The 5G era is nearly upon us, and poses several challenges for system designers; one important question is how the (soon to be standardized) mmWave bands of wireless mobile access can coexist harmoniously with optical links in fixed telecom networks. To this end, we present a Radio-over-Fiber (RoF) backhauling concept, interfaced to a 60-GHz indoor femto-cell via a field-installed optical fiber link. We successfully demonstrate generation of a RoF signal up to 1 Gb/s and transmit it optically over 43 km of deployed Single Mode Fiber (SMF), as well as investigate the performance of the 60-GHz access link as a function of distance. The optical link introduces negligible degradation, contrasting the effect of multipath fading in the 60-GHz wireless channel; the latter requires adaptive equalization using offline DSP. The proposed scheme is further validated by demonstration of a 60-GHz Remote Antenna Unit (RAU) concept, handling real traffic from commercial Gigabit Passive Optical Network (GPON) equipment. Proper RAU operation at 1.25 Gb/s is achieved, accommodating true data packets from a Media Converter emitting at 1310 nm through an in-building fiber link. System performance is confirmed through Bit Error Rate (BER) and Error Vector Magnitude (EVM) measurements. EVMs of ~11 and 19% are achieved with BPSK signals, for distances of 1 and 2 m respectively. As standardization of mmWave technologies moves from 5G testbeds to field-trial prototypes, successful demonstration of such 60-GHz wireless access scenarios over a telecom operator’s commercial fiber infrastructure is even more relevant.
Proc. SPIE. 9775, Next-Generation Optical Networks for Data Centers and Short-Reach Links III
KEYWORDS: Digital signal processing, Modulation, Digital filtering, Receivers, Single mode fibers, Modulators, Pulse shaping, Data conversion, Electro optics, Singular optics, Forward error correction, Optical interconnects, Signal detection, Nyquist pulse
Faced with surging datacenter traffic demand, system designers are turning to multi-level optical modulation with direct
detection as the means of reaching 100 Gb/s in a single optical lane; a further upgrade to 400 Gb/s is envisaged through
wavelength-multiplexing of multiple 100 Gb/s strands. In terms of modulation formats, PAM-4 and PAM-8 are
considered the front-runners, striking a good balance between bandwidth-efficiency and implementation complexity. In
addition, the emergence of energy-efficient, high-speed CMOS digital-to-analog converters (DACs) opens up new
possibilities: Spectral shaping through digital filtering will allow squeezing even more data through low-cost, low-bandwidth
In this work we demonstrate an optical interconnect based on an EAM that is driven directly with sub-volt electrical
swing by a 65 GSa/s arbitrary waveform generator (AWG). Low-voltage drive is particularly attractive since it allows
direct interfacing with the switch/server ASIC, eliminating the need for dedicated, power-hungry and expensive
electrical drivers. Single-wavelength throughputs of 180 and 120 Gb/s are experimentally demonstrated with 60 Gbaud
optical PAM-8 and PAM-4 respectively. Successful transmission over 1250 m SMF is achieved with direct-detection,
using linear equalization via offline digital signal processing in order to overcome the strong bandwidth limitation of the
overall link (~20 GHz). The suitability of Nyquist pulse shaping for optical interconnects is also investigated
experimentally with PAM-4 and PAM-8, at a lower symbol rate of 40 Gbaud (limited by the sampling rate of the AWG).
To the best of our knowledge, the rates achieved are the highest ever using optical PAM-M formats.
Datacenter traffic is exploding. Ongoing advancements in network infrastructure that ride on Moore’s law are unable to
keep up, necessitating the introduction of multiplexing and advanced modulation formats for optical interconnects in order
to overcome bandwidth limitations, and scale lane speeds with energy- and cost-efficiency to 100 Gb/s and beyond. While
the jury is still out as to how this will be achieved, schemes relying on intensity modulation with direct detection (IM/DD)
are regarded as particularly attractive, due to their inherent implementation simplicity. Moreover, the scaling-out of
datacenters calls for longer transmission reach exceeding 300 m, requiring single-mode solutions.
In this work we advocate using 16-QAM sub-cycle Nyquist-SCM as a simpler alternative to discrete multitone (DMT),
but which is still more bandwidth-efficient than PAM-4. The proposed optical interconnect is demonstrated at 112 Gb/s,
which, to the best of our knowledge, is the highest rate achieved in a single-polarization implementation of SCM. Off-the-shelf
components are used: A DFB laser, a 24.3 GHz electro-absorption modulator (EAM) and a limiting photoreceiver,
combined with equalization through digital signal processing (DSP) at the receiver. The EAM is driven by a low-swing
(<1 V) arbitrary waveform generator (AWG), which produces a 28 Gbaud 16-QAM electrical signal with carrier frequency
at ~15 GHz. Tight spectral shaping is leveraged as a means of maintaining signal fidelity when using low-bandwidth
electro-optic components; matched root-raised-cosine transmit and receive filters with 0.1 excess bandwidth are thus
employed. Performance is assessed through transmission experiments over 1250 m and 2000 m of SMF.