Browsing by Subject "Orthogonal frequency division multiplexing."

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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
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
FPGA based implementation of IEEE 80211a physical layer
(2010) İnce, Mustafa
Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission technique, in which a single bitstream is transmitted over a large number of closely-spaced orthogonal subcarriers. It has been adopted for several technologies, such as Wireless Local Area Networks (WLAN), Digital Audio and Terrestrial Television Broadcasting and Worldwide Interoperability for Microwave Access (WiMAX) systems. In this work, IEEE802.11a WLAN standard was implemented on Field Programmable Gate Array (FPGA) for being familiar with the implementation problems of OFDM systems. The algorithms that are used in the implementation were firstly built up in MATLAB environment and the performance of system was observed with a simulator developed for this purpose. The transmitter and receiver FPGA implementations, which support the transmission rates from 6 to 54 Mbps, were designed in Xilinx System Generator Toolbox for MATLAB Simulink environment. The modulation technique and the Forward Error Coding (FEC) rate used at the transmitter are automatically adjusted by the desired bitrate as BPSK, QPSK, 16QAM or 64QAM and 1/2, 2/3 or 3/4, respectively.The transceiver utilizes 5986 slices, 45 block RAMs and 73 multipliers of a Xilinx Virtex-4 sx35 chip corresponding to % 39 of the resources. In addition, the FPGA implementation of the transceiver was also tested by constructing a wireless link between two Lyrtech Software Defined Radio Development Kits and the bit error rate of the designed system was measured by performing a digital loop-back test under an Additive White Gaussian Noise (AWGN) channel.
Open Access
Implementation of an OFDM based underwater acoustic modem
(2011) Demirörs, Emrecan
In this thesis we designed and implemented an underwater acoustic (UWA) communication system employing multicarrier modulation in the form of orthogonal frequency division multiplexing (OFDM). UWA communication became more popular as there has been a growing interest in transmitting real-time data, such as video and sonar images. There are many applications where these transmissions are used. These applications are underwater wireless sensor networks(UWSN) and unmanned underwater vehicles (UUVs) for military and scientific purposes. Therefore, building an efficient UWA communication system which has a high data rate can improve these applications’ performance signifi- cantly. Currently, many underwater communication systems use single carrier modulation which have limited data rate due to complexity of their receivers, as frequency selectivity of the channel increases when the symbol rate increases, so we preferred to use multicarrier modulation in UWA communication in order to increase data rate of our system. In this thesis, we considered a system that uses zero-padded (ZP) OFDM modulation. Based on ZP-OFDM, we used a receiver model that performs pilot-tone based channel estimation, carrier frequency offset compensation based on least squares (LS) fitting error or null subcarriers if they occur and data demodulation for each OFDM block individually. We used MATLAB environment for implementing our system. The MATLAB scripts generate a data burst that contains OFDM blocks, and then it is transmitted to the hardware from a laptop by using a Data Acquisition (DAQ) Card. At the other side of the system, the receiver laptop gets the data by using a DAQ Card. As the data is received, MATLAB scripts are used for demodulating it. As we built our system, we performed underwater experiments at Bilkent Lake Facility to investigate its performance in a real UWA channel. In our test, a data rate of 13.92 kbps has been achieved with quadrature phase shift keying (QPSK) modulation while the bit-error-rate (BER) was less then 9x10−2 without using any coding.
Open Access
Low complexity equalization for OFDM in doubly selective channels
(2009) Pamuk, Alptekin
In current standards Orthogonal Frequency Division Multiplex -OFDM- is widely used for its high resistance to multi-path environments and high spectral ef- ficiency. However since the transmission duration is longer, it is affected from time variations of the channel more than single carrier systems. Orthogonality of sub-carriers are lost within an OFDM symbol and intercarrier interference(ICI) occurs as a result of time variation of the channel. Channel estimation and equalization become problematic, because the classical structures like MMSE require very complex operations. This thesis studies the channel equalization problem, as separate from the channel estimation problem. The thesis assumes that the channel coefficients are perfectly known and focuses on the estimation of data transmitted on each OFDM carrier. First, a survey of existing algorithms on channel equalization is given and simulations are provided to compare them in terms of complexity and performance under an OFDM system scenario that is consistent with the present WiMAX system parameters and operating conditions. As a novel contribution, the thesis proposes two new equalization methods by amending existing algorithms and shows that these modified algorithms improve the state-of-the-art in channel equalization in terms of complexity andperformance under certain high-mobility scenarios. Finally it is shown that the intercarrier interference cancellation problem remains a major impediment to the implementation of OFDM in high-mobility environments.
