The idea of using unlicensed spectrum over LTE spectrum services is to improve downlink throughput by operators. Let’s look at Unlicensed LTE technologies such as LTE-U, License Assisted Access (LAA, or LTE-LAA) and eLAA, LWA and CBRS and its deployment globally.
LAA is a 3GPP standard finalised 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 adds 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.
TDD and FDD modes are supported in the standard. Fair coexistence in the unlicensed channel is provided by a region-specific listen-before-talk (LBT) capability to ensure channels are clear before transmission. LBT is mandatory in the EU and Japan.
LTE-U is a pre-Release 13 technology pioneered in the USA (notably by Verizon and T-Mobile) and is also being trialled in other countries including South Korea, South Africa and Saudi Arabia.
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.
Unlicensed LTE Spectrum Technologies Snapshot:
- 26 LAA trials, deployments and launches
- 10 LTE-U trials, deployments and launches
- 2 LWA planned deployments
- CBRS trials launched in the USA
- 1 eLAA trial
- 21 commercially available chipsets and modules
- 70 small cells and smartphones announced
LWA (LTE Wireless-LAN Aggregation) is a 3GPP Rel-13 standardised technology, which aggregates carriers at the PDCP layer and uses the Dual Connectivity feature from 3GPP Rel-12. LWA supports downlink aggregation only; eLWA in Rel-14 adds 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.
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. There are other approaches of interest. MulteFire leverages LAA for the downlink and eLAA for the uplink, with additional enhancements to operate without a licensed spectrum anchor.
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 for use in their LTE networks. Access to the spectrum is prioritised for government/military users and after them for Priority Access Licensed (PAL) users that have acquired one of the many regional three-year licences in spectrum auctions; everyone else is able to dynamically request access to use the spectrum via licensed SAS (spectrum access server) operators. The CBRS Alliance is working on technical aspects of making CBRS work in the USA, including interoperability and coexistence specifications.
GSA has been able to identify two commercial LAA networks, both located in the USA, one operated by AT&T and one by T-Mobile. AT&T deployed LAA on its network in parts of Indianapolis, Indiana in November 2017. T-Mobile launched LAA in the fourth quarter of 2017. In February 2018 the operator partnered with Nokia to achieve speeds of 1.3 Gbps, using LAA, 5C CA, 256 QAM and 4X4 MIMO.
Elsewhere, Bell Mobility in Canada has announced plans to support LAA technology by aggregating unlicensed 5 GHz spectrum with LTE in licensed bands to provide higher transfer speeds. Seven other telecom operators have disclosed similar levels of LAA-readiness. These include China Mobile, Vodafone Turkey, South Africa’s MTN and Vodacom, Verizon Wireless in the USA, Singapore’s Singtel and TIM in Italy.
There have been two LTE-U network deployments. T-Mobile USA’s network was launched in June 2017 covering Bellevue, WA; Brooklyn, NY; Dearborn, MI; Las Vegas, NV; Richardson, TX; and Simi Valley, CA. The operator has previously announced plans to continue to roll the network out through 2018 using carrier aggregation, 4×4 MIMO and 256QAM in the 5 GHz unlicensed band, but interestingly T-Mobile announced in November 2017 that it plans to shift its focus away from LTE-U toward LAA deployment in the rst half of 2018.
Vodacom in South Africa deployed LTE-U in its network on a very limited scale in Midrand in 2016, but it too has recently announced plans to deploy LAA (currently under test in the same location) on a national scale.
In August 2017, AIS in Thailand launched a network called AIS NEXT G that uses multipath TCP technology (MP-TCP protocol). This is an alternative way of enabling the use of combinations of licensed and unlicensed bands; it does not use standardised LAA, LTE-U or LWA technology.
In yet another approach, Redzone Wireless in the USA has announced plans for a FWA broadband service capable of Gigabit speeds that combines both licensed LTE (at 2.5 GHz) and unlicensed wireless broadband technologies (at 5 GHz and 60 GHz) on a single tower and integrates them with a proprietary operational support system (OSS). It has already launched a network combining LTE at 2.5 GHz with unlicensed spectrum at 5 GHz to deliver speeds of 450 Mbps.
Croatian telecom VIPNet has announced it is ‘experimenting with LAA technology. The operator stated its intent to use LAA to avoid the need for more licensed spectrum. Rogers Wireless in Canada demonstrated Quad-band LAA on Gigabit LTE to show how LAA provides high bandwidth simultaneously across several devices. GSA has also become aware of a LAA trial conducted by Mobily in Saudi Arabia in partnership with Nokia, and in April 2018 Telenor Myanmar became the latest operator to announce a LAA trial.
GSA has not been able to identify an eLAA trial conducted by SK Telecom in South Korea and Qualcomm Technologies. Both companies claimed it was the first over-the-air technology demonstration of Enhanced Licensed Assisted Access (eLAA) and LAA, utilising 2.6 GHz band and the unlicensed spectrum.
GSA has been able to identify six US-based telecom operators trialling or requesting licences for trials in the 3.5 GHz CBRS spectrum band. AT&T Mobility, Charter Communications, T-Mobile US and Verizon Wireless are those staging CBRS trials. They could soon be joined by US Cellular and Comcast. Both these operators have applied to the US Federal Communications Commission (FCC) for special temporary authority (STA) to conduct trials.