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Mix Magazine

This installment of The Bitstream column appeared in the August 2005 issue of Mix Magazine.

The Bitstream

This column discusses UWB and MIMO…

Finding Captain MIMO

Last month, I talked a bit about the wackiness that is UWB or Ultra–Wideband and, this month, I’m going to continue with more new, wireless networking stuff…Seems the boys and girls in the lab have been wee busy of late, muddying the waters of digital communications. By now, you’re all familiar with 802.11 or WiFi but, the networking folks at the Eye Triple EE, aka IEEE 802 Working Group (Institute of Electrical and Electronics Engineers), have been cranking out new extensions to existing standards to keep up.

Take, for example, 802.15.3a, the 480 Mbit per second standard being built. Officially, the IEEE 802.15 WPAN High Rate Alternative PHY TG3a is, according to their site, “working to define a project to provide a higher speed PHY enhancement amendment to 802.15.3 for applications which involve imaging and multimedia.” As Homer Simpson would say, “Umm, wireless multimedia. is there nothing it can’t do.” This standard–in–waiting is the backbone of Wireless USB (WUSB), and Intel wants everyone to have it.

Let’s decode the group’s name in an effort to get a handle on all this; “Wireless Personal Area Network High Rate Alternative Physical Layer Task Group 3a” tells us they’re hashing out a standard for high speed, short distance or “personal” wireless networking over an “alternative” or new PHY. The new PHYsical layer will be radio, maybe some form of UWB, but the fun part is, even at 30 feet, the data rate should exceed 100 Mbps, not too shabby for a wireless connection.

For those of you who still have long term memory, recall that last month I mentioned how UWB, low frequency UWB, can more readily pass through solid objects than traditional radio methods. Also remember that “wireless” implies broadcast, with anyone able to listen; not good for security. With Bluetooth getting a bloody nose from its many security stumbles, the WUSB folks are determined to prevent any future embarrassment.

The ultrawide part of UWB means that the radio signals are spread over an ultrawide range of frequencies, far wider than traditional channel coding techniques. Because the total energy is so spread out, there’s not a lot at any particular frequency. To an “untrained listener,“ a UWB transmission can easily be made to look like the random background radio noise that pervades our environment. This feature makes it inherently more secure than continuous wave (CW) transmissions that, on a spectrum analyzer, stick out from background noise like the proverbial sore thumb.

Here are two features of UWB that are good for you and me:

Large RF bandwidth
extremely short duration pulses, millions of times per second
pulses are randomly spread in time, ie: “Time Hopping”
data is time modulated, coded as Pulse Position Modulation
Minimal “signal profile”
minimal pulse amplitude at high repetition rate
noise–like signal means low probability of interception & detection

Here’s a fun fact; transmitted signal bandwidth is inversely proportional to pulse duration. Since UWB is pulsed, not using a continuous wave carrier, this results in a typical bandwidth of >1.5 GHz and, with a low duty cycle, the result is a low average energy density. For comparison, classic narrowband communication, like AM radio, may have a 30 kHz bandwidth while spread spectrum techniques, like 802.11a with its 5 MHz bandwidth, is much wider. For UWB, the energy is spread so broadly that there’s little energy in any specific band. This ability of a UWB signal to hide in the underbrush bodes well for the security wonks in the crowd. Reduced detection, spoofing and jamming are all good reasons, along with improved forensics, logistics, materials science and medicine, why our military has found UWB so attractive. UWB signaling techniques also require lower power than CW techniques to get the job of data carriage done, meaning longer battery life or smaller, more efficient power supplies.


Figure 1: This antenna, usually a cumbersome component, illustrates just how small UWB gear can be…

Let’s go on to another issue that you may come across from time to time and that’s the current crowded radio spectrum. If you’ve ever had to set up a gaggle of reliable radio mics, you know it can be a challenge. You may also recall that 802.11 works in the same frequency band as wireless phones and other appliances, making it sometimes difficult to set up solid WiFi networks. Because UWB doesn’t use the relatively large power requirements of CW transmission systems and the total energy is spread very far, it exhibits higher “spatial capacity” than even spread spectrum. In other words, less crowded airwaves.

Another difference between UWB and CW transmissions is that UWB transceivers do not exhibit Rayleigh Fading, whereby multipath interference causes signal reduction or cancellation. Ah, multipath, the bane of rabbit ear–equipped couch potatoes the world over, which brings us to MIMO, the hero of this month’s title.

