In this study, the performance of energy harvesting systems in the presence of unreliable backhaul links is investigated over independent but not necessarily identically distributed Nakagami-m fading channels. In particular, the energy-constrained relay uses the amount of harvested energy from the best small-cell transmitter based on time switching-based relaying protocol to process for the next hop transmission. The authors derived the closed-form expressions of the outage probability and the effective throughput with two distinct transmission schemes: (i) delay-limited transmission, and (ii) delay-tolerance transmission are attained. In order to assess the impacts of unreliable backhaul links, they thus obtain the asymptotic expression of the outage probability in high signal-to-noise ratio (SNR) regime. The numerical results are conducted to analyse the effects of energy harvesting fraction time, energy efficiency, backhaul reliability, and the fading parameters on the system performance. The authors' results show that under the unreliable backhaul links the outage probability yields the error-floor in the high SNR regime which demonstrates the significant impact of the backhaul unreliability.

In this study, the secrecy performance of cooperative half-duplex cognitive relay networks (CRNs) comprised of multiple primary users (PUs) and multiple eavesdroppers under the effect of unreliable wireless backhauls is studied. The authors consider relay networks with single relay assisting multiple sources under power constraints at PUs and secondary transmitters, and based on the availability of channel-state information at the sources, they propose two best source selection schemes, namely: (i) optimal source selection (OSS) and (ii) sub-optimal source selection (SoSS). For both scenarios, they obtain exact closed-form and asymptotic expressions for the system's outage probability and carry out numerical simulations to justify their analyses. From the results, source selection is proven to be capable of counteracting the negative impact of unreliable backhaul on CRNs. The secrecy performance limitations are also verified to be influenced by the backhaul reliability. Furthermore, OSS is more desirable compared to SoSS in low signal-to-noise ratio (SNR) regime, while the secrecy performance for both schemes converge to an asymptotic limit in high-SNR ranges.

The authors investigate pilot contamination in a multicell massive multiple-input multiple-output system, in which the estimated channel state information (CSI) is significantly different from the real CSI. Pilot contamination is an obstacle to achievable rates for users. The authors first propose an uplink training strategy to limit the effect of intercell pilot contamination on channel estimation with low complexity. As a result, the downlink data rate through the reciprocal channels is significantly improved. However, as the conventional uplink training, this strategy suffers from intra-cell pilot contamination when the length of pilots is smaller than the number of users in a cell, while increasing the length of pilots degrades the spectral efficiency. The authors, therefore, propose a pilot optimisation that can be incorporated into the proposed training strategy. Based on the minimum mean square error criterion, they derive a closed-form equation to determine the optimal pilots for all users in a multicell system. The joint pilot optimisation and proposed uplink training strategy is found to outperform other schemes, especially when the number of users exceeds the length of pilots in a cell. Numerical results verify that the downlink achievable rate with the proposed training strategy and pilot optimisation outperforms that with conventional approaches.

The authors propose an efficient spectrum sharing scheme in cooperative cognitive radio networks, where an energy-constrained secondary transmitter (ST) first scavenges radio frequency (RF) energy from the received primary signals, and then the ST assists the primary transmission to obtain the opportunity of spectrum access. Specifically, the ST can forward the primary signal with its own signal by adopting both the Alamouti coding technique and superposition scheme only if the harvested energy is sufficient while the primary data is decoded correctly by the ST. Otherwise, the ST will continue to harvest RF energy. The authors use the discrete Markov chain to model the processes of charging and discharging of the battery. Moreover, two different joint decoding and interference cancellation schemes are employed at the receivers to restore the desired data. Closed-form expressions of outage probabilities for both the primary and secondary systems are derived. Aiming to minimise the outage probability of the secondary system with guaranteeing the primary transmission, an optimal power allocation factor for the ST is determined by Monte-Carlo simulation. Numerical results demonstrate that the proposed scheme can effectively improve the transfer performance of the secondary system while realising the transfer requirement of the primary system.

In this study, the authors study a cognitive multiple-input-single-output network, where the secondary base station (BS) communicates with a secondary destination scheduled from K candidate users. Assuming the scheduled space–time block coding scheme, they investigate the system performance and obtain the exact expressions for the outage probability (OP), symbol error rate (SER) and the achievable rates, taking into account the imperfect channel state information in the secondary BS to secondary receiver Rx link and secondary BS to primary Rx link. Furthermore, the asymptotic expressions for OP and SER in high sigal-to-noise ratio region are obtained, from which the coding and diversity gains are readily deduced. Finally, extensive performance evaluation results accompanied with equivalent ones obtained by simulations are provided to verify their analysis.

