The network densification with dense small cell deployment is one of promising and essential technologies to increase capacity and extend coverage for 5G networks. In this dissertation, a study on network mobility management schemes in heterogeneous mobile networks which are composed of multi-tier small cells overlapping the coverage of macro cells, are proposed. Network mobility procedures cause various overheads including network delay, service interruption time and control message exchanges. These overheads may degrade the performances of network not only on the physical layer but also on the upper layers. This performance degradation may be more serious, especially under complex wireless interference condition in the heterogeneous mobile network (HMN) with dense small cells. In this dissertation, mobility management schemes are proposed to enhance network performance in HMNs which include multi-tier dense small cells overlaying coverage of macro cells.
In the macro cell environment, UEs search neighboring cells by measure the signal strength and receiving synchronization signals. However, UEs need to acquire system information which is essential to connect a certain cell, during neighboring cell search in a dense small cell deployment scenarios; A UE needs closed subscriber group (CSG) information to confirm whether the given cell is available for the UE or not. Moreover, physical ID from synchronization signals is not a unique identity because it can be reused by other base stations. Therefore, UEs have to acquire system information of cells by receiving and decoding the master information block and system infomation block packets by itself. Moreover, a UE searches multiple neighboring cells up to eight at a neighboring cell search procedure. This system information acquisition causes delay and service interruption times. In the conventional system, UEs acquire system information cell by cell, i.e. serial manner. Therefore, a parallel system information acquisition scheme is proposed to minimize performance degradation due to system acquisition.
Network mobility procedures, for example handovers, cell re-selction, connection re-establishment after a radio link failure, involve network overehads such as control message exchanges, delay and service interruption times as mentioned above. In conventional mobility scheme, cell selection for mobility procedure execution performed mainly according to signal strength; reference signal received power (RSRP) in the 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) system. It is unavoidable and inherent characteristic of highly dense HMNs, that frequent network mobility procedures are triggered for mobile UE. Frequent network mobility procedures will cause severe performance degradation. In this dissertation, the time of stay estimation based on users moving speed and transmission power of base stations, is introduced into the cell selection scheme. The time of stay is the duration from the time when a UE associate with a certain base station of certain cell to the time when the UE disconnects from the given base station. Therefore, UEs avoid unnecessary association which may cause very short time of stay.
Since the proposed schemes consider practical heterogeneous mobile networks, and the results show remarkable performance enhancements, it is expected that proposed schemes can be applied to not only 3GPP LTE and LTE-Advanced system but also to the other future mobile network systems.
