LTE-U By Any Other Name is Licensed-Assisted
I recently read the rather interesting announcement from NTT DoCoMo where they demonstrated LTE running over the unlicensed 5GHz band. They report a 60% increase in spectral efficiency over IEEE 802.11n Wi-Fi. The article also noticed that LTE-U is now referred to as Licensed-Assisted Access using LTE or LAA-LTE. This caused me to pause, and think about several things.
The comparison between LAA-LTE and Wi-Fi is not (and should not be) about spectral efficiency. Rather it is about several other factors:
- A robust network with a diverse client ecosystem (does anybody reading this blog own an IP device without Wi-Fi? How many of those devices contain LTE? How many are Wi-Fi only, without any SIM card?)
- The ability to support neutral host deployments (are stadium owners willing to deploy LAA-LTE if it only supports one operator?)
- The ability to co-exist in a multi-operator environment (how would LAA-LTE operate in dense environments when it has to co-exist with LAA-LTE APs from other operators?)
- The ability to co-exist in a multi-technology environment (would if it adversely affects the existing and extensive deployments of Wi-Fi infrastructure?)
- A number of Mobile operators have agreements with Wi-Fi providers for offloading cellular traffic (how does one enable such a scenario with LAA-LTE?)
And if you really want to talk about speeds and feeds, it’s interesting that the test was done against 802.11n, when 802.11ac is now widely available, providing speeds that exceed LTE-Advanced speeds of 1 Gbps (IEEE 802.11 ac Wave 1 provides maximum speed of 1.3 Gbps and Wave 2 provides maximum speed of 3.5 Gbps). It’s also important to note that LAA-LTE has not been defined yet and so it’s very likely that the LAA-LTE implementation tested here does not have the politeness mechanisms required in certain regulatory domains like Europe. These mechanisms allow fair usage of the unlicensed spectrum by allowing other users an opportunity to transmit and share the spectrum. These mechanisms already integrated into Wi-Fi will add additional overhead to LAA-LTE that will reduce its spectral efficiency, a factor that needs to be taken into account in any comparison.
Hype is interesting, but let’s take stock of where the industry is today. The main goal for LTE in unlicensed spectrum is to leverage the relatively large amount of unlicensed spectrum in order to provide a better mobile service. Given the transmit power restrictions imposed on unlicensed bands, it is expected to be primarily deployed in small cells. No standards exist for LTE in unlicensed spectrum. However, the mobile industry is discussing certain technical proposals. These describe an LTE carrier in unlicensed spectrum combined with a LTE carrier in a licensed spectrum using either carrier aggregation (CA) or supplemental downlink (SDL) where all control channels are on the licensed spectrum LTE carrier. Specifically, the industry proposals do not describe a “stand-alone” LTE in unlicensed spectrum system where there is no licensed LTE carrier. Operators will not be able to leverage someone else’s unlicensed LTE hotspot, they will need to deploy their own (unlike with Wi-Fi). It cannot be overstated that LTE in unlicensed spectrum is meant as supplementary technology to LTE (which is by definition operable only in licensed spectrum).
I’ll just conclude by noting that Cisco is supportive of the LAA-LTE initiative and is participating in 3GPP standards work on this topic. However, I don’t think that LAA-LTE replaces carrier grade Wi-Fi because of the factors I mentioned earlier. As a result, it is vitally important for the industry that the LAA-LTE standard ensures fair usage of unlicensed spectrum with all other technologies using the same spectrum like Wi-Fi.
We contributed to the 3GPP workshop in June, and will work closely with the industry and in the standards to ensure that LAA-LTE co-exists with Wi-Fi in order to ensure that the global investment operators and Enterprises have made in Wi-Fi, in the billions of dollars, is enhanced by LAA-LTE, and not disrupted. It goes without saying that the mobile networks of the future will use all aspects of licensed and unlicensed bands and technology to deliver the best mobile experience possible in any given environment.
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