A new high-speed delta-sigma modulator (DSM) topology is proposed by cascading a bit reduction process with a multi-stage noise shaping MASH-1-1 DSM. This process converts the two-bit output sequence of the MASH-1-1 DSM to a singlebit sequence, merely compromising the DSM noise-shaping performance. Furthermore, the high clock frequency requirements are significantly relaxed by using parallel processing. This DSM topology facilitates the design of e.g. wideband software defined radio (SDR) transmitters and delta-sigma radio-over-fiber transmitters. Experimental results of the FPGA implementation show that the proposed low-pass DSM can operate at 21 GS/s, providing 520 MHz baseband bandwidth with 42.76 dB signal-to-noiseand-distortion ratio (SNDR) or 1.1 GHz bandwidth with 32.04 dB SNDR (based on continuous wave measurements). An all-digital transmitter based on this topology can generate 218.75 MBd 256-QAM over 200 m OM4 multimode fiber in real-time, with 7-GS/s sampling rate and an error vector magnitude below 1.89%.

Cell-free massive multiple-input multiple-output (MIMO) has attracted wide attention as wireless spectral efficiency has become a 6G key performance indicator. The distributed scheme improves the spectral efficiency and user fairness, but the fronthaul network must evolve to enable it. This work demonstrates a fronthaul network for distributed antenna systems enabled by the bit-interleaved sigma-delta-over-fiber (BISDoF) concept: multiple sigma-delta modulated baseband signals are time-interleaved into one non-return-to-zero (NRZ) signal, which is converted to the optical domain by a commercial QSFP and transmitted over fiber. The BISDoF concept improves the optical bit-rate efficiency while keeping the remote unit complexity sufficiently low. The implementation successfully deals with an essential challenge—precise frequency synchronization of different remote units. Moreover, owing to the straightforward data paths, all transceivers inherently transmit or receive with fixed timing offsets which can be easily calibrated. The error vector magnitudes of both the downlink and uplink data paths are less than 2.8% (–31 dB) when transmitting 40.96 MHz-bandwidth OFDM signals (256-QAM) centered around 3.6 GHz. (Optical path: 100 m multi-mode fibers; wireless path: electrical back-to-back.) Without providing an extra reference clock, the two remote units were observed to have the same carrier frequency; the standard deviation of the relative jitter was 9.43 ps.