Degree distribution has a great influence on the performance of Luby Transform (LT) codes. In this study, the authors study a new degree distribution for LT codes. First, improved Poisson distribution (IPD) is proposed by refining the PD. The successful decoding rate for IPD reaches more than 90% when the decoder receives less encoding symbols, while robust soliton distribution (RSD) performs better as received encoding symbols grow. Then they combine IPD and RSD by a parameter to derive the function of combined Poisson RSD (CPRSD). Moreover, the golden section algorithm is introduced to obtain the optimal parameter for CPRSD. The expected ripple size is employed to present the objective function for this algorithm, and their objective is to increase the mean of ripple size and reduce its variation at the same time. Simulation results show that when compared with RSD and other optimised degree distributions, the CPRSD achieves at least 20.1% (20.1%–30.4%) reduction in overhead, and saves 28%–46.2% consuming time per encoding/decoding process.

This study proposes a progressive algebraic Chase decoding (PACD) algorithm for Reed–Solomon (RS) codes. On the basis of the received information, 2 ^η (η > 0) interpolation test-vectors are constructed for the interpolation-based algebraic Chase decoding. A test-vector reliability function is defined to assess their potential for yielding the intended message. The algebraic Chase decoding will then be performed progressively granting priority to decode the test-vectors that are more likely to yield the message, and is then terminated once it is found. Consequently, the decoding complexity can be adapted to the quality of the received information. An enhanced-PACD (E-PACD) algorithm is further proposed by coupling the PACD algorithm with the adaptive belief propagation (ABP) decoding. The ABP decoding generates new test-vectors for the PACD algorithm by enhancing the received information. It improves the Chase decoding performance without increasing the decoding complexity exponentially. It is shown that the E-PACD algorithm's complexity can be significantly reduced by utilising the existing interpolation information of the previous Chase decodings'. Our performance evaluations show that the two proposed decoders outperform a number of existing algebraic decoding approaches. Complexity and memory analyses of the PACD algorithm are also presented, demonstrating that this is an efficient RS decoding strategy.

By introducing amplify-and-forward relaying into a cognitive radio system, typical cognitive relay networks are studied for the optimisation problems of the spectrum and power allocation. By applying the mixed spectrum access of overlay and underlay approaches, an interference evaluation strategy is proposed to use different spectrum and power allocation methods while the secondary users (SUs) are located in different service regions of the primary users (PUs). In the interference evaluation strategy, the service area of the PUs is divided according to the possible interference strength of the PUs from the SUs compared with the location-aware strategy in which the service area is divided only by the location information. An optimal power allocation algorithm is developed to maximise the throughput of the SUs under the condition of anti-interference performance of the PUs and the total power constraints of the SUs. A type of computing algorithm that joins the subgradient and the Newton's method is used in order to resolve the complex optimisation problem. Numerical results show that the performance using the interference evaluation strategy, such as the throughput, the power consumption, and the energy efficiency, outperforms that using the location-aware strategy.

In this study, an overlay spectrum sharing scheme has been proposed for a cognitive radio system. To achieve the desired quality of service for the licenced (i.e. primary) system and spectrum access for unlicenced (i.e. secondary) system, a three-phase cooperative decode and forward relaying is used. Furthermore, space–time block coding is used in order to cancel the interference at primary as well as secondary receiver. It has been shown that the proposed scheme helps in achieving diversity gain of three and two for primary and secondary systems, respectively. The performance of both primary and cognitive (secondary) system is appraised by deriving closed-form expression for bit error rate and outage probability. The theoretical and simulation results validate that the proposed scheme improves the performance of both systems as compared with the earlier proposed schemes.

In this study, the authors consider the downlink radio resource allocation for heterogeneous traffic in generalised frequency division multiplexing (GFDM)-non-orthogonal multiple access (NOMA) based heterogeneous cellular networks. In this scheme, multiple number of users can be allocated on each subcarrier. Two types of traffic are considered, elastic and streaming. The problem of maximising the weighted sum-rate of elastic users is addressed subject to streaming users minimum rate in addition to subcarrier and transmit power constraints. This problem is a non-convex NP-hard optimisation problem. To solve this problem, the authors divide it into two subproblems, subcarrier allocation and power allocation then an iterative algorithm is proposed. Subcarrier allocation is updated by solving an integer linear program, where a successive convex approximation approach is adopted to transform the power allocation subproblem to a sequence of convex subproblems, using one of the three methods, successive convex approximation for low ComplExity, arithmetic-geometric mean approximation (AGMA) and difference of two concave functions to find the power allocation optimal solutions. Numerical experiments show that the proposed algorithms can improve the system performance. Furthermore, they show that AGMA can achieve a sum-rate near to the global optimal solution, at the expense of more computational time.

