Maximum likelihood (ML) algorithms, for the joint estimation of synchronisation impairments and channel in multiple input multiple output–orthogonal frequency division multiplexing (MIMO–OFDM) system, are investigated in this work. A system model that takes into account the effects of carrier frequency offset, sampling frequency offset, symbol timing error and channel impulse response is formulated. Cramér–Rao lower bounds for the estimation of continuous parameters are derived, which show the coupling effect among different impairments and the significance of the joint estimation. The authors propose an ML algorithm for the estimation of synchronisation impairments and channel together, using the grid search method. To reduce the complexity of the joint grid search in the ML algorithm, a modified ML (MML) algorithm with multiple one-dimensional searches is also proposed. Further, a stage-wise ML (SML) algorithm using existing algorithms, which estimate less number of parameters, is also proposed. Performance of the estimation algorithms is studied through numerical simulations and it is found that the proposed ML and MML algorithms exhibit better performance than SML algorithm.

The present work analyses and compares two of the most popular specifications for data transmission over power line networks: PoweRline Intelligent Metering Evolution (PRIME) and G3-power line communication (G3-PLC). A simulation model has been built using Matlab software in order to evaluate their respective performances with special focus on impulsive noise environments. For this purpose, Middleton's Class-A noise model was used in conjunction with measured noise parameters reported in the literature for the narrow-band spectrum. The performance is measured in terms of bit error rate versus signal to noise ratio. Simulation results show how G3-PLC outperforms PRIME when the channel is impaired by such type of noise. Moreover, although the use of compressive sensing to cancel impulsive noise in communications has already been proposed in other studies, this paper details a modification based on Partial Fourier Matrix indexing according to difference sets. Results from simulations report an almost complete cancellation of the impulsive noise effects. An advantage of this technique is that no redundancy is added to the message; therefore no decrement in the transmission rate is experienced.

In this study, the authors propose a diversity-combining scheme with transmit antenna selection (TAS) and output-threshold generalised selection combining (OT-GSC) receiver. It is shown that the proposed TAS/OT-GSC can achieve similar performance as compared to the TAS/GSC with a reduced number of channel estimates when the predetermined signal-to-noise ratio (SNR) threshold is optimised. The outage probability, average output SNR, and symbol-error rate (SER) expressions of the proposed TAS/OT-GSC scheme are derived and presented over independent and non-identically distributed Rayleigh-fading channels. The analytical expressions are validated by simulation results. The trade-off comparisons in terms of the SER and number of channel estimates between the TAS/OT-GSC and TAS/GSC are discussed. Lastly, the capacity between the TAS/OT-GSC and TAS/GSC is compared.

In a multicell scenario, two competitive beamforming design schemes are proposed to maximise the worst signal-to-interference-plus-noise ratio (SINR) in each cell for downlink transmissions. The first design adopts interference control. The authors use different interference control weighting factors to suppress the inter-cell interference effectively as each cell may experience different level of inter-cell interference. The other design is based on power control, where the authors can adjust the power of each base station to improve the system performance. Both the two improved designs keep the distributive nature of the competitive design as well as improve the worst SINR of the system. The existence of Nash equilibrium of the proposed designs is proved and the system overhead is analysed for the comparison with other distributive algorithms. Simulation results show that both the proposed competitive designs have improved performances, and the competitive design with power control outperforms that with the interference control as the transmission power increases.

In this study, we analyse the error performance of decode and forward (DF) and amplify and forward (AF) multi-way relay networks (MWRNs). The authors consider a MWRN with pair-wise data exchange protocol using binary phase shift keying (BPSK) modulation in both additive white Gaussian noise (AWGN) and Rayleigh fading channels. The authors quantify the possible error events in an L-user DF or AF MWRN and derive accurate asymptotic bounds on the probability for the general case that a user incorrectly decodes the messages of exactly k (k ∈ [1, L − 1]) users. They show that at high signal-to-noise ratio (SNR), the higher order error events (k ≥ 3) are less probable in AF MWRN, but all error events are equally probable in a DF MWRN. They derive the average BER of a user in a DF or AF MWRN in both AWGN and Rayleigh fading channels under high SNR conditions. Simulation results validate the correctness of the derived expressions. The authors results show that at medium to high SNR, DF MWRN provides better error performance than AF MWRN in AWGN channels even with a large number of users (e.g. L = 100). Whereas, AF MWRN outperforms DF MWRN in Rayleigh fading channels even for much smaller number of users (e.g. L > 10).

