02 Nov 2012

Rapid Rise of TD-LTE in Global Networks

Paul Beaver, LTE Business Unit Director at Anite looks at the critical role of device testing in the adoption of TD-LTE

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, with several countries declaring an active interest in the technology.

TD-LTE was developed by the international standards body, the 3rd Generation Partnership Project (3GPP), with China Mobile being a material driving force, and large-scale contributor, to the technology.

China has a long history of pioneering home-grown technology, and is keen to be seen as a global technological innovator. In 2011, China Mobile, in conjunction with likeminded operators, launched the global TD-LTE initiative (GTI), which has the primary aim of drawing together the world’s leading mobile operators in order to promote TD-LTE as a dominant standard for mobile broadband technology.

As of April 2012, there were 37 trial TD-LTE networks in place, with 7 countries having launched TD-LTE. Since May 2012, 46 Operators have joined the GTI, emphasising the fact that TD-LTE is gaining popularity in the market.

A different kind of LTE

In both TD-LTE and FDD-LTE, transmitted signal is divided into subframes of 1ms duration, with ten subframes making up an individual radio frame. An individual subframe is typically comprised of 14 Orthogonal Frequency-Division Multiplexing (OFDM) symbols. Despite these similarities in frame structure, there are some notable differences between TD-LTE and FDD-LTE; particularly the way in which subframes are assigned for uplink and downlink transmission in the respective technologies.

FDD-LTE makes use of two carrier frequencies, assigning an individual frequency for uplink and downlink respectively, and the transmission and reception of data is performed simultaneously. As a result of this, FDD-LTE involves 10 uplink subframes and ten 10 downlink subframes. 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. The use of a single carrier frequency means that the base station and the mobile terminals must interchange between transmission and reception. Because a subframe is designated to either uplink or downlink, this means that for TD-LTE, a radio frame consists of fewer than ten subframes in each direction.

However, TD-LTE is not a technology that is superseding FDD-LTE. Each technology has its own unique set of advantages. In actuality, TD-LTE could be viewed as a complimentary technology, existing 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.

The rise of TD-LTE

A key reason for TD-LTE’s increased market traction comes directly from China Mobile’s vigorous drive to promote the standard. A key element of this promotional activity has been the foundation of the GTI. One of the GTI’s primary aims is to encourage the synthesis of TD-LTE and FDD-LTE technology, in order to maximise economies of scale. Undoubtedly, TD-LTE’s reputation has been buoyed by the backing of the world’s largest mobile operator.

TD-LTE’s unique features have also played an important part in the technology’s growing stature in the market. Because TD-LTE makes asymmetrical use of unpaired spectrum, for both uplink and downlink, it is a spectral efficient technology. Spectrum is a valuable commodity for mobile operators, especially those who operate in countries where there is a limited amount of available FDD spectrum; or where only single unpaired frequency is available. Driven by its spectral efficiency, TD-LTE is now increasingly being viewed as an attractive proposition in markets of this type.

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. Africa, for example is a continent with a lack of fixed line networks. The remote nature of some African countries makes the deployment of fixed line networks unviable. However, because of the spectral efficiency and flexibility of TD-LTE, it can act as an effective solution for last mile connectivity, keeping communities in Africa connected.

TD-LTE has appeal in other global markets as well, in the United States for example, TD-LTE is becoming increasingly popular. The progression path in the U.S. is led by LTE and spectrum that had previously been allocated to operators for WiMAX deployments can be easily converted to TD-LTE. This trend has already been 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.

TD-LTE is evidently appealing to a wide range of markets. In Japan, for example, Softbank has launched a TD-LTE network and Hi3G Access AB has also gone live with TD-LTE in Denmark and Sweden. In India, Bharti Airtel has recently launched its own commercial TD-LTE network, while Sky TV in Brazil has also adopted the technology. TD-LTE’s flexibility, convenience and spectral efficiency combine to give it a broad market appeal. When this is coupled with China Mobile’s proactive drive to promote the standard on a worldwide stage, it is not surprising that TD-LTE is rapidly emerging as a genuine force in the industry.

TD-LTE: The crucial nature of testing

Whilst TD-LTE continues to develop in the market, the technology is still not as widely deployed as FDD-LTE. One reason for this is the limited amount of mobile devices in the market that currently support TD-LTE. Any vendor looking to develop TD-LTE devices must adopt a rigorous programme of development, conformance and interoperability testing.

The official certification programme for TD-LTE devices is managed by organisations such as the Global Certification Forum (GCF). By achieving certified compliance with the 3GPP TD-LTE standards, device manufacturers can advance TD-LTE 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. Crucially, given TD-LTE’s broad international appeal, devices will also function efficiently when roaming onto foreign networks. This, in turn, will ensure that subscribers are fully satisfied with the TD-LTE services that they are receiving, both at home and when abroad.

As well as conformance testing, industry players in the mobile ecosystem must also invest heavily in interoperability testing to ensure that devices can work with chosen network configurations, including interworking between different network technologies – for example, FDD LTE and TDD LTE. The importance of interoperability testing was underlined by research from Informa last year.

In a survey of 250 global operators, Informa found that 33 per cent of those surveyed were planning to deploy the TD-LTE and FDD-LTE standards alongside each other in networks. The GTI is actively promoting the synthesis between TD-LTE and FDD-LTE technology. Operators will require chipset and device manufacturers to integrate both TD-LTE and FDD-LTE capability in the same products. However, this presents a series of major challenges for all parties. In order to overcome this, operators, chipset and device manufacturers must undertake a rigorous programme of testing in order to ensure device interoperability and performance.

Operators need to be assured that finalised 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 meet consumer expectations. However, these simulated network environments must provide a comprehensive range of potential conditions to ensure TD-LTE’s commercial success.

TD-LTE: The future

TD-LTE’s market position has been driven by China Mobile and its GTI initiative, which has brought together a number of global players such as Vodafone, Softbank and Bharti Airtel. This industry collaboration has produced giant strides in the commercial adoption of TD-LTE.

It should be noted though that TD-LTE is not applicable for all markets and FDD-LTE remains a more suitable choice for many operators. However, the spectral efficiency and flexibility that TD-LTE offers means it is ideal for markets where operators that lack FDD-LTE spectrum and for those that lack a fixed line network, but require a technology to provide last mile network connectivity.

TD-LTE boasts a series of unique benefits for certain markets, but devices supporting this standard must interoperate with FDD-LTE and other cellular technologies, requiring comprehensive testing. Testing for device interoperability, conformance and development has a critical part to play in the successful integration and deployment of TD-LTE. The introduction of TD-LTE, untested, into a complex network environment, featuring a diverse range of devices, presents significant challenges for operators. By undertaking lab-based testing, operators and manufacturers alike can pinpoint issues in TD-LTE devices and solve them at an early stage. This helps in reducing costs and ensures a more rapid time to market for TD-LTE devices; providing a solid platform for TD-LTE to increase its market traction.

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