We develop a method to control the group delay of electromagnetic waves continuously using a doubly resonant metasurface. The method is based on the dependences of i) the group velocity in a medium featuring two resonance lines on the resonance linewidths and ii) the resonance linewidth of a metasurface composed of split-ring resonators on an incidence angle of electromagnetic wave. To verify this method for group-delay control, we design a terahertz metasurface composed of two split-ring resonators with different resonance frequencies and numerically analyze the transmission characteristic of the metasurface. Double resonance lines are observed for oblique incidence and the resonance transmission dips become deeper and broader with increasing the incidence angle. The group delay at around the center frequency of the double resonance lines is found to vary in the range about from 0 s to 20 times the period of the incident wave with the incidence angle. In contrast with a previously reported method for variable control of group delay using electromagnetically-induced-transparency-like metamaterials, a high transmittance is achieved for a small group delay condition.
This paper develops the first prototype hardware for TDD two-way multi-hop relay network with MIMO network coding. Since conventional wireless multi-hop relay networks have a drawback of low data rate, TDD two-way multi-hop relay networks have been studied as a solution to realize high data rate recently. In the networks, forward and backward streams are spatially multiplexed by using interference cancellation techniques such as MIMO beamforming or MIMO network coding. In this paper, demonstration system for TDD two-way multihop relay network with MIMO network coding (called 2-way relay network hereafter) is developed by using the prototype hardwares. In the demonstration system, each transmitter and receiver performs network coded broadcast and MIMO multiple access respectively. By using the demonstration system, network throughput is measured in a real indoor environment to prove the realization and effectiveness of the 2-way relay network. The network throughput of the 2-way relay network is about twice as large as that of the simple TDD one-way multi-hop relay network (1-way relay network), and higher than that of the direct link at all average end-to-end SNRs. From the result, the realization and effectiveness of the 2-way relay network is proved in the real indoor environment.
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