LTE in Unlicensed Spectrum
Mobile broadband traffic loads are increasing rapidly which is leading to the need for additional radio spectrum to satisfy the demand. LTE networks are mostly deployed in spectrum from 700 MHz to 2.6 GHz. While LTE mobile broadband in licensed spectrum is highly efficient due to its exclusive occupancy of the spectrum, the amount of available licensed spectrum can be limited and expensive.
The amount of unlicensed spectrum (especially in the 5 GHz band) assigned or currently planned to be assigned is comparable to, or even more than, the amount of licensed spectrum available. Many operators have already deployed Wi-Fi access points to offload some cellular traffic to the unlicensed spectrum bands.
LTE is currently the most advanced mobile technology and many operators are upgrading their networks to LTE and planning a roadmap toward LTE-A. LTE operating in licensed spectrum will be the main deployment, but to further expand LTE capacity to meet future traffic demands, proposals are underway to adapt the LTE air interface to operate in the unlicensed spectrum.
Extending LTE-A to unlicensed spectrum is the latest in a series of innovations to address the challenge in the growth of mobile data. Bringing LTE-A to unlicensed spectrum is a basic concept but with many benefits. It involves taking advantage of the large number of small cells that are being deployed and employing CA (Carrier Aggregation) of unlicensed spectrum with the licensed spectrum. The existing core network is unaffected and in essence, the whole system works as a unified LTE network to take advantage of the capacity of both licensed and unlicensed spectrum bands efficiently.
Since bringing LTE-A to unlicensed spectrum is an alternative to carrier Wi-Fi, the effect on existing Wi-Fi networks is an important consideration. LTE in unlicensed spectrum must be designed to coexist harmoniously with Wi-Fi and in many cases it may be a better neighbour than Wi-Fi itself. The 3GPP is studying the coexistence of both technologies, while conforming to all the regulatory requirements of the different spectrum bands.
Cell providers ordinarily rely on licensed spectrum to ensure stable and high-quality LTE services, but with increasing mobile traffic, they are exploring the use of unlicensed frequencies to expand capacity. Technologies like License Assisted Access (LAA) allow cellular networks to deploy LTE in the unlicensed band, particularly in the 5 GHz spectrum, by aggregating it with licensed spectrum for improved efficiency.
One key feature of LTE in unlicensed spectrum is Carrier Sensing Adaptive Transmission (CSAT), which ensures fair coexistence with existing Wi-Fi networks. Unlike Wi-Fi, which operates on a contention-based access model, LTE uses scheduled transmissions, which can lead to more efficient use of spectrum while minimizing interference. Organizations such as the LTE-U Forum have worked to standardize and refine these technologies to ensure compatibility.
Beyond handling not just phone calls, LTE in unlicensed spectrum enhances broadband services, including home and business connectivity. By integrating with home broadband networks, cellphone carriers can extend high-speed mobile data access to more users while reducing congestion on primary licensed spectrum bands. As demand for mobile data grows, LTE in unlicensed spectrum will play a crucial role in expanding cellular networks and supporting next-generation connectivity.
Author: Paul Waite
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