China Mobile Ltd. is on the verge of building what could ultimately be the world's largest Internet access network as it starts the construction of an LTE TDD-based 4G radio access network that will comprise macro and small cells.

The operator, which has begun commercial trials of its 4G services, aims to roll out a network -- focused on densely-populated cities -- that is far more competitive than its existing 3G network, which was based on China's home-grown TD-SCDMA system.

To put it into perspective, China Mobile expects to have 200,000 LTE TDD macro base stations in service by the end of 2013 and 350,000 by the end of 2014. That compares with the 350,000 base stations in total it has for 3G, and the 600,000 2G GSM sites that today carry the bulk of its mobile traffic.

A mere 15 percent of China Mobile's 730 million mobile customers are on its undersized 3G network and the operator is determined its 4G network will do better. To achieve its rollout targets will require considerable investment, which is why the carrier has allocated billions of dollars from its 2013 capex budget towards the 4G rollout. (See China Mobile's Capex Blowout.)

The operator is working with a number of vendor partners to plan its next-generation mobile network, including Alcatel-Lucent, which has teamed up with China Mobile to develop technology for LTE TDD metro small cells.

Most 4G deployments globally will utilize the FDD flavor of LTE and that's where the majority of vendor R&D has been focused. But while there are "slight differences" between the two strands of LTE, says Michael Schabel, vice president of small cells at Alcatel-Lucent, the fundamental technologies are the same.

"We have modularized our [radio access] solution. We can basically build up the technology as needed. The form factor is similar; the mounting is similar; the backhaul is similar. We have tried to keep as much of the design as possible," he told Light Reading.

Metro Radio on trial
The product developed in partnership with China Mobile is called Metro Radio, part of AlcaLu's lightRadio portfolio, was launched at Mobile World Congress in February and is being tested now in three of the operator's 13 trial networks. (See MWC13 Hot Network Techs: Alcatel-Lucent and Who's Big in Small Cells?)

Schabel says China Mobile is opting for an architecture, known as cloud-based RAN (C-RAN), that includes the baseband elements of the traditional base station being housed in a central location and connected to distributed antennas via fiber, a plan that allows smaller cell sites. (See China Mobile Steps Up Cloud RAN Efforts.)

"If we can take that [baseband] modem and keep it at the data center, then we can pool that modem resource. You can effectively get more opportunities to do advanced features with the pooled modem because you are doing shared scheduling," says Schabel.He said the approach is not unique to China Mobile and Metro Radio. Operators might apply it in a high-density environment such as a sports stadium, where it makes sense to centralize the baseband and deploy multiple small antennas.

Spectrum and planning
It's not only the network architecture that China Mobile needs to consider: As Qiu Heng, vice president for LTE and WiMax products at Huawei Technologies Co. Ltd., says, the operator also has to work around its spectrum assets.

The operator is rolling out LTE TDD in three different frequency bands: at 1900MHz In Shanghai, for example; and at 2300MHz and 2600MHz in other cities.

But Qiu predicts that, in the future, China Mobile and other operators around the world that are deploying LTE TDD will operate and manage their networks differently from their FDD counterparts because it enables the asymmetric use of unpaired spectrum (that is, the downlink and uplink transmissions share the same frequency band). FDD LTE networks run over paired spectrum, where different frequencies are used for the downlink and uplink.

The asymmetric nature of LTE TDD will allow greater flexibility in capacity planning and allocation, especially as the volumes of data traffic increase, believes Qiu.

Like other vendors, Huawei offers pico and femtocell solutions, including one that combines Ethernet in the power cable to help cut the exorbitant cost of installing kit in Chinese urban buildings.

Huawei has also developed a small-cell management offering called CloudBB, which works with its outdoor product "to share baseband unit resources so [that] outdoor and indoor networks can better work together," so cutting down on handover areas.

— Robert Clark, contributing editor, special to Light Reading