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In this paper, we investigate the two-way communication between two users assisted by a reconfigurable intelligent surface (RIS). The scheme that two users communicate simultaneously over Rayleigh fading channels is considered. The channels between the two users and RIS can either be reciprocal or non-reciprocal. For reciprocal channels, we determine the optimal phases at the RIS to maximize the signal-to-interference-plus-noise ratio (SINR). We then derive exact closed-form expressions for the outage probability and spectral efficiency for single-element RIS. By capitalizing the insights obtained from the single-element analysis, we introduce a gamma approximation to model the product of Rayleigh random variables which is useful for the evaluation of the performance metrics in multiple-element RIS. Asymptotic analysis shows that the outage decreases at $\left ({\log (\rho)/\rho }\right)^{L}$ rate where $L$ is the number of elements, whereas the spectral efficiency increases at $\log (\rho)$ rate at large average SINR $\rho $. For non-reciprocal channels, the minimum user SINR is targeted to be maximized. For single-element RIS, closed-form solution is derived whereas for multiple-element RIS the problem turns out to be non-convex. The latter one is solved through semidefinite programming relaxation and a proposed greedy-iterative method, which can achieve higher performance and lower computational complexity, respectively. [ABSTRACT FROM AUTHOR]

In this paper, power allocation problem is investigated for an LTE-U (long-term evolution unlicensed) system, in which a cellular system will transmit data on the unlicensed spectrum occupied by Wi-Fi system. One base station and multiple mobile users are considered for both uplink scenario and downlink scenario. To access the base station in uplink scenario or to transmit information to multiple mobile users in downlink scenario, orthogonal multiple access (OMA) and non-orthogonal multiple access (NOMA) are studied respectively. The transmit power of the base station or multiple mobile users over multiple channels in the unlicensed spectrum are optimized to maximize the system throughput while imposing a probabilistic constraint on the interference to Wi-Fi receiver only with a number of samples of interference channel's gain. Although being in non-closed-form, the formulated probabilistic constraint is transformed to be two types of closed-form constraints. Then the originally formulated optimization problems fall into convex optimization problems. In addition, it is proved that the OMA mode can achieve the same performance with the NOMA mode for uplink scenario, and one simple solution is developed under OMA mode for downlink scenario. Numerical results are demonstrated to show the performance of our proposed methods. [ABSTRACT FROM AUTHOR]

In a multihop wireless network, the traffic at each node and the channel over each link may fluctuate with time. Thus, traditional optimal resource allocation needs to be computed for each moment with instantaneous information of channel states over all links and the traffic rates at all nodes, leading to huge communication overhead and computation cost. To solve this challenge, in this correspondence, we propose to use robust resource allocation, in which the only needed information is the mean and variance of the wireless channels and the traffic rates. In the formulated problem, there are probabilistic constraints, which are difficult to handle. Effective methods are provided that can transform the probabilistic constraints to convex constraints. As the resource allocation does not need instantaneous channel state information or instantaneous traffic rate information, it is robust to channel and traffic variations, with very little communication and computation overhead. [ABSTRACT FROM AUTHOR]

This paper studies dynamic spectrum leasing in a cognitive radio network. There are two spectrum sellers, who are two primary networks, each with an amount of licensed spectrum bandwidth. When a seller has some unused spectrum, it would like to lease the unused spectrum to secondary users. A coordinator helps to perform the spectrum leasing stage-by-stage. As the two sellers may have different leasing periods, there are three epochs, in which seller 1 has spectrum to lease in Epochs II and III, while seller 2 has spectrum to lease in Epochs I and II. Each seller needs to decide how much spectrum it should lease to secondary users in each stage of its leasing period, with a target at revenue maximization. It is shown that, when the two sellers both have spectrum to lease (i.e., in Epoch II), the spectrum leasing can be formulated as a noncooperative game. Nash equilibria of the game are found in closed form. Solutions of the two sellers in their leasing periods are then derived. [ABSTRACT FROM AUTHOR]

