Beam-steer feasibility study for a moving broadcast source (#137)#148
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…lative) Quantifies the go/no-go: an N-element array buys up to 10log10(N) dB directive gain, but it must point, and a broadcast FPV downlink has no per-packet CSI — only a slow, coarse return-channel direction, on a moving platform. The sim models the array-factor roll-off vs pointing error and a pointing-error budget (rate x update-latency + CSI resolution), and compares the realised beam gain against the measured feedback-free STBC gain (2.5 dB, no pointing). Verdict: for a moving drone (e.g. 90 deg/s, 0.2 s return-channel latency, 10 deg CSI resolution) the ~28 deg pointing error lands the beam in a null (-17 dB) — far worse than a single antenna. Beam-steering a moving broadcast source is a no-go; feedback-free diversity (STBC/MRC, measured) wins. Only a slow / known-geometry case (fixed relay, stabilised gimbal) clears STBC. tools/precoder/beamsteer_feasibility_sim.py + pytest; --sweep and --self-test. Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
josephnef
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This was referenced Jul 2, 2026
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Summary
The last (speculative) roadmap item, #137 — a feasibility study for TX
beam-steering a moving broadcast source, grounded in a small sim rather than
hand-waving.
The question
Coherent N-element beamforming buys up to 10·log10(N) dB directive gain (and its
EIRP-per-PA-watt scales with N — the #135 energy lever at its strongest). But it
must point, and a broadcast FPV downlink has no per-packet CSI — only a slow,
coarse direction from the return channel, on a moving platform. Does the
realised gain, after the pointing error a moving source suffers, beat the
feedback-free diversity (STBC/MRC) we already measured — which needs no pointing?
Model & result
Array-factor roll-off vs pointing error, and a pointing-error budget
= rate × update-latency + CSI resolution, compared to the measured STBC gain(2.5 dB).
pointing error 28° → beam lands in a null, -17 dB — far worse than a
single antenna.
Verdict
No-go for a moving broadcast source — the pointing budget blows long before
the beam pays, so feedback-free diversity (STBC/MRC, measured to work) wins.
Beam-steering is only viable for a slow or known-geometry case (fixed relay,
stabilised gimbal). This closes #137 as assessed.
Idealisation caveat: an ideal λ/2 ULA — an upper bound, since real adapter
antennas aren't a calibrated phased array.
Contents
tools/precoder/beamsteer_feasibility_sim.py(+ pytest):--sweep,--self-test.--self-testgreen; 5 pytest pass. This completes the spatial-diversityroadmap (#127): #128–#136 landed, #137 assessed no-go.
🤖 Generated with Claude Code