In the quest for substantial increases to the capacity and throughput of mobile networks, unlicensed radio spectrum is expected to have an important role in 5G systems from 2020. However, work is already underway in 3GPP to allow 4G LTE-Advanced networks to take advantage of unlicensed spectrum much sooner. The Licensed Assisted Access (LAA) feature will enable LTE-Advanced to use unlicensed frequencies as an adjunct to licensed bands, to boost capacity for network operators and performance for service users.
LAA will build on existing carrier aggregation capabilities of LTE-Advanced, with new features introduced to achieve fair co-existence with WiFi and other users of unlicensed spectrum. 3GPP will complete a study of the requirements in mid-2015 before moving on to define the necessary changes to standards by mid-2016. However, equipment manufacturers and network operators are already pressing ahead with LAA developments, and pre-standard equipment and services are likely to start appearing before the end of 2015, with standards-compliant versions following in 2016 and 2017.
In principle, LAA could have major benefits for network operators and users of LTE-Advanced services. However, with algorithms for fair co-existence still undefined and the scope of LAA deployment still unclear, the WiFi community has expressed concerns about the implications for the technical performance and commercial outlook for WiFi. In practice, it is likely that LAA products will be rather more expensive than WiFi and this may naturally limit the circumstances in which LAA is used, leaving WiFi to benefit from ever-closer integration with LTE-Advanced.
Extending LTE to unlicensed spectrum at 5GHz is an enticing prospect
It is common practice for governments to set aside certain parts of the radio spectrum for use without a licence. WiFi, Bluetooth, RFID and many other technologies take advantage of unlicensed spectrum of this sort, for example in the popular 2.4GHz band. Of particular interest is unlicensed spectrum at 5GHz, where large bandwidths are freely available in many regions of the world. For example, Europe and Japan offer 455MHz and the USA 580MHz. These frequencies are subject to local regulations on emitted power and co-existence with other technologies, such as WiFi and radar, but in most situations the spectrum is largely unoccupied. Given that most 4G networks are currently constrained to a few tens of MHz, these large bandwidths offer the prospect of a substantial uplift in the spectrum available.
The WiFi capabilities of wireless devices are already used to offload substantial volumes of data traffic from licensed mobile networks to unlicensed frequencies, for example in homes and business premises. However, there are situations where it would be beneficial for LTE-Advanced to use unlicensed spectrum directly, for example in indoor or outdoor hotspot locations where licensed spectrum is at a premium. Using the same technology in licensed and unlicensed spectrum would simplify the task of offering seamless services across the two and would allow network operators to use common core network and network management systems. Furthermore, LTE-Advanced applies various technical innovations that could achieve performance benefits over WiFi, including greater spectral efficiency and more robust performance under heavy load. Hence, deploying LTE in unlicensed spectrum could relieve the load on both the licensed LTE and unlicensed WiFi spectrum. LAA could also take advantage of the robust security features of LTE-Advanced.
LAA uses unlicensed spectrum opportunistically
Extending LTE-Advanced to unlicensed spectrum is a major feature of 3GPP Release 13, due to be frozen in March 2016. Previously this was referred to as LTE-Unlicensed (LTE-U), but 3GPP uses the name LAA to reflect the role of licensed spectrum in its operation. A fundamental principle of LAA is that LTE-Advanced services remain firmly anchored in licensed spectrum, with unlicensed frequencies used opportunistically to boost throughput and capacity. The approach is illustrated in the figure below, in which a primary cell uses licensed spectrum to provide a robust connection for control signalling, mobility and user data, and a secondary cell uses unlicensed spectrum to carry best effort user data, to provide a potentially substantial but variable speed boost.
3GPP is considering a number of different deployment scenarios for LAA, including the integration of small cells operating in unlicensed spectrum with macro cells operating in licensed spectrum, and the integration of co-located small cells operating in both unlicensed and licensed spectrum. The former approach would enable network operators to use lower-frequency licensed LTE spectrum for wide area coverage and penetration into buildings, and higher-frequency unlicensed spectrum for a speed boost in local areas, such as indoors. The latter approach would provide a cost-effective solution for high-capacity indoor or outdoor hotspot locations. There is no intention to support standalone operation of LTE in unlicensed spectrum and this is one of the reasons for the change in name from the rather open-ended LTE-U to the more specific (although not necessarily any more enlightening) LAA.
