Τεχνικές διαχείρισης ραδιοπόρων σε συστήματα OFDMA με χρήση πολλαπλών κεραιών στο σταθμό βάσης και στη συσκευή του χρήστη

Abstract

This doctoral thesis attempts to provide an analysis and performance investigation on the operation of OFDMA networks. The research analysis of an OFDMA network can certainly involve different aspects of its operation. However, in the framework of this thesis, the interest lies in the resource allocation algorithms that these networks apply along with the different array antenna techniques that can also be implemented in such networks in order to endure their performance. In the context of the radio resource allocation, six subcarrier allocation algorithms with gradually increasing difficulty, are examined. Their performance is also investigated when adaptive modulation is applied. The adaptive modulation technique is combined with OFDMA systems since it has been proved to optimize their performance. Nev- ertheless, this technique involves important feedback information so less demanding techniques are usually preferred. A new subcarrier allocation algorithm is proposed with frequency reuse one that is built upon the well-known cell splitting concept. This technique presents improved performance compared to the already applied ones, while it maintains the complexity in sufficiently low levels. In the next step array antenna techniques are applied at the network base stations. The technique of switched-beam smart antennas systems is investigated in the context of a typical OFDMA network and a study on the impact of the user channel length is also held. This study concerns only systems that enable adaptive modulation and attempts to provide an insight on the suitable channel length (in terms of system subcarriers) that the network should allocate to its users according to their requested service. The array antenna at the base station can alternatively be used for random beamforming. This technique provides additional fluctuations in the user channel as a means to increase the multiuser diversity gain. Finally, the link between the transmitter and the receiver of the OFDM system that was used in the aforementioned studies is simulated. The SNR-SER mapping values for the specific OFDM parameters are now calculated with the use of channel models that simulate small scale fading. Next, the OFDM link simulation is extended to the point that MIMO encoding techniques are also included. The simulated MIMO techniques are stemmed from the extended Alamouti orthogonal designs and involve two and four transmit antennas with two antennas at the receiver end. The simulation results of the MIMO-OFDM link are provided to the OFDMA network simulation environment so as the performance of the MIMO-OFDM encoding techniques to be evaluated in the context of a large scale OFDMA network.