Fast Solution of l1-Norm Minimization Problems When the Solution May Be Sparse

The minimum `1-norm solution to an underdetermined system of linear equations y = Ax, is often, remarkably, also the sparsest solution to that system. This sparsity-seeking property is of interest in signal processing and information transmission. However, general-purpose optimizers are much too slow for `1 minimization in many large-scale applications. The Homotopy method was originally proposed by Osborne et al. for solving noisy overdetermined `1-penalized least squares problems. We here apply it to solve the noiseless underdetermined `1-minimization problem min ‖x‖1 subject to y = Ax. We show that Homotopy runs much more rapidly than general-purpose LP solvers when sufficient sparsity is present. Indeed, the method often has the following k-step solution property: if the underlying solution has only k nonzeros, the Homotopy method reaches that solution in only k iterative steps. When this property holds and k is small compared to the problem size, this means that `1 minimization problems with k-sparse solutions can be solved in a fraction of the cost of solving one full-sized linear system. We demonstrate this k-step solution property for two kinds of problem suites. First, incoherent matrices A, where off-diagonal entries of the Gram matrix AA are all smaller than M . If y is a linear combination of at most k ≤ (M−1 + 1)/2 columns of A, we show that Homotopy has the k-step solution property. Second, ensembles of d × n random matrices A. If A has iid Gaussian entries, then, when y is a linear combination of at most k < d/(2 log(n)) · (1 − n) columns, with n > 0 small, Homotopy again exhibits the k-step solution property with high probability. Further, we give evidence showing that for ensembles of d×n partial orthogonal matrices, including partial Fourier matrices, and partial Hadamard matrices, with high probability, the k-step solution property holds up to a dramatically higher threshold k, satisfying k/d < ρ̂(d/n), for a certain empirically-determined function ρ̂(δ). Our results imply that Homotopy can efficiently solve some very ambitious large-scale problems arising in stylized applications of error-correcting codes, magnetic resonance imaging, and NMR spectroscopy. Our approach also sheds light on the evident parallelism in results on `1 minimization and Orthogonal Matching Pursuit (OMP), and aids in explaining the inherent relations between Homotopy, LARS, OMP, and Polytope Faces Pursuit.