Multiple Antennas Increase WLAN Data Rate
August 17, 2004
When TV Technology Editor Tom Butts forwarded a press release to me on a new Wi-Fi technology called "WWiSE", I was skeptical. Many new technologies fail shortly after reaching the press release phase. After a little research, it was clear it was worth looking into the technology the WWiSE consortium and the other groups are working on. Specifically the consortium is working with the IEEE 802.11 Task Group N efforts to develop a wireless networking standard that increases reliability and throughput using MIMO (multiple input, multiple output) technology. Data to be transmitted is split among multiple antennas. Multiple antennas are used to receive the signal and apply it to circuitry to extract the multiple data streams. The more antennas that are used, the higher the data rate.
In my 2001 RF Technology column Exotic Modulation -- Beyond 8-VSB I described modulating time (ultra-wideband -- UWB -- as implemented at that time) and modulating space (the Bell Labs "BLAST" technology that forms the basis for the 802.11 Task Group N efforts).
It is amazing to see that in less than four years this complicated technology has gone from the lab to a practical implementation that will be available in mid-October at a list price of under $310 for a wireless router and client card. Belkin says their wireless Pre-N system will provide four times the coverage and four times the speed of 802.11g equipment while remaining compatible with 802.11b and 802.11g devices.
Belkin's Pre-N system uses Airgo Networks "True MIMO(tm)" technology. Many companies and organizations submitted proposals for a MIMO standard to IEEE 802.11 Task Group N before last week's deadline. For information on the standards process, see the IEEE 802.11 Web site and search on "Task Group N". Airgo Networks is part of the WWiSE alliance that also includes Bermai, Broadcom, Conexant, STMicroelectronics and Texas Instruments.
Agere is proposing its own MIMO system to Task Group N that would achieve a raw data rate up to 500 Mbps. While the Belkin unit operates at 2.4 GHz, Agere is focusing its efforts on the 5.8 GHz band. For more about the competing proposals, see Unstrung.com and the article 802.11n Slap fest Ahead.
To learn more about how MIMO works, refer to Chapter 7 of UC Berkley EECS 224B: Fundamentals of Wireless Communications. The WWiSE IEEE 802.11n Proposal Technical summary covers the main features of the WWiSE proposal. For a more detailed description without as much math as the UC Berkley tutorial, see Research on Multi-Antenna Receivers and MIMO Systems in Digital Mobile Radio.
A Google search on the phrase ""spatial multiplexing" MIMO will provide links to several additional articles and papers on this interesting technology.