Following on from this week’s Small Cell World Summit, the Small Cell Forum hosted an open event where several leading analysts discussed their views on proposed use of unlicensed spectrum alongside or with LTE. The main options were explained and views exchanged, mostly oriented with a technical focus to enable better-educated debate on their relevance and value to the industry.
Running through the options
The Small Cell Forum has published an updated paper on the technical options as part of this week’s Release 5.1 documentation, so I won’t repeat that all here. Mark Grayson of Cisco, who led the work, walked through the main options:
- Today’s standalone Small cell
- Adding Wi-Fi data path using TWAG
- Adding Wi-Fi data path using ePDG, enabling Wi-Fi Calling which was built into iOS8
- LAA (Licensed Augmented Access): Adding extra LTE bands running unlicenced 5GHz bands
- LWA (Licensed Wi-Fi Access) using PDCP integration
- Multi-path TCP using standard Wi-Fi Access Point in parallel with cellular (e.g. Apple Siri does this and it’s similar to Alcatel-Lucent’s Wi-Fi Boost proposal).
Just this week, Qualcomm has thrown another spanner in the works by proposing to run LTE standalone in the unlicenced 5GHz band, with or without a SIM card, called MuLTEfire. Previously, they've always argued to retain a control link in licenced spectrum so it would only be available to existing mobile network operators.
The cynical view
So there’s no shortage of technical alternatives to choose from.
What I felt was missing was more focus on defining and agreeing exactly what the specific problem being solved was, and who would pay for it. A cynical view is that it’s more about how the industry can create growing demand for yet more chipsets.
A key theme was about addressing the insatiable desire for ever more wireless traffic rather than higher speeds (Wi-Fi would surely outperform LAA on peak speed, because 802.11ac can access a full 160MHz of bandwidth vs 4x LTE 20Mhz carriers). Monica Paolini observed that the proportion of smartphone data traffic sent over Wi-Fi is predicted to grow (referencing Cisco VNI). So why should Wi-Fi lose some of its spectrum capacity for use by those with their own licenced spectrum?
Unlicenced sounds very attractive because it's both abundant and free - see chart below from Real Wireless.
Simon Saunders analysed three specific high capacity cases, including an open workspace, a conference room and a lunch room/ cafeteria, where many people close together want to consume data. The workspace and many other use cases would be more than satisfied for years to come with today’s dual-carrier LTE small cells, especially where Wi-Fi is also available. He predicted that within five years time, the lunch room might need the additional capacity that LAA could bring. All I can say is that it’s pretty sad if we’re all addicted to watching HD videos during our lunch break rather than talking to each other across the table.
Although unsaid, that would limit the market to a relatively small number of places with very high traffic scenarios and not the mainstream. Nobody was suggesting this would be incorporated into your residential femtocell at home and I heard little or nothing about outdoor use.
Unlicensed bands have no protection
Simon commented that unlicensed access comes with many benefits (not least free spectrum). There are regulatory constraints including RF power level, duty cycle limitations and “politeness” by sensing/adapting to other users nearby. But its not exclusive to property owners (there was a recent court case where a Marriot hotel settled a case with the FCC for blocking Wi-Fi personal hotspots for $600K). While regulators aren’t required to protect the installed base, it would be churlish to abandon or ignore them.
His simulation studies have shown that at low levels of activity, both can work well alongside each other. However as usage increases, LTE-U becomes “a very effective jammer”. A polite “Listen Before Talk” feature is required, and with one simulation both were shown to work co-operatively for mutual benefit. Still at a very early stage of standardisation (there was some lively debate between chipset vendors about exactly what stage a 3GPP work item had reached), there are many nuances about what “polite” means and how that will be embodied in the final agreed standard. Only then would the simulation studies and results reflect the true impact.
Monica Paolini pointed out that most Wi-Fi traffic is still going over the limited 2.4GHz band, so there’s plenty of scope for Wi-Fi growth as we adopt 5GHz and 802.11ac technology available today. She also couldn’t see customers running to the store just to buy LAA handsets. Venue owners would want to protect investments in their own Wi-Fi systems. She felt that LTE-U had to “be nice to Wi-Fi” or it has no future. But there is a price to pay for “being nice” (e.g. implementing Listen Before Talk), and that will have some impact on the overall performance benefit.
The primary focus of LAA today is the global 5GHz band. It was thought that while the 3.5GHz band might be interesting to many in the USA, it’s highly fragmented around the world making it difficult to introduce on a global scale.
Many questions unasked or unanswered
The format of the event limited the number and range of questions. The Forum will continue to evolve the discussion. Just a few topics I could think of which weren’t covered:
- Wi-Fi continues to evolve rapidly. Gigabit speeds will be mass deployed using Wi-Gig (Wi-Fi at 60GHz) long before LAA hits the streets.
- Why should consumers pay for LAA but not Wi-Fi use, and whether they should be able to discriminate?
- Why would a building owner pay for installation of LAA capable kit (locked to a network operator) rather than just buy some more Wi-Fi and/or small cells?
- How will multi-operator installations work?
- Given the radically different RF footprint of 5GHz band, how many LAA small cells would be needed vs LTE vs Wi-Fi only ones.
- What percentage of users would need to adopt LAA to make a significant difference?
- Many operators still have plenty of licenced spectrum still unused. I could point to much of the 2.6 TD-LTE bands in Europe for example. Shouldn’t they be used first?
Monica Paolini's conclusions are shown below.
I’d say attendees found the session useful and it helped move forward and educate about some of the technical aspects. Attendance was good (estimate over 70 attendees).
It’s clearly very early days for this proposal, with a lot of hard debate to be had in the 3GPP standards meetings. Without a firmer definition of how LTE would be adapted to become polite (adding Listen Before Talk etc), it’s difficult to make accurate assessments of how it would perform.
I felt little attention was given to the business drivers, the value to the venue owners or differentiated service to the end users. Is this simply a trade-off between deploying a few more small cells vs re-equipping smartphones to access spectrum in a different way? Little time was spent on the simpler option of using Wi-Fi, such as LWA.
It reinforced the Small Cell Forum as the place to go for technical explanations of upcoming technology options, but not necessarily which one(s) to choose.