Non-orthogonal multiple access (NOMA) enables power-domain multiplexing via successive interference cancellation (SIC) and has been viewed as a promising technology for 5G communication. The full ...benefit of NOMA depends on resource allocation, including power allocation and channel assignment, for all users, which, however, leads to mixed integer programs. In the literature, the optimal power allocation has only been found in some special cases, while the joint optimization of power allocation and channel assignment generally requires exhaustive search. In this paper, we investigate resource allocation in downlink NOMA systems. As the main contribution, we analytically characterize the optimal power allocation with given channel assignment over multiple channels under different performance criteria. Specifically, we consider the maximin fairness, weighted sum rate maximization, sum rate maximization with quality of service (QoS) constraints, and energy efficiency maximization with weights or QoS constraints in NOMA systems. We also take explicitly into account the order constraints on the powers of the users on each channel, which are often ignored in the existing works, and show that they have a significant impact on SIC in NOMA systems. Then, we provide the optimal power allocation for the considered criteria in closed or semi-closed form. We also propose a low-complexity efficient method to jointly optimize channel assignment and power allocation in NOMA systems by incorporating the matching algorithm with the optimal power allocation. Simulation results show that the joint resource optimization using our optimal power allocation yields better performance than the existing schemes.
To investigate wheel-rail contact stick-slip characteristics (SSCs) under coupling damages of rail corrugation and wheel flat and explain the co-action mechanism of the two damages, a vehicle-track ...system model was built, and the SSCs were analyzed by employing the index of the overall dispersion rate of adhesion coefficient (ODRAC). The results show that both inner and outer wheel-rail systems (WRSs) on the curve have a propensity for stick-slip motion. Since the values of SSCs of the inner WRS are 1.204 to 2.963 times higher than those of the outer one, the inner WRS is easier to stick-slip vibration (SSV). When the rail corrugation is fixed, the overall dispersion rates of adhesion coefficients (ODRACs) progressively decrease with the increase of flat depth and length, and the decrease range is 2.766–41.775%. When the wheel flat is fixed, the ODRACs also progressively decrease as the corrugation wavelength increases, and the decrease ranges from 7.129% to 42.474%; the ODRACs incrementally increase together with the increase of the corrugation wave depth, and the increase ranges from 10.030% to 30.380%. The presence of flat will attenuate the level of SSV of the original system with rail corrugation, whilst the presence of corrugation will intensify the level of SSV of the original system with wheel flat.
Non-orthogonal multiple access (NOMA) and mobile edge computing (MEC) have been recognized as promising technologies for the beyond fifth generation networks to achieve significant capacity ...improvement and delay reduction. In this paper, the technologies of hybrid NOMA and MEC are integrated. In the hybrid NOMA MEC system, multiple users are classified into different groups and each group is allocated a dedicated time slot. In each group, a user first offloads its task by sharing a time slot with another user, and then solely offloads during a time interval. To reduce the delay and save the energy consumption, we consider jointly optimizing the power and time allocation in each group as well as the user grouping. As the main contribution, the optimal power and time allocation is characterized in closed form. In addition, by incorporating the matching algorithm with the optimal power and time allocation, we propose a low complexity method to efficiently optimize user grouping. Simulation results demonstrate that the proposed resource allocation method in the hybrid NOMA MEC systems not only yields better performance than the conventional OMA scheme but also achieves quite close performance as global optimal solution.
A carbody chattering phenomenon accompanied by bogie hunting movement was investigated. The online test results showed that the vehicle at a large tread conicity state exhibited insufficient ...stability when running at higher speeds, causing the hunting frequency of the bogie to continuously increase and approach the diamond-shaped deformation modal frequency of the carbody, consequently causing the carbody to chatter. To improve the running stability of the vehicle and to suppress the carbody chattering, a two-step progressive optimisation method that combined analytical optimisation and simulation optimisation to optimise the parameters of the anti-yaw damper was proposed. Furthermore, the effect of optimisation was experimentally verified. Results showed that after optimisation, the critical speed of the vehicle under the condition of worn wheel was significantly enlarged, the bogie exhibited no obvious hunting motion, carbody chatter was significantly alleviated, and both the ride quality and the curve passing performance were improved.