In this paper, the problem of dynamic pricing over multiple rounds of spectrum leasing is investigated. One primary network is considered, which is the spectrum seller that would like to lease its unused channels to secondary users. To accommodate different arrival instants of secondary users' spectrum requests, spectrum leasing is performed in multiple rounds (stages), and in each stage, a separate spectrum price is set. First, we consider the case that, for each specific price value, the spectrum demand (the number of channels requested by secondary users) is a random variable. An optimization problem is formulated to set up the spectrum prices in the multiple stages, with the purpose of maximizing the total revenue of the primary network. The solving method of the formulated optimization problem is presented. Additionally, some interesting properties of the optimal solution are also presented, such as monotonicity and convexity of the maximal total revenue with respect to the stage index and lower/upper bounds of the maximal total revenue. Furthermore, we consider the case that, for a specific price value, the spectrum demand is nonrandom and can be solely determined by the price. An incremental algorithm is given to find out the optimal price values at the stages. We also demonstrate the monotonicity of the optimal price value with respect to the stage index. Numerical results are provided to verify the research findings and compare them with existing work. [ABSTRACT FROM AUTHOR]

In this paper, the average rate maximization problem given transmission power constraints in a decode-and-forward (DF) relay network over slow fading channels is studied. For the problem, either a long- or short-term power constraint is imposed on the source and relays. In each combination of the power constraints at the source and relays, the original optimization problem is decomposed into subproblems, each corresponding to a specific channel gain realization. For each subproblem, a fast algorithm with closed-form solutions is provided. The case with additional peak power spectrum density constraints is also investigated. Numerical results are presented to demonstrate the effectiveness of the algorithms. [ABSTRACT FROM PUBLISHER]

Wireless multimedia sensor networks (WMSNs) are expected to support multimedia services such as delivery of video and audio streams. However, due to the relatively stringent quality-of-service (QoS) requirements of multimedia services (e.g., high transmission rates and timely delivery) and the limited wireless resources, it is possible that not all the potential sensor nodes can be admitted into the network. Thus, node admission is essential for WMSNs, which is the target of this paper. Specifically, we aim at the node admission and its interaction with power allocation and link scheduling. A cross-layer design is presented as a two-stage optimization problem, where at the first stage the number of admitted sensor nodes is maximized, and at the second stage the network lifetime is maximized. Interestingly, it is proved that the two-stage optimization problem can be converted to a one-stage optimization problem with a more compact and concise mathematical form. Numerical results demonstrate the effectiveness of the two-stage and one-stage optimization frameworks. [ABSTRACT FROM AUTHOR]

We consider a cognitive radio system, in which a secondary transmitter harvests energy from a primary transmitter's RF signal. The secondary transmitter, which provides decode-and-forward relaying service for the primary system, transmits its own data by using downlink non-orthogonal multiple access (NOMA). A time-switching protocol is used by the secondary transmitter to harvest energy and decode the primary transmitter's information. Our objective is to achieve maximal secondary throughput, by optimally selecting the time portion used for energy harvesting and the secondary transmitter's power allocation in NOMA transmission. In this paper, two optimization problems are formulated, in which the secondary receiver performs or does not perform successive interference cancellation (SIC), respectively. Although the two problems are nonconvex, we devise a method to transform the problems into equivalent problems under different cases. Then, we theoretically prove that the objective functions of the equivalent problems are quasiconcave, based on which we develop two-level bisection search algorithms to solve the equivalent problems. Interestingly, we show that performing SIC at the secondary receiver does not always guarantee a higher secondary throughput than the case without performing SIC. Computer simulation demonstrates that our algorithm performs better than an equal power allocation algorithm and an orthogonal multiple access algorithm. [ABSTRACT FROM AUTHOR]

This paper investigates the tradeoff between information rate and harvested energy in a simultaneous wireless information and power transfer system under power splitting (PS) scheme and time switching (TS) scheme. A single-input-multiple-output transmitter–receiver pair is considered. For the channel from the transmitter to the receiver, only the mean and covariance matrix are known, but the instantaneous channel information or the channel gain distribution is unknown. We define robust outage capacity that applies for all possible channel distributions with the given mean and covariance matrix, and study the tradeoff between the robust outage capacity and harvested power level. By solving optimization problems, we find the PS ratio setting for PS scheme and TS ratio setting for TS scheme to achieve the outer boundary of robust outage capacity-energy region (which includes all possible pairs of achievable robust outage capacity and harvested power level). The PS and TS ratio setting are fixed over fading blocks, and thus can avoid communication overhead and computation overhead over each fading block, and facilitate a simple implementation in a practical system. We also prove that the robust outage capacity-energy regions in PS scheme and TS scheme are both convex. Further, we prove that the PS scheme outperforms the TS scheme in terms of the rate-energy tradeoff. Numerical results are also presented to verify the correctness of our theoretical results. [ABSTRACT FROM PUBLISHER]

