Cost-effective Solutions Enabling Seamlessly Converged Fibre-Wireless Access Networks for 6G and Beyond

Abstract

To efficiently support rapidly evolving services and applications with diverse demands on traffic characteristics, in comparison with current and previous mobile networks, 6G networks must satisfy more stringent requirements, including higher data rates, higher connection densities and lower latencies. Achieving these targets requires significant technological advancements to be made across all layers of the network, of which critical technical bottlenecks in the physical layer include the relaxation of mobile fronthaul bandwidth demands, the reduction of latency-inducing processes such as intermediate digital signal processing (DSP) and optical-electrical-optical (O-E-O) conversions, cost-effective accommodation of massive device connectivity, and more importantly, the seamless convergence of fibre and radio access networks allowing continuous ultra-low latency signal flows across the entire access network without O-E-O conversions and DSP at intermediate nodes. To address the aforementioned technical challenges, the PhD dissertation research is undertaken to propose and experimentally demonstrate cost-effective technical solutions at device, system and network levels.
At the device level, the thesis proposes and experimentally verifies a low-cost optical switching device, termed the soft reconfigurable optical add/drop multiplexer (soft-ROADM), featuring a new dual-arm drop element. The proposed dual-arm soft-ROADM drop element achieves an O-E-O conversion-free dynamically reconfigurable drop operation across wavelength, sub-wavelength and orthogonal I and Q sub-bands levels with excellent resilience to both I/Q crosstalk and drop RF signal phase offsets. As such, soft-ROADMs incorporating the new dual-arm drop element can be employed in cost-sensitive access networks to eliminate the need for high-latency intermediate O-E-O conversion-based electrical domain switching, thus enabling seamless and dynamic fine-granularity optical switching to efficiently accommodate highly dynamic traffic.
At the system level, the thesis experimentally demonstrates the dynamic reconfigurability of both adaptive access network operation and corresponding performance by using a simple free-running laser at each remote radio head (RRH) for photonic millimetre wave (mmWave) signal generation via optical beating between the RRH-laser and the baseband unit (BBU)-transmitted optical signal, and by using a passive electrical envelope detector at the user equipment (UE) to realise cost-effective local oscillator (LO)-free mmWave signal detection. The system performance is extensively evaluated, validating that the solution offers a cost-effective solution for seamlessly converging fibre and radio domains, enabling continuous ultra-low latency signal transmissions across the entire access network (e.g., between BBUs and UEs) without latency-inducing O-E-O conversions and DSP at intermediate nodes.
To enhance the traffic routing flexibility in the aforementioned seamlessly converged fibre-wireless transmission system, without any intermediate O-E-O conversions or DSP, soft-ROADMs are deployed at intermediate nodes between BBUs and RRHs. The solution offers ultra-low-latency flexible user access (optical or radio) and ultra-dense dynamically reconfigurable “just-the-right-size” on-demand BBU-UE connections at different locations.
At the network level, to gain an in-depth understanding of the tolerance to timing jitter, in the context of 6G massive device connectivity, and by taking into account the impacts of performance-limiting timing jitters arising from low-cost clock sources in network end-devices, combined with the multiple access nature of access networks, high-accuracy timing jitter models are developed and numerically validated. Based on these models, the impacts of both white and coloured digital-to-analogue converter (DAC) and analogue-to-digital converter (ADC) timing jitters are analysed in hybrid OFDM digital filter multiple access passive optical networks. Additionally, a simple DSP technique termed joint sideband processing (JSP) is proposed and verified to enhance the network performance robustness against the timing jitter and significantly reduce timing jitter-induced optical power penalties. The work confirms that the JSP technique is highly effective in relaxing DAC/ADC clock jitter requirements, thereby lowering the overall system implementation costs.
In conclusion, the PhD dissertation research has strong potential to significantly contribute to the realisation and economical deployment of practical 6G networks.

Date of Award	2025
Original language	English
Awarding Institution	Bangor University School of Computer Science & Engineering
Supervisor	Jianming Tang (Supervisor) & Roger Giddings (Supervisor)