Massive multi-input multi-output (MIMO) is envisioned as a key technology for the emerging fifth generation of communication networks (5G). However, considering the energy consumption of the large number of radio frequency (RF) chains, massive MIMO poses a problem to energy efficiency (EE) requirement of 5G. In this paper, we propose an energy-efficient power allocation method for millimeter-wave (mmWave) beamspace MIMO non-orthogonal multiple access (NOMA) systems, where there may be multiple users in each selected beam. First, according to the beam selection (BS) results, we get the precoding matrix through zero-forcing (ZF) beamforming method. Second, we formulate the energy efficiency (EE) maximization optimization problem as a fractional programming. Through sequential convex approximation (SCA) and second-order cone (SOC) transformation, the original optimization problem can be transformed to a convex optimization problem. By using iterative optimization algorithm, we can get the power allocation results. Then, we analyze the convergence of our proposed iterative optimization method and get that the solution in each iteration is a suboptimal solution to the original non-convex optimization problem. Simulation results show that the proposed energy-efficient power allocation scheme has better EE performance comparing with the conventional methods when the transmitted power exceeds the power threshold.
The layered transition metal dichalcogenide has attracted tremendous attention for its unique structure and electrical and optoelectronic properties. As an emerging two-dimensional material, PdSe2 plays a key role in optoelectronic applications due to its distinct optical and tunable electrical properties. The carrier dynamic and low-frequency phonon modes and how they evolve with the number of layers are important for future device fabrication in photonics, optoelectronics, and nanomechanics. Here, by employing ultrafast optical pump–probe spectroscopy, we have investigated systematically the photocarrier dynamics as well as the thickness dependent interlayer coherent phonon modes in PdSe2 films. Two low-frequency phonon modes in PdSe2 films are identified after photoexcitation at 780 nm. The higher-frequency mode is ascribed to the interlayer breathing mode, and the lower one is assigned to the standing wave mode, and both of the mode frequencies decrease with increasing the number of layers of films. Analysis based on simple one-dimensional chain model produces interlayer force constant K = 5.74 × 1019 N/m3 for the interlayer breathing mode, and sound velocity of v = 8.27 × 104 cm/s for the standing wave mode in PdSe2 film. Our experimental finding paves the way for designing and developing PdSe2-based optoelectronic and nanomechanic devices.
The Internet of Things (IoT) and machine-to-machine (M2M) communication will play an important role in future communications, but there is currently no ultra-reliable low-latency wireless communication theory that guides its design. For highly mobile vehicle-to-vehicle (V2V) wireless communication scenarios, asynchronous transmission should also be accepted. Therefore, it is particularly urgent to study ultra-reliable low-latency wireless transmission technology that satisfies asynchronous transmission. Universal Filter Multi-Carrier (UFMC) is a new type of filtering wireless transmission mechanism that meets this character. Although some of its properties have been explored in recent years, there are few articles that systematically evaluate its performance. In this paper, firstly, the performance of the UFMC system is fully evaluated in terms of spectral efficiency (SE), bit error rate (BER), peak-to-average power ratio (PAPR), carrier frequency offset (CFO), as well as various multipath fading channels, effects of time delay (TD), etc. Meanwhile, a mathematical analysis model is established for the BER of UFMC system, and some exact closed-loop expressions of bit error probabilities for UFMC are derived. Moreover, the equivalent form of the transmitter in the frequency domain is derived. Finally, the Monte Carlo simulation results related to UFMC are presented. The results reveal that UFMC suffers from the same problems as other multi-carrier system, including higher PAPR, affected by CFO, but it has its own inherent advantages such as insensitivity to time delay or energy-efficient, and the main negative factor of UFMC is inter-carrier interference (ICI) not inter-symbol interference (ISI), which may play an important role in future M2M and V2V communication.INDEX TERMS UFMC, spectral efficiency, BER, PAPR, carrier frequency offset, time delay.
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