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Both Device-to-device (D2D) and full-duplex (FD) have been widely recognized as spectral efficient techniques in the fifth-generation wireless communications systems. By enabling FD mode in D2D communications, the attainable spectrum efficiency can be (in theory) up to twice as high as that of conventional half-duplex (HD) mode D2D technology, provided that the selfinterference signal in the former can be sufficiently suppressed. Considering the fact that FD technology does not always outperform the HD technology in arbitrary channel condition, we will delve into the FD gain as well as its preconditions for acquisition in this paper. In particular, the performance of D2D-aided underlaying cellular networks is investigated by assuming that the D2D users are capable of operating at both HD and FD modes. Under a given user workload, the sum throughput in each DU is shown to be always improved by activating D2D links despite an extra DU-induced interference could be imposed on its cospectrum cellular users. Numerical results show that the FD-D2D mode exhibits its superiority in terms of sum throughput than the HD-D2D mode in a light-workload scenario even if a non-ideal SI cancellation has been implemented in the former, but the latter pulls back a game in a heavy-workload scenario. Furthermore, to maximize the sum throughput, an appropriate mode-selection scheme (i.e., by choosing either FD or HD mode in the current time-slot for each individual D2D user) should be implemented for sufficiently exploiting the FD gain according to the instantaneous radio frequency environment.

The hybrid nonlinear transceiver optimization problem of reconfigurable intelligent surface (RIS)-aided multi-user multiple-input multiple-output (MU-MIMO) downlink is investigated. Specifically, the Tomlinson-Harashima precoder (THP) and the hybrid transmit precoder (TPC) of the base station are jointly optimized with the linear digital receivers of mobile users. The triangular feedback matrix of the THP is optimized and the optimal solution is derived in closed form based on a matrix inequality. Moreover, in order to tackle the nonconvexity of the constant-modulus constraints imposed on the analog TPC, the Majorization-Minimization (MM) based reconfigurable optimization framework is proposed, which strikes a trade-off between the implementation complexity and system performance in a reconfigurable manner. Explicitly, our MM-based reconfigurable optimization framework is capable of optimizing the analog TPC in a dynamically reconfigurable manner on an element-byelement, column-by-column, row-by-row or block-by-block basis. Moreover, an MM-based reconfigurable algorithm is proposed for the optimization of the phase shifting matrix at RIS, which also suffers from constant-modulus constraints. In the proposed MM-based reconfigurable algorithm, the RIS can be partitioned into a series of subarrays for striking different performance vs. complexity tradeoffs. Finally, our numerical results demonstrate the performance advantages of the proposed nonlinear hybrid transceiver optimization techniques.

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In this paper, a novel model is proposed to characterize superheated steam-air (SSA) pressure and temperature distribution in horizontal wells with toe-point injection technique. Firstly, the mathematical model of SSA flow in both tubing and annulus is established based on the mass, momentum and energy conservation equations, and it is solved by employing finite difference method. The mass and heat transfer are coupled with injection of SSA into formation and heat exchange between tubing and annulus by iterative technique. Then, the proposed model is validated by field data. Finally, the effect of mass fraction of air and injection temperature on SSA temperature, pressure, mass flow rate in annulus and heat transfer rate from annulus fluid to formation are conducted. Results indicate that: (1). The SSA temperature in tubing decreases while flowing from heel point to toe point and the temperature gradient decreases with distance to heel point. (2). The SSA temperature in annulus first decreases and then turns to increase while flowing from toe point to heel point. (3). The SSA pressure in tubing decreases linearly from heel point to toe point, while the SSA pressure gradient in annulus decreases with the distance to toe point. (4). The SSA temperature in both tubing and annulus decreases with the increasing of mass fraction of air. This is because the enthalpy of air is smaller than superheated steam. (5). Increasing injection temperature can decrease the SSA pressure in both tubing and annulus by reducing SSA density and increasing frictional loss. Based on the presented model, the SSA pressure and temperature in both tubing and annulus can be accurately predicted with the relative error less than 5%. The theoretical studies in this paper can be taken as a reference for engineers in optimization of injection parameters and provide following researchers with the very basic theory for the application of toe-point injection technique.