Microwave Engineering Europe - May 2008 - (Page 26)

26 RADIO Raising the bar for the radio: Making 802.11n work Power ampliﬁer design for a WLAN radio poses a challenge when meeting speciﬁcations. It is more challenging than ever to deﬁne and achieve the levels of radio-frequency performance required to meet the 802.11n standard in increasingly small portable and mobile applications. By Darcy Poulin and Gord Rabjohn, SiGe Semiconductor, www.sige.com s the next generation in wireless local area networking (WLAN) development, 802.11n aims to open up a new world of high-speed local networking. This new technology will have the bandwidth required for high-deﬁnition video, high-quality audio and data-intensive applications such as interactive gaming. It will likely drive a change in the way networks are used throughout homes and ofﬁces. For designers of the power ampliﬁer (PA) in the 802.11n radio, it is more challenging than ever to deﬁne and achieve the levels of radio-frequency performance required to meet the 802.11n standard in increasingly small portable and mobile applications. Since the development of the 802.11a protocol (1999), WLANs have deployed orthogonal frequency-division multiplexing (OFDM). In this technique, the usable bandwidth is precisely divided into a large number of smaller bandwidths, or “subcarriers.” The high-speed information is then divided onto these multiple lower-speed signals, which are transmitted simultaneously on different frequencies in parallel. The resulting low-datarate carriers are more tolerant of fading because of multiple reﬂections. The challenge for the PA lies in the highly variable amplitude of RF signals that is the sum of the orthogonal subcarriers. Because each subcarrier has an essentially random amplitude and phase, if the vector sum of all of the subcarriers happens to add in phase, a large peak (in the time domain) occurs. To avoid excessive distortion, the PA must accommodate these peaks, which can range more than 10 dB higher than the average. In reality, some distortion is permitted on the highest, most infrequent peaks, so typically a 7-dB peak needs to be accommodated. Traditionally, WLAN PAs have been designed using GaAs heterojunction bipolar transistor (HBT) technology. Recently, however, equivalent performance in the 2.5-Hz band has been achieved using SiGe bipolar transistors in a BiCMOS silicon-based process A technology. The advantages of using silicon technology are numerous and include higher levels of integration (allowing designers to build in advanced control circuitry) and lower cost. There has been much discussion about manufacturing CMOS PAs for 2.5 GHz, which would allow integration with transceivers. Although this technology may have potential, the results have been disappointing: low output power and efﬁciency on the order of 10 percent. Nonetheless, we can expect that CMOS PAs will be regularly used in certain low-power applications. Many 802.11n WLAN designs using 2.5 GHz are in development, but the new standard also covers 5 GHz. Currently, all of the 5-GHz designs use GaAs PAs. However, SiGe PAs are also in development for this bandwidth, and we can expect to see them in the near future. The availability of siliconbased PAs for 5 GHz might provide a boost to 5-GHz 802.11n, which is relatively expensive to deploy. As the 2.5-GHz band becomes even more saturated, there will likely be a shift to 5 GHz, and silicon-based PAs could be an important enabler. As a WLAN standard, 802.11n’s primary focus is on improving throughput. To achieve this, it departs from the time-, frequency- and code-division schemes that came before it. Microwave Engineering Europe ● May 2008 ● www.mwee.com http://www.sige.com http://www.mwee.com

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Microwave Engineering Europe - May 2008 News Contents Comment Cover Feature: How to Succeed as a GaAs Foundry Wireless Networking: Wireless Coverage Where Everybody WINS Wireless Networking: Achieving Good Coexistence in the 2.4 GHz ISM Band GPS and Satellite: GPS developments: Galileo Moves Forward with Successful Giove-B Satellite Launch — Broadcom Targets AGPS in Mobile Phones and Devices Raising the Bar for the Radio: Making 802.11n Work Reducing Power Consumption in Ultrawideband Chips WiMax Catches Second Test Wave Products Calendar

Microwave Engineering Europe - May 2008

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