Target position estimation of radar system has attracted much attention. Researchers have proposed a variety of joint direction-of-departure (DOD) and direction-of-arrival (DOA) estimation algorithms over the last few decades for this well-known problem. However, traditional estimation algorithms require a pairing process between the DOD and DOA estimation. In this study, the authors propose an improved unitary estimation of signal parameters via rotational invariance technique (ESPRIT) algorithm for joint DOD and DOA estimation without a pairing operation. The waveforms are transmitted by an array with M sensors and received by two detached sub-arrays with and sensors, respectively. Specifically, the proposed algorithm eliminates a pairing process via sharing the eigenvectors of DOD and DOA. Theoretical derivation demonstrates that the proposed algorithm requires less computational complexity than two-dimensional multiple signal classification (MUSIC), reduced-dimension MUSIC, and ESPRIT. Simulation results show that the proposed algorithm can effectively enhance the accuracy of identifying and locating targets for the bistatic multiple-input multiple-output radar system.

Mobile ad-hoc network (MANET) is an emerging technology that comes under the category of wireless network. Even though the network assumes that all its mobile nodes are trusted, it is impossible in the real world as few nodes may be malicious. Therefore, it is essential to put forward a mechanism that can provide security by selecting an optimal route for data forwarding. In this study, a goal programming model is designed using a hybrid optimisation algorithm, called M-LionWhale, for secure routing. M-LionWhale is an optimisation algorithm that incorporates lion algorithm (LA) into whale optimisation algorithm (WOA) for the optimal selection of the path in MANET. The multi-objective optimisation model considers several quality of service (QoS) parameters, namely energy, distance, link lifetime, delay, and trust. With the estimated multi-objective parameters, a fitness function is developed for the best selection of routes. The performance of the proposed algorithm is evaluated using three metrics, such as packet delivery ratio (PDR), throughput, and energy and is compared with that of existing trust-based QoS routing model, LA, and WOA. The proposed M-LionWhale algorithm could attain the maximum performance with 24.1313% residual energy, throughput of 0.2966 kbps, and PDR of 0.3051 at maximum simulation time.

Compared with the conventional degrees of freedom, the orbital angular momentum (OAM), which describes the helical phase structure of electromagnetic wave, provides a new degree of freedom. As a new multiple access scheme, mode division multiple access (MDMA) is constructed in millimetre wave frequency band utilising the orthogonality and high dimensionality in this study. Various traditional resources such as frequency, time and code pattern have been shared. Therefore, addresses of signals from different terminal users can be distinguished by OAM mode to realise multi-address connection. In this study, the theoretical analysis of the number of terminals in MDMA scheme is carried out. According to the analysis results, infinite terminals can be connected together in the ideal case. Moreover, the simulation results show that compared with the conventional multi-input multi-output millimetre wave communication systems, the performance indicators of MDMA millimetre wave communication systems are improved remarkably.

The transmission capacity (TC) of the wireless ad-hoc network is one of the key properties to ensure the quality of network service. The main goal of this study is to explore the mobility of the network to establish a node mobile model with a saturated state and determine the TC and delay for the network based on the bidirectional full-duplex (BFD) system. Considering the outage probability, packet arrival rate, and failure probability of packet attempting transmission, the node mobility models are established based on the node mobility and network self-organization. In analysis, this study determines the mobile state, static state, communication state, and outage stat by defining the mobile rule and range of nodes, then the steady-state probability of each state is obtained by calculating the state transition matrix. After optimising the existing saturated Markov model, the authors determine the network delay and TC based on the mobile nodes, contention window size, maximum number of given packet attempting retransmission, back-off time and signal-interference ratio (SIR) etc. The theoretical analysis and numerical simulation show that the TC of the bidirectional ad-hoc network can be significantly improved by using space resources in two directions, and the introduction of the BFD system is indeed to reduce the network delay.

The purpose of this study is to investigate an M/M/R retrial queue with geometric loss and Bernoulli feedback, in which all servers are subject to breakdowns and starting failures. After the completion of service, unsatisfied customers can join the retrial group with probability p or depart from the system with probability 1 − p. All servers may breakdown at any time, and the failed server undergoes repair immediately when a breakdown occurs. An arriving customer finding all servers unavailable (busy or down), will either join the orbit with probability b or leave the system with probability 1 − b. For such a queuing model, the authors apply the matrix-geometric method to compute the stationary probabilities and develop system performance measures in the steady state. Moreover, they construct a cost model and formulate an optimisation problem of minimising the expected cost per unit time. Finally, numerical results are given for illustrative purposes.

