The authors consider secret communication through a relay-assisted cognitive interference channel in which primary and secondary base stations (SBSs), respectively, communicate with the primary and secondary receivers (PU-Rx and SU-Rx) in the presence of multiple eavesdroppers. An SBS is allowed to transmit simultaneously with the primary base station (PBS). They propose three cooperative jamming (CJ) schemes based on the available channel state information at the base stations and the relay. The proposed CJ schemes are designed to create additional interference in the direction of eavesdroppers without creating any interference to the PU-Rx and the SU-Rx. A combined approach of beamforming and zero-forcing precoding is developed at the PBS, the SBS and the relay to cancel out jamming interference at the PU-Rx and the SU-Rx. The secrecy rate of SU-Rx is calculated with the constraint of maintaining interference temperature at the PU-Rx under a certain threshold. Compared with the direct transmission schemes that are available in the literature, the authors’ results show that the approach which combined beamforming and CJ significantly improves the secrecy rate of the cognitive interference channel.

In this study, the problem of energy-efficient power allocation (EEPA) for secrecy wireless information and power transfer in a massive multiple-input multiple-output relay aided secure communication system is well addressed. The relay forwards the signal sent from a source to a legitimate destination with the harvested energy based on a decode-and-forward relaying protocol, while a passive eavesdropper intends to intercept the message. The authors first derive a closed-form expression for the secrecy energy efficiency of the considered system under practical conditions, i.e. no instantaneous eavesdropper channel state information (CSI) and only imperfect legitimate CSI. Then, they propose an EEPA scheme for maximising the secrecy energy efficiency. Finally, simulation results validate the effectiveness of the proposed scheme.

In this study, the effects of pilot asynchronism on channel estimation of multi-user multiple-input multiple-output systems are analysed. Classical linear sequences such as Gold, Walsh–Hadamard and quaternary were deployed as pilots in large-scale MIMO context aiming to trace their behaviour in asynchronous scenarios. Through this study, it was derived a closed form expression for the channel estimate error, suggesting that asynchronism on channel estimation may introduce deviations from the actual channel and spatial correlation, which decreases the overall performance. Numerical results also demonstrated performance degradation of the single-cell massive MIMO system as asynchronism increases, besides of highlighting the effect of the bit-error-rate floor as the number of base stations antennas increases in such scenarios.

The authors present efficient frequency-domain channel estimation methods based on the intra-symbol frequency-domain averaging (ISFA), minimum mean squared error (MMSE) and weighted inter-frame averaging (WIFA) schemes for the orthogonal frequency division multiplexing (OFDM) visible light communications (VLC) system. OFDM-VLC with quadrature phase shift keying, 16- and 64-quadrature amplitude modulation mapping is experimentally demonstrated. Compared with the conventional least square channel estimation method, ISFA, MMSE and WIFA offer improved performance with MMSE offering the best performance in terms of the error vector magnitude but at the cost of high complexity. The authors show that the WIFA can improve the estimation accuracy of time-varying VLC optical channel.

To improve the energy efficiency of multi-cell massive multiple-input–multiple-output system while guaranteeing the information transmission quality, this study proposes a dual-layer coordinated beamforming scheme. In the proposed scheme, the beamformer at each evolved node B (eNB) is divided into a cell-layer beamformer and a user-layer beamformer. The cell-layer beamformer is used to mitigate inter-cell interference (ICI) by exchanging long-term channel state information (CSI) among eNBs. The user-layer beamformer serves user according to the local real-time CSI at each eNB. On the basis of the dual-layer structure, the cell-layer beamformers, the user-layer beamformers, and power allocation are jointly optimised in order to minimise the total transmit power across all the eNBs subject to the signal-to-interference-plus-noise ratio requirements and single-antenna power constraints. To make the original problem solvable, the ICI is replaced by its upper bound. Then, the problem is partitioned into two convex sub-problems, and two iterative algorithms are proposed in order to find the sub-optimal solution to the original optimisation problem. Simulation results show that the proposed scheme performs better than two reference schemes including the existing zero-forcing scheme and coordinative multiple point schemes.

