UWB Wireless Access Could Be Here Soon

fluppy88 writes: "802.11b doesn't have anything on UWB. With a potential of 1000M bits/sec it blows the pants out of 802.11b and doesn't eat up the tightly controlled spectrum. This article on CNN gives an interesting introduction to UWB, another candidate in the future of wireless." It was mentioned here a while ago, but much more mired in controversy about whose idea it was. Now there are several companies which seem anxious to get products based on UWB to market -- if it's approved.

Comments

Affects GPS

[mESSDan]

The University of Texas ran a battery of tests last December looking at the UWB's impact on GPS. The data were handed over to the FCC last February with several other independent studies. UWB can have an effect, but whether it is unacceptable depends on how one defines it, Cummings says. The tests also found the effects vary widely depending on how the UWB transmitter is tuned, timed and powered.

I wonder what that means? The FCC is going to kill this quick if it messes with GPS, which is IMO, more important.

All of the subjectivity in (how one defines unacceptable) will probably cause everyone to take a few more years before they start doing anything with this.

Intel

[jedwards]

Another technical and practical article at Intel - Ultra-Wideband Technology for Short- or Medium-Range Wireless Communications

Re:Gotta pick one or the other...

[tzanger]

Where do they get these guys? First he says that it doesn't use any spectrum...then he says that anything below 2 GHz will interfere with existing Nav and Comm systems. Gotta be one or the other. Can't be both.

Yes, it can be both

UWB works by sending single-cycle pulses. The information is carried by when the pulse is transmitted with respect to a reference.

Since there is no carrier, it doesn't affect a specific part of the spectrum. However, since there is no carrier, it affects all parts of the spectrum by adding to the noise floor. That is what the big problem with this technology is and why the FCC is looking so closely at it. The UWB Consortium has more information.

Personally I don't see a problem with raising the noise floor for this technology because, as I understand it, it raises the floor uniformly and, if I understand this correctly, the actual number of devices transmitting doesn't play into this.

I've been interested in this for a while. Time Domain (warning, flash-heavy site) is a company which has been playing with this for a long long time. I was rather skeptical of this when I first heard of it but my opinions on it are changing. Hell even EDN had an article on it.

The only thing I don't quite grok is how they can get two devices to have such rock-solid stable time references (we're talking sub-picosecond jitter) without secondary clock transmitters and keep them that way. If anyone out there can help shed some light on it I'd love to hear from you.

This is simply Spread Spectrum radio

[mesocyclone]

There has been a lot of hype about UWB technology. First, some myth busting:

1. It does occupy specturm. No radio system can occupy zero spectrum except an infinite duration CW signal (which is of merely academic interest).
   
2. It does not rely on any new physical principles. It is a radio system.
   
3. It is a variation of a widely studied and widely deployed form of radio communications: Spread Spectrum.
   

UWB is a clever form of spread spectrum technology. Spread spectrum (SS) is defined as any radio communications system in which the occupied bandwidth is much wider than the baseband (information rate) signal. The most common forms of SS include FM broadcast radio, where 200kHz is used to transmit about 50kHz of signal, and CDMA - cellular phone spread spectrum invented by Qualcomm. Spread spectrum was actually invented by the actress Heddy Lamar for use during World War II and was used for secure communications between Roosevelt and Churchill.

Spread spectrum has a parameter called "spreading gain" which is the ratio (expressed in DB) of the occupied bandwidth to the baseband signal. UWB is a form of spread spectrum with an extremely high spreading gain - it occupies a whole lot of spectrum (contrary to some claims) to transmit a relatively small amount of information. However, because the signal is spread over a very wide frequency range, very little signal is required on any given frequency (or technically, in any given narrowband channel). Thus the signal appears to ordinary receivers as an increase in background noise, and under most circumstances will not do so in a noticeable way.

Traditional spread spectrum uses one of two modulation techniques to mix the information signal with a spreading signal: direct sequence (DSS) and frequency hopping (FH).

Direct sequence uses a bandwidth constrained random noise generator (typically a pseudo-random digital bit stream) and multiplies the baseband signal by this. It is also band limited, either/or by filters or the spectral characteristics of the pseudo-random noise. DSS is used in CDMA cellular phones.

Frequency hopping involves moving the carrier frequency frequenly, typically in a pseudo-random manner. In fact, usually the frequency changes a number of times for each bit transmitted.

Both techniques allow reception of the transmitted information by synchronous detection - the spreading signal is duplicated in the receiver, and used to recover the baseband signal. In the case of DSS, you generate a precisely timed replica of the transmitter's pseudo-random sequence, and multiply it by the input from the antenna (or in the intermediate frequency stages - dependinng on receiver design). Low pass filtering (integration) of the output yields the original signal.

Spread spectrum systems have some or all of the following characteristics:

1. Low probability of intercept - if you don't know the spreading sequence, and you are not close to the transmitter, you probably cannot even detect its existence.
   