Multi-band transmitter systems are evolving to support the smooth transition from 4G to 5G communication systems. Moreover, recent developments of multi-band and ultra-wideband power amplifiers have led to a possible scenario where the second carrier signal is transmitted at the harmonic frequency of the first carrier signal. This results in harmonic interference from the first carrier signal as well as additional cross-modulation and intermodulation distortion (IMD) components, which cannot be filtered out. The computational and memory requirements for digital predistortion (DPD) in such scenario increase drastically to include all interference terms. This study presents a novel two-dimensional curtailed harmonic memory polynomial (2D-CHMP) model to capture harmonic interferences, cross-modulation and IMDs. The model complexity and memory requirement of 2D-CHMP are very less as compared to the state-of-the-art two-dimensional harmonic memory polynomial (2D-HMP) model. For proof-of-concept, it is shown with two different measurement setups that the proposed 2D-CHMP DPD provides similar linearisation performances as compared to the 2D-HMP DPD with less number of coefficients and computational complexity. As a study, it is shown that the proposed model can be further adapted to a low-precision (low-bit) environment by utilising principal component analysis.

This communication presents the gain enhancement of a dual-band WLAN microstrip antenna loaded with left-handed split-ring resonator metamaterials. A diagonal repeating pattern of metamaterial unit cells is integrated into the right corner of the substrate to enhance the gain in the lower WLAN band without distorting the upper band characteristics. The proposed antenna covers the different operating bands: 2.4–2.484 and 5.15–5.85 GHz (IEEE 802.11a and b/g/n) for WLAN applications. The gain improvement in the lower operating band is obtained around 3.69 dB which is explained by estimating the Green function of the metamaterial diagonal periodic pattern at the lower resonant frequency. The antenna provides high gain and efficiency and easily printable on a low-cost FR-4 substrate. The proposed antenna prototype is developed and tested and experimental results show an excellent agreement with the simulated one.

In this study, the authors propose a simple yet tighter approximations for the special two-dimensional Gaussian Q functions using the Trapezoidal rule of numerical integration. This enables a simplified and accurate symbol error probability (SEP) approximation of the hexagonal-quadrature amplitude modulation (HQAM) in additive white Gaussian noise channel. The proposed approximation further simplifies the SEP calculation of HQAM in parametric , , and fading distributions. Also, the SEP of HQAM over log-normal shadowing is calculated in this study. The accuracy of the analytical framework is verified using computer simulations.

The transmission capacity of a vehicular network is still an interesting and challenging problem as capacity is impacted by transmission distance between both vehicles, density of vehicles, relay among vehicles, noise and interference. This study analyses the transmission capacity of vehicular device-to-device relay networks with multi-users (service vehicle (SV) and helper vehicle (HV)) in the overlay mode. SVs coexisting with HVs share the spectrum resources. Utilising stochastic geometry theory, SVs and HVs are modelled as the Poisson point process in vehicular networks, respectively. Subsequently, the HVs existence probability and the expectation distance with HV link are derived to calculate the successful transmission probabilities for SV to SV communication. With this characteristic HV relay transmission signals exceed transmission distance, the authors further calculate the transmission capacities with SV to SV communication assisted by HVs and reveal the influence of vehicles' density and transmission power. Besides, the relationship between transmission capacities, the variable direct link distance of SV to SV communication is also analysed. Simulation results indicate that transmission capacity can be improved by an assisted HV and influenced by a few system parameters, including the direct link distance between source SV and destination SV, SVs density and HVs density.

This study presents a new hybrid scheme that effectively reduces the peak-to-average power ratio (PAPR) of discrete cosine transform based single-carrier frequency division multiple access (DCT-SCFDMA) signal as well as improves the bit error rate (BER) performance of system. The method is to consolidate pulse shaping process with a novel hyperbolic tangent companding technique so that the PAPR of the SCFDMA signals can be further reduced by enabling the transformation of the amplitude of modulated signals into uniform distribution. Through extensive computer simulations, it has been shown that proposed scheme achieves significant improvements in BER and PAPR reduction over the existing standard pulse shaping methods when used alone in DCT-SCFDMA system as far as the lowest PAPR is concerned without degrading the error performance of the system.

