A Plotkin-based polar coded space–time spatial modulation scheme is proposed and its bit error rate (BER) performance over quasi-static Rayleigh fading channel is evaluated. The exacerbated complexity in receiver design is ameliorated by employing a low complexity soft demapper for their proposed Plotkin-based polar coded scheme. As there is a natural split present in the design of the considered Plotkin's construction, therefore the authors effectively extended their proposed scheme to coded-cooperative scenarios. In any coded-cooperative communication, the relay node always plays a pivotal role in determining the BER performance of communication system. Therefore, a rational selection criterion for selecting the information bits at the relay node is presented. The authors have also devised nested polar coded-cooperative space–time spatial modulation (NPCCST-SM) scheme as a useful benchmark. Monte-Carlo simulated results revealed that their proposed coded-cooperative scheme has triumphed conventional NPCCST-SM scheme by a gain of 0.6–0.8 dB under identical conditions. Moreover, the proposed cooperative scheme outperforms its corresponding non-cooperative counterpart scheme by a gain of 1–1.2 dB under identical conditions.

This study investigates outage probability (OP) of a generalised decode-and-forward-based multiuser spectrum sharing relay system. In practice, the secondary users are exposed to the primary transmitters’ signals which can measurably affect the OP in the secondary system. In this context, analytical and asymptotic outage analysis are presented for two different relay selection techniques, namely partial relay selection (PRS) and opportunistic relay selection (ORS). In terms of the ORS, deriving an exact analytical expression for the end-to-end (e-t-e) OP is contingent upon solving a very complicated integral. To resolve this problem, they propose a simple modification under which the OP can be obtained with preferable accuracy. Numerical simulations are presented where the excellent agreement of the simulation results with the analytical results validates the mathematical derivations and confirms that the proposed simplification is accurate. Findings suggest that the ORS has three main advantages over the PRS. First, it is more robust against the primary interference. Second, it achieves the full diversity order, whereas the PRS limits the diversity order to the number of secondary destinations plus one. Third, the gain of enhancement achieved by increasing the number of relays is significantly larger.

Massive multiple-input multiple-output (MIMO) technology employs hundreds of antennas at the base station (BS) of a cellular system. Activating hundreds of antennas requires hundreds of radio-frequency (RF) chains that lead to high cost and high energy consumption. Antenna selection techniques reduce the number of RF chains while curtailing the resulting performance loss. In this study, the authors consider massive-MIMO downlink with zero-forcing (ZF) precoding in a single cell. In ZF precoding with antenna selection, the number of users must always be less than or equal to the number of selected antennas, which may require adopting user selection (scheduling) algorithms. They propose a joint user and antenna selection algorithm to increase BS's energy efficiency (EE) and show that its performance is very close to that of the optimum exhaustive-search based scheme. Using an analytical approximation for EE of the proposed scheme in asymptotically large numbers of users region, they find the optimum number of antennas to be selected.

This study investigates the spectral efficiency (SE) and energy efficiency (EE) of a multipair two-way massive multiple-input multiple-output amplify-and-forward relaying system over Ricean fading channels. Both maximum-ratio combining/maximum ratio transmission and zero-forcing transmission/zero-forcing reception beamforming matrices are considered at the relay with imperfect channel state information (CSI). The asymptotic signal-to-interference-plus-noise ratio expressions (in the number of relay antennas M) are derived. Moreover, four power scaling schemes are proposed and the asymptotic SE and EE based on the proposed power scaling schemes are obtained analytically. Theoretical analyses and simulation results show that with imperfect CSI when M tends to infinity, the transmit power should be scaled down to different proportions for the Ricean channel with and without line-of-sight components to maintain a desirable rate. However, when M tends to infinity, SE is independent of the Ricean K-factor with perfect CSI.

A cyclic switching weighted bit-flipping (CS-WBF) decoding algorithm for low-density parity-check (LDPC) codes is proposed in this study. By carefully choosing two selection criteria of the bit to flip and cyclically switching from one criterion to the other according to some rules during decoding, the proposed scheme can effectively break decoding loops, which often appear for weighted bit-flipping (WBF) algorithms and greatly degrade decoding performance. Simulation results show that CS-WBF provides 0.7–1.3 dB coding gain over various simple improved WBF algorithms, such as reliability ratio-based WBF and improved modified WBF, with almost the same complexity for both regular and irregular LDPC codes. Compared with WBF algorithms that break decoding loops and the belief propagation-based algorithm, CS-WBF can also provide a better complexity-performance trade-off.

In this study, the pilot-assisted synchronisation method for a random communication system (RCS) has been proposed. The pilot symbol, which has alpha-stable distribution, has been used to establish synchronisation and to maintain covertness in the RCS. The introduced synchronisation block (SB) consists of fractional lower-order covariance-based correlators (FLOCCs), threshold detectors (TDs) and the synchronisation control block. To measure the performance of the proposed SB, the performance criterion, i.e. confidence ratio (CR), has been proposed. The reliability of the proposed SB can be enhanced by altering the CR and the achieved CR by using the FLOCCs and TDs in SB.