2. Interference to both narrowband systems and other spread spectrum systems takes the character of an increase in background noise. A high spreading gain SS system will put a relatively tiny amount of signal into the bandwidth of a typical non-spread spectrum receiver (NBFM, AM, SSB).
   
3. Reduced sensitivity to multiphath distortion.
   

To get back to UWB, it uses very narrow pulses as its spreading signal. The Fourier spectrum of a very narrow pulse shows a very flat distribution of energy over a very wide bandwidth. In this sense, UWB is spread spectrum. Likewise, it recovers the signal in a similar manner as other spread spectrum signals - it uses a regenerated narrow band pulse to synchronously sample (a form of multiplication) the radio spectrum, thus recovering the original pulse (minus pulse spreading caused by reflections and frequency dispersion).

AFAIK one could duplicate the behavior of a UWB system by using an extremely wide band direct sequence system. It would provide the precise ranging, see-through wall radar characteristics. It would have the low detectability. It would have the low interference to narrower-band signals. However, the UWB system appears to be much easier and inexpensive to build.

Re:This is simply Spread Spectrum radio

[mikeb]

Spot on! Some of the posts here say stuff like "I am not a radio engineer" - well I am (or was) before picking up software as a better paid and more interesting job. UWB is, as many have said, just a variant on spread-spectrum transmission. It isn't spectacularly new and - I'll bet real money - will eventually disappoint. There is no free lunch. Claude Shannon at Bell Labs did the basic theoretical work: you either either burn spectrum or use power to get the data through. Each has its up and down side. If everyone pumps the bandwidth side, you are forced into a power race, so UWB works until somebody else starts the same game. It has been used for years, especially in clandestine communications. The theory is well understood and this stuff REALLY isn't new at all.

UWB defies standard spectrum management

[Bobs2paksVegaSwirled]

UWB systems produce RF emissions across a vast bandwidth, exceeding 1 GHz in some cases. Many devices don't have a conventional carrier frequency, but are characterized by a "maximum in the power spectrum envelope." Within any given conventional frequency band, the receivable power from a single UWB device is so low that it is far below the noise threshold of the conventional devices that operate in that band. The emissions are not receivable even by sensitive measurement equipment unless the UWB device is within few meters. For these types of wideband emissions, the potential for interference is determined entirely by the nuances of the "victim" receiver implementation. Conventional spectrum management techniques rely on the existence of an interference threshold -- a power level which may be measured independently of a particular receiver implementation. This threshold does not exist in the same sense for UWB devices; a separate value and measurement technique would have to be defined for every receiver implementation in the entire emission rage of the UWB device itself. The UWB industry claims (and has some evidence to support) that such an exhaustive list of values is not necessary given the low power level of the devices.

The important questions is how potential victim receivers will cope with an aggregate of many UWB emitters operating at the same time. If this technology is widely adopted, will there be an aggregate noise effect that is significant? Much work has already been done to cope with the noise properties of microwave ovens, which are centered at 2.45 GHz. See this report , p. 48 of the pdf. The large hump near 2.45 GHz is due to emissions from microwave ovens, and is measurable anywhere there is a sizable population (town > 20,000 people) in the U.S. Microwave ovens are very different than UWB devices -- they emit several orders of magnitude more power, and are bandwidth limited, but there are many technologies that operate within this band despite their emissions (802.11b is one of them). These technologies were designed specifically to operate in the noise environment generated by microwave ovens, and the band itself is designated to be a kind of "free for all" frequency range known as an ISM (Industrial, Scientific and Medical) band. Existing receivers in the bands where UWB devices produce emissions were not designed in such a manner.

Nevertheless, the potential increase in communication capacity offered by UWB devices demands that it be scrutinized for interoperability with these existing receivers, and given a chance to fulfill its promise.

Not just the noise floor...

[Ungrounded Lightning]

Where do they get these guys? First he says that it doesn't use any spectrum...then he says that anything below 2 GHz will interfere with existing Nav and Comm systems. Gotta be one or the other. Can't be both.

Yes, it can be both

No it can't.

There is only so much spectrum. The faster you change the signal, the broader the chunk of spectrum you use.

You can use it for a shorter time, and end up with the same time-bandwidth product.

You can control signal intensity more finely to encode more bits, until the ambient noise (or weaker interfering signals) would confuse the decoder.

You can direct your signal so that most of the energy goes toward the receiver rather than spreading out uniformly (though this gets harder as the bandwidth gets wider).

You can restrict its polarization to one of a complimentary pair, leaving the complimentary polarization's half of the spectrum free (or using it for a second or a return signal).

But that's IT.

If two transmitters are hitting a receiver with energy in the same chunk of spectrum they interfere. Spread the actual bits around so they're transmitted redundantly in different parts of the spectrum (not just hop the carrier to put a burst of bits in one chunk and move on) and you might be able to pull them out of some interference. But then you've used up several times as much spectrum in the first place - as have the interfering signals from other users of the same scheme.

