LTE-WLAN aggregation (LWA) is a technology defined by the 3GPP. In LWA, a mobile handset supporting both LTE and Wi-Fi may be configured by the network to utilize both links simultaneously. It provides an alternative method of using LTE in unlicensed spectrum, which unlike LAA/LTE-U can be deployed without hardware changes to the network infrastructure equipment and mobile devices, while providing similar performance to that of LAA. Unlike other methods of using LTE and WLAN simultaneously (e.g. Multipath TCP), LWA allows using both links for a single traffic flow and is generally more efficient, due to coordination at lower protocol stack layers.
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For a user, LWA offers seamless usage of both LTE and Wi-Fi networks and substantially increased performance. For a cellular operator, LWA simplifies Wi-Fi deployment, improves system utilization and reduces network operation and management costs. LWA can be deployed in collocated manner, where the eNB and the Wi-Fi AP or AC are integrated into the same physical device or in non-collocated manner, where the eNB and the Wi-Fi AP or AC are connected via a standardized interface referred to as Xw. The latter deployment option is particularly suitable for the case when Wi-Fi needs to cover large areas and/or Wi-Fi services are provided by a 3rd party (e.g. a university campus), rather than a cellular operator.
LWA has been standardized by the 3GPP in Release-13. Release 14 Enhanced LWA (eLWA) adds support for 60Ghz band (802.11ad and 802.11ay aka WiGig) with 2.16 GHz bandwidth, uplink aggregation, mobility improvements and other enhancements.
Background
Cellular networks have been designed for licensed spectrum. However, as usage patterns changed from voice-centric to data-centric and data usage surged, operators started looking into unlicensed spectrum opportunities. Using WLAN does not only allow operators to increase peak data rate and system capacity, but also to offer services for non-cellular devices, such as laptops.
To cater to operators demand, 3GPP have defined various methods of LTE-WLAN inter-working, ranging from Non-Seamless WLAN Offload (NSWO), to loosely coupled methods such as S2b, to somewhat more advanced methods such as LTE WLAN Radio Level Integration with IPsec Tunnel (LWIP), to ultimately LTE-WLAN Aggregation (LWA).
With LWA, WLAN finally becomes fully integrated into cellular operator's network.
LWA in depth
From the network perspective, there are two options that provide flexibility when looking at deploying LWA - collocated and non-collocated.
LWA design primarily follows LTE Dual Connectivity (DC) architecture [3] as defined in 3GPP Release 12, which allows a UE to connect to multiple base stations simultaneously, with WLAN used instead of LTE Secondary eNB (SeNB).
In the user plane, LTE and WLAN are aggregated at the Packet Data Convergence Protocol (PDCP) level. In the downlink, the eNB may schedule PDCP PDUs of the same bearer to be delivered to the UE either via LTE or WLAN. In order to perform efficient scheduling and to assign packets to LTE and WLAN links in the most efficient manner, the eNB can receive radio information about both links, including flow control indication. In order to avoid changes to the WLAN MAC, LWA uses an EtherType allocated for this purpose, so that LWA traffic is transparent to WLAN AP.
In the control plane, Evolved Node B (eNB) is responsible for LWA activation, de-activation and the decision as to which bearers are offloaded to the WLAN. It does so using WLAN measurement information reported by the UE. Once LWA is activated, the eNB configures the UE with a list of WLAN identifiers (referred to as the WLAN Mobility Set) within which the UE can move without notifying the network. This is a tradeoff between fully network controlled mobility and fully UE controlled mobility.
Even though WLAN usage in LWA is controlled by cellular network, UE has the option to "opt out" in order to use home WLAN (in case UE does not support concurrent WLAN operation).
LWA performance
It has been demonstrated that LWA can provide up to 50% gains compared to other LTE/WLAN integration methods (S2b, LWIP, MP-TCP), depending on the load, backhaul latency and other factors.
Deployments
On 19 August 2016 Singapore's M1 announced Singapore’s first commercial HetNet (Heterogeneous Network) rollout, including LWA. Through leading edge LTE-WiFi Aggregation (LWA) technology, M1 expects to deliver peak download speeds of more than 1Gbps by 2017.
Chunghwa Telecom (CHT) will open a commercial LTE/Wi-Fi Aggregation (LWA) network on February 23, making it the world's first telecom operator to launch LWA, according to CHT. LWA technology standards were approved by 3GPP in June 2016.