Though radio engineers have been toiling over new cauldrons to high tech soup, they haven’t pushed the existing skillet on the back burner. The incredibly popular 802.11 standard is being improved, to compete with the likes of WUSB and other networking darlings, and the new 802.11n standard hopes to address earlier shortcomings. One of the key technologies in the upgrade formula is MIMO, or “Multiple In, Multiple Out,” a kind of diversity method whereby multiple transmitters, distributed in space and frequency, transmit simultaneously. MIMO is sort of “the more, the merrier” with its distributed approach, and is much less susceptible to the vagaries of transceiver location, with its associated multipath, interference and signal attenuation issues, than existing 802.11 products. Along with improved range, MIMO provides increased throughput by “bonding” multiple data streams together into a bigger, virtual pipe.

Multipath interference exists wherever and whenever a transmitted electromagnetic signal takes more than one path to arrive at the receiver. In the case of radio, that means reflections off solid objects, like tall buildings, arriving at a receiver after the direct, straight line signal. Reception in the presence of multipath interference is the holy grail for most modern communications systems, whether you’re talking WiFi or DTV.


Figure 2: Multipath Bad — Compare the green transmitted signal with the blue actual received signal

You don’t have to be a Jedi master to know that signal reflections lead to anger, anger leads to hate and, it’s all down hill from there. That incorrectly terminated AES signal path mentioned in the sidebar also causes a kind of multipath in wires, with the same degradation of signal integrity. The cause is transmission line reflections, where the digital audio signal bangs into a discontinuity, any connectors or patchbays that’re not the proper 110 Ω impedance, and reflects back down the line. The original signal, along with attenuated reflections, all make their way to the receiver, which then has to sort the mess out.

802.11n is being designed to replace the a, b and g varieties but, like consumer UWB–based gear in general, isn’t ready for prime time. That hasn’t stopped vendors from selling “pre-N” products but, interoperability issues are already causing consternation and, we already know what poor interoperability did for Fibre Channel and other new technologies. My advise is to hold off on any new networking gear that mentions “802.11n” on the box until the standard actually gets ratified and vendors settle down a bit.

According to Forrester Research, the market for “visible network” mobile devices, like cell phones and WiFi, is somewhere around 100 million to 1 billion units. For “invisible networks,” machine–to–machine communication without human intervention, the market is estimated to be much larger; 20 billion to 200 billion units. UWB, MIMO and other new technologies will enable the next generation of networking tech and that has lots of folks salivating.

Sidebar

We don’t need no stinking pulse duration!

Let me restate that terse comment made earlier: regardless of PHY, transmitted signal bandwidth is inversely proportional to pulse duration. Next time you patch AES/EBU digital audio using old school pathways designed for analog audio, you should reconsider…According to some well trusted folks, an analog audio pathway should exhibit a bandwidth 2 times that of the signal you are moving on that path. This guarantees negligible effect on phase or frequency response from the pathway itself on the signal. But, what about digital audio? Because the data has been channel coded for optimal transmission over any PHY, the signal passing over your wires is very different from its analog cousin. Channel coding, according to the EBU, “describes the method by which the binary digits are represented for transmission through the interface,” with the interface being the wire or fiber optic connection. Basically the data is channel coded as carefully timed square wave–like voltage swings representing either of two states or bits; up or down, ones or zeros. The transitions themselves need to happen as quickly as possible for a receiver to “read” the state correctly at the proper time. If the signal hasn’t reached the proper voltage, one/high or zero/low, at the proper time, then the data gets scrambled.

AES/EBU is clocked at anywhere from 44.1 to 192 kHz so, since signal bandwidth is inversely proportional to pulse duration, the bandwidth required to pass even a 44.1 kHz digital audio signal cleanly is several times that of its analog counterpart. Multiply that fact by the increasing use of double and quad–speed interfaces, for 96 and 192 kHz work, and you can see why that mini TT patchbay ain’t so good for your AES audio. Better still is impedance controlled connectors and 110 ohm wiring as specified in the 1995 standard, so that the relatively high frequencies involved pass without attenuation. I’ve said it before and I’ll say it again: If you treat digital audio, either AES/EBU or DSD, as if it were analog video, in terms of wiring and connectors, you and your signal will be happy campers.

Bio

OMas is the marcom director at Sonic Studio and, in his past life, presented a paper on UWB technology which became the basis of these recent columns. If you’d like a PDF version, give me a shout…See you next month!