hours - and dollars -- later on. Many times, however, there is not a lot of choice in using a sole-sourced component, especially with technology changing as rapidly as it is today. If you strive to stay on the leading technological edge of product designs, you are more likely to run into unavoidable component shortage situations.

• You may run across a single new component that can replace several components that were used in the past in a particular circuit design. Use of this new part may result in substantial savings in cost and increased reliability, but its availability may be suspect. If you have room on the printed circuit board, you can include the older standard design using the additional components in "parallel" with the newer design circuitry. If enough of the new parts can be found, the "extra" components can be left off during manufacturing. Where high demand for the new component results in shortages, however, the parallel circuit can really "save the bacon" for the product manufacturer. In this case, you simply don't use the newer component while it's in short supply, but instead populate only the additional "paralleled" circuit instead.

• When selecting a component with a small footprint, make certain that it is readily available. Larger sized versions of the same part are sometimes easier to obtain. If possible, leave enough room on the circuit board so you can utilize more than one size of the same component if necessary.

• Be careful when choosing two or more manufacturers of the same part. Sometimes one of the pins on one of the components does not have the same functionality as the others!

• Be careful when using specialty integrated circuits that are not widely available from several different manufacturers. Components that have been on the market for a number of years are likely to become obsolete during the lifetime of your product. Special integrated circuits that were a hot item at one point in time are especially likely to become obsolete quickly, as newer technology replacements will be developed first for high-demand applications. Backfitting an existing design with new parts is probably one of the worst nightmares in manufacturing.

• Most importantly, work with manufacturers' representatives and distributors up front to find out about any potential component availability problems that may be on the horizon. Develop a close working relationship with these representatives and distributors early and maintain this endeavor throughout the design process. Give them the initial bill of materials list up front and keep them notified of any changes to it that they need to know about. Ask them to keep you posted of any upcoming changes with any of the components on your list. You will be rewarded many times over in the future for this additional effort.

All this extra research, liaison, and design workarounds do take more time, but may make the difference in turning your product idea into reality in the marketplace! At Pegasus Technologies, we help our customers go from concept to an actual working design. We are concerned not only about a good product design, but also are genuinely interested in manufacturability and continuing product support for our customers. For this reason, we verify to the best of our ability that the components that we use in our designs will be there so your product will be ready to ship on schedule, both now and in the future.

Anyone who has spent more than ten minutes researching digital communications has run across the cryptic notation E_b/N_o. Usually this shows up when discussing bit error rates or modulation methods. You may have a vague feeling that it represents something important about a digital communication system, but can't really put a finger on what or why. So let's take a look at just what this E_b/N_o thing is and why it's important.

First of all, how do you pronounce E_b/N_o? Most engineers that I know say "E bee over en zero," though some of the more fastidious ones say "E sub bee over en sub zero". At any rate, even though "No" is usually written with an "Oh" instead of a zero, it is not pronounced as the word "no".

E_b/N_o is classically defined as the ratio of Energy per Bit (Eb) to the Spectral Noise Density (No). If this definition leaves you with a empty, glassy-eyed feeling, you're not alone. The definition does not give you any insight into how to measure E_b/N_o or what it's used for.

E_b/N_o is the measure of signal to noise ratio for a digital communication system. It is measured at the input to the receiver and is used as the basic measure of how strong the signal is. Different forms of modulation -- BPSK, QPSK, QAM, etc. -- have different curves of theoretical bit error rates versus E_b/N_o as shown in Figure 1. These curves show the communications engineer the best performance that can be achieved across a digital link with a given amount of RF power.

Figure 1. BER vs E_b/N_o
(Thanks, Intersil for this figure)

In this respect, it is the fundamental prediction tool for determining a digital link's performance. Another, more easily measured predictor of performance is the carrier-to-noise or C/N ratio.

So let's pretend that we are designing a digital link, and see how to use E_b/N_o and C/N to find out how much transmitter power we will need. Our example will use differential quadrature phase shift keying (DQPSK) and transmit 2 Mbps with a carrier frequency of 2450 MHz. It will have a 30 dB fade margin and operate within a reasonable bit error rate (BER) at an outdoor distance of 100 meters. Hold on to your hat here! Remember that when we play with dB or any log-type operation, multiplication is replaced by adding the dBs, and division is replaced by subtracting the dBs.

Our strategy for determining the transmit power is to:

• Determine E_b/N_o for our desired BER;
• Convert E_b/N_o to C/N at the receiver using the bit rate; and
• Add the path loss and fading margins.

We first decide what is the maximum BER that we can tolerate. For our example, we choose 10^-6 figuring that we can retransmit the few packets that will have errors at this BER.

