الفهرس 
Development and Application of OFDM
Orthogonal Frequency Division MultiplexingOnly 14 pages are availabe for public view
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Abstract
Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier transmi-ssion technique in wireless environments, and can be seen as a multi-carrier digital modulation or multi-carrier digital multiplexing one as well. A large number of orthogonal sub-carriers are used to transmit information. OFDM systems have high utilization of the frequency spectrum and satisfactory capabilities of reducing multi-path inference. So, OFDM has been considered as one of the core techno-logies of 4th generation (4G) wireless communication systems.
Channel estimation plays a very important role in OFDM systems. It can generally be separated into two methods: pilot-based channel estimation and blind channel estimation. Pilot-based channel estimation, which is the focus of this thesis, estimates the channel information by obtaining the impulse response from all sub-carriers by pilot. Compared to blind channel estimation, which uses statistical information of the received signals and is not considered in this thesis, pilot-based channel estimation is a practical and effective method.
This thesis covers the basic principles of the OFDM system, system construction and the advantages and disadvantages of OFDM systems are considered also. It also offers a brief overview on signal propagation, channel parameters and the basic principles of channel estimation in OFDM systems.
The great challenge of channel estimation methods is to compromise between low complexity and high performance. In this thesis three improved methods of channel estimation are introduced. These methods are based on pilot-aided OFDM system with the arrangement employed in the DVB-T2 standard in time-varying frequency-selective fading channels. The first and second proposed methods (low complexity and improved low complexity methods, respectively) are modified methods based on the Domain Transform Least Square Estimation (DTLSE) method; they reduce the computational complexity by avoiding the use of the matrix inversion. The estimation matrix size for obtaining Channel Impulse Response (CIR) depends only on the length of the channel rather than the number of pilot sub-carriers or the size of OFDM symbols.The third proposed method (high performance method) is based on a combination of the proposed low complexity method and the Two Dimensional Linear Interp-olation (2-DLI) method. Similar to the previously proposed methods, the estim-ation matrix size for obtaining CIR depends only on the length of the channel. The three proposed methods are compared with three other methods: the DTLSE, 2-DLI and Minimum Mean Squared Error (MMSE). The first and second proposed methods prove to be less complex, with less computational load than the DTLSE and MMSE methods, and have higher robustness to Doppler shifts than the 2-DLI method. The second method also, proves to give BER performance comparable to MMSE method. The third method offers lesser complexity than the MMSE met-hod, and a BER performance close to it and substantially better than the first proposed, DTLSE and 2-DLI methods.
Keywords:
The second generation for Digital Video Broadcasting - Terrestrial system (DVB-T2) , Domain Transform Least Square Estimation (DTLSE) , Two Dimensional Linear Interpolation ( 2-DLI) , Minimum Mean Square Estimation (MMSE).