This study presents an analytical method that shows how spatial correlation degrades the overall performance of cooperative spectrum sensing. In a cooperative spectrum sensing, where all secondary users (SUs) report their sensing information to the fusion centre (FC), sensing information is correlated for closely located users due to the similar surrounding environment. Considering the soft combining at the FC, this study shows how detection performance of correlated SUs is different from uncorrelated SUs. Furthermore, an algorithm is proposed which selects best uncorrelated SUs among all SUs of the network based on received power to the FC from SUs. The simulation results confirm the performance analysis of cooperative spectrum sensing with spatial correlation on the sensing data.

In this study, the performance of decode-and-forward relay-assisted free-space-optical (FSO) communication systems under atmospheric turbulence-induced fading and misalignment errors is investigated. To mitigate the adverse effects of the atmospheric turbulence, the aperture-averaging receivers are considered both at the relay and destination sides. The atmospheric turbulence-induced fading is modelled via the exponentiated-Weibull distribution, which has recently been proposed to characterise an FSO link in the presence of finite-sized receiver aperture. The expression for the moment generating function (MGF) of the instantaneous signal-to-noise ratio is derived. Furthermore, new closed-form expression for the outage probability is obtained. Moreover, the new expression for the average symbol error rate of the subcarrier intensity-modulated M-ary phase-shift keying is obtained using the MGF-based approach. Finally, numerical examples are discussed and all the derived analytical results are corroborated by Monte Carlo simulations.

As a prevailing concept in 5G, virtualisation provides efficient coordination among multiple radio access technologies (RATs) and enables multiple service providers to share different RATs’ physical infrastructures. This study proposes a generic framework for virtualising heterogeneous networks with different RATs. A novel virtual medium access control concept is introduced to realise resource offloading (allocation) and mobility management of the framework. The offloading and handover protocols are designed in detail. A novel resource offloading strategy is devised: First, to model the fact that different RATs possess different adaptability to different services, an ‘adaptability ratio’ concept is introduced and calculated using grey relational analysis. After that, a matching theory based framework is proposed to formulate the resource offloading problem and a ‘deferred acceptance’ algorithm is introduced to solve it. Through simulation, it is proved that the proposed protocols can efficiently reduce the service interruptions and improve the resource usage.

This study considers the optimisation of multi-edge type low-density parity-check (MET-LDPC) codes to maximise the decoding threshold. The authors propose an algorithm to jointly optimise the node degree distribution and the multi-edge structure of MET-LDPC codes for given values of the maximum number of edge-types and maximum node degrees. This joint optimisation is particularly important for MET-LDPC codes as it is not clear a priori which structures will be good. Using several examples, they demonstrate that the MET-LDPC codes designed by the proposed joint optimisation algorithm exhibit improved decoding thresholds compared with previously reported MET-LDPC codes.

For a mobile multihop relay (MMR) system, a centralised packet scheduling method is compared with a distributed packet scheduling method. The performance of the MMR system considering the joint effect of a finite-length queue and an adaptive modulation and coding scheme for base station and relay station is analysed. A finite state Markov chain (FSMC) describing the queue and channel state pairs’ transitions for the MMR system is built. Moreover, a method of reducing the state space of the FSMC, which enables a hop-by-hop performance analysis of the MMR system without explosive growth in the number of states, is presented. Numerical and simulation results show that the proposed FSMC modelling is accurate and the performance of centralised scheduling becomes degraded because of increased mismatches between a scheduled modulation and coding scheme (MCS) level and the actual MCS level.

Two-path successive relaying (TPSR) improves spectral efficiency of relay under half-duplex constraint. However, when the relay pair is operating in non-orthogonal channels, the capacity of TPSR can be severely degraded by inter-relay interference (IRI). Relay selection has been proposed in the existing literature to mitigate the IRI. However, existing opportunistic TPSR (OSR) schemes limit the pool of potential relay pair choices. Eventually, the performance of existing OSR schemes is underestimated. This study proposes four OSR schemes to improve the achievable performance of OSR schemes in terms of ergodic capacity and outage probability. The proposed schemes select a pair of relays jointly from the N potential relays. This increases the pool of potential relay pairs quadratically. Acknowledging the fact that OSR is only good for certain operating conditions, adaptive schemes that dynamically switches between OSR and opportunistic half-duplex relaying (OHR) are proposed. The simulation results show that the proposed schemes outperform the existing OSR, OHR and full-duplex relaying schemes significantly. The asymptotic ergodic capacity analysis of the proposed schemes increases logarithmically with the quadratic of N. The diversity and multiplexing tradeoff analysis reveals that the proposed schemes achieve the full diversity and multiplexing gain given a large number of potential relays.

