Calin Vladeanu

Design of a fast and robust chaos-based crypto-system for image encryption

In this paper, we propose a fast and robust scheme for image encryption including a new chaotic generator. The proposed crypto-system uses a variable block cipher length (8, 128, and 512) with different modes: cipher block chaining (CBC), output feedback (OFB), cipher feedback (CFB) and counter (CTR). The fastness and robustness properties of our crypto-system are determined by the chaotic generator using a 32-bits finite precision with integer representation to facilitate hardware implementation, and by a keystream permutation using a 2D-cat map with chaotic control parameters. The chaotic generator is constructed with two non linear (the skew tent map) IIR filters. To quantify the security level of the proposed cryptosystem, we analyze the global dynamical properties of the chaotic generator using the NIST (National Institute of Standards and Technology) test, and we show that, the algorithm can resist the statistical and differential attacks; it also passed the key sensitivity test. Moreover, the algorithm has a large key space. The experimental results indicate that the scheme is secure, efficient, and faster than conventional advanced encryption standard (AES).

Turbo trellis-coded spatial modulation

In this paper, a turbo trellis-coded spatial modulation (TTCSM) transmission scheme is proposed. In trellis-coded spatial modulation (TCSM) scheme, the information bits used to select the transmit antenna index are encoded. In order to increase the performances of the TCSM scheme, a parallel concatenated encoding scheme can be used. Therefore, the TCSM scheme is modified by introducing two identical convolutional encoders, with proper interleaving and puncturing, as in the case of conventional parallel turbo trellis-coded modulation (TTCM) scheme. The punctured TTCSM scheme, using the symbol-by-symbol log-MAP iterative decoding algorithm in the receiver, is developed. The bit error rate (BER) is estimated by simulations for PSK-TCSM transmissions over stationary Rayleigh, Rician, and spatially correlated (SC) fading channels with additive white Gaussian noise (AWGN). As an example, a scheme with a spectral efficiency of 3b/s/Hz using eight transmit and receive antennas and a BPSK signal is considered. It is shown that the TTCSM scheme offers a coding gain over the corresponding non-iterative TCSM scheme of at least 6 dB for the Rayleigh fading channel, 3 dB for the Rician channel, and 2 dB for the SC fading channel, respectively.

Chaotic Digital Encoding for 2D Trellis-Coded Modulation

In this paper, nonlinear digital filters with finite precision are analyzed as recursive systematic convolutional (RSC) encoders. An infinite impulse response (IIR) digital filter with finite precision (wordlength of N bits) is a rate 1 RSC encoder over a Galois field GF(2N). The Frey chaotic filter is analyzed for two different wordlengths N, and it is demonstrated that this encoder can be used for trellis-coded modulation (TCM) schemes. A definition for the encoding rate is provided in the context of the new structure. The Frey encoder scheme is modified in order to reduce the encoding rate from 1 to 1/2. In fact, this modification consists in increasing the number of encoder outputs, using the same wordlength as for the input. The resulted encoders are used for two-dimensional (2D) TCM schemes. Also, the signal sets are partitioned following the Ungerboeck’s rules. The symbol error rate (SER) is estimated for all proposed structures and the results show the expected coding gains as compared to their equivalent non-encoded and linear versions.

Dynamic OCDMA coding for indoor wireless optical communications

In this paper we present a new “dynamical” spectral encoding method for the optical code division multiple access (OCDMA) indoor wireless communication system. In contrast with the classical OCDMA systems, where one user transmits all the information bits using the same spreading code sequence, in the proposed “dynamical” case one user transmits a fixed number of bits using one code sequence, and thereafter the code sequence is changed. According to obtained results, the performances of the system are better because of multipath interference reduction.

Optimum PAM-TCM schemes using left-circulate function over GF(2 N )

In this paper, optimum recursive systematic convolutional (RSC) encoders over Galois field GF(2N) are designed using a nonlinear function, i.e., left-circulate function (LCIRC). The LCIRC function performs a bit left circulation over the representation word; it is used in microprocessors as an accumulator operation, and in chaotic sequence generators working in finite precision. Different encoding rates are obtained for these encoders when using different representation wordlengths at the input and the output, denoted as Nin and N, respectively. A generalized 1-delay GF(2N) RSC encoder scheme using LCIRC is proposed for performance analysis and optimization, for any possible encoding rate, Nin/N. The minimum Euclidian distance is estimated for these optimum encoders and a general expression is found as a function of the wordlengths Nin and N. The symbol error rate (SER) is estimated by simulation for a quaternary pulse amplitude modulation - trellis-coded modulation (PAM-TCM) transmission over an additive white Gaussian noise (AWGN) channel. The simulation results confirm the expected coding gains determined theoretically.

