Different from a terrestrial network, novel un-manned aerial vehicle (UAV) networks are necessary to efficiently serve aerial users (AUs) as well as ground users (GUs). In this paper, we model a base station (BS) antenna power gain, which is determined by the BS antenna tilt angle and the horizontal distance between BS and user (GU or AU). In order to efficiently serve both GUs and AUs, we propose two BS service schemes, inclusive-service base station (IS-BS) and exclusive-service base station (ES-BS) schemes. For each BS service scheme, we derive a network outage probability by taking into account the characteristics of the BS antenna power gain and channel components for line-of-sight (LoS) and non-line-of-sight (NLoS) environments. Finally, we show that the effect of the total BS density, the interfering BS density, and the user density on the optimal BS tilt angle and our proposed scheme.

In this paper, we propose transmission strategies in multiple-input-single-output (MISO) cooperative communications with two relay nodes in cases when the relay nodes have different trust degrees, where the trust degrees represent how much the relay nodes can be trusted for cooperation. For the given trust degrees and channel conditions, we first derive a relay selection strategy that maximizes the expected achievable rate. We then propose a cooperative transmission strategy of relays with an optimal cooperative beamforming vector that maximizes the expected achievable rate, which is a linear combination of weighted channel vectors. Finally, we derive the optimal transmission strategy, which is a mixed strategy between the relay selection and cooperative transmission strategies with respect to the trust degrees. Our analysis and numerical results show that the proposed transmission strategies increase the expected achievable rate by exploiting the trust degrees of the relay nodes, along with the channel conditions.

In smart grid, the local data aggregator of an area collects the total electrical consumption of the area by aggregating the measurements of smart meters (SMs). A great number of schemes based on the cryptography have been proposed to guarantee the privacy of the individual measurements of the SMs. However, the cryptographic systems require the complicated computations and key-management infrastructural support as well as introduce information leaking risks. In this paper, we propose a novel physical channel-based scheme for privacy preservation in data aggregation, which can work without a cryptographic system. The SMs preserve the privacy of the measurements by adding the jamming signals, which are constructed by scaled channels. The jamming signals are designed to be cancelled with each other at the aggregator. Concurrently, our scheme can resist different types of attacks such as eavesdropping, compromising, and differential attacks even in the case that they are colluding. The simulation results show that the mean squared error (MSE) of the total measurements is significantly lower than that of the traditional scheme while the privacy of the individual measurements is high.

For a user cooperation system with multiple antennas, we consider trust degree-based cooperation techniques to explore the influence of the trustworthiness between users on the communication systems. For the system with two communication pairs, when one communication pair achieves its quality of service (QoS) requirement, they can help the transmission of the other communication pair according to the trust degree, which quantifies the trustworthiness between users in the cooperation. For given trust degree, we investigate the user cooperation strategies, which include the power allocation and precoder design for various antenna configurations. For single-input-single-output and multiple-input-single-output (MISO) cases, we provide the optimal power allocation and beamformer design that maximize the expected achievable rates while guaranteeing the QoS requirement. For a single-input-multiple-output (SIMO) case, we resort to semidefinite relaxation technique and block coordinate update method to solve the corresponding problem, and guarantee the rank-one solutions at each step. For a multiple-input-multiple-output (MIMO) case, as MIMO is the generalization of MISO and SIMO, the similarities among their problem structures inspire us to combine the methods from MISO and SIMO together to efficiently tackle MIMO case. Simulation results show that the trust degree information has a great effect on the performance of the user cooperation in terms of the expected achievable rate, and the proposed user cooperation strategies achieve high achievable rates for give trust degree.

For a confidential communication, we develop the transmission strategies by taking into account trust degree among users, which shows how much a user can be trusted in terms of an eavesdropping attack. For a single potential eavesdropper (PE) case, we derive an optimal power allocation for confidential message and jamming signal to maximize an expected secrecy rate with respect to the trust degree. In order to further increase the expected secrecy rate, by exploiting relaying transmission via PE, we derive the optimal transmission strategy and corresponding power allocation based on the threshold of trust degree. Furthermore, to investigate the effect of the accuracy of trust degree, we analyze the expected secrecy rate loss caused by an estimation error of the trust degree and provide the robust power allocation based on imperfect trust degree. Finally, we extend the results to the case of confidential communication in the presence of multiple PEs. Our analytical and numerical results show that the proposed strategies achieve substantially higher secrecy rates than that without exploiting the trust degree of users.

This paper models the cooperative transmission via caching helpers (cache-based joint transmission), which cache one of the K most popular files, in wireless cellular networks and analyzes its performance in terms of the file transmission success probability in the stochastic geometry framework. For given caching placement, the caching helpers can either provide a file diversity gain by serving different files or a cooperative gain by jointly transmitting the same files. In order to account for the tradeoff between the file diversity gain and the cooperative gain, we derive the cache hit probability and the rate coverage probability. We also find the optimal caching placement balancing the tradeoff and investigate the effects of various system parameters, such as the cooperative region, the density of caching helpers, and the file popularity exponent, on the optimal caching placement.

For communication secrecy, we develop the cooperative transmission strategies by taking into account the degree of trust between users as well as physical wireless channels. For a given trust degree information, we derive an optimal power allocation for confidential message and jamming signal to maximize an expected secrecy rate with respect to the trust degree information. In order to further increase the expected secrecy rate, by adopting cooperative transmission via user relaying, we derive the optimal transmission strategy and corresponding power allocation based on the trust degree information. Our analytical and numerical results show that the proposed strategies achieve substantially higher secrecy rates than that without exploiting the trust degree information of users.