LAA relies on extending existing LTE-Advanced carrier aggregation features to the combination of licensed and unlicensed bands. However, it also requires a number of additional functions to satisfy regulatory requirements and to ensure fair co-existence between LTE-LAA and WiFi systems and between LTE-LAA services from different network operators operating in the same unlicensed spectrum.
Over a period of time 3GPP has already introduced a series of carrier aggregation capabilities in LTE-Advanced, which will have a role to play in LAA. Among other things these include the aggregation of carriers in different frequency bands and the aggregation of paired carriers (normally used for Frequency Division Duplex (FDD) operation) with unpaired carriers (normally used for Time Division Duplex (TDD) operation). Initially LAA will focus on using unlicensed spectrum to boost the downlink only, using the LTE-Advanced supplemental downlink feature, because this has less onerous requirements on mobile devices, but later this may be extended to include uplink aggregation in unlicensed spectrum.
LAA must, as a minimum, satisfy local regulations such as the maximum transmission power in specific bands and the avoidance of protected services. However, a further guiding principle for LAA is that LTE should not cause any more interference to WiFi than a typical WiFi system operating on the same carrier. This is particularly important because LTE is more tolerant to interference than WiFi and therefore could cause significant disturbance to WiFi if it was not designed to avoid unnecessary conflict. These factors require a number of new features in LTE-Advanced. For example, LAA systems should select the carriers that are least occupied in their local areas and they must dynamically change operating frequency to avoid conflict with protected systems, such as radar. LAA systems must also apply Listen Before Talk (LBT) or Clear Channel Assessment (CCA) techniques, to check that a channel is free before making a transmission. Exactly how these decisions are made will be a crucial aspect of the LAA system design.
LAA is developing at a pace but will face obstacles
3GPP aims to complete its study of LAA requirements by the middle of 2015, before moving on to draft the relevant standards material for approval in 2016. However, a number of companies are already moving ahead with developments prior to the completion of the work. Huawei and NTT DoCoMo have been collaborating on LAA since 2014 and Ericsson has been working with SK Telecom, T-Mobile USA and Verizon. Qualcomm has announced the launch of pre-standard chipsets before the end of 2015 and Alcatel-Lucent, Ericsson, Huawei, Nokia and Samsung are all planning the introduction of LAA into their product ranges. T-Mobile USA plans to trial pre-standard 5GHz LAA from Qualcomm and Alcatel-Lucent at the end of 2015, with a commercial launch following in 2016. Similarly Verizon plans to deploy pre-standard LAA at 3.5GHz and 5GHz in its network in 2016.
On the face of it, LAA is positioned as an adjunct to LTE-Advanced, which will provide a capacity and performance boost to existing services and will be introduced in accordance with local regulations in such a way as to achieve fair co-existence with WiFi. However, the WiFi community has expressed concerns about exactly how fair the arrangement will be, particularly in pre-standard implementations that may not be as tightly controlled as the final standard. LTE operators may be in a position to pour as much or as little traffic into unlicensed spectrum as they want at any given time, leading to unpredictable changes in WiFi quality of service. Also, while 3GPP has stated that there is no intention for standalone operation of LTE-Advanced in unlicensed spectrum, it is not clear how far LTE might move beyond its traditional role and start impinging on the WiFi business. 3GPP is collaborating with the WiFi Alliance and IEEE with the aim of developing a mutually-acceptable approach. In any case, LAA products are likely to be rather more expensive than WiFi and this may naturally limit the circumstances in which LAA is a cost-effective solution. 3GPP is already introducing a series of features to improve the integration of WiFi with LTE-Advanced and in many situations network operators may prefer to exploit unlicensed spectrum this way instead of deploying LAA.
It remains to be seen how widely LAA is installed in practice, but given the immense pressure on wireless spectrum it will be a welcome further addition to the tool bag of many mobile operators as we approach the end of the decade.