The Future Role of Transport Networks in 6G

In the last post we looked at the role of the All-Photonics Network (APN) in IOWN and its relevance to future 6G systems. That discussion highlighted how photonic transmission and the removal of optical-electrical-optical conversions could significantly improve latency, efficiency and bandwidth. Building on that theme, Nokia has recently published a new white paper titled 'The Future Role of Transport Networks in 6G'. The following is a technical summary of its key points.

Evolution of Architectures

The shift towards cloud-native and disaggregated radio access networks has already begun in 5G. Functions traditionally concentrated within the base station are being decomposed into radio, distributed and centralised units. These units can be further separated into user and control plane instances, virtualised and run in cloud environments. Such flexibility supports cost efficiency and new service models, but it also places a far greater burden on the transport layer. Every new functional split introduces additional interfaces that require reliable, low-latency and synchronised connections across varying distances.

Transport networks are no longer limited to backhaul alone. With the introduction of fronthaul and midhaul, collectively referred to as xHaul, the network must now support highly diverse requirements ranging from time sensitive nanosecond-level synchronisation to terabit-per-second interconnects between data centres hosting cloud RAN functions. This makes the transport network an active and critical element in end-to-end performance rather than a passive connectivity layer.

6G Transport Requirements

The move to higher frequency bands and wider channel bandwidths in 6G will increase spectral bandwidth from the 100 MHz channels of today to as much as 400 MHz. Together with advanced multiple antenna systems this could multiply per-cell capacity by a factor of twenty compared with 5G. Such growth will demand backhaul and midhaul links beyond 10 Gbps and fronthaul connections capable of carrying traffic at 100 Gbps with end-to-end latencies below 200 microseconds. Ultra-reliable low latency services will require end-to-end latencies under one millisecond across radio, core and transport domains combined.

Urban densification will further multiply the number of xHaul connection points. New split options, O-RAN interfaces, and legacy interworking will increase the complexity of managing and orchestrating the network by an order of magnitude. At the same time, operators will face financial pressures that necessitate energy efficient and scalable solutions. Transport networks must therefore be designed with automation, programmability and intelligent management at their core.

Key Technologies

Optical fibre remains the preferred medium for xHaul and will extend closer to the antenna with fibre-to-the-antenna and related deployments. Dense wavelength division multiplexing will be essential to maximise capacity, while tunable optical components will introduce flexibility and energy efficiency. Coherent optical transmission and photonic integration are pushing individual channel rates beyond 1 Tbps, with research into spatial division multiplexing promising even greater future capacity. Nevertheless, economic constraints in the access domain will often cap throughput to around 1 Tbps per fibre, making efficient use of wavelengths and modulation schemes a priority.

Where fibre deployment is impractical, microwave and millimetre wave systems provide vital alternatives. Traditional microwave bands between 6 and 40 GHz will continue to be used, but E-band, W-band and eventually D-band frequencies will extend capacity to 100 Gbps with extremely low latencies. These links are particularly attractive for dense urban deployments where fibre trenching is prohibitively expensive. Advances in system-on-chip integration, glass substrate antennas and phased arrays will further enhance performance and reduce form factor.

Passive optical networks (PONs) are evolving to meet the needs of fronthaul and backhaul, with 25G PON already available, 50G PON beginning deployments from 2025, and 100G PON expected within the 6G timeframe. Cooperative transport interfaces (CTI) defined by O-RAN will enable time sensitive traffic to be carried over these networks with dynamic bandwidth allocation.

Non-terrestrial networks will complement terrestrial transport solutions by providing backhaul, emergency coverage and rural connectivity through low earth orbit satellites and high-altitude platforms. These systems will increasingly host selected radio access functions onboard, enabling direct mobile device access and new approaches to end-to-end latency management.

Intelligent Orchestration

The scale and complexity of future xHaul systems necessitate complete automation. Software-defined networking separates control and user planes, allowing centralised controllers to configure thousands of forwarding instances in real time. End-to-end orchestration across radio, core and transport domains will ensure optimal use of network resources and support large-scale network slicing. Artificial intelligence will further enhance operations by predicting traffic, optimising parameters and troubleshooting automatically, improving efficiency and reducing costs.

Digital twins represent another transformative concept. By creating real-time virtual replicas of transport networks, operators can simulate, test and optimise configurations before deployment. This avoids costly lab testing, accelerates innovation and provides closed-loop control by continuously synchronising virtual and physical systems. Digital twins can also support energy optimisation, training and security analysis, extending their value beyond operations.

Conclusion

The path to 6G will require as much innovation in transport as in the radio and core domains. Sustainable, flexible and intelligent transport networks are essential to support cloud-native architectures, ultra-dense deployments, massive capacity growth and stringent latency requirements. Optical, microwave and satellite technologies will combine with automation, artificial intelligence and digital twins to create a unified and programmable foundation.

Nokia’s white paper The Future Role of Transport Networks in 6G provides a detailed discussion of these themes and is available for anyone interested in exploring the topic further.

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