Theses and Dissertations
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Item Open Access Security Enhancement using Channel State Information for Multicarrier Modulation(2021) Sindhav, Nidhi K.; Mekala, Priyanka; Goel, SupriyaWireless technologies of the fifth generation (5G) are vital to advancing Internet-of- Things (IoT) networks in the future. Information security in wireless networks is one of the most difficult problems due to the open nature of the wireless medium. A promising pproach to ensure strong security has been characterized as physical layer security, which exploits the differences between the physical properties of signal channels to provide a resilient and effectively degraded signal at an eavesdropper that cannot be recovered regardless of the processing of the signal. This thesis presents a coherent framework for a secure transmission and receiver for IoT devices. The proposed framework will operate only in the physical layer. A study of symbol shuffling using channel state information (CSI) in OFDM (Orthogonal Frequency Division Multiplexing) GFDM(Generalized Frequency Division Multiplexing) are presented here. BER simulations of the multi-carrier modulation schemes with proposed Physical Layer Security(PLS) technique. Further, simulations of OFDM with the proposed PLS technique have been performed in order to compare different equalizers, tap lengths, and modulations. Devices with limited resources can benefit from these proposed PLS technique.Item Open Access Digital implementation of orthogonal frequency division multiplexing(Dhirubhai Ambani Institute of Information and Communication Technology, 2017) Bhanushali, Artiben; Pillutla, Laxminarayana; Bhatt, Amit"OFDM (Orthogonal Frequency Division Multiplexing) is the most important part of 4G and 5G technology which is adopted by many standards because of its various advantages. In this thesis, firstly prototype of OFDM system is designed according to 802.11a standard in MATLAB to check its performance and then to implement the same in Verilog using Xilinx ISE 14.4 design suite. FFT (Fast Fourier Transform) and IFFT (Inverse Fast Fourier Transform) are the most complex part of the design which are implemented using radix-2 algorithm. For channel estimation, scattered pilot arrangement is used to insert pilots at regular period in OFDM frame. At receiver side, least-square estimation is used to estimate the channel’s impulse response at known pilot tones. The system analysis is done using MATLAB-Verilog co-simulation in which Verilog transmitter and receiver are connected with MATLAB channel using testbench to generate the text files. This files are used as connecting platform between MATLAB and Xilinx. Performance of channel estimation at different number of multipath is observed which shows that the combination of time interpolation followed by frequency interpolation performs better over only frequency interpolation. In later part, BER (bit error rate) vs. SNR (signal to noise ratio) is analyzed for different number of multipath. From graphs, it is clearly denoted that, BER performance improves with respect to increase in SNR. Finally, fixed point and floating point comparison is carried out which depicts that fixed point system implemented in Verilog performs almost similar to the floating point implementation done in MATLAB."Item Open Access Adaptive channel estimation and loading for OFDM based two-way relay systems(Dhirubhai Ambani Institute of Information and Communication Technology, 2013) Joy, Arun; Chakka, Vijay KumarIn this thesis we introduce a low complexity Recursive Least Square (RLS) based Orthogonal Frequency Division Multiplexing (OFDM) channel estimation for two-way relay system. Using the estimate of the channel, a bit and power loading scheme for OFDM based two-way relay system is implemented. In a wireless communication scenario, the channel frequency response is usually correlated across both time and frequency. Hence an optimal Minimum Mean Square Error (MMSE) estimator for OFDM channel is one which considers the correlation in the dimension of time and frequency. These estimators are known as 2D estimators. Since the channel is time varying and the channel statistics are not available, we implement a 2D-RLS adaptive filter based channel estimator. The 2D-RLS filter used for channel estimation is based on the principle of fast array based algorithms. The advantage of this algorithm is that it has a computational complexity comparable to that of 2D-Normalized Least Mean Square (2D-NLMS) algorithm while having same convergence rate as the conventional 2D-RLS algorithm. The reason for the low complexity of fast array algorithm is due to the fact that it considers the time shift structure in the input data vector while updating the inverse of covariance matrix. In the case of OFDM channel, this time shift structure is not directly evident. Hence we propose a simple reordering of the input data vector so as to bring in the notion of time shift structure. The adaptive filter so