This preprint consolidates the second study in a research program on phase-robust finite-alphabet superposition. It connects exact constellation geometry to coded-service feasibility for two-user 16-QAM superposition under a fixed-frame AWGN model.
The manuscript develops (i) a radial reduction for worst-phase labeled separation, (ii) a phase-uniformity analysis, (iii) service-feasibility converses that progress from Fano/Gaussian capacity through finite-alphabet and finite-blocklength bounds, and (iv) an explicit amplitude-ratio feasibility interval. At the Paper 2 operating point, the geometric ratio r = 0.19074357 is outside the retained necessary region; a code-specific moment refinement gives a stronger decoder-independent lower bound for that allocation. A dense oracle study and a paired interference-removal/decoder-iteration study then locate a narrow, unresolved-to-supported boundary for the tested finite code and five oracle phase values.
The evidence package includes the 27-page manuscript, LaTeX source, complete project archive, verification and audit files, checksums, protocols, and reproducibility notes. The release does not claim an exact coded optimum, a practical continuous-phase receiver, an RF or hardware advantage, an energy saving, or commercial superiority. The QAM ratio is discussed with its relevant prior-art context. No new physical or RF trials are included.