URSI GASS 2023, Sapporo, Japan, 19 – 26 August 2023

Photonics for next generation radio access network (RAN)
M. Nakazawa
Tohoku University, Japan; nakazawa@riec.tohoku.ac.jp
The capacity of the optical communication infrastructure in backbone networks has increased a thousand-foldover the last twenty years. In addition to this rapid progress, Internet traffic is continuing to grow at an annual rateof 40%. This means that in twenty years, we will need Petabit/s or even Exabit/s optical communication. Inaddition, the traffic capacity of recent wireless communication systems is also increasing greatly as seen with 5G
and 6G systems, where their peak rates reach as high as ~ 10 Gbit/s and ~ 100 Gbit/s, respectively. Therefore,
there is an urgent need to incorporate advanced optical communication technology in the recently developed radio
access network (RAN) as this will make it possible to realize a high-capacity and high-speed global
communication network.
In this talk, I will first present recent challenges and efforts toward achieving a hardware paradigm shift toovercome the capacity limitation imposed by the current optical communication infrastructure. I will overview
the latest advances on the three “multi” (3M) technologies, i.e., multi-level transmission with ultrahigh spectral
efficiency, multi-core fibers for space division multiplexing, and multi-mode fibers for mode divisionmultiplexing with multiple-input multiple-output (MIMO).
In the latter part, I will describe an optically and wirelessly linked fully coherent access system for next
generation RAN that uses 3 M technology. I will highlight the importance of the fully coherent system and present
in detail what we have been examining experimentally. In this system, we treat optical and wireless regions as
one unified electromagnetic-wave transmission region although the IF carrier frequencies of the optical andwireless waves differ greatly. This method makes it possible to realize a simple and high performance mobile
fronthaul (MFH) inexpensively by correcting the data errors that occur in the optical transmission region with astrong wireless forward error correction (FEC) scheme. That is, no FEC scheme was installed in the optical region.
Since the present system operates by means of an analogue transmission between the wireless and optical regions,
we can remove several A/D, D/A, and even common public radio interface (CPRI）framer/deframer devices.
To achieve such an interesting scheme, I will point out the important role played by an optical injection locking
technique, which can achieve precise phase-locking between optical and local oscillator (LO) signals. I will showthat the use of one laser diode (LD) makes it possible to phase-lock the LO and eventually achieve a 256 QAM
MFH transmission. Lastly, I will present optical and electrical devices that can accelerate the development of afully coherent MFH system including integrated optoelectronic devices.

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