For an energy harvesting wireless network (EHWN), power oscillation will occur in uplink signal-to-interference-plus-noise ratio-based power control if some energy-non-supported nodes exist. Power oscillation will destroy power control algorithm's convergence and influence the system's stabilisation no matter the EHWN is feasible or infeasible. Unfortunately, existing algorithms cannot avoid the power oscillation. Therefore, the authors propose a new distributed algorithm which contains a soft removal mechanism to solve this problem in this study. Some energy harvesting nodes should be removed softly both in terms of their energy state information and channel state information. The convergence of the authors’ proposed algorithm can be guaranteed and power oscillation can be avoided in both feasible and infeasible EHWN. Simulation results verify their analysis and show that their algorithm brings less outage ratio than other algorithms.

The authors propose a generalised multi-layer architecture for the phase shifter network (PSN) in hybrid analogue/digital beamforming (BF), which provides flexible trade-offs between the performance and implementation complexity. By changing the layer number, the authors can adjust the complexity of the PSN and the achievable performance. A two-stage BF algorithm is adopted, in which the analogue and digital BF matrices are designed separately. Analogue BF is adaptive to the second-order channel statistics. A simple iterative algorithm is proposed to maximise an upper bound of the average rate of the effective channel after analogue BF. Digital BF is adaptive to the effective channel for multiuser interference cancellation. Both ideal phase shifters and discrete phase shifters with limited phase and amplitude resolutions are considered in the algorithm design. Theoretical analysis and simulation are used to demonstrate the good performance and flexibility of the proposed technique. In a 64-antenna system, it can use 16 radio-frequency chains to perform very closely to full digital BF with a fully-connected PSN, or achieve 70–80% performance with a simple PSN. It is also robust against the limited phase shifter resolution. It enables us to select the proper performance and implementation complexity according to the requirement and capability of real systems.

This study presents the average bit error rate (BER) performance analysis of free space optical (FSO) systems employing subcarrier intensity modulation with binary phase-shift keying and avalanche photodiode (APD) receiver. Intensity fluctuations of received optical signal are assumed to originate from both the gamma–gamma atmospheric turbulence and pointing errors. Besides theoretical derivation of the average BER expression, numerical integration is performed assuming specified relative calculation errors. Numerical results are presented and validated by Monte Carlo simulations. Furthermore, closed-form expressions are derived for thermal noise and shot noise limited conditions, which represent special cases of the system under investigation. On the basis of the presented results, effects of the FSO link and receiver parameters on BER performance are investigated and discussed. It is shown that optimal values of laser beam waist and average APD gain exist for minimising the average BER. The optimal values of laser beam waist depend strongly on the normalised jitter standard deviation, while the optimal values of APD gain are mostly dependent on receiver noise temperature, bit rate and atmospheric conditions.

The outage probabilities (OPs) of cognitive amplify-and-forward systems with the conventional and the switching-based partial relay selection (PRS) schemes over independent but not identically distributed Rayleigh fading channels are evaluated. The conventional PRS scheme always depends on the instantaneous channel quality of the first hop, while the switching-based PRS scheme is based on the estimated average channel state information (CSI) of the first and second hops. For the switching-based PRS scheme, the instantaneous CSI for links with the smaller average channel power in the first and the second hops for each end-to-end path is used for relay selection. Thus, the switching-based PRS scheme counts on the instantaneous CSI of either the first or second hop. The tight lower bounds and asymptotic expressions of the OP are derived. The feedback overheads of both schemes are discussed. Simulation results substantiate the theoretical analysis and also reveal that the switching-based PRS outperforms the conventional one over the cognitive relay networks in terms of OP.

This study considers a multi-pair massive multiple-input multiple-output two-way relay network where the M-antenna relay simultaneously serves K pairs of single-antenna users in the same time–frequency resource. For a more general case of Ricean fading channel, the authors propose a fixed-gain maximum ratio combing/maximum ratio transmission relay scheme. The approximate expressions on the ergodic sum rate with such scheme are derived in two cases: (i) M, which is much larger than the users’ Ricean factors, is large enough; and (ii) if M is large enough but bounded, the Ricean factors of all the users go to infinity. Simulation results show that the approximate results are very tight. Based on the result for the first case, they further discuss the power-scaling laws, which reveal that despite the Ricean fading channel and the interference at the relay, the achievable sum rate can still remain unchanged if the transmitted power at each user or at the relay is or both are made inversely proportional to M.

