Wi-Fi has enjoyed a quasi-monopolistic usage of the 5 GHz in the last few years but that is changing as Multefire and LTE Unlicensed in general are making inroads in once considered “Wi-Fi” bands. We thus feel it is about time we ask how these new technologies will impact Wi-Fi performance. It is hard to find a neutral opinion in this highly debated topic but I sat down with Veli-Pekka Ketonen, founder and Chief Innovation Officer at 7signal to pick his brain and help us understand where we are today.
There are conflicting views about how various flavors of LTE Unlicensed may impact the performance of Wi-Fi networks. Depending on who you ask you will get a different answer. Can you provide some clarity into this debate?
Any additional traffic and RF energy thrown at current Wi-Fi channels will reduce the airtime and spectrum available for existing Wi-Fi networks. All unlicensed LTE variants add traffic to 5 GHz Wi-Fi channels. Variants using LTE modulation at 5 GHz channels will add further inefficiencies to Wi-Fi radio operation.
However, LTE Unlicensed variants which aggregate a normal Wi-Fi carrier at 5 GHz band to LTE anchor do guarantee fair co-existence. These variants include LWIP (LTE-Wi-Fi aggregation using IPSec tunnel) and LWA (LTE-WLAN aggregation) with the latter being more efficient and capable. eLWA is an evolution version of LWA which also supports WiGig (60 GHz). However the market does not seem to favor LWIP and LWA at this time.
There are of course, LTE Unlicensed variants which use LTE carrier at the 5 GHz band. These include LTE-U, LTE-LAA (Licensed Assisted Access) and Multefire. LTE uses only central scheduling. It has been reliably shown that high duty cycle LTE transmissions completely override Wi-Fi traffic. Wi-Fi will stay out of occupied channels by design. LTE-U and LAA (used also by Multefire) technologies add features which help to share airtime more fairly. Reaching a true fair spectrum sharing is still a big challenge.
What are the differences between LTE and Wi-Fi LBT mechanisms?
Just talking about using LBT (Listen Before Talk) can be misleading. There are significant differences between LTE Unlicensed, 3GPP and Wi-Fi LBTs. Devil is in the details here. The exact implementation and parametrization of LBT defines completely how fairly LTE co-exists with Wi-Fi. Using one of the 3GPP LBT variants does not yet mean fair co-existence. This is a common misconception.
It’s mandatory for Wi-Fi to use two different thresholds for channel occupancy detection. Wi-Fi’s threshold for ED (Energy Detection) is -62 dBm, this applies to any energy, Bluetooth, ZigBee, LTE, etc.. Wi-Fi also needs to detect Wi-Fi pre-ambles, typically at 5 GHz down to -78 dBm. This threshold varies between vendors and can go down to -90 dBm range. If either of these indicates the channel is busy, Wi-Fi transmitter will hold back from transmitting to prevent frame corruption.
3GPP LBT only specifies use of ED threshold. LTE-LAA will hold back from transmitting only if transmitter detects energy above ED threshold. With 3GPP specifications ED is defined –72 dBm. However, this is high compared to -78 – -90 dBm range used Wi-Fi pre-amble detection. Note that 6 dB difference doubles the cell radius in open space. It’s very important to remember that USA, China and Korea do not require LBT at all. If LBT is used, it’s mechanism and any related parameters are not defined. If LTE ED is higher than Wi-Fi pre-amble detection threshold, this means LTE will end up transmitting during Wi-Fi frames and there will be corrupted frames. These lead to retransmissions, reduces channel efficiency and Wi-Fi quality. The opposite is also true, as LTE frames may get corrupted as well. Since Wi-Fi RF power levels are generally lower, Wi-Fi is at disadvantage.
LTE-U does not use LBT but uses CSAT which relies on controlling duty cycle of LTE scheduling based on average Wi-Fi energy in the channel. CSAT is also based only on ED. It does help, but it’s not very effective. Overall, it seems LTE-U was a stepping stone to LAA and may have shorter life than LAA.
It’s worth mentioning that the standard does not prevent LTE-U and LAA products using Wi-Fi pre-amble detection to ensure co-existence. It may also be a competitive advantage in environments where owner of the space wants to minimize risk on Wi-Fi operation.
What studies do you see as most complete or reliable?
3GPP has defined fairness as follows. “The capability of an LAA network not to impact Wi-Fi networks active on a carrier more than an additional Wi-Fi network operating on the same carrier, in terms of throughput and latency.”
In practice, results of co-existence fairness testing are heavily impacted by the way the tests are done.
The Co-existence Test Plan developed with Wi-Fi Alliance’s leadership is an important benchmark. The first version was completed in September 2016. This work was done in collaboration with a number of industry players, like Qualcomm, Broadcom, Ericsson, Verizon, T-Mobile, Huawei, HP Enterprise, Google and CableLabs.
Unfortunately, to my knowledge, there are no public test reports against this test plan. Neither are there any companies claiming to be compliant with the test plan. The test plan is not a standard either, so compliance is not mandatory. Furthermore, compliant once does not mean product needs to be compliant after the testing has passed. It’s not like FCC or ETSI RF emissions requirements with strict requirements.
Which vendors have committed to ensure coexistence between these 2 technologies?
Committing to ensure co-existence means to me that implementation is compliant with the Wi-Fi Alliance co-existence test plan. I have not seen anyone stating that yet. I ’d be happy to be mistaken here.
In which environments do you expect LTE to impact Wi-Fi performance? How can it be prevented?
It does not seem to me that Unlicensed LTE suppliers are actively promoting fairness. The mobile industry seems to consider LTE more efficient, thus preferable. In any case, additional traffic in shared channels will limit Wi-Fi capacity and quality.
Environments requiring the highest mobile network capacity will continue be first ones in focus. These include for example stadiums, arenas and dense city centers. There are already a number of areas and some common devices supporting different flavors of unlicensed LTE. Impact on Wi-Fi increases as more terminals start to support unlicensed LTE. Press releases on unlicensed LTE are published regularly. Often a key theme is record speed.
Unless an organization controls what carriers and technologies are used in their physical space, appearance of LTE carrier in unlicensed spectrum happens without any upfront notice. Unlike in mobile networks, performance management has not been common practice in Wi-Fi, while recently it has started to gain more attention. Having proper visibility to network performance and user experience is important. There is no guaranteed way to just prevent the impact completely. One way is to reduce impact on Wi-Fi is to use different 5 GHz channels than unlicensed LTE. For DAS customers with indoor Wi-Fi and LTE network coverage, it’s important to be aware of what kind of carriers are transmitted inside the buildings.
What are the conclusions you have drawn from your research?
Unlicensed LTE is already happening. There will be no big announcements to Wi-Fi users that LTE is now sharing the spectrum. When you hear about Gigabit LTE network roll out, unlicensed LTE is most likely used.
After all the effort put into co-existence test plan, it’s disappointing that it seems not to be in active use.
There will be impact on Wi-Fi. The impact will grow as more unlicensed LAA and Multefire are deployed and more terminals start to support it. Companies providing Wi-Fi products and organisations operating Wi-Fi networks seem unprepared for this. It’s important to make sure Wi-Fi continues to operate reliably in hospitals, warehouses, universities, enterprises and other locations which rely on its good performance.
Veli-Pekka Ketonen is the founder and Chief Innovation Officer at 7signal, The Wi-Fi Performance Company. Prior to 7signal, Veli-Pekka worked at Nokia Networks leading base station R&D and optimizing performance of Nokia radio network products.
