Recent progress in cognitive radio networks (CRNs) promises to meet device-to-device communication requirements for spectrum utilisation and power control to support billions of machines/devices to be connected worldwide. The architecture of the CRN must maintain a high data rate (throughput) at low power consumption, which requires both spectrum efficient and energy efficient system design. To this aim, the proposed work adopts a CRN model, which operates in an interweave mode that allows spectrum sensing followed by opportunistic secondary user (SU) data transmission over the unused bandwidth of the primary user (PU) in a non-overlapping frame structure. Closed form expressions of the optimal spectrum sensing duration, bandwidth, and power allocation of each secondary node are obtained to maximise the sum throughput of the overall CRN while maintaining the constraints of sensing reliability of PU, individual SU transmission outage probability, permissible interference, and residual bandwidth. Numerical results highlight that the proposed scheme maximises the sum network throughput by and over the other existing techniques.

Technologies such as cognitive radio and dynamic spectrum access rely on spectrum sensing which provides wireless devices with information about the radio spectrum in the surrounding environment. One of the main challenges in wireless communications is the interference caused by malicious users on the shared spectrum. In this manuscript, an artificial intelligence enabled cognitive radio framework is proposed at system-level as part of a cyclic spectrum intelligence algorithm for interference mitigation in wideband radios. It exploits the cyclostationary feature of signals to differentiate users with different modulation schemes and an artificial neural network as classifier to detect potential malicious users. A dataset consisting of experimental modulated and dynamic signals is recorded by spectrum measurements with an in-house software defined radio testbed and then processed. Cyclostationary features are extracted for each detected signal and fed to a neural network classifier as training and testing data in a complex and dynamic scenario. Results highlight a classification rate of in most of cases, even at low transmission power. A comparison with two previous works with hand-crafted features, which employ an energy detector-based classifier and a naive Bayes-based classifier, respectively, is discussed.

Compared with traditional radio frequency identification (RFID), computational RFID (CRFID) tag has more powerful computing capability, but it shows poor performance when it follows EPC Class-1 Generation-2 protocol especially transmitting large amounts of data. In fact, the operation of the CRFID tag entirely depends on the state of energy. For this point, this study proposes an optimised protocol called LILAC. LILAC allows tags to select the communication time slot according to their current voltage value measured by using an analogue-to-digital converter rather than randomly selecting, the authors proposed a more reasonable time slot mapping algorithm for LILAC that increases the success rate of tag responding. In addition, they design a data transmission format that can be retransmitted to improve the uplink throughput. Finally, they implemented LILAC on a CRFID platform and practically measured several parameters to compare with the existing well-known protocol. The results of experiments show that LILAC increases the maximum communication distance by more than half in the access phase and doubles the goodput of the backscatter link in both the inventory and access phase.

The traditional fully-digital beamforming is realised by the full radio-frequency (RF) chain configuration, which will be impracticable in massive multiple-input multiple-output (MIMO) systems because of the overburden energy consumption of RF chains at millimetre wave frequencies. To address this issue, a series of hybrid beamforming schemes have been proposed to reduce the number of RF chains. Although the hybrid beamforming schemes for single-user MIMO (SU-MIMO) have been studied extensively, the performance of hybrid beamforming for multi-user MIMO (MU-MIMO), especially for partially-connected hybrid architecture, still has room for improvement. In this study, the authors propose a hybrid beamforming scheme for MU-MIMO systems. Specifically, they focus on the design of analogue beamforming and optimise the analogue RF precoders and combiners jointly. The simulation results show that the proposed hybrid beamforming in fully-connected structure can achieve a better performance compared with the existing hybrid beamforming schemes in both SU-MIMO and MU-MIMO systems. It is also observed that the proposed hybrid beamforming can achieve significant performance advantages compared with the existing hybrid beamforming schemes in partially-connected structures.

Spectral efficiency, energy efficiency, and security are cornerstones for the upcoming 5G systems. In this study, the issue of how the energy and spectral efficiency of multiuser massive multiple-input multiple-output (Ma-MIMO) systems are affected in the presence of a secrecy constraint is addressed. The performance of the two most prominent linear precoding techniques, the matched filter (MF) and zero-forcing (ZF) precoders, for secure downlink multiuser Ma-MIMO in the presence of multi-antenna passive eavesdropper is investigated. The authors consider three performance metrics, namely, the achievable ergodic secrecy rate, the secrecy spectral efficiency (SSE), and the secrecy energy efficiency (SEE), assuming perfect and imperfect channel state information. The tradeoff between SSE and SEE is also studied. Moreover, the authors derive tight lower bounds on the achievable ergodic secrecy rate for MF and ZF precoding techniques. The derived lower bounds provide insights on the tradeoff between the SSE and SEE. It is shown that ZF precoder outperforms MF precoder at high transmit power, whereas at very low transmit power, MF outperforms ZF. Moreover, it is shown that using large number of transmit antennas can improve the SSE and the SEE with orders of magnitude compared to a single-input single-output system.

