A guide to test procedures for use on wireless USB via UWB - Embedded.com

A guide to test procedures for use on wireless USB via UWB

The initial public awareness of UWB came about in February 2002 whenthe U.S. Federal Communications Commission (FCC) allocated 7.5GHz ofspectrum – 3.1-10.6GHz – for use by UWB devices, enabling thispreviously classified military technology to be commercialized as hadhappened with CDMA years before.

The unique ability of UWB signaling to operate at the noise floorenables UWB devices to peacefully co-exist and share spectrum withtraditional wireless services (Figure1 below ).

Figure1: UWB operates in the noise floor of traditional wireless applicationsand is able to share the already allocated spectrum with other serviceswhile only negligibly raising their noise floor.

The low transmit power authorized by the FCC curtailed the range ofUWB links to about 10m, limiting this technology to wireless personalarea networking (WPAN) applications. This range is not a fundamentallimitation of UWB technology itself.

If transmit power limits were increased, the range of UWB wouldincrease as well. The FCC approved the UWB spectrum allocation andtransmit power limit but did not specify air interface, modulation ormedia access controller (MAC) specifications that were undertaken bythe IEEE 802.15 committee in December 2002 and abandoned in January2006.

Today, UWB implementations are not constrained to any particular MACor PHY and have the flexibility of using any MAC and PHY layers as longas they comply with the FCC spectrum mask limits.

Many of the companies originally working on the IEEE 802.15 standardjoined the WiMedia Alliance creating their own specification of UWBbased on OFDM PHY and a distributed USB-like MAC.

This WiMedia specification was published as the European ComputerManufacturers Association ECMA-368 standard. Pulse~LINK developed andenhanced their original impulsebased UWB signaling and implementedtheir solution based on the IEEE 802.15.3b MAC.

UWB applications
While the original goal of 802.15.3 was wireless video distributionwith QoS, the WiMedia Alliance has chosen to focus on the PC-centricwireless USB application. Pulse~LINK, an early pioneer of UWBtechnology, focused on the original consumer electronics application ofUWB ” high definition (HD) video distribution.

The company's approach has an interesting twist in that they havedeveloped their CWave architecture to work on both wireless and wiredmedia such as coax, powerline and phone line.

An innovative aspect of the CWave architecture is that any deviceusing the Pulse – LINK chipset is capable of supporting wireless,coaxial and powerline transmissions under a single 802.15.3b MAC,enabling HD video transport throughout the entire house on whatevermedia are available.

The isochronous 802.15.3b MAC, with QoS builtin from the ground up,is designed to support whole-home networking of streaming video,multichannel audio and high data rate networking.

Comparing PC-centric
WiMedia products with CE-centric Pulse~LINK products may at first seeminappropriate, but with the rapid convergence of PC and CE devices themission of both solutions is to move bits faster and with QoS thatsupports high-quality video, audio and data. It is the speed andquality of UWB transport that we set out to test.

UWB video distribution
While Pulse~LINK persisted with the initial goal of 802.15.3 -streaming and distribution of HD content and multichannel Audio -theWiMedia group has at least initially strayed from this goal.

Only two WiMedia vendors, Tzero and Sigma Designs, announced HDvideo distribution architectures. In addition, while both companieshave announced availability of UWB silicon as far back as InternationalConsumer Electronics Show in 2005, neither of them have commerciallyavailable products in the market.

Our understanding is that WiMedia may embrace the video applicationsin the near future, but today most of the commercial WiMedia productsare implementations of wireless USB.

Table1: Throughput requirements for common video formats and are listed.

One exception is Toshiba Corp.'s port replicator that supports USB,Gigabit Ethernet and an A/V link over a single UWB link. This link isbased on wireless digital video (WiDV), the WiMedia-compatible UWBtechnology from WiQuest Communications Inc. Video content istransported and stored in a compressed format. Most broadcast and cableTV transmissions and conventional DVDs use MPEG-2 compression.

H.264/MPEG-4 and JPEG 2000 are the emerging video compressionformats that roughly double the efficiency of video transport andstorage afforded by MPEG-2 (Table 1above ).

The video transport media in a typical home include coaxial, twistedpair, powerline and wireless. Wired video transmission technologies,such as HomePlug and HomePNA operate within a spectral mask below 30MHzin order to meet the FCC emissions limit. Pulse~LINK pioneered the useof UWB over these wired media. The wide frequency band of UWB enablesCWave to outperform HomePlug and HomePNA on their native media.

