Abstract-Certain optimization problems in communication systems, such as limited-feedback constant-envelope beamforming or noncoherent M -ary phase-shift keying (M PSK) sequence detection, result in the maximization of a fixed-rank positive semidefinite quadratic form over the M PSK alphabet. This form is a special case of the Rayleigh quotient of a matrix and, in general, its maximization by an M PSK sequence is N P-hard. However, if the rank of the matrix is not a function of its size, then the optimal solution can be computed with polynomial complexity in the matrix size. In this work, we develop a new technique to efficiently solve this problem by utilizing auxiliary continuousvalued angles and partitioning the resulting continuous space of solutions into a polynomial-size set of regions, each of which corresponds to a distinct M PSK sequence. The sequence that maximizes the Rayleigh quotient is shown to belong to this polynomial-size set of sequences, thus efficiently reducing the size of the feasible set from exponential to polynomial. Based on this analysis, we also develop an algorithm that constructs this set in polynomial time and show that it is fully parallelizable, memory efficient, and rank scalable. The proposed algorithm compares favorably with other solvers for this problem that have appeared recently in the literature.Index Terms-Algorithms, maximum likelihood detection, MIMO systems, noncoherent communication, optimization methods, phase shift keying, Rayleigh quotient, sequences.
I. PROBLEM STATEMENT, PRIOR WORK, AND CONTRIBUTION A. Problem statementWe consider the optimization problemwhere is the Euclidean 2 -norm. Problem P in (1) can be recast as the special case of the maximization of the Rayleigh quotient of VV H over the M PSK alphabet, i.e.,and solved by an exponential-complexity exhaustive search among M N −1 length-N sequences.1 However, such a solver would be impractical even for moderate values of the problem size N .Of particular interest is the case where V in P is fullcolumn-rank (i.e., it is a "tall" matrix) and its rank D is independent of its row dimension N , which appears in certain optimization problems in communication systems, such as limited-feedback multiple-input multiple-output (MIMO) beamforming which has polynomial cardinality |S(V)| and can be built with polynomial complexity. After S(V) is constructed, the optimal sequence s opt can be identified by a polynomial-complexity exhaustive search among the elements of S(V).In this work, we present a new algorithm to solve P for any even 2 M and arbitrary D and prove that it has lower complexity than the current state of the art [7], [9], [12], is fully parallelizable and rank-scalable, and requires minimum memory resources.
B. Prior workCase (i): M = 2. A lot of effort has been made to solve P when M = 2, i.e., s is a binary 3 sequence, and V is a realvalued matrix. We note that, for the special case D = 1, V becomes a N ×1 vector and the solution of P is simply s opt = sgn(V). 4 Equivalently, we can say that S(V) = {s...