Enhanced licensed assisted access (eLAA) is a new LTE Release 14 mode of operation that provides the necessary technology for cellular operators to fully integrate the unlicensed spectrum into their networks. An enhancement to the Downlink-only LTE Release 13 LAA, the eLAA technology enables both uplink and downlink operation of LTE in unlicensed bands.
Listen before Talk (LBT)
In eLAA, channel access in both downlink and uplink rely on the listen-before-talk (LBT) feature. A wireless device or a base station must first “sense” the communications channel to find out there is no communications prior to any transmission. The “channel sensing” procedure relies on detecting the energy level on the communications channels. The LBT parameters (such as type/duration, clear channel assessment parameters, etc.) are configured in a wireless device by the base station.
Frame Structure type 3
The new frame structure type 3 has been introduced by the 3rd Generation Partnership Project (3GPP) for operations in the unlicensed communications channels. Similar to the LTE time division duplex (TDD), the uplink and downlink operations using frame structure type 3 are on the same frequency channel but are separated in time. However, unlike in LTE TDD, a subframe is not configured as a downlink subframe or an uplink subframe and may be used by either the base station or the wireless device.
Interlace Resource Allocation
The resource allocation framework in the eLAA system has some major difference with the legacy LTE. For example, the basic unit of resource allocation for the LTE unlicensed channels is an interlace which is composed of ten equally spaced resource blocks within a 20 MHz frequency bandwidth.
In the eLAA system, a base station can allocate resources in maximum of four multiple consecutive subframes to a wireless device using the multi-subframe grants. The eLAA system specifies two new downlink control information (DCI) formats (i.e., format 0B and 4B) that the base station can transmit on the physical downlink control channel (PDCCH) to schedule resources for a wireless device in up to four consecutive subframes.
In the legacy LTE, a wireless device needs four subframes to process an uplink grant. The eLAA system enables fast resource allocation by specifying the triggered grants. The base station may first transmit a resource allocation command to the wireless device and may later transmit a trigger, with a short processing time, to indicate to the wireless device the time for transmission. The timing between resource allocation command and the trigger can be flexible.