A new orthogonal frequency division multiplexing (OFDM) with reduced peak-to-average power ratio (PAPR) based on coded discrete cosine harmonic wavelet packet transform (DCHWPT), is proposed. In this method, tree pruning is done for the selection of a tree structure with minimum PAPR. To identify different tree structures, Gray code is used which is communicated as side information to the receiver. Also to improve bit error rate (BER) performance and reduce PAPR, Hadamard codes are employed. The proposed method based on DCHWPT exploits the desirable discrete cosine transform (DCT) properties like energy compaction/low leakage, better frequency resolution and its real nature, compared with those of discrete Fourier transform. Further, the harmonic wavelet employed has built in decimation without any associated antialiasing filters and easy reconstruction by concatenation of different scales in frequency (DCT) domain with no image rejection filters and complicated delay compensation makes the proposed wavelet packet simple in implementation. The proposed OFDM discrete cosine harmonic wavelet packet (DCHWP-OFDM) which leverages the advantages of DCHWPT has been used for BPSK modulated signals. The DCHWP-OFDM provides an improvement of PAPR reduction of 2.4 and 3.4 dB as compared with those of Haar and Daubechies-4 wavelet packet OFDM, respectively. This reduction in PAPR is with a BER performance in the range of 0.001–0.0001 at an signal-to-noise ratio of 10 dB.

In this study, the authors investigate the linear minimum-mean-square-error beamforming design for relay-assisted cloud radio access network (C-RAN). A standard C-RAN architecture separates baseband processing units and wireless radio units in order to save energy cost. To further enhance network coverage, several relay nodes (RNs) are also deployed. Regrading the per-antenna power constraints at both of the remote radio heads (RRHs) and the RNs in the author's work the beamforming matrices at the RRHs and RNs are ‘jointly’ optimised for the considered relay assisted C-RAN. The considered optimisation problem is a non-convex and multiple variable optimisation problem which is in general very hard to solve. In order to make the design suitable for large scale networks exploiting to the problem structure a novel two stage decomposition algorithms are proposed. Finally, a detailed mean-square-error performance comparison is given by the simulations.

The authors consider a half-duplex asynchronous code division multiple access cooperative network with N source–destination (SD) pairs and a number of relay nodes where the nodes of all pairs have to exchange data in two hops via assistance of one of the available relays. In this study, they minimise the total transmit power and derive the closed-form solution for choosing the best relay, the best relay gain and the transmit powers of all sources where some predefined signal-to-interference plus noise-ratios (SINRs) are guaranteed. Interestingly, the feasibility condition of the problem depends only on the required SINRs, the number of SD pairs and the maximum cross-correlation of users’ codes. They suggest two control procedures for admitting or dropping of users to the network to satisfy the feasibility condition. For a reciprocal environment, the best relay and its gain are proved to remain unchanged for reversing the communication directions. In addition, the authors’ power control algorithm can be directly applied to the case of two-hop two-way relaying. Computer simulations are used to demonstrate the system performance.

In previous studies on the capacity of orthogonal frequency division multiple access (OFDMA) systems, it is usually assumed that co-channel interference (CCI) from adjacent cells is a Gaussian-distributed random variable. However, very-little work shows that the Gaussian assumption does not hold true in OFDMA systems. In this study, the statistical property of CCI in downlink OFDMA systems is studied, and spectral efficiency of downlink OFDMA system is analysed based on the derived statistical model. First, the probability density function (PDF) of CCI in downlink OFDMA cellular systems is studied with the considerations of path loss, multipath fading and Gaussian-like transmit signals. Moreover, some closed-form expressions of the PDF are obtained for special cases. The derived results show that the PDFs of CCI are with a heavy tail, and significantly deviate from the Gaussian distribution. Then, based on the derived statistical properties of CCI, the downlink spectral efficiency is derived. Numerical and simulation results justify the derived statistical CCI model and spectral efficiency.

This paper studies amplify-and-forward (AF) relaying schemes for layered relay networks. The design of the network-wide optimal AF scheme in terms of maximum end-to-end transmission rate is mathematically formulated as a non-convex optimisation problem which is computationally intractable in general. Therefore, the authors restrict it to a localised optimisation problem and develop a second-order cone programming-based approach to efficiently solve it, which yields localised optimal AF schemes for the relays at a specific layer. To improve the performance of a given network-wide AF scheme, a low-complexity alternating algorithm is proposed based on the localised optimisation technique. To reduce the computational complexity, they also derive localised suboptimal schemes in closed form. With relays at one layer applying this scheme and the other relays transmitting with the maximum allowable powers, a simple network-wide AF scheme is derived, which is proved to be asymptotically optimal in the generalised high-SNR regime. Finally, simulations are used to illustrate the validity of the proposed schemes.