proposed is called Fast Array Multichannel 2D-RLS (FAM 2D-RLS) filter. In order to reduce the number of training symbols, the OFDM adaptive channel estimator is implemented based on the principle of Decision Directed Channel Estimation (DDCE). The standard literature on adaptive filter usually assumes that the data to be estimated is a scalar quantity while the weight is a vector. But in our proposed OFDM channel estimation method, the data to be estimated is a vector and the weight is a matrix. The steady state analysis of the RLS filter with weight matrix is derived and verified using simulations. The proposed steady state analysis is derived based on the fact that any adaptive filter can be viewed as an iterative equation solver, with RLS algorithm being a special case. Hence this method could be used for deriving the steady state analysis of any adaptive filter algorithm which has a weight matrix. A two-way relaying scheme is a spectrally efficient relaying scheme. Using FAM 2D-RLS, an OFDM-DDCE is proposed for this relaying scheme. It is observed that even a simple case of relaying involving nodes and relay with single antenna requires the concepts of Multiple Input Multiple Output (MIMO) systems for estimating the channel. The performance of the FAM 2DRLS for estimating pedA, pedB and vehA channel in case of two-way relay are analyzed. The complexity of FAM 2D-RLS can be further reduced, if instead of considering the correlation of the frequency response across all the frequency samples, we consider only a block of the frequency response, i.e the channel frequency response vector is grouped into M subvectors and is estimated using M parallel FAM 2D-RLS filters. This estimation algorithm is called Block FAM 2D-RLS (BFAM 2D-RLS). The computational complexity of BFAM 2D-RLS is lesser compared to that of FAM 2D-RLS by a factor of 1 M . It is observed that even though BFAM 2D-RLS does not consider the correlation across all the frequency response samples, the Mean Square Error (MSE) of the estimate is comparable to that of FAM 2D-RLS. It is also observed that MSE of channel estimation for BFAM 2D-RLS with large M is lesser compared to FAM 2D-RLS for the case of highly frequency selective channels like pedB and vehA. The capacity and Bit Error Rate (BER) of a communication system can be improved if the modulation scheme and transmitted power is adapted with respect to the variation of the channel. In order to perform adaptive modulation, the channel State Information (CSI) should be available at the transmitting node. In the case of Adaptive OFDM (AOFDM) systems, based on Signal to Noise Ratio (SNR) of each subchannel, power and bit could be assigned. This is known as power and bit loading algorithms. In a two-way relay system, if the channel estimation is performed at the node, the channel response so obtained pertains to that of the overall channel between the two transmitting nodes. In order to implement loading algorithms, the nodes require the CSI of individual channel, i.e channel between source - relay and relay - destination. Since our proposed channel estimation method is implemented in the frequency domain, the individual channel can be easily obtained from the estimated combined channel with a sign ambiguity. The effect of channel estimation error on loading algorithms for two-way relay is also analyzed. All the computer simulations are performed using MATLABR . As a summary of this section, we point out the unique features of this thesis, 1. A low complexity adaptive filter called FAM 2D-RLS filter is proposed for DDCEOFDM. 2. Steady state equations for RLS filter with matrix weight is derived. 3. FAM 2D-RLS based DDCE-OFDM is implemented for two-way relay systems. 4. Complexity of FAM 2D-RLS is further reduced by BFAM 2D-RLS. 5. A loading algorithm is implemented for OFDM based two-way relay systems. 6. Effect of channel estimation error on loading algorithm for two-way relay system is analyzed.Item Open Access Frequency offset estimation in OFDM signals using particle filtering along with resampling methods(Dhirubhai Ambani Institute of Information and Communication Technology, 2008) Jayanthi, V. Ravisankar; Vijaykumar, ChakkaFrequency offset is one of the main effects the Bit Error Rate(BER) in orthogonal frequency division multiplexing (OFDM). There are many applications which depend on OFDM. The present method describes the estimation of frequency offset for OFDM irrespective of the distribution of the channel noise. This method uses Particle Filtering for the estimation of frequency offset. As Particle Filter is a non-linear and non-Gaussian estimator, there will be no constraint on the channel noise. This method also uses the resampling methods, which are essential in modifying the weight approximates during the iterations. Here the performance of the Particle Filter in estimating the frequency offset for the OFDM is observed along with the resampling methods.Item Open Access Multirate signal processing in digital communications(Dhirubhai Ambani Institute