Analysis of maximum ratio transmission based cooperative ofdm system

Multipath fading, which is caused by enviromental conditions, is one of the the most corruptive and important problems to deal with in wireless telecommunication systems that are developing rapidly. Space diversity via multiple antennas on transmitter and/or receiver is one of the most effective solutions to overcome multiple fading. Using multiple antennas allows the transmitter to send the signal by using statistically independent paths. As a result of that, the receiver obtains the diverse information. Maximum Ratio Transmission (MRT) is one of the several beamforming methods that provides full diversity by using multiple antennas at transmistter. Using MRT with wireless local area networks, downlink connections of cellular communications and similar systems is beneficial and simple because MRT does not require complexity at receiver side. Multiple antenna systems operating with Orthogonal Frequency Division Multiplexing (OFDM) ensures the diversity gain also for frequency selective channels which is experienced in actual enviromental conditions. Cooperative communcation systems provide space diversity as multiple imput multiple output systems. Like multiple antenna systems, cooperative communication systems provides transmitter to send information over independent paths by using other users as a relay. Consequently, the information that is sent by source, acquired more accurately if proper combining techniques are used. There are two type of basic cooperative communication technique investigated in literature: Amplify-Forward (AF) and Decode-Forward (DF). In this thesis, OFDM based cooperative systems, where source and relays are equipped with multiple transmit antennas and the use of MRT are studied. Moreover AF and DF based cooperative systems are modelled and error performance of these systems are simulated on fading channels. The results clearly shows that, on the AF based systems, using multiple antennas at the transmitter side of source and relays and applying MRT increases system performance and reliability, also decreases error probability. Moreover, the diversity gain increases significantly when the number of relays and transmit antennas are increased. These systems become resistant to frequency selective fading channels by using OFDM. On the other hand, DF based systems which use MRT at source and relays, can't show the desired error performance because of the error propagation at source-relay link. To overcome this problem, two different types of combining methods are applied at the destination. Results show that, cooperative maximum ratio combining method increases the error performance of the DF based systems.