Ambient backscatter technology that utilizes the ambient radio frequency signals to enable the communications of battery-free devices has attracted much attention recently. In this paper, we study the problem of signal detection for an ambient backscatter communication system that adopts the differential encoding to eliminate the necessity of channel estimation. Specifically, we formulate a new transmission model, design the data detection algorithm, and derive two closed-form detection thresholds. One threshold is used to approximately achieve the minimum sum bit error rate (BER), while the other yields balanced error probabilities for “0” bit and “1” bit. The corresponding BER expressions are derived to fully characterize the detection performance. In addition, the lower and the upper bounds of BER at high signal-to-noise ratio regions are also examined to simplify a performance analysis. Simulation results are then provided to corroborate the theoretical studies. [ABSTRACT FROM PUBLISHER]

This paper offers an investigation into a solar-powered airplane with electric motor-propeller propulsion, secondary battery and autopilot system. Topics covered include the absolute ceiling of an eternal solar-powered airplane, the relation between wing loading and maximum flight altitude as well as the relation between wing loading and technical parameters and the rule on the maximum flight altitude.

To support wireless Internet of things (IoT) devices, this paper presents a new solution which combines wireless power transfer and mobile edge computing. Specifically, we consider one mobile device, which first harvests energy from radio frequency signals sent by a base station and then offloads all or part of its data to be processed to the base station. The process of energy harvesting and offloading span over multiple fading blocks. The target is to maximize the average amount of processed data in unit time. To achieve this target, we optimize the stopping rule for energy harvesting (i.e., when to stop energy harvesting and start offloading) and the number of fading blocks for data offloading. To solve the formulated problem optimally, we decompose it into two levels. In the lower level, the stopping rule for energy harvesting is optimized given a fixed number of fading blocks for offloading. The associated lower-level problem is solved optimally based on a series of special properties of the problem. In the upper level, the number of fading blocks for offloading is optimized. Efficiency of our work with fully offloading mode and partially offloading mode is shown by using simulation. [ABSTRACT FROM AUTHOR]

NOMA has been recognized as a highly promising FRA technology to satisfy the requirements of the fifth generation era on high spectral efficiency and massive connectivity. Since the EE has become a growing concern in FRA from both the industrial and societal perspectives, this article discusses the sustainability issues of NOMA. We first thoroughly examine the theoretical power regions of NOMA to show the minimum transmission power with fixed data rate requirement, demonstrating the EE performance advantage of NOMA over orthogonal multiple access. Then we explore the role of energy-aware resource allocation and grant-free transmission in further enhancing the EE performance of NOMA. Based on this exploration, a hybrid NOMA strategy that reaps the joint benefits of resource allocation and grantfree transmission is investigated to simultaneously accomplish high throughput, large connectivity, and low energy cost. Finally, we identify some important and interesting future directions for NOMA designers to follow in the next decade. [ABSTRACT FROM AUTHOR]

We investigate the power-allocation problem in a two-tier femtocell network including a macrocell and multiple femtocells. Due to shadowing and fading effects, at each cell (macrocell or femtocell), the power levels of desired signal and interference signals vary with time. Under this circumstance, one method to achieve transmission quality in the cells is to first get channel state information (CSI) of all desired links and interference links and then perform power allocation. This method has very large communication and computation overhead. In this paper, we focus on power allocation in which the transmit power levels of the users in the cells do not need to change when the wireless channels fluctuate. We formulate a power-allocation problem subject to bounded outage probability in any cell (macrocell or femtocell). The formulated problem has probabilistic constraints. It is hard to have closed-form expressions of the probabilistic constraints. To solve this, we propose novel transformations of the constraints, based on which we obtain constraints in the format of worst-case value-at-risk. Such constraints can be converted to convex constraints, and thus, the research problem is transformed to a convex problem. For the convex problem, we provide an iterative algorithm that converges quickly. We also investigate the case when the channel gain distributions are unknown. Our proposed schemes have the merits of very small communication and computation overhead and are particularly useful in fast-fading environments. [ABSTRACT FROM AUTHOR]