Improving performance of differential pulse coding modulation based digital mobile fronthaul employing noise shaping.

Abstract

A differential pulse coding modulation with noise shaping (NS-DPCM) is proposed to achieve better error vector magnitude (EVM) performance in a digital mobile fronthaul. Compared to previous DPCM based digital mobile fronthaul, a feedback loop combined with a finite-impulse-response (FIR) filter is added to the quantizer of DPCM encoder to operate as a quantization noise shaping technique block. The noise shaping technique increases the signal-to-quantization noise ratio by reshaping the spectrum of the quantization noise. Therefore, the noise power is at a lower level in subcarriers of OFDM signal where data is modulated and at a higher level in the subcarriers where no data is modulated. Different from the noise shaping of delta-sigma modulation, the proposed scheme utilizes the existence of unused subcarriers of OFDM signal, thus does not require oversampling at the transmitter and low pass filter at the receiver. In the experiment, the proposed NS-DPCM based mobile fronthaul transmission is demonstrated in a 25-Gb/s PAM-4 intensity modulation-direct detection optical link. Compared to the existing DPCM based mobile fronthaul, significant EVM performance improvement is achieved using the same number of quantization bits.