Βελτιωση Συστηματος Ds/Cdma Μιαδικης Ορθογωνιας Σηματοδοσιας Για Κινητες Επικοινωνιες

Abstract

Personal communications is the base of Telecommunication in the 21 century. Increasing demand for higher capacity and high quality service has motivated research to introduce new schemes. Two candidate schemes are the alternatives for the problem : Time Division Multiple Access amd Code Division Multiple Access. The Spread Spectrum techniques are well known from the decade of 40's but only for military applications. The commercial applications of Spread Spectrum techniques began during 1980. The following thesis investigates spread spectrum techniques in M-ary orthogonal systems for mobile communications. The study focus on the design of a noncoherent M-ary orthogonal receiver and the improvement of different systems parameters.At the outset, a presentation of the most popular channel models and the principles of spread spectrum techniques is given. At the following, a modified RAKE receiver is proposed and is thoroughly examined in detail for the criteria of error probability and the outage probability. Also, a simulation model in C++ is developed and a comparative study between the numerical and the simulation results is presented.Then we propose a wide range of alternative chip waveforms instead of the classic pulse waveform in a frequency selective channel. The channel model is Rayleigh or Nakagami-m.At the following, the mitigation of the shadowing phenomenon is presented through the use of macrodiversity techniques, namely a number of distributed geographical base stations. Antennas are located at the hexagon corners and two cases of the correlation among base stations is examined : uncorrelated and correlated case. The additional complexity and cost are justified completely by the benefits (from the user's aspect) that could be provided.Finally, we extend the previous study, analyzing a cluster for both the links (uplink and downlink). The proposed system is evaluated in terms of Bit Error Probability. Also, the imperfect power control case has been studied giving us an in depth insight of the system.