Browsing by Subject "Multiple Input Multiple Output (MIMO)"

Now showing 1 - 4 of 4

Open Access
Application and improvement of SAGE algorithm for channel parameter estimation
(2008) Bodur, Harun
In recent years, Multiple Input Multiple Output (MIMO) systems have gained importance due to the improvements on the performance of radio systems. Channel parameter estimation is an important factor in the design and optimization of MIMO systems. In this thesis, channel parameters such as delay, angles (azimuth and elevation) of arrival (AoA) and departure (AoD), Doppler frequency and polarization are estimated from measurement data using Space Alternating Generalized Expectation-Maximization (SAGE) algorithm. One of the focuses of this thesis is to reduce the computational complexity of the algorithm by using Particle Swarm Optimization (PSO) technique. Additionally, a new initialization procedure is proposed to get better estimates and to improve the processing time of the algorithm. Moreover, performance of the SAGE algorithm and improvements on the algorithm are tested via extensive measurement data. It is found that SAGE algorithm is a powerful tool for channel estimation and it can further be improved by the aforementioned propositions.
Open Access
Base station cooperation in multiple input multiple output orthogonal frequency division multiple access systems
(2009) Tokel, Turgut Barış
Newly emerging advancements such as multiple input multiple output (MIMO) and orthogonal frequency division multiple access (OFDMA) techniques become indispensable parts of today’s wireless systems such as WiMAX (IEEE 802.16 standard) since they can increase the supportable data rates significantly. However, achieving the maximum spectral efficiency in a MIMO system requires perfect channel state information (CSI) at the transmitter side and multicarrier nature of OFDMA systems increase the necessary CSI feedback from users to base stations remarkably. To further increase the supportable data rates, using frequency reuse factor of 1 in the system is also mandatory. Unfortunately, this results in significant cochannel interference (CCI) observed especially by the users near cell edges, which can severely degrade the system spectral efficiency. To cope with this problem, base station cooperation may play an important role. In this thesis, the problem of cooperative data transmission from base stations to users in multicellular MIMO-OFDMA systems is considered. An efficient cooperative scheduling and data transmission scheme, requiring limited CSI feedback from users to base stations and also limited information exchange between the base stations, is proposed. The numerical results demonstrate that, the proposed algorithm offers considerable spectral efficiency gains compared to conventional frequency reuse and noncooperative schemes, under severe CCI conditions
Open Access
Performance analysis of diversity techniques for OFDM and base station cooperation
(2010) Üzeler, Hande
The main goal of the next generation wireless communication systems is to provide high data rate services. In order to deal with performance-limiting challenges that include frequency selective fading channels, power and bandwidth constraints, multiple input multiple output (MIMO) and orthogonal frequency division multiplexing (OFDM) techniques have been proposed as effective techniques to combat fading and to provide high rate reliable transmission. In this thesis we first give an overview of WiMAX as an example of an OFDM system and study the performance of the WiMAX physical layer under different MIMO techniques. We also analyze space-frequency coding and propose a threaded algebraic space-time (TAST) based code. Secondly, since the mobile bandwidth is an expensive and scarce resource, it seems likely that a high frequency reuse will be employed in the future cellular networks to increase spectral efficiency. This means that base stations (BSs) will operate in the same frequency band and therefore cause cochannel interference (CCI) to the users at other cells. CCI is an important performance degrading factor. Therefore our second aim is to investigate BS cooperation techniques to mitigate CCI. We assume that channel state information (CSI) is available at the cooperating BSs and analyze the performance gains due to cooperation when used in conjunction with Alamouti space-time coding.
Open Access
Polarization included geometry based channel modeling for MIMO systems
(2008) Akkaya, Keziban
Most of the studies in the literature about channel modeling do not include the polarization. Aiming to develop a more realistic geometric model including polarization, the channel characteristics are examined using measurement data. Each multipath in the measurement data is modeled with a scatterer. Locations of scatterers are determined in the geometry based single bounce model. Then, each scatterer is replaced by a thin impedance disc. Electrical properties, sizes and orientations of discs are obtained using physical optics approximation. Using the channel model, XPD characteristics of the environment are examined. As a result of this study, a channel model for characterizing the general scenarios as much as possible is developed.