PRODUCT HOW-TO: LTE Handset Antenna Implementation in 700 MHz Band

Many industry leaders think that LTE is the 4G solution that will naturally follow today's 3G UMTS technology. LTE promises significantly higher data rates for both upload and download, thereby enabling a wide variety of Internet Protocol (IP) services such as VoIP and online gaming. The ability to deliver high data rates to a variety of consumer devices including Mobile Internet Devices (MIDs) and smartphones will largely drive the success of LTE.

The industry is painfully aware of the issues that surround implementing LTE in small mobile devices with already limited space. To make matters worse, LTE will likely be yet another service that the device must support. Therefore, LTE will compete with existing functions and services for PCB real estate and battery power, potentially increasing the cost of the handset.

For mobile handset antenna development, significant challenges arise when some of the complex implementation schemes of LTE are compounded by the longer wavelengths of the 700 MHz frequency band. This article explores these challenges and offers a solution.

700 MHz LTE ntenna Design Challenges
In the current LTE standard as defined by 3GPP, developers must implement multiple-in, multiple-out (MIMO) antenna technology and a number of advanced signal processing techniques to achieve the maximum data rate.

For effective MIMO, multiple independent antennas must operate simultaneously in the 700 MHz frequency band. Although there are several possible implementation schemes, this article focuses specifically on a 2x2 MIMO configuration with two independent transmit and receive paths.

The typical antenna specification for 2x2 MIMO has the following goals:

1. Number of independent antenna ports: 2
2. Radiation efficiency: As high as possible
3. Gain balance ratio (ratio of the gain at each antenna port): As high as possible, approaching 1
4. Correlation coefficient (envelope correlation coefficient between the two antenna ports): As low as possible, approaching 0

Other typical antenna parameters such as impedance bandwidth, compliance to SAR, and HAC are a given. Following is a detailed look at the implications for each of these requirements.