Browsing by Subject "orthogonal frequency division multiplexing (OFDM)"

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Open Access
Capacity and spectrum efficiency analysis of an asymmetric PMR system with DAB downlink
(2003) Şengül, Ersin
Different trunked Private Mobile Radio (PMR) systems have been designed over the last several decades, all of which have symmetric downlink and uplink channel capacities. Due to this symmetry, those systems may not be spectrally efficient in case of different types of services which are only supported by PMR systems, such as group (acknowledged or unacknowledged) and broadcast calls, either voice or data. In this thesis, a new asymmetric trunked PMR system comprising an OFDM based broadband, wide-area downlink and a narrowband cellular uplink, is proposed to achieve a higher capacity and higher spectral efficiency than current digital trunked PMR systems have. This thesis concentrates on the system capacity analysis of the proposed system associated only with the downlink part for voice communications, as well as the spectrum efficiency comparison of the proposed system with the Terrestrial Trunked Radio (TETRA) system, which is accepted as the spectrally most efficient PMR system. In this study, we study the performance and capacity of the proposed system using Digital Audio Broadcasting (DAB) downlink. In particular, we study the capacity of such a system for voice calls using voice activity detection and statistical multiplexing. Moreover, we show that, the capacity of the system can significantly increase, if the incoming calls, which cannot find an available channel, are allowed to wait a certain amount of time before occupying a channel. The system is shown to have high trunking efficiency since all users are assumed to use the pool of channels available in the wideband downlink. Spectral efficiency of the proposed system and a standard TETRA system are compared using numerical case studies against different traffic loads, cell sizes and number of clusters. The optimum point, with respect to number of clusters, up to which the proposed system is more efficient, is determined. It is shown that for a realistic PMR scenario the proposed system is more efficient up to 5 clusters, i.e. 35 cells, and therefore it can be concluded that the proposed system can be used efficiently in realistic situations.
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.