Contents Introduction
With the advent of the 5th Generation era (5G) for mobile networks, there has been an increasing demand for high data rates. 5G mobile communication networks employ a centralized-radio access network (C-RAN) which is composed of a centralized baseband unit (BBU) pool, remote radio heads (RRHs), and a distributed optical transmission network connecting the BBUs and RRHs. Since the fronthaul capacity scales proportionally with the radio bandwidth and number of RRHs, a large number of high-speed optical transceivers are required in the existing CPRI-based fronthaul. To this end, analog radio-over-fiber (RoF) transmission is gaining increasing momentum as a more spectrally efficient transport solution, inherently supporting lower latency and reduced RRH complexity [1]. To extend the capacity of the RoF-based fronthaul and add an additional degree of freedom for multiplexing, slicing or routing, wavelength division multiplexing (WDM) [2], [3], space division multiplexing (SDM) [4], mode division multiplexing (MDM) [5], and polarization division multiplexing (PDM) [6] have been proposed for RoF links. However, all of the previous multiplexing schemes require additional optical transceivers. In this paper, we propose and demonstrate a novel multiplexing scheme to implement uplink transmission for mobile fronthaul without the need of additional optical transceivers. In the remote cell site, two RRHs can be connected to one optical transmitter which includes a dual-drive Mach-Zehnder modulator (DD-MZM). In the BBU pool, an optical receiver based on coherent detection is employed to demultiplex the two uplink signals from the two RRHs. Thanks to the use of coherent detection, this novel transmission link offers a two-fold increase in capacity and at the same time an increase in the receiver sensitivity.
