TD-LTE evolves into mainstream contender


Part 1: LTE to follow very different path from 3G
Part 2: BRIC taking leading role in 4G

The influence of China Mobile has promoted TD-LTE from sideline technology to mainstream contender. This trend will only accelerate as operators run short of spectrum and start to look at the neglected, and web-suitable, TDD frequencies. Also, TD-LTE provides a migration path for Wimax operators who want to tap into the broader device ecosystem and economies of scale promised by LTE.

The TDD flavor will allow some non-traditional players, such as TV operators and rural providers, to enter the mobile space, but its main function is likely to be as a second wave technology for cellcos whose FDD spectrum starts to get overstretched.

At this stage, they will either deploy or buy their own TDD licences, or offload traffic to a TDD-based partner. The biggest example is Clearwire in the US, which will migrate much of its Wimax network to TD-LTE. The resulting system will be able to provide wholesale and offload services to key customer Sprint, to complement its own FD-LTE build-outs in its 800MHz and PCS bands.

A similar approach is seen in Japan, where KDDI has a Wimax/TD-LTE subsidiary in the shape of the UQ Communications joint venture, and Softbank is planning a secondary network in the TDD spectrum it acquired from Willcom. And in Europe, some major operators like Orange are already testing TD-LTE with a view to activating their TDD holdings when they see their initial 4G net-works coming under strain. Some carriers will also look to use TDD bands for separate purposes to the FDD frequencies – perhaps to run a dense mesh of femtocells, offloading traffic from and FDD-based macro network to ease congestion; or to support specific services such as M2M.

The second dual-mode FDD/TDD network in the world went live last week, at Hutchison's Hi3G unit in Sweden. While many operators will wait until their current capacity is exhausted before adding TDD, Hi3G has taken this view at a far earlier stage than most of its peers and invested in both types of frequencies right at the beginning, seeking to steal a march on larger and more established rivals in the Nordic regions. It bought both types of licences in Sweden's 4G auctions and is now deploying services in Stockholm, Gothenburg and Malmo, using equipment from ZTE.

Its plan – like the other dual-mode projects at Aero2 and the Australian NBN broadband initiative – shows TD-LTE moving beyond its first customer bases, among Wimax carriers looking to migrate to the newer standard, to the future when it will also be part of the network mix for mainstream, traditionally FDD-focused cellcos.  The key to TDD success will be a broad device ecosystem, and China Mobile, with key ally Vodafone, has worked hard to persuade chipmakers and OEMs to support both modes as a default in their designs. They have not quite achieved that, but most silicon suppliers at least have TDD and dual-mode on their roadmaps, and the operator strategies outlined above will make affordable dual-mode devices essential from 2013. 

According to the GSA industry association, as of August there were 31 trial TD-LTE networks and one live. As well as the Wimax/LTE developments going on in the US, Japan, Korea and elsewhere, there is already a commercial TD-LTE carrier (Mobily in Saudi Arabia) plus the dual-mode rollouts. These deployments are proving a major boost for the Chinese vendors, whose Wimax activities and closeness to China Mobile have created a headstart in TDD technologies – Huawei (with Samsung) provides the kit for Mobily and Aero2, while ZTE is the partner for Hi3G. Samsung is also well placed for TDD because of its Wimax leadership.

Clearwire CTO John Saw sees a benefit of TD-LTE – in addition to its support for asymmetric data loads – being a more unified spectrum picture than in FDD. The technology is nearly always deployed in the area between 2.3GHz and 2.7GHz. "If you look at the entire map for 4G, those frequencies are the closest thing to what I call a 4G world band since they are used in many countries," Saw said. But TDD carriers will also need dual-mode devices in order to roam with FDD networks.

The rise of the TDD option shows how operators will need to adopt some non-traditional spectrum options to keep up with mobile broadband demand, and how LTE will help them do this. It is a flexible standard with a significant ability to span different types of  spectrum, but this comes with its downsides, notably the fragmentation of the device ecosystem. With well over 30 bands theoretically supported, many vendors will hesitate before launching a product unless its band combination is certain of a mass market.

