When it comes to spectrum, current cellular networks that are designed well can still keep up with the ever rising demand for bandwidth. In addition, not all of the spectrum below 3 GHz that has been assigned to cellular operators is used so capacity can be further increased by adding additional carriers per base station for a few years. In addition, there is still spectrum below 3 GHz that could be assigned to cellular networks in the future but compared to what's already assigned it is not that much. So sooner or later, alternatives are required to keep up should bandwidth demands continue to grow at current rates of 50-80% per year.
Small Cells to Escape The Bandwidth Crunch On High Frequencies
There are a couple of options to escape the dilemma: Smaller cell sizes and spectrum in higher frequency bands. Smaller cell sizes and the use of licensed spectrum allow a higher re-use factor and hence increase the overall capacity of the network. There is also no way around very small cells when licensed or unlicensed spectrum beyond 3 GHz are considered for future use as the higher signal attenuation limit practical cell sizes to a few tens of meters from the cell's center. So one way or another, going beyond current network capacity by an order of magnitude requires smaller cells. How that can be done economically is a matter of debate and I won't dwell on this particular point in this blog post.
What I would like to take a closer look at today, however, is the potential use of the license free 5 GHz band which is pretty much exclusively used by Wi-Fi today. So while the Wi-Fi camp is probably not very happy about the LTE camp considering this band, it is a logical evolution of the LTE ecosystem and the 300+ MHz of available spectrum in the band at zero cost is quite irresistible as well.
A number of industry players are doing quite a lot to push the idea in the media at the moment. Even 3GPP published an article back in September 2014 on their web site stating that LTE in the license free 5 GHz band, referred to as LAA-LTE (License Assisted Access LTE), is seen as a major RAN feature for 3GPP Release 13. [http://www.3gpp.org/news-events/3gpp-news/1628-rel13].
What is License Assisted Access?
So why this strange name “License Assisted Access”? The 3GPP Study Item on the topic (see links below) is quite clear that the aim, at least initially, is not to have independent LTE cells in the 5 GHz band. Instead, a primary cell transmitting on a carrier in a licensed chunk of spectrum a network operator has paid for is complemented with an additional LTE carrier in the unlicensed 5 GHz band. The Study Item even goes as far as saying that in a first instance, the 5 GHz band shall only be used for downlink transmission, i.e. as a Secondary Cell (SCell) in a classic Carrier Aggregation (CA) configuration. All signaling and control information is only sent on the Primary Cell (PCell) which is operated in a licensed band. I'm not quite sure why that limitation has, at least for now, been put into place but it sounds like there is a fair amount of politics involved to appease some players with a vested interest in Wi-Fi. What such a setup would of course do is to keep a cellular signaling link in place all the time so an ongoing data transfer can quickly be pulled away from a LAA-LTE carrier back to the primary carrier that potentially covers a larger area when the signal deteriorates.
And this immediately brings us into the domain of the various HetNet (Heterogeneous Network) and CoMP (Coordinated Multipoint) features that have been specified in recent 3GPP releases but are not used in practice so far. So perhaps at the beginning small eNodeBs devices may transmit both the PCell and SCell parts. But I am sure at some point the industry might get more daring and also think about a split between the PCell being transmitted in licensed spectrum from an overlay macro cell while the SCell using unlicensed spectrum is transmitted from a small cell. An interesting aspect of such a scenario would be that a small cell that only uses unlicensed spectrum is easy to set-up as the site does not have to be registered as a cellular transmitter. After all, it's only using unlicensed spectrum and has to adhere to the same transmit power limitations and regulations as Wi-Fi access points.
Dealing with Interference and Being a Fair Player
Obviously as someone operating a Wi-Fi access point in the 5 GHz band at home for high bandwidth media streaming I'd be very unhappy if a nearby LAA-LTE cell would significantly interfere with my transmissions. Fortunately there is enough space in the 5 GHz band, at least until 160 MHz Wi-Fi channels defined by 802.11ac replace the 40 Mhz and 80 MHz channels used by 802.11n and 802.11ac products today, so networks can go out of each other's way. LAA-LTE carriers will be limited to 20 MHz channels but Carrier Aggregation (CA) would allow to bundle several channels in addition to the PCell channel in a licensed band. Today, 2×20 MHz Carrier Aggregation in licensed bands are used in practice and 3×20 MHz Carrier Aggregation is just around the corner. By the time LAA-LTE might be ready for deployment, perhaps in the 2018-2019 timeframe, it might be even more.
Also, the Study Item promises to have a close look at how a “Listen before Transmit” scheme can be implemented for the LAA-LTE cell so it can detect Wi-Fi networks in the same spectrum and either change to a different section of the band, reduce it's transmit power or to coordinate transmissions with the Wi-Fi networks it detects. The promise is that a LAA-LTE carrier would no more interfere with a Wi-Fi network than other Wi-Fi networks in the area. A nice promise but a heavily loaded nearby Wi-Fi network would interfere quite a lot with my own Wi-Fi network.
It's going to be interesting to see how this particular part will be standardized. Today, an LTE cell does not look out for interference and uses the licensed spectrum assigned to a carrier whenever it likes. Wi-Fi on the other hand has an interference and collision detection scheme with backoff times and retries. So if you will, LTE without any enhancements is not really a fair player when it comes to competing for the same spectrum with other transmitters in real time because it did not have to compete for access to a channel so far. Also a LAA-LTE cell has to take care that it doesn't interfere with a LAA-LTE cell in 5 GHz band of another network operator that has also decided to put a small cell in the area.
Why Compare Spectral Efficiency of LTE vs. WiFi?
Sure, one could use Wi-Fi access as part of a cellular network but there have been so many approaches to include Wi-Fi access into a cellular network infrastructure that have failed that it's not worth to think about yet another flavor. What makes LTE so attractive over Wi-Fi for cellular use is not its spectral efficiency but that LTE is a cellular technology while Wi-Fi is a hotspot technology without mobility in mind. So while Wi-Fi is great for homes, hotels and offices for stationary or nomadic Internet use, it can't compete with LTE in full mobility scenarios in which it is important to have automatic subscriber authentication, integrated backhaul and seamless mobility to and from larger macro cells. Also, Wi-Fi is not specified 3GPP so they have little influence over potential enhancements required to fully integrate it into an LTE network. As I said above, it's been tried before…
To make sure I still remember how it this activity started here are some dates. From what I can tell, the Study Item started in December 2014 and Ericsson envisages the study to be finished by mid-2015 with the specification to be finished mid 2016. Deployment usually takes 2-3 years after that.
And here are a number of great links to the details:
- Felllow blogger Zahid Ghadialy already picked up on the topic back in June 2014. Zahid also links to his blog posts on LTE-U (-Unlicensed) that show activity around using LTE in the 5 GHz unlicensed band as far back to December 2013.
- And here's a link to the report from the 3GPP workshop on 'LTE in Unlicensed Spectrum” held in June 2014 in Sophia Antipolis.
- The LAA-LTE Study Item description from September 2014, RAN#65, Edinburgh.
- And the final Study Item Description as approved in December 2014 in the 3GPP RAN#66 plenary meeting.
- The latest document is TR 36.889, created in December 2014 at RAN#66 that will be updated continuously to hold the result of the study item.