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Peek inside the first commercial UWB hub

Posted: 26 Jul 2007 ?? ?Print Version ?Bookmark and Share

Keywords:first commercial UWB hub teardown? WiMedia?

When Wisair won the contract to be at the heart of Belkin's UWB-based Cable-Free USB Hub and donglethe first such product on the marketit was a bit of a mixed blessing. For sure, winning the design socket was a coup, but the circumstances were a bit less than ideal.

The main drawbackor challengewas that the form factor of the hub had already been determined by Belkin. It was originally based around a direct-sequence UWB chipset from Freescale that wasn't delivered. The second challenge came from the market and peer pressures that Wisair faced as it implemented the design. As the first widely available consumer device to be based on the multiband orthogonal frequency division multiplex (MB-OFDM) form of UWB being proposed by WiMedia, the onus was clearly upon the design team to make it the best design they could to reinforce that standard, as well as Wisair's already strong standing among its peers.

"This was the very first implementation and we wanted to ensure the best quality," said Serdar Yurdakul, director of marketing and business development at Wisair. With regard to the form factor, David Menshulam, Wisair's vice president of engineering, said the biggest challenge was the predetermined profile. "We had to have everything in, including the internal antennas and filtering and still get the performance [Belkin needed]."

So how'd it fare?
To see how well Wisair overcame the challenges I did some tests. The kit comes with the HWA531 USB dongle, a sleekly designed 4-port USB hub, an optional docking station for the USB dongle and the appropriate driver software. The advertising on the package promoted ranges of 9.144m (30ft) and file download rates of "up to 480Mbps." The former is credible, the latter has to be taken for what it is: over-zealous marketing. Few in their right mind would expect that from the very first such UWB implementation.

The set-up was fairly straightforward, though I ended up ignoring the manual in the end due to endless redundancies. Anyone familiar with installing new equipment would be better off following their instinct: install the software, reboot, connect the equipment and follow the install wizards from there, using the manual only for quirks like enabling the hub via the system tray.

Belkin's USB Hub and dongle

Figure 1: Belkin's slick box design meant Wisair had to contend with a low profile and no external antennas.
(View Belkin's USB Hub and dongle teardown.)

For peripherals, I chose a Canon Eos Rebel digital SLR and a Maxtor OneTouch back-up drive. The camera connected fine, but connecting the Maxtor caused my system (IBM Thinkpad) to crash. Wisair suspected that it might be a setting on the Maxtor that initiated a back-up too quickly, before the hub had a chance to register the peripheral. In any case, I tried a Western Digital 250Gbyte My Book external hard drive and that connected first time. For the tests, I chose the My Book as the data source given the access-speed limitations of the camera's compact flash storage. For test files, I used multiple JPEG images that amounted to 50.2Mbytes or 401.6Mbits. The tests were performed in a single room, as UWB's low power and high frequencies do not make it suitable for inter-room communication. The test ranges were 1-, 6-, 12-, 20- and 30ft. Three measurements were made for each test range and the average was calculated.

With the dongle inserted in the docking station and the transmit rate set to 'auto', the averaged data rates at the given ranges were 29.7, 32.18, 30.3, 28.7 and 24.6Mbps. It's important to note that this is the actual payload rate, not factoring in the USB/WiMedia overhead, which would increase the overall throughput. With the dongle attached to the laptop directly, the data payload rates were 30.9, 22.5, 20, 17.9 and 12.5Mbps, respectively.

All in all, the data rates seem to be currently on a par with 802.11g, which touts 54Mbps but has a typical payload rate of between 30 and 39Mbps. Not bad for the first implementation of a new technology, though it's still a long way from the goal of an overall throughput of 480Mbps and at $200 it's still on the pricey side.

Figure 2: Back-of-board layout for dongle and back of hub board.
(Click image for larger view)

So how'd they do that?
The design is based entirely upon a Wisair UWB hub reference design, with form-factor-related modifications. It uses Wisair's board, 502 RF front end chip and its 531 media access control (MAC) and baseband chip, both of which were announced in December of 2005. According to Yurdakul, the chips are not yet WiMedia certified so Wisair classifies them as "pre-standard." The frequency range supported is 3.1 to 4.8GHz with three sub-bands of 528MHz each. The maximum power output is 80?W or -41.25dBm/MHz. For the dongle's USB controller, Wisair chose, strictly for cost reasons, the low-power Cypress CY7C68013A with a USB PHY and DMA for the dongle, as well as an integrated enhanced 8051 MCU. "The next version of the dongle will have the USB controller integrated," said Gadi Shor, Wisair's chief technology officer. For the hub's USB controller it went with Taiwan-based Star's STR9104. "This is normally used for network processing," said Menshulam, "but it also has a USB hub."

The back of the hub's board comprises a Hynix 128Mbyte DDR SDRAM (part number HY5DU281622ETP-5), an SMSC USB2504A-JT 4-port hub controller and 32Mbits of Spansion Mirrorbit flash memory, part number S29GL032M.

The importance of antenna design
While Wisair has spent years perfecting its RF and MAC/baseband chips and has lots of intellectual property and patents surrounding its signal acquisition, detection, automatic gain control and timing on the front end, when it came down to system implementation, antenna design became the make-or-break factor.

Figure 3: Final antenna patterns. Placing the two omni antennas at right angles helped mitigate orientation and environmental effects.

"We had to redesign the antenna to meet the real estate and had to make sure the antenna worked in such close proximity to the board," said Shor. The design team also had to cope with meeting expectations for a slick design with a low profile, no external antennas and with pre-defined locations for the connectors. According to Menshulam, Wisair's partnership with antenna maker In4tel had already given them a head start with a low-cost design based on FR4 material, small size and positive gain that was flat over the 3- to 5GHz band (Figure 4, Figure 5 and Figure 6). While FCC rules require a transmit EIRP of 0dBi, having a positive gain in the receive path improves the link budget. "We have 2dBi gain, that's very impressive: the norm is 0-1Bi," said Yurdakul.

However, for a real-world implementation, the team had to make sure the antenna worked on different surfaces and in different environments. "To do this, we had to reshape the antenna, check it in free space and then optimize it for the design," said Menshulam. The result is an omni-like effective antenna that allows the user to place the hub in different orientations and which also keeps positive gain despite the close proximity to the to the board. The company also leveraged its own antenna diversity expertise and In4tel's internal antenna design expertise. Despite the compromises made in the redesign to make it effective in the real world of varying surfaces, orientations and environments, Yurdakul said the team was able to keep a positive gain of approximately 2dBi. For antenna diversity, the team placed the two redesigned wideband omni antennas at 90 to each other which made the design less sensitive to the environment. Additional acceptance tests were added to make sure the effective pattern from the two antennas would be omni-like.

- Patrick Mannion

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