Multiple access techniques with periodic clock



Bogdan Cristea

thesis abstract

  • Motivation

    This thesis has begun in November 2003 and represents a continuation of the works realized in our laboratory (four thesis already presented). The research subject of these thesis is the use of Linear Periodic Time-Varying (LPTV) filters in digital communications. The aim of our research consists in the application of Periodic Clock Changes (PCCs) for the design of multiple access systems. Our study is focalized on a class of PCCs: block interleavers (matrix interleavers, circular interleavers and random permutations). First a bibliographical study on LPTV filters and existing multiple access systems is realized and then we study two multiple access systems based on block interleavers.

  • Multiple access with LPTV filters

    A first multiple access system based on deterministic block interleavers (matrix interleavers and circular interleavers) is proposed. After the pulse shaping, the users are modulated with orthogonal carrier frequencies in order to ensure users' orthogonality. The samples of the modulated signal are then interleaved by a matrix interleaver, the same for each user. At the interleaver output a spread spectrum signal is obtained. In multipath channels the transmission chain with matrix interleavers is equivalent with a sum of PCCs. Based on this result, a cancellation technique of the time-varying nature of the sum of PCCs was proposed. It can be shown that there is no multi-user interference, when the users are quasi-synchronous. Equalization techniques adapted for our multiple access system were proposed. System performances were compared with other multiple access techniques (DS-CDMA and CIBS-CDMA). Based on the equivalence of the modulator and matrix interleaver with an LPTV filter, a synchronization algorithm was proposed. This algorithm uses the variation of the received power as a function of the desynchronisation between the emission and reception systems. One can use a delay-locked loop in order to synchronize the receiver.

  • Multiple access with random permutations

    A second multiple access system based on random permutations was studied. The information bits are coded by a repetition code (shaping filter) and then interleaved by a random permutation. The permutations are different for each user and make it possible to obtain users' orthogonality. In multipath channels a Zero Padding (ZP) technique allows the elimination of the interference between successive blocks of chips after multipath propagation. The system performances were studied using a Rake type receiver. The last part of the thesis is devoted to iterative reception techniques adapted to the proposed multiple access technique. Initially, turbo equalization techniques in a mono-user context are studied, then turbo detection techniques are approached in a multi-user context. The convergence of the iterative reception techniques is evaluated by EXtrinsic Information Transfer (EXIT) charts.

    The results obtained in this thesis have been published in several conferences: ICASSP 2004, Montreal, Canada; EUSIPCO 2004, Vienna, Austria; EUSIPCO 2005, Antalya, Turkey; Communications 2006, Bucharest, Romania; EUSIPCO 2006, Florence, Italy.