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Title. Uplink channel estimation for multi-user OFDM-based systems. Carlos Ribeiro M. Julia Fernández-Getino Garcia Víctor P. Gil Jiménez Atílio Gameiro Ana García Armada. Department of Signal Theory and Communications Communications Group.

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  1. Title Uplink channel estimation formulti-user OFDM-based systems • Carlos Ribeiro • M. Julia Fernández-Getino Garcia • Víctor P. Gil Jiménez • Atílio Gameiro • Ana García Armada Department of Signal Theory and Communications Communications Group © 2006, motion | mobile systems communication group | all rights reserved

  2. Outline • Objectives of proposed method; • OFDM overview; • TD downlink Channel Estimation; • TD multi-user uplink Channel Estimation; • Simulation results; • Conclusions.

  3. The TD Uplink ChEst • Specify low-complexity pilot-assisted channel estimation scheme (suited for both downlink and uplink); • To be used in OFDM-based networks with centralized architecture; • Good performance in comparison with reference methods with no complexity trade-off; • Processing performed immediately on the time-domain (TD) received vector.

  4. OFDM system structure • Some OFDM symbols, s, convey pilots at sub-carriers {0,Nf,2Nf,…}; remaining sub-carriers convey data symbols: s=d+p

  5. TD pilot-assisted channel estimationDownlink • Channel estimator uses pilot symbols to perform the estimation of the channel’s frequency response (CFR); • The transmission of p through a noisy wireless multipath channel results in a received signal made-up of Nfnoisy channel impulse response (CIR) replicas in TD: where

  6. TD pilot-assisted channel estimationDownlink (2) • Example of received vector r with Nf=4 • The vector is made-up of 4 CIR replicas. • Each node in the network builds new vector based on r with L most significant taps (MST) of each CIR [ref. Minn et al], • CFR estimate, , for each node is the result of the Nc-points DFT of r,

  7. TD pilot-assisted channel estimationUplink • Nodes share same sub-carriers for pilots in the same symbols (perfect overlap in the receiver in FD);

  8. TD pilot-assisted channel estimationUplink (2) • Different pilot sequences for each node in FD; Node u (out of U) pilot sequence, Pu[k] : • An example for U=4 nodes,

  9. TD pilot-assisted channel estimationUplink (3) • Nodes different pilot sequences • Central Point channel estimator builds a vector for each node keeping L MSTs of each CIR, starting at position ,  time-shifted CIR replicas

  10. TD pilot-assisted channel estimationUplink (4) • CFR estimate, , for each node is the result of the Nc-points DFT of ru, • Performance Evaluation / Simulation scenario • QPSK modulated OFDM system with Nc=1024 carriers; • U=4 nodes; • Pilot separation Nf=4; • Sampling time t=25ns; • BRAN-A wireless multipath channel;

  11. TD pilot-assisted channel estimationUplink (5) • Performance Evaluation (2) • Channel estimation MSE averaged over all nodes for different L • Channel estimation MSE dependence on pilot separation Nf

  12. Conclusions • Conclusions • Low-complexity uplink multi-user OFDM channel estimator; • Simple TD averaging to obtain user’s CIR; • Can be used in acquisition (preamble-based) or tracking ( pilot-tones based); • Provided the pilot sequences that allows identification of the user’s CIR in a multi-user scenario; • Performance substantiated by computer simulation using BRAN-A channel model.

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