UWB works by sending single-cycle pulses. The information is carried by when the pulse is transmitted with respect to a reference.

Since there is no carrier, it doesn't affect a specific part of the spectrum. However, since there is no carrier, it affects all parts of the spectrum by adding to the noise floor. That is what the big problem with this technology is and why the FCC is looking so closely at it. The UWB Consortium [uwb.org] has more information.

This scheme doesn't "not use spectrum". It uses the ENTIRE spectrum - up to the limit of the transmitting equipment. The shorter the pulse (so you can space them more closely and send more bits) the higher the limit. When it talks it steps on EVERYTHING - one pulse, one "POP" in a radio, one fleck of snow on a TV screen, one bright spot on a radar. The has to be above the noise floor itself to be heard - and the "noise floor" includes all those other signals it's interfering with.

If your data rate is low you can keep the signal weak - at the receiver. The pulses spread out their energy over an enromous chunk of bandwidth, so your reciever can measure the energy over the whole specrtum and recover the desired data from the other signals-assuming there is only ONE transmitter using this scheme, of course.

But electromagnetic signals fall off with inverse-square. Near the transmitter you're not just "raising the noise floor". You're generating a continuous lightning bolt.

Want to send data a hundred miles? Imagine you were doing this with VISIBLE light. You're modulating an arc light at your transmitter, bright enough that the output of a solar cell a hundred miles away has more signal from your arc light than from all the other light sources (including any other arc lights) combined. Now imagine somebody on your block trying to read morse-code flashes from a distant colored lightbulb, using his own solar cell and a color filter.

Run a protocol so the radio versions of your "arc lights" take turns and you can run a network. (Think of running "Ethernet" in the real aether.) But that's ALL you'll be able to run. Goodbye TV, goodbye AM and FM radio, goodbye aircraft band, police band, CB, ... A radar (with its very directional antenna) will show only a wedge of light in the direction of your transmitter. (Which is how the authorities will find you to shut you down - if they get there before the neighbors with the torches and pitchforks.)

Meanwhile, just as the time-limited signal of time-domain modulation schemes selectively "punch a hole" in the time distribution of the signals of the frequency-domain modulation services, the frequency-domain modulation signals selectively punch holes in time-domain modulation signals. The "hole" is in the form of pattern sensitivity - selective interference with those bit patterns that correspond to the energy distribution of the frequency-domain signal. Your pulse strength has to be great enough to "shout down" this interfering signal, or those bit patterns just don't go through uncorrupted.

Time-domain and frequency-domain signals don't play well together. And the time-domain kids got to the playground first. Do you want ANOTHER war with broadcast media? Remember, if they lose it's their death, so they'll fight REAL HARD.

You can avoid the war by shaping your pulses so the energy stays in a limited band (at the cost of limiting your data rate correspondingly). But within that band you can only use time-domain schemes. You've "divided the playground". And the smaller the hunk of playground you got, the lower your data rate. Shaping your pulses stretched them out - and you have to move them farther apart to tell them apart at the receiver. How much playground do you think you can get for your gang's exclusive use?

Personally I don't see a problem with raising the noise floor for this technology because, as I understand it, it raises the floor uniformly and, if I understand this correctly, the actual number of devices transmitting doesn't play into this.

Each one of those "arc lights" raises the noise floor - by a bunch. More noise means you can't measure the signal from the desired "arc light" as accurately - which means you get less bits-per-second from it.

The total number of bits-per-second available to ALL transmitters at any given receiving antenna is a constant described by Nyquist: 2 * bandwidth in cycles-per-second * base-2 log of the signal-to-noise ratio.

But the distribution in space of the "raised noise floor" is a face-down morning-glory flower. Like those surfaces where they roll a ball bearing to demonstrate orbits, but upside-down.

Imagine a rubber sheet: You grabbed it at your transmitter and stretched it WAY up - until the sheet at the distant receiver was raised enough that the receiver could detect you yanking. And your neighbor's TV antennas are up on the peak with you. (And so is your network antenna...)

The only thing I don't quite grok is how they can get two devices to have such rock-solid stable time references (we're talking sub-picosecond jitter) without secondary clock transmitters and keep them that way. If anyone out there can help shed some light on it I'd love to hear from you.

You sacrifice a part of your bandwidth to send a pre-defined, typically repetitive, synchronization signal, to keep the receiver synchronized with the transmitter. Think of the start/stop bits on a serial line, the framing bits in T1, T3, or SONET, or the vertical and horizontal sync bars in a TV signal.

The more bandwidth you sacrifice, the faster your receiver's clock syncs up when reception starts. If you're transmitting bursts you can put most of the sync at the start of the burst to get things locked quickly (and identify the signal and its start), then use just enough to keep the receiver locked for the rest of the burst.