Figure2: Channel tilt – the wider the channel the greater the attenuationtilt between high and low frequencies in the channel. To correct thetilt distortion, equalization can be performed in the receiver orreverse tilt pre-distortion can be done in the transmitter.

Further advantage of the multi-interface CWave architecture is thata single device can simultaneously support multimedia, includingpowerline now supported by HomePlug and coax and twisted pair nowsupported by HomePNA. CWave's TDMA MAC can effectively bridge thesedisparate media by time-slicing the traffic over multiple networkinterfaces.

UWB technologies
Today there are two UWB solutions in the market -WiMedia and CWave. Thechallenge for both technologies is to maximize the dynamic range of thelink while still meeting the very low FCC transmit power threshold.

Due to the wide spectrum of UWB, frequency-dependent tilt (Figure 2above ) severely compromises the dynamic range of the link. Since RFattenuation increases with frequency, the wider the frequency band themore tilted the receive spectrum and the more dynamic range is lost toreceive equalization or transmit pre-distortion. WiMedia

The WiMedia specification broke up the available UWB spectrum intofive Band Groups that are further subdivided into 528MHz sub-bands (Figure 3 below ). Data transmissionscan be frequency hopped among the three subbands to reduce the averagetransmit power while maximizing the instantaneous power of symboltransmissions.

Figure3: WiMedia MB-OFDM channel assignment in the 3.1-10.6GHz band is shown.Most existing products support Band Group 1.

For example, the OFDM signal can be pulsed in the time domain overany of the three frequency sub-bands with 1/3 duty cycle, therebyreducing the average transmit power by a factor of 3 or 4.77dB.

The WiMedia techniques for spreading the power include what WiMediacalls Time- Frequency Interleaving (TFI) and Fixed FrequencyInterleaving (FFI). TFI is essentially a technique of frequency hoppingthe 528MHz wide OFDM pulses over three bands. The FCC relaxed the-41.3dBm/MHz limit to -36.5dBm/ MHz for peak power in the 528MHzsub-bands since the 1/3 duty cycle averages to -41.3dBm/MHz.

To avoid the Unlicensed National Information Infrastructure (UNII)band 5.8GHz interference from Wi-Fi, the current generation of WiMediaproducts operate in Band Group 1.

WiMedia uses MB-OFDM with data rates of 53.3, 80, 106.7, 160, 200,320, 400 and 480Mbit/s. QPSK modulation is used for data rates up to200Mbit/s and dual-carrier modulation (DCM) is used for data rates of320Mbit/s and higher.

On the TX side a single 4- to 6bit DAC running at 1GHz is typicallyused to generate the 528MHz TX spectrum and on the RX side two 4bit,1GHz A/D converters (one for “I”, the other for “Q” component) aretypically required to detect and recover the MB-OFDM subcarriers.

One only has to look at the power consumption for these componentsalone to see this is not a low power technology and that it hassubstantial complexity in both the TX and RX sections.

Figure4: A CWave modulation scheme is shown with a single carrier BPSK usingan XOR gate as the modulator. This example shows a 4GHz carrier and themodulating waveform of 1.3GHz. The integer multiple of the carriercycles per data symbol assists with carrier recovery and enhances therobustness of this scheme.

CWavePulse~ LINK 's CWave signaling scheme uses simple baseband pulses ofabout 750ps to spread a bit's total energy over the entire 1.35GHz ofspectrum. WiMedia's more complex architecture uses longer 242ns pulsesrequiring the baseband to calculate a 128 point FFT on 528MHz ofspectrum.

CWave's considerably simpler architecture may explain why CWave'soverall performance appears to be an order of magnitude higher thanWiMedia.

Pulse~LINK
claims much lower power consumption than WiMedia since theirimplementation avoids the use of power-hungry converters. CWave usessingle-carrier BPSK (binary phase shift keying) modulation, whichrequires less stringent equalization (Figure4 above ) than QPSK or DCM and thus can operate more robustlyover a wide frequency band.

With a 4GHz carrier the CWave sin(x)/x shaped spectrum has nulls at2.7- and 5.3GHz. The CWave spectrum can be moved anywhere within the3.1-10.6GHz FCC band by changing the carrier frequency. The bandwidthcan be expanded or contracted by varying the frequency of themodulating signal (data rate). The current CWave reference designoperating band is 3.3-4.7GHz centered around 4 GHz.