In this study, the authors investigate the problem of energy-efficient packet transmission with arbitrary arrival instants and deadline constraints over a point-to-point additive white Gaussian noise channel. This is different from previous work where it is assumed that the packets follow a first-in–first-out (FIFO) order in that a packet that arrives earlier has a deadline that is also earlier. They first investigate the necessary and sufficient conditions of the optimal offline transmission schedule. They then propose an algorithm which finds the transmission schedule of each packet. Next, they show that their algorithm is optimal by proving it satisfies the sufficient conditions of the optimal offline transmission schedule, and further obtain the computational complexity in the worst case: 𝒪(N ³), where N is the number of packets. In addition, based on the proposed optimal offline policy, an efficient heuristic online policy which assumes only the causal arrival information of the packets is proposed. Finally, simulation results show that the proposed offline policy can achieve significant energy savings compared with the existing FIFO offline policy, and the proposed online algorithm can achieve a comparable performance with the proposed optimal offline policy.

The tradeoff between decreasing interference to primary user (PU) and increasing secondary user's (SU's) throughput is of great importance for cooperative sensing in cognitive radio networks. Non-ideal spectrum sensing in PHY and multiple SUs' access contention in MAC jointly impact SUs' transmission and the tradeoff. In this study, the authors investigate the joint impact from a cross-layer perspective. First, they quantify the reliability of cooperative sensing and compute SUs' transmission probability under the conditions of non-ideal sensing and access contention. Closed-form expressions of the interference probability to PU and SUs' throughput are derived. Specially, two widely-used contention-based MAC protocols, i.e. slotted Aloha and distributed coordination function, are studied. Then, they formulate the sensing-throughput tradeoff problem by using interference probability to PU, rather than the detection probability, as the constraint. Finally, a 2-dimension search algorithm is proposed to obtain the optimal solution, including the optimal fusion rule, sensing duration and detection threshold. Simulation results validate the outperformance of the cross-layer scheme. They also demonstrate how the optimal solution varies with some key parameters, i.e. PU's signal-to-noise ratio and the number of contending SUs.

As the shortage of spectrum is becoming a serious problem to the wireless communication, visible light communication (VLC) is attracting widespread attention. However, the access to the backbone network is likely to completely modify the communication network, thus leading to a great cost. Fortunately, the technology of power line communication (PLC) is a perfect match to VLC, which provides power supply and connection for VLC to the backbone network. In this study, the authors take an integration of PLC and VLC into consideration and mainly focus on a communication framework with multi-service. In the case of not making a big modification to the origin network, a direct re-transmission concept is considered and three implementation schemes which differ in frequency domain, time domain, and bit division multiplexing (BDM) are proposed. The demonstration platform and the simulation results suggest that the proposed schemes have the ability to support multi-service transmission with no big modification of current power line network. Moreover, the BDM scheme can accommodate different requirements of quality of service with better performance than conventional multi-service schemes.

In distributed multiple-input multiple-output (D-MIMO) systems, the improvements of spectral and energy efficiencies rely heavily on the accuracy of the channel state information (CSI). In order to enhance the accuracy of CSI acquisition, the problem of pilot design in a D-MIMO system is addressed in this study. In particular, the authors focus on the optimisation of pilot power adaptation in a single-cell D-MIMO system with multiple users served in orthogonal resources. Under the concept of tomographic channel estimation, the problem of pilot power adaptation aiming at maximising the lower bound of the sum capacity is formulated. As the computational complexity of solving the problem is relatively high due to the mutual coupling constraints, the dual decomposition technique is introduced to decouple the constraints and reduce the complexity via parallel computation. An effective pilot power adaptation scheme is further proposed by using the projected subgradient method. The superiority and effectiveness of the proposed scheme are illustrated by the simulation results.

