MIMO Multiway Relaying with Pairwise Data Exchange: A Degrees of Freedom Perspective

In this paper, we study achievable degrees of freedom (DoF) of a multiple-input multiple-output (MIMO) multiway relay channel (mRC) where $K$ users, each equipped with $M$ antennas, exchange messages in a pairwise manner via a common $N$-antenna relay node. % A novel and systematic way of joint beamforming design at the users and at the relay is proposed to align signals for efficient implementation of physical-layer network coding (PNC). It is shown that, when the user number $K=3$, the proposed beamforming design can achieve the DoF capacity of the considered mRC for any $(M,N)$ setups. % For the scenarios with $K>3$, we show that the proposed signaling scheme can be improved by disabling a portion of relay antennas so as to align signals more efficiently. Our analysis reveals that the obtained achievable DoF is always piecewise linear, and is bounded either by the number of user antennas $M$ or by the number of relay antennas $N$. Further, we show that the DoF capacity can be achieved for $\frac{M}{N} \in \left(0,\frac{K-1}{K(K-2)} \right]$ and $\frac{M}{N} \in \left[\frac{1}{K(K-1)}+\frac{1}{2},\infty \right)$, which provides a broader range of the DoF capacity than the existing results. Asymptotic DoF as $K\rightarrow \infty$ is also derived based on the proposed signaling scheme.Comment: 13 pages, 7 figure

A novel wideband dynamic directional indoor channel model based on a Markov process

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Harmonic chirp imaging method for ultrasound contrast agent

Coded excitation is currently used in medical ultrasound to increase signal-to-noise ratio (SNR) and penetration depth. We propose a chirp excitation method\ud for contrast agents using the second harmonic component of the response. This method is based on a compression filter that selectively compresses and extracts the second harmonic component from the received echo signal. Simulations have shown a clear increase in response for chirp excitation\ud over pulse excitation with the same peak amplitude. This was confirmed by two-dimensional (2-D) optical observations of bubble response with a fast framing camera. To evaluate the harmonic compression method, we applied it to\ud simulated bubble echoes, to measured propagation harmonics, and to B-mode scans of a flow phantom and compared it to regular pulse excitation imaging. An increase of approximately 10 dB in SNR was found for chirp excitation. The\ud compression method was found to perform well in terms of resolution. Axial resolution was in all cases within 10% of the axial resolution from pulse excitation. Range side-lobe levels were 30 dB below the main lobe for the simulated bubble echoes and measured propagation harmonics. However,\ud side-lobes were visible in the B-mode contrast images

Design and Projected Performance of a Flapping Foil AUV

The design and construction of a biomimetic flapping foil autonomous underwater vehicle is detailed. The vehicle was designed as a proof of concept for the use of oscillating foils as the sole source of motive power for a cruising and hovering underwater vehicle. Primary vehicle design requirements included scalability and flexibility in terms of the number and placement of foils, so as to maximize experimental functionality. This goal was met by designing an independent self-contained module to house each foil, requiring only direct current power and a connection to the vehicle’s Ethernet local area network for operation. The results of tests on the foil modules in the Massachusetts Institute of Technology (MIT) Marine Hydrodynamics Water Tunnel and the MIT Ship Model Testing Tank are both used to demonstrate fundamental properties of flapping foils and to predict the performance of the specific vehicle design based on the limits of the actuators. The maximum speed of the vehicle is estimated based on the limitations of the specific actuator and is shown to be a strong function of the vehicle drag coefficient. When using four foils, the maximum speed increases from 1 m/s with a vehicle Cd of 1.4 to 2 m/s when Cd = 0.1, where Cd is based on vehicle frontal area. Finally, issues of vehicle control are considered, including the decoupling of speed and pitch control using pitch-biased maneuvering and the tradeoff between actuator bandwidth and authority during both the cruising and hovering operation