Mobile operators are starting to implement carrier aggregation to increase capacity and make better use of disparate frequencies. However, they are also looking for additional spectrum, usually in bands where LTE will need to share with other technologies. A particularly contentious option is in parts of 5GHz, where Qualcomm is pushing an LTE-Unlicensed option in WiFi’s traditional band.
But despite objections on the grounds of interference issues and regulatory barriers, the chip giant has recently revised its proposals to the 3GPP and claims it has now addressed many of the issues, especially when it comes to the critical markets of China and the US.
Last year, Qualcomm spearheaded the LTE-U proposal, which effectively uses 5GHz licence-exempt spectrum for supplemental downlink (SDL), adding capacity to a primary cell in a licensed band. Similar SDL techniques have been pioneered in other bands – for instance, Qualcomm tested SDL with AT&T, using 700MHz unpaired frequencies, and Orange has trialled the approach using mobile satellite spectrum in 2GHz to add downlink capacity to LTE.
However, the 5GHz approach has greater complexities, and greater political sensitivity, because the band is unlicensed and the main area where WiFi has been expanding in capacity and capability. The current gigabit standard, 802.11ac, is mainly focused on 5GHz, as is the future 802.11ax for ultra-dense deployments. All this has made 5GHz the flagship band for those who campaign for more unlicensed spectrum and, more importantly, open technologies to harness it. Shortages in wireless broadband capacity can then be addressed by a wide range of service providers with flexible models and affordable prices, not a small group of carriers with closed networks.
Qualcomm, the great flagwaver for the cellular status quo, hits directly at this argument by proposing LTE-U, which gives mobile operators more spectrum capacity, for free, while maintaining their control in a way that WiFi offload does not. LTE-U might be running in unlicensed frequencies, but because it acts as a secondary system under the control of the primary network controller, it is still fully within the carrier’s walls.
There are all kinds of barriers in the way, however, not least that LTE-U is now part of the 3GPP Release 13 work, and so will not be fully standardized until early 2016. By that time, the build-out of advanced WiFi, and the power of the WiFi lobby worldwide, will be even greater than now. Indeed, advances by the WiFi sector will help make LTE-U less appealing to cellcos. Technologies which allow WiFi hotspots to be managed from their mobile cores address some of the issues of bringing non-3GPP technologies into their mix – offloading low value traffic is fine, but carriers fear losing the ability to monitor and monetize their subscribers once they roam into the unlicensed wilderness. That is starting to be addressed by common management and single sign-off techniques, such as Hotspot 2.0, Next Generation Hotspot and 3GPP work.
These developments indicate how most mobile operators acknowledge that they need WiFi, and would be shooting themselves in the foot if they compromised its performance. Coexistence between two technologies which work completely differently is hard, and LTE-U, while it may be part of the long LTE-Advanced menu of features, is very much the newcomer and could be left on the shelf.
There are echoes of the difficulties WiMAX faced in its 5GHz implementation, and even in the technology’s heyday, that band went largely ignored in favor of licensed 2.5GHz and 3.5GHz. Like WiMAX, LTE transmissions are scheduled by the base station, while WiFi implements ‘listen before talk’ (carrier sense multiple access) to support sharing between different users.
That has led to opposition, even within the 3GPP, on the basis that LTE-U would interfere with or even block WiFi transmissions (other objections revolved around different regulatory requirements in different regions for 5GHz). But Qualcomm argues that it will be simpler for cellcos to manage a 5GHz extension of their main network, rather than a separate WiFi system, and claims its simulations have shown LTE-U delivering twice the performance of a similar network of WiFi access point in 5GHz.
The company has acknowledged the interference fears though, and has updated its proposal with several coexistence features, designed to reduce the risk to WiFi, particularly in China, Korea and the US, whose regulations make conflict particularly concerning.