Mobility Robustness and Multi-Connectivity in 5G Mobile Networks
Buch in deiner Nähe kaufen
Beschreibung
The mobile communication industry has been working hard to satisfy the requirements of various generations of information and communication technologies. The 4 th Generation (4G) mobile technology, which is commonly termed as Long Term Evolution (LTE), has revolutionized mobile communication by providing high data rate broadband services through all-IP networks. While the deployment of 4G networks is rolling out and its operation is settling, the International Telecommunication Union (ITU) has put forward the requirements of 5 th Generation (5G) networks for the information society in 2020 and beyond. The three pillars of the requirements of 5G networks are providing ultra-high data rates via enhanced broadband services, supporting nonhuman-centric machine type communication, and achieving ultra high reliability and low latency for applications such as mission-critical services.
In particular, low latency and ultra-high reliability are expected to be some of the major requirements of future generation wireless communication applications such as vehicular communications where mobility is still required and accidents play a critical role on human life. Generally, enabling low latency can involve reducing end to end communication delay, transmission time interval, transmission round trips and call setup times. On the other hand, ultra-reliability mainly refers to continuous provisioning of a wireless application with low probability of service interruption. Some of the major risks of low latency and ultra-reliability in wireless communication networks are low received power due to fading processes such as shadowing and fast fading, interference, system failures such as hardware or software failures, and service interruption due to mobility events such as handovers and connection failures. The various components of the aforementioned challenges are the subjects of ongoing research in wireless communication industries and academia. Herein, this thesis focuses on addressing the risks of low latency and ultra-reliability that arise from mobility-related service interruptions.
Mobility in cellular networks such as LTE is performed via handover procedures. A conventional handover in LTE incurs a certain service interruption as it involves a number of procedures such as handover preparation, handover execution and handover completion phases, before a User Equipment (UE) has a fully-fledged change of serving cell. In particular, a UE experiences service interruption due to the delay to have initial access to the target new cell because this is usually achieved via random access channel procedures that require a certain time to be completed. Moreover, the backhaul delay in transferring residual packets from the source cell to a target cell contributes to the service interruptions. More prominently, mobility-related connection failures incur long service interruption due to the timers required for monitoring failures and the delay in connection re-establishment procedures. To deal with such connection failures, 3 rd Generation Partnership Project (3GPP) has specified Mobility Robustness Optimization (MRO) for LTE networks based on the requirements of 4G networks. However, MRO has not been evaluated based on the more stringent 5G ultra low latency and ultra-high reliability requirements. This work investigates 3GPP-based MRO algorithms based on 5G requirements. Moreover, it proposes advanced and practical context-aware MRO algorithms, showing its performance evaluation in a concrete and elaborated mobility scenario.
Evaluation of context-aware MRO has shown that more applications with higher reliability requirements can be supported as compared to a conventional handover. However, it is observed that context-aware MRO in single-connectivity does not fulfill 99.999% at extremely low latency which is a typical reliability requirement for future applications. Consequently, multi-connectivity schemes are investigated focusing on the mobility related challenges. The major components of multi-connectivity schemes involve pro-active preparation of a set of serving cells which are termed as Active Set (AS), and enabling the serving cells to perform a co-ordinated wireless transmission to the UE. The main criteria in preparing AS for a UE is to include the currently strongest cell(s) in terms of the slow changing channel. However, UEs are not able to directly measure the slow changing channel with sufficient accuracy and sufficient measurement delay because of fast fading and noise. A novel AS Management (ASM) strategy is proposed taking into account the inaccuracy and delayed availability of UE measurements. With support of theoretical and practical foundation, the proposed ASM strategy is evaluated based on the requirements of 5G wireless communication networks.
Once the set of serving cells is prepared using the proposed ASM strategy, the serving cells need to be co-ordinated for wireless transmission to the UE. Such multi-cell transmission is not a new topic, and it has been discussed in 3GPP’s LTE specifications under the umbrella of Co-ordinated Multi-Point (CoMP) transmission. However, multi-cell transmissions are performed only for data signals and mobility-related transmission of control signals are performed from only one cell which is updated based on conventional handover procedures in single-connectivity. Unlike LTE CoMP, this work proposes that multi-cell transmission is performed for both control and data signals to exploit the co-ordination gain not only on data signals, but also on control signal that in turn leads to a robust mobility that fulfills the requirements of future generation networks. One of the proposed transmission schemes is termed as Single Frequency Network (SFN) transmission. Herein, transmission of both control and data signals from all the cells of an AS, are aggregated at the UE non-coherently. This reduces the major overhead of coherent joint transmission which has very high requirements in antenna calibration. With SFN transmissions, one UE is allowed to block resources in multiple cells which risks throughput performance. As a result, concrete scheduler model is designed and used to monitor the throughput performance along with mobility performance which is the primary target of the investigation. The performance evaluation in a concrete mobility scenario has shown that the proposed transmission scheme provides not only full reduction of connections failures which are the main mobility-related ultra-reliability challenges, but also it provides gain in the throughput of cell-edge UEs due to the coordinated transmission gain on the data signals.
Some of the entailed challenges of multi-connectivity scheme with SFN transmission are requirement of multi-cell co-ordination to aggregate the received signals at the UE, and multi-cell resource scheduler. An alternative transmission scheme with lower overhead in multi-cell co-ordination is proposed with the primary target of reducing connection failures. The proposed transmission scheme is termed as Fast Cell Select (FCS) transmission. In this case, the strongest cell among the AS is selected based on feedback from the UE, and the selected cell is used for transmission of both control and data signals. Hence, the proposed scheme provides a UE the flexibility to change serving cells among the AS faster as compared to conventional handover which is slower. Performance evaluations show that even though FCS transmission exhibit limitations for noise-limited UEs, it provides a considerable reduction of connection failures with lower co-ordination complexity as compared to the SFN transmission scheme.
Generally, this work and the related contributions have been documented in 9 conference papers [TZV + 14] [TZV + 16] [KWT + 15] [TZV + 15] [TAV + 15] [TAV + 16a] [TAV + 16b] [TGVR13] [TAV + ed], 2 journals [TAV + 16c] [TAV + ed], and 3 filed patent ap- plications [VTA + 62] [TAV39] [ATV14]. Moreover, this work has contributed [Nok16a] to 3GPP discussion on URLLC and mobility events, and a demo at Mobile World Congress 2015.
Autor*in
Fasil Tesema