Wireless Systems Design

Third Method Of UWB Solves The Home Networking Problem

And you thought you knew what Ultra-Wideband really was.

Like many of the technologies we have grown to love, Ultra-Wideband (UWB) has experienced several poignant changes leading up to its current state. UWB is that weird and wacky wireless technology that spreads the signal over at least 500 MHz of bandwidth in the 3.1- to 10.6-GHz range for accomplishing blazing data rates over short ranges (the wireless solution to interconnecting HDTV and other consumer products in home entertainment systems. What we ended up with is three really different types of UWB and only the beginning of the home entertainment and networking solution. This is a look at the third and more recent version of UWB that promises to fully meet the needs of the home networking dilemma.

What ever happened to impulse radio?

Before UWB became commonly known as UWB it went through a bunch of changes. Prior to UWB, impulse radio was the common terminology, and to reach back a bit farther, it also has roots in the 1960s and 70s—then you may have heard it called carrier-less radio, when it was first discovered and developed for military radar and secure radio. Most UWB transmitted data by sending very short impulses called monopulses or wavelets, sort of like one cycle or a little more of a sine wave (but not exactly a sine wave). Pulse position modulation (PPM) or a version of binary phase shift keying (BPSK) modulation was used to achieve data rates up to and well over 1 Gb/s. Direct sequence (DS) coding is used to improve, gain and sort out the multiple users.

The big problem with impulse UWB is that it generated a very wide bandwidth so it had to be restricted to fit into the microwave range. As you may recall, UWB was blessed as an official wireless technology by the FCC in 2002. When the FCC approved of UWB it had the impulse format in mind, but had the good sense to make the definition broad enough to include any other technique that blatantly used up bandwidth like there was no tomorrow. The basic definition is any radio modulation technique with a minimum bandwidth of 500 MHz or at least 20% greater than the center frequency of operation.

A number of companies developed impulse or DS coded systems and chips. Perhaps the most aggressive was Xtreme Spectrum who created the Trinity chip-set that did a great job of implementing this form of DS-UWB. Xtreme Spectrum was acquired by Motorola (now Freescale) and fought valiantly to get this form of UWB selected to be the IEEE's 802.15.3a standard. But that was not to be, and the second form of UWB came about.

OFDM Lands One More Wireless Standard

I am not sure why DS-UWB didn't cut it. It certainly was not the technology, which proved to be reliable, robust and super fast (version over 1 Gb/s data rates) up to 10 meters. I suspect it had to do with all the patents and IP problems vendors encountered, and they surely weren’t going to pay for that. Raw politics, I’m sure, also played a major role. Furthermore, Intel and Texas Instruments became entrenched in orthogonal frequency division multiplexing (OFDM), the current darling of all new wireless technologies and services. TI and Intel put forth a second form of Multiband or OFDM UWB as a candidate for the IEEE standard. It got lots of support and attention. Yet, because of the contentiousness of the standard battle, no technology came away with the standard and the IEEE abandoned the effort entirely. But with huge support for the OFDM approach, the WiMedia Alliance was formed to develop and certify the OFDM version of UWB.

The OFDM type of UWB does away with the impulse idea completely and divides up the spectrum from 3.1 to 10.6 GHz into big chunks with up to 14 defined bands 528-MHz wide. There are 128 carriers 4.125-MHz wide for each of these bands. Currently, only the three lower bands from 3.168 to 4.952 GHz are used. Standard BPSK or QPSK is used per carrier and data rates from 53 to 480 MHz are easily achieved. The lower speed signals traveling 10 meter or more and the highest speeds limited to about 3 meters.

Today, the center of the universe for UWB is Multiband OFDM and the WiMeda method. There are several key vendors such as Alereon, Focus Enhancements, T-Zero, WiQuest, Wisair, and a few others making chipsets. The target is wireless USB connections. Some of these companies recently received blessing from the USB Implementers Forum as being fully compatible with USB 2.0 standards. This will no doubt be a seriously successful niche. Afterall, who doesn’t want a wireless USB connection?

This band of wireless USB advocates have not totally given up on the potential for using their UWB for home networking. Yet, many have doubts about range and reliability, rather than the data rate. If the reliable range is only a few meters, 10 meters maximum, then UWB is not the home networking solution that everyone was hoping it would be. T-Zero has added MIMO to its UWB version making things a bit better, but most of these UWB vendors backed off now that 802.11n standard Wi-Fi products are available (and even more will ensue after the final standard is blessed early next year). Wi-Fi 11n products are fast and have the range to address home networking, yet even then that does not seem to cut it for those who need solid video-capable home networking technology now. That is why organizations such as MoCA and Home PNA with their wired solutions to home networking have emerged as the clear winners with the likes of AT&T, who is implementing Internet protocol TV (IPTV) across the country. The Multimedia over Coax Alliance (MoCA) promotes their standard that puts video and other multimedia on the installed base of cable TV coax existing inside most homes. The HomePNA advocates a similar method with home installed telephone wiring, and the Home Plug Alliance promotes fast home networking over AC power lines. All of these systems work well and are significantly reliable. Unlike wireless, they don't have to fight the battle of multipath reflections, penetration of walls, ceilings and floors, or radio noise. That is not to say that they will kill off any wireless effort in the home, but they will certainly take a chunk of that market.

The Light At The End Of The Tunnel Pulse~Link, one of the pioneers of original impulse UWB, did not get caught up in the Multimedia OFDM UWB whirlwind. Instead, they created a third method of UWB that has some very interesting features that distinguishes it from the other methods. Their product, CWave, uses a continuous sine wave carrier modulated with a special version of BPSK. It starts with a 4.05 GHz carrier that is XORed with the serial digital data rate at 1.35-GHz. Timing of the data-pulses is such that it produces three cycles of carrier for each bit or symbol using BPSK (180 degree shift between symbols). The resulting signal is certainly enough to fit the definition of true UWB, extending from 2.7 to 5.3 GHz for a bandwidth of 2.6-GHz— its power level meets the FCC’s strict -41 dBm ceiling as well.

The really interesting thing about CWave is that it is not just a wireless technology. It was also created to match up to cable TV coax within the home. Using the same standard, you can put together a hybrid wireless/cable TV coax system compatible with the exact equipment you have and where it is located.

The Pulse~Link solution is implemented in a three-chip-set called the PL3100. It will work in a wired or wireless product. The PL3120 RF transceiver is unique in that it directly digitizes the incoming signal with a sampling rate of 10.7 GS/s, parallelizes it and sends it to the PL3130 baseband chip for down conversion, demodulation, filtering and other functions. The baseband chip handles part of the modulation tasks, but the final modulation takes place in the RF chip which also includes the transmit power amplifier. On-chip PLL synthesizers set the operating frequency. A separate LNA chip designated to the PL3110 is used ahead of the receiver to provide extra sensitivity.

The baseband chip is the workhorse of the set. It implements a full 802.15.3b media access controller (MAC). It can handle asynchronous or isochronous data and provides full quality of service (QoS) traffic support. Other features include an advanced forward error correction (FEC) method known as low density parity check (LDPC) and a piconet coordinator for wireless connectivity. The physical (PHY) data rate is selectable from 21 Mb/s to 675 Mb/s depending upon the level of forward error correction (FEC) needed to achieve the desired QoS. The chip-set works with Ethernet, IEEE 1394 and HDMI high-speed serial interfaces. These external interfaces attach to the baseband chip's PCI bus.

Whether you are designing an HDTV set, DVD, DVR, media center PC, high-end audio system, set-top box, or gaming system, the CWave chip-set is an interesting option to consider.

Pulse~Link

TAGS: Freescale
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