In this study, the authors consider secure communications in multiuser wireless networks where full-duplex (FD) jamming operates to enhance physical layer security. The considered multiuser system is equipped with FD legitimate receivers in contrast to conventional frameworks where a half-duplex (HD) receiver is at hand. This study investigates an alternative solution in which the authors take advantage of FD capability of the receivers to send jamming signals against the eavesdropper. Under these assumptions, the impact of self-interference and channel interference on physical layer security is investigated. They derive the expressions of secrecy outage probability and ergodic secrecy rate in the proposed system. Two FD jammer selection schemes are proposed to further improve the security. In addition, they apply a reinforcement learning technology, called Q-learning, to model the interaction between the source and multiple jamming users. The preliminary results show that the application of FD jamming and user selection scheme leads to a significant improvement in the wireless network security.

Massive multiple-input multiple-output (MIMO) as a promising technology to achieve high spectral efficiency encounters the problem of high cost and complexity of hardware implementation for digital signal processing. One proposed solution is hybrid analogue–digital processing to reduce the number of radio-frequency (RF) chains required at massive antenna arrays. In this study, hybrid analogue–digital preprocessing aided spatial modulation is introduced to the downlink of multi-user massive MIMO systems. Linear zero-forcing (ZF) and regularised ZF preprocessors which are near-optimal in massive MIMO systems and a low-complexity two-stage preprocessing scheme based on block-diagonalisation are proposed. These full-digital schemes are implemented in hybrid RF-baseband domain through a low-dimensional processor based on the baseband effective channel and phase controllers at the RF domain. The spatially correlated and sparsely scattered millimetre wave channels are adopted to consider the proposed preprocessing schemes. Under an imperfect channel state information at the transmitter, the robust design of full digital and hybrid analogue-digital preprocessors is presented using stochastic robust approximation. Furthermore, the hybrid RF-baseband preprocessors are realised using heavily quantised angles and the preprocessing schemes are investigated from the data rate point of view. The simulation results show desirable performance and close to that of full digital preprocessing is achieved by proper configuration of system.

Composite fading channel modelling of terrestrial wireless propagation is crucial for the design of communication system and its performance analysis. In this study, a systematic characterisation of Rayleigh-inverse Gaussian (RIG) distribution is provided for modelling the channel of distributed multiple-input multiple-output (MIMO) systems with zero-forcing (ZF) receiver in the presence of transmit antenna correlation. At the outset, the closed-form expression for probability density function of instantaneous signal-to-noise ratio (SNR) is derived followed by its associated moments and cumulative density function. In the subsequent analysis, exact closed-form analytical expression of ergodic capacity with its bounds are derived and their asymptotic expressions are obtained in high and low SNR regime. Then the closed-form expressions are deduced for average symbol error rate (ASER) and outage probability (OP). Additionally, the authors propose to formulate simplified expressions for asymptotic ASER as well as OP and assess their approximation accuracy. Numerical results are presented to compare the performance of RIG model with the existing generic models and illustrate that RIG is superior in terms of ergodic capacity, ASER and OP. Finally, the derived analytical expressions are validated through Monte Carlo simulations which demonstrate the effectiveness of RIG model in the diverse environmental scenario for various system and channel parameters.

Non-orthogonal multiple access (NOMA) is a strong candidate for next generation radio access networks due to its ability of serving multiple users using the same time and frequency resources. Therefore, researchers in academia and industry have been recently investigating the error performances and capacity of NOMA schemes. The main drawback of NOMA techniques is the interference among users due to the its non-orthogonal access nature, that is usually solved by interference cancellation techniques such as successive interference cancellation (SIC) at the receivers. On the other hand, the interference among users may not be completely eliminated in the SIC process due to the erroneous decisions in the receivers usually caused by channels. In this study, for the first time in the literature, the authors derive an exact closed-form bit error rate (BER) expressions under SIC error for downlink NOMA over Rayleigh fading channels. Besides, they derive one-degree integral form exact BER expressions and closed-form approximate expressions for uplink NOMA. Then, the derived expressions are validated by simulations. The numerical results are depicted to reveal the effects of error during SIC process on the performance for various cases such as power allocation for downlink and channel quality difference for uplink.

This study outlines a non-orthogonal multiple access (NOMA) scheme for cooperative multicasting in a cognitive radio network. The network consists of a common base station (BS), which serves a primary user multicast group (PU-MG) and a secondary user MG (SU-MG). The SU-MG is located in between the BS and the PU-MG. Their proposed spectrum sharing protocol comprises of three transmission phases. The first phase is utilised by the BS for multicasting both PUs' and SUs' signals, using NOMA transmission. In the second phase, to overcome fading impairments, the BS exploits spatial diversity among SUs, by seeking their cooperation to relay PUs' message signal. The BS, as an incentive, leases a time slot in the third phase, exclusively for supporting multicast to SU-MG over PUs' licenced spectrum. Analytical expressions for outage probabilities of both PU-MG and SU-MG are derived and validated using simulations. Furthermore, outage performance of PUs and ergodic capacity of SUs are compared with a prominent recent scheme.