of Information and Communication Technology, 2008) Arya, Krishna Kumari; Chakka, VijaykumarFundamentals of multirate signal processing and perfect reconstruction filter banks are revisited. An attempt is made to develop a good understanding of concepts by connecting the filter bank design issues to the concepts of linear algebra. An interpretation of multicarrier systems in the form of transmultiplexers is provided and the notion of resampling of channel is introduced. A pulse shaping filter for the OFDM systems is proposed to remove the constraint of Cyclic Prefix. A comparison of performance of OFDM and filter bank based multicarrier systems in terms of stop band attenuation is provided. Notion of MIMO biorthogonal partners and the conditions for existence of FIR biorthogonal partners are studied. Emphasis is put on finding out the Greatest Right Common Divisor (GRCD) of polynomial matrices.Item Open Access Model based channel shortening technique for IEEE 802.11a OFDM system(Dhirubhai Ambani Institute of Information and Communication Technology, 2007) Patidar, Pravin; Chakka, VijaykumarChannel shortening tehniques for IEEE802.11a OFDM system are studied. Their performance is evaluated for various standard channel models. A model based channel shortening technique is presented, which reduces the computational comlexity, both during initialization and data transmission. The method is based on modeling of channel impulse response by a pole-zero system, and using the denominator coefficients as the equalizer. Further, it is shown that for channel shortening, calculation of zeros of the model is not required. Comparison of proposed technique with MMSE and MSSNR channel shortening techniques is presented.Item Open Access Performance evaluation of OFDM technique for high speed communication applications(Dhirubhai Ambani Institute of Information and Communication Technology, 2005) Kabra, Mukul; Maskara, S. L.The Internet revolution has created the need for wireless technologies that can deliver data at high speeds in a spectrally efficient manner. However, supporting such high data rates with sufficient robustness to radio channel impairments requires careful selection of modulation techniques. The demand for high-speed mobile wireless communications is rapidly growing. OFDM technology promises to be a key technique for achieving the high data capacity and spectral efficiency requirements for wireless communication systems of the near future. Orthogonal frequency division multiplexing (OFDM) is a special case of multi-carrier transmission, where a single data stream is transmitted over a number of lower rate sub-carriers. OFDM is currently being used in Europe for digital audio and video broadcasting. The IEEE standardization group decided to select OFDM as the basis for their new 5-GHz standard, targeting a range of data stream from 6 up to 54 Mbps. This standard was the first one to use OFDM in packet-based communications, while the use of OFDM until now was limited to continuous transmission systems. OFDM is also being considered as a serious candidate for fourth generation cellular systems. In this project, transmitter, channel and receiver were simulated with various parameters, to evaluate the performance and different possibilities in the implementation. Also, some considerations about forward error correction coding, interleaving, synchronization and channel estimation are given to improve the system performance.Item Open Access Enhancing data efficiency in OFDM (IEEE 802.11a) using equalization (11A)(Dhirubhai Ambani Institute of Information and Communication Technology, 2005) Reddy, S. Gangadhar; Chakka, VijaykumarIEEE 802.11a OFDM signal passed through a dispersive channel introduces ISI & ICI. To avoid ISI & ICI, cyclic prefix (guard interval) is introduced between the OFDM symbols. But the cyclic prefix contributes about 20% (0.8us/4us) of the total symbol duration. Transmission of OFDM symbols without cyclic prefix increases the data efficiency, but introduces ICI & ISI. The ISI & ICI can be removed by equalization at the receiver. Spill over in the frequency domain is same as ICI & ISI. This spill over can be observed at the unused carriers also (carriers numbered from 0-6, 33, 59-64 are unused in IEEE 802.11a). Equalization is done in the frequency domain by making use of spill over in unused carriers. A relationship is brought between the length of the channel impulse response and the number of unused carriers for perfect equalization in the absence or very little AWGN-channel noise. To avoid the noise amplification caused by Zero Forcing equalization and to satisfy the relationship derived, a methodology using Time domain equalizer (TEQ) is proposed and simulation results are presented supporting it. Alternatively, computational efficient algorithm for indoor environment is suggested by transmitting data in unused carriers with the combination of used carriers. By this method, ICI & ISI is removed at the transmitter itself and it even doesn’t amplify the noise.