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In this thesis, we investigate a wireless communications system with distributed transmit antennas. Under such system scenario, the received signal has multiple carrier frequency offsets (CFOs) since each transmitter has its own oscillator, leading to serious inter-carrier interference (ICI) at the receiver end. Therefore, an ICI self-cancellation scheme is proposed in this thesis, where two different relay nodes use different sub-carriers. When the signals from different relay nodes are combined at the destination node, the ICI self-cancellation can be achieved. In addition, the quality of the received signal can be further improved if the residual CFO can be properly compensated. Traditionally, the medium value of the various CFOs is taken for compensation because of its simplicity. However, a medium value does not result in the optimal performance. In this thesis, a close form expression of optimal CFO is derived to maximize the average signal to interference power ratio. It is shown that the optimal CFO compensation is a function of channel state and individual CFOs. Simulation experiments are conducted to investigate the performance of the proposed scheme. It is shown that the system bit error rate can be substantially improved when the CFO is less than 0.3 subcarrier spacing.

Ingeniero Civil Eléctrico / Los constantes y cada vez más acelerados avances tecnológicos, han generado que los sistemas de comunicación se optimicen considerablemente con el transcurso de los años. Dentro de los cambios más importantes que se han visto en el último tiempo, destaca la disminución del uso de los clásicos sistemas de telecomunicaciones basados en portadoras únicas, dándose paso a sistemas más complejos en donde la información se transmite utilizando múltiples portadoras. Dentro de este último grupo de técnicas, uno de los que más sobresale es orthogonal frequency division multiplexing (OFDM), el cual ha sido y es, ampliamente utilizado en múltiples aplicaciones o estándares de comunicación. El uso extensivo de OFDM se debe a varias ventajas que esta técnica posee, tales como: alcanzar altas tasas de transmisión de datos, generar señales robustas ante canales inalámbricos, tener una alta eficiencia espectral, entre otros. Pero, aun cuando éstos exhiben múltiples ventajas, también presentan ciertos inconvenientes que deben ser tratados como lo son: los altos niveles de peak-to-average power ratio (PAPR) que caracterizan a las señales OFDM y la sensibilidad a errores originados por la desincronización entre el transmisor y receptor. Esto último, facilita la generación de interferencia entre portadoras (inter-carrier interference, ICI) y, por ende, un aumento en la probabilidad de error. En este trabajo se examina el funcionamiento de los sistemas basados en OFDM, partiendo desde la generación de la señal, hasta su comportamiento espectral. Pero por otro lado, también se analiza como la implementación de pulsos que cumplen con el primer criterio de Nyquist (Nyquist-I), favorece al rendimiento de esta clase de sistemas. El uso de pulsos Nyquist-I para combatir los inconvenientes típicos mostrados por los sistemas basados en OFDM ha sido propuesto por múltiples investigadores. En este trabajo se estudia en detalle una nueva familia de pulsos Nyquist-I llamada improved parametric linear combination pulses (IPLCP), la cual se propone para combatir un completo listado de aspectos perjudiciales mostrados por sistemas OFDM reales, a diferencia de otros pulsos que solamente buscan solucionar uno de ellos. Para analizar el rendimiento de la nueva familia de pulsos, se le compara con otras ya conocidas en términos del ICI, la razón señal a interferencia (signal-to-interference ratio, SIR), el PAPR y la tasa de probabilidad de error de bit (bit error rate, BER). Finalmente, el análisis demuestra que la nueva familia es la que mejor se desempeña en promedio en términos de los parámetros de estudio recién mencionados, dejando en claro que la implementación del IPLCP favorece al rendimiento de los sistemas de comunicación basados en la tecnología OFDM.

Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme that provides efficient bandwidth utilization and robustness against time dispersive channels. This thesis deals with self-interference, or the corruption of desired signal by itself, in OFDM systems. Inter-symbol Interference (ISI) and Inter-carrier Interference (ICI) are two types of self-interference in OFDM systems. Cyclic prefix is one method to prevent the ISI which is the interference of the echoes of a transmitted signal with the original transmitted signal. The length of cyclic prefix required to remove ISI depends on the channel conditions, and usually it is chosen according to the worst case channel scenario. Methods to find the required parameters to adapt the length of the cyclic prefix to the instantaneous channel conditions are investigated. Frequency selectivity of the channel is extracted from the instantaneous channel frequency estimates and methods to estimate related parameters, e.g. coherence bandwidth and Root-mean-squared (RMS) delay spread, are given. These parameters can also be used to better utilize the available resources in wireless systems through transmitter and receiver adaptation. Another common self-interference in OFDM systems is the ICI which is the power leakage among different sub-carriers that degrades the performance of both symbol detection and channel estimation. Two new methods are proposed to reduce the effect of ICI in symbol detection and in channel estimation. The first method uses the colored nature of ICI to cancel it in order to decrease the error rate in the detection of transmitted symbols, and the second method reduces the effect of ICI in channel estimation by jointly estimating the channel and frequency offset, a major source of ICI.

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Title from PDF of title page. / Document formatted into pages; contains 93 pages. / Thesis (M.S.E.E.)--University of South Florida, 2003. / Includes bibliographical references. / Text (Electronic thesis) in PDF format. / ABSTRACT: Orthogonal Frequency Division Multiplexing (OFDM) is a multi-carrier modulation scheme that provides efficient bandwidth utilization and robustness against time dispersive channels. This thesis deals with self-interference, or the corruption of desired signal by itself, in OFDM systems. Inter-symbol Interference (ISI) and Inter-carrier Interference (ICI) are two types of self-interference in OFDM systems. Cyclic prefix is one method to prevent the ISI which is the interference of the echoes of a transmitted signal with the original transmitted signal. The length of cyclic prefix required to remove ISI depends on the channel conditions, and usually it is chosen according to the worst case channel scenario. Methods to find the required parameters to adapt the length of the cyclic prefix to the instantaneous channel conditions are investigated. / ABSTRACT: Frequency selectivity of the channel is extracted from the instantaneous channel frequency estimates and methods to estimate related parameters, e.g. coherence bandwidth and Root-mean-squared (RMS) delay spread, are given. These parameters can also be used to better utilize the available resources in wireless systems through transmitter and receiver adaptation. Another common self-interference in OFDM systems is the ICI which is the power leakage among different sub-carriers that degrades the performance of both symbol detection and channel estimation. Two new methods are proposed to reduce the effect of ICI in symbol detection and in channel estimation. The first method uses the colored nature of ICI to cancel it in order to decrease the error rate in the detection of transmitted symbols, and the second method reduces the effect of ICI in channel estimation by jointly estimating the channel and frequency offset, a major source of ICI. / System requirements: World Wide Web browser and PDF reader. / Mode of access: World Wide Web.

The growing interest for high data rate wireless communications over the last few decades gave rise to the emergence of a number of wideband wireless systems. The resulting scarcity of frequency spectrum has been forcing wireless system designers to develop methods that will push the spectral elciency to its limit. One such method is to have multiple systems utilize the same spectrum by allowing some unavoidable interference to occur between them. The idea of co-channel systems is tested in the industrial, scientific and medical (ISM) bands and it is found to be a very beneficial approach. Therefore, it can be foreseen that co-channel systems might be a potential solution to the growing spectral crowding problem. Besides the systems that are designed to be co-channel, it is sometimes also possible to encounter that multiple systems occupy the same band undesirably. This kind of unintentional co-channel system scenarios might occur especially due to the dense re-use of available frequency bands. Another reason for unwanted co-channel usage might be the coexistence of third generation (3G) and fourth generation (4G) systems. Since 4G systems will probably be targeting to use the same frequency bands as their 3G counterparts, and since the transition from 3G to 4G will take some time, unintentional co-channel scenarios might be observed between the 3G and 4G systems. This dissertation consists of baseband receiver algorithms for OFDMA-based systems that target at handling the potential co-channel interference (CCI) in various co-channel system scenarios. Three CCI avoidance and two CCI cancellation algorithms are proposed that can be applied to intentional and unintentional co-channel systems. Femtocells, which have recently been introduced as a new class of personal-use base stations that can coexist with macrocell networks in a shared spectrum manner, might constitute an appropriate example for both types of co-channel systems. Therefore, they are considered to be one of the co-existing systems in most of the algorithms presented.

