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Modern communications systems use multiple-input multiple-output (MIMO) and high-order QAM constellations for maximizing spectral efficiency. However, as the number of antennas and the order of the constellation grow, the design of efficient and low-complexity MIMO receivers possesses big technical challenges. For example, symbol detection can no longer rely on maximum likelihood detection or sphere-decoding methods, as their complexity increases exponentially with the number of transmitters/receivers. In this paper, we propose a low-complexity high-accuracy MIMO symbol detector based on the Expectation Propagation (EP) algorithm. EP allows approximating iteratively at polynomial-time the posterior distribution of the transmitted symbols. We also show that our EP MIMO detector outperforms classic and state-of-the-art solutions reducing the symbol error rate at a reduced computational complexity. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a new beamforming technique that maximizes the end-to-end signal-to-noise ratio (SNR) foramplify-and-forward multiple-input–multiple-output cooperative relaying systems with direct link between thesource and the destination. Instead of conventional schemes resorting to an iterative method, such as a gradient ascentalgorithm, our scheme provides a simple closed-form solution for source-relay joint beamformer designs. To this end, wefirst derive a new expression of the end-to-end SNR for the cooperative relaying systems and its lower bound, which isgiven as the harmonic mean of two individual SNRs. Then, a new beamforming scheme, which adaptively optimizes one of thetwo SNRs depending on the channel condition, is proposed. In addition, we perform a diversity order analysis of theproposed scheme and show that our scheme achieves a full diversity order of relaying systems with direct link. It isconfirmed by simulation results that the proposed technique obtains almost identical performance to the gradient ascentalgorithm with much reduced complexity, and our analytical work accurately predicts the numerical results. [ABSTRACT FROM AUTHOR]