Abstract
Future cellular systems need to cope with a huge amount of data and diverse service requirements in a flexible, sustainable, green and efficient way with minimal signalling overhead. This calls for network densification, a short length wireless link, efficient and proactive control signalling and the ability to switch off the power consuming devices when they are not in use. In this direction, the conventional always-on service and worst-case design approach has been identified as the main source of inefficiency, and a paradigm shift towards adaptive and on-demand systems is seen as a promising solution. However, the conventional radio access network (RAN) architecture limits the achievable gains due to the tight coupling between network and data access points, which in turn imposes strict coverage and signalling requirements irrespective of the spatio-temporal service demand, channel conditions or mobility profiles. This suggests a new clean slate RAN architecture with a logical separation between the ability to establish availability of the network and the ability to provide functionality or service. This separation of control and data planes provides a framework where limitations and constraints of the conventional RAN can be overcome. In this context, the aim of this thesis is to investigate the control/data separation architecture (CDSA) for futuristic RANs where data services are provided by data base stations (DBSs) under the umbrella of a coverage layer supported by control base stations (CBSs). A comprehensive literature survey of the CDSA is provided in this thesis. The concept, general structure and basic operation are discussed along with the separation framework and approaches. In addition, limitations of the conventional architecture are pointed out and superiority of the CDSA is discussed whilst focusing on futuristic deployment scenarios. Furthermore, the CDSA technical challenges and enabling technologies are identified, and preliminary standardisation proposals related to this research vision are presented. Three areas, namely energy efficiency, signalling overhead and latency, and mobility management, are identified as promising dimensions that can be substantially improved under CDSA configuration. Focusing on the signalling overhead dimension, a correlation-based adaptive DBS pilot signalling scheme is proposed by exploiting the separation property and the one-to-one nature of the DBS link. The proposed scheme considers channel estimation pilots in the downlink of a multi-carrier DBS air interface, and it depends on estimating the actual channel correlation function to redistribute the pilot signals dynamically. Simulation results show that the proposed adaptive scheme provides a significant saving of 74%-78% in pilot signalling overhead without (or with a marginal) performance penalty as compared with the conventional worst-case design approach. In addition, the out-of-band signalling related to mobility management is investigated by exploiting the relaxed constraints offered by the CDSA. In particular, the active state handover (HO) signalling in the DBS layer is tackled by proposing two predictive DBS HO signalling schemes with minimal HO latency. These include a history-based predictive DBS HO scheme that predicts future DBS HO events based on the user history, and a measurement-based context-aided predictive DBS HO scheme that predicts future DBS HO events along with the expected HO time by combining DBS signal measurements to physical proximity and user contextual information. In both schemes, the prediction outcome is utilised to perform the HO-related DBS RAN signalling in advance, resulting into light-weight HO procedures. Simulation results show that these predictive schemes can remarkably reduce the DBS HO signalling latency w.r.t. the benchmark. Precisely, up to 34% reduction in the HO signalling latency is achieved. Moreover, the dual connection feature of the CDSA and the large CBS footprint are utilised to minimise the HO-related core-network (CN) signalling load by proposing a CN-transparent HO signalling scheme. In the latter, the CBS is used as a mobility anchor point for the users and as a data plane anchor point for the DBSs. Thus, the control plane remains unchanged as long as the user mobility is within the same CBS, while the data plane is switched locally at the CBS. Furthermore, the additional data plane backhaul latency induced by the CDSA is modelled and an upper bound for the DBS density under latency constraints is derived. Numerical and simulation results show that the CDSA-based CN-transparent HO signalling scheme significantly outperforms the conventional architecture-based CN-visible HO approaches in terms of CN signalling load. In dense deployment scenarios, the CN-transparent HO scheme is found to be more beneficial where the gains reach 90% reduction in the CN signalling load. Additionally, the CN-transparent HO scheme is integrated with the predictive HO techniques, and simulation results show that the integrated scheme doubles the gains of the predictive-only HO approach in terms of reduction in DBS HO signalling latency.