A Survey on Channel Estimation Techniques Based on Pilot 基于导频信道估计技术的研究.ppt

A Survey on Channel Estimation Techniques Based on Pilot Arrangement in OFDM Systems by Mustafa Ergen Authors: Sinem Coleri, Mustafa Ergen {csinem,ergen}@eecs.berkeley.edu Berkeley Web Over Wireless Group University of California Berkeley Outline Motivation for OFDM OFDM System Architecture Channel Estimation Techniques Performance Analysis Conclusion Motivation for OFDM Disadvantages of FDMA Bad Spectrum Usage Disadvantages of TDMA Multipath Delay spread problem OFDM: Use of Frequency Spectrum Efficient use of spectrum Overlap in frequency spectrum of subcarriers Null point of all other subcarriers at the center frequency of any particular subcarrier Multipath Delay Spread Multi-path delay spread definition Time spread between the arrival of the first and last multipath signal, seen by the receiver. Received radio signal consisting of a direct signal, plus reflections from objects Multi-path delay spread effect Inter-Symbol Interference (ISI) when the delayed multipath signal overlaps with the symbols following it OFDM: Eliminating ISI Cyclic Prefix Prepend the last part of the signal to the beginning of the signal Duration of the CP larger than multipath delay spread Orthogonality of the carriers not affected. OFDM Overview Divides high-speed serial information signal into multiple lower-speed sub-signals. Transmits simultaneously at different frequencies in parallel. Modulation ( BPSK, PSK,QPSK,16QAM, …). Pilot subcarriers used to prevent frequency and phase shift errors. Benefits of OFDM Higher data rates Overlap of subcarriers Lower bandwidth than spread spectrum. High spectral efficiency Lower multi-path distortion Usage of cyclic prefix Our OFDM System Assumptions Usage of cyclic Prefix Impulse response of the channel shorter than Cyclic Prefix. Slow fading effects so that the channel is time-invariant over the symbol interval. Rectangular Windowing of the transmitted pulses Perfect Synchronization of transmitter and receiver Add