TD-LTE Technology And Its Measurements

By Yvonne Liu and Bai Ying, Agilent Technologies, Inc.

Abstract
LTE, and particularly TD-LTE, brings new challenges to developers and to the vendors of design and test tools. New RF modulation schemes, MIMO antenna configurations, higher system bandwidths and capacity, and lower latency are just some of them. New measurement methods, including crossing the digital and RF domains, combined with a reduction in test point accessibility mean there's a steep learning curve for both system and test developers. Successful technical introduction is a must, with analysts predicting between 30 and 80 million LTE subscribers and well over $100 billion in operator revenues within 5 years.

Third-generation (3G) wireless systems are deployed all over the world. W-CDMA maintains a mid-term competitive edge by providing high speed packet access (HSPA) in both downlink and uplink modes. Typical cell maximum data rate today is around 7.2 Mbps, and typical single-user data rates of around 1.5 Mbps can be expected. To ensure competitiveness into the future, the long-term evolution (LTE) of the 3rd Generation Partnership Project's (3GPP) UMTS is first specified in release 8 of the 3GPP specification, and covers the emerging needs of "mobile broadband" into the next decade with cell data rates of over 300 Mbps expected when the system is fully functional.

The majority of work to date on LTE has focused on the frequency division duplex (LTE FDD) variant. Following the integration of the Chinese TD SCDMA standard, based on time division duplex (TDD), into the 3GPP specifications for LTE, chipset and device designers are now working to include TDD capability. Now known as TD-LTE, the standard allows carriers to make use of the unpaired spectrum that many of them already own.

Compared to previous standards such as GSM/EDGE and W-CDMA, the timescale from first-generation standards documents to commercial release for LTE in general is short, and for TD-LTE in particular is shorter, due to its later addition into the standards. For handsets and data cards, LTE's maximum specified RF bandwidth of 20 MHz has driven a change in block diagram and the emergence of standard connections, while the requirement for multi-format devices which include compatibility with legacy systems may lead designers to the increased use of software-defined radios. New designs need more analog/digital cross-domain measurement and "digital-in, RF-out", meaning designers need new tools and measurement methods.