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.

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.

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.