A simultaneous packet transmission (SPT) technique is proposed to increase the throughput of a communication system. A SPT scheme employs multiple levels and employs multiple streams at each level above the first level. Every stream of the SPT has its assigned partition on the overall constellation. Symbols of an N-level SPT are formed by taking , bits from a selected stream of every level. The SPT scheme can employ higher code rates on streams at higher levels to increase the system throughput. Numerical results show that the SPT offers 15–30% throughput enhancement in turbo-coded LTE systems.

In this study, the authors consider the secrecy communication of one source–destination pair in a wireless multi-hop relay network, where each node is equipped with multiple antennas. They use the method of interference alignment for secrecy transmission in the presence of an eavesdropper. Firstly, they propose a decode-and-forward (DF) relaying protocol and give a new result on the joint probability density function of the first, second, …, kth () largest eigenvalues of the complex Wishart matrices , where is an matrix whose elements are independent identically distributed complex Gaussian variables with zero mean and unit variance. They then utilise this result to characterise the legitimate outage probability and the diversity order of the proposed DF protocol. The case where the relay works in the amplify-and-forward (AF) mode is considered at last, and the diversity order of the proposed AF protocol is given.

The Neighbour Discovery Protocol and the Address Resolution Protocol are important protocols in the data link layer. Their functions include Internet Protocol (IP) address configuration, resolving the correspondence between an IP address and a medium access control address, and duplicate address detection (DAD). In DAD, the new address that the node is going to use is public, and thus, it is vulnerable to malicious node attacks. Moreover, address configuration is inefficient because only one address is generated and detected each time. In this study, the authors propose a multi-address generation and DAD scheme called MAGD. MAGD generates a set of addresses each time, but only discloses a part of the set during DAD, thereby reducing the risk of being attacked. DAD will only fail when all the addresses are in conflict, and thus, the efficiency of node's address configuration is enhanced. Experiments show that the additional overhead in the CPU and memory caused by MAGD's multiple address configuration is within an acceptable range. When subjected to denial-of-service (DoS) attacks, MAGD performs better than traditional encryption schemes.

In this study, the authors present and evaluate the performance of two mode selection schemes for device-to-device (D2D) enabled unmanned aerial vehicle-based wireless networks. The proposed schemes are based on a threshold received signal strength and an average threshold D2D distance to select the D2D mode. The focus of the two schemes is either to enhance the quality of the signal or the connectivity in case of emergency situations. To evaluate the performances of the schemes, they derive the corresponding expressions of the probability of using D2D mode and ergodic capacity. Numerical results show the advantage of the presented schemes in off-loading traffic from aerial platforms and shed lights on the effect of the environment on the performance of D2D enabled aerial networks.

With the rapid development of dynamic adaptive streaming over HTTP (DASH) services, how to satisfy the requirements of DASH clients has attracted more and more attention. This study was carried out to focus on the quality of experience (QoE) modelling and the model-based cross-layer design which consists of segment adaptation and resource allocation, improving the playback experience of the clients. First, the key factors are investigated, which can affect the clients’ subjective satisfaction. A characteristics and playback information of the segments-based QoE (CPIQ) model is established by employing the curve-fitting method based on a large amount of subjective experimental results of these factors. Then with the CPIQ model as the objective function, a cross-layer design CPIQ model-based joint algorithm of segment request and resource allocation (CJRRA) model is formulated to maximise the total QoE of all the clients subject to the network resource and segment representation constraints. Segment adaptation and resource allocation strategy can be determined jointly by the authors developed low complexity solution. Finally, the simulation results show that the overall performance of CPIQ model and CJRRA significantly outperforms other compared models or algorithms in terms of accuracy, linearity and stability.

