Design of Wireless Communication Systems -- Issues on Synchronization, Channel Estimation and Multi-Carrier Systems
Abstract: This thesis deals with certain aspects in the design of wireless communications systems. It is focused on problems related to the mobile or wireless channel: synchronization, channel estimation and design of wireless orthogonal frequency division multiplex (OFDM) systems. There is a short introduction to the field of wireless systems and a deeper review of pervious work and the state of the art in each of the research fields. Throughout the thesis the goal has been to analyze the problems analytically, deriving expressions for the resulting bit error rate (BER) or synchronization performance. The work on channel estimation is focused on the influence of the pilot pattern in OFDM systems and on the possibility of compensating the signal in advance for impairments introduced by the channel. The BER is derived and compared when using different pilot patterns in an OFDM system. For the pre-compensated system the BER, maximum block length, and resulting peak-to-average power ratio are investigated when compensating the power attenuation and/or the phase rotation introduced by the channel. The work on wireless OFDM system design is directed towards preamble-based synchronization and optimization of the sub-channel bandwidth. A new synchronizer structure is introduced for time and frequency synchronization. In this synchronizer the order of correlation and multiplication has been changed compared to conventional synchronizers, which results in significantly improved detection performance. The sub-channel bandwidth is optimized based on the expected Doppler frequency and excess delay of the channel. The use of superimposed pilot sequences is introduced for time and frequency synchronization in OFDM systems, as well as for channel estimation in flat fading channels. As opposed to conventional pilot-based systems, the pilot sequence is linearly added to the data sequence and therefore transmitted simultaneously and in the same band as the data sequence. The performance when using superimposed pilot sequences for synchronization and for joint channel estimation and detection is analyzed. The scheme is also compared to other well known pilot-based and differential detection methods.
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