  A coherent transmission methodology for a continuous-variable quantum key
distribution (CV-QKD) system based on quantum-heterodyne measurement through a
coherent intradyne receiver is experimentally demonstrated in the framework of
5G mobile fronthaul links. Continuous optical carrier synchronization is
obtained through training information, which is multiplexing to the quantum
signal as pilot tone in both, frequency and polarization. Spectral tailoring by
means of optical carrier suppression and single-sideband modulation is adopted
to simultaneously mitigate crosstalk into the quantum channel and
self-interference for the pilot tone, thus allowing for a high signal-to-noise
ratio for this training signal. Frequency offset correction and optical phase
estimation for the free-running local oscillator of the receiver is accurately
performed and guarantees low-noise quantum signal reception at high symbol
rates of 250 MHz and 500 MHz with additional Nyquist pulse shaping. A low
excess noise in the order of 0.1% to 0.5% of shot-noise units is obtained for
fiber-based transmission over a fronthaul link reach of 13.2 km. Moreover,
co-existence with 11 carrier-grade classical signals is experimentally
investigated. Joint signal transmission in the C-band of both, quantum signal
and classical signals, is successfully demonstrated. Secure-key rates of 18 and
10 Mb/s are obtained under strict security assumptions, where Eve has control
of the receiver noise, for a dark and a lit fiber link, respectively. Moreover,
rates of 85 and 72 Mb/s are resulting for a trusted receiver scenario. These
secure-key rates are well addressing the requirements for time-shared CV-QKD
system in densified 5G radio access networks with cloud-based processing.
