LTE standards for unlicensed spectrum

The idea of using unlicensed spectrum to offer LTE services and improve downlink throughput has been around for some years, and there are alternative standards like LAA, LTE-U, LWA, LWIP and CBRS.

LAA is a 3GPP standard finalized in Release 13, (Rel-13) and involves a licensed LTE band as the primary carrier and Carrier Aggregation (CA) used to add a carrier in unlicensed 5 GHz spectrum for the downlink only. In Rel-14, eLAA will add uplink support. High priority traffic can use the licensed band, while lower priority traffic can use the Wi-Fi channel. The decision on traffic allocation is made by the LAA enabled base station/small cell, and there is no impact on the core network infrastructure.

LTE-U is a pre-Release 13 technology pioneered in the US (notably by Verizon and T-Mobile), and also being trialled in other countries including South Korea. It is similar to LAA with Carrier Aggregation; no core network changes; decisions on spectrum use made by base station/small cell – except that LTE-U uses Carrier-Sensing Adaptive Transmission (CSAT) rather than LBT to determine when the 5 GHz unlicensed Wi-Fi spectrum is in use to maintain fair coexistence.

Development of the industry standard has been led by the LTE-U Forum. In February 2017, the USA’s FCC authorized LTE-U devices for use in the 5 GHz unlicensed band. LTE-U has an upgrade path to LAA and eLAA.

LWA (LTE Wireless-LAN Aggregation), a 3GPP Rel-13 standardized technology, aggregates carriers at the PDCP layer, and uses the Dual Connectivity feature from 3GPP Rel-12. LWA supports downlink aggregation only; eLWA (a work item for Rel-14) will add uplink support. Separate nodes (a WLAN access point and an LTE base station) are used, and these can be physically separate or collocated in a single unit. The eNodeB makes the decision on activation of LWA and the use of the bearers.

Operators Update on LTE Unlicensed Spectrum

15 operators are trialling or deploying LAA

1 operator has commercially launched LTE-U and 7 more are trialling or deploying

14 base stations and 7 smartphones support LAA, LTE-U or LWA

LBT is used to ensure fair coexistence in the unlicensed band. Both 2.4 GHz and 5 GHz unlicensed spectrum bands can be used

LWIP (LTE WAN Integration with IPSec tunnel) is a Rel-13 feature similar to LWA but performing aggregation and switching at the IP layer. It has been designed to require no changes to existing WLAN infrastructure, and in its Rel-13 version can support uplink data transmission as well as downlink.

CBRS (Citizens Broadband Radio Service) was set up by the FCC in the USA in April 2016 to use the 3.5 GHz band (3550-3700 MHz) as a space where a variety of users could share bandwidth. There have been a number of releases of CBRS-compatible products in recent months, including ip.access’s base station software Viper and Accelerran’s small cell base station E1000. The CBRS Alliance promotes the use of LTE-based CBRS solutions.

Live Operator Networks

One live LTE-U network deployment. T-Mobile USA’s network launched in June 2017 covering Bellevue, WA; Brooklyn, NY; Dearborn, MI; Las Vegas, NV; Richardson, TX; and Simi Valley, CA

Verizon’s LTE-U network in the USA is expected to go live during 2017

No commercial LAA or LWA networks, though several are close to such status. Significant among these are: MTN’s trial of indoor LTE-LAA in Pretoria, South Africa; Vodafone Turkey’s “LAA-ready” network; Chunghwa Telecom’s LWA network; and M1 Singapore’s LWA network scheduled for commercial launch during 2017.

Chipsets and Modules

The silicon required to deliver LTE-U, LAA, LWA and other related network features comes in a number of forms. Key are the modems and mobile processors that can handle aggregation of carriers in both licensed and unlicensed spectrum, and the bandwidth enabled by such aggregation. Baseband SoCs (System on Chip) also need to be capable of supporting the relevant technologies on the base station.

Presently, commercially available modules supporting LWA, LTE-U or LAA is not there but we understand that the next generation of Sierra Wireless’s EM and MC pattern modules will include Qualcomm’s X16 modem and that LTE-U/ LAA will be supported. The modules will be available as samples this year, but a shipping date has not been announced.

In addition there are modems and mobile processors which are not yet commercial:

• Intel’s XMM7560 modem, announced in February 2017, is at pre-commercial stage will support LAA
• Qualcomm’s X20 modem, announced in February 2017, is at pre-commercial stage will support LAA and LTE-U
• MediaTek’s Helio P10 mobile processor is used in HTC’s U Play smartphone; Chunghwa Telecom states this device is LWA-capable
• MediaTek’s Helio X35 mobile processor, currently pre-commercial (announced November 2016) will support relevant technologies

Devices

The most significant development is smartphones are now shipping with Qualcomm’s LAA- and LTE-U capable Snapdragon 835 mobile processors. T-Mobile USA said in April 2017 that it would announce its first LTE-U handsets soon. Cisco resells SpiderCloud’s entire portfolio of small cell products.

Ericsson has confirmed that its Radio Dot System is being developed for LAA (but not LTE-U); that the Radio 2205 outdoor small cell will be available from Q3, 2017 with LAA capability (it can also be deployed indoors), and that the Vault Radio concept will also have similar capabilities to the Radio 2205.

Nokia’s Femtocell Multi-band SoHo uses a Qualcomm chipset (FSM99xx) that can be enabled for LAA/LTE-U, but Nokia has not con rmed that the product supports those features. It has confirmed that it is developing LWA capability for its AirScale Wi-Fi base station (announced March 2017).

Conclusion

Global ecosystems for a number of technologies supporting the use of unlicensed spectrum for LTE services are building, and commercial services using these technologies are now being launched.