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LTE-U(LTE-Unlicensed) / LAA / LWA / MultiPath / MulteFire /LTE-R (LTE-Railway)


Recently (from 2014 as far as I remember), there has been pretty much discussion (even real testing going on) on LTE being extended to non-standard mode of operation (Non 3GPP yet at least up to this point, as of Jan 2015).

Note : In early second half of 2016, I saw a company to try LTE-U device for the first timie, and as it approaches to the end of the year, I saw many companies testing LTE-U devices.  Now in early 2017 we are at the stage of testing LAA.


Two of the technology falling into this category is LTE-U and LTE-R. LTE-U has been proposed and start being implemented in the industry for about a year and LTE-R just start being proposed as Integrated wireless railway network to be commercialized .


As of Jun 2016, I don't see/hear anything about LTE-R.. but I saw the drastic evolution of unlicensed LTE that created several different technologies (names) to the list. It started with LTE-U and evolved to LAA, eLAA and recently we are hearing of new branches like MulteFire and LWA. I don't know how many other technologies are waiting to be newly evolved and which of these technologies will survive to the end.. some of them are now pretty well defined but there are a lot of other features to be defined technically. I will keep following them up and try to write down whatever has been clarified.



Followings are the brief descriptions for each of these technologies. I will put further details under separate section if more technical details are published. Also, I put many whitepapers and YouTube links under Reference section. I would recommend you to go through those materials when you have free time and use this page as reminder afterwards. (The contents of this table is mostly based on Ref [7] )


< Comparative Tables on LTE Unlicensed Technologies >




  • First initated out side of 3GPP, but formalized in 3GPP Rel 12 later.
  • Downlink only
  • Support CSAT but no LBT support.


  • Defined in 3GPP Rel 13
  • Downlink Only
  • Dynamic Channel Selection
  • Support LBT


  • Defined in 3GPP Rel 14
  • Downlink and Uplink
  • Dual Connectivity : Aggregation across non-collocated nodes


  • Using Wi-AP as a part of LTE transmission point
  • Transmitting LTE Packet (PDCP) is encapsulated in WiFi Packet (802.11 MAC Frame)
  • LTE eNB and WiFi AP needs enhancement to support the special interface Xw-C / Xw-U


  • Not much known in terms of technology as of now (Jun 2016)
  • Main focus is to make it easy to depoly LAA like system


Followings are the list of topics that gives you further details of various LTE-Unlicensed technologies.





LTE-U stands for LTE-Unlicensed. This is the first real implementation of the whole LTE-Unlicensed family as far as I remember. Since this is already implemented and tested relatively widely and a lot of details are known, I thought I got enough information to write this subject in a separate page. Refer to LTE-U page for the details.



Channel Selection : How to find proper channel to use ?


In LTE, every transmission within a specified spectrum is well managed/scheduled both for downlink and uplink. So Network can figure out exactly when to transmit something and when UE will transmit something.. but in Unlicensed band transmission and reception by the existing WiFi device is not as predictable as in LTE. Basically anybody can transmit in anytime as long as it complies with CSMA/CA procedure.  


In this kind of situation, how LTE network can schedule transmission in Unlicensed band ?

One method is for LTE cell to perform a special channel selection procedure and allocate the channel only when it is clean.


In channel selection process, network measures the energy level within the allowed band and use those area in which the energy level is under a certain threshold (e.g, -62 dBm over 20 Mhz BW).

The Cell performs this kind of channel selection algorithm in on-going mode and if needed it will select more suitable channel and switch to it.


What if you (a Cell) fail to find any clean channel at the initial channel selection process ?

In this case, the cell use another mechanism called CSAT (Carrier-Sensing Adaptive Transmission). Technically, CSAT is very similar to CSMA or LBT(Listen Before Talk), the main difference is that CSAT monitors(senses) the medium for much longer duration (around 10 s of msec to 200 msec) comparing to normal WiFi CSMA, so that it would not interfere not only with WiFi data transmission (QoS traffic) but also with more sporadic transmission like discovery signals.



Coexistence strategy


According to Reference [5], there seems to be roughly three levels of Coexistence strategy regarding LTE-U. Most of these techniques are already mentioned in previous section, but let me rewrite in a little bit different words.


Channel Selection : This says "Use WiFi band only when it is not being used (i.e, the band is clean)". To figure out whether it is being used or not, you have to measure the engergy level of the target band as described above.


