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802.11n chip touts extended range

Posted: 01 Feb 2007 ?? ?Print Version ?Bookmark and Share

Keywords:Ralink? 802.11n? PCI? MiniPCI? CardBus?

Ralink Technology Corp. is jumping into the 802.11n game with a Draft 2.0-compliant chipset, which it claims handily beats early movers in the market at extended ranges. Taiwan-based Ralink ramped up its RT2800 chipset series in December, targeting access points and clients bought by consumers. In such applications, people will be most concerned about the first 100m of throughput, and in that range, Ralink says its solution is on par with those of Broadcom and Marvell.

Ralink has added some optional .11n extensions to improve range, including spatial expansion and space-time block coding. It also claims an improved ability to demodulate and decode data in a way that lowers the baseband's signal-to-noise requirement, further boosting range.

The past several months have been chaotic in the .11n market, with predraft versions of the spec competing for market share and consumers getting burned on interoperability. "New technology was killed by the technology companies, not the users. So now, people are hesitant when you say it is pre-.11n," said Sheng Lee, chief technology officer of Ralink.

Lee believes things are getting better as the handful of companies selling .11n chips refocus on interoperability. "We waited longer because we knew that there was not a consensus," he said. "Now we think Broadcom, Atheros and Marvell have largely resolved the interoperability issue, making it easier to cross-pollinate devices from different vendors."

Interface support
Ralink's RT2800 consists of a transceiver and an integrated baseband and media-access control (MAC) device. Both support industry-standard interfaces including PCI, MiniPCI, CardBus, USB 2.0, PCIe and MiniCard. The chipset complies with .11n Draft 2.0; the final spec can be installed via a software upgrade.

There are a number of derivatives of the RT2800, depending on the interface. In general, the transceiver can support the 5GHz and 2.4GHz bands, using three receivers and two transmitters for a PHY-layer transfer speed of 300Mbps. After protocol overhead, that will be 180Mbps or less. A version intended for compact form factors supports only the 2.4GHz band. Both transceivers are powered from a single 3.3V supply and use a 40MHz crystal that controls a built-in PLL to synthesize the transmit carrier frequencies, Ralink said.

The baseband/MAC includes six ADCs and two DACs. The digitized data is routed through security and QoS accelerators, then off chip via a direct-memory access controller. To reduce power, the chip shuts down unused portions.

The chipset consumes 480mA under an MCS15 modulation coding scheme and with an interpacket gap equal to 50?s and 40MHz during transmit. It draws 400mA during receive under roughly the same conditions.

Ralink uses a patented antenna design to squeeze the .11n spec into a small USB dongle. The .11n drivers are offered on an optional on-board flash memory chip.

Currently, Ralink's .11n chips are based on SiGe, but designers are in the process of porting them to RF CMOS. That will enable a single-chip solution.

The RL2800 series starts at $20 per chipset.

- Mike Clendenin
EE Times

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