The optimisation of a communication system becomes a difficult objective if distinct techniques are utilised to resolve different problems of the same system. Accordingly, the main target of future mobile communication systems (e.g. fifth generation) is to integrate different techniques under a unified framework for the optimisation purpose. This study proposes a downlink air interface that targets maximum capacity and bit error rate (BER) performance enhancement. The proposed system is an integration of a full loaded multi-code transmission orthogonal frequency code division multiple access and space-time spreading that exploits transmit diversity needed for BER enhancement and data rate optimisation. Further BER improvement was achieved through applying an effective iterative interference cancellation in the space domain combined with multi-code interference suppression algorithm at the receiver. The authors present a system performance analysis in addition to simulation results for the proposed system. The BER performance improvement was emphasised through comparing the proposed system with a similar system that uses joint iterative detection. Finally, the effect of frequency domain spreading factor with different number of iteration loops was investigated for further BER system performance enhancement. The achieved BER performance outperforms the maximal ratio receive combining diversity system performance with 1Tx and 8Rx.

To bridge the gap between the resource-constrained mobile devices and the resource-demanding applications, mobile cloud computing (MCC) emerges for offloading complex tasks to a cloud server. Based on this concept, cloudlets, which move available resource to the vicinity of the mobile network, enhance further the system accessibility and performance. Moreover, to strengthen the network capacity in traffic intensive area, dense small cell network (DSCN) is proposed as one of the promising solutions. In this study, the operation of cloudlets and DSCN is collaboratively studied in order to further improve the system performance. On the one hand, users can select a cloudlet and dynamically adapt the connection according to the performance and the cost, which is referred to as a user-essential dynamic cloudlet selection problem. On the other hand, a cloudlet needs to set the optimal selling price and the size of resource for the users, which is considered as a cloudlet resource allocation problem. To jointly address the problems of dynamic cloudlet selection and resource allocation, the authors propose a hierarchical evolutionary game to maximise the utilities. Simulation studies are carried out to demonstrate the effectiveness of the proposed algorithms, which, indeed, improve the entire system performance significantly.

Rateless codes (RCs) have the ability of successfully recovering the message from any subset of the fixed number of encoded symbols, making them a good candidate for delay-tolerant systems. However, most of the applications are delay-constrained. In this study, quality of violation probability (QVP) is used as the performance metric which includes reliability, secrecy, considering delay-constraint conditions. The performance of the RC in the presence of an eavesdropper is studied, and a relay selection technique is used to minimise the QVP in a system with regenerative relays. The authors derive an expression for the QVP when adequate mutual information (MI) is obtained at the receiver, and compares it to the energy accumulation (EA) receiver. Further, based on the direct link channel state information available, relay selection scheme is analysed. They observe that MI accumulation reduces QVP significantly in comparison to the EA. It is also observed that initially on increasing the maximum number of allowed channel usages, the QVP gets reduced and later gets saturated. They observe an improvement in outage probability and intercept probability due to diversity gain. A closed-form expression of QVP for relay selection is determined for the EA and the MI accumulation receiver.

Cognitive radio enabled vehicular ad-hoc networks (CR-VANETs) are one of the promising architectures in the future vehicular ad-hoc networks. In this study, the joint spatial–temporal correlation is exploited to improve decision accuracy of cooperative spectrum sensing (CSS) while reducing overhead introduced by cooperation in the CR-VANETs. Firstly, a theoretical method is presented to analyse the impact of spatial–temporal correlation on cooperative sensing performance when using soft combining in the CR-VANET. Then, the expression of the optimal probability of detection with respect to spatial–temporal correlation is given for a target probability of false alarm by employing likelihood ratio test. Additionally, the user selection problem is formulated as an efficient double-threshold optimisation problem by considering both sensing accuracy and stability to achieve the optimal probability of detection. Finally, a hybrid CSS scheme based on spatial–temporal correlation is designed for the CR-VANET. Simulation results reveal that the proposed scheme could achieve significant sensing performance gain by selecting a subset of secondary users for combination, and could reduce user selection frequency by employing spatial–temporal diversity.

The performance of coordinated multi-point (CoMP) transmission system depends heavily on the quality of information exchange via backhaul links. However, the limited capacity of backhaul leads to quantisation error and delay of shared channel state information. In this study, the authors propose a novel hybrid CoMP transmission strategy to mitigate the impacts of the backhaul delay for data sharing and switch the transmission modes adaptively based on the current arrivals of the shared user data. Furthermore, they present analytical expressions for the average sum rate in an interference alignment network, in which the combining effects of quantisation from limited capacity and connection uncertainty introduced by finite delay are considered. With the analytical results, they evaluate the sum rate performance under limited backhaul for the conventional CoMP transmission modes and the proposed strategy. The simulation results demonstrate that the proposed strategy functions better on overcoming the impacts of capacity limited backhaul than the conventional ones.