abstract: Underwater acoustic communications face significant challenges unprecedented in radio terrestrial communications including long multipath delay spreads, strong Doppler effects, and stringent bandwidth requirements. Recently, multi-carrier communications based on orthogonal frequency division multiplexing (OFDM) have seen significant growth in underwater acoustic (UWA) communications, thanks to their well well-known robustness against severely time-dispersive channels. However, the performance of OFDM systems over UWA channels significantly deteriorates due to severe intercarrier interference (ICI) resulting from rapid time variations of the channel. With the motivation of developing enabling techniques for OFDM over UWA channels, the major contributions of this thesis include (1) two effective frequencydomain equalizers that provide general means to counteract the ICI; (2) a family of multiple-resampling receiver designs dealing with distortions caused by user and/or path specific Doppler scaling effects; (3) proposal of using orthogonal frequency division multiple access (OFDMA) as an effective multiple access scheme for UWA communications; (4) the capacity evaluation for single-resampling versus multiple-resampling receiver designs. All of the proposed receiver designs have been verified both through simulations and emulations based on data collected in real-life UWA communications experiments. Particularly, the frequency domain equalizers are shown to be effective with significantly reduced pilot overhead and offer robustness against Doppler and timing estimation errors. The multiple-resampling designs, where each branch is tasked with the Doppler distortion of different paths and/or users, overcome the disadvantages of the commonly-used single-resampling receivers and yield significant performance gains. Multiple-resampling receivers are also demonstrated to be necessary for UWA OFDMA systems. The unique design effectively mitigates interuser interference (IUI), opening up the possibility to exploit advanced user subcarrier assignment schemes. Finally, the benefits of the multiple-resampling receivers are further demonstrated through channel capacity evaluation results. / Dissertation/Thesis / Ph.D. Electrical Engineering 2011

Orthogonal Frequency Division Multiplexing (OFDM) is an efficient multi- carrier modulation scheme, which has been adopted for several wireless stan- dards. Systems employing this scheme at the physical layer are sensitive to frequency offsets and that causes Inter Carrier Interference (ICI) and degra- dation in overall system performance of OFDM systems. In this thesis work, an investigation on impairments of OFDM systems will be carried out. Anal- ysis of previous schemes for cancellation of the ICI will be done and a scheme for estimating and compensating the frequency offset based on Unscented Ka- man Filter (UKF) and Extended Kaman Filter (EKF) will be implemented. Analysis on how the UKF improves the Signal to Noise Ratio (SNR); and how well it tracks the frequency offset estimation under Additive White Gaussian Noise (AWGN) channel and flat fading Rayleigh channel will be carried on.

This thesis present a new receiver framework for the cyclic-prefix free (CP-free) MIMO-OFDM system, equipped with the space-time block coded (ST-BC) uplink transmission over (slowly) time varying multipath channels. Usually, without CP in the OFDM system the inter-carrier interference (ICI) could not be removed, effectively, at the receiver, when the inter-symbol-interference (ISI) has to be taken into account. In this thesis, by exploiting the spatial and frequency resources, we propose a novel frequency-domain decision-feedback equalizer, associated with the oblique projection (OB), to combat the effects of ISI and ICI, simultaneously. The OB is a non-orthogonal projection and is very useful to deal with the structure noise (e.g., the ISI term). From computer simulations, we observe that the performance of propose scheme can perform very close to the conventional CP-based MMO-OFDM with the ST-BC.