Cellular networks with device-to-device (D2D) communication capabilities offer increased spectrum utilisation and system throughput. However, these performance gains depend on the channel and power allocation strategy used. In this study, the authors have chosen underlay D2D as it is more advantageous in terms of spectrum efficiency, but introduces additional interferences. To cope up with these challenges, they propose new channel assignment and power allocation schemes with and without Quality of Service (QoS) constraint for D2D pairs. The authors' schemes allow multiple channels to be shared by each D2D pair without degrading the performance of the existing cellular system. First, they formulate the joint channel and power allocation problem without considering the QoS for D2D pairs which results in a non-convex mixed-integer non-linear programming problem and cannot be solved in polynomial time. Then, the problem of channel and power allocation is reformulated and optimised, individually. Following this, they introduce QoS constraints for each D2D pair by specifying the minimum number of channels that should be allocated to each D2D pair during the assignment. The effectiveness of their scheme is verified through numerical results.

Motivated by recent theoretical challenges for 5G, this study aims to position relevant results in the literature on code-domain non-orthogonal multiple access (NOMA) from an information-theoretic perspective, given that most of the recent intuition of NOMA relies on another domain, that is, the power domain. Theoretical derivations for several code-domain NOMA schemes are reported and interpreted, adopting a unified framework that focuses on the analysis of the NOMA spreading matrix, in terms of load, sparsity, and regularity features. The comparative analysis shows that it is beneficial to adopt extreme low-dense code-domain NOMA in the large system limit, where the number of resource elements and number of users grow unboundedly while their ratio, called load, is kept constant. Particularly, when optimum receivers are used, the adoption of a regular low-dense spreading matrix is beneficial to the system achievable rates, which are higher than those obtained with either irregular low-dense or dense formats, for any value of load. For linear receivers, which are more favourable in practice due to lower complexity, the regular low-dense NOMA still has better performance in the underloaded regime (load ), while the irregular counterpart outperforms all the other schemes in the overloaded scenario (load ).

In this research, the authors investigate the outage probability (OP) of the secondary network in a cognitive wireless powered communication network. Energy-constrained secondary users harvest energy from a hybrid access-point (H-AP) and the primary transmitter in the first phase. In the second phase, they select a secondary user based on two different schemes, namely the best uplink channel selection (UCS) and the minimal interference channel selection (MICS), to transfer information to the H-AP. The secondary network can share the same spectrum with the primary network ensuring that a desired OP constraint in the primary network is always met. This constraint represents the quality-of-service (QoS) of the primary network. The analytical expressions and asymptotic expressions of the OP of the secondary network are derived. The result shows that increasing the number of secondary users can considerably improve system performance. The transmit power of the selected secondary user, energy harvesting time and relaxing the QoS constraint of the primary network have a significant impact on the OP of the secondary network. The results show that UCS outperforms MICS.

As the number of antenna elements increases in multiple-input multiple-output (MIMO) systems, an efficient feedback and beamforming strategy becomes more important to achieve the desired bandwidth efficiency required by the next-generation wireless systems. The authors investigate the statistical characteristics of the MIMO channel generated by the three-dimensional spatial channel model to recognise some of the limitations inherent in the existing codebooks adopted by the current third generation partnership project specifications. Such limitations include unnecessary co-phasing combinations for cross-polarised (X-pol) antenna arrays and, in particular, long-term codevector selections, which are not best suited to the channel distributions. In this study, the observed shortcomings are improved by proposing a new multi-rank codebook applicable to the X-pol array of antenna elements supporting up to the quadruple rank. The proposed beamforming codevectors are presented using variable parameters so that they can be adapted to changing channel characteristics. The efficiency of the proposed codebook is further improved by eliminating redundant co-phasing values. The performance of the proposed codebook is evaluated using the TR 36.873 urban macro environment and its effectiveness is demonstrated by quantifying its gain over the existing codebooks.