This study proposes rate-estimation-based relay selection scheme (RERSS) for large-scale wireless networks (LSWNs) that effectively distributes traffic of packets from a source to a sink. In an LSWN, direct data transmission from a source and a sink has been becoming a challenge. This is because of longer communication distance and more interference caused by more adjacent nodes. Hence, in order to transmit data packets to the sink, the source exploits neighbour nodes as relay nodes around it to forward packets through single-hop or multi-hop until reaching the sink. Different from the conventional cooperative relaying scheme, RERSS introduces a rate-estimation parameter A to select the forwarder for relaying messages. After determining A value of each neighbour relay node using a link rate stored in the advertising packet, the relay node whose A value meets the requirement of RERSS can be qualified as a forwarder of the source or the other relay node. Simulation results indicate the potential to not only increase throughput but also decrease energy consumption and end-to-end delay.

The solar powered energy harvesting sensor node is a key technology for Internet of Things (IoT), but currently it offers only a small amount of energy storage and is capable of harvesting only a trivial amount of energy. Therefore, new technology for managing the energy associated with this sensor node is required. In particular, it is important to manage the transmission interval because the level of energy consumption during data transmission is the highest in the sensor node. If the proper transmission interval is calculated, the sensor node can be used semi-permanently. In this study, the authors propose an energy prediction algorithm that uses the light intensity of fluorescent lamps in an indoor environment. The proposed algorithm can be used to accurately estimate the amount of energy that will be harvested by a solar panel using a weighted average for light intensity. Then, the optimal transmission interval is calculated using the amount of predicted harvested energy and residual energy. The results from the authors’ experimental testbeds show that their algorithm's performance is better than the existing approaches. The energy prediction error of their algorithm is approximately 0.5%.

This study investigates the framework of associating multiple-input–multiple-output (MIMO) beamforming with secure code to achieve unconditional secure communications in the wireless passive eavesdropping environment. The schemes are based on a two-step method under Wyner's wiretap channel model. First, with MIMO transmit beamforming, one can utilise the spatial degree of freedom to cripple eavesdroppers’ interceptions even when he does not know the eavesdropper's channel state information. Consequently, by taking the threshold characteristics of the secure code, the legitimate receivers will continue to extend an average bit error rate advantage over eavesdroppers when they share similar conditions (background noise power and channel gains). By this way, the proposed system could achieve almost zero information obtained by the eavesdroppers while still keeping rather lower error transmissions for the main channel. A profound theoretical analysis for the MIMO advantage channel and the exact closed-form expressions of secrecy outage probability for the secure code joint system are presented. The authors launch extensive experiments to verify the proposed security systems and demonstrate its feasibility and implement ability.

In this study, the authors revisit energy detection-based spectrum sensing cognitive radio systems operating over generalised fading channels. In particular, they derive closed-form exact expressions as well as low- and high-signal-to-noise ratio asymptotic expansions for the misdetection probability over the Fox's H-function fading channel. The closed-form expression is given in terms of the bivariate Fox's H-function and subsumes most of the expressions previously presented in the literature. Also, the obtained asymptotic expressions are very easy to compute and can be used to get various performance insights. We verified, theoretically and numerically, the validity of the exact expression for important special cases previously reported in the literature, namely the Nakagami-m and the extended generalised-K fading distributions. Numerical results also demonstrate the high accuracy of the asymptotic expansions.

A complex number whose real and imaginary parts are both integers is called a Gaussian integer. A Gaussian integer sequence is said to be perfect if it has an ideal periodic autocorrelation function (PACF) where all out-of-phase values are zero. Further, the degree of a Gaussian integer sequence is defined as the number of distinct non-zero Gaussian integers within one period of the sequence. Recently, the perfect Gaussian integer sequences have been found important practical applications as signal processing tools for orthogonal frequency-division multiplexing systems. The present article generalises the authors’ earlier paper by Lee et al. (2015) related to the Gaussian integer sequences with ideal PACFs. By the applications of two-tuple-balanced binary sequences and cyclic difference sets, a number of new degree-2 perfect Gaussian integer sequences with different periods are obtained.