A novel key-based Direct Sequence Spread Spectrum (DSSS) technique is proposed as a safety function for Communications-Based Train Control (CBTC) systems. According to well-known railway standards, namely IEEE 1474, IEC 61508, and EN 50128, CBTC systems should satisfy acceptable safety, security, and reliability levels. However, due to their inherent low-power features, prevalent CBTC systems cannot meet those levels perfectly in front of co-band interferences or attackers. Thus, a safety function is required which can be implemented without consuming much energy and restricted resources of CBTC systems. In this paper, the DSSS technique is based on a Cryptographically Secure Pseudo-Random Number Generator (CSPRNG). Comprehensive time-domain analysis shows how this safety function enhances the safety, security, and reliability of CBTC systems concerning the standards mentioned above. It is proved that any inauthentic signal turns into a Gaussian term with significantly mitigated power at the output of the CBTC receiver. It is also established how to determine appropriate communications parameters based on railway properties. The proposed scheme’s failure rate performance is compared with that of similar works. Additionally, computer simulations and experiments verify the efficiency of the proposed safety function.

Recently, bionic signals have been used to achieve covert underwater acoustic communication (UWAC) with high signal-to-noise ratios (SNRs) over transmission systems. A high SNR allows the attackers to proceed with their mischievous goals and makes transmission systems vulnerable against malicious attacks. In this paper we propose an improved Merkle hash tree based secure scheme that can resist current underwater attacks, i.e., replay attack, fabricated message attack, message-altering attack, and analyst attack. Security analysis is performed to prove that the proposed scheme can resist these types of attacks. Performance evaluations show that the proposed scheme can meet UWAC limitations due to its efficiency regarding energy consumption, communication overhead, and computation cost.

Sum of fading signals arises in several communication applications such as maximal-ratio combining (MRC) used in multiple antenna systems and eliminating the inter-symbol interference in high speed wireless networks. The exact formulation of the statistics of the sum is known to be very complicated. This article presents two simple closed-form approximations to the probability density function (PDF) and cumulative distribution function (CDF) of the sum of $\alpha -\mu $ variates, resembling an urban mobile communication environment. By direct integration, it is demonstrated that the PDF of the sum of $\alpha -\mu $ random variables can be approximated by a weighted sum of few identical $\alpha -\mu $ distributions including order one coefficients. The proposed approximations are of variable-order to achieve the desired accuracy and also have specific parameters using which, multiple-input multiple-output (MIMO) diversity systems can be explicitly analyzed as simply as a single-input single-output (SISO) system. In addition, a MIMO wireless system utilizing orthogonal space-time block coding technique (OSTBC-MIMO) in an $\alpha -\mu $ frequency nonselective fading environment is analyzed, and expressions for outage probability (OP) and bit error rate (BER) of digital binary modulations as well as the capacity are obtained. Moreover, the asymptotic OP and BER expressions are derived from the declared approximate expressions. The asymptotic OP and BER performances clearly indicate a diversity order equal to the product of the $\frac {\alpha }{2}$ parameter, the $\mu $ parameter, the number of transmit antennas and the number of receive antennas. Analytical results are verified by simulation.

In this paper, the problem of interference management is investigated in the downlink of multi‐user multiple‐input multiple‐output two‐hop interference channel under various scenarios of channel state information (CSI) and antenna correlation. Interference alignment (IA) in the space dimension is a powerful tool to solve this problem. Iterative IA algorithms are adopted in both the first and the second hops transmissions independently. After aligning interfering signals, the recovery of the transmitted signal vector is performed based on zero‐forcing and semi‐definite relaxation detectors. Furthermore, the impacts of the imperfect CSI and antenna correlation on sum‐rate are investigated for decode‐and‐forward (DF) and amplify‐and‐forward relaying protocols. When the system is proper but IA is not completely feasible, the performance of minimum interference leakage (MIL) is compared with rank‐constrained rank minimisation (RCRM) algorithm in DF protocol. RCRM algorithm is more sensitive to CSI such that the channel uncertainty easily leads to an increase in the interference matrix rank. According to the simulation results, for the case of perfect CSI, RCRM makes further interference‐free dimensions which leads to better performance than MIL, but for the case of imperfect CSI, close performance is observed.

