Introduction: The research team led by Nan Chi and Junwen Zhang of Fudan University conducted in-depth research on the challenges faced by traditional access networks due to increased bandwidth demand. They proposed a next-generation flexible coherent optical access solution, taking into full consideration the requirements of low cost, wide coverage, and high flexibility, thereby powerfully promoting the development of next-generation coherent optical access exceeding 100G.

Research Background and Challenges

With the advent of the sixth generation fixed network era, a host of transformative functionalities are becoming a reality. The emergence of an array of new services is driving a significant increase in bandwidth demand for optical access networks and a more flexible service format, setting higher performance requirements for the foundation of the optical access network communication system - the physical layer. Traditional intensity modulation/direct detection struggle to meet the power budget and flexibility needs of over 100G broadband optic access, desperately requiring a transformation in physical layer communication technology. The novel low-cost, wide-coverage, high-flexibility single-channel over 100G coherent optical access technology has become a current research hotspot both domestically and internationally.

Next generation advanced access network technology

Figure 1 - Three major improvements for the next generation access network: low-cost, wide-coverage, and high-flexibility.

The research team led by Zhang Junwen at Fudan University has conducted in-depth research on advanced technologies for future access network systems, and proposed a comprehensive typical next-generation coherent optical access solution, taking into account the needs for low cost, widespread coverage, and high flexibility. The results were published in Chinese Optics Letters Vol.22 No.4 (S. Xing, et al., Low-cost, large-coverage and high-flexibility coherent PON for next generation access network: advances, challenges, and prospects) and were selected as the cover for that issue.

For the "low-cost" requirement, researchers have delved into and organized ultra-simple coherent and low-complexity DSP technology solutions, which can effectively reduce system complexity from the aspects of transmitters, receivers, and algorithms, significantly reducing the deployment cost of next-generation coherent PON networks. For the "wide-coverage" requirement, researchers have also combined coherent technology with various latest advanced DSP technologies, realizing a larger dynamic range at high speeds, breaking through high sensitivity weak signal detection, power budget enhancement, and large power difference coherent detection balancing technology, thus ensuring widespread network coverage. For the "high-flexibility" requirement, researchers have used flexible coherent access modulation coding and multi-dimensional multiplexing technology nearing the Shannon limit to overcome the "time-frequency-entropy" and other multi-dimensional cooperative mechanisms. They have also analyzed and compared various system solutions to realize flexible rate access, laying a solid theoretical foundation for increasing the system's overall throughput.

Figure 2 - Principle of next-generation coherent access network system based on time-frequency multiplexing system architecture.

Future Prospects

Looking ahead, with continued research and development, staff anticipate that further innovations will see coherent access network technology more widely adopted. Among these, emphasis will be placed on the integration of low-cost, wide-coverage and high-flexibility. With these advanced technologies, staff expect Sixth Generation Fixed Networks (F6G) to be poised not only to meet growing bandwidth demand, but can also make fiber to the home, office, building, neighborhood and other end points feasible. F6G is ready to cope with the unique challenges posed by these applications, such as strict power budget constraints and demands for adaptive data rates. Low-cost, wide-coverage, high-flexibility coherent optical access technology is on a track of continuous improvement and adaptation, which will provide significant support for the construction of F6G.