Spectral and Energy Efficiency of Multi-pair Massive MIMO Relay Network with Hybrid Processing

We consider a multi-pair massive multiple-input multiple-output (MIMO) relay network, where the relay is equipped with a large number, N, of antennas, but driven by a far smaller number, L, of radio frequency (RF) chains. We assume that K pairs of users are scheduled for simultaneous transmission, where K satisfies 2K = L. A hybrid signal processing scheme is presented for both uplink and downlink transmissions of the network. Analytical expressions of both spectral and energy efficiency are derived with respect to the RF chain number under imperfect channel estimation. It is revealed that, under the condition N > 4L^2/pi, the transmit power of each user and the relay can be respectively scaled down by 1=sqrt(N) and 2K=sqrt(N) if pilot power scales with signal power, or they can be respectively scaled down by 1=N and 2K=N if the pilot power is kept fixed, while maintaining an asymptotically unchanged spectral efficiency (SE). While regarding energy efficiency (EE) of the network, the optimal EE is shown to be achieved when Pr = 2KPs, where Pr and Ps respectively refer to the transmit power of the relay and each source terminal. We show that the network EE is a quasi-concave function with respect to the number of RF-chains which, therefore, admits a unique globally optimal choice of the RF-chain number. Numerical simulations are conducted to verify our observations.Comment: 32 pages, to appear in IEEE Transactions on Communications, early access availabl

Performance Analysis of Mixed-ADC Massive MIMO Systems over Rician Fading Channels

The practical deployment of massive multiple-input multiple-output (MIMO) in future fifth generation (5G) wireless communication systems is challenging due to its high hardware cost and power consumption. One promising solution to address this challenge is to adopt the low-resolution analog-to-digital converter (ADC) architecture. However, the practical implementation of such architecture is challenging due to the required complex signal processing to compensate the coarse quantization caused by low-resolution ADCs. Therefore, few high-resolution ADCs are reserved in the recently proposed mixed-ADC architecture to enable low-complexity transceiver algorithms. In contrast to previous works over Rayleigh fading channels, we investigate the performance of mixed-ADC massive MIMO systems over the Rician fading channel, which is more general for the 5G scenarios like Internet of Things (IoT). Specially, novel closed-form approximate expressions for the uplink achievable rate are derived for both cases of perfect and imperfect channel state information (CSI). With the increasing Rician $K$-factor, the derived results show that the achievable rate will converge to a fixed value. We also obtain the power-scaling law that the transmit power of each user can be scaled down proportionally to the inverse of the number of base station (BS) antennas for both perfect and imperfect CSI. Moreover, we reveal the trade-off between the achievable rate and energy efficiency with respect to key system parameters including the quantization bits, number of BS antennas, Rician $K$-factor, user transmit power, and CSI quality. Finally, numerical results are provided to show that the mixed-ADC architecture can achieve a better energy-rate trade-off compared with the ideal infinite-resolution and low-resolution ADC architectures.Comment: 11 pages, 11 figures, to appear in IEEE Journal on Selected Areas in Communication

Learning Oriented Cross-Entropy Approach to User Association in Load-Balanced HetNet

This letter considers optimizing user association in a heterogeneous network via utility maximization, which is a combinatorial optimization problem due to integer constraints. Different from existing solutions based on convex optimization, we alternatively propose a cross-entropy (CE)-based algorithm inspired by a sampling approach developed in machine learning. Adopting a probabilistic model, we first reformulate the original problem as a CE minimization problem which aims to learn the probability distribution of variables in the optimal association. An efficient solution by stochastic sampling is introduced to solve the learning problem. The integer constraint is directly handled by the proposed algorithm, which is robust to network deployment and algorithm parameter choices. Simulations verify that the proposed CE approach achieves near-optimal performance quite efficiently

Light-Neutrino Exchange and Long-Distance Contributions to 0ν2β0\nu2\beta Decays: An Exploratory Study on ππ→ee\pi\pi\to ee

We present an exploratory lattice QCD calculation of the neutrinoless double beta decay $\pi\pi\to ee$. Under the mechanism of light-neutrino exchange, the decay amplitude involves significant long-distance contributions. The calculation reported here, with pion masses $m_\pi=420$ and 140 MeV, demonstrates that the decay amplitude can be computed from first principles using lattice methods. At unphysical and physical pion masses, we obtain that amplitudes are $24\%$ and $9\%$ smaller than the predication from leading order chiral perturbation theory. Our findings provide the lattice QCD inputs and constraints for effective field theory. A follow-on calculation with fully controlled systematic errors will be possible with adequate computational resources.Comment: 6 pages, 3 figures. V2: version accepted by PRL; minor changes compared to v