Looking at Figure 1, we find that for DQPSK modulation, a BER of 10^-6 requires an E_b/N_o of 11.1 dB.

OK, great. Now we convert E_b/N_o to the carrier to noise ratio (C/N) using the equation:

Where:
fb is the bit rate, and
Bw is the receiver noise bandwidth.

So for our example, C/N = 11.1 dB + 10log(2x106 / 1x106) = 11.1 dB + 3dB = 14.1dB.

Since we now have the carrier-to-noise ratio, we can determine the necessary received carrier power after we calculate the receiver noise power.

Noise power is computed using Boltzmann's equation:

Where:
k is Boltzmann's constant = 1.380650x10^-23 J/K;
T is the effective temperature in Kelvin, and
B is the receiver bandwidth.

Therefore, N1 = (1.380650x10^-23 J/K) * (290K) *(1MHz) = 4x10^-15W = 4x10^-12mW = -114dBm

Our receiver has some inherent noise in the amplification and processing of the signal. This is referred to as the receiver noise figure. For this example, our receiver has a 7 dB noise figure, so the receiver noise level will be:

We can now find the carrier power as C = C/N * N, or in dB C = C/N + N.

This is how much power the receiver must have at its input. To determine the transmitter power, we must account for the path loss and any fading margin that we are building in to the system.

The path loss in dB for an open air site is:

Where:
PL is the path loss in dB;
d is the distance between the transmitter and receiver; and
λ is the wavelength of the RF carrier (= c/frequency)

This assumes antennas with no gain are being used. For our example,

Finally, adding our 30 dB fading margin will give the required transmitter power:

Our result, 55 mW, is well within a reasonable power level for spread spectrum links in the 2.4 GHz band. So we see that, in this example, our 100 meter range is a very reasonable expectation.

So, what is all this E_b/N_o stuff? Simply put, it's one of the "secrets" used by top RF design engineers to evaluate options for digital RF links, and is a crucial step in the design of systems that will meet performance expectations.

Bibliography & Related Links

Intersil Tutorial on Basic Link Budget Analysis, by Jim Zyren and Al Petrick Adobe Acrobat format -- 80K

OFDM stands for Orthogonal Frequency Division Multiplexing and is an up and coming modulation technique for transmitting large amounts of digital data over a radio wave. The main proponent and inventor of OFDM is Wi-LAN of Calgary, Alberta. They have a very interesting simplified description of how OFDM works on their website>.[Editor's note 2/2/09: Sigh. No longer. Their website is substantially useless these days for educational purposes.]

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 recently 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 preceeding paragraph and multiply it 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). This is now so common that many people just assume that coding will be used, so they drop the "C".

For additional information on this technology, refer to OFDM Forum page [Editor's Note 2/2/09: this page now leads one to Wilan's essentially useless site]. We've also added a new page on OFDM to SSS Online, and will keep you posted on new developments by this means.

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 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 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!

Editor's Note: This is the first in a series of SSS Online articles that will focus on USB, its uses, and the internals of its operation.

With increasing use of the Internet, cellular telephones, and communications in general, communications interconnectivity has been growing at a rate that no one could have imagined five years ago. As a result, problems have arisen in the wireless industry related to limited bandwidth as well as connections to "wired" devices. The RS-232D standard has been a staple in "wired" communications for a number of years. Generally, this standard limits the rate of communication speed to 115,000 bps, which is not adequate for today's technology. Also, RS-232 does not allow daisychaining multiple devices that are attached to one main device, unless a special design is implemented for the purpose.

Of the many new serial protocols that have popped up in response to these problems, USB (Universal Serial Bus) currently seems to be reigning supreme. One of the reasons that USB was implemented was to replace existing serial and parallel ports on computers. USB has several advantages for this application, which is why it has been included in most of the new PCs that have been shipped since Windows 98 was released in late June of 1998:

• It uses a much higher data transfer rate than many common serial data formats.
  
  
• It allows a large number of devices to be attached to a single host USB connector. Up to 127 devices can theoretically be used on a single USB port, but realistically this could cause bandwidth problems and other potential complications.
  
  
• It simplifies the connection to external devices. USB supports "plug and play" -- the operator does not need to be heavily involved in the set-up process. When a device is connected to a host's USB bus, it is immediately recognized by the host, dynamically enumerated, and assigned an address by the host. Once the host knows what kind of device has been plugged into it, it interrogates the device to understand how to communicate with it. While a device driver needs to be loaded on the host PC, some operating systems have "generic" drivers embedded in them that will work for some common USB devices such as keyboards.

