A new function split option 9 based on delta-sigma modulation is proposed for the next generation fronthaul interface (NGFI). All existing low layer split (LLS) options, such as 6 (MAC-PHY), 7 (high-low PHY), and 8 (CPRI) require a complete RF layer implemented in the analog domain at remote cell sites; whereas the proposed option 9 can implement RF layer in the digital domain and moves the split point between the distributed unit (DU) and remote radio unit (RRU) into the RF layer. With the help of delta-sigma modulation, high-RF layer functions are centralized in the DU and lowRF layer distributed in RRUs. Although it splits at a lower level than conventional option 8 (CPRI), option 9 offers improved spectral efficiency and reduced fronthaul traffic than CPRI. Moreover, it implements all RF functions in the digital domain and eliminates the need of analog devices required by other LLS options, such as DAC, local oscillator, mixer, power amplifier. Therefore, option 9 enables all-digital RF transmitter and RF layer virtualization, which makes low-cost, energy-efficient, and small-footprint cell sites possible for the wide deployment of 5G small cells.
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