Offshore assets in the South China Sea, such as oil and gas platforms, offshore wind farms, maritime supervision nodes, and emergency response bases, are increasingly moving toward deep-water, far-offshore, and long-duty operations, which require backhaul links with high throughput, low latency, and high availability. Conventional line-of-sight microwave is limited by range, and geostationary Earth orbit satellites suffer from high latency. Therefore, this study develops and validates a practical, over-the-horizon (OTH) microwave communication solution that leverages evaporation ducts within the marine atmospheric boundary layer. The objectives are threefold: (ⅰ) establish a predict-then-deploy workflow that integrates a parabolic-equation (PE) propagation model with environmental data and engineering constraints; (ⅱ) quantify capacity-stability co-design via polarization multiplexing and link aggregation for capacity and spatial diversity for stability; and (ⅲ) verify engineering applicability using long-range field measurements from representative South China Sea links.

We develop a PE-based maritime radio channel model that incorporates refractivity profiles of the marine boundary layer, rough-sea impedance boundary conditions, and coastal/terrain data to predict transmission loss along candidate paths. The modeling follows a forward-propagating, narrow-angle PE with split-step fast Fourier transform marching, using antenna patterns as initial conditions and an absorbing layer aloft to mitigate spurious reflections. The transmission loss is referenced to free space at 1 m and sampled at the transmitter/receiver heights, adhering to standard engineering link-budgeting conventions. On the system side, we design a "microwave-primary, scatter/satellite-assisted" architecture for offshore production networks. Capacity enhancement is achieved through dual-polarization multiplexing and physical-layer link aggregation, and stability is improved using spatial diversity with combined reception to counteract sea-surface multipath, evaporation-duct variability, and platform motion. We instrument a typical land-to-platform link (135.3 km; land site altitude ≈ 420 m) for one month, recording adaptive modulation/coding states, received signal level (RSL), transmit power, bandwidth occupancy, and antenna-combining modes. End-to-end user datagram protocol (UDP) tests (iperf) are conducted to validate whether the configured capacity is realized at the service layer.

The PE model accurately reproduces long-range evaporation-duct propagation. For the 135.3 km link, the measured maximum path loss is 209.89 dB, and the PE-predicted maximum is 208.59 dB, resulting in a difference of 1.30 dB under synchronized environmental conditions, confirming the model's predictability and applicability for planning and operations. Long-term statistics indicate that combined reception (spatial diversity) is the predominant mode during most periods, enhancing link availability and maintaining the decision margin required for higher-order adaptive modulation. Time-occupancy analysis indicates that high-order modes (16/32/64QAM) dominate, whereas low-order modes (QPSK/4/8QAM) only appear briefly during adverse channel conditions. Consistent with these distributions, daily RSL ranges remain high on most days, with occasional troughs attributable to boundary-layer changes, sea-surface multipath, or weather transitions. The system' s capacity envelope shows a stable lower bound (≈ 26.355 Mb/s under conservative ACM settings) and a peak near 184.079 Mb/s at 64QAM. When aggregating three links with a 1∶ 1 split between uplink and downlink, the configured aggregate capacity reaches ≈ 1.1 Gbps, which is verified at the service layer: end-to-end UDP throughput achieves ~557 Mb/s uplink and ~551 Mb/s downlink (total ~1.108 Gb/s) under 40 MHz channels, 64QAM, and combined reception as the normal operating mode. These results confirm that the "capacity via polarization/aggregation + stability via spatial diversity" co-design achieves high spectral efficiency and availability on long OTH paths in realistic sea states.

By integrating a PE-based, environment-coupled propagation model with an engineering architecture that optimizes capacity and stability, we link propagation predictability to operational utility for OTH microwave communication in the South China Sea. The modeling-measurement agreement (≤ 1.30 dB on maximum path loss at 135.3 km) provides a foundation for scenario-specific planning and parameter selection. In continuous operation, spatial-diversity combining emerges as the default mode, significantly enhancing availability and sustaining higher-order modulation over extended periods, whereas polarization multiplexing and link aggregation yield near-linear capacity scaling up to the tested ~1.1 Gb/s aggregate. The proposed workflow and parameter baselines are reproducible for deep-water, far-offshore facilities requiring low-latency, high-availability backhaul, offering a transferable template for the deployment and evolution of microwave-centric offshore networks in evaporation-duct-prone maritime environments.