Microwave Engineering Europe - January/February 2009 - (Page 20)

20 4G TECHNOLOGY FOCUS Addressing PA efﬁciency for multi-mode wideband handset applications By Jean-Pierre Joosting W ith traditional ﬁnal RF stage power transistors the input power is constant while the output radio signal varies in amplitude. Consequently, the difference between power in and power out of a power ampliﬁer (PA) is dissipated as heat. In order to address PA efﬁciency there are various techniques such as Doherty ampliﬁers, but such conventional techniques for improving PA efﬁciency are inherently narrowband, and are unable to span more than a single band. Building Doherty solutions, which are now common in base stations, would require variants to cover each different frequency, and PAPR value, as these designs have inherently narrow bandwidth. Doherty also typically needs Digital Pre Distortion (DPD) to achieve the linearity speciﬁcations. The DSP processing required by DPD would mop up much, if not all, the power saved in the relatively low power handset transmission circuits. In an industry where cost is important these techniques are not suitable. High Accuracy Tracking (HAT) Nujira High Accuracy Tracking (HAT™) has become popular in the network, and is now available in the handset too. Instead of optimizing a ﬁnal RF stage power transistor supplied by constant voltage, the supply is changed dynamically, modulated in synchronisation with the envelope of the incoming RF signal. HAT can make a signiﬁcant contribution to the power efﬁciency of the PA, improving this from the 15 percent of ‘traditional’ PAs to 50 percent and more. Coolteq™-l, which is being unveiled at Mobile World Congress migrates the company’s successful HAT technology from the basestation to the terminal. More than saving power — the 4G challenge So far, ten different FDD frequency bands and four different TDD frequency bands have been deﬁned in 3GPP that can be used for LTE, and it is likely that more bands will be added to this list such as 700 MHz in the US. A 4G handset will be required to transmit on the appropriate band anywhere in the world. This implies seven or more conventional PAs for a broadband cellular RF interface seeking to cover all fourteen LTE bands — adding several dollars to the bill of materials (see Figure 1). A further issue is that 3G and 4G standards use complex modulation schemes that increase data throughput in the operators’ expensive Figure 1: Handset RF subsystem covering all 14 LTE bands without Nujira HAT technology. Figure 2: Possible design after using a Nujira HAT power modulator. Microwave Engineering Europe ● January/February 2009 ● www.mwee.com http://www.mwee.com

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Microwave Engineering Europe - January 2009 News Contents Comment Using KPIs to Ensure Quality in a Converging Network Amplifier Error Vector Magnitude Characterisation Using High-Speed Modular PXI Instruments GPS: Making a Play for Femtocells Accelerating Global WiMAX Adoption: The Move to Picocell and Femtocell Base Stations Addressing PA Efficiency for Multi-Mode Wideband Handset Applications Wi-Fi: Mobile Feature or Fundamental RAN? Products Calendar

Microwave Engineering Europe - January/February 2009

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