M Tech Dissertations

Permanent URI for this collectionhttp://ir.daiict.ac.in/handle/123456789/3

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    Channel estimation in OFDM systems
    (Dhirubhai Ambani Institute of Information and Communication Technology, 2010) Rathore, Hitesh; Joshi, Manjunath V.
    Orthogonal frequency division multiplexing (OFDM) is not a new concept, but its receiver design has constant improvement. OFDM uses available spectrum efficiently and the system provides high data rates. Many standards such as Digital video broadcasting (DVB), Wireless local area network (WLAN), 3GGP LTE uses OFDM as standard. Most of the digital communication systems nowadays are wireless, where information bits are transmitted over a radio channel. These radio channels are generally multi path channel which cause inter symbol interference (ISI). To remove the effect of ISI from received signal equalization is done, which requires the channel impulse response. A good channel estimation technique can reduce the complexity of equalizer by large factor. Channel estimation can be done by sending pilot symbols at regular interval and at the receiver the channel impulse response is estimated using these received pilot symbols.

    In this thesis various pilot based channel estimators are studied. We propose a new channel estimator, which is based on regularized least squares method and uses some prior information of the channel. The performance of existing channel estimators and proposed channel estimator is compared. The criteria for performance are on the basis of mean squared error (MSE) and symbol error rate (SER). Various adaptive channel estimators are also studied. We also propose a new adaptive channel estimator, based on two-dimensional normalized least mean squared (NLMS) algorithm with variable step size. The performance of existing adaptive channel estimators is compared with the proposed adaptive channel estimator. Performance comparison is based on MSE and the bit error rate (BER).

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    Executable specification design and simulation of OFDM based communication system
    (Dhirubhai Ambani Institute of Information and Communication Technology, 2009) Sa, Sudhir Kumar; Dubey, Rahul
    The communication system using the OFDM principle is today one of the most important application in communication field. This system has various applications from broadband to 3G and digital TV to Radio LANs. This master’s thesis project deals with the implementation of certain specification, algorithmic exploration for WVAN (wireless video area network) in Simulink®. The implemented model and its specification is the reference for the hardware designing and verification. The model used in OFDM based QPSK/16-QAM modulated communication system for the WHD WVAN standard at the High Rate Physical (HRP) layer. This thesis project describes the functionality of the various communication blocks and the method of data transmission through these blocks. The main purpose of this model is to calculate the BER (Bit Error Rate). The final design which supports the different HRP mode for different code rate and different Modulation scheme can take different length of sub-packets which depends on the HRP mode of Transmission. This model also implements the radix-2 FFT algorithm for fixed point FFT processor. Since the FFT processor cannot be used standalone, so in this thesis it is employed in an OFDM Transmitter and Receiver. The goal of this report is to outline the knowledge gained during the master’s thesis project, to describe a design methodology for the OFDM based communication system for high throughput and best error protection. The functionality of each block of the communication system is written in „C‟ code and the output data of each block of Simulink Model is compared to 'C' code written output for the same input.
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    Design and implementation of 128-point fixed point streaming FFT processor for OFDM based communication system
    (Dhirubhai Ambani Institute of Information and Communication Technology, 2009) Verma, Sunil Kumar; Dubey, Rahul
    Fast Fourier Transform (FFT) processors are today one of the most important blocks in communication systems. They are used in every communication system from broadband to 3G and digital TV to Radio LANs. This master’s thesis project deals with the pipelined, radix-2 algorithmic exploration and the hardware solution for the FFT processor with the FFT size of 2N points, the selection of the scaling schemes based on application requirement is discussed. The designed architecture is functionally verified in Simulink® and the Xilinx® ISE simulator. How to encapsulate the C++ coded algorithms or functions into the Simulink. This FFT processor is used in OFDM based BPSK modulated communication system for the WHD WVAN standard at the Low Rate Physical (LRP) lay. This thesis project presents the design of the 128 point fixed–point F streaming processor. The final architecture used is the SDF (single path with delay feedback) that implements the radix-2 FFT algorithm. Since the FFT processor can’t be used standalone, so in this thesis it is employed in an OFDM Transmitter and the performance is measured for SNR over a range of PAPRs. The goal of this report is to outline the knowledge gained during the master’s thesis project, to describe a design methodology for the fixed point pipelined FFT processors, the scaling choices and how to encapsulate the existing C code into the Simulink environment to measure the performance of fixed-point systems.
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    Pulse shaping design for PAPR reduction in OFDM
    (Dhirubhai Ambani Institute of Information and Communication Technology, 2009) Jain, Akansha; Vijaykumar, Chakka
    Future mobile communications systems reaching for ever increasing data rates require higher bandwidths than those typical used in todays cellular systems. By going to higher bandwidth the (for low bandwidth) at fading radio channel becomes frequency selective and time dispersive. Due to its inherent robustness against time dispersion Orthogonal Frequency Division Multiplex (OFDM) is an attractive candidate for such future mobile communication systems. OFDM partitions he available bandwidth into many subchannels with much lower bandwidth. Such a narrowband subchannel experiences now almost at fading channel. However, one potential drawback with OFDM modulation is the high Peak to Average power Ratio (PAPR) of the transmitted signal: The signal transmitted by the OFDM system is the superposition of all signals transmitted in the narrowband subchannels. The transmit signal has then due to the central limit theorem a Gaussian distribution leading to high peak values compared to the average power. system design not taking this into account will have a high clip rate. Each signal sample that is beyond the saturation limit of the power amplier suersither clipping to this limit value or other non- linear distortion, both creating additional bit errors in the receiver. One possibility to avoid clipping is to design he system for very high signal peaks. However, this approach leads to very high power consumption (since the power amplifier must have high supply rails) and also complex power amplifiers. The preferred solution is therefore to apply digital signal processing that reduces such high peak values in the transmitted signal thus voiding clipping. These methods are commonly referred to as PAPR reduction. APR reduction methods can be categorized into transparent methods here the receiver is not aware of the reduction scheme applied by the transmitter and on-transparent methods where the receiver needs to know the PAPR algorithm applied by the transmitter. This master thesis would focus on transparent PAPR reduction algorithms. The pulse shaping mechanism is used to reduce PAPR. he ct is analyzed in terms of BER.
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    Enhancing data efficiency in OFDM (IEEE 802.11a) using equalization (11A)
    (Dhirubhai Ambani Institute of Information and Communication Technology, 2005) Reddy, S. Gangadhar; Chakka, Vijaykumar
    IEEE 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.