LTE unlicensed: coexist or perish

By Monica Paolini, Founder and President, Senza Fili Consulting

LTE unlicensed is clearly a hot topic these days – and one that often draws a strong emotional response. It was one of the key themes in London last week at the Small Cell World Summit and at the Carrier Wi-Fi World Summit. The consultation from the FCC also attracted a wide set of contributions.

Last Friday, at the end of the conferences last week in London, the Small Cell Forum gave us the opportunity to reflect on LTE in 5 GHz during a workshop on LTE unlicensed (LTE-U, and licensed-assisted access or LAA) that was very engaging even after three days of packed with small cells and Wi-Fi presentations. Caroline Gabriel did an excellent job at moderating the panel and providing a perspective on the topic. I had the pleasure to share the panel side with Simon Saunders and Mark Grayson. And we had great questions from an expert audience.

Many mobile operators deploying small cells find LTE unlicensed attractive as it gives them additional capacity at a low per-bit costs. The marginal costs of adding a new wireless interface to existing small cells is low because, being collocated, the installation and operating costs are largely shared by licensed and unlicensed LTE. In this perspective, LTE unlicensed greatly enhances the business case for LTE small cells, and it can provide a substantial push to move to dense small-cell deployments.

As a special case of carrier aggregation (CA), LTE unlicensed is organically integrated within existing LTE networks. This makes it even more valuable to mobile operators as they can manage traffic and services across the licensed and unlicensed spectrum more effectively than they do today with basic Wi-Fi offload. For operators, it is becoming increasingly valuable to integrate multiple wireless interfaces and spectrum assets to leverage network resources more effectively – i.e., to maximise QoE within their existing network – and to do so leveraging license-exempt band. In this perspective, LTE unlicensed as one solution in this direction, but it is not the only one.

LTE and Wi-Fi aggregation (LWA) and other mechanisms to improve integration between Wi-Fi and cellular network are also gaining traction, and represent a powerful alternative to LTE unlicensed, especially for operators already invested in Wi-Fi. LWA achieves most of LAA does, but without requiring a new wireless interface in the 5 GHz band.

The advantages of LTE unlicensed to mobile operators – full integration within the LTE network, and higher spectral efficiency –, however, are not sufficient to ensure the success of the technology. Wi-Fi users and vendors have expressed their worries that LTE unlicensed may degrade Wi-Fi performance, and correctly advocate that LTE unlicensed adopt mechanisms for fair coexistence that enable LTE unlicensed to treat Wi-Fi networks in the same way Wi-Fi networks treat each other. Unless these concerns are adequately addressed, LTE unlicensed is unlikely to succeed. Mobile operators and vendors are aware of this, and are working within 3GPP on the LAA version of LTE unlicensed, which is designed to coexist fairly with Wi-Fi, by using LBT mechanisms that are similar to those that Wi-Fi uses.

LTE has the potential to be nasty to Wi-Fi because it is a technology designed to have exclusive control of the licensed channel in which it operates, using a scheduled transmission model that treats any signal in its channel as interference. But are mobile operators likely to use this aggressive approach – even in countries like the US where regulation does not have strict co-existence requirements – and to antagonize the Wi-Fi community? I doubt they would do that, especially since most subscribers use Wi-Fi, and Wi-Fi access accounts for most traffic in mobile devices.

The listen-before-talk (LBT) mechanisms that 3GPP is working on to include in Release 13 arguably benefit the Wi-Fi ecosystem as much as they benefit the mobile operator ecosystem. Without robust coexistence mechanisms, mobile operators will have to demonstrate that LTE unlicensed does not harm Wi-Fi more than Wi-Fi. If the enterprise and real-estate managers do not trust LTE to fairly coexist with Wi-Fi, they are unlikely to allow mobile operators to deploy LTE unlicensed in their premises.

In the short term some operators may carefully deploy LTE-U networks that do not support LBT (and use other coexistence mechanisms that many in the Wi-Fi community are suspicious of). However, in the long term we expect LBT-based LAA to dominate because it can gain the needed support among the entire wireless ecosystem, including the Wi-Fi section. If it doesn’t play nice, it is difficult to see how LTE unlicensed can succeed in an environment where Wi-Fi is already fully established, and its presence and relevance continue to grow.

(The slides from the panel are available here. They summarize the results from a report which you can download here)