USB Implementers Forum, Inc. is a non-profit corporation formed by a group of companies that developed the initial USB specification. Among their activities is the development of a testing and certification program for compliance with the USB specification. Before a device can use the USB logo or icon, it must undergo rigorous testing and be certified as USB compliant.

While this compliance testing goes a long way toward ensuring device compatibility, there are no guarantees, however, that all USB certified devices will be able to work together compatibly over a particular USB bus. This is not only because of differences in interpreting and implementing the USB standard and failure by some manufacturers to adhere to the standards, but also because of the rapid development of technology itself. For example, because of the limited bandwidth of the USB 1.x standard, care must be exercised when combining devices compliant with that specification where data receipt is time-sensitive -- such as several devices on one bus that all transfer video simultaneously.

There are several different editions of the USB standard that have been released:

• USB 1.0, the first edition, was released in January 1996. It supported 1.5 Mb/s (low speed) and 12 Mb/s (high speed) transfer rates. Note that this is Megabits per second and not MegaBytes per second -- a common misunderstanding. A percentage of this data rate is reserved for USB protocol overhead, so the actual data transfer is less than the indicated speed. How much less depends on the transfer type and the packet sizes.
  
  
• USB 1.1 was released in September 1998. This edition fixed many of the problems in release 1.0.
  
  
• USB 2.0 was released in early 2000 and has increased the maximum transfer speed by a factor of 14 up to 480 Mb/s! USB 2.0 is backwards compatible with USB 1.x. Although the USB 2.0 specification has been released, operating programs for personal computers are not expected to have USB 2.0 support until about the fourth quarter of 2001. A few peripherals supporting USB 2.0 have already begun to show up on the market in late 2000.
  

Windows 95 (and earlier versions of Windows) has no USB support. A sub-release of Windows 95 (OEM Service Release 2) was issued to computer manufacturers only and it added somewhat limited support for the USB protocol. Windows 98 added additional support and fixed some problems that were in the 95 OEM Service Release 2. Windows 98se (98 second edition) released in early June of 1999 had more robust support for USB. Both Windows 2000 and Windows Me released in early 2000 added additional features. Apple Computer's OS 9.0.4 was released late summer of 2000 and added much better support for USB for the Mac. Many problems associated with USB can be solved by using the latest version of the appropriate operating system.

In this article, the term USB includes all the above revisions as a general protocol. However, the operating details described below refer to USB 1.x (both USB 1.0 and 1.1) unless otherwise specified. Also, when a "device" is mentioned here, it is referring to a USB-compliant peripheral.

USB uses a four-wire cable interface. Two of the wires are used in a differential mode for both transmitting and receiving data, and the remaining two wires are power and ground. The source of the power to a USB device can come from the host, a hub, or the device can be "self powered." There are two different connector types on each end of a USB cable. One of these connectors is for upstream communications, and the other for downstream. Each cable length is limited to about 5 meters.

USB has four types of communication transfer modes:

• control,
• interrupt,
• bulk, and
• isochronous.

In interrupt mode, interrupts do not occur in the usual sense. As in control mode, the host has to initiate the transfer of data. Interrupt mode works by the host querying devices to see if they need to be serviced.

Bulk mode and isochronous mode complement each other in a sense. Bulk mode is used when data accuracy is of prime importance, but the rate of data transfer is not guaranteed. An example of this would be disk drive storage. Isochronous mode sacrifices data accuracy in favor of guaranteed timing of data delivery. An example of this would be USB audio speakers.

These four modes will be discussed in more detail below.

Above is an example of USB ports found on PCs and on some USB peripherals including keyboards and monitors.
Thanks, USB Forum, for this picture!

The PC host typically has connections for two external USB ports. Each of these two connectors on the PC is actually a connection to a separate root hub inside the PC. If either of the two root hubs needs to have more than one device connected to it, a downstream USB hub is required to expand connections. Hubs are used to add to the number of devices that can be connected to one USB port. They can be considered to be a repeater of sorts and also a controller. When a device is connected downstream of a hub, the hub does the connect detection of the new device and notifies the host.

Hubs can be inside the device itself -- for example, in a keyboard that may have an additional two downstream USB connectors for additional devices. A hub can have a combination of high and low speed devices connected to it, up to a maximum of four additional hubs downstream from itself. A hub's upstream port to the PC must be high speed. The hub acts as a traffic cop, handling communication to downstream devices as either high or low speed. A hub can ignore a downstream device that is not behaving properly. Hubs can be either self-powered or receive power from the USB bus. USB 1.x hubs support both low and high-speed data transfers.

