The generalized WAX decomposition validity conjecture

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Let A{\boldsymbol{A}} be given by the Kronecker construction in the source, with A~T=ITP\widetilde{{\boldsymbol{A}}}_\mathrm{T}=\mathbf{I}_{T_{\mathrm{P}}}, and let A~B\widetilde{{\boldsymbol{A}}}_\mathrm{B} be constructed through the stated generalized algorithm. For each iteration i=1,…,NQi=1,\dots,N_\text{Q}, let QiQ_i be the quotient defined by that algorithm, and let NQN_\text{Q} be the iteration at which the dimensions are exhausted. Define

Qtot=Q1+1Q2+1⋱+1QNQ.Q_\mathrm{tot}=Q_1+\frac{1}{Q_2+\frac{1}{\ddots+\frac{1}{Q_{N_\text{Q}}}}}.

Generalized WAX decomposition validity conjecture. A randomly chosen matrix H{\boldsymbol{H}} admits WAX decomposition with probability 11 for the given A{\boldsymbol{A}} if

L≥K1+Qtot.L\geq \frac{K}{1+Q_\mathrm{tot}}.

Moreover, in the regime TP<MP/2+1T_{\mathrm{P}}<M_{\mathrm{P}}/2+1, the first column of A~B\widetilde{{\boldsymbol{A}}}_\mathrm{B}, written as [α1,…,αΦ]T[\alpha_1,\dots,\alpha_\Phi]^\mathrm{T}, should satisfy the same restrictions as in Proposition 2 of the source. The conjecture is motivated by the proposed generalization of earlier valid constructions. A formal proof for general NQN_\text{Q} was not found, although extensive simulations reportedly found no exceptions and produced valid matrices exactly when the stated condition was satisfied.

References

Primary source

Juan Vidal Alegría and Fredrik Rusek, “Trade-Offs in Decentralized Multi-Antenna Architectures: Sparse Combining Modules for WAX Decomposition”, arXiv:2309.04297 (2023).

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