Antenna Systems & Technology - Summer 2015 - (Page 12)

FEATURE ARTICLE Impedance Matching is No Match for Aperture Tuning By Paul Tornatta and Lars Johnsson | Cavendish Kinetics LTE smartphones shipments, network deployments and spectrum allocations are expanding rapidly around across the globe, and the 3GPP standards organization has assigned more than 40 different spectrum bands for use with LTE. In addition, the networks are already so loaded with users and traffic that operators such as AT&T and Verizon are beginning to deploy LTE-Advanced Carrier Aggregation to increase network speed and capacity. Moreover, 3GPP has identified more than 60 possible spectrum band combinations covering both inter and intra band combining. Consequently, smartphones need to be able to support the growing number of spectrum allocations and carrier aggregation scenarios. For the LTE radio inside the smartphones this means that it has to "tune into" any of these spectrum bands, and that requires antennas that are highly efficient across all spectrum bands. Unfortunately antenna efficiency is easier required than designed. In the early days of cellular, the antenna was an after-thought for RF system designers. Handsets were Figure 1. Evolution of Cellular Phones and Their Antenna Efficiencies big, data rates were low and the number of spectrum bands for global use was limited to just four. This allowed the antennas in early generation cell phones to achieve high efficiency. Fast forwarding to 2015, the cell phone has morphed into a smartphone dominated by a large screen and big battery, OEMs are adopting antenna tuning technologies as they try to deliver acceptable LTE radio performance across all spectrum bands. The most critical part of the LTE radio is the RF front-end (RFFE), which includes the antenna and all analog signal processing. The power amplifiers, filters and switches in the RFFE are designed to operate at peak efficiency at 50Ω, which is also the targeted impedance level at the feed point where the antenna connects to the RFFE. The antenna impedance at the feed point depends on the antenna type. The most common antenna used in mobile handsets is a dual band PIFA. The feed point impedance of the antenna is purely resistive at the resonance frequency (PIFAs are around 90Ω, Dipoles 72Ω and Monopoles 36Ω). To maximize radiation efficiency, this is matched to 50Ω with a simple fixed matching circuit so that the power delivered to the antenna is also radiated. Figure 2. LTE RFFE (Radio Frequency Front End) Block Diagram With respect to antenna tuning there are two fundamentally different approaches to tuning the antenna:Tunable Impedance Matching (TIM) and Antenna Aperture Tuning (AAT). 12 Antenna Systems & Technology Summer 2015 www.AntennasOnline.com http://www.AntennasOnline.com

Table of Contents for the Digital Edition of Antenna Systems & Technology - Summer 2015

Editor’s Choice
The Future of DAS - In-Building and Dynamic Capacity
Impedance Matching is No Match for Aperture Tuning
All Band VSAT Antenna Radomes: A New Perspective
Antennas
Components/Subsystems
Software / System Design
Test & Measurement
Industry News
Marketplace
DAS and Small Cells, Revisited

Antenna Systems & Technology - Summer 2015

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