High-Speed Optical Communications Systems for Future WDM Centralized Radio Access Networks

Several modulation techniques aimed at ensuring high capacity and low latency for next generation of mobile transport networks are discussed. Centralized radio access networks are considered based on WDM transmission to ensure long distance, sufficient number of antenna sites and segregation of heterogeneous connectivity services over the same fiber infrastructure. Of all the proposed techniques, attention is paid to those allowing for low-cost transceivers. With increasing bit-rate, when considering distances in the order of 20 km, the most significant impairment is the fiber chromatic dispersion. The reasons dispersion compensating fiber modules are to be avoided in a mobile transport network are discussed and modulation formats resilient to fiber chromatic dispersion are examined to check for the highest achievable bit-rate when using direct-detection receivers. It is shown that, when using binary formats, the combined amplitude-phase shift (CAPS) codes allow to bridge a distance of 15 km or more with a significant power budget advantage over PAM-4. However, higher modulation formats such as PAM-4 allow for slower electronics speed. When halving the speed of electronics is desirable and optical amplification is employed, a bipolar PAM-4 format allows for an improved power budget. One of the results of this paper is that, without any signal processing or dispersion compensators and without exploiting polarization modulation, direct-detection formats cannot be employed to bridge distances of the order of 20 km at a bit-rate greater than about 50 Gb/s. To achieve higher rates, coherent detection is needed and we also discuss a low-cost implementation avoiding a local oscillator for detection.