Sun-Heui Ryoo

On the trade-off between handover failure and small cell utilization in heterogeneous networks

To enhance the performance of a heterogeneous network, where many small cells are deployed in a macro cell, we consider two technical challenges. First, seamless handover between the same or different types of cells should be supported with a low handover failure rate. Second, small cells should use their radio resources as much as possible to maximize a cell splitting gain. It was well known that there is a trade-off between the low handover failure rate and the high small cell utilization. To find a better operation considering such a trade-off, we propose a state-dependent handover decision algorithm. In the proposed scheme, a user sets a “state” for each small cell, and performs a “state transition” to distinguish between macro-to-small and small-to-macro handovers, and then uses suitable handover parameters for each handover type. Through simulation, we observe that the proposed scheme can reduce the handover failure rate significantly without degrading the small cell utilization and the throughput experienced by the user.

Mobility enhancement of dense small-cell network

Achieving significant throughput enhancement is a fundamental technical challenge for next generation cellular communication systems. In order to meet the projected 1000 fold traffic increase by the year 2020, the cellular systems are intensely focusing on ultra-densification by deploying a large number of small cells in a geographical area as a practical means for aggressively increasing the areal capacity. However cell densification is inherently limited by the high inter-cell interference that it inadvertently generates which seriously jeopardizes its gain and renders it practically unsuitable to meet the projected huge traffic explosion. Furthermore, increased densification leads to increased cell edges which in turn lead to rugged user experience. In order to overcome these daunting limitations and make densification a practical reality, we propose novel mobility robustness solutions which significantly decrease the number of handover failures and minimize the interruption in the ongoing services thereby leading to an appreciably better quality of experience for the user. The performance of the proposed schemes are evaluated using computer simulations and also in the drive test with a commercial handset and network equipment in the Gang-nam station area where the network configuration is close to the dense small cell environment. The proposed schemes enhance mobility performance up to 77% and reduce the service outage by 38% in the simulation with typical handover configurations. The enhancement is also observed from the drive tests in the commercial LTE small cell network around Gang-nam station area.

Battery life extension for WLAN-LTE aggregation

To enlarge the cell capacity of mobile network, LTE network offloads its traffic to WLAN network. However, User Equipment (UE) can run out of batteries rapidly due to the independent operation of multi-Radio Access Technologies (RAT), LTE and WLAN. In this paper, we propose new Multi-RAT power saving scheme, Single-RAT Monitoring which remove the duplication of periodic wake up operation. Because UE has both LTE and WLAN modem, LTE modem can monitor the traffic status for WLAN link and then activate or deactivate the WLAN modem. Our performance analysis shows that the WLAN modem applying proposed Single-RAT Monitoring consumes 11%~99% less energy with various traffic pattern.