With eXtensible Markup Language (XML) becoming a ubiquitous language for data storage and transmission in various domains, effectively safeguarding the XML document containing sensitive information is a critical issue. In this study, the authors propose a new access control model with filtering privacy. Based on the idea of separating the structure and content of the XML document, they provide a method to extract the main structure of the XML document and use matrix to save the structure information, at the same time, the start–end region encoding is used to combine the corresponding structure and content skillfully. These not only save the storage space but also efficiently speed up the search and make it convenient to find the relevant elements, especially the finding of the related content. In order to evaluate the security and efficiency of this model, the security analysis and simulation experiment verify its performance in this work.

Relay technology has been included in long term evolution (LTE) cellular system through 3GPP Release 10. Relay-based LTE cellular system is one of the most promising technologies to enhance the throughput and coverage of access network. Deployment of relays in a cell opens many issues of frequency/time domain radio resource allocation and frequency planning. In this study, the authors consider the uplink of LTE relay-assisted network and formulate the joint scheduling and intra-cell interference mitigation as a non-linear optimisation problem. They take into account the projection of the gradient of user's utility function over the user's rate vector based on Lagrangian method. They propose a novel transformation of scheduling problem into combinatorial Knapsack optimisation to convert the optimisation problem into binary integer non-linear program. Finally, they present an alternate solution through a low complexity algorithm to allocate resources based on group of resource blocks, whereas considering the contiguity constraint. Simulation results show that the proposed scheme is near optimal with a much lower complexity order.

The authors propose a novel localised discrete Fourier transform (DFT) spread M-ary amplitude shift keying (M-ASK) orthogonal frequency division multiplexing (OFDM) system with the Hermitian symmetry (DFT-spread M-ASK HS OFDM) to reduce the peak-to-average power ratio (PAPR). Compared with the conventional OFDM and localised DFT-spread OFDM, the proposed system has lower PAPR not only because of the DFT-spread scheme but also its lower modulation order than other OFDM systems. The lower modulation order of the proposed system can be achieved by using the Hermitian symmetry of DFT. The simulation result shows that the localised DFT-spread M-ASK HS OFDM provides a significant improvement in bit-error-rate and reduced PAPR.

The increasing demand of high-speed and secure wireless broadband networks has generated significant interests in the optimisation and the analysis of energy-efficiency in orthogonal frequency division multiple access (OFDMA) system. In this study, the authors present an in-depth mathematical analysis of the maximisation of energy-efficiency by taking into consideration the quality of service (QoS). By using optimality conditions, they have shown that the optimal solutions can be obtained analytically. Furthermore, it has been proved in this study that this optimisation problem is strictly concave with the existence of a global maximum. Case studies with multiple subchannels validate the consistency of numerical results with the results obtained from derivative-free method like genetic algorithm. Graphical illustrations also validate and confirm the numerical values obtained from the mathematical analysis. Therefore the solution is optimal with respect to the OFDMA model adopted in this study. The authors proposed approach can be used for practical applications because of its simplicity and efficacy with QoS guaranteed for efficient energy consumption.

Three schemes are designed for joint relay selection and link adaption in amplify and forward relay network which are called RS-CAP (joint relay selection with continuous adaptation of powers), RS-DAP (joint relay selection with discrete adaptation of powers) and OTD-DAP (optimised time division with discrete adaptation of powers). In all schemes spectral efficiency is optimised using adaptive modulation, coding and transmit power subject to average power constraint of each node and a bit error rate constraint of detection. The main idea behind all designs is to save power of relays in inactive time to be utilised for active time. In RS-CAP and RS-DAP, optimised relay selection is done based on channel state information of all corresponding paths, however, in OTD-DAP just an optimised time percentage is allocated to each relay to cooperate in signal transmission. Considerable less channel estimation load is imposed on the receiver in the OTD-DAP scheme compared with the others. In RS-CAP power of nodes are continuously adapted, however, in RS-DAP and OTD-DAP, powers are discrete adaptive. Thus there is a considerable decrease in feedback rate in the RS-DAP and OTD-DAP schemes in comparison with RS-CAP. A trade-off between the performance and channel estimation load and feedback rate is seen in numerical evaluations. Noticeable improvement in achievable rates is seen by using proposed schemes compared with the previous works.