An empirical stochastic discrete tapped delay line (DTDL) power delay profile (PDP) model is presented. It is used for characterising the multipath effects under indoor stair environment. In this model, the amplitude at each DTDL tap and stair step follows the Nakagami distribution. Its scale parameters are lognormally distributed, and its shape parameters are distance and propagation delay dependent. Then, the procedure for simulating the PDPs is given. In addition, the average PDP, root mean square delay spread and capacity extracted using the measured and simulated channels are compared to validate the accuracy of the proposed model. Finally, a measurement-based channel simulator is developed by implementing an orthogonal frequency division multiplexing communication procedure on the simulated channel. These works can provide important information about the designs of the physical layer algorithms in small cells scenarios.

Partial transmit sequence (PTS) is an effective scheme to reduce high peak-to-average power ratio (PAPR) for multicarrier modulation (MCM) signal transmission systems. This approach produces side information (SI) data as a result of the MCM signal optimisation process. The generated SI data are required to be transmitted with the original data over the channel for successful data recovery at the receiver. An effective method for SI data transmission has not yet been identified and research is still ongoing. Hence, the authors introduce a technique that embeds SI data into the original data frame. In this study, a wavelet packet (WP)-based PTS (WP-PTS) scheme is selected as the MCM transmission method. The proposed scheme is called WP-PTS with embedded SI data. In addition, a suitable scheme for reconstructing the original data is developed. Simulation result shows that the PAPR performance of the proposed scheme improves by up to 2.5 dB at a complementary cumulative distribution function level of 10⁻⁴ compared with the original WP-orthogonal frequency-division multiplexing system without the PAPR reduction scheme when the number of selected disjoint subblocks is 16.

In mobile ad hoc networks (MANET), the existing route discovery may result in traffic overflow and overhead. In order to overcome these issues, in this study, the authors propose an ant based multipath backbone routing for load balancing in MANET. When the source wants to transmit data towards destination, it selects the multiple routes with maximum path preference probability using swarm based ant colony optimisation technique. The path preference probability is estimated based on next hop availability, delay and bandwidth. During route discovery, the nodes subjected to faults are found and the relevant path is skipped. Then the network load on the routes is balanced by an index by each backbone node to distribute the data traffic equally on the links from source to destination. By simulation results, the authors show that proposed technique reduces the network load.

In vehicular ad hoc network (VANET), the accuracy of the channel estimation plays a key role toward obtaining considerable improvement to the physical layer performance. In this study, least square (LS) and minimum mean square error (MMSE) estimation techniques are discussed and their performances in different VANET transmission scenarios are thoroughly investigated. Furthermore, a modified scheme of these techniques to improve the performance of the system and reduce the complexity in such a way that suits VANET communications is developed. Discreet Fourier transform and singular value decomposition techniques are companied with LS and MMSE to develop a channel estimation scheme which suits VANET channel conditions and ensures performance improvement and remarkable complexity reduction. Comprehensive simulations results show that the developed scheme greatly improves the performance of the system compared with LS and substantially reduces the complexity of MMSE.

In this study, the authors propose an improved multiple feedback successive interference cancellation (IMF-SIC) algorithm and an ordered IMF-SIC (OIMF- SIC) algorithm for near-optimal multiple-input multiple-output (MIMO) detection. In particular, the multiple feedback (MF) strategy in successive interference cancellation (SIC) detector is based on the concept of shadow region, where, if a decision falls in the shadow region, then multiple neighbouring constellation points are used in the decision feedback loop followed by the SIC technique, and the best candidate symbol is selected by using maximum likelihood cost. However, while deciding the best symbol, the shadow condition is not checked in the subsequent layers which may result in an unreliable decision. Thus, to improve the accuracy of a decision, the authors propose an improved MF strategy where the shadow region condition is checked recursively. Further, the authors also propose an OIMF-SIC algorithm where the log likelihood ratio based dynamic ordering is utilised for ordering the detection sequence. Simulation results validate superiority of the proposed algorithms over the other SIC based detection techniques. In addition, to validate robustness of the proposed algorithms, BER performance is computed and compared under channel state information mismatch.