An equalization technique for high rate OFDM systems

In a typical orthogonal frequency division multiplexing (OFDM) broadband wireless communication system, a guard interval using cyclic prefix is inserted to avoid the inter-symbol interference and the inter-carrier interference. This guard interval is required to be at least equal to, or longer than the maximum channel delay spread. This method is very simple, but it reduces the transmission efficiency. This efficiency is very low in the communication systems, which inhibit a long channel delay spread with a small number of sub-carriers such as the IEEE 802.11a wireless LAN (WLAN). To increase the transmission efficiency, it is usual that a time domain equalizer (TEQ) is included in an OFDM system to shorten the effective channel impulse response within the guard interval. There are many TEQ algorithms developed for the low rate OFDM applications such as asymmetrical digital subscriber line (ADSL). The drawback of these algorithms is a high computational load. Most of the popular TEQ algorithms are not suitable for the IEEE 802.11a system, a high data rate wireless LAN based on the OFDM technique. In this thesis, a TEQ algorithm based on the minimum mean square error criterion is investigated for the high rate IEEE 802.11a system. This algorithm has a comparatively reduced computational complexity for practical use in the high data rate OFDM systems. In forming the model to design the TEQ, a reduced convolution matrix is exploited to lower the computational complexity. Mathematical analysis and simulation results are provided to show the validity and the advantages of the algorithm. In particular, it is shown that a high performance gain at a data rate of 54Mbps can be obtained with a moderate order of TEQ finite impulse response (FIR) filter. The algorithm is implemented in a field programmable gate array (FPGA). The characteristics and regularities between the elements in matrices are further exploited to reduce the hardware complexity in the matrix multiplication implementation. The optimum TEQ coefficients can be found in less than 4µs for the 7th order of the TEQ FIR filter. This time is the interval of an OFDM symbol in the IEEE 802.11a system. To compensate for the effective channel impulse response, a function block of 64-point radix-4 pipeline fast Fourier transform is implemented in FPGA to perform zero forcing equalization in frequency domain. The offsets between the hardware implementations and the mathematical calculations are provided and analyzed. The system performance loss introduced by the hardware implementation is also tested. Hardware implementation output and simulation results verify that the chips function properly and satisfy the requirements of the system running at a data rate of 54 Mbps

Volumetric Phased Arrays for Satellite Communications

The high amount of scientific and communications data produced by low earth orbiting satellites necessitates economical methods of communication with these satellites. A volumetric phased array for demonstrating horizon-to-horizon electronic tracking of the NASA satellite EO-1 was developed and demonstrated. As a part of this research, methods of optimizing the elemental antenna as well as the antenna on-board the satellite were investigated. Using these optimized antennas removes the variations in received signal strength that are due to the angularly dependent propagation loss exhibited by the communications link. An exhaustive study using genetic algorithms characterized two antenna architectures, and included optimizations for radiation pattern, bandwidth, impedance, and polarization. Eleven antennas were constructed and their measured characteristics were compared to those of the simulated antennas. Additional studies were conducted regarding the optimization of aperiodic arrays. A pattern-space representation of volumetric arrays was developed and used with a novel tracking algorithm for these arrays. This algorithm allows high-resolution direction finding using a small number of antennas while mitigating aliasing ambiguities. Finally, a method of efficiently applying multiple beam synthesis using the Fast Fourier Transform to aperiodic arrays was developed. This algorithm enables the operation of phased arrays combining the benefits of aperiodic element position with the efficiency of FFT multiple beam synthesis. Results of this research are presented along with the characteristics of the volumetric array used to track EO-1. Experimental data and the interpretations of that data are presented, and possible areas of future research are discussed.Ph.D.Committee Chair: Steffes, Paul; Committee Member: Durgin, Gregory; Committee Member: Peterson, Andrew; Committee Member: Roper, Robert; Committee Member: Williams, Dougla

Detecção multiutilizador do domínio da frequência para sistemas DS-CDMA

Doutoramento em Engenharia ElectrotécnicaOs Sinais de Espalhamento de Espectro de Sequência Directa exibem propriedades cicloestacionárias que implicam redundância entre componentes de frequência espaçadas por múltiplos da taxa de símbolo. Nesta tese, é apresentado um cancelador de interferência multiutilizador (Cancelador por translação na frequência - FSC) que tira partido desta propriedade. Este cancelador linear opera no domínio da frequência no sinal espalhado de tal forma que minimiza a interferência e ruído na saída (Critério do Mínimo Erro Quadrado Médio). Além de testado para o caso de antena única, são avaliadas as performances das configurações de antenas múltiplas para o caso de beamforming e canais espacialmente descorrelacionados considerando sistemas síncronos e sistemas com desalinhamento no tempo dos perfis de canais (ambos UMTS-TDD). Essas configurações divergiam na ordem da combinação temporal, combinação espacial e detecção multiutilizador. As configurações FSC foram avaliadas quando concatenadas com o PIC-2D. Os resultados das simulações mostram consideráveis melhorias nos resultados relativamente ao RAKE-2D convencional e PIC-2D. Foi atingida performance próximo ao RAKE de utilizador único quando o FSC foi avaliado concatenado com PIC-2D em quase todas as configurações. Todas as configurações foram avaliadas com modulação QPSK, 8-PSK e 16-QAM. Foi introduzida codificação Turbo e identificou-se as situações da vantagem de utilização do FSC antes do PIC-2D. As modulações 8-PSK e 16-QAM foram igualmente testadas com codificação.Direct Sequence Spread Spectrum signals exhibit cyclostationary properties which imply redundancy between frequency components separated by multiples of the symbol rate. In this thesis a Multiple Access Interference Canceller (Frequency Shift Canceller - FSC) that explores this property is presented. The linear frequency domain canceller operates on the spreaded signal so as to minimize the interference and noise at the output (Minimum Mean Squared Error Criterium). Moreover the FSC was tested with single antenna, the performance of multisensor configurations for the cases of beamforming and uncorrelated spatial channels was evaluated considering both synchronous and time misalignment systems (both UMTS-TDD). Those configurations diverge in temporal combining, spatial combining and multiuser detection order. The FSC configurations were concatenated with PIC-2D structure and evaluated. The simulation results show considerable improvement relative to the conventional RAKE-2D and PIC-2D receiver. A performance close to the single user RAKE case was a achieved when it (FSC) was evaluated jointly with PIC-2D. All the configurations with modulation QPSK, 8-PSK and 16-QAM were evaluated. Turbo Codes were introduced and it was identified the situations which it is advantageous to use the FSC before the PIC-2D. The 8-PSK and 16-QAM modulations were equally tested with coding