In this study, the authors consider the secrecy of a one-receiver, two-eavesdropper broadcast channel (BC) with three degraded message sets. Consider a three-receiver BC with three messages, where the first message is decoded by all the receivers. The second message is decoded by the first and the second receivers and is to be kept secret from the third receiver. The third message is decoded by the first receiver and is to be kept secret from the second and the third receivers. The authors consider the imperfect secrecy condition at the second receiver, that is, it is allowed to partially decode the third message. However, the perfect secrecy condition at the third receiver, does not allow it to decode the confidential messages. The coding scheme for this model requires decoding strategy for finding the messages at different destinations. The authors propose a coding scheme which uses indirect decoding. The authors also obtain an outer bound and use it to determine the secrecy capacity region of some classes of one-receiver, two-eavesdropper BCs with three degraded message sets. The authors extend our results to the Gaussian case and evaluate the achievable region.

Localisation in wireless sensor networks (WSNs) has received much attention, where most studies focus on mitigating the effects of measurement noise under the assumption of accurate anchors’ positions. However, anchors’ positions could be inaccurate for the inevitable errors in practical observations. This paper studies the sensor localisation with both inaccurate anchors’ positions and noisy range measurements in WSNs. To solve the intractable integrals in likelihood function, the authors propose to use expectation-maximisation (EM) algorithm to obtain the maximum likelihood (ML) estimation iteratively. The ‘a posteriori’ distribution of the anchor's position uncertainty is approximated to a circularly symmetric Gaussian distribution by minimising the Kullback-Leibler divergence between them. Building on this, the authors derive the expectation step in a closed-form expression. In the maximisation step, based on the Taylor expansion of the confluent hypergeometric function of the first kind presented in the expectation step, analytical solutions are obtained. Simulation results show that the proposed EM estimator significantly outperforms the approximated ML estimator. The performance gain by using the EM estimator becomes larger as the increase of anchors’ position uncertainties. Moreover, the performance of the EM estimator is close to that of the Monte Carlo-based estimator with much less computational complexities.

In this study, a new iterative interference cancellation (IIC) multiuser detection method is proposed for overloaded multiple-input multiple-output (MIMO) coded orthogonal frequency division multiplexing systems. A channel analyser is used to detect those channels for which the IIC detector will not converge. For these we propose two adaptive methods to enable detection. The first is to use a lower code rate for these channels. The second is to divide the transmitters into several groups which transmit in separate slots. Low-complexity matched filter is applied, and both successive interference cancellation and parallel interference cancellation are considered for interference reconstruction. The simulation results show that the proposed technique can achieve near-optimum bit error rate performance and optimum frame error rate performance, with lower complexity than optimum maximum-likelihood methods.

The authors consider centralised cooperative spectrum sensing under correlated shadowing in this study. Formulating the spectrum sensing problem as a Gauss–Gauss hypothesis test, they use a linear quadratic rule and show that it is the optimal detector under Bayesian criterion. They derive the upper and lower Bhattacharyya bounds and investigate the error performance of spectrum sensing by studying the behaviour of these upper and lower bounds. They also study the asymptotic error performance in two different scenarios of finite and infinite area networks. They show that by increasing the number of nodes the sensing error probability approaches zero in both cases but with different decay rates. The lower the correlation between nodes or the larger the network area, the faster the decay.

Sleep mode control is essential to the energy efficiency of small cell networks. However, frequently switching on/off small cell base stations (SBSs) may cause the degradation to the quality-of-service of their users and the increase of network operational cost as well. In this study, the authors propose a novel dual-threshold-based sleep mode control strategy for small cell networks. The motivation of using dual-thresholds to control the sleep mode is to minimise the network energy consumption while avoiding the frequent mode transitions of SBSs at the same time. They utilise the Markov chain method to analyse the performance of the proposed strategy. Optimisation problems are formulated to achieve the optimal dual-thresholds for two different scenarios: the homogeneous threshold scenario in which uniform dual-thresholds are applied to all SBSs and the heterogeneous threshold scenario where different dual-thresholds are assigned to SBSs. For the homogeneous threshold scenario, they develop an optimal solution which is based on exhaustive searching. A reinforcement learning-based algorithm and a heuristic algorithm are proposed for the heterogeneous threshold scenario, respectively. Simulation results are presented to demonstrate the performance of the author's proposed algorithms.