Recently, a hybrid remote sensing network constituted by satellites in constellation and Unmanned Aerial Vehicles (UAVs) in formation attracts a lot of interests, benefiting from the flexible architecture and excellent rapid responsiveness. Considering frequently intermittent connectivity and limited resource onboard, Disruption-Tolerant Networking (DTN) develops a feasible solution for the remote sensing scenarios. However, the intrinsic motion models of multifarious nodes lead to deterministic or semi-deterministic contacts, which makes finding a reliable end-to-end routing path for timely data delivery difficult, with typical routing strategies such as Contact Graph Routing (CGR). To cope with such routing challenge in the hybrid network, a Probabilistic Contact Graph (PCG) is designed, taking the diverse node properties into consideration. In particular, a probability prediction model for semi-deterministic contacts between the UAV nodes is proposed, with a semi-Markov motion model for the UAV nodes. Besides, a Markov Decision Process based Routing (MDPR) algorithm is designed to search for a feasible data transmission path with a series of hybrid deterministic and semi-deterministic contacts. Through the numerical and experimental simulations with Interplanetary Overlay Network (ION), the proposed MDPR algorithm shows excellent routing performance concerning delivery delay and delivery ratio, compared with the typical CGR strategy.

Channel estimation in time division duplexing (TDD)-based massive multiple-input multiple-output (MIMO) systems is heavily hampered by the pilot contamination, which constitutes a major bottleneck on the overall system performance. This study considers the pilot contamination problem in multi-cell TDD-based massive MIMO systems, and analytical expressions are presented on the normalised mean square error (NMSE) of the minimum mean square error channel estimation algorithm. Based on the obtained NMSE, this study proposes an optimal pilot assignment strategy to minimise the effect of pilot contamination. In order to further improve the system performance, a pilot design-based channel estimation scheme is proposed, where Chu sequences with perfect auto-correlation property are employed to design the optimal pilot sequences aiming at acquiring the accurate channel state information. Simulation results show that the proposed pilot assignment strategy outperforms the random pilot assignment method, and approaches to the performance of the exhaustive search method which requires high computational complexity. Moreover, the performance gain of the pilot design-based channel estimation scheme is verified in massive MIMO systems.

In decentralised cognitive radio (CR) networks, establishing communication sessions between a communicating pair requires them to meet each other on a common channel via a ‘rendezvous’ process. Devising distributed CR rendezvous protocol is a challenging task as cognitive nodes are not necessarily synchronised, and may have different perceptions of channel availability. In this study, the authors present M-Rendezvous, an order-optimal rendezvous protocol exploiting the performance gain brought by having multiple radios at cognitive nodes. As a distinguished feature, M-Rendezvous is a unified rendezvous protocol that can operate in both homogenous case where both of the rendezvous nodes are equipped with only one radio or multiple radios, and heterogeneous case where one of the rendezvous nodes has single radio and the other has multiple radios. In both cases, by rigorous analysis, the authors demonstrate that M-Rendezvous can guarantee rendezvous over every channel with bounded and order-minimal delay even when rendezvous nodes have asynchronous clocks and asymmetrical channel perceptions.

In this work, the authors consider phase shift keying (PSK)-type signals with very compacted spectrum and low envelope fluctuations obtained through the use of magnitude modulation (MM) techniques and stringent roll-off pulse shaping filtering. The analytical bit error rate (BER) of PSK-type signals combined with MM is assessed, for both flat fading and time-dispersive channel scenarios. The authors use a Gaussian model to approximate the statistical distribution of the distortion error introduced by MM to enable a closed-form evaluation of the BER. The proposed expression depends only on the constellation order and the Kullback–Leibler divergence of the MM factors' probability density function from the Gaussian one, and it was found to be very accurate.

Prolonging the network lifetime for a reasonable period while meeting users' needs is key in developing a data gathering algorithm for WSNs. Clustered sensor networks are considered as an efficient approach for extending the network lifetime of WSNs. In clustering, sensor devices are arranged in clusters. Inside the cluster, non-cluster head nodes transmit data to the cluster head (C _H) nodes while the C _H forward the message to the sink. Irrespective of how the clusters are formed, a clustered WSN requires sturdy energy-balanced (EB) and energy-efficient (EE) communication protocol. Different from the previous clustered based routing protocol, this paper develops and analyses a selective-path priority table EB and EE clustering based routing protocol, which employs a mobile sink. The priority table is formed by prioritising the two shortest paths to the C _H or sink according to some simple but efficient rules. The rules are derived using some routing metrics such as transmission range/power and residual energy. The simulation results show that the proposed routing protocol can be integrated with most clustering formation algorithms, even in large scale WSNs. Besides, results show that this routing protocol improve the network lifetime and throughput as compared to similar schemes.