The capability of exploiting Orthogonal Space-Time Block Codes (OSTBC) for increasing physical layer security of wireless systems is studied. A technique named “hidden OSTBC” is introduced, in which, a pseudorandom sequence is utilized by both transmitter and legitimate receiver to provide required security. Traditionally, employing pseudorandom sequences with methods such as spread spectrum or cooperative jamming involves huge amount of bandwidth or transmission power constraints, which are major challenges for wireless systems. Without requiring additional power or bandwidth, this study is designed to address exploitation of a pseudorandom antipodal sequence as a precoder. Elements of this sequence are multiplied to each antenna's transmitting symbol, and legitimate receiver employs the same sequence upon its combining rule. Mathematical analysis and simulations prove that an eavesdropper who does not know the pseudorandom sequence suffers from a degraded equivalent channel. Security enhancement is studied by investigating eavesdropper's higher error rate compared with that of legitimate receiver. Also, by employing lattice-based codebooks, a lower-bound is drawn for secrecy capacity, implying the achievability of nearly perfect secrecy regarding information theoretic analysis.

In a K ‐user single input‐single output interference channel, it is proved that the upper bound for multiplexing gain (MG) is . Based on interference alignment technique, Cadambe and Jafar introduced a scheme known as Original Interference Alignment (IA). This scheme is improved by Choi–Jafar–Chung (CJC) in which for reduced channel extension, higher MG is achievable. These designs requirean infinite number of channel extension to achieve the upper bound. In this study, as a common generalisation of Original IA and CJC, the authors focus on designing new precoding matrices. With the same conditions as in Original IA and CJC, by changing the values of defined variables ( t and s ), different cases could be generated so that different MGs with different channel extensions become possible. Broadly speaking, setting appropriate values to the assigned variables, the proposed scheme yields the same results as Original IA and CJC schemes. For all other values of t and s parameters better results than Original IA are obtained but for some special t and s parameters better results than CJC scheme are achieved. Thus, it is possible to simultaneously achieve a higher MG with lower channel extension (i.e. the reduction of computational complexity) in comparison with previously designed schemes.

New precoder matrices are designed to estimate an unknown vector parameter considering stochastic channels for multiple‐input multiple‐output wireless sensor network (MIMO‐WSN) with decentralised sources. Two types of WSNs are considered, with and without total power network limitations. Within this context, two closed‐form relations are derived for the precoder matrices and mean square error (MSE) of the unknown vector parameter is obtained. To cancel any operations at a fusion centre to estimate the aforementioned parameter, a distortionless constraint is added. In simulation section, the authors compare the MSE performances of the proposed method in stochastic MIMO channels with those in deterministic MIMO channels. The simulation result indicates that the MSE performance in the stochastic channels is decreased by almost 1.3 dB at low signal‐to‐noise ratios. Therefore, the reliability of the unknown vector parameter estimation is enhanced using the MIMO model. Ultimately, effects of bandwidth and number of sensor nodes on the MSE criterion are also analysed.

This study investigates the spectrum shaping observed in ultra-wideband (UWB) communications. An auxiliary independent signal (AIS) to limit undesired spectral lines because of periodicity or unbalanced data sources (i.e. non-uniformly distributed) and or non-symmetric data modulation schemes are introduced. Each transmitted symbol is represented by a preamble AIS pulse followed by a set of transmitted data pulses that are weighted, delayed and summed in accordance with a predefined modulation. The use of the fifth derivative Gaussian pulse ensures that the UWB spectrum mask established by the Federal Communications Commission (FCC) is met, and a preamble signal is designed to eliminate spectral lines at specific frequencies that appear mainly by existing balanced or unbalanced binary data sources. A unified spectral analysis is adapted for general uncorrelated and correlated quaternary bi-orthogonal modulations. An optimisation problem is also introduced in the case of unbalanced data sources in order to minimise undesired spectral lines subject to comply with the FCC spectrum mask and maximise spectral utilisation.

Abstract In this paper, it is proved that Linet al.'s scheme that tried to strengthen the Hill cipher against the known‐plaintext attack has several security flaws and is vulnerable to the chosen‐ciphertext attack. This paper also introduces a secure and efficient symmetric cryptosystem based on affine transformation. The proposed cryptosystem includes an encryption algorithm that is an improved variant of the Affine Hill cipher, and two cryptographic protocols that are introduced for the proposed cryptosystem. Copyright © 2009 John Wiley & Sons, Ltd.

This paper presents a classical MIMO transmission with pilot aided sequences based on adaptive LS channel estimation in a correlated time varying. The complexity reduction in performance is obtained for the various forgetting factors derived through the simulation. It is realized that, as long as the correlation of the channel parameters at different time indexes exceeds, the value of forgetting factor has to be increase, reaching maximum value. The performance of channel estimation is considered by the MSE criterion of the estimated channel coefficients. The MSE of the channel coefficients is formulated analytically, employing the distinct pilot sequences to confirm how it matches the simulated result. It can be emphasized that the results verify a final relation that is independent of the number of antennas.

