Digital Mobile Fronthaul Based on Adaptive Differential Pulse Code Modulation

A novel signal quantization scheme based on adaptive differential pulse code modulation (ADPCM) is proposed and experimentally demonstrated. By adaptively adjusting the quantization step size using a small number of quantization bits (QBs), the spectral efficiency (SE) can be improved by a factor of 3∼5 while maintaining a high signal-to-quantization-noise ratio (SQNR), compared to the methods based on PCM and DPCM used in Common Public Radio Interface (CPRI)/ enhanced CPRI (eCPRI) architectures. A proof-of-concept experiment is conducted to verify the feasibility. After applying 3-bit and 4-bit quantization to an OFDM RF signal at 5 GHz with a bandwidth of 500 MHz, the resulting SQNR are 24.9 dB and 30.81 dB, respectively. Compared to PCM and DPCM methods, the ADPCM 3-bit and 4-bit schemes provide 14 dB and 7.5 dB improvements in SQNR, respectively. The bitstream generated by the ADPCM 3-bit and 4-bit quantization schemes are transmitted 10 km using 4-level Pulse Amplitude Modulation (PAM4) with intensity modulation/direct detection (IM/DD), and then transmitted over 2m via RF wireless transmission. Additionally, the flexibility of the proposed scheme is also evaluated.