LTE Flavors in Unlicensed Spectrum

Guest post by Faris Alfarhan*

The unprecedented increase in demand for high-speed broadband requires a bundle of solutions to satisfy the demanded capacity. Unlicensed spectrum is increasingly considered by cellular operators, internet service providers, and businesses as part of solution set. Unlicensed spectrum cannot match the quality of licensed spectrum, as the interference profile is much more stochastic. However, unlicensed spectrum offers a complimentary solution to licensed carriers for operators, and an opportunity to cable companies and internet service providers – who typically don’t own any licensed spectrum – to deploy wireless networks and hotspots.

As mentioned in an earlier post, parts of the 5 GHz and 3.5 GHz frequency bands provide an opportunity for operating LTE in small cell deployments. Propagation characteristics at these frequencies have the advantage of high frequency re-use and penetration losses, thus naturally limiting interference. In the 5 GHz frequency range, depending on the country, there could be as much as 500 MHz of unlicensed bandwidth available. Unlicensed spectrum has been dominated by Wi-Fi, but LTE is emerging as an attractive competitor for better capacity and coverage offered and better mobility and handover aspects. From a capacity perspective, LTE could offer as much as double the capacity of Wi-Fi under the same physical assumptions (number of antennas and users). From a coverage point of view, LTE could expand the coverage area by a factor of 2, thanks to the extended range of error-correcting turbo codes used in LTE. Wi-Fi, on the other hand, has a restricted choice of coding and modulation schemes. Using the lowest Wi-Fi modulation and coding scheme corresponds to a data rate of 6.5 Mbps in a 20 MHz channel, which restricts its coverage significantly compared to LTE at the same carrier frequency and channel bandwidth. The lowest LTE error-correction code rate is as low as 1/9, while the lowest Wi-Fi coding rate is 1/2 in comparison. Finally, in many cases, LTE is deemed to be a better neighbor to Wi-Fi than Wi-Fi itself. A study by Qualcomm shows that in a scenario where two operators operate Wi-Fi in the same geographical region on adjacent and orthogonal unlicensed carriers, switching one of the operators to LTE actually improves the capacity of the Wi-Fi operator.

Operating LTE in the 5150-5925 MHz range has made it to standards of release 13 of LTE, and is commonly referred to as LTE-U. The standardized part relies on carrier aggregation, whereby an anchor carrier (a primary cell – PCell) is selected from the operator’s licensed carriers for robustness of control and system access channels, and a secondary carrier (SCell) is aggregated from unlicensed spectrum to boost data rates. This is known as “Licensed-Assisted Access” in 3GPP standards, whereby the PCell mainly carries control signals and some retransmission data, while the SCell carries data signals only. It is important for LTE to coexist with other existing and future wireless technologies. Thus, proper interference sensing and avoidance is vital for system efficiency in unlicensed spectrum, as well as proper interference management with respect to neighboring access points of other technologies in adjacent carriers. For LTE-U/LAA networks, this is maintained through the following interference avoidance schemes, which mostly depend on region:

• Listen before talk (LBT) (Clear channel assessment)
  LBT is a procedure by which an access point applies a clear channel assessment (CCA) to ensure channel clarity before using it. CCA determines the presence or absence of other signals on a channel in order to determine if a channel is occupied. European and Japanese regulations mandate using LBT in unlicensed bands. Apart from regulatory requirements, carrier sensing via LBT is one way for fair sharing of the unlicensed spectrum and hence it is considered to be a vital feature for fair and friendly operation in the unlicensed spectrum in a single global solution framework. LBT is part of Rel. 13 standards of LTE-LAA.

• Adaptive duty cycle
  When LTE-U access points operate in congested channels, they use Carrier Sense Adaptive Transmission (CSAT). In CSAT mode, LTE-U cells are configured with TDMA-based on/off frame splits, where the on/off ratio is configurable according to the loading levels of the technologies sharing the spectrum. This interference management approach is possible in USA, China, Korea, India, and countries in the same regulatory region. CSAT is part of Rel. 10-12 standards.

A slightly different flavor of LTE in unlicensed spectrum just announced by Qualcomm is MuLTEfire, which is a version of LTE-U that doesn’t require an anchor carrier in licensed spectrum, thus offering the opportunity of deploying LTE in unlicensed spectrum for non-cellular operators. MuLTEfire could be attractive to any business owning fixed internet access, from small business in the service industry to large cable companies.  The performance of critical system control signals remains unclear under MuLTEfire in congested carriers. Control signals that contain information on the LTE radio frame configuration, channel state, admission control, cell detection, mobility, and HARQ feedback, are transmitted on the same frequencies as data signals, which are vulnerable to variable interference conditions in unlicensed spectrum. Nonetheless, MuLTEfire should be suitable for small cell deployments, especially in dense-urban or indoor environments where interference could be contained at higher frequencies.

* Faris is wireless systems engineer in the research and specifications team at InfoVista. His domain of interest and expertise include radio access network design and optimization, performance simulations, and advanced technologies.