Browsing by Subject "Orthogonal Frequency Division Multiplexing (OFDM)"

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Open Access
Analysis of using OFDM for short-range, multı-user, underwater acoustic communication
(2006) Öktem, Kemalettin Kerem
Acoustic waves are being used in several underwater applications, such as SONARs, underwater communication systems. Most of already developed and deployed underwater communication systems use narrow band communication and lacks layered communication approach. In this thesis, we propose a spread spectrum, layered architecture for underwater communication system, such as for SCUBA divers. The communication device shall be designed such that divers can communicate with each other in shallow water, short range in a multi-user fashion and provide not only voice communication but also data transmission as well. The device shall use Orthogonal Frequency Division Multiplexing (OFDM) as a spread spectrum technique. The OFDM technique is selected from other spread spectrum techniques due to it’s inherent ability to combat the channel impairments and flexibility of implementing the communication system using software defined radio (SDR). The spread spectrum system shall operate in 100 kHz to 300 kHz frequency band using wideband acoustic transducers. In this work, we studied a layered architecture for the communication device. We mainly studied the application layer, data link layer and physical layer in order to analyze the achievable data rate and performance. In this work, we tried to find the optimal communication parameters to achieve guaranteed communication performance for possible scenarios. The communication parameters are set in order to achieve best performance for the worst condition. Using the optimal parameters, the system shall occupy 5 users voice and data communication at the same time using the entire frequency band at the same time, however with certain Grade of Service (GOS) the capacity shall be increased. The capacity of the system shall further be increased if the system uses adaptive communication parameters that are adapted to changing channel and user conditions. The system using adaptive communication parameters shall provide at most 16 users’ voice and data communication using the entire frequency band at the same time.
Open Access
Estimation of receiver sampling clock timing impurity impact on channel orthogonality in OFDM based communication systems
(2009) Tanyeri, H. Onur
The growing need for high-speed wireless communication systems has led communication engineers to design and implement communication systems at higher frequencies where more bandwidth is available, use digital modulation schemes with more complex constellations and place carriers closer together with little guard-band in the pursuit of designing communication systems closer to the channel capacity. These new designs have placed tighter constraints on the performance of oscillators and timing devices of transceivers. In this work, the effects of timing clock jitter on the receiver Analog-to-Digital Converter (ADC) of Orthogonal Frequency Division Multiplexing (OFDM) based communication systems are examined and Inter-Carrier Interference (ICI) effects are quantified in order to prevent unnecessary over designs in OFDM ADC circuitry. In this respect, a simulation tool that synthesizes jitter processes with defined spectral characteristics is prepared. The generated jitter processes are utilized in an OFDM simulation tool that quantifies the ICI levels caused by receiver ADC sampling jitter. Using these two tools, ICI levels of certain OFDM systems are examined and guidelines for OFDM ADC circuitry design are proposed.
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.