As global LTE deployments continue to increase, a Chinese pioneered version of the technology, Time Division Duplexing-LTE (TD-LTE) is also gaining market traction.
TD-LTE was developed by international standards body 3GPP, with China Mobile a large-scale contributor. In 2011, China Mobile and like-minded operators launched the global TD-LTE initiative (GTI), focused on drawing together the world’s leading mobile operators in order to promote TD-LTE. By April 2012 there were 37 trial TD-LTE networks in place covering seven countries, and GTI membership had swollen to 46 operators including Vodafone, Softbank and Bharti Airtel.
The rapid rise of TD-LTE means device testing is now a critical element in the technology’s future development. However, such work involves more than simply porting existing FDD-LTE test cases over to TD-LTE handsets.
Despite some similarities in frame structure, there are some notable differences between TD- and FDD-LTE, particularly in terms of uplink and downlink transmission. FDD-LTE makes use of two carrier frequencies, assigning an individual frequency for uplink and downlink respectively, and transmitting and receiving data simultaneously. Conversely, TD-LTE makes use of a single carrier frequency to support uplink and downlink transmission, but the time for transmitting and receiving is always different.
However, TD-LTE is not a technology that is superseding FDD-LTE - each one has its own unique set of advantages. In actuality, TD-LTE could exist alongside FDD-LTE in the network. The fact that chipset and device manufacturers are building TD-LTE and FDD-LTE into reference designs and devices, combined with the requirement for interworking with legacy technologies, means that interoperability, conformance and development testing of TD-LTE devices is critical in ensuring the commercial success of the technology.
Driven by its spectral efficiency, TD-LTE is now increasingly being viewed as an attractive proposition in countries where there is a limited amount of available FDD spectrum; or where only single unpaired frequency is available. Equally, TD-LTE is gaining significant traction in markets where there is a lack of fixed line infrastructure, as it can work to provide effective ‘last mile’ connectivity.
TD-LTE has appeal in other global markets as well. In the United States for example, spectrum that had previously been allocated to operators for WiMAX deployments is being converted to TD-LTE. This trend is evidenced by the recent actions of U.S. wireless providers Clearwire and Sprint, when the former identified 5,000 initial WiMAX cell sites that it plans to convert to TD-LTE.
Despite the growing attraction of TD-LTE, it is still less widely deployed than FDD-LTE. One reason for this is the limited amount of compatible mobile devices in the market. Any vendor looking to develop TD-LTE devices must adopt a rigorous program of development, conformance and interoperability testing. The official certification program for TD-LTE devices is managed by organizations such as the Global Certification Forum (GCF). By achieving certified compliance with the 3GPP TD-LTE standards, device manufacturers can advance the technology in the market much more rapidly. Equally, they can be assured that their TD-LTE devices will deliver first class performance on the network.
Research by Informa Telecoms & Media highlights the growing popularity of TD-LTE. A third of 250 global operators quizzed by the firm plan to deploy TD-LTE and FDD-LTE alongside each other in networks, which will require chipset and device manufacturers to integrate both versions of the technology in the same products. However, this presents a series of major challenges for all parties that requires a rigorous program of interoperability testing to overcome.
Operators need to be assured that commercial TD-LTE devices will work as anticipated. By putting devices through a comprehensive series of tests, incorporating their network configuration and scenarios designed to mirror their network operation, they can ensure TD-LTE will meet industry and operator standards and, crucially, consumer expectations.
Paul Beaver is products director at device testing firm Anite