Using multiple antennas at both ends of a wireless link, known as multiple-input multiple-output (MIMO) wireless, has been shown to offer significant improvements in link reliability and spectral efficiency through space-time coding and spatial multiplexing respectively. Our main objective in this contribution is to propose a novel relaying strategy that uses multiple input multiple output (MIMO) fixed relays with linear processing to support multiuser transmission in cellular networks. We apply a transmit preprocessing technique at the transmitter in order to eliminate the multiple-user interference. The fixed relays process the received signal with linear operations and forward it to multiple users. The effective channel from transmitter to the users can be modeled as a broadcast channel. This paper optimizes the relay amplification matrix and transmitted power according to achievable sum rate at the users, neglecting the direct links from the base station to the users and with certain structure of relays. The results show that the sum rate achieved by the our proposed system architecture are close to the sum rate achieved by the decode-and-forward relaying though decoding at the relay is not required.

The Hill cipher is a classical symmetric encryption algorithm that succumbs to the know-plaintext attack. Although its vulnerability to cryptanalysis has rendered it unusable in practice, it still serves an important pedagogical role in cryptology and linear algebra. In this paper, a variant of the Hill cipher is introduced that makes the Hill cipher secure while it retains the efficiency. The proposed scheme includes a ciphering core for which a cryptographic protocol is introduced.

Density evolution (DE) is a technique for tracking the distribution of the log likelihood ratio (LLR) messages exchanged between the variable nodes and the check nodes in a bipartite graph. It is widely assumed that these distributions are close to Gaussian. However, in many scenarios, this assumption is not valid, for example, the case that the signal to noise ratio is low, or the degree of variable nodes exceeds a certain threshold. A new (suboptimal) method for DE algorithm in low-density parity-check codes is introduced. We provide a more accurate model for the distribution of message bits (as compared to Gaussian) through matching the first n statistical moments. An iterative message passing algorithm is proposed to compute these moments from the graphical representation of the underlying code. It shown that the proposed algorithm results in an improved estimate of the underlying EXIT chart as compared to using a Gaussian assumption. In this respect, the proposed method achieves a performance very close to that of the best earlier methods, while it offers a much lower complexity.

Nonlinear Tomlinson–Harashima precoding (THP) is an attractive solution for a scenario where the transmission system employs multiple antennas at transmitter and multiple users with a single antenna at the receiver, so that the cooperation among the receive antennas are impossible (downlink scenario). THP solution based on zero forcing (ZF) and minimum mean square error (MMSE) criteria is one of the important techniques to achieve near multiple input multiple output channels capacity with reasonable complexity. In this paper, the effect of channel imperfection on THP is considered. At first, the achievable rate of THP with respect to ZF criterion in an imperfect channel state information (CSI) scenario is calculated. Moreover, based on MMSE criterion, a new robust solution is derived which provides a significant improvement with respect to the conventional optimisation method. Then, the effect of channel estimation error on THP is considered as an improved optimisation where THP filters are optimised together with a channel estimator. Spatial power loading is found to be important to the THP performance. This loading for robust/joint optimisation of MMSE THP is developed by minimum average symbol error rate sense. Simulation results show the capacity loss, the performance advantage attained by the robust/joint optimisation and the power loading in an imperfect CSI scenario.

Orthogonal frequency division multiplexing (OFDM) is a promising technique for the broadband wireless communication system. However, the inter-sub-carrier- interference (ICI) produced by the phase noise of transceiver local oscillator is a serious problem. Bit error rate (BER) performance is degraded because the orthogonal properties between the sub-carriers are broken down. In this paper, the number of sub-carriers is chosen according to the consideration of the trade off between the amount of ICI and data rate of the system. ICI self-cancellation of Mapping Method introduced here is also studied to reduce ICI effectively. Then the system performance of the Mapping Method is compared with those of the original OFDM and the conventional data-conversion method.

Served as one of the most widely used wireless air link interfaces, code division multiple access (CDMA) has been identified as a major technique for 3G wireless communications. Here, a new method based on applying encryption algorithm over spreading codes is proposed to improve the security of CDMA. The security produced in this way is primarily associated with the complexity of the employed encryption algorithm. Because the encryption algorithm's security is highly reliable, it is suitable for any kind of data communications. Since the multi-user detection is the inherent characteristic of the CDMA, the multi-user interference must be studied carefully. For this reason, the cross-correlation between outputs of encryption algorithm, causing the multi-user interference, is studied thoroughly. Then, the combination of encrypted and unencrypted M-sequence is used as a spreading code to mitigate the system performance, and the advantage of this combination is considered from the interference level and the security point of view. A compatible algorithm for the key exchange is produced.