Transmit beamforming (precoding) is a powerful technique for enhancing the channel capacity and reliability of multiple-input and multiple-output (MIMO) wireless systems. The optimum exploitation of the benefits provided by MIMO systems can be achieved when a perfect channel state information at transmitter (CSIT) is available. In practices, however, the channel knowledge is generally imperfect at transmitter because of the inevitable errors induced by finite feedback channel capacity, quantization and other physical constraints. Such errors degrade the system performance severely. Hence, robustness has become a crucial issue. Current robust designs address the channel imperfections with the worst-case and stochastic approaches. In worst-case analysis, the channel uncertainties are considered as deterministic and norm-bounded, and the resulting design is a conservative optimization that guarantees a certain quality of service (QoS) for every allowable perturbation. The latter approach focuses on the average performance under the assumption of channel statistics, such as mean and covariance. The system performance could break down when persistent extreme errors occur. Thus, an outage probability-based approach is developed by keeping a low probability that channel condition falls below an acceptable level. Compared to the aforementioned methods, this approach can optimize the average performance as well as consider the extreme scenarios proportionally. This thesis implements the outage-probability specification into transmit beamforming design