This paper presents a practical algorithm for antenna selection in multiple-input multiple-output wireless communication systems employing space-time block codes (STBC). It first shows that maximizing the channel Frobenius norm helps maximize the mutual information for both orthogonal STBC and quasi-orthogonal STBC. However, the computational complexity for finding the optimal antenna subset grows exponentially with the number of antennas. This paper identifies that the channel Frobenius norm maximization problem can be formulated as a quadratically constrained quadratic programming (QCQP) problem. Then, despite the fact that the problem is non-convex, a semidefinite relaxation of QCQP enables the problem to be solved approximately by semidefinite programming in polynomial time. Simulation results indicate that the loss of semidefinite relaxation is negligible. It is also shown that although the combination of STBC and antenna selection is not always beneficial, it is a robust transmission strategy in the high SNR regime when only imperfect channel information is available.

This paper proposes a resource-allocation technique for Gaussian multiple-input multiple-output, orthogonal- frequency-division-multiplexing (MIMO-OFDM) multiple-access channel (MAC). Zero-forcing generalized decision-feedback equalizers (ZF-GDFE) are employed at the receiver to cancel completely the interferences produced by all the other users, while a novel matrix decomposition method based on the generalized channel singular-value-decomposition (SVD) is used to find the precoding matrices at each user's transmitter. The achievable rate region is obtained by solving a series of weighted sum-rate maximization problems, and the associated users' power distributions are efficiently found by a Lagrange dual- decomposition method along with a per-tone-based exhaustive search of decoding orders among users. The complexity can be further reduced by replacing the exhaustive search with a greedy algorithm. Simulation results show that the achievable rate region obtained by the greedy algorithm is very close to that achieved by the exhaustive search. Therefore, with a significant computational complexity reduction over the canonical minimum-mean- squared-error (MMSE) -based GDFE, the proposed algorithm provides an ideal scheme for practical systems such as uplink wireless MIMO-OFDM systems and upstream vectored digital subscriber lines (DSL).

This correspondence presents new rate-1 space-time block codes (STBCs) attending full diversity over every quadrature amplitude modulation (QAM) constellation when the number of Tx antennas is a power of two. From the simulation results, our design performs very closely to the quasi-orthogonal code with constellation rotation over 4-QAM and 16-QAM in the case of four Tx antennas over quasi-static Rayleigh fading channels. Moreover, the proposed codes would not cause any constellation expansion over QAM symbols in contrast with the quasi-orthogonal codes with constellation rotations

In upstream vectored DSL transmission, the far-end crosstalk (FEXT) can be completely cancelled by using zero- forcing generalized decision-feedback equalizers (ZF-GDFE). When the spatially correlated alien crosstalk is present, the achievable data rates of DSL lines with ZF-GDFE depend on their decoding orders at each DMT tone. Given a weighted sum-rate maximization problem, the optimal orderings for all DMT tones can be found by the Lagrange dual decomposition method. However, the computational complexity of such approach grows with the factorial of the number of users, which makes the optimal search infeasible with a large number of vectored lines. This paper presents a modified greedy algorithm (MGA) that performs close to the optimal search of decoding orders. The complexity of MGA is only proportional to the cube of the number of users, which is the same as it of QR decomposition. With a significant reduction of complexity, MGA is a promising technique for practical DSL systems.

It has been shown that the quasi-orthogonal space-time block codes (QOSTBCs) can achieve full diversity by rotating the constellations of half the transmitted symbols. Although the rotation angle has been analytically derived for QAM constellations, for arbitrary M-PSK, the coding gain maximizing rotation angle can be found only by computer search. In this paper, a set of QOSTBCs for four transmit antennas that achieves full diversity for M-PSK modulations is presented. The orthogonal designs for real constellations and several QOSTBCs are also included in our formulation. With the same maximum-likelihood decoding complexity, the new codes have BER performance identical to the QOSTBC with constellation rotations. Without any constellation expansion, our scheme is superior to the QOSTBC with constellation rotations in implementations.

It has been shown that a complex orthogonal design that provides full diversity and full transmission rate is not possible for more than two transmit antennas. The paper presents a new class of quasi-orthogonal space-time block codes using a group-theoretic method. The new designs can achieve full diversity for quadrature phase-shift-keying modulation, like the system using rotated constellations. Based upon the analysis of the new codes, a relaxed designing viewpoint for full diversity is proposed for arbitrary signal constellations.