We propose a novel cooperative user relaying scheme for a two-user multiple-antenna downlink cellular system where each user has to receive a certain required amount of information. A user who successfully receives its required amount of information is supposed to help the other user in receiving its required amount of information through cooperative user relaying. For the proposed cooperative user relaying scheme, we jointly design linear beamformers at the base station over three transmission phases to minimize the total transmission time required for both users to receive their respective required amounts of information, which turn out to be approximated equivalent to the maximization of the sum throughput. In addition, considering a practical hybrid automatic repeat request (HARQ) protocol with user relaying, we modify the proposed scheme to minimize the required number of retransmissions. Our numerical results show that the proposed cooperative user relaying scheme achieves substantial gains over conventional transmission without user relaying in terms of both the average sum throughput and the transmission failure probability.

This paper investigates device-to-device (D2D) communication nested in a cellular network, where a pair of D2D users directly exchanges their information using the uplink frequency band of the cellular network. When the D2D user treats the interference from the cellular user as noise, power control at the cellular user is optimal for maximizing the rate of the cellular user while controlling the interference to the D2D user. However, if the D2D user can perform successive interference cancelation (SIC), the cellular user needs to adjust both transmit power and rate to maximize its rate, because the decodability of the interfering signals at the D2D user depends not only on the signal power but also on the rate of the cellular user. To control the interference from the cellular user, we propose a joint transmit power and rate control scheme at the cellular user. Forcing the cellular user to transmit with a reduced data rate compared with the maximum possible rate, given its transmit power, the proposed joint power and rate control scheme efficiently enables SIC at the D2D user. To reduce the computational complexity, we also propose a near-optimal scheme that employs either power control or rate control depending on the channel conditions.

This paper proposes a user cooperation scheme to improve the achievable rate of a cell edge user in a two cell model. In the proposed user cooperation, one user helps decoding of the other user by either desired signal forwarding (SF) or interference forwarding (IF). We derive an achievable rate region of the proposed user cooperation and compare it with that of a conventional two-cell system without user cooperation. It is shown that the proposed user cooperation exploiting either SF or IF is beneficial in terms of achievable rate region compared to the conventional two-cell system.

We characterize the Pareto boundary of the achievable rate region of the two-user symmetric Gaussian interference channel when transmitters control their transmission power and receivers have successive interference cancellation capability. By identifying the shape of achievable rate region according to the strength of interference channels, we find combinations of decoding and power allocation strategies achieving the Pareto boundary. We also characterize the Pareto boundary of the achievable rate region when time sharing is additionally considered.

A novel user relaying scheme based on bit concatenation is proposed for multiple-input multiple-output (MIMO) broadcast channels. In a broadcasting phase, a part of information for a destination user is concatenated to data for a relaying user and transmitted to the relaying user. The destination user receives the part of information via the relaying user in a relaying phase in addition to the data in the broadcasting phase. An achievable rate region for the proposed scheme is derived and an algorithm to characterize the boundary of the rate region is proposed. Our analysis shows that the proposed user relaying in a MIMO BC expands the achievable rate region for the zero-forcing dirty paper coding (ZF-DPC) in the MIMO BC without relaying. It is also shown that the proposed user relaying scheme always outperforms a superposition coding (SC) based user relaying scheme. Optimal and suboptimal user pair scheduling algorithms are investigated for a two-user MIMO BC and the proposed suboptimal user pair scheduling scheme is shown to approach the optimal scheduling when the relaying channel gain is large.

A novel user relaying scheme based on bit concatenation is proposed for a two-user multiple input multiple output(MIMO) broadcast channel (BC). In broadcasting phase, a part of information for a destination user is concatenated to data for a relaying user and transmitted to a relaying user. The destination user receives the part of information via the relaying user in relaying phase in addition to the data in broadcasting phase. Achievable rate region of the proposed scheme is derived and an algorithm to characterize the boundary of the rate region is proposed. Our analysis and simulations show that the proposed user relaying in a MIMO BC expands the achievable rate region of a zero-forcing dirty paper coding (ZF-DPC) in a MIMO BC without relaying.

This paper proposes a linear precoding technique for multiple-input multiple-output (MIMO) decode-and-forward (DF) based relay communications. The proposed precoder is constructed by linearly combining two independently designed precoders that maximize data rates at relay and destination, respectively. Contrary to conventional precoding in DF relay communications, the proposed precoding balances direct and relay links and maximize the achievable rate of the overall system. We also propose a distributed precoder design method using limited information. The proposed algorithms are shown to significantly reduce computation complexity of precoder design. The numerical results show that the proposed precoders with full information and limited information achieve significantly higher data rates than conventional precoding neglecting a direct link and comparable data rate to the optimal precoding even when the coefficients for linear combining are restricted to real numbers.

This paper proposes a novel partial decode-and-forward (DF) relaying strategy with multiple antennas. In the first phase, the source broadcasts data streams consisting of non-forwarding and forwarding data streams. In the second phase, a relay node forwards only forwarding data streams to a destination node, and a destination node decodes both non-forwarding and forwarding data streams by successive interference cancellation (SIC). We provide an analytical framework of achievable rate and design a linearly combined precoding matrix for rate maximization. Our results show that the proposed partial DF relaying with a linearly combined precoding matrix achieves substantially higher rate than a conventional DF relaying scheme.

A simple linear preceding scheme to cancel interference is proposed for downlink multiuser cellular relay networks where the available information is asymmetric between the base station and the relay node. The interference signal from the relay is eliminated by linear pre-pocessing at the base station without any cooperation between the base station and the relay. In the information asymmetric environment, it is shown that the proposed scheme outperforms a time division multiple access (TDMA) scheme and approaches an ideal scheme based on dirty paper coding (DPC) in terms of capacity. An mathematical framework for the analysis of the proposed technique is also presented.