In this paper, we investigate the outage performance for simultaneous wireless information and power transfer (SWIPT) relaying systems in the presence of direct link between the source and the destination. For the SWIPT relaying system, we employ a power splitter (PS) at the relay, which splits the received signal into the information transmission and the energy harvesting parts. First, we provide an analysis of the outage probability as a closed-form based on a high signal-to-noise ratio approximation. From the analysis, it is recognized that the diversity order of the SWIPT relaying systems equals that of the non-SWIPT cases regardless of the energy harvesting efficiency. The closed-form outage expression also enables us to obtain a simple expression for the PS factor which minimizes the outage probability. Simulation results demonstrate the accuracy of the derived analysis and the efficiency of the proposed PS scheme.

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In this paper, we investigate minimum mean squared error (MMSE) based amplify-and-forward cooperative multiple antenna relaying systems where a non-negligible direct link exists between the source and the destination. First, we provide a new design strategy for optimizing the relay amplifying matrix. Instead of conventional optimal design approaches resorting to an iterative gradient method, we propose a near optimal closedform solution which provides an insight. As relay systems with a direct link incur a non-convex problem in general, we exploit the decomposable property of the error covariance matrix and a relaxation technique imposing a structural constraint on the problem. Next, we study the error performance limit of the proposed scheme using diversity-multiplexing tradeoff analysis, which leads to several interesting observations on MMSEbased cooperative relaying systems. Finally, through numerical simulations, we confirm that the proposed solution shows the performance very close to the optimum with much reduced complexity and the analysis closely matches with simulation results.