LAA is in 3GPP SI and WI since 2013. Though it is not absolutely a 5G topic, it is expected to see improvements in LAA when 5G is rolled out.
Spectrum is limited resource. Hence to support growing high speed data rate demands operators purchase Frequency spectrum. Since most of the frequencies up to 6 GHz are licensed and already occupied (2G/CDMA/3G/4G), operators/academics are looking at unlicensed spectrum. As most of you know WiFi devices operate around 2.4 GHz unlicensed (free) bands and also operating in 5 GHz bands.
In LTE, as you know, carrier aggregation comprises of one Pcell and one or more Scells in the same band or different bands. In normal LTE CA case, Scell is operated in licensed spectrum. In LAA (Licensed Assisted Access), Scell(s) is operated in unlicensed bands.
In LAA operation, eNBs send data on Scell(s) and UE acknowledges in Pcell UL only. Since WiFi devices operate in 5 GHz band fair co-existence between LAA DL transmissions on Scell and WiFi is expected. One concept called LBT (Listen Before Talk) is adopted in LAA implementation, where the channel is monitored for some time before transmit and send data when it is idle by the eNB. LBT is not mandatory in some countries.
To provide assistance to UEs to detect the LAA cells PSS/SSS and for measurement purpose reference signals (DRS Discovery Reference Signals) are sent by the eNB. DRS transmission should also follow LBT. DRS can transmitted periodically in DMTC frame (DRS Measurement Timing Configuration) windows. DRS is of 6 ms duration and DMTC is configurable at 40/80/160 ms. Since LAA Scell data transmission can happen anytime, new frame structure Type 3 is proposed.
UE measures LAA Scell average RSSI to reflect the channel occupancy/interference instead of LTE style.
The extension of LAA is called eLAA or Evolved LAA or UL LAA (Uplink LAA). In LAA, supplemental DL is supported from WiFi unlicensed spectrum where as in eLAA, supplemental UL is supported in the same WiFi unlicensed spectrum. It is part of 3GPP Rel-14 specifications. More details of eLAA and challenges are nicely explained in https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7925553
The extension of LAA is called eLAA or Evolved LAA or UL LAA (Uplink LAA). In LAA, supplemental DL is supported from WiFi unlicensed spectrum where as in eLAA, supplemental UL is supported in the same WiFi unlicensed spectrum. It is part of 3GPP Rel-14 specifications. More details of eLAA and challenges are nicely explained in https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=7925553