A new ML detector for trellis-coded spatial modulation using hard and soft estimates

In this paper we propose a new detector for the spatial modulation (SM) receiver. In trellis coded spatial modulation (TCSM) schemes, the joint detection is used for identifying both the antenna index and the transmitted symbol. We define a hybrid maximum-likelihood (ML) SM detector, which determines the transmit antenna index soft estimate and the transmitted symbol hard estimate. The antenna indexes soft estimates are decoded using the logarithmic maximum a posteriori probability (log-MAP) algorithm. It is shown that for at least four receive antennas, the hybrid ML-SM detector offers a coding gain of at least 2 dB over the hard-output solutions in spatially correlated (SC) channels. The proposed detector is less complex than soft-output one, with a negligible bit error rate (BER) performance decrease. The BER is estimated by simulation for QPSK-TCSM transmissions over stationary Rayleigh and SC fading channels with additive white Gaussian noise (AWGN).

Optical spatial modulation for indoor wireless communications in presence of inter-symbol interference

In this paper, we analyze an indoor wireless communications system using optical spatial modulation (OSM) when is subject to inter-symbol interference (ISI). An array of light emitting diodes (LEDs) is used in transmitter and an array of photo diodes (PDs) is used in receiver. Each group of k information bits selects a unique transmitting LED from the 2k total number of available LEDs. In the receiver, an optimal maximum likelihood (ML) detector is used to identify the transmitting LED. A channel model with ISI is introduced and the OSM system performances are tested in presence of additive white Gaussian noise (AWGN). The signal-to-interference-and-noise ratio (SINR) is estimated as function of channel path gains. The bit error rate (BER) is analyzed by simulation as function of SINR for different distances between transmitter and receiver. The simulation results reveal that for SINR values closer to the upper limit, SM receiver performs better than in the only presence of AWGN.

On the Behavior of LMS Adaptive Algorithm in MMSE Receivers for DS-CDMA Systems

This paper analyzes the behavior of the Least Mean Square (LMS) adaptive algorithm in the minimum mean-squared error (MMSE) single user adaptive receiver for the asynchronous direct sequence-code division multiple access (DS-CDMA) system. It is known that in this context the adaptive algorithm can be iterated several times during the same bit interval in order to achieve a faster convergence rate for the receiver, which further reduces the length of the training sequence. Instead of using such multiple iterations we propose a single equivalent formula for updating the receiver coefficients, saving significant time processing. The simulation results prove the equivalency of these approaches in terms of convergence rate performances.

New Recursive Convolutional GF(2N) Encoders for Parallel Turbo-TCM Schemes

In this paper, parallel turbo phase shift keying – trellis coded modulation (Turbo PSK-TCM) schemes are designed using recursive convolutional encoders over Galois field GF(2^N). These encoders are designed using the nonlinear left-circulate (LCIRC) function. The LCIRC function performs a bit left circulation over the representation word. An optimum 1-delay GF(2^N) recursive convolutional encoder scheme using LCIRC (RC-LCIRC) is proposed for PSK-TCM schemes. The minimum Euclidian distance is estimated for these PSK-TCM schemes and it is shown that these structures offer the maximum coding gains. However, the RC-LCIRC encoders are less complex than the corresponding binary encoders. The optimum RC-LCIRC encoder is used as component encoder of a parallel turbo PSK-TCM transmission scheme, using the iterative multilevel log-MAP algorithm in the receiver. The bit error rate (BER) is estimated by simulation for the proposed Turbo PSK-TCM transmissions over an additive white Gaussian noise (AWGN) channel, and the results are similar to the conventional Turbo-TCM schemes.

Practical variable step-size adaptive algorithms for echo cancellation

The most common algorithms used for echo cancellation are the normalized least-mean-square (NLMS) and the affine projection (AP). The classical versions of these algorithms use a constant step-size parameter and need to establish a tradeoff between several performance criteria e.g., high convergence rate versus low misadjustment. In this paper, we present a class of variable step-size NLMS and AP algorithms, which are designed to recover the near-end signal from the error of the adaptive filter. The simulation results indicate a robust behavior of these algorithms against the presence of the near-end signal.