In this study, the authors explore two-way relaying having a full-duplex decode-and-forward relay with two finite buffers. Chiefly, they propose a novel adaptive protocol (that maximises the cumulative network throughput) based on the combination of the buffer states, lossy link, and the outage probability; a decision is generated as to whether it can transmit, receive or even simultaneously receive and transmit information. Towards this objective, first, based on the queue state transition and the outage probability, an analytic Markov chain model is proposed to analyse this scheme, and the throughput and queueing delay are derived. Second, they evaluate the performance of the throughput according to the variation of the self-interference. The authors' numerical results reveal exciting insights. First, the adaptive protocol is optimal when the length of the buffer is superior to a certain threshold. Second, they demonstrate that the adaptive protocol can boost the transmission efficiency and prevent buffer overflow.

This study addresses spectral efficiencies (SEs) in a massive multiple-input multiple-output cellular network with pilot contamination. Both uplink and downlink SEs are investigated with practical channel state information. A downlink precoder minimising signal detection error and power leakage with considerations on spatial correlation has been derived. Also, a low-complexity covariance matrix estimation method has been proposed and compared with the existing estimation method in terms of normalised mean-squared error. It has been demonstrated via simulations that both uplink and downlink SEs using the proposed covariance matrix estimation method can grow with the number of antennas even with a relatively small number of pilot symbols.

This study examines the linear source and relay precoder and destination combiner design for multiple-input multiple-output full-duplex (FD) relay communication systems. The effect of the residual interference due to the imperfect loop interference cancellation is considered in the design. Two design algorithms are proposed to minimise the mean squared error of the received signal at the destination. The first is a tri-step alternating iterative algorithm while the second is a bi-step iterative algorithm, which has lower complexity and performance comparable to that of the first algorithm. The convergence of these algorithms is evaluated. Results are presented, which show that the proposed FD relay system can provide approximately double the achievable rate of the corresponding half-duplex system if the residual interference is not high.

The geo-location database (GDB) driven is the enforcement method for dynamic spectrum sharing in TV White Space and 3.5 GHz spectrum bands, as well as, a preferred option for the other spectrum sharing applications. Although providing accurate and reliable spectrum information services, the GDB driven spectrum sharing suffers from a critical security threat of spoofing attack. Under a spoofing attack, an adversary could spoof either the identification (ID) or the location information in its request messages. This breaks the fairness and reduces the efficiency of the GDB driven spectrum sharing system. To counteract the location and ID spoofing attacks, the authors consider the location verification of request messages and the ID verification of communicating data. Since a resource manager and an adversary are independent and self-interested, they formulate two corresponding surveillance games to analyse the conflict interaction between the spoofing attack and the countermeasures. By expressing the surveillance game on requests' location in a strategic form and representing the surveillance game on data ID in a sequence form, they find out Nash equilibrium. The analytical and numerical results show that a resource manager can mitigate the spoofing attack by adequately adapting its penalty policy and surveillance strategy.

The radio frequency spectrum has become very congested and scarce. A response to this dilemma is the implementation of visible light communications (VLC) technology, which is enabled by the dramatic development of light emitting diodes (LEDs) and laser diodes (LDs). VLC has become the most popular emerging telecommunication technology. This is due to the interesting advantages that it provides, which include an unlicensed and large bandwidth, a high throughput capability and a low implementation cost. VLC needs a pivot network in medium and long distance transmissions. The amplify-and-forward scenario between the backbone network and VLC is cost-effective owing to its simplicity when compared to the decode-and-forward scenario. MPSK-CSK is a typical example of cascaded systems in AF scenario with VLC where the backbone channel utilises PSK to convey the message. In this paper, we investigate a forward error correction scheme for cascaded MPSK-CSK systems. A convolutional encoder is added to the MPSK-CSK scheme to construct the hybrid trellis coded modulation scheme. Selected 8HTCM systems using set partitioning are investigated and results for a Gaussian channel are presented. Numerical results reveal that 8HTCM achieves up to 2.129 dB of asymptotic coding gain when compared to the corresponding un-coded MPSK-CSK system.

