LTE-U and Wi-Fi Coexistence
Since other technologies such as Wi-Fi also operate in unlicensed spectrum, it is critical for LTE-U to support coexistence and fair sharing of the unlicensed band. From its very inception, LTE-U has been designed to coexist with Wi-Fi and other unlicensed technologies. LTE-U exceeds the minimum regulatory requirements of unlicensed spectrum, to ensure fair coexistence. The criterion for good coexistence is that the impact of LTE-U interference on a given neighboring Wi-Fi node is no more than the impact from other Wi-Fi nodes in the same setup.
There are three key features that help LTE-U achieve good coexistence with Wi-Fi and other unlicensed technologies:
1. Channel Selection: LTE-U small cells always try to select a clean channel to avoid interference with nearby nodes, based on continuous channel measurements. Considering that multiple 20 MHz channels are available in 5 GHz, channel selection is very effective in low to medium density scenarios.
2. Time-domain coexistence techniques are needed when there is no clean channel available. Based on unlicensed band regulations in specific markets, there can be multiple techniques to share the channel fairly with Wi-Fi. For non listen-before-talk (LBT) markets such as the US, South Korea, China and India, CSAT (Carrier Sensing Adaptive Transmission) can be used without changing Rel-10 Carrier Aggregation protocols for co-channel coexistence. For LBT markets such as Europe and Japan, Rel-13 LAA will enable channel sharing with Wi-Fi by performing clear channel assessment (CCA) based channel availability sensing and adapting the transmission duration on a fine timescale, ranging from 1ms-10ms. Both solutions achieve co-existence through adaptive on/off transmission based on channel utilization measurements. The choice between the two options will depend on the regulatory requirements in a specific region and time-to-market considerations.
3. Opportunistic SCell operation: In LTE-U, unlicensed carriers are designed to operate as secondary cells (SCells), with the principle that LTE-U small cells would release the unlicensed carriers and fall back to the anchor carrier in licensed spectrum at low traffic load.
Coexistence performance has been thoroughly studied by Qualcomm and other companies with numerous simulation results showing that LTE-U can provide higher spectral efficiency and better user experience while maintaining the performance of neighboring Wi-Fi users.
In addition, Qualcomm has tested LTE-U performance in an over-the-air test bed with commercial Wi-Fi equipment including a hyper-dense stress test chamber. The test results confirmed the fair sharing behavior of LTE-U and show that LTE-U is often a better neighbor to Wi-Fi than Wi-Fi itself. Qualcomm will showcase two sets of LTE-U demos, without and with listen-before-talk (LBT) capability, at MWC 2015.