Delta-Sigma Modulation for Next Generation Fronthaul Interface

A new function split option for the next generation fronthaul interface (NGFI) is demonstrated based on all-digital RF transmitter using bandpass delta-sigma modulation. Different from other low layer split (LLS) options, such as option 6 (MAC-PHY), 7 (high-low PHY), and 8 (CPRI), the proposed option 9 implements RF functions in the digital domain, and splits within the RF layer, with high-RF layer centralized in the distributed unit (DU) and low-RF layer distributed in remote radio units (RRUs). A proof-of-concept all-digital RF transmitter based on real-time delta-sigma modulation is implemented using a Xilinx Virtex-7 FPGA. A 5-GSa/s delta-sigma modulator is demonstrated to encode LTE/5G signals with bandwidth up to 252 MHz and modulation format up to 1024-QAM to a 5-Gb/s OOK signal, which is transmitted over 30-km single-mode fiber from DU to RRU. To relax the FPGA speed requirement, a 32-pipeline architecture is designed. Two-carrier aggregation of 5G and 14-carrier aggregation of LTE signals are demonstrated with error vector magnitude (EVM) performance satisfying the 3GPP specifications. Compared with option 8 (CPRI), although the proposed option 9 split occurs at a lower level, it offers improved spectral efficiency and reduced NGFI data rate than CPRI. Moreover, other LLS options, such as 6, 7, and 8, all require a complete RF layer implemented in the analog domain at remote cell sites; whereas option 9 realizes high-RF layer in the digital domain at DU, and eliminates the need of analog RF devices, such as DAC, local oscillator and mixer at RRU, which not only makes low-cost, energy-efficient, and small-footprint cell sites possible for the wide deployment of small cells, but also paves the road toward software defined radio (SDR) and virtualization of DU and RRU for improved compatibility and reconfigurability among multiple radio access technologies (multi-RATs). Given its centralized architecture and deterministic latency, option 9 is suitable for radio coordination applications, and has potential in low-frequency narrowband scenarios with cost, power, and/or size sensitive cell sites, such as massive machine type communication (mMTC) and narrowband internet of things (NB-IoT).