LTE is a cellular system operating in licensed bands, where an operator has the exclusive usage of the allocated frequencies. The rising demand for mobile data creates the need for the use of more spectrum. A common way to increase the available spectrum is to use WiFi offloading, where instead of using the cellular network, data connectivity is provided through WiFi in unlicensed bands, and in particular the 5 GHz unlicensed spectrum.
However using WiFi for the unlicensed spectrum has several drawbacks: WiFi is not as spectrally efficient as LTE, and using WiFi requires integrating two different technologies. Some estimations put LTE as being twice as spectrally efficient as WiFi, therefore using LTE could double the available bandwidth using the same amount of spectrum resources. For this reason there is a growing interest in using LTE in unlicensed bands, which may be considered as part of LTE release 13 (the currently deployed LTE release being 9, or 10 in the most advanced deployments). For this LTE should be modified to share the unlicensed spectrum with other technologies, mainly WiFi, in a fair manner.
LTE is designed for licensed bands, so there is an assumption that an LTE channel is fully dedicated to LTE. As such, there is no current mechanism to share the spectrum with other users, and possibly other technologies. Conversely, in unlicensed bands there is a requirement to share the available capacity with other users and technologies in a fair manner.
The key principle behind this fair coexistence on an unlicensed channel is “Listen Before Talk” (LBT). With LBT, in order for a device to transmit, it will listen to the channel and only start transmission if no other transmission is on-going. However, two (or more) such devices could listen at the same time to an unused channel and decide to start transmission at the same time, leading to a collision and a failed transmission. LBT cannot avoid this issue, but is still an important mechanism to avoid collisions and share the spectrum between uncoordinated users and technologies.
Although not required, a number of technologies using unlicensed bands can start transmitting at any time, in order to quickly use the spectrum as soon as it is available.
LTE uses a completely different scheme: the channel usage is split into 1 millisecond subframes, with fixed time synchronization, and the eNodeB base station (eNB) centrally schedules the spectrum usage by explicitly allocating part of each subframe subcarriers to different devices (UE, user equipment) as would be understood by the skilled person. Typically, the eNB sends allocation order to a device over the PDCCH (Physical Downlink Control CHannel) to describe an allocation in a given subframe PDSCH or PUSCH (Physical Downlink/Physical Uplink Shared CHannels). An allocation provided by the eNB has to be used.
The LTE allocation scheme is inflexible and, as is, is not suitable to share unlicensed spectrum in a fair manner. A recently developed mechanism, carrier aggregation (CA), provides a little more flexibility in that a secondary channel can be activated/deactivated within a few milliseconds (up to 8), and so does not need to be used all of the time. A primary cell (PCell) in a licensed band combined with a secondary cell (SCell) in an unlicensed band would bring more flexibility than the regular, ‘always on’ LTE used on the primary cell. But once a secondary cell is activated on the license-exempt band, the regular LTE framing and eNB scheduling is used, and is still not flexible enough. It is noted that the secondary cell could be downlink (DL) only as denoted for instance in 3GPP as supplementary DL. In the context of LTE-U (referring to LTE that has been modified and extended to work in unlicensed bands), it is assumed that the secondary cells used in the license-exempt spectrum is DL only.
There is a need for a scheme to use the LTE waveform in unlicensed bands, with minimal modification on the device side to help implementation and adoption. There is also a need to make the LTE allocation more flexible when an unlicensed band is used, while preserving most of the LTE waveform and its high bandwidth efficiency.