Characterization of Photonic Frequency Conversion Techniques for mmWave Systems

The increasing demand for higher bandwidth in 6G and future use case scenarios is driving the exploration of untapped spectrum within the mmWave frequency bands. As frequencies rise, coverage per cell diminishes, prompting a transition towards denser cell deployments. To accommodate this shift, a robust radio access network (RAN) must be implemented, capable of efficiently serving a large number of cells while remaining cost-effective and energy-efficient. Optical fiber technology emerges as a viable solution for wireless signal transport in this context [1]. For mmWave operation, heavy signal processing and rigorous requirements for high-speed electronics introduce complexity on signal generation and increased cost. Photonic-aided Frequency Conversion (PFC) techniques may be considered to replace the electronic hardware for mmWave generation in a Centralized RAN scenario, where all the baseband signal processing is done at a central office (CO) and the analog radio signal is sent over the fiber to the Remote Radio Head, greatly reducing the hardware complexity at the remote site. To increase the capacity of the optical channels, subcarrier multiplexing (SCM) can be used to serve multiple users over the same link [2]. PFC techniques for the downlink (DL) channel have been explored in two-stage architectures where the multiplying tone is applied to a Mach-Zehnder modulator (MZM) biased at the null-point (NP) for frequency multiplication by two, while the data is modulated either directly over the laser current [2] or in a separate MZM biased at the quadrature point [3]. This work is devoted to the study of PFC for the DL channel obtained by simultaneous data modulation and frequency multiplication by a factor of three in a single MZM, with subcarrier multiplexing to enable multi-user support in a back-to-back configuration. Results based on simulations of the basic scheme shown in Figure 1 are presented and discussed.