97 research outputs found

    Full-duplex wireless communications: challenges, solutions and future research directions

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    The family of conventional half-duplex (HD) wireless systems relied on transmitting and receiving in different time-slots or frequency sub-bands. Hence the wireless research community aspires to conceive full-duplex (FD) operation for supporting concurrent transmission and reception in a single time/frequency channel, which would improve the attainable spectral efficiency by a factor of two. The main challenge encountered in implementing an FD wireless device is the large power difference between the self-interference (SI) imposed by the device’s own transmissions and the signal of interest received from a remote source. In this survey, we present a comprehensive list of the potential FD techniques and highlight their pros and cons. We classify the SI cancellation techniques into three categories, namely passive suppression, analog cancellation and digital cancellation, with the advantages and disadvantages of each technique compared. Specifically, we analyse the main impairments (e.g. phase noise, power amplifier nonlinearity as well as in-phase and quadrature-phase (I/Q) imbalance, etc.) that degrading the SI cancellation. We then discuss the FD based Media Access Control (MAC)-layer protocol design for the sake of addressing some of the critical issues, such as the problem of hidden terminals, the resultant end-to-end delay and the high packet loss ratio (PLR) due to network congestion. After elaborating on a variety of physical/MAC-layer techniques, we discuss potential solutions conceived for meeting the challenges imposed by the aforementioned techniques. Furthermore, we also discuss a range of critical issues related to the implementation, performance enhancement and optimization of FD systems, including important topics such as hybrid FD/HD scheme, optimal relay selection and optimal power allocation, etc. Finally, a variety of new directions and open problems associated with FD technology are pointed out. Our hope is that this treatise will stimulate future research efforts in the emerging field of FD communication

    Physical Layer Security in Integrated Sensing and Communication Systems

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    The development of integrated sensing and communication (ISAC) systems has been spurred by the growing congestion of the wireless spectrum. The ISAC system detects targets and communicates with downlink cellular users simultaneously. Uniquely for such scenarios, radar targets are regarded as potential eavesdroppers which might surveil the information sent from the base station (BS) to communication users (CUs) via the radar probing signal. To address this issue, we propose security solutions for ISAC systems to prevent confidential information from being intercepted by radar targets. In this thesis, we firstly present a beamformer design algorithm assisted by artificial noise (AN), which aims to minimize the signal-to-noise ratio (SNR) at the target while ensuring the quality of service (QoS) of legitimate receivers. Furthermore, to reduce the power consumed by AN, we apply the directional modulation (DM) approach to exploit constructive interference (CI). In this case, the optimization problem is designed to maximize the SINR of the target reflected echoes with CI constraints for each CU, while constraining the received symbols at the target in the destructive region. Apart from the separate functionalities of radar and communication systems above, we investigate sensing-aided physical layer security (PLS), where the ISAC BS first emits an omnidirectional waveform to search for and estimate target directions. Then, we formulate a weighted optimization problem to simultaneously maximize the secrecy rate and minimize the Cram\'er-Rao bound (CRB) with the aid of the AN, designing a beampattern with a wide main beam covering all possible angles of targets. The main beam width of the next iteration depends on the optimal CRB. In this way, the sensing and security functionalities provide mutual benefits, resulting in the improvement of mutual performances with every iteration of the optimization, until convergence. Overall, numerical results show the effectiveness of the ISAC security designs through the deployment of AN-aided secrecy rate maximization and CI techniques. The sensing-assisted PLS scheme offers a new approach for obtaining channel information of eavesdroppers, which is treated as a limitation of conventional PLS studies. This design gains mutual benefits in both single and multi-target scenarios

    Constructive Interference Beamforming for Cooperative Dual-Hop MIMO Relay Systems - Invited Paper

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    In this paper, we consider the downlink transmission for a dual-hop amplify-and-forward (AF) multiple-antenna relay systems, where we propose beamforming techniques for interference exploitation on a symbol level. Based on the constructive interference (CI), we firstly propose a joint source/relay precoding, where the precoding matrices at the source and the output signals at the relay are jointly optimized. To alleviate the high computational costs and circumvent the difficulty of practical implementation of the joint design, we further propose a low-complexity decoupled approach, where a closed-form linear precoding method is first employed at the source, and we then optimize the beamforming matrix at the relay for interference exploitation. It is revealed by numerical results that the proposed approaches that exploit the instantaneous interference can achieve an improved performance over the conventional case with a linear approach