In this paper, the performance of hybrid carrier (HC) systems based on weighted fractional Fourier transform (WFRFT) is investigated in an uplink non-orthogonal multiple access (NOMA) scenario. NOMA is a promising technology to improve the system capacity, in which two users (far-user and near-user relative to a base station) are allocated to use the same time-frequency resources, and the successive interference cancellation (SIC) technique is implemented to decode signals at the receiver. Considering the actual error decoding in the SIC process (i.e. imperfect SIC), NOMA cannot avoid the inter-user interference (IUI) and residual interference (or error propagation). Therefore, firstly IUI and residual interference are analysed, and signal to interference plus noise ratio (SINR) of the far-user is expressed mathematically considering the residual interference. Then, based on the analysis of IUI, considering different WFRFT orders, a near-user BER expression over additive white Gaussian noise (AWGN) channels is derived. Furthermore, the optimal WFRFT order selection to minimise the interference influence in the uplink is formulated and solved efficiently. Simulation results have verified the mathematical expression of SINR, the near-user theoretical BER expression, and the proposed optimal WFRFT order selection to obtain the maximum sum spectral efficiency.

In this study, the authors consider the simultaneous wireless information and power transfer (SWIPT) protocol design for the massive multiple-input multiple-output (MIMO) system in the beam-domain. In this system, the base station (BS) simultaneously serves a set of half-duplex energy-constrained terminals that are uniformly distributed within its coverage area. Based on the beam-domain distributions of channels, the BS can intelligently schedule terminals to mitigate the interference between terminals and improve the transmission spectral efficiency. The entire protocol can be divided into two phases. The first phase is designed for terminals energy harvesting as well as downlink training. During this phase, the BS transmits energy signals to the terminals. The terminals utilise the received energy signals for energy harvesting and downlink channel estimation. In the second phase, the BS forms the receive beamformers to receive signals transmitted by terminals. The transmit powers at the BS and the time switching ratio are optimised under the constraints of the current available energy and minimum transmission rate of terminals, so that the system can achieve the maximum sum-rate performance. Simulation results show that compared with traditional massive MIMO SWIPT protocols, the proposed SWIPT protocol can achieve better spectral efficiency performance.

The asymmetry traffic between downlink (DL) and uplink (UL) in massive machine-type communication (mMTC) systems is so prominent that it makes the traditionally fixed frame protocols insufficient to handle. Meanwhile, the dynamic time-division duplexing (D-TDD) is a promising and attractive technology since its number of time slots for the DL and UL can be asymmetric and adjusted dynamically. In the cellular and mMTC co-existing network, to balance the discrepancy of the UL/DL ratio and alleviate the interference as well, in this study, the authors design a D-TDD-based transmission frame structure to first fulfil the basic transmission requirements of the human-type communication (HTC) users with a low power almost blank subframe. Herein, the stochastic geometry methods are adopted to calculate spectral efficiencies of the HTC user equipment. Then, focusing on the worst queue state in mMTC, they devise the slot allocation problem with the min–max objective of the UL/DL queues and utilise the sub-gradient descent (SGD) method for a solution. Simulation results show that the proposed traffic aware sub-frame configuration is more appropriate for the dynamical asymmetry environment. Meanwhile, the adopted dynamic step size SGD algorithm can achieve a trade-off between the worst-case queue and the network throughput.

In this study, the authors assess the performance of a cognitive multiple-input single-output (MISO) network, where the secondary base station schedules a secondary destination to communicate from K candidates, by employing scheduled maximal ratio transmission (MRT) scheme. A practical scenario is considered where imperfect channel state information (CSI) is taken into account. Specially, assuming Rayleigh fading channels, the exact expression for the cumulative distribution function of the received signal-to-noise ratio (SNR) at is derived, from which the outage probability (OP) at is obtained. An asymptotic expression for the OP at high SNR region is also presented, indicating the diversity gain that the scheduled MRT scheme can achieve. Furthermore, the interference OP at the primary receiver is discussed due to imperfect CSI in the link from base station to primary receiver link. Finally, extensive performance evaluation results accompanied with equivalent ones obtained by computer simulations are presented to corroborate the proposed analysis.

Device-to-device (D2D) communication is seen as an important emerging technology and a hot research area for (5th Generation) cellular networks that takes advantage of the physical proximity of communicating devices. However, its implementation faces some technical issues such as interference management, resource allocation, mobility management, security and so on. Recently, many research works have tried to deal with those problems. In this study, the authors focus on D2D mobility management issue and they give a comparative study on the existing techniques in cellular networks managing the mobile devices (MDs) connections and maintaining service continuity when moving between coverage areas. First, they describe the different D2D handover management techniques. Second, through analysing and categorising a wide range of the latest research works on Distributed Mobility Management, they select some approaches that could be suitable for D2D mobility management in 5G cellular networks. Finally, they mathematically analyse the handover performance of the selected solutions, and compare them and they conclude that Software Defined Networking based solution outperforms the others in terms of handover delay, packet loss, ping-pong effect as well as the handover failure probability.