Spatial modulation is a promising transmission scheme for massive multiple-input multiple-output (MIMO) systems to improve energy efficiency. In this study, considering the low-complexity hybrid structure, the authors propose a joint spatial modulation and beamforming scheme for a hybrid massive MIMO system in both single user and multi-user scenarios. Specifically, a design criterion of the channel control parameter is provided to reduce the high correlation among different channel coefficients in the hybrid massive MIMO system. In addition, with the statistical channel state information acquired at the base station, the optimum analogue beamforming vectors are designed with the goal of maximising the signal to leakage and noise ratio in the multi-user scenario. Furthermore, the upper bound, the lower bound as well as the closed form approximation of achievable spectral efficiency of the proposed scheme are derived for both single user and multi-user scenarios. Numerical simulation results demonstrate that the proposed scheme outperforms the conventional MIMO scheme.

As an issue under heated discussion, spectrum sensing (SS) exhibits its particular significance in various application scenarios due to its limited spectrum resource and low spectrum utilisation. For a given SS scheme, detection probability, false alarm probability and available throughput jointly determine its performance. To consider these three factors as a whole, the possible performance improvement of an SS scheme will make a great difference in active demand for idle spectrum, especially in Internet of things-based domains. Motivated by this, this study proposes a new strategy on SS to further improve its sensing performance, where the optimal sensing performance is demonstrated to be better than traditional schemes when signal-to-noise ratio is higher than 1.76 dB. Additionally, the authors extend the proposed scheme to multiple secondary user case and give optimal N for N-out-of-K fusion rule. Both theoretical derivation and simulation experiments validate the effectiveness of the proposed scheme.

Multiple-input multiple-output is being considered as a highly interesting topic for the new generation of wireless communication systems. The major challenge is how to enhance the link reliability while neither affecting the spectral efficiency nor the quality of service. To reduce the complexity of the maximum likelihood (ML) receiver, efforts have been focused on zero forcing (ZF) decoder using lattice reduction-aided (LRA) techniques. In this study, the authors propose a method that improves performance using the LRA-based precoding technique without increasing the decoder complexity. Their approach is to exploit the channel matrix to generate matrices for both the precoding and LRA detection technique. This makes it possible to achieve maximum orthogonality for the generated channel matrices while reducing considerably the noise effect. They focus on the use of the LRA-ZF for the ease of its implementation and low complexity. The proposed method outperforms LRA-minimum mean square error-based receiver and gets closer to ML receiver performance. The performance and complexity of the proposed scheme are discussed.

Indoor localisation has become increasingly important for many services. When the environment is not crowded, i.e. most links between the target and anchors do not suffer from non-line-of sight (NLOS) offsets, the centralised time-difference-of-arrival localisation can efficiently estimate the target position. However, such a method does not work in crowded environments unless a large number of anchors are installed, which is unrealistic for many applications. In this case, the objects that block the LOS links can serve as dynamic anchors (e.g. with round trip time-of-arrival localisation) to improve positioning accuracy. However, such a method tends to include too many unreliable dynamic anchors in estimation, which will significantly increase the computational complexity (e.g. to detect and discard such unreliable anchors in the NLOS mitigation stage). This study proposes a coarse NLOS detection algorithm based on discrete power levels to efficiently achieve the coarse NLOS mitigation, which automatically discards most unreliable dynamic anchors. The simulation results show that the proposed method can achieve very similar estimation performance as the optimal estimation method in the crowded environment, while analysis shows that it can significantly decrease the computational complexity.

Based on outdoor microcellular measurements at 26 and 32 GHz, the path loss models are modified by a proposed dynamic rain model to see the difference of the path loss models between clear and rainy air with respect to rainfall intensities and transceiver distances. Moreover, a dynamic rain cell model for a multi-user system is developed to investigate the total rain attenuation. The results show that the parameters for floating-intercept (FI) model have a bigger change with different rainfall intensities than close-in (CI) model. When considering the dynamic rain model with a different radius and moving speeds, the larger rain cell radius, the larger path loss exponents in CI and FI models, the smaller path loss intercept in FI model and system capacity will be achieved. For a fixed rain cell radius, the larger moving speed of the rain cell, the shorter effective time on the path loss and system capacity will be achieved. In addition, the rain cell has a bigger effect on the path loss exponent, intercept and system capacity at higher frequency band with respect to its size, moving speed, and rainfall intensity.

In this work, the authors have explored the average bit error rate (BER) and ergodic capacity (EC) of a frequency-selective channel consisting of multi-hop and multi-branch (MHMB). Each hop consists of multiple paths and signal travels in successive hops using decode and forward (DF) relaying. The channel is assumed to be log-normal distributed. Numerical integration (NI) formulations based on the Gauss-quadrature rule representation of the moment generating function of log-normal distribution have been derived for both average BER and EC for multi-hop and MHMB cooperative communication framework using maximal ratio combining and selection combining (SC) techniques at receiver. Results are obtained for a variable number of hops and branches. Excellent matching between Monte Carlo simulation and NI plots validate the accuracy of the proposed formulations. The contribution of this work to the existing research scenario is two-fold: (i) to the best of the authors' knowledge, this is a first attempt to study a generic MHMB framework of a log-normal distributed frequency-selective channel. (ii) DF which is a more practical relaying protocol has not been explored till date for analysis of a log-normal distributed MHMB network. Both of these aspects make this work pertinent.

