Microwave Engineering Europe - July/August 2008 - (Page 25)

WiMAX DESIGN 25 Why is PA efﬁciency so low for WiMAX? State of the art WiMAX PAs achieve an efﬁciency of 20 percent. Many GSM system designers may look at 20% efﬁciency with disdain because GSM PAs easily achieve efﬁciencies that are greater than 50 percent. Why do WiMAX PAs have efﬁciencies that are so much lower? The answer is that GSM PAs can be driven much harder, since GSM linearity requirements are signiﬁcantly relaxed compared to WiMAX. Combined with this, the OFDM modulation used in WiMAX results in a signal with a peak to average power ratio (PAPR) of 6 to 7 dB, compared to a PAPR of 0 dB (constant envelope) for GSM. Of course, OFDM has a large number of advantages, including high spectral efﬁciency and excellent immunity to fading, which are critical for the high bandwidth, mobile applications like mobile WiMAX. For example, when transmitting at +23 dBm with a 10 MHz channel bandwidth, achieving -25 dBm/MHz requires a net rejection of: 23 - 10log(10)+25 = 38 dB rejection. Transmitting at 24.5 dBm requires 39.5 dB rejection. Therefore, it becomes increasingly difﬁcult to meet regulatory requirements as output power is increased. To reduce IMD distortion, the PA must operate more linearly, and the result is that PA efﬁciency will drop as the output power target is increased. Recognizing the tradeoffs Undoubtedly, higher transmit power is important for mobile WiMAX networks. Networks are currently being deployed specifying that the minimum transmit power is +23 dBm. Each user who enters a network transmitting at powers greater than +23 dBm increases overall network efﬁciency. However, delivering higher transmit powers comes at a cost to power consumption. As a result, power ampliﬁer efﬁciency becomes more important as higher output powers are used. About the author Darcy Poulin brings to SiGe Semiconductor more than 15 years of experience in RF engineering and IC design. He is currently Senior Systems Engineer, and is responsible for RF systems work, standards development, and regulatory compliance. Darcy has made a number of key contributions to RF areas of the 802.16 standard. He has also been heavily involved in the development of the WiMAX Mobile Radio Conformance Test (RCT) speciﬁcation, particularly in the development of spectral masks and spurious emissions limits. Footnotes [1] IEEE Std 802.16e-2005, section 8.4.13. [2] Table 11 of Mobile WiMAX-Part 1Overview and Performance. [3] Mobile WiMAX “ Part I: A Technical Overview and Performance Evaluation, section 5.2. (Published by the WiMAX Forum). [4] FCC 47CFR 27.53(l.). Leader in Discrete and Integrated RF Solutions Microwave Engineering ● July/August 2008 ● www.mwee.com http://www.skyworksinc.com http://www.skyworksinc.com http://www.mwee.com

Table of Contents Feed for the Digital Edition of Microwave Engineering Europe - July/August 2008

Microwave Engineering Europe - July/August 2008 Contents News Comment Cover Feature: Effective EM Simulations with Micro−λ Resolution in Macro-λ Objects — General Huygens Box Implementation RF CMOS: Programmable Transceiver IC Minimises OEM Inventory for Femtocells CAD/EDA: Software-Defined Radio Platforms CAD/EDA: Cadence Enhances RF Verification While AWR Delivers an Improved Microwave Office How to Meet the Design Challenges of WiMAX Power Amplifiers Products Calendar

Microwave Engineering Europe - July/August 2008

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