LAA specification work starting in 3GPP
A global standard for Licensed Assisted Access is agreed to be included in Release-13, the next release of the LTE specifications by standardization organization 3GPP. Fair co-existence with other technologies operating in unlicensed spectrum is a fundamental design principle for LAA. Extensive investigations of how LAA can co-existence with itself and Wi-Fi systems have been done over the last 9 months within 3GPP. This together with a continued large industry interest formed the basis for the decision to start a work item. This blog post will give you some important details about the study and its results.
LTE is an extremely successful platform for meeting the demand for wireless broadband – something that’s proven by the rapid uptake of LTE in different regions of the world. Existing and new spectrum licensed for use by IMT technologies allows provisioning of seamless wide-area coverage, achieving high spectral efficiency, and ensuring high reliability of cellular networks. To meet the ever increasing data traffic demand (e.g., video streaming) from users, particularly in densely populated buildings or hot spots, more mobile broadband bandwidth will be needed. Given the large amount of spectrum available in the unlicensed bands around the globe, unlicensed spectrum is increasingly considered by cellular operators as a complementary tool to augment their service offerings in licensed spectrum. Some operators already deploy Wi-Fi equipment in unlicensed bands to handle areas with high traffic load.
For the last three quarters, 3GPP has studied enhancements to LTE for operation in the 5 GHz band, using the technique referred to as Licensed Assisted Access (LAA) to unlicensed spectrum. We discussed this topic in two earlier blog posts back in February, Licensed Assisted Access: Operation Principles and Licensed Assisted Access: Practical Coexistence Solutions. Based on the principle of carrier aggregation, the LAA designs aim to combine the best of the licensed and unlicensed band opportunities while reducing operators’ operating costs.
The system starts with a Primary Cell (PCell) in licensed spectrum that is used for the exchange of essential control messages. The PCell also provides a robust link over licensed spectrum that is always available for delivery of real-time or high-value traffic. The LAA Secondary Cells (SCells) carry data transmissions in unlicensed spectrum with assistance from the PCell. With this approach, operators only need to manage and maintain one radio access network (instead of two in the case of Wi-Fi offloading), which can be beneficial in reducing operating costs and simplify service provisioning.
In the LAA Study Item in 3GPP, coexistence mechanisms for LAA and Wi-Fi systems were investigated extensively by over 40 contributing companies. The evaluations followed the guideline suggested by the document guiding the study referred to as the study item description (SID), which suggests that LAA should not impact Wi-Fi services more than an additional Wi-Fi network on the same carrier. The coexistence evaluation was based on a rigorous two-step methodology derived from this principle. In step 1, performance metrics for two coexisting Wi-Fi networks – A and B – were evaluated and documented. In step 2, Wi-Fi network B was replaced with an LAA network, and coexistence was evaluated and documented. A comparison of the performance metrics for Wi-Fi network A in step 1 and step 2 were then used to evaluate the effectiveness of co-existence between LAA and Wi-Fi in the unlicensed band.
The 3GPP study received over 1500 technical contributions and considered a diverse set of performance metrics considering both DL and UL traffic: (1) user file throughput distributions, (2) user file delay distributions, (3) network buffer occupancy, (4) network served-to-offered traffic ratios, and (5) VoIP user outage. Network load was varied by increasing the file arrival rate or by increasing the number of active devices in the network. The Technical Report includes coexistence evaluation reports with 10, 20 and 50 devices per network per 20 MHz carrier. Networks in indoor and outdoor deployments were both considered. Networks sharing one 20 MHz unlicensed channels as well as four 20 MHz unlicensed channels were investigated.
Over 380 coexistence evaluation reports were submitted and included in the Technical Report. Each of the coexistence evaluation reports can contain 80 to 225 performance metrics depending on the evaluated scenario. A majority of sources contributing to this extensive set of evaluations showed at least one listen-before-talk scheme for LAA that does not impact Wi-Fi more than another Wi-Fi network (offering the same traffic to the same users). The study concluded that when an appropriate channel access scheme is used, it is feasible for LAA to achieve fair coexistence with Wi-Fi, and for LAA to coexist with itself based on the evaluated scenarios. While it was found that a diverse range of energy detection based listen-before-talk algorithms can achieve coexistence, the Technical Report recommends a listen-before-talk framework that includes random backoff and variable contention windows at least for downlink data transmissions. The technical report produced by 3GPP that captures all the work performed in the study item is available from the 3GPP website, if you want further details.
Based on the findings from the study, 3GPP RAN decided to approve a work item on LAA. The work item targets the creation of a global standard for LAA as part of Rel-13 which is scheduled to produce specifications by March 2016. The scope of the work item is to specify LAA operations in the downlink. The Rel-13 design should be forward compatible so that UL support can be added in a later work item without any modifications to downlink operation. Fair coexistence with other technologies operating in unlicensed bands is a fundamental design consideration for the LAA work item, as it was for the designs considered in the study item.
Ericsson is one of the companies leading the work on LAA in 3GPP to ensure that LAA will be able to achieve fair coexistence with other technologies operating in the 5 GHz unlicensed band (including Wi-Fi). We believe that development of LAA as a technology alternative can serve to boost innovation in the use of unlicensed spectrum, something that will bring significant benefits to wireless data users worldwide.
Daniel Larsson and Thomas Cheng, Ericsson Research