Energy-Efficient Mobility for Small-Cell Overlaid Cellular Networks

Abstract

To satisfy the ever-growing demand for enhanced data rate in future cellular networks, an ultra-densification approach is introduced to shrink the coverage of base stations (BSs) and improve frequency reuse. A gain in system capacity is anticipated by increasing the density of BSs; however this gain is expected to come at the expense of a high number of handovers (HOs), increased HO delays, increased HO failures (HOFs), and a high ping pong (PP) rate, which for moderate-to-high-speed users implies significant signaling overhead traffic resulting in an unsatisfactory user experience.In this thesis, we provide a simulation analysis to study the performance of current 3GPP cellular networks (e.g. Long Term Evolution (LTE)/ New Radio (NR)) with a legacy downlink handover (DL-HO) procedure by taking into account the cell sizes and user mobility. In particular, the potential problems of HOFs are highlighted especially in the case of ultra-densification. Moreover, this work derives a power consumption model and addresses the signaling overhead and power consumption that results from the transmission and reception of HO signaling both at the BS and at the User Equipment (UE) during DL-HO. The DL-HO analysis exhibits that the measurement report (MeasReport) transmission is the largest contributor to air-interface signaling and its power consumption is higher than random access channel (RACH) signaling and the signaling confirming the HO. In order to cope with the problem of high MeasReport signaling and effectively reduce the associated power consumption, a handover that is based on the uplink (UL) reference signal, referred to here as UL-HO, is proposed that exploits uplink (UL) reference signals (RSs), namely the sounding reference signal (SRS), transmitted by UEs. The performance of UL-HO is compared with DL-HO to quantify the potential benefits in terms of reduction in HO rates, HOFs, PPs, UE, and BS power consumption. After this, we then highlight another major challenge of today's cellular networks which is the increasing demand for voice and data services in fast-moving public transportation (i.e. bus, tram, train, subway, etc.), especially in urban areas. To this end, we investigate the utilization of mobile relay nodes (MRNs) in vehicles to facilitate efficient group HO and reduce the energy consumption for all on-board UEs. In this thesis, we also address the DL-HO performance of an MRN to identify the causes of MRN HOFs towards the donor BS (DBS) that are more critical for the on-board UEs.To sort out the problem of MRN HOF to the DBS, we extend the applicability of the UL-HO scheme for the MRN to eliminate the MeasReprot signaling during MRN HO to the DBS. Therefore the HO delay can be reduced, decreasing the chances of single point of failure (SPoF) and thus, uninterrupted services can be provided to on-board UEs. Moreover, we analyze the impact of on-board UE cluster size on HO performance and the associated power consumption.