This paper investigates the beamforming design for multiple-input single-output nonorthogonal multiple access (NOMA) downlink systems. The NOMA beamforming design is formulated as a weighted sum rate ...maximization (WSRM) problem with decoding order constraints on the beamforming vectors and quality-of-service constraints for each user. We first investigate the feasibility of the NOMA WSRM problem and then propose an efficient method to achieve the optimized beamforming vectors. Furthermore, we consider the situation where user channels are homogeneous, which is particularly suitable for using NOMA. In this case, we show that the NOMA WSRM beamforming problem admits the favorable convexity and thus the optimal beamforming solution can be efficiently found. We further show that, with homogeneous channels, the optimal power allocation of the users can be analytically characterized.
In this paper, we propose a downlink multiple-input single-output (MISO) transmission scheme, which is assisted by an intelligent reflecting surface (IRS) consisting of a large number of passive ...reflecting elements. In the literature, it has been proved that nonorthogonal multiple access (NOMA) can achieve the same performance as computationally complex dirty paper coding, where the quasi-degradation condition is satisfied, conditioned on the users' channels fall in the quasi-degradation region. However, in a conventional communication scenario, it is difficult to guarantee the quasi-degradation, because the channels are determined by the propagation environments and cannot be reconfigured. To overcome this difficulty, we focus on an IRS-assisted MISO NOMA system, where the wireless channels can be effectively tuned. We optimize the beamforming vectors and the IRS phase shift matrix for minimizing transmission power. Furthermore, we propose an improved quasi-degradation condition by using IRS, which can ensure that NOMA achieves the capacity region with high possibility. For a comparison, we study zero-forcing beamforming (ZFBF) as well, where the beamforming vectors and the IRS phase shift matrix are also jointly optimized. Comparing NOMA with ZFBF, it is shown that, with the same IRS phase shift matrix and the improved quasi-degradation condition, NOMA always outperforms ZFBF. At the same time, we identify the condition under which ZFBF outperforms NOMA, which motivates the proposed hybrid NOMA transmission. Simulation results show that the proposed IRS-assisted MISO system outperforms the MISO case without IRS, and the hybrid NOMA transmission scheme always achieves better performance than orthogonal multiple access.
Driven by the explosive growth of Internet of Things (IoT) devices with stringent requirements on latency and reliability, ultra-reliability and low latency communication (uRLLC) has become one of ...the three key communication scenarios for the 5th generation (5G) and 6G communication systems. In this paper, we focus on the beamforming design problem for the downlink multiuser uRLLC system. Since the strict demand on the reliability and latency, in general, short packet transmission is a favorable way for uRLLC systems, which indicates the classical Shannon's capacity formula is no longer applicable. With the finite blocklength transmission, the achievable rate is greatly influenced by the reliability and finite blocklength. Using the developed achievable rate formula for finite blocklength transmission, we respectively formulate the problems of interest as the weighted sum rate maximization, energy efficiency maximization, and user fairness optimization by considering the maximum allowable transmission power and minimum rate requirement. These problems considered are non-convex and are hard to obtain the global optimal solution, even for the local optimal solution. To overcome these difficulties, some important insights have been discovered by analyzing the function of achievable rate. For example, an analytical solution of the minimum rate requirement is provided with respective to the signal-to-interference-plus-noise ratio. Based on the discovered results, we provide algorithms to optimize the beamforming vectors and power allocation, which are guaranteed to converge to a local optimum solution to the formulated problems with low computational complexity. Our simulation results reveal that our proposed beamforming algorithms outperform the zero-forcing beamforming algorithm with equal power or water filling allocation widely used in the existing literatures.
This work focuses on the beamforming design for downlink multiple-input single-output (MISO) nonorthogonal multiple access (NOMA) systems. The beamforming vectors are designed by solving a total ...transmission power minimization (TPM) problem with quality-of-service (QoS) constraints. In order to solve the proposed nonconvex optimization problem, we provide an efficient method using semidefinite relaxation. Moreover, for the first time, we characterize the optimal beamforming in a closed form with quasi-degradation condition, which is proven to achieve the same performance as dirty-paper coding (DPC). For the special case with two users, we further show that the original nonconvex TPM problem can be equivalently transferred into a convex optimization problem and easily solved via standard optimization tools. In addition, the optimal beamforming is also characterized in a closed form and we show that it achieves the same performance as the DPC. In the simulation, we show that our proposed optimal NOMA beamforming outperforms OMA schemes and can even achieve the same performance as DPC. Our solutions dramatically simplifies the problem of beamforming design in the downlink MISO NOMA systems and improve the system performance.
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