CWave implemented a new cutting edge error correction algorithmknown as Low-Density Parity Check Coding (LDPC) with FEC rates of 1/2,and 3/4. They claim it gives them lower power consumption and asubstantial performance improvement over the traditional Reed-Solomon/Viterbi FEC used by WiMedia. CWave is capable of 1.35Gbit/s of raw datarate. In our tests, we were able to demonstrate actual data throughputapproaching 900Mbit/s at close range.

In addition to the wireless medium, CWave supports transport over75? coaxial cabling and CATV RF splitters installed in most homes.Pulse~LINK claims support at similar data rates for transport overpower lines and twisted pair cabling including telephone lines.octoScope did not test performance over power lines or twisted pair,but we look forward to testing these media the near future.

Furthermore, CWave's isochronous 802.15.3b MAC and PHY have beendown-selected by the membership of the Firewire 1394 Trade Associationfor extending 1394 functionality over coaxial networks within the home.

Architecturally, CWave appears to offer a significant advantage overthe status quo of video transport products requiring disparate MACs tosupport different media:

WiMedia UWB or Wi-Fi for Wireless
HomePNA for twisted pair
MoCA (Multimedia over Coax Alliance) or HomePNA for Coax
HomePlug for powerline

It's a compelling idea to have one chip that is capable ofsupporting all the above media with one common platform:

One chipset supports wireless, coax, power-line andphone-line.
One common MAC for a uniform QoS across all PHY media types
MAC supports streaming high quality audio and HD video
PHY layer bridging is inherent in the TDMA access scheme
Up to 1Gbit/s throughput on all PHY media
Whole home connectivity

The CWave 802.15.3b MAC using its TDMA channel access scheme cantime-slice traffic, enabling a single multiport device to route videoand data streams among disparate media in the home.

Given the ample throughput of CWave, several simultaneous 1,080pstreams can be sent around the house time-multiplexed on multiplenetwork interfaces and over multiple media.

Thus, a single CWave chipset can replace multiple network chips fortransport of digital content wirelessly, over coax, power-line andphone-line. Pulse~LINK's CWave 802.15.3b MAC was designed from theground up to support the QoS demands of isochronous streaming of audio,HD video and high data rate digital networking across all available PHYtransports media within the home.

Test results
Most of the WiMedia devices in the test were implementations ofwireless USB with the exception of the Toshiba R400 laptop. It featuresa built-in UWB link to its port replicator. This link is based on WiDV,the WiMedia compatible UWB technology from WiQuest.

The port replicator supports Gigabit Ethernet, wireless USB (W-USB),display and audio over a single WiMedia link to the laptop.Pulse~LINK's CWave was the only UWB device capable of multistream HDvideo transport and the only device supporting coaxial cabling inaddition to wireless.

The throughput of Belkin International Inc.'s FSU302 WUSB link wasthe highest W-USB throughput measured with around 50Mbit/s at closerange.

The Gigabit Ethernet only, the USB only and the combined throughputmeasurements of the Toshiba port replicator were under 25Mbit/s atclose range. The fact that the throughput of the GbE port drops duringthe file transfer on the USB port may mean that Toshiba purposefullymanages bandwidth allocation among the Gigabit Ethernet, USB and A/Vinterfaces sharing the WiMedia link.

It is difficult to judge how much bandwidth on the WiDV interface isallocated to the video stream. At optimum antenna orientations videolinks were achievable up to distances of 24-30 inches, but the qualityof the display at this distance was sub-optimum exhibiting a wavinessthat makes reading the text difficult. The waviness becomesimperceptible at the distance of about 12 inches.

The WiQuest WiDV chipset used to implement the Toshiba WiMediainterface uses a proprietary video compression that may be based on thewavelet method, similar to Analog Devices Inc.'s JPEG 2000.

WiQuest claims a factor of five video compression reducing the rawSXGA video throughput of 1.8Gbit/s (1,280 x 1,024 x 24 bits x 60Hz =1.8Gbit/s) down to 377Mbit/s. We were unable to verify the actualthroughput on the video link.

However, the distortion of the image observed at 24-30 inches ofdistance between the port replicator and the laptop was symptomatic ofwavelet video compression at a throughput limited to approximately30-40Mbit/s. In order to optimize display quality, the Toshiba portreplicator documentation specifies a distance limit of 0.5m..

Fanny Mlinarsky is President andJohn Ziegler is Data Communications Software Development Consultant at octoScope Inc.

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