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Yayın Cooperative strategies, achievable rates and resource allocation for OFDMA channels(Işık Üniversitesi, 2011-07-28) Bakım, İsmail Sezi; Kaya, Onur; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Doktora ProgramıThe design of next generation wireless communication systems brings along new challenges, since the degrading factors such as fading and multi-user interference become harder to deal with as the number of users and the bandwidth require- ments increase. Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple accessing technique which provides a solution to both of the problems above: it provides a relatively simple way of assigning available bandwidth to users, while avoiding interference; and at the same time, it converts a frequency selective fading channel, to parallel flat fading subchannels, hence reducing the effects of intersymbol interference. However, in wireless channels, what is tradi- tionally considered as interference is in fact side information, and combined with the diversity created by the orthogonal subchannels in OFDMA, this side infor»mation can be carefully taken advantage of to increase the rates achievable by the users. In our thesis, without imposing any prior constraints on subchannel allocation, we investigate cooperation strategies, achievable rates and resource allocation for OFDMA channels. We propose new cooperative encoding strategies for wireless communication net- works över OFDMA channels. We particularly focus on a two user cooperative OFDMA system, based on block Markov superposition encoding (BMSE). We obtain expressions of the resulting achievable rate regions for ali proposed coroperative encoding strategies. We show that, by allowing for re-partitioning and re-encoding of the cooperative messages across subchannels, it is possible to better exploit the diversity created by OFDMA, and higher rates can be achieved. In order to take full advantage of the diversity created by OFDMA, we then introduce a channel adaptive cooperation strategy for OFDMA, and optimize the transmit powers as a function of the channel states. We provide the optimality conditions that need to be satisfied by the powers associated with the users' codewords and derive the closed form expressions for the optimal powers.Yayın Design and performance analysis of a novel trellis coded space-time-frequency transmission scheme(Işık Üniversitesi, 2004-07) Oğuz, Onur; Panayırcı, Erdal; Aygölü, Hasan Ümit; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Yüksek Lisans ProgramıThis work is concerned with a transmitter diversity technique for wireless communications over frequency selective fading channels. The proposed technique utilizes orthogonal frequency division multiplexing (OFDM) to transform a frequency selective fading channel into multiple flat fading sub channels on which space-frequency processing is applied and then additional space-time coding is applied in order to increase the diversity. In order to improve the performance of the system, a convenient trellis coding scheme is investigated and design criteria are established. Performance of proposed space-time-frequency OFDM (STF-OFDM) transmitter diversity technique has been examined by computer simulations.Yayın Design of reed-solomon and convolutional coded OFDM system and performance analysis(Işık Üniversitesi, 2003) Bal, Ufuk; Panayırcı, Erdal; Işık Üniversitesi, Fen Bilimleri Enstitüsü, Elektronik Mühendisliği Yüksek Lisans ProgramıIn this study, OFDM and error-correcting codes, which have an important place in digital communication systems, have been studied. Literature research has been made on OFDM and error-correcting codes like Reed-Solomon and convolutional codes. Reed-Solomon and convolutional coded OFDM system has been designed. Furthermore, system's performance analysis has been made. OFDM is special case of multicarrier transmission. The basic principle of OFDM is to split a high-rate datastream into a number of lower rate streams that are transmitted similtuanesly over a number of subcarriers. One of the main reason to use OFDM is to increase the robustness against frequency selective fading or narrowband interference. In a single carrier system, a single fade or interferer can cause the entire link to fail, but in a multicarrier system, only a small percentage of the subcarriers will be affected. Error correcting codes can then be used to correct the errors. In error correcting, combining convolutional and block codes in a concatenated code is particularly powerful technique. In this study Reed-Solomon codes are used as an outer code and convolutional codes are used as an inner code.
