Theses and Dissertations

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    Channel estimation and tracking OFDM and MIMO systems
    (Dhirubhai Ambani Institute of Information and Communication Technology, 2009) Singh, Smriti; Vijaykumar, Chakka
    In this thesis, To estimate and track the slow Time varying channels in OFDM and MIMO systems. In first part: we have used a two-dimensional recursive least square adaptive channel estimation technique is used. In orthogonal frequency division multiplexing (OFDM) system, time- and frequency-domain two-dimensional minimum mean square error (2D-MMSE) channel estimation is optimum. However, accurate channel statistics, which are often time varying and unavailable in practice, are required to realize it.2DRLS adaptive channel estimation does not require accurate channel statistics, and at the same time can make full use of time and frequencydomain correlations of the frequency response of time-varying wireless channels. With properly chosen parameters, 2D-RLS adaptive channel estimation can converge into the steady state in only several OFDM symbols time. Although the 2D RLS algorithm creates adaptive letters with a fast convergence speed, this algorithm diverges when the inverse correlation matrix of input loses the properties of positive definiteness or Hermitian symmetry. The diverging of the 2D RLS algorithm same as standard RLS limits the application of this algorithm. We proposed a QR decomposition-based 2DRLS (inverse QR-2DRLS) algorithm, which can resolve this instability. Instead of propagating inverse of correlation matrix of the input signal, it propogates square root of inverse correlation matrix of the input signal. Therefore, this algorithm guarantees the property of positive definiteness and is more numerically stable than the standard RLS algorithm. The parallel implementation of the inverse QR-2DRLS algorithm permits a direct computation of the least squares weight coefficients matrix MATLAB simulations demonstrate that performance of QR-2D-RLS adaptive channel estimation is same as of 2D-RLS adaptive channel estimation and is very effective and suitable for a broad range of channel conditions. In the second part of the thesis: Since, In MIMO systems, accurate channel estimation is necessary to fully exploit the benefits of spatial diversity offered by such systems. And for time-varying channels, these channel estimates should also be updated accordigly to track the variation of channel. we have used One such method of channel estimation using adaptive SVD updates for channel estimation and tracking of slow-time varying channels in MIMO system. The channel estimates are then further used for symbol detection using V-BLAST/ZF detection algorithm which ensures interference reduction and give better BER vs. SNR performance than SVD based MIMO system.
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    Performance of MIMO in correlated rayleigh flat fading channels
    (Dhirubhai Ambani Institute of Information and Communication Technology, 2005) Jain, Alok; Jadhav, Ashish
    The recent interest in Multiple Input Multiple Output (MIMO) communication systems was initiated by the theoretical work of Telatar. The development in the field of random matrix theory is the basis of these studies. The assumption in the mathematical model for studying MIMO turned out to be too idealistic for realistic (correlated) channel condition. This lead to an exaggeration of performance of MIMO. This thesis deals with quantification of performance of MIMO in realistic channel conditions with the emphasis on study of working of MIMO receivers. Our definition of MIMO contains both diversity systems and spatial multiplexing systems. For the simulation of realistic channel conditions a correlated fading channel model with lack of rich scattering is used. To perform simulations for N transmit and M receive MIMO systems there is a need for having N x M correlated fading envelops. We propose a novel and simplified technique to extend the algorithm for two correlated fading envelops given by Richard and Jeffery. Various analytical studies on the effect of correlated multi-path channel on performance of spatial multiplexing have been done. These studies focus on capacity cumulative distribution function. Effect of various degree of correlation on other performance measures like diversity gain and error probability have been not done. This is due to lack of simple method of generation of correlated Rayleigh fading envelops. By making use of the extended algorithm we have been successful in carrying out simulation to observe the effects on the performance. Recently there is interest in extending the concept of MIMO in wireless communication for sensor networks. The motivation behind it is energy efficiency of sensor nodes. Our results have implications on this venture.