Signs of momentum for TD-LTE in 3.5-GHz

Caroline Gabriel/Wireless Watch

Rethink

The 3.5-GHz band is currently the dark horse of LTE. Traditionally used for fixed local loop and Wimax services, it has many potential benefits for more modern applications, notably small cells, especially as it is a near-global band. Some companies are placing high hopes on a future flourishing of this spectrum, including small cell start-up Accelleran.

The Belgium-based company points to the importance of the FCC’s support for 3.5-GHz as an international small cell band, which could be valuable to add a layer of dense capacity to LTE services. It has upgraded its M101 TD-LTE small cell to support the US spectrum, even though the rules are not yet finalized.

The product is in line with the FCC proposal, issued on April 23 and labelled US Citizens Broadband Radio, to earmark 3.5-GHz as an “innovative band” for small cells. It claims its design has sufficient flexibility to adapt as the FCC decides on the specifics of its licensing scheme and band plan.

This is only at the preliminary stage, and the FCC still has to finalize many details, but the general objective is to open up 3.5-GHz via a flexible spectrum sharing model, which could be the model for other frequencies too.

As first-wave LTE deployments, most of them in paired FDD spectrum such as 1.8-GHz, start to reach capacity, many operators plan to ‘densify’ using small cells and TDD spectrum, which is particularly suited to downlink-heavy applications and for minimizing interference in multilayer HetNets. In the US, this prospect, and pressure from spectrum reformers, has led to moves to open up 3.5-GHz, even though much of it is occupied by government users.

The FCC supports a three-tiered model, with spectrum sharing in the government frequencies, to open up a total of 150 MHz for consumer access (including mobile), carrier small cells, backhaul and fixed wireless. The agency initially targeted the frequencies from 3550-MHz to 3650-MHz back in 2012. These are used by federal and other bodies and would require spectrum sharing, with the incumbents having some form of priority access. In April the regulator sought comments on adding a further tranche in 3650-MHz to 3700-MHz, which is used by WISPs to provide rural broadband. Interoperability would be required across all three tiers within the 150 MHz, to help drive economies of scale.

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