Electronic Design

Fast MIMO 802.11n Chips Reach Customers

The IEEE 802.11n standard for the next level of Wi-Fi performance hasn’t been ratified yet. But companies already are shipping “draft specification” chips to customers who are anxious to get high-speed multiple-input/multiple-output (MIMO) wireless routers, gateways, and access points to customers.

The 11n Task Group approved the version 1.0 draft with an approximately 87% majority during its March meeting. That’s a pretty good showing, and only a few glitches still need to be ironed out. Coming to an agreement on this standard certainly hasn’t been easy, with widely differing opinions and intellectual property issues.

But it’s slowly being resolved as chip vendors hunker down and make some concessions and compromises so they can push forward as quickly as possible into what’s sure to be a very hot market. The standard must be complete enough to let the chip companies finalize the silicon, and the equipment companies must have enough confidence to move forward with a final product.

The 802.11n standard calls for a minimum data rate of 100 Mbits/s in a Wi-Fi system with a range up to 100 m. To achieve this performance, the standard adopted MIMO. Yet it also must be compatible with the orthogonal frequency-division multiplexing (OFDM) of previous 11a/g designs.

MIMO refers to the process of using multiple transmitters, receivers, and antennas. With two transmitters and receivers, the serial data can be divided into two separate streams, effectively doubling the data rate. Three data streams essentially triple it, and so on. The 11n standard provides for up to four separate streams.

One of the major sticking points has been whether to use 20-MHz channels only or provide for the use of two channels to create a 40-MHz band to further boost speed. While most developers agree the standard should have the 40-MHz option, how a device determines that a second 20-MHz channel is available for use needs to be resolved. In high-density areas, the second 20-MHz band may be occupied. That problem has yet to be fully resolved.

With two transmitters and receivers (2-by-2 MIMO format) on one 20-MHz channel, speeds are expected to be as high as about 150 Mbits/s. Moving to a 40-MHz channel doubles this to 300 Mbits/s. Using 40 MHz and all four possible streams (4 by 4), a theoretical data rate of 600 Mbits/s is possible.

In practice, the real rate is much slower as the system adjusts the speed downward to compensate for noise, range, and other factors. Besides, who really needs 150 Mbits/s, much less 600 Mbits/s? Video would be one target application.

The 11n standard also includes a section defining this technology for the 5-GHz band as well as the more widely used 2.4-GHz band. With more spectrum and less traffic in the 5-GHz spectrum, it could become a popular option in more densely populated areas.

The really good news is that by using MIMO, the effects of diversity and multipath combining come into play. MIMO reduces the multipath problems and produces an overall greater range and reliability. This may be the larger benefit of 11n products. The range will be two or three times farther, and overall link reliability will be superior. In both home and business applications, robustness and range are far more important than speed.

Atheros’ XSPAN family of wireless chips includes the AR5008 chip set, which serves the 11n market. There are two RF chip options. The AR2133 contains three transmitters (TX) and three receivers (RX) and operates in the 2.4-GHz band. The AR5133 has three TX and three RX (3 by 3) but operates in the 2.4- or 5-GHz bands. As for the baseband chips, the AR5416 has a PCI host interface, and the AR5418 has a PCI Express host interface.

The Atheros Signal-Sustain Technology (SST) dramatically increases link robustness and throughput, simultaneously transmitting across three spatially diverse signal paths and incorporating information for three receivers simultaneously within the signal processing at the receiver. This 3-by-3 format with SST promises up to 300 Mbits/s with a sustained actual user rate of 150 to 180 Mbits/s.

Marvell’s Top Dog wireless local-area network (WLAN) products include the Top Dog chip set. Its 88W8060 RF radio chip has two transmitters and three receivers (2 by 3) that can operate in the 2.4- or 5-GHz bands. The 88W8360 is the baseband chip. The 88W8361 is a combined baseband and RF chip.

The company’s 88W8660 high-performance networking chip contains the 500-MHz Feroceon processor, which runs the ARE v5TE instruction set. It makes a good addition to products that need to interface to fast Ethernet switches. The Marvell chips also support the 20- and 40-MHz bandwidths and can sustain a peak rate of 300 Mbits/s, with the more typical rate ranging from 140 to 180 Mbits/s.

Wi-Fi equipment vendors D-Link and Netgear already have selected the Marvell chip sets for their products. Meanwhile, the IEEE will need about another year to iron out the minor problems before it can ratify the 11n standard, currently scheduled for July 2007.

Atheros Communications


Marvell Semiconductor

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