Walsh-Hadamard Sequences Research Articles

UCHT-based complex sequences for asynchronous CDMA system

The use of orthogonal spreading codes has attracted much attention due to their ability to suppress interference from other users, compared with the nonorthogonal sequences in the synchronous case. In this paper, new sets of orthogonal sequences derived from the unified complex Hadamard transforms (UCHTs) are investigated. Various correlation properties of the sequences are mathematically derived and analyzed. It is shown that some UCHT sequences provide better autocorrelation properties than orthogonal Walsh-Hadamard sequences. Performance comparisons between UCHT sequences, Gold, small set of Kasami, and m-sequences show that some UCHT sequences outperform these well-known spreading sequences under simulation of systems in the presence of multiple access interference and additive white Gaussian noise.

Modified Walsh‐Hadamard sequences for DS CDMA wireless systems

AbstractWe propose a simple but efficient method for modifying Walsh–Hadamard sequences to achieve correlation properties suited for asynchronous DS CDMA applications. The proposed method can be used to minimize the mean square value of aperiodic cross‐correlation or the mean square value of aperiodic autocorrelation, the maximum value of aperiodic cross‐correlation functions, merit factor or other properties of the sequence set. The important feature of the method is that it modifies correlation properties of the sequence set, while preserving their orthogonality for the perfect synchronization. The proposed method can be applied to obtain bipolar, quadri‐phase, or general polyphase sequences. Copyright © 2002 John Wiley & Sons, Ltd.

Performance of DS/CDMA systems with differential M-ary orthogonal modulation and RS-coding for LEO satellite communications

This paper presents a coded modulation scheme based on M-ary orthogonal modulation by means of Walsh–Hadamard (WH) sequences, suitable for low-earth-orbit (LEO) direct sequence/code division multiple access (DS/CDMA) satellite communication systems. Based on the IS-95 scheme, we consider Reed–Solomon (RS)-coded M-ary orthogonal modulation with error or erasures decoding, which presents good performance enhancement with low complexity. LEO satellite links are characterized by large Doppler frequency shifts caused by the difference in velocity between the satellite and the earth mobile terminal, which make conventional non-coherent detection ineffective. In order to overcome the phase shift variations during the symbol period, which result in orthogonality loss of the WH sequences, we applied a differential encoding process to the spreading sequences or the WH chips prior to transmission. A special diversity process suitable for the environment under consideration is also applied. Simulation results show that the proposed diversity/coding/modulation scheme attains very good performance at low transmitter/receiver complexity. © 1998 John Wiley & Sons, Ltd.

Performance of DS/CDMA systems with differentialM-ary orthogonal modulation and RS-coding for LEO satellite communications

This paper presents a coded modulation scheme based on M-ary orthogonal modulation by means of Walsh–Hadamard (WH) sequences, suitable for low-earth-orbit (LEO) direct sequence/code division multiple access (DS/CDMA) satellite communication systems. Based on the IS-95 scheme, we consider Reed–Solomon (RS)-coded M-ary orthogonal modulation with error or erasures decoding, which presents good performance enhancement with low complexity. LEO satellite links are characterized by large Doppler frequency shifts caused by the difference in velocity between the satellite and the earth mobile terminal, which make conventional non-coherent detection ineffective. In order to overcome the phase shift variations during the symbol period, which result in orthogonality loss of the WH sequences, we applied a differential encoding process to the spreading sequences or the WH chips prior to transmission. A special diversity process suitable for the environment under consideration is also applied. Simulation results show that the proposed diversity/coding/modulation scheme attains very good performance at low transmitter/receiver complexity. © 1998 John Wiley & Sons, Ltd.

Design of Test Signals for System Identification in Distributed Systems

Three different test signals (a pseudo-white gaussian signal, a second order auto-regressive process, and a binary non-white sequence generated from the Walsh-functions) are investigated on a fixed-bed catalytic reactor subject to flow-rate disturbances in order to compare their ability to estimate the frequency response function, especially in a frequency range where gain and phase oscillations occur. It is concluded that the binary Walsh-Hadamard sequences are best suited for this purpose, since their maximum deviations are less than those obtained with the stocastic signals, and the three signals give equally reliable estimates in the frequency range of interest.

Dynamic modelling of a gas phase catalytic fixed bed reactor—III: Identification using binary pulse sequence test signals

The dynamics of a gas phase catalytic reactor is identified experimentally using broad bandwidth Walsh-Hadamard sequences as test signals. The binary test signals are designed from Walsh functions as they appear in Hadamard matrices. Each element of the matrix is multiplied with a fundamental signal with zero mean. The input and output time series are Fourier analysed to give local frequency responses. The reliabilities of the estimated gain and phase angels are evaluated and provide a useful guide for experimental design and interpretation. Results from a Walsh-Hadamard sequence are compared to results from a square wave signal and found to give similar accuracy. Good agreement between experimental values and theoretical predictions is obtained. The test signal period should be at least three times the thermal residence time or generally the dominating time delay.