    Spectral, Energy and Computation Efficiency in Future 5G Wireless Networks

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    Wireless technology has revolutionized the way people communicate. From first generation, or 1G, in the 1980s to current, largely deployed 4G in the 2010s, we have witnessed not only a technological leap, but also the reformation of associated applications. It is expected that 5G will become commercially available in 2020. 5G is driven by ever-increasing demands for high mobile traffic, low transmission delay, and massive numbers of connected devices. Today, with the popularity of smart phones, intelligent appliances, autonomous cars, and tablets, communication demands are higher than ever, especially when it comes to low-cost and easy-access solutions. Existing communication architecture cannot fulfill 5G’s needs. For example, 5G requires connection speeds up to 1,000 times faster than current technology can provide. Also, from transmitter side to receiver side, 5G delays should be less than 1ms, while 4G targets a 5ms delay speed. To meet these requirements, 5G will apply several disruptive techniques. We focus on two of them: new radio and new scheme. As for the former, we study the non-orthogonal multiple access (NOMA) and as for the latter, we use mobile edge computing (MEC). Traditional communication systems allow users to communicate alternatively, which clearly avoids inter-user interference, but also caps the connection speed. NOMA, on the other hand, allows multiple users to transmit simultaneously. While NOMA will inevitably cause excessive interference, we prove such interference can be mitigated by an advanced receiver side technique. NOMA has existed on the research frontier since 2013. Since that time, both academics and industry professionals have extensively studied its performance. In this dissertation, our contribution is to incorporate NOMA with several potential schemes, such as relay, IoT, and cognitive radio networks. Furthermore, we reviewed various limitations on NOMA and proposed a more practical model. In the second part, MEC is considered. MEC is a transformation from the previous cloud computing system. In particular, MEC leverages powerful devices nearby and instead of sending information to distant cloud servers, the transmission occurs in closer range, which can effectively reduce communication delay. In this work, we have proposed a new evaluation metric for MEC which can more effectively leverage the trade-off between the amount of computation and the energy consumed thereby. A practical communication system for wearable devices is proposed in the last part, which combines all the techniques discussed above. The challenges for wearable communication are inherent in its diverse needs, as some devices may require low speed but high reliability (factory sensors), while others may need low delay (medical devices). We have addressed these challenges and validated our findings through simulations

    Beamforming Design for Multiuser Transmission Through Reconfigurable Intelligent Surface

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    This paper investigates the problem of resource allocation for multiuser communication networks with a reconfigurable intelligent surface (RIS)-assisted wireless transmitter. In this network, the sum transmit power of the network is minimized by controlling the phase beamforming of the RIS and transmit power of the BS. This problem is posed as a joint optimization problem of transmit power and RIS control, whose goal is to minimize the sum transmit power under signal-to-interference-plus-noise ratio (SINR) constraints of the users. To solve this problem, a dual method is proposed, where the dual problem is obtained as a semidefinite programming problem. After solving the dual problem, the phase beamforming of the RIS is obtained in the closed form, while the optimal transmit power is obtained by using the standard interference function. Simulation results show that the proposed scheme can reduce up to 94% and 27% sum transmit power compared to the maximum ratio transmission (MRT) beamforming and zero-forcing (ZF) beamforming techniques, respectively.Comment: RIS as transmitter for multiuser transmission, accepted in IEEE Transactions on Communication

    Simultaneous Wireless Information and Power Transfer Based on Generalized Triangular Decomposition

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    The rapidly growing number of wireless devices has raised the need for designing self-sustained wireless systems. Simultaneous wireless information and power transfer (SWIPT) has been advocated as a promising solution. Various approaches have emerged to design wireless systems that enable SWIPT. In this thesis, we propose a novel approach for spatial switching (SS) based SWIPT using the generalized triangular decomposition (GTD) for point-to-point multiple-input-multiple-output (MIMO) systems. The GTD structure allows the transmitter to use the highest gain subchannels jointly for energy and information transmissions and these joint transmissions can be separated at the receiver. We first derive the optimal GTD structure to attain optimal performance in SS based SWIPT systems. This structure is then extended to design three novel transceivers where each transceiver achieves a certain objective and meets specific constraints. The first transceiver focuses on minimizing the total transmitted power while satisfying the energy harvesting and data rate constraints at the receiver. The second transceiver targets the data rate maximization while meeting a certain amount of energy at the receiver. The third transceiver considers the energy harvesting maximization and guarantees to satisfy the required data rate constraint. The proposed transceivers are designed assuming two transmitted power constraints at the transmitter; the instantaneous total transmit power and the limited transmit power per subchannel. For each designed transceiver, optimal and/or suboptimal solutions are developed to obtain joint power allocation and subchannel assignment under a linear energy harvesting model. Additionally, a novel extension to the SS based SWIPT system is proposed considering a non-linear energy harvesting model. Thereafter, the case of maximizing the energy harvesting for a given data rate and instantaneous total transmitted power constraints is studied. A solution is developed that obtains jointly the optimal power allocation and the subchannel assignment alongside the optimal and/or suboptimal split ratios at the energy harvesters. The theoretical and simulation results show that our novel proposed GTD designs for both linear and non-linear energy harvesting models outperform the state-of-the-art singular value decomposition (SVD) based SWIPT designs
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