This paper explores photonic-based analog fronthaul solutions for 6G, highlighting their effectiveness in meeting the RF requirements of standards, supporting future distributed-MIMO networks, and providing insights into prospective solutions for radios in potential 6G bands.

We experimentally demonstrate a room-temperature LWIR FSO link with a 9.1-μm directly modulated QCL and an MCT detector. Net bitrate of up to 16.9 Gb/s is achieved at both 15°C and 20°C over a 1-meter distance.

We experimentally demonstrate a room-temperature LWIR FSO link with a 9.1-μm directly modulated QCL and an MCT detector. Net bitrate of up to 16.9 Gb/s is achieved at both 15°C and 20°C over a 1-meter distance.

This study investigates the potential of long-wave infrared (LWIR) free-space optical (FSO) transmission using multilevel signals to achieve high spectral efficiency. The FSO transmission system includes a directly modulated-quantum cascade laser (DM-QCL) operating at 9.1 µm and a mercury cadmium telluride (MCT) detector. The laser operated at the temperature settings of 15°C and 20°C. The experiment was conducted over a distance of 1 m and in a lab as a controlled environment. We conduct small-signal characterization of the system, including the DM-QCL chip and MCT detector, evaluating the end-to-end response of both components and all associated electrical elements. For large-signal characterization, we employ a range of modulation formats, including non-return-to-zero on-off keying (NRZ-OOK), 4-level pulse amplitude modulation (PAM4), and 6-level PAM (PAM6), with the objective of optimizing both the bit rate and spectral efficiency of the FSO transmission by applying pre- and post-processing equalization. At 15°C, the studied LWIR FSO system achieves net bitrates of 15 Gbps with an NRZ-OOK signal and 16.9 Gbps with PAM4, both below the 6.25% overhead hard decision-forward error correction (6.25%-OH HD-FEC) limit, and 10 Gbps NRZ-OOK below the 2.7% overhead Reed-Solomon RS(528,514) pre-FEC (KR-FEC limit). At 20°C, we obtained net bitrates of 14.1 Gbps with NRZ-OOK, 16.9 Gbps with PAM4, and 16.4 Gbps with PAM6. Furthermore, we evaluate the BER performance as a function of the decision feedback equalization (DFE) tap number to explore the role of equalization in enhancing signal fidelity and reducing errors in FSO transmission. Our findings accentuate the competitive potential of DM-QCL and MCT detector-based FSO transceivers with digital equalization for the next generation of FSO communication systems.

This article highlights the potential of photonic-assisted analog fronthaul solutions, particularly analog radio-over-fiber (ARoF) and analog radio-over-free-space-optics (ARoFSO), as prospective alternatives for the development of 6G applications. First, we present (New-Radio) NR/5G conformance testing of ARoF and ARoFSO fronthaul links, including the assessment of the error vector magnitude (EVM) and adjacent channel leakage power ratio (ACLR) to demonstrate compliance with the minimum transmitter requirements outlined by the 3rd Generation Partnership Project (3GPP) standards. Then, with focus on future 6G Distributed-MIMO (D-MIMO) networks, we conduct experimental validations of coherent joint transmissions (CJT) using ARoF and ARoFSO fronthaul links in a 2-transmitter D-MIMO network, demonstrating MIMO gains of up to 5.35 dB and that these links meet the stringent synchronization demands for CJT. These tests represent the first realizations of CJT utilizing ARoF and ARoFSO links. Finally, for consistency, we validate CJT in a 4-transmitter D-MIMO network with ARoF fronthaul links, with MIMO gains up to 9.4 dB and confirming our previous results. This evidence indicates that these technologies hold significant potential for applications in future 6G systems.

We experimentally validate coherent joint transmission (CJT) in a D-MIMO system with four transmitters using analog fronthaul and RoF links, fulfilling CJT stringent synchronization requirements. MIMO gains close to theoretical values are demonstrated.

We experimentally validate coherent joint transmission (CJT) in a D-MIMO system with four transmitters using analog fronthaul and RoF links, fulfilling CJT stringent synchronization requirements. MIMO gains close to theoretical values are demonstrated.

We summarize our recent experimental studies of free-space communications enabled by directly modulated quantum cascade lasers at both MWIR and LWIR regions. Different detector types with different characteristics are compared.

We summarize our recent experimental studies of free-space communications enabled by directly modulated quantum cascade lasers at both MWIR and LWIR regions. Different detector types with different characteristics are compared.

The sixth generation (6G) of mobile networks will enable novel services and applications but new challenges are expected to arise as well. As a result, novel implementation techniques need to be developed. Among these, distributed multiple-input multiple-output (D-MIMO) is a promising wireless technology that can provide higher coverage and spectral efficiency, both of which are essential to fulfill some of the new 6G requirements. To demonstrate the feasibility of DMIMO technology, we experimentally validate coherent joint transmission (CJT) in a D-MIMO network that uses analog radio-over-fiber (ARoF) fronthaul links and centralized processing, achieving signal-to-noise ratio (SNR) gains of up to 10.54 dB.