Tuesday, September 8, 2015

Unlicensed Spectrum

Wi-Fi, an unlicensed wireless technology, has experienced huge success due to high throughput rates, ease of use for consumers, extensive deployment by businesses, widespread availability in public places, and large amounts of available spectrum.

For mobile operators, Wi-Fi can offload data traffic, relieving some stress from capacity demands. To make offload work more effectively, the industry is working to more tightly bind Wi-Fi functionality with cellular operation, as discussed below in more detail under “Wi-Fi Integration and Data Offload.”

Wi-Fi uses spectrum efficiently because its small coverage areas result in high-frequency reuse and high data density (bps per square meter). Less efficient are white-space unlicensed networks, sometimes called “super Wi-Fi,” that have large coverage areas, because the throughput per square meter is much lower. While white-space networks may be a practical broadband solution in rural or undeveloped areas, they face significant challenges in urban areas that already have mobile and fixed broadband available.34 See the section on “White Space Networks” in the appendix for further details.

Advocates argue that unlicensed spectrum unleashes innovation and that government should allocate greater amounts of unlicensed spectrum. Although Wi-Fi has been successful, the core elements that make unlicensed spectrum extremely successful are also the source of inherent disadvantages: local coverage and its unlicensed status. Local coverage enables high data density and high frequency reuse but makes widespread continuous coverage almost impossible. Similarly, unlicensed operation facilitates deployment by millions of entities but results in overlapping coverage and interference.

Wi-Fi cannot replace networks built using licensed spectrum. The two are complementary and helpful to each other, as summarized in Table 1
Table 1: Pros and Cons of Unlicensed and Licensed Spectrum
Unlicensed Pros
Unlicensed Cons
Licensed Pros
Licensed Cons
Easy and quick to deploy
Potential of other entities using same frequencies
Huge coverage areas
Expensive infrastructure
Low-cost hardware
Difficult to impossible to provide wide-scale coverage
Able to manage quality of service
Each operator has access to only a small amount of spectrum

Some operators, such as Republic Wireless, offer a “Wi-Fi first capability under which devices always attempt to use a Wi-Fi connection and fall back to a cellular connection only if no Wi-Fi is available. Such cellular backup is essential because Wi-Fi, due to low- power operation in many bands, is inherently unsuited for providing continuous coverage. The sharp drop-off in signal strength makes coverage gaps over large areas inevitable, especially outdoors.

Figure 1: Propagation Losses of Cellular vs. Wi-Fi

A capability being discussed for Release 13 is LTE operating in unlicensed bands. Carrier aggregation would combine a licensed carrier with an unlicensed 20 MHz carrier in the 5 GHz band as a supplemental channel. LTE uses channels differently than Wi-Fi, so engineers are evaluating how LTE could be a fair neighbor in unlicensed bands and how it could meet varying regulatory requirements for unlicensed bands in different parts of the world.

LTE operating in unlicensed bands could eliminate handoffs to Wi-Fi, possibly creating a more seamless user experience. Under heavy load, LTE is spectrally more efficient than Wi-Fi, since it uses more sophisticated over-the-air scheduling algorithms.

A capability being discussed for Release 13 is LTE operating in unlicensed bands. Carrier aggregation would combine a licensed carrier with an unlicensed 20 MHz carrier in the 5 GHz band as a supplemental 

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