Microwave over-the-horizon high-speed communication technology for offshore facilities in the South China Sea

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.