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1 5 Adaptive Sequence Detection using T algorithm for Multipath Fading ISI Channels Heung No Lee and Gregory J Pottie Electrical Engineering Department University of California at Los Angeles Box 951594 Los Angeles CA 90095 Email pottie icsl ucla edu Phone 310 825 8150 FAX 310 206 8495 Abstract We develop an adaptive low complexity tree search receiver using the T algorithm for multipath fading ISI channels Unlike previous research on sequence based detection a symbol spaced channel is not given a priori rather the receiver utilizes the feedforward channel estimation to derive the matched filter and the symbol spaced statistics Then to enhance the efficiency of the T algorithm the use of a mean square whitening filter MSWF is proposed We also propose the use of per survivor processing which brings a further SNR advantage and reduction of the average computations required by the T algorithm receiver A substantial SNR benefit over a correct decision feedback DFE is achieved at a moderate increase in complexity I INTRODUCTION The receiver technique developed in this paper is intended to facilitate the implementation of a reliable adaptive and highly bandwidth efficient communication link over time varying dispersive channels The receiver must cope with the unknown channel exhibiting Doppler spreading frequency selective fading and shadowing In addition since the radio spectrum is dear use of a large signal set modulation is highly desirable We therefore consider system with a large signal set up to 64 QAM together with an explicit antenna diversity combining and adaptive equalization For equalization we investigate a low complexity sequence search receiver using the T algorithm for uncoded use which achieves a performance very close to that of maximum likelihood sequence detection MLSD A great deal of research has been undertaken to reduce the computational complexity required to achieve the performance of MLSD Research in this arena includes reduced state sequence

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