Suppressing phase decoherence of a single atom qubit with CPMG sequence

We experimentally demonstrate the strong suppression of dephasing of a qubit stored in a single \textsuperscript{87}Rb atom in an optical dipole trap by using Carr-Purcell-Meiboom-Gill(CPMG) pulse sequences. Regarded as a repetition of spin echo, CPMG sequence is an optimal choice for suppressing both inhomogeneous and homogeneous phase decoherence mechanisms. In the trap with 830 nm wavelength and 0.7 mK potential depth, the spin relaxation time of single atoms is showed to be 830.8 ms. We obtain the reversible inhomogeneous dephasing time of $T_{2}^{\ast}$=1.4 ms. The homogeneous dephasing time is $T_{2}^{\prime}$ =102.7 ms in the spin echo process, by employing CPMG sequence with pulse number n = 6 the homogeneous dephasing is further suppressed by a factor of 3, and its corresponding coherence time is extended to $T_{2}^{\prime}$=304.5 ms.Comment: 7 pages, 5 figure

Multipair Massive MIMO Two-Way Full-Duplex Relay Systems with Hardware Impairments

Hardware impairments, such as phase noise, quantization errors, non-linearities, and noise amplification, have baneful effects on wireless communications. In this paper, we investigate the effect of hardware impairments on multipair massive multiple-input multiple-output (MIMO) two-way full-duplex relay systems with amplify-and-forward scheme. More specifically, novel closed-form approximate expressions for the spectral efficiency are derived to obtain some important insights into the practical design of the considered system. When the number of relay antennas $N$ increases without bound, we propose a hardware scaling law, which reveals that the level of hardware impairments that can be tolerated is roughly proportional to $\sqrt{N}$. This new result inspires us to design low-cost and practical multipair massive MIMO two-way full-duplex relay systems. Moreover, the optimal number of relay antennas is derived to maximize the energy efficiency. Finally, Motor-Carlo simulation results are provided to validate our analytical results.Comment: 6 pages, 3 figures, accepted by IEEE Globecom 201

Experimental nonlocal steering of Bohmian trajectories

Interpretations of quantum mechanics (QM), or proposals for underlying theories, that attempt to present a definite realist picture, such as Bohmian mechanics, require strong non-local effects. Naively, these effects would violate causality and contradict special relativity. However if the theory agrees with QM the violation cannot be observed directly. Here, we demonstrate experimentally such an effect: we steer the velocity and trajectory of a Bohmian particle using a remote measurement. We use a pair of photons and entangle the spatial transverse position of one with the polarization of the other. The first photon is sent to a double-slit-like apparatus, where its trajectory is measured using the technique of Weak Measurements. The other photon is projected to a linear polarization state. The choice of polarization state, and the result, steer the first photon in the most intuitive sense of the word. The effect is indeed shown to be dramatic, while being easy to visualize. We discuss its strength and what are the conditions for it to occur.Comment: 10 pages, 3 figure

Non-classical correlation of cascaded photon pairs emitted from quantum dot

We studied the quantum correlation between the photon pairs generated by biexciton cascade decays of self-assembled quantum dots, and determined the temperature behavior associated with so-called sudden change of the quantum correlation. The relationship between the fine structure splitting and the sudden change temperature is also provided. Our study indicates that this correlation behavior sudden change temperature is independent on the back ground noise in the system and far lower than entanglement sudden death temperature, therefore it should be easier to observe the phenomenon of correlation sudden change in experiments than to observe entanglement sudden death

Model-Driven Deep Learning for Physical Layer Communications

Intelligent communication is gradually considered as the mainstream direction in future wireless communications. As a major branch of machine learning, deep learning (DL) has been applied in physical layer communications and has demonstrated an impressive performance improvement in recent years. However, most of the existing works related to DL focus on data-driven approaches, which consider the communication system as a black box and train it by using a huge volume of data. Training a network requires sufficient computing resources and extensive time, both of which are rarely found in communication devices. By contrast, model-driven DL approaches combine communication domain knowledge with DL to reduce the demand for computing resources and training time. This article reviews the recent advancements in the application of model-driven DL approaches in physical layer communications, including transmission scheme, receiver design, and channel information recovery. Several open issues for further research are also highlighted after presenting the comprehensive survey.Comment: 20 pages,6 figure

Experimental Demonstration of Robust Bidirectional Quantum Optical Communications

We experimentally realized a new method for transmitting quantum information reliably through paired optical polarization-maintaining (PM) fibers. The physical setup extends the use of a Mach-Zehnder interferometer, where noises are canceled through interference. This method can be viewed as an improved version of the current decohernce-free subspace (DFS) approach in fiber optics. Furthermore, the setup can be applied bidirectionally, which means that robust quantum communication can be achieved from both ends. To rigorously quantify the amount of quantum information transferred, optical fibers are analyzed with the tools developed in quantum communication theory. These results not only suggests a practical means for protecting classical and quantum information through optical fibers, but also provides a new physical platform for enriching the structure of the quantum communication theory.Comment: 15 pages, 4 figure