There are several hardware requirements for devices that are placed on the USB bus. Five volts is the nominal supply voltage on the bus. A device that requires 100mA or less can be powered from the host or any hub, provided that the total available power hasn't already been exhausted by other devices. A device on the bus can draw up to 500mA from it. However, not all USB hosts (especially a battery powered PC) or bus-powered hubs will allow a device to draw more than 100mA from the bus. For this reason, a USB device that draws more than 100mA should, in most cases, be self-powered .

A device tells the host how much current is required for its operation. Self-powered devices usually get their power from a separate power supply or batteries. A battery-powered device plugged into the bus can get its power from the bus if it meets the tests above, and it can then switch back over to battery power when it is disconnected from the bus or when the host is shut down. When a device is in suspend mode, it cannot draw any more than 500uA from the bus if it is bus-powered. Also, if a device has not seen any activity on its bus in 3 mS, it needs to go into suspend mode. A host can initiate a resume command to a device that is in suspend mode. A device can also issue a remote wakeup to an inactive host to make it active.

All devices have endpoints, which are memory buffers. An endpoint can be as simple as an addressable single register, or it can be a block of memory that is used to store incoming and/or outgoing data. There may be multiple endpoints inside a device. Each device has at least one endpoint -- "endpoint 0"-- which is used as a control endpoint. It must be able to both send and receive data, but can only communicate in one direction at a time. Typically, when a device receives data such as an Out or Setup command from the host, this data is stored in the endpoint and the device's microprocessor is interrupted and works on this data. When a device receives an In command that is addressed to it from the host, data for the host that is stored in the endpoint is sent to the host.

The host is considered to be the master in most all cases. One exception is when a device issues a remote wakeup to the host as discussed above. There are time limits for both the host and device to respond to each other. For example, if the host requests data from a device using an In command, the device must send the data back to the host within 500mS, in some cases. Depending on the transaction type, the host and/or the device may respond to data received with an acknowledgement. Data transfer involves quite a bit of error-checking and handshaking. The different types of data packets sent and received use different ways to verify correct data transfer.

A logical connection link needs to be set up between the host and a device before a transaction can occur. This connection is referred to as a Pipe. It is set up as soon as possible after a host has recognized a device as being connected. When the host responds to a connect signal from the device, one of the parameters that is sent to the host is the device's required data transfer type and speed. The host can refuse to establish a Pipe if the host does not have enough bandwidth to support the device's request or if its power requirements cannot be met. The device at its discretion can lower its requested data rate and try again until the host accepts it and initiates a Pipe.

When a device is connected, it also sends to the host descriptor information on the types of endpoints in the device, the type of data transfer it uses, size of data packets, endpoint addresses within the device, and if used, the time required between data transfers.

The following describes a typical data flow for a device when it is initially plugged into a host's bus while the host is active. Remember here that the host has an internal USB hub, and additional hubs may be connected downstream from the host's hub.

1. The host recognizes that a device has been attached to one of its USB hubs. It realizes this by a simple resistive divider that is connected to the differential data pair of wires in the USB bus. These resistors are inside the USB hubs and devices.
   
   
2. The host sends a Get_Port_Status request to the hub to find out more about what has been plugged in. It could be another hub, a device connected directly to the host hub, or a device that has been plugged into one of the downstream hubs.
   
   
3. After receiving a response from the hub, the host issues a Set_Port_Feature command in which the hub issues a reset over the data pair but only to the newly connected device on the USB bus.
   
   
4. The host then checks to see if the device has come out of the reset state by issuing a Get_Port_Status command to the hub. After reset, the device is in the Default state and can only draw a maximum of 100mA. In Default state, the device can communicate with the host through Endpoint 0.
   
   
5. The hub now detects the device's speed by using the resistive dividers that are attached to the USB bus. The hub sends the speed of this device back to the host.
   
   
6. The host then sends a Get_Descriptor command to the hub in which the hub gets the packet size needed from this particular device and sends the result back to the host.
   
   
7. The host now issues a Set_Address command to the hub which sends this information to the device. The device in turn acknowledges the command back through the hub to the host and sets up this address internally.
   
   
8. To learn more about this device, the host sends a Get_Descriptor command to the address that the device has been given. The information that is returned to the host consists of various details of the device that the host needs to know for its operation. These queries by the host continue two more times to retrieve all the information needed.
   
   
9. Based on the information received from the device, the host determines the best device driver to use for communications with it.
   
   
10. The device driver in the host now takes over by requesting a Set_Configuration command. There can be several configurations for one device, and the device driver determines which to use based on information received from the device in response to the Get_Descriptor command.
    
    
11. The device is now ready for use.
    
    

As you can see, the USB protocol is a fairly complex arrangement. This strict pattern of query and response, however, is important in alleviating potential conflicts on the bus.

In part two of this article, we will continue our exploration of USB basics.