In this study, the authors first propose a sequence with the impulse-like autocorrelation property named zero autocorrelation code (ZAC) for orthogonal frequency division multiplexing (OFDM) based on offset quadrature amplitude modulation (OQAM) systems. The ZAC sequence can be generated by many prototype filters such as physical layer for dynamic spectrum access and cognitive radio and isotropic orthogonal transform algorithm filters. Thus, it has a wide application range. The authors then propose a synchronisation algorithm based on the ZAC. The proposed algorithm only needs two symbols to compose the preamble while existing algorithms need at least four symbols. They further assess the synchronisation performances via computer simulation. The simulation results show that the proposed algorithm can achieve better performance in terms of root mean square error of the timing and frequency offset estimation, with a 10 dB improvement on timing offset estimation performance over Saeedi-Sourck's algorithm when signal-to-noise ratio is 0 dB.

This study introduces a three-dimensional (3D) model for K-tier heterogeneous networks (HetNets) in order to evaluate the performance of using millimetre wave (mmWave) and radio-frequency (RF) bands. The model is based on 3D stochastic geometry that describes the different cells' positions in the network with realistic constraints. Based on the 3D model, the successful transmission probability, the average throughput, and the average bit error rate expressions of down-link HetNets are detailed and derived for both considering bands. Using numerical results, the analytical derived expressions are evaluated, and the advantages of using mmWave and RF bands for HetNets are investigated.

Efficient use of energy has recently become an essential research topic for wireless communications. Channel state information (CSI) feedback has been well recognised to have great significances for the energy-efficiency (EE) of the closed-loop wireless networks. However, works on EE focus mostly on the feed-forward data link by performing precoding and upper layer resource allocation and scheduling approaches. Unlike traditional work, the authors propose to study the EE by controlling the CSI feedback, where the mobile transmitter and receiver pairs exchange data by precoding for co-channel interference reduction. The EE maximisation problem can be formulated in the analytical settings of a game-theoretic framework by a two-level Stackelberg-type CSI feedback control game (SCFC), which can balance the power consumptions and the bandwidth. Specifically, the leader maximises the EE objective by introducing a price factor in the higher level. In lower level, multiple mobile receivers compete for the feedback channel utilisation to maximise their own utilities. The existence of the equilibriums is corroborated and the convergence behaviour is discussed. Simulation results demonstrate that the proposed distributed SCFC game can greatly enhance the EE performance by adjusting the pricing factor which can achieve close optimal performances in comparison with the centralised scheme.

The vehicle-to-grid (V2G) technology enables electric vehicles to deliver electricity into power systems, providing them supplementary capacity. On the other hand, a new set of security threats are brought to smart grid participants by V2G networks. However, security and privacy in V2G networks have so far received little attention, despite a rich literature on the design of conceptual structures or the impact of V2G networks on the current grid. In this study, the authors explore the features of V2G communication networks and identify their security challenges for communication functions. The authors then establish a novel and secure communication framework for V2G networks to achieve a balance among security, privacy preservation, efficiency and accountability without relying on any trusted third party. The feasibility of the framework is demonstrated by experimental results.

Wireless powered communications allow sustainable operations of low-power applications by supplying wireless power remotely. In this study, the performance of wireless powered communications is evaluated in terms of achievable rate and bit error rate (BER), for applications where the downlink and the uplink are correlated, in contrast to previous works that assume independent uplink and downlink. Semi-closed expressions for the achievable rate and series expressions for the BER are derived in Nakagami m fading channels, based on which the effect of link correlation is examined. Numerical results show that the link correlation has a significant impact on the achievable rate. Consequently, the optimum system parameter for correlated links is very different from that for independent links, showing the usefulness of the authors' results. Also, the link correlation has a noticeable effect on the BER, depending on the system parameters considered.

In this study, the authors propose a power allocation strategy for full-duplex relay (FDR) networks, where two cases are considered according to whether the direct link is desired or not. The power allocation is studied in two scenes under the amplify-and-forward and decode-and-forward protocols, respectively. Aiming at maximising the system capacity, the authors derive the optimal power allocation strategy under the condition that the source and relay have the same maximum power constraint and the total power is limited. The effects of global and individual power constraints for FDR on system capacity are analysed. Simulations verify the theoretical analysis and indicate that the proposed power allocation strategy enhances the system performance greatly compared with uniform power allocation.

User-centric network (UCN) organises a dynamic femto access point group (FAPG) for each user to meet the exponential data demands in 5G. However, there exists spatial correlation in the scheduling of femto access points (FAPs) due to the overlap of different users' FAPGs, which has not yet been analysed in existing literature. In this study, considering this spatial correlation by using Matérn-like model, simple yet accurate semi-closed form expressions of rate coverage probability and area average goodput of power control strategy in UCN are derived, respectively, using stochastic geometry tools. In this user-centric power control strategy, each user equipment (UE) determines its FAPG radius considering the distance to the nearest FAP, and FAPs within the FAPG employ power control to mitigate dominant interference for the UE. Further, Matérn-like model guarantees that two FAPs in the same FAPG will not transmit at full power simultaneously, and thus whether a FAP performs power control is correlated with other FAPs that belong to the same FAPG. The analytical results demonstrate significant performance gains of power control strategy in UCN compared with the non-coordination baseline, e.g. area average goodput is improved by about 44% when FAP-to-UE density ratio equals 1.

