TX beam-steer feasibility study for a moving broadcast source (speculative)

[josephnef]

Part of the spatial-diversity roadmap #127. Depends on #A, #H. Speculative.

Goal

Assess whether transmit beam-steering is feasible for a moving broadcast
source (the drone), where coherent multi-chain combining could focus energy toward
the ground station — the strongest form of the #H energy lever (EIRP per PA-watt
scales with N).

The hard part

Beamforming needs direction/CSI, and a one-way video broadcast has no per-packet
feedback. The adaptive-link return channel gives slow/coarse direction at best, and
the source is moving. This issue is a feasibility study, explicitly downstream
of the feedback-free diversity techniques (#C–#E), not a commitment to implement.

Scope

• Characterize what direction/CSI quality is achievable from the existing return
  channel and airframe attitude, and how stale it is for a moving platform.
• Model the achievable beam-steer gain vs pointing error and update rate.
• Decide go / no-go vs simply lighting up more chains for (feedback-free) diversity
  at the same baseline cost.

[josephnef]

Feasibility assessed → PR #148: no-go for a moving broadcast source. An N=4 array's 6 dB beam gain needs pointing within ~12° to beat STBC, but a moving drone (90°/s, 0.2 s return-channel latency, 10° CSI resolution) has a 28° pointing error → the beam lands in a null (-17 dB), worse than a single antenna. Feedback-free diversity (STBC/MRC, measured) wins; beamforming is only viable for a slow / known-geometry case (fixed relay, gimbal). Closing as assessed.