It is defined for six carrier bandwidths ranging from 1.4 to 20MHz, although it is estimated that it will be most widely used in higher bandwidths, similar to FDD. The telecom community is already working on additional definitions for backward compatibility aspects and reselection. In addition the introduction of LTE to other markets such as public safety and health care brings the evolution of the TDD technology full circle--TDD actually started in these markets.
There’s no shortage of TDD devices either. Qualcomm, the dominating player in the handset chipset market in the U.S., is ready to launch a multi-mode LTE FDD/LTE TDD chipset along with backward compatibility in 3G services. ZTE and many other companies are also working to ensure that devices will be available with FDD and TDD support at no extra cost.
TDD spectrum is already allocated in numerous countries. Several UMTS mobile operators in Europe and Asia received small chunks of TDD spectrum in the 2.1GHz band. The spectrum was allocated at the same time as larger channels of UMTS FDD spectrum and in most cases the TDD spectrum went unused. However, most operators are opting to roll out TDD on 2.3 GHz and 2.6 GHz. These bands offer the largest contiguous blocks of spectrum enabling the best possible performance.
While TDD is selling quickly, unpaired bands are still available and are not as expensive as FDD. For countries in Europe, Asia and Latin America, following the recommendations of the European Conference of Postal and Telecommunications Administrations (CEPT), this means that 50MHz of TDD should be integrated alongside 2x70MHz FDD for 2.6GHz auctions. Other countries are allowing operators to decide which technology to opt for. In addition, some markets distributed licenses that were technology-neutral, allowing licensees to use their spectrum for LTE TDD.
In addition, vendors around the world have conducted interoperability tests (IOTs), making TDD technology more reliable, and many have announced plans to deploy LTE (FDD or TDD). For example, TDD is an option for WiMAX operators or greenfield operators with 2.3GHz or 2.5GHz spectrum, so there has been a trend for WiMAX operators to switch to LTE. The technical expertise gained from WiMAX development is also a valuable asset when it comes to developing LTE TDD solutions because both technologies are all-IP OFDM-based. These similarities provide an easy path for WiMAX operators transitioning to LTE.
TDD’s introduction into different markets and regions
TDD is already seeing momentum in Asia and Europe as many launches in these regions are including TDD in their network roll out plans. UK Broadband (UKB) is building the first LTE network in London using Huawei’s LTE TDD solution. This is the first LTE TDD 3.5GHz deployment in the world and the first commercial LTE TDD deployment in the UK. UKB will operate a wholesale model beginning in May 2012, working with partners to offer commercial services to businesses, consumers and the public sector. In large cities like Beijing, many people live in high-rise apartment buildings made of concrete, which creates both coverage and quality challenges for mobile operators transitioning to frequency technologies such as 4G/LTE. To improve service, operators are circumventing the local FDD macrocell base station and using residential TDD femtocells to transmit calls over the household’s broadband connection.
China and India are committed to deploying this technology and other operators in various countries are supportive primarily due to the economies of scale. TDD represents the future of China Mobile’s broadband strategy and India’s broadband wireless access (BWA) auction boosted the industry’s interest in LTE TDD. Demand for equipment supporting TDD started primarily in China and Japan due to greater flexibility in how frames are transmitted over an open air interface. These countries are requesting TDD-based femtocells because it can more easily manage capacity and reduce interference compared to FDD. With three of the largest regions selecting LTE TDD deployments, it is likely that other countries will also vie for their share in this market. For example, Clearwire and Sprint have already taken TDD into the Americas.
Radisys Trilium LTE software supports both FDD and TDD. In a recent case-study a Tier-1 TEM incorporated Trillium TotaleNodeB LTE Femtocell Software and contracted PHY/MAC integration assistance from Radisys Services to save time and development costs compared to doing everything in-house, allowing them to get to market in six months compared with two years or more. The Trillium software was designed to support scaling from small to large deployments and has been optimized to run a single processor with limited resources on multiple architectures.
Figure 4. Radisys offers LTE TDD solutions along with complete end-to-end LTE solutions.
With plenty of data offload techniques, the introduction of the HetNet concept, the improvements in spectral efficiency and the innovative data plans now being offered, the mobile broadband dream of providing a wireline-like experience to wireless users is becoming a reality. TDD technology emergence is a new beginning of the hybrid LTE TDD/FDD deployment model, opening up a new way for capacity addition and data offload. The industry is gravitating toward mixed LTE deployments where femtocells and small cells employ TDD and macro base stations use FDD to create a HetNet topology that eliminates interference issues. Implementing TDD on femtocells can lead to improved quality signal and optimized bandwidth allocation thus delivering a high quality experience to the end users. TDD has already demonstrated its potential to overcome the challenges wireless networks are facing today. Due to its affordable spectrum it will become a respite in congested areas, adding capacity for the offload of asymmetric data like video and M2M applications.
About the Author
As a principal systems architect at Radisys, Renuka Bhalerao is primarily focused on 3G and LTE wireless technologies and associated network architectures. Renuka has extensive experience working in telecom software and systems with expertise in wireless, small cells and VoIP areas.
1 QUALCOMM whitepaper, A Comparison of LTE Advanced HetNets and Wi-Fi, October 2011. http://www.qualcomm.com/media/documents/comparison-lte-advanced-hetnets-and-wifi
2QUALCOMM presentation, LTE TDD, The Global Solution for Unpaired Spectrum, September 2011. http://www.qualcomm.com/media/documents/lte-tdd-global-solution-unpaired-spectrum
3Julien Grivolas, Principal Analyst, OVUM whitepaper, Global opportunities for LTE TDD, February 2011. http://www.lte-tdd.org/sites/default/files/Global%20Opportunities%20for%20LTE-TDD.pdf