The low-density parity-check (LDPC) code design for three-terminal (namely, source, relay, and destination) relay network while considering decode-and-forward protocol is studied. Numerous works have been done on LDPC relay code design with parity bits forwarding approach, where additional parity bits are generated at the relay and forwarded to the destination. Most of the previous works assume that the forwarded parity bits are received perfectly at the destination and hence ignore the impact of relay–destination channel. This assumption is unrealistic in practical/finite-length codeword scenario. In this study, a protograph based LDPC code design is proposed while lifting the above unrealistic assumption. A Gaussian approximated density evolution is presented for the proposed scheme, which considers that the overall codeword experiences source–destination and relay–destination channels. For various designed codes with different rates, the authors show that the asymptotic thresholds of the designed codes are very close to the corresponding capacity bounds. Asymptotically, our designed code gives similar decoding threshold compared to the existing optimised irregular LDPC codes, while protograph codes are simpler to optimise and encode than the irregular LDPC codes. More importantly, proposed code performs better than the existing LDPC-based relay codes for finite-length scenarios.

The authors design a transceiver for a multiuser (MU) multiple-input multiple-output communication system with a full-duplex relay (FDR) station. Such a system contains several types of interference, such as MU interference, parallel streams, and self-interference. To cancel this interference and achieve reliable MU communication, the authors use an equivalent FDR station model, in which the block diagonalisation (BD) scheme, which includes a base station, a relay, and a mobile station, can be used to derive the linear precoders and postprocessing filter. This work also studies link quality enhancement for the full-link system, including MU direct link and relay links communications. More interference will exist in a full-link system. They proposed a novel separable precoder and postprocessing algorithms to cancel all interference over the downlink FDR system. Simulation results demonstrate that the proposed transceiver design can provide a reliable sum rate with linear growth performance under the MU scenario with multiple interference and channel estimation errors and obtain diversity gain to improve the full system link quality.

The software-defined wireless sensor networks (SDWSNs) have been proposed recently to solve the energy limitation of sensor nodes and extend the lifetime of the wireless sensor networks by fast node reconstruction and dynamical resource allocation. In this study, the authors investigate an energy-efficient resource allocation algorithm in SDWSNs, in which radio resource allocation could be handled at central controllers with powerful storage and computation capacity. In this algorithm, the authors formulate an optimisation problem to minimise the energy consumption, under the individual constraint of quality of service. Then, the initial optimisation problem is transformed using semidefinite relaxation, to achieve centralised adaptive bandwidth and power allocation (CABPA). Additionally, two special cases are derived to reveal the performance of the CABPA. Furthermore, an OpenFlow-based scheme is proposed for information exchanging and updating to realise the centralised resource allocation. Meanwhile, a distributed scheme with limited information about the whole network is developed to serve as a performance benchmark for the CABPA in the SDWSN. Finally, the simulation results reveal that the proposed CABPA performs better than the other algorithms, and it balances the power and bandwidth utilisation.

In this study, a cooperative non-orthogonal multiple access (NOMA) scheme is investigated in an underlay cognitive radio network. With this aim, a number of secondary users are concerned in the cooperative NOMA, in which a user with strong channel gains is properly selected (to act as a relay) by a multi-antenna base station () for assisting another user with poor channel gains in the presence of a primary network. Closed-form expressions for the outage probability of the secondary network are derived, based on which the asymptotic outage behaviours of each secondary user assuming various realistic cases are discussed. Our analytical results clearly reveal that an outage floor may exist in the outage probability of the secondary users, being determined by the interference constraint and the number of antennas at the . The impact of the multiple antennas and the number of cooperative NOMA users on the system performance is examined, and it shows that the cell-edge user under poor channel gains can benefit from both the cooperative NOMA and opportunistic relay transmission.

The authors propose an adaptive cell-breathing (ACB) technique to improve the energy efficiency (EE) of a downlink cellular network consisting of small-cell base stations (BSs), wherein each BS adaptively adjusts its transmission power such that the received signal strength of the worst-case user is larger than a pre-defined threshold. They also propose an aggressive BS on–off (ABO) technique in which the small-cell BSs having a number of users smaller than a certain value, , are turned off, whereas conventional techniques only turn off the empty BSs. They adopt a stochastic geometry for modelling the locations of both BSs and users. Simulation results show that the ACB technique yields a much better EE than the power on–off technique with a fixed power, including the ABO technique. In particular, the EE of the ACB technique is proportional to , where denotes the BS density and the exponent c denotes the increasing ratio of the EE to in the domain. The EE of the ABO technique tends to increase as increases.