Microwave Engineering Europe - May 2008 - (Page 27)

RADIO 27 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. 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 peakto-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 and antennas in MIMO conﬁgurations. Today’s state-of-the-art PAs for 2.5-GHz WLANs operate with 20 percent efﬁciency, which means that delivering 100 mW of RF power requires 500 mW of total power. An ideal linear PA produces only the original frequency from the input signal. In real-world implementations, PA nonlinearities introduce new frequencies, or intermodulation distortion (IMD), which generates out-of-band signals that interfere with adjacent users (referred to as spectral regrowth or spectral leakage). Nonlinearities also generate in-band products that can cause errors in the data stream. The amount of out-of-band signals is measured as adjacent channel power (ACP) leakage. The FCC strictly regulates the amount of ACP emitted from a Wi-Fi transmitter. To adhere to these guidelines, the PA must be linear. Unwanted in-band signals are tracked using a parameter called error vector magnitude (EVM) that quantiﬁes the modulation accuracy of a transmitter. It is the measure of the difference between the ideal performance and the actual performance, with each point in the EVM constellation representing a particular magnitude and phase of one symbol in the OFDM packet. In 802.11n, EVM requirements are stricter because the noise from each radio will add noise to the other(s) in the system, further degrading performance and EVM. PA designers for 802.11n face a conundrum. Nonlinearity leads to more ACP leakage and poor EVM. Greater linearity requires a PA designed to operate at average power levels of 7 dB (peak-to-average) below its maximum output power, decreasing efﬁciency. Finding the optimal solution requires a careful assessment of trade-offs, as well as clever engineering and circuit design. A PA’s linearity is determined by factors such as the semiconductor technology used, ampliﬁer design, use of earlier stages, predistortion of later stages, bias circuits, matching networks, and even the impedance seen at harmonic frequencies. Designers have proprietary techniques that allow improvements in linearity beyond “textbook” designs. For instance, most designers use standard Class AB uncorrected PAs. However, some specialized techniques traditionally used in larger systems (such as predistortion) are being explored to improve PA performance. It is theoretically possible to make the PA more efﬁcient at the expense of linearity and afterward improve linearity by means of predistortion circuitry. However, at the relatively low RF power levels used in WiFi, the extra power required to implement predistortion is often greater than the power savings achieved in the PA. As geometries used in the baseband processor move to 65 nm and below, this balance may tip, making it quite possible that future wireless PAs will use predistortion techniques. PAs for 802.11n also face signiﬁcant size constraints. The expectation is that four power ampliﬁers (two for 2.4 GHz and two for 5 GHz) will ﬁt in the same space occupied by the two PAs used in a non-MIMO, dual-band conﬁguration. The best approach is to design the PAs as part of a highly integrated package, ﬁtting more dice in the package and spacing them closer together. Some of the latest FEMs for 802.11n integrate multiple PAs, LNAs, power detectors, ﬁlters, diplexers (frequency selective splitters that divide a signal into separate 2-GHz and 5-GHz paths), and switches for switching between receive paths and transmit paths. These ﬂexible building blocks can be designed with connections that allow multiple modules to be cascaded or stacked for use in MIMO applications of various conﬁgurations. At these levels of integration, the footprint for the PA and its associated circuitry is comparable to current 802.11g solutions. Every country has its own regulatory requirements for WLAN emissions. The United States and Japan, generally speaking, have the most stringent demands, so most WLAN solutions are designed to satisfy the requirements of those countries. Because an 802.11n WLAN device is likely transmitting twice as much power as a nearby 802.11g device (because there are now two transmit channels), the amount of ACP leakage has to be well managed in order to comply. If the PA emits too much power on an adjacent channel, it could interfere with other services operating in that channel, thus failing to comply with regulatory requirements. Such risks are best minimized by using linear PAs. Even with the best-available linearity specs, there can still be some coexistence issues with other wireless technologies. 802.11n is designed to be compatible with previous 802.11 standards, so there are no coexistence issues there. Most cellular frequencies operate far enough away from the ones used by 802.11n to avoid any problems, and their standard frequency ﬁltering mitigates any possible interference. However, when an 802.11n device and cellular radio operate within a single device (as in a cell phone with WLAN capability), additional ﬁltering is required. Bluetooth and WiMax, for example, operate in the same bands as 802.11n, so avoiding interference issues will be more challenging. The ﬁnal 802.11n standard is not expected to be ofﬁcially released until October, yet there is already signiﬁcant activity in product development based on the draft standards. About the authors Gord Rabjohn (gr@sige.com) specializes in GaAs and power ampliﬁers at SiGe Semiconductor while Darcy Poulin (dp@sige. com) is an RF specialist. Microwave Engineering ● May 2008 ● www.mwee.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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