The energy-constrained devices such as the mobile terminals and nodes of the Internet of Things make lightweight security schemes an urgent need. The traditional identity authentication techniques can provide protection for the user's privacy and information to a certain extent, but they suffer from heavy cost. Non-cryptographic authentication mechanisms based on the physical layer characteristics are new techniques, which have a higher security level. The recognition technique of radio transmitter based on radio-frequency fingerprint (RFF) is one of the non-cryptographic authentication techniques. The authors propose a lightweight one-time password (OTP) authentication scheme based on RFF (RFF-OTP), which is a novel cross-layer secure authentication scheme and can provide mutual authentication between the mobile terminal and server, by combining RFF recognition algorithm with a hash encryption algorithm. By theoretical analysis and Syverson and van Oorschot logic verification, they prove that the RFF-OTP scheme is simple, efficient, flexible and independent of trusted-party while it also can resist the cloning attack and satisfy the anonymity compared with the OTP authentication scheme. Besides, it only requires the password to log into the system in the authors’ scheme in comparison to the OTP scheme that needs both ID and password for the same purpose.

This study addresses the statistically optimal channel estimation for orthogonal frequency division multiplexing systems in the presence of phase noise (PHN). The maximum a posteriori (MAP) cost function for the joint estimation of channel transfer function (CTF) and PHN is presented, which utilises the prior statistical knowledge of PHN spectral components without the assumption of small PHN. The frequency-domain estimation of unknown frequency selective fading makes the method simpler, compared with the estimation of channel impulse response in time domain. An iterative cyclic gradient descent optimisation algorithm is derived to minimise the cost function over the unknown CTF and PHN. Cramer-Rao lower bound is analysed and compared with the simulation results to illustrate that the proposed joint MAP estimation can achieve near optimum performance.

A novel concatenated physical-layer encryption (CPLE) scheme is proposed in this study, where better security can be achieved as well as reliability advantage. In CPLE scheme, the encryption is embedded in rateless encoding. The secret key, which is generated from wireless channel, controls the random degree and random linear combination. The rateless codes are also concatenated with other conventional channel codes, which can achieve better reliability. Different from security codes, both the security and reliability in the proposed scheme can be improved. Furthermore, there is no extra computation requirement on decoding process. Security analysis is presented including time complexity comparison and attacks resistance in practical application. Compared to other PLE schemes, the proposed scheme achieves lower complexity, and higher channel adaptation and performance advantage. On the basis of National Institute of Standards and Technology statistical test suite, CPLE scheme can achieve randomness as the traditional cipher system such as advanced encryption standard. Furthermore, simulation results show that the CPLE scheme outperforms the conventional concatenated codes at the same coderate.

In a communication system, security and reliability have an important impact on the quality of services. In this study, the authors investigate power and subcarrier allocation of a transmitter over the downlink of an orthogonal frequency division multiplexing access system with security considerations among users in the presence of a jammer. This study considers the interaction between the jammer and the transmitter as a zero-sum game where the objective function is the transmitter's secrecy sum-rate. To allocate the available resources, they solve the optimisation problem of each player to characterise its optimal strategies. Considering the intractability of the closed form Nash equilibrium for general power regime, they study a matrix game for obtaining maxmin and minmax solutions and demonstrate the validity of inequality . Moreover, they show the existence of pure Nash equilibrium point for low total power regimes of the transmitter and the jammer and attain an expression for each regime. Finally, they examine the jamming power effect on the secrecy sum-rate and acquire lower and upper bounds of this rate using simulations.

In this study, the design of an adaptive medium access control (MAC) protocol based on the space-division–time-division multiple access (SD-TDMA) mechanism is presented to improve the utilisation and fairness of the time-slot allocation scheme in the vehicular ad hoc network. The SD-TDMA mechanism dynamically allocates time-slots owing to a dynamic topology. This protocol reduces transmission collisions through user detection procedures. Most users can thus access the service channel (SCH) without conflict and acquire time slots according to the mapping between geographic locations and time slots. Users failing to access the channel in user detection procedures can send request access packets in security channels to again participate in SCH time-slot allocation during the SCH interval. Simulations are conducted to evaluate the performance of the proposed method in urban scenarios. The SD-TDMA mechanism is compared with an SDMA-based MAC protocol, the IEEE 802.11p distributed coordination function and an MAC protocol based on transmission sequences. Owing to the decreased rate of transmission collisions and the dynamic time-slot allocation scheme, it is shown that the SD-TDMA mechanism provides a significantly higher utilisation of time slots and better fairness of data transmission than existing approaches.