Time-domain coexistence techniques : Even when it fail to find any clean channel in channel selection stage, there is still some chance. Even though there is no clean channel at the point of channel selection, there might be some empty slots if you look at the time domain scheduling with relatively high granularity. To find out this kind of time domain opportunity and in order not to make an interference at any time, they are considering several different technology like CSAT, eCSAT, LBT. My personal question were 'do we have to support all these technology everywhere (every devices)?'. This question was answered by Reference [5] as follows.

For non listen-before-talk (LBT) markets such as the US, South Korea, China and India, CSAT (Carrier Sensing Adaptive Transmission) can be used without changing Rel-10 Carrier Aggregation protocols for co-channel coexistence. For LBT markets such as Europe and Japan, Rel-13 LAA will enable channel sharing with Wi-Fi by performing clear channel assessment (CCA) based channel availability sensing and adapting the transmission duration on a fine timescale, ranging from 1ms-10ms


Opportunistic SCell operation : To me, this sounds like a rule (not a technology). Basically this says "Use the unlicensed band only when it is really necessary (e.g, in real demand on high throughput or when the primary cell is overloaded). Release the unlicensenced and fall back to anchor carrier when the demand for unlicenced band is not critical.


Interplay of these coexistance technical components are well illustrated as follows in Ref [2].




LWA (LTE-WLAN Aggregation)


In other LTE-Unlincensed technology, LTE borrow only radio spectrum from WLAN and all the transmission/reception technology except radio spectrum purely based on LTE. However, in LWA LTE borrows not only WLAN radio spectrum but also WLAN Access Point for transmitting data. This key difference is illustrated as below. To implement this key difference, we would need to implement many other technical components. The most important part would be to implmenet special interface between LTE eNB and WLAN AP. Then you may ask.. at which layer the eNB and WLAN will be connected ? It would be quicker to interact if they can get connected at lower layer (e.g, PHY/MAC), but LTE PHY/MAC and WLAN PHY/MAC is completely different.. so hard to make any connection at this layer. So they will use higher layer (specifically PDCP) for this connection. As you know, there is no PDCP layer in WLAN. So connecting at PDCP layer might be a little bit misleading. More accruately speaking, LTE PDCP packet will be encapsulated into WLAN MAC frame.

Now LWA is defined in 3GPP specification and I created a separate page for 3GPP LWA because it will be a long story. Refer to LWA page for the 3GPP details.






Around the end of 2016, I start hearing of implementing this both on UE modem and Network, UE testing equipment side. Now (as of early 2017) I am hearing of testing this feature on real UE. So I decided to create separate page for this. Read LAA page for the details.



Multipath Aggregation


Putting very simple way, Multipath Aggregation (more formally Multipath TCP Aggregation : mptcp) is LTE-WLAN aggregation at TCP level as illustrated below. The key component in this architecture would be MP Proxy (Multipath Proxy) and this part is being defined in IETF (Ref [12] )






As of now (Jan 2015), I don't have any specific information other than Integrated wireless railway network to be commercialized .



Reference :


[1] LTE-U/LAA, MuLTEfire and Wi-Fi; making best use of unlicensed spectrum (Qualcomm)

[2] Qualcomm Research LTE in Unlicensed Spectrum: Harmonious Coexistence with Wi-Fi (Qualcomm)

[3] 3GPP TR 36.889 - Study on Licensed-Assisted Access to Unlicensed Spectrum

[4] 3GPP LAA Workshop Document

[5] Achieving good coexistence.- Qualcomm

[6] LTE Aggregation and Unlicensed Spectrum - 4G America

[7] #WiFiNOW: 8 LTE in Unlicensed Bands Latest developments - Qualcomm (YouTube)

[8] LTE-U/Wi-Fi coexistence with CableLabs - Wi-Fi Now Episode 13 (YouTube)

[9] Cracking Google Project Fi & LTE/Wi-Fi Aggregation with Qualcomm - Wi-Fi Now Episode 5 (YouTube)

[10] The LTE-U vs. Wi-Fi debate continues empowering carriers with seamless Wi-Fi - Wi-Fi Now Episode 15 (YouYube)

[11] Ruckus Wireless LTE Operation in Unlicensed Spectrum (YouTube)

[12] Multipath TCP (mptcp)  

[13] Licensed-Assisted Access to Unlicensed Spectrum in LTE Release 13

[14] Real-time LTE/Wi-Fi Coexistence Testbed (National Instruements)