Previous works on spectrum sensing use either samples free of secondary user interference based on dedicated sensing period or samples with partly ‘cleaned’ secondary user interference based on specific characteristics of orthogonal frequency division multiplexing signals or decoded signals. In this article, collaborative spectrum sensing using samples corrupted by secondary user interference without dedicated sensing period or ‘cleaning’ procedure is studied, and its performance is analysed. Two different cases of primary user signals are considered. In the first case, the primary user signal suffers from fast fading, whereas in the second case, the primary user signal suffers from slow fading. New detectors for these two cases are derived, based on which their performances are analysed. Numerical results show that the diversity gain is lost because of the secondary user interference. For fast-fading primary user signal, the sensing performance deteriorates quickly when the number of interfering secondary users increases, whereas for slowly fading primary user signal, an error floor exists when the number of interfering secondary users increases. On the other hand, by choosing appropriate system parameters, high sensing accuracy can still be achieved with acceptable loss in detection probability, but with long transmission time when no dedicated sensing period or ‘cleaning’ procedure are used.

The performance of ad-hoc networks is greatly affected by interference, particularly interference from nearby nodes. Based on stochastic geometry, this work studies a form of multiple-input–multiple-output (interference alignment (IA) that eliminates interference from transmitters within a range and treats the remaining interference as a shot noise process. Adapting to the bursty nature of ad-hoc network traffic, the authors introduce a novel distributed ad-hoc network with renewal process, whereby cooperation between interferer and receiver is implemented in one-way process, which results from the difficulties for the node to change the beamforming or receiving matrix whereas communicating with a proposed transmitter. In addition, this scheme takes advantage of IA, leaving the desired signal with higher degree of freedom (DoF) compared with the partially zero-forcing method. Outage probability and transmission capacity are derived with and without Rayleigh fading. Simulation results show the effect of IA in ad-hoc network, including the incremental in the DoF of the desired signal as time goes by. Monte Carlo simulations are implemented and show that IA outperforms successive interference cancellation by at least 10 dB in terms of the signal-to-interference ratio with large path-loss exponent.

Two knapsack-based public key cryptosystems were proposed recently, in which the entries of the secret knapsack sequences are composed of products of random integers and the knapsack problems have nonbinary solutions. These features make the cryptosystems to be secure against low density attacks. In this study, the authors present lattice-based complete private key recovery attacks on these two schemes. The authors attacks firstly find short vectors related to the public key and with some orthogonality to the secret knapsack sequences and then recover some components of the private key by computing common factors of the entries of the short vectors. Especially, the authors attack can both completely recover the unique key for these two schemes. The attacks are of practical complexities and verified by experiments.

To provide increasing multimedia broadcast/multicast services (MBMSs), the third generation partnership project modified long-term evolution release nine documents to further support evolved MBMSs (e-MBMS). This study designed a data-broadcasting method called evolved broadcast scheduling algorithm (eBSA). This eBSA provides new scheduling technology for e-MBMS and a new information searching method for user equipments (UEs) that simultaneously conserves UE energy and maintains favourable access latency of broadcast messages. The eBSA constructs index and message channels based on the message popularities, using the index channel to guide UEs to rapidly locate the position of the desired message in the message channel. If searching for unpopular messages, the index channel guides UEs directly to conduct a linear search. Consequently, a short mean access time and mean turning time can be attained. The results of numerical analysis verify that when access messages possess skewed-access characteristics, the proposed eBSA method performs exceptionally. Additionally, by adequately adjusting the parameter provided by the eBSA method, the optimal tradeoff between energy conservation and access latency performance can be attained.

In this study, new transmission schemes are designed for the spectral efficiency optimisation in the amplify and forward relay network with diversity. The proposed schemes utilise both adaptive rate and adaptive power transmission with individual average power constraints for the relay and the transmitter. To obtain an analytical insight into the network performance, schemes with continuous rate adaptation are designed at first. Next, transmission schemes with discrete rate and power adaptation are developed for practical systems. An amplify–forward relaying scenario with diversity is considered in which signals at the destination are received from both the transmitter and the relay. Albeit, the schemes could be simplified for the network without diversity in which there is no path for the signal from the transmitter to the receiver. Schemes with constant power transmission are also designed and analysed. Numerical evaluations show that there is a considerable performance gain in using adaptive power transmission compared to the constant power scheme at the cost of a reasonable complexity increase.