LTE spectrum fragmentation

According to Wireless Intelligence, there will be more than 200 LTE networks in the world by 2015, but these will span 38 frequencies. As more carriers build out, the range of bands will increase – and some will use different and incompatible frequencies within the same band, hindering roaming, as we are already seeing in the US 700MHz spectrum.

There, smaller holders of 700MHz licences are asking the FCC to mandate that Verizon and AT&T support roaming onto their different blocks of spectrum. Even the big two are not consistent – Verizon uses 746-787MHz while AT&T has 704-746MHz.  Of course, some bands are far more popular than others and will attract the support of the big names. According to the most recent figures from the GSA (Global mobile Suppliers Association), in November there were 197 LTE devices announced by 48 manufacturers worldwide, of which 27 were smartphones. Routers were the largest category, with 70 models, followed by dongles, netbooks, tablets, notebooks and PC cards.

In frequency terms, the main choices are 700MHz (digital dividend in the Americas and elsewhere); 800MHz (digital dividend in Europe and other parts); 1.8GHz (key 2G band for refarming, outside the US); 2.6GHz (new 4G-specific allocation in many areas though already widely avail-able in TDD mode in the Americas); and the US 3G band, AWS. Because of the influence of Verizon and AT&T, the largest buyers of LTE devices to date by a long way, 700MHz boasts the biggest number of products, at 106, and most of the handsets. 

OEMs targeting non-US markets are also starting to think in terms of multiple bands, since most European and Asian cellcos will look for multimode 3G/4G devices as well as deploying LTE in two different frequencies. Already, the GSA has counted 36 gadgets launched for the most popular multiband profile – 800MHz/1.8GHz/2.6GHz, as demanded in much of Europe. As Orange and Telstra have made very clear, their minimum requirement for a device which could roam effectively would be quad-band GSM/EDGE, triband HSPA and triband LTE (in Europe, 800MHz,  1.8GHz and 2.6GHz).

And that doesn't take into account the rising interest in refarming the 900MHz GSM band too, a trend led by the Middle East and important in key European markets like the UK (because of Vodafone's and O2's lack of 1.8GHz for refarming). A recent survey by Rethink Technology shows how the OEMs will be forced to shift their priorities, in terms of band support, as the LTE industry evolves (the full results will be published in January in a report on LTE spectrum fragmentation and its impact on the device market).

Not only will they need to support three or even four 4G bands in most smartphones, but they will need to keep an eye on which frequencies are moving to the fore in carrier plans. The graph shows the number of branded smartphones which will support each of the main bands (many as one of several frequencies in a multiband gadget). This indicates how, as the US loses its premier position with the rise of Asian rollouts, in particular, the 700MHz frequency will lose its lead to more international frequencies, especially 2.6GHz.

The risks of over-hasty LTE roll-out

Overall, 248 operators have committed to investing in LTE (185 network commitments and 63 pre-commitment trials), while 35 commercial systems have gone live, a figure which should rise to 103 by the end of 2012. While this indicates impressive year-one progress, fast action comes with some risks too. The major European multinationals like to tout the benefits of waiting until the technologies are more stable and tested in real world conditions, and the prices are lower.  Of course, they have the luxury of extensive HSPA+ to fill the gap in the meanwhile.

Orange's LTE/EPC program director Rémi Tho-mas, speaking at Layer123's recent EPC and Converged Mobile Backhaul conference in London, said the key reason to deploy LTE was when capacity was running short, and that gave carriers the option to wait until the time is right in most markets. "LTE is not driven by a killer application, but it will essentially be driven by capacity needs," he said. "We want to launch LTE first time right."

Among the downsides of early roll-out, he said, are effective interworking and roaming with 2G and 3G; optimization of some aspects of the radio network; implementing fallback or VoIP solutions for voice; and implementing QoS. He also thinks the device offerings are immature and inadequate for many purposes as yet – Orange's shopping list, which echoes that of other major HSPA users like Telstra, includes a far wider range of multiband options plus fallback and VoLTE voice capabilities and self-organizing network features.

 In the new year, Rethink Technology will publish a comprehensive research report on the LTE spectrum bands – which will be the most important and where they will be supported. For more details and to receive an executive summary in January, please email Caroline Gabriel on [email protected]

Part 1: LTE to follow very different path from 3G
Part 2: BRIC taking leading role in 4G