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VI INTERNATIONAL TELECOMMUNICATIONS SYMPOSIUM (ITS2006), SEPTEMBER 3-6, 2006, FORTALEZA-CE, BRAZIL10 0 P TX[dB]10 −1Downlink Beamforming (DB)Alamouti (TD)Eigen−BeamformingmBER−TD−DBGain [dB]100−10mBER−TD−DBl=1l=210 −2−20BER10 −310 −4−30−80 −60 −40 −20 0 20 40 60 80Angle100Eigen−Beamforming l=1l=210 −5Gain [dB]−10−2010 −60 1 2 3 4 5 6 7 8 9 10Fig. 5: Performance for 2-path LOS channel and 4-QAMmodulation as a function of the transmit power P TX .B. LOS scenarioThe LOS scenario corresponds to a flat-fading channel asin the NLOS case. However, it presents a direct path (or LOSpath) which is Rice distributed, while all the other paths areRayleigh distributed. We have considered a 2-path channel,with a LOS path and a Rayleigh path that have the samerelative delay. The LOS path has a Rician factor K Rice = 20 dBand is 6 dB stronger than the Rayleigh path. The DOA of theLOS path is 0 ◦ while the Rayleigh path has a DOA of 40 ◦ .Fig. 5 shows the BER for all simulated techniques as afunction of the transmit power P TX . We see that the proposedmBER-TD-DB outperforms all other techniques, includingthe Eigen-Beamforming, which is no longer optimal in thisscenario due to Rician fading. Even DB performs betterthan Eigen-Beamforming, since it concentrates the transmittedpower in the strongest path (which is Rician), mitigating thefading created by the second path. The Eigen-Beamforming,however, tries to mitigate the fading as if the paths wereRayleigh distributed. So it tries to combine both paths togenerate a milder fading path. But, by doing so, it only bringsfading to the Rician path, which is practically a constantmodulus path. This can be seen in the radiation patternson the bottom of Fig. 6, where the dashed lines representeach path of the channel. The upper part of the figure showsthe radiation patterns for the proposed mBER-TD-DB, whichpresents the expected behavior in order to reduce the BER. Thefirst layer radiates almost only in the direction of the Rician(and strongest) path, resulting in a robust Rician channel, whilethe second layer can be neglected since it is 25 dB below thefirst one. Hence, in this scenario, the use of only one layerwould be optimal. However, the optimum purely spatial filterwould not be the DB solution, but the proposed one.V. CONCLUSIONWe have showed that the proposed technique is equivalentto Eigen-Beamforming [1] for Rayleigh channels. However, inall other scenarios such as Rician channels, the mBER-TD-DBoutperforms Eigen-Beamforming since it is based on a moregeneral criterion that does not make any assumption about−30−80 −60 −40 −20 0 20 40 60 80AngleFig. 6: Radiation pattern of the precoder layers formBER-TD-DB and Eigen-Beamforming for atransmit power of 10 dB in the LOS scenario.the channel distribution. This makes the proposed techniquean attractive solution for the general application, where thechannel type is not known a priori.Future work will involve analyzing the cost function inregard of its minimums and investigating the convergenceof the proposed algorithm, as well as the influence of theestimation of the DCCMs on the performances.ACKNOWLEDGMENTSThe authors would like to thank professor Didier Le Ruyetof the Laboratory of Electronics and Communications at theCNAM, Paris-France, for pointing their attention to [1].REFERENCES[1] S. Zhou and G. B. 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