This study investigates the information security problem in amplify-and-forward (AF) satellite transmission with eavesdroppers at both the transmitter and the receiver. Two multi-antenna earth stations aim to exchange messages through a multi-antenna satellite in two hops. The satellite channels (both uplink and downlink) are assumed to be shadowed-Rician fading. In the first hop, the multiple term weighted fractional Fourier transform (MWFRFT) is adopted to secure the messages from the eavesdropper at the transmitter. To overcome the simplicity of parameters in MWFRFT, the authors enhance the MWFRFT with frequency hopping and propose the frequency hopping multiple term weighted fractional Fourier transform (FH-MWFRFT) to improve the secrecy. In the second hop, the satellite broadcasts the signal to the receiver after null space beamforming (NSBF). Since the energy at the satellite is confined, the NSBF is simplified by applying the singular value decompose to avoid the complicate optimisation calculations at the satellite. The one-way and two-way transmission scenarios with full channel state information (CSI) and statistic CSI are both considered in this study. Simulation results demonstrate the proposed method could achieve a better security at the transmitter while maintain a similar secrecy performance to the optimisation method at the legitimate receiver.

Cognitive radio (CR) has been proposed for spectral efficiency improvement while avoiding interference with licensed users. The offset quadrature amplitude modulation-based filter bank multicarrier (OQAM-FBMC) modulation is a promising candidate for CR-based systems. Although it behaves better in many aspects than orthogonal frequency division multiplexing (OFDM), the application of some techniques with FBMC becomes more challenging due to the imaginary interference, as in the case of multiple-input multiple-output (MIMO) technique. Here, the authors first propose a new MIMO-based FBMC transceiver structure for CR systems. The proposed structure is based on QAM transmission to handle the interference presented in conventional OQAM-FBMC systems. The authors also introduce two different methods to satisfy the orthogonality conditions and cancel the residual interference in QAM-FBMC systems. Both methods are based on discrete cosine transform (DCT) to achieve superior spectrum confinement and hence better spectral efficiency. Moreover, the authors present a resource allocation algorithm for the proposed structure. Simulation results demonstrate that the proposed structure outperforms the conventional FBMC-based MIMO techniques in terms of bit error rate (BER) and computational complexity. The results also show that the spectral efficiency of the proposed structure could be further improved by applying the proposed resource allocation algorithm.

Routing design is an effective mechanism to improve the energy-efficiency in packet transmissions for wireless multihop networks. Meanwhile, the total energy consumption for packet transmissions over a path comprising of several unreliable links depends on the power assignment on each link. In this study, the authors address the energy-efficient routing, which integrates with the optimal power assignment, in end-to-end (E2E) retransmission systems. The process of packet transmission across a path in E2E systems is modelled as a random walk. The expected energy consumed for the packet transmission is then calculated. As the expected energy consumption closely relies on the transmit power assigned on the path, it is necessary to integrate the power assignment into the energy-efficient routing. The optimal power assignment on the candidate path is analytically derived for minimising the expected energy consumption. Specially, the energy-efficient routing integrated with power sssignment (EERPA) algorithm is developed to find the optimal path among all candidate paths with all possible power assignments. Simulations demonstrate the efficiency of the authors' EERPA algorithm.

The issues of the physical layer security are studied, where one transmitter (Alice), one full-duplex (FD) legitimate receiver (Bob) and one multi-antenna jammer (Charlie) are considered to defend against multiple passive and non-colluding eavesdroppers (Eves), for multiple-input multiple-output wiretap channels. With the assumption that the complete channel state information (CSI) for main channels and the statistical CSI for Eves' channels are available, an improved secrecy transmission design is proposed to minimise the secrecy outage probability (SOP), under the minimum secrecy rate requirement. In addition to integrating artificial noise aided transmit beamforming and multi-antenna cooperative jammer to impair Eves' channels, which is widely used in many existing works, a FD legitimate receiver is applied together to further enhance physical layer security. Based on the basic probability theory and convex optimisation, the accurate closed-form expression of the SOP is derived, and the impacts on secrecy performance are described, including the qualities of main channels, the Alice's transmit power and the number of Eves. Numerical results show that the proposed scheme using the FD Bob can significantly enhance the system security, as compared with that employing the half-duplex Bob.