Pre-filtered and post-filtered 1-bit delta sigma modulator for fronthaul downlinks

The fifth generation (5G) mobile fronthaul transmission system is being investigated, and one potential solution is to use 1-bit bandpass delta–sigma modulation, thus a bandpass filter (BPF) can be used to convert a digital 1-bit signal back to original analog RF signal at remote radio units. It is inevitable that this bandpass filter can induce phase and amplitude distortion to the signal, which may severely distort the signal integrity. In this work, two novel delta sigma modulators (DSM), called pre-filtered and post-filtered DSM, are proposed to compensate for the above both phase and amplitude distortion. The two designs are verified by using both simulation and experimentation with radio-over-fiber (RoF) links up to 20 km. The 5G signal used in the simulation and experimentation is a 200 MHz 64-QAM orthogonal frequency-division multiplexing signal centered at 2.5 GHz. An error vector magnitude is improved by up to ∼19 dB using the above two DSMs.