Abstract
Cell-free massive multiple-input multiple-output (MIMO) has attracted wide attention as wireless spectral efficiency has become a 6G key performance indicator. The distributed scheme improves the spectral efficiency and user fairness, but the fronthaul network must evolve to enable it. This work demonstrates a fronthaul network for distributed antenna systems enabled by the bit-interleaved sigma-delta-over-fiber (BISDoF) concept: multiple sigma-delta modulated baseband signals are time-interleaved into one non-return-to-zero (NRZ) signal, which is converted to the optical domain by a commercial QSFP and transmitted over fiber. The BISDoF concept improves the optical bit-rate efficiency while keeping the remote unit complexity sufficiently low. The implementation successfully deals with an essential challenge—precise frequency synchronization of different remote units. Moreover, owing to the straightforward data paths, all transceivers inherently transmit or receive with fixed timing offsets which can be easily calibrated. The error vector magnitudes of both the downlink and uplink data paths are less than 2.8% (–31 dB) when transmitting 40.96 MHz-bandwidth OFDM signals (256-QAM) centered around 3.6 GHz. (Optical path: 100 m multi-mode fibers; wireless path: electrical back-to-back.) Without providing an extra reference clock, the two remote units were observed to have the same carrier frequency; the standard deviation of the relative jitter was 9.43 ps.
Highlights
As communication systems evolve from one generation to another, the demands on system capacity, energy efficiency, and cost continuously become more challenging [1,2]
The distributed unit (DU) (Figure 5a) comprises a personal computer (PC) and a Hitech Global HTG-930 board, which connects to the PC via the peripheral component interconnect express (PCIe) interface and has a Xilinx Virtex UltraScale+ FPGA (VU13P). (FPGA: field-programmable gate array.) The board is connected to a four-port QSFP FMC module. (QSFP: quad small form-factor pluggable; FMC: FPGA mezzanine card.) This PC is used for performance monitoring
On the DU FPGA, sixteen parallel low-pass sigma-delta modulation (SDM) are implemented to serve two remote radio unit (RRU) as four I-Q pairs are required by each RRU; the SDMs modulate the baseband signals at 3.6864 Gbps
Summary
As communication systems evolve from one generation to another, the demands on system capacity, energy efficiency, and cost continuously become more challenging [1,2]. This work demonstrates an SDoF-based fronthaul network in which multiple sigmadelta modulated baseband signals are time-interleaved and transmitted over fiber. In [12], the sigma-delta modulated I and Q signals are digitally up-converted [17] before being transmitted over fiber. If analog filters are added (Figure 3b), digital up-conversion is not possible and an analog up-converter must be included In this option, fΣ∆ can be chosen freely based on the signal quality requirement, denoted as f in the figure. For the ease of implementation, the proposed network has the same architecture for both directions
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