Microwave Engineering Europe - January/February 2008 - (Page 12)

12 RADIO Raising the bar for the radio: Making 802.11n work By Darcy Poulin and Gord Rabjohn, SiGe Semiconductor A 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. The SE2546A is a dual-band, dualstream front end module (FEM) designed for MIMO. A FEM designed for MIMO can shrink the footprint of the PA and necessary passive circuitry. 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 silicon-based 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. Key performance metrics of 802.11n 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. This protocol aims to send simultaneous data on the same frequency. Normally, these signals would interfere with each other. However, 802.11n can take advantage of a concept called “spatial diversity,” which separates the data streams by using multiple antennas and radios. This is referred to as multiple input, multiple output (MIMO). Spatial diversity uses these multiple paths (multipath was previously an impairment) to effectively increase throughput. Thus, it is ideal for cluttered indoor and urban environments. CMOS process technology 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 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 Because of the MIMO conﬁguration often used in 802.11n, the major performance metrics for the PA are output power, efﬁciency and linearity (or error vector magnitude, which is closely linked to linearity). The 802.11n standard requires higher performance in these areas without allowing for increase in size or component count. Typical 2.5-GHz 802.11g PAs can deliver about +19 dBm of output power. There is a 7-dB peak-to-average power ratio for OFDM, which requires a PA capable of about +26-dBm peak power. This poses the ﬁrst design challenge: +26 dBm delivered into a 50-ohm load is equivalent to 13 volts peak-to-peak. Because WLAN PAs are typically operated from a 3.3-V supply, a large voltage transformation is required. This must be done using passive components, and space requirements force integration of these passives into the PA or the front-end module. In PAs, efﬁciency is the measure of the average RF power out versus the average dc power in. Despite its need to handle large peaks, the PA operates at a lower power (about 7 dB below peak) 90 percent of the time. Standard classes and topologies of PAs generally offer their best efﬁciency at high output power, and efﬁciency decreases as power is backed off. Although there are exotic PA designs that claim to offer high efﬁciency over a range of powers, PA designers typically opt for Class AB, which offers good efﬁciency at reasonable cost. PA efﬁciency has been a major target for designers of 802.11g systems. Now, for 802.11n, this challenge becomes even more difﬁcult because of the use of multiple radios Microwave Engineering Europe ● January/February 2008 ● www.mwee.com 012_014_MWEE.indd 12 23/01/08 12:16:34 http://www.mwee.com

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Microwave Engineering Europe - January/February 2008 Contents News Comment Radio: Raising the Bar for the Radio: Making 802.11n Work Cover Feature: The RF-System-In-Package Trend - Efficient Design with Advanced Design System 2008 Wireless Sensor Networks: The Zigbee PRO Feature Set: More of a Good Thing Very Fast Measurements of Wireless Devices with Small Antennas in Reverberation Chambers WiMAX Update 2008 Bridging the Gap from the CMOS DSP to the Antenna in OFDM Systems Products Calendar

Microwave Engineering Europe - January/February 2008

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