LTE in Unlicensed Spectrum Research
While licensed spectrum provides the highest spectral efficiency among all spectrum types and remains the top priority for operators, unlicensed spectrum is playing an increasingly important role, allowing for opportunistic data offload. Qualcomm Research has conducted extensive studies to explore the feasibility and validate the benefits of extending LTE Advanced to unlicensed spectrum (LTE-U).
Higher user throughput.
By aggregating unlicensed carriers with licensed FDD or TDD carriers in small cells, LTE-U augments existing LTE mobile networks to provide an enhanced mobile broadband experience to consumers. A unified LTE network utilizing licensed and unlicensed spectrum assets offers improved mobility, higher data capacity and median user data rates, and improved coverage. In dense deployment scenarios, Qualcomm simulations show 2X or higher user throughput gain for an LTE Heterogeneous Network augmented with LTE-U, compared to the case of aggregating Wi-Fi 802.11ac with LTE.
There are two fundamental reasons that LTE-U offers better performance and spectral efficiency over carrier Wi-Fi.
- The LTE radio link is more robust and efficient with features such as Hybrid ARQ (HARQ), more efficient and granular Modulation and Coding Schemes (MCS) and better receiver performance.
- LTE-U has a higher MAC layer efficiency with robust protection of control channels, intelligent network based scheduling and rate control based on CSI feedback, and interference coordination with synchronized operation within the same deployment.
These link and MAC level features allows LTE-U to provide better capacity as the network gets denser and interference becomes the dominant factor of performance. LTE-U is likely to stimulate further technology innovation in unlicensed spectrum, including both LTE-U and Wi-Fi evolution, which is expected to ultimately improve unlicensed band efficiency and provide consumers with more choices.
Unified LTE network.
For cellular operators that already have an LTE network in licensed spectrum, LTE-U can be bundled easily with LTE carrier(s) in licensed spectrum using Supplemental Downlink (SDL) or Carrier Aggregation (CA).
- Supplemental Downlink: The unlicensed spectrum is used solely for downlink traffic
- Carrier Aggregation: Both downlink and uplink traffic are supported in the unlicensed spectrum.
In both cases, applications that have strict QoS requirements and important signaling traffic can be routed through the licensed carriers, thus ensuring high reliability. The licensed and unlicensed carriers can be managed conveniently as one unified network, and data traffic can be routed flexibly through the licensed and unlicensed carriers as needed.
Technically, LTE-U can work in any unlicensed spectrum. However, the initial target band is 5 GHz, due to the wide amount of bandwidth available, which makes it conducive to spectrum sharing by different technologies and users.
Coexistence and fair sharing of the unlicensed band with other technologies such as Wi-Fi are key requirements for LTE-U. Depending on the regulatory requirements in different geographical regions and time-to-market considerations, there are two flavors of LTE-U that operators can choose from: 3GPP Rel-10 based LTE-U and Rel-13 Licensed Assisted Access (LAA).