Ultra wideband (UWB) systems transmit signals across a much wider frequency than conventional systems and are usually very difficult to detect. UWB systems tend to be useful for short-range indoor applications. However, because of the short duration of UWB pulses, it is easier to engineer extremely high data rates. In multi-band orthogonal frequency division multiplexing (MB-OFDM) architecture, designers gain flexibility, reduction in power consumption, high spectrum efficiency and cost. In the earlier study, people shadowing has been considered to model attenuation in a packet-level UWB channel model. However, in dense short-range indoor applications, the effect of scatterers, diffractors and reflectors cannot be ignored. This happens mostly when the transmit antenna is omni-directional. Compared with the earlier work which incorporated only shadowing effects, this study discusses a packet-level UWB channel model subject to both multipath fading and people shadowing, which can arise even in an indoor channel (e.g. inside a hall of dimensions ∼ 10 m), and is incorporated in an MB-OFDM system. The performance analysis of the link is made based on link throughput and delay in servicing packets. The numerical results show the relative effects of various fading distributions, viz., Rayleigh, Rician, Weibull and Nakagami, together with people shadowing on the MB-OFDM system.

Recently, improving energy efficiency has been a new tendency in wireless relay communication. However, the introduction of relay technology may increase the system capacity, but is often followed by a more severe energy-efficiency problem when compared with traditional single-hop system. In this study, based on orthogonal frequency division multiplexing-based frequency-selective channel of amplify-and-forward relay link, a suboptimal two-step power allocation algorithm of subcarriers is proposed to maximise the energy efficiency. The proposal includes two steps. First, a suboptimal subcarrier matching is achieved for introducing a virtual direct link, which may reduce the dimensionality of the power allocation. Second, the allocation problem of relay network is simplified as that in traditional cellular network. Thus, the optimal power allocation of virtual direct link can be obtained by one-dimensional search method. Furthermore, the effects of rate and power constraints on the energy-efficiency problem are analysed. Simulation shows that the suboptimal solution of the proposed algorithm is very close to the optimal solution, and the proposed energy efficient power allocation method can achieve the highest energy efficiency, compared with the rate adaptive and margin adaptive optimisations.

In this study, the minimal polynomials and the linear complexity of interleaved binary sequences are investigated. Both the linear complexity and the minimal polynomials of low correlation zone sequences constructed by Zhou et al. are completely determined. Besides, an open problem proposed by Li and Tang is discussed. At last, a sufficient condition and a necessary condition are presented about when the linear complexity of the interleaved sequences constructed by Tang et al. attains the maximum.

In this study, the authors discuss energy-efficient power allocation for training-based multiple-input multiple-output (MIMO) system with and without feedback. Firstly, they consider the non-feedback MIMO system, where data power is equally allocated to transmit antennas. With pilot power fixed, the energy efficiency (EE)-optimal data power is derived based on the pseudo-concavity of the EE expression. When data power and pilot power are both variable, a convergent alternating optimisation algorithm is proposed to obtain EE-suboptimal data power and pilot power. Then, they consider the feedback MIMO system where data power is water-filling allocated to transmit antennas. Although the effective transmit antennas number at this scenario is discrete, they propose an algorithm to calculate it and its corresponding EE-optimal data power with pilot power fixed. When data power and pilot power are both variable, a similar convergent alternating optimisation algorithm is given. The numerical results demonstrate the EE performance of the training-based MIMO system with and without feedback. The impacts of multi-antennas configuration, circuit power and block length are also shown and analysed.

In this study, the authors consider the problem of signal detection in cognitive radio under the cases of known and unknown noise levels. The analysis is focused on maximum eigenvalue-based method which requires the knowledge of noise variance and the ratio of maximum eigenvalue to the trace of covariance matrix. The analytical formulas are derived for threshold calculation and probability of detection for the considered methods. Performance of the considered methods is measured through simulation analysis. The performance comparison between the considered methods and other existing methods is provided. Simulations based on independent and identically distributed signals and wireless microphone signals are presented to verify the various sensing methods.