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
Time-of-arrival estimation in OFDM-based cognitive radio systems
(2010) Karışan, Yasir
Cognitive radio (CR) systems can efficiently utilize the radio spectrum due to their ability to sense environmental conditions and adapt their communications parameters (such as power, carrier frequency, and modulation) so as to enable dynamic reuse of the available spectrum. In this thesis, theoretical limits on time-of-arrival (TOA) estimation are derived for CR systems in the presence of interference. Specifically, closed form expressions are obtained for Cramer-Rao bounds (CRBs) on TOA estimation in orthogonal frequency division multiplexing (OFDM) based CR systems in various scenarios. Based on the CRB expressions, an optimal power allocation strategy that provides the best possible TOA estimation accuracy is proposed. This strategy considers the constraints imposed by regulatory emission mask and the sensed interference spectrum. The maximum likelihood (ML) TOA estimator is derived for an OFDM-based signalling scheme, and its performance is investigated against the theoretical limits offered by the CRB expressions. In addition, numerical results for the CRBs and ML TOA estimator are obtained and the effects of the optimal power allocation strategy on the accuracy of ML TOA estimator are examined in the absence/presence of interference. The use of optimal power allocation strategy instead of the conventional power assignment scheme is demonstrated to provide significant gains in terms of the TOA estimation accuracy. Analysis of the performance sensitivity of the optimal power allocation strategy to the uncertainty in spectrum estimation is performed, and the performance of optimal power allocation is observed to be consistently superior to that of the uniform power allocation even for substantially high values of spectral estimation errors.
Open Access
Underwater acoustic modem using OFDM
(2012) Yüksel, Mine Merve
This thesis is about design, simulation and testing of an underwater acoustic modem using OFDM. The thesis work combines a theoretical part, whose objective is to understand the appropriate techniques to deal with the characteristics of the targeted channel, simulations and a practical part regarding the system deployment and experimental tests. There has been a great growing interest in transmitting real-time data and video. Unmanned underwater vehicles (UUVs) for military and scientific applications have become important. Building distributed and scalable underwater wireless sensor networks (UWSN) that will bring significant advantages and benefits to underwater applications, such as ocean observation for scientific exploration, commercial exploitation, coastline protection and target detection in military events has been in the scope of researchers. Based on these, designing a concrete system with high data rate will benefit many underwater acoustic (UWA) applications. The existing systems in literature use single carrier transmission and rely on linear or non-linear equalization techniques to suppress inter-symbol interference (ISI), however this requires complex equalizers and results in low data rates. Therefore we concentrate on multicarrier modulation. In this thesis ZP-OFDM (Zero Padded-Orthogonal Frequency Division Multiplexing) receiver is built, where CFO (Carrier Frequency Offset) compensation, pilot-tone based channel estimation, and data demodulation are carried out on the basis of each OFDM block. The implemented OFDM system has been developed in MATLAB. MATLAB scripts generate a data burst that contained OFDM blocks, and then they are transmitted to the hardware from a laptop by using a Data Acquisition (DAQ) Card. At the other side of the system, the receiver laptop gets the data by using a DAQ Card. As the data is received, MATLAB scripts are demodulated and data is detected. Simulations aim to provide correct implementation of all the algorithms by coupling the generated OFDM signal to a channel using Bellhop underwater channel model and noise addition algorithm, that artificially introduces some of the real channel effects into the signal. The method is tested in a shallow-water experiment at Bilkent Lake. Over a bandwidth of 12 kHz, the data rate is 13.92 kb/s with QPSK modulation, when the number of subcarriers was 1024. Bit-error-rate (BER) is less than 9x10−2 without any coding.