Page 1 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability. Title Carlos Ribeiro M. Julia Fernández-Getino Garcia Víctor P. Gil Jiménez Atílio Gameiro Ana García Armada Uplink channel estimation for multi-user OFDM-based systems © 2006, motion | mobile systems communication group | all rights reserved Department of Signal Theory and Communications Communications Group

Page 2 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability. Outline  Objectives of proposed method;  OFDM overview;  TD downlink Channel Estimation;  TD multi-user uplink Channel Estimation;  Simulation results;  Conclusions.

Page 3 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability. 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.

Page 4 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.  Some OFDM symbols, s, convey pilots at sub-carriers {0,Nf,2Nf,…}; remaining sub-carriers convey data symbols: s=d+p OFDM system structure

Page 5 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.  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 Nf noisy channel impulse response (CIR) replicas in TD: where TD pilot-assisted channel estimation Downlink

Page 6 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.  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, TD pilot-assisted channel estimation Downlink (2)  Example of received vector r with Nf=4 –The vector is made-up of 4 CIR replicas.

Page 7 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.  Nodes share same sub-carriers for pilots in the same symbols (perfect overlap in the receiver in FD); TD pilot-assisted channel estimation Uplink

Page 8 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.  Different pilot sequences for each node in FD; Node u (out of U) pilot sequence, Pu[k] :  An example for U=4 nodes, TD pilot-assisted channel estimation Uplink (2)

Page 9 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.  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 TD pilot-assisted channel estimation Uplink (3)

Page 10 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.  CFR estimate,, for each node is the result of the Nc-points DFT of r u,  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; TD pilot-assisted channel estimation Uplink (4)

Page 11 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.  Performance Evaluation (2) –Channel estimation MSE averaged over all nodes for different L TD pilot-assisted channel estimation Uplink (5) –Channel estimation MSE dependence on pilot separation Nf

Page 12 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability. 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.

Page 13 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.  Data effect on the Channel Estimation TD pilot-assisted channel estimation Uplink

Page 14 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.  Data effect on the Channel Estimation TD pilot-assisted channel estimation Uplink

Page 15 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability.  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 TD pilot-assisted channel estimation Uplink (3)

Page 16 The information in this document is provided as is and no guarantee or warranty is given that the information is fit for any particular purpose. The user thereof uses the information at its sole risk and liability. TD pilot-assisted channel estimation Uplink (3)