Delay and security are both highly concerned in massive internet of things (IoT) which is a promising application scenario in 5G wireless communication system. In this study, the authors develop an integrated framework to study delay performance and secrecy performance in massive IoT. First, stochastic geometry and queuing theory are applied to model the spatially location of IoT devices and the temporal arrival of packets. To enhance the physical layer security of massive IoT with limited overhead increment at IoT devices, a low-complexity noise injection scheme is applied. Then, the packet delay and packet secrecy outage probability are derived to characterise delay performance and secrecy performance. It is demonstrated that the IoT device intensity and power allocation coefficient arouse a tradeoff between the delay and security. Furthermore, optimal power allocation coefficient that maximises secrecy transmission rate can be derived, which can improve the delay-security tradeoff. The analytical and simulation results show the effects of power allocation coefficient and IoT device intensity on the tradeoff between delay performance and secrecy performance.

In future cellular 5G, the network operators have recognised the spectrum scarcity as one of the major problems. Therefore, expanding or even re-utilising the current bandwidth requires an advanced technique to accommodate the enormous communicating devices. In this paper, energy efficiency (EE) maximization is jointly carried out with respect to the following design parameters: base station density, number of users, number of attached antennas, and power control coefficient based on analytical channel estimates and feasible pilot reuse. Although dense network performance is restricted to the accumulation of both user and inter-cell interferences, the reasons of mutual selection among the optimal design parameters will be clearly discussed to discover their effects on mitigating interference. Simulation results validate that the diversity of the design parameters achieves the maximum EE given a predefined quality of service constraints. Also, the study of hardware distortion will expose the relevant influence on the optimised EE. Moreover, the results will be extended to study the energy consumption in terms of optimised design variables. Finally, our results will be confirmed through a fair comparison with the alternating optimisation based on Monte Carlo simulation, wherein, our proposal shows a reality and fast convergence.

Low-density spreading multiple access (LDSMA) was proposed for supporting massive connectivity and low latency scenarios in 5th generation systems owing to its low implementation complexity. In this study, the authors analyse the performance loss of conventional LDSMA systems from two aspects: the one from low-density spreading constellations, and the other from the mismatch between multi-user detector and channel decoders. For the first aspect, they propose a new multi-dimensional constellation designed via simple symbol remapping with the same constellation as in LDSMA in each dimension. And for the second aspect, newly designed Raptor-like low-density parity-check codes are proposed to help retrieve the performance loss. Simulation results show that the performance of the proposed enhanced system is 1.4 dB and above superior to the conventional LDSMA system and less than 1.2 dB away from Shannon limit of Gaussian multiple access channel with symmetric users at sum rate of 1.5 and 2.25 bits per channel use.

Transmission reliability, security, and efficiency are of great importance to free space optical (FSO) communication. For the first time, the authors obtain optimal parameters for the FSO/code division multiple access (CDMA) system taking into account the transmission reliability, security, and efficiency simultaneously. By establishing the eavesdropping model of the FSO/CDMA binary symmetric wiretap channel (BSWC), the performance of physical-layer security and reliability are evaluated simultaneously. Intercept probability is derived for FSO/CDMA BSWC, considering atmospheric attenuation, atmospheric turbulence, shot noise, thermal noise, background noise and multiple access interference (MAI). The effects of transmission distance, code length, transmission power and a number of interfering users on the physical-layer security and reliability are discussed. It is shown that, in order to meet the legitimate user's reliability and security simultaneously, the number of interfering users and code length must be in a certain interval. Furthermore, the maximum spectral efficiency can be obtained by selecting the transmission rate. These results open a new way of researching the FSO/CDMA system, where security, reliability, and spectral efficiency should be taken into account simultaneously.

This study presents a novel idea for information hiding at the physical layer level in order to securely transmit confidential cognitive radio data in cooperative relaying systems. Considering a cognitive radio network in which the secondary user cooperates with the primary system in relaying its data, in the proposed model, confidential cognitive information is hidden into the primary signal as a low-power noise at a symbol level such that it would be invisible by malicious users. The new hiding algorithm can be utilised along with any standard relaying scheme as long as the primary information signal is used as the cover data without significant degradation of the primary system performance. In the presence of cognitive data hiding, both the frame error rate and throughput performance of the primary and secondary systems are derived analytically for well-known relaying schemes such as fixed and incremental relaying protocols, when a selection-combining rule is utilised at the primary receiver for symbol detection. Using simulation results the correctness of the proposed theoretical analysis is verified. Moreover, using numerical results the existing tradeoff between the hiding cognitive capacity of the proposed model and the performance degradation of the primary system is highlighted.