title: Multi-Connectivity Techniques for Improved performance in Mobile Communications
Creators: name: Adamu, Paul
Publisher: name: University of Liverpool Repository
Year: 2023

description: Multi-Connectivity (MC) techniques play a significant role in enhancing the performance and reliability of mobile communication systems. These techniques aim to improve network connectivity by utilising multiple simultaneous connections between a mobile device and base stations or access points. Among other multiple benefits, MC techniques can provide increased data rates and enhanced link reliability, both of which are of extreme relevance to the new use cases introduced in the Fifth Generation (5G) of mobile communication systems, namely enhanced Mobile Broadband (eMBB), Ultra-Reliable Low-Latency Communication (URLLC) and massive Machine Type Communication (mMTC). MC techniques are equally important for the future Sixth Generation (6G) technology services and applications, where data rates and link reliability requirements will be even more demanding than in the previous generation. In this context, this thesis proposes some novel MC techniques with the potential to significantly improve both aspects (i.e., data rate and reliability) in mobile communication systems. First, a MC framework based on Carrier Aggregation (CA) is proposed. CA was originally proposed as a technique to combine spectrum from different bands into a single virtual chunk of spectrum for the higher layers of the protocol stack. In this thesis, a novel framework where CA is exploited as a diversity technique is proposed. The idea is to divide a large block of contiguous spectrum into a number of adjacent sub-blocks, where each of them is treated as an individual Component Carrier (CC) according to CA and therefore runs an independent transmission process. The motivation for this approach is to benefit from the diversity experienced at different frequencies. This idea is evaluated by means of simulations and the obtained results indicate that CA can effectively be exploited as a diversity technique to increase network capacity, with the optimum number of CCs depending on the radio propagation scenario. The simulation results indicate that the use of CA as a diversity technique, as proposed in this thesis, can enhance the obtained data rate up to almost five times with respect to the case where only one CC is employed. Subsequently, a mathematical model is developed to characterise the performance of CA as a diversity technique. The model is first used to characterise the channel capacity as a function of the number of CCs and other relevant parameters, which is shown to explain and corroborate the findings derived from simulation results. Capitalising on the developed mathematical model, the impact of various relevant configuration parameters on the performance of CA as a diversity technique is then evaluated. In such a study, not only the ergodic capacity but also the secrecy capacity are both considered and investigated. The results demonstrate that the proposed mathematical modelling approach can correctly predict the performance of CA as a diversity technique as well as the impact of various relevant configuration parameters. Finally, a hybrid transmission scheme for improved link reliability is also proposed. The proposed hybrid system benefits from the range of frequency bands available in mobile communication systems and their complementary characteristics. Higher-frequency bands tend to provide larger bandwidths (i.e., higher data rates) but are also characterised by more challenging propagation conditions (i.e., lower link reliability), while the opposite is true in general for lower-frequency bands. To exploit these complementary characteristics, a hybrid system is proposed that dynamically switches between both bands according to the instantaneous channel quality. The obtained results demonstrate that such a hybrid scheme not only improves dramatically the transmission reliability but also has the potential to simultaneously increase the capacity while efficiently exploiting the resources in both bands. Concretely, for a reliability requirement of 10−5, the proposed scheme can provide an 8/9-fold increase in the communication range of the main link in the higher-frequency bands by only reducing 2%-4% the availability of the backup link in the lower-frequency bands. Moreover, the attained high level of link reliability is not obtained at the expense of the link capacity, which is indeed improved by applying the proposed scheme. These findings suggest that the proposed scheme is a suitable technique to effectively meet the URLLC requirements for 5G/6G in a resource-efficient manner. In summary, the MC techniques developed in this thesis can provide significant improvements in terms of enhanced data rates and reliability for current and future 5G/6G mobile communication systems.