Microwave Engineering Europe - July/August 2008 - (Page 20)

20 WiMAX DESIGN How to meet the design challenges of WiMAX power ampliﬁers Power ampliﬁer (PA) designers and those selecting PAs need to ﬁnd the optimal balance between high power and high efﬁciency in order to ensure robust links, high data rates, and good range. By Darcy Poulin, SiGe Semiconductor s designers turn their attention to mobile WiMAX devices, they are quickly learning that there are some speciﬁc design challenges regarding power ampliﬁers. For Wave 2 mobile WiMAX products, the mobile device needs to efﬁciently deliver +23 dBm output power with high linearity from a 3.3 VDC supply. Managing power in mobile WiMAX is quickly shaping up to be vitally important as ﬁrst-generation designs are tested and deployed. One of the challenges of designing for mobile WiMAX is its long range, since WiMAX networks typically achieve coverage of about 1 km per cell. To achieve these ranges, WiMAX must have an optimized power proﬁle — from the base station right down to the components in the mobile device. High transmit power, then, is important. But how high can WiMAX go and what are the limitations imposed by regulatory bodies, technological limits, and usage models? Designers of the power ampliﬁer (PA) and those selecting PAs need to ﬁnd the optimal balance between high power and high efﬁciency in order to ensure robust links, high data rates, and good range for their WiMAX services. The nature of WiMAX What makes WiMAX challenging for designers is that it is an access technology with a unique set of constraints. As a result, power ampliﬁcation circuits that were used for cellular or Wi-Fi applications cannot simply be dropped into WiMAX designs and tweaked to perform adequately. In many ways, WiMAX can be considered a hybrid technology because it shares aspects of both cellular and Wi-Fi networks. Mobile WiMAX is very similar to cellular; it is meant to be used in highly mobile devices and it uses licensed frequency bands (so users expect high reliability). It also A Figure 1: Achievable modulation versus distance with +23 dBm transmit power. employs transmit power control techniques, much like CDMA cellular does. However, it differs from cellular because it operates at much higher data rates (resulting in more stringent linearity requirements) and must simultaneously handle voice over Internet Protocol (VoIP), data, and video transmissions. Managing the bandwidth and priority of transmission for these types of services requires a quality of service (QoS) component that is not required for mobile voice alone. On the other hand, WiMAX is also similar to Wi-Fi. For instance, it offers high data rates, uses orthogonal frequency division multiplexing (OFDM) with modulations from BPSK to 64-QAM, and is an all-IPbased network. However, it differs from Wi-Fi because it uses a fully-scheduled service, unlike the collision-based carrier sense multiple access (CSMA) technique used by Wi-Fi. This gives WiMAX a signiﬁcant advantage over Wi-Fi. As the number of users increases in a CSMA network, overall capacity drops dramatically since each collision requires a subsequent retransmission. With a scheduled service, overall network capacity is unaffected as the number of users increases, since the basestation manages each user’s access to the network efﬁciently. WiMAX network coverage Wi-Fi networks typically cover ranges that are measured in the tens or hundreds of meters for each access point (AP). However, WiMAX networks will achieve coverage of about 1 km per basestation (BS). In order to achieve this, mobile WiMAX networks employ a number of techniques to achieve longer range, including high transmit power, subchannelization, and adaptive modulation. Simply put, RF power translates directly into range, so higher power equals longer range. To achieve long range, WiMAX basestations transmit at power levels of approximately +43 dBm (20 W), as compared to Wi-Fi APs, which typically transmit at +18 dBm (60 mW). A WiMAX mobile station (MS) typically transmits at +23 dBm (200mW), as compared to +18 dBm (60 mW) for Wi-Fi. Cellular Microwave Engineering Europe ● July/August 2008 ● www.mwee.com http://www.mwee.com

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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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