LTE-U Release 10
In markets where there is no LBT (Listen Before Talk) regulation, such as the US, South Korea, China and India, LTE-U can be deployed based on LTE Rel-10/11/12 Carrier Aggregation protocols. LTE-U small cells (eNB) and devices (UE) need to support 5 GHz as a new LTE RF band. Coexistence with Wi-Fi or other unlicensed technologies can be achieved with radio resource management techniques without changing R10 protocols. The initial deployment of R10-based LTE-U will focus on Supplemental Downlink (SDL) operation, so only eNB’s will transmit in 5 GHz band and the coexistence features will be mostly implemented by the small cells. With minor modifications to current LTE product design, R10-based LTE-U can leverage the large LTE ecosystem and enable early deployments in non-LBT markets.
In addition to the channel selection and opportunistic SDL features, time-domain coexistence techniques are needed when there is no clean channel available. More specifically, for a LTE-U system that is designed based on Rel-10/11/12, a technique called CSAT (Carrier Sensing Adaptive Transmission) would sense the channel utilization by neighboring nodes and adapt LTE-U’s on/off duty cycle on secondary cell(s) in the unlicensed band on the order of tens to hundreds of msec. The duty cycle is chosen adaptively based on the number of active Wi-Fi APs and their channel usage to allow fair sharing with other devices using the same channel. In many scenarios such as high density cases, CSAT allows LTE-U to be a better neighbor to Wi-Fi as LTE-U vacates the channel deterministically, as compared to CSMA which is prone to collisions in dense scenarios. The number of nearby LTE-U small cells in the same channel from other operators is also taken into consideration when CSAT sets its duty cycle. There are also short transmission gaps during the LTE-U “ON” period to allow for latency sensitive applications such as VoIP in neighboring Wi-Fi.
R10/11/12-based LTE-U will meet all applicable regulatory requirements for 5 GHz unlicensed band in non-LBT regions. LTE-U commercial equipment will need to pass a minimum performance and conformance specification on 5 GHz, as well as a comprehensive coexistence specification. The coexistence specification will cover both LTE-U/Wi-Fi and inter-LTE-U deployment coexistence scenarios. The test scenarios and passing criteria are being designed to be more comprehensive and stringent than the existing Wi-Fi coexistence test.
Qualcomm has deployed an LTE-U over-the-air test network on its San Diego campus comprising of multiple Wi-Fi access points and LTE-U small cells in a hyper dense network to evaluate their real-life performance. Extensive testing concluded that LTE-U can not only provide superior performance, but fairly coexist with Wi-Fi and, in many cases, improve the performance of neighboring Wi-Fi access points due to LTE’s improved efficiencies in the use of unlicensed spectrum. These results are shown at MWC 2015.