This paper presents outage probability analysis and a practical algorithm for antenna selection in multiple-input multiple-output wireless communication systems employing space-time block codes (STBC). First, to minimize the outage probability in these systems, a satisfactory antenna selection criterion for an STBC is to maximize the channel Frobenius norm. Analysis shows that the more receive antennas are selected, the better the performance. However, the performance of transmit antenna selection heavily depends on how fast the channel changes. When the channel changes slowly, since STBC averages the channel gains of the selected transmit antennas, selecting more transmit antennas causes lower coding gain and thus higher outage probability. When the channel is fast changing, it is shown analytically that the system can no longer provide transmit selection diversity in the high SNR regime. Since the transmit diversity can be still provided by using STBC, the best STBC scheme varies with SNR. Although the outage analysis helps determine the STBC scheme, finding the optimal antenna subsets with maximum channel Frobenius norm for each fading state is still a challenging problem. This is because solving the problem optimally requires an exhaustive search with exponentially growing complexity. When the numbers of antennas are large, the problem becomes intractable. To reduce the complexity, this problem is formulated as a quadratically constrained quadratic programming (QCQP) problem. Despite the fact that the problem is nonconvex, a semidefinite relaxation of QCQP enables the problem to be solved approximately in polynomial time. Simulation results indicate that the loss of semidefinite relaxation to optimal selection is negligible.Index Terms-Antenna selection, multiple-input-multipleoutput (MIMO), outage analysis, space-time block codes (STBC).

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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 zeroforcing 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 sumrate 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.

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