Dynamic pilot allocation (DPA) for discrete Fourier transform (DFT)-based channel estimation in multi-input–multi-output orthogonal frequency division multiplexing (MIMO-OFDM) systems with spatial multiplexing can significantly improve the bit error rate performance compared with systems with uniform pilot allocation. However, the exhaustive search for optimum pilot allocation leads to very high complexity. The authors devise a multi-input–multi-output iterative pilot search (MIPS) algorithm applied with different MIMO-OFDM receivers (linear, successive interference cancellation (SIC) and maximum likelihood (ML)), which significantly reduces the complexity of DPA. Exact derivations are also given based on the receivers. They also propose a novel stacked vector quantisation technique to reduce feedback burdens for DPA in MIMO-OFDM system. Simulation results illustrate that the proposed MIPS algorithm with limited feedback can improve the performance of MIMO-OFDM systems.

This study considers the joint transmission precoding design for downlink multicell multiuser multiple-input single-output systems where imperfect channel variances are available at the base stations. The authors aim to tackle the robust signal-to-interference-plus-noise ratio (SINR) balancing problem to maximise the minimum worst-case user rate. To solve the non-convex problem, a duality relationship between the downlink max–min worst-case SINR optimisation problem and the virtual uplink min–max worst-case SINR optimisation problem is first revealed. Based on this, a new algorithm is developed to solve the virtual problem by using jointly the sub-gradient method and the geometric programming methods, whose achieved solution is finally converted to the downlink. Their analysis shows that the proposed algorithm is guaranteed to converge and has lower computational complexity than conventional approaches. Moreover, computer simulations validate the effectiveness of the proposed method and show that the proposed algorithm has fast convergence and achieves a performance close to that of the brute search method.

The underwater acoustic channel is considered as one of the most challenging channels for reliable communications. Long delay spread and Doppler effect bring a great challenge to directly apply the algorithms in radio communications to underwater communications, which calls for modification and enhancement. In this study, a pilot-assisted linear turbo equalisation algorithm termed P-TE is proposed. Unlike the existing pilot methods that insert pilot into the modulated symbols to facilitate the channel estimation, the authors insert the known bits into the uncoded binary sequence to assist equalisation as well as decoding with the careful design of the bit positions. A second-order least mean square adaptive algorithm as well as adaptive step size method is introduced for weights update. Furthermore, a modified low-density parity-check decoding algorithm is presented to describe the pilot-assisted decoding algorithm, which does not increase the complexity. Moreover, pilot extraction from error-free blocks is proposed for enhanced performance. Experimental results have indicated that P-TE can significantly improve the performance and reduce the required number of the iterations, leading to lower complexity. Furthermore, experiments show that the effective throughput of P-TE can be higher than the method without pilot at a signal-to-noise ratio of 10 dB.

Estimation of the multipath channel in 60-GHz communications is challenging, because the channel may be sparse or dense during beam training. Specifically, because of the variation of the number of non-zero taps, it is hard for common estimators to obtain robust and prominent performance. In order to address this problem, the authors propose a sparse/dense channel estimation with non-zero tap detection (SDCE-NTD). The estimation is conducted in a three-stage fashion, including initial estimation with the unstructured least-square (LS) algorithm, non-zero-tap detection with the generalised likelihood ratio test approach, and posterior estimation with the structured LS algorithm. The false-alarm and detection probability of the tap detector, as well as the mean square error (MSE) of SDCE-NTD, are derived and confirmed via simulations. Comparisons are conducted between SDCE-NTD and the common estimators in the beam training scenarios, where both dense and sparse channels exist. Results show that SDCE-NTD reveals a significant gain in terms of MSE over both the conventional LS algorithm, which does not exploit the sparse nature of the channel, and the matching pursuit algorithm, which endeavours to exploit the sparsity. In addition, it is also demonstrated that the proposed estimator can approach the lower bound with high signal-to-noise ratio.

In ‘Sparse block circulant matrices for compressed sensing’, in order to apply Lemma 4, every off-diagonal element of Gram matrix for the sparse block circulant matrix was separated into two component sums to make sure the terms in each sum are independent. In this comment, however, the authors show that separating every element into two sums is not sufficient to guarantee the independency of the terms in each sum. The authors also prove that the entries should be split into three parts instead of two to satisfy the requirements of Lemma 4. Finally, the authors modify the deduction and the result.