LTE operation in unlicensed spectrum is not limited to its aggregation with WiFi (like with LWA, LWIP and RCLWI)¹. Another approach to utilize the unlicensed spectrum combined with the MNO-controlled LTE networks is based on a specialized version of LTE system designed to cope with the unlicensed spectrum’s requirements. This is referred to as Licensed Assisted Access (LAA) and has been addressed with the introduction of LTE-Advanced Pro within Rel-13. There are however, two other technologies to achieve that, not-standardized by 3GPP, namely LTE-Unlicensed (LTE-U) and MuLTEfire.
LTE accessing unlicensed spectrum
The access of the LTE system to the unlicensed spectrum has been addressed by the following three “technologies”:
- Licensed-Assisted Access (LAA) [1], is a 3GPP Rel-13 feature where the resources from unlicensed spectrum, handled by the modified version of the LTE radio interface, are aggregated utilizing Carrier Aggregation feature. Using this technology, the legacy licensed LTE carrier acts as serving as a Primary Component Carrier (PCC), and up to four DL Secondary Component Carriers (DL SCC) can be aggregated on the 5GHz unlicensed band with the specialized Frame Type 3. The 3GPP in its Rel-14 study is working on enhanced LAA (eLAA) which considers addition of the UL LAA carriers. To assure “fairness” of using the unlicensed spectrum LAA utilizes Listen Before Talk (LBT) mechanism where the Tx, prior to transmission, senses the channel to verify if it’s occupied or free. Using this mechanism, it can be deployed globally provided it fulfills the regulatory requirements.
- LTE-Unlicensed (LTE-U) [2], is a proprietary technology (developed before the release of LTE-Advanced Pro), where the unlicensed spectrum is aggregated with the licensed spectrum PCC by means of CA in DL. However, the standard LTE frame type is used, not supporting the LBT scheme. LTE-U instead uses the Channel Selection and Carrier Sensing Adaptive Transmission (CSAT), where once the specific channel is empty the regular LTE transmission is used, while when it’s not empty, the adaptive duty cycle is used during which the LTE Tx is switched ON and OFF for specific periods of time with the durations adapted to the channel occupancy by the other systems. However, due to no support of LBT, it is not allowed in many countries due to regulatory restrictions.
- MuLTEfire [3], is also a proprietary solution reusing LAA and eLAA design with LBT, however in a standalone mode, i.e., no licensed PCell is needed. As a result additional aspects need to be taken into account such as mobility, paging and system information. This allows for using the unlicensed spectrum with neutral host concept, where multiple operators share the MuLTEfire resources.
Summary
As described above, there are 3 methods with slight differences between them enabling access to the unlicensed 5GHz spectrum within the LTE framework:
- LAA is standardized, globally usable technology with LBT scheme, requiring an anchor licensed carrier, supporting DL (and Rel-14 eLAA considering UL);
- LTE-U is non-standardized, non-globally usable technology, but introduced earlier than LAA, and supporting only DL direction, requiring an anchor licensed carrier;
- MuLTEfire is non-standardized, globally usable technology with LBT scheme, operating in standalone mode for both DL and UL.
References:
[1] 3GPP TS 36.300
[2] www.lteuforum.org
[3] www.multefire.org
¹LWA (and other schemes with WiFi) are addressed in those posts: LTE Advanced Pro with Wifi Ran Level Integration, LTE Advanced Pro: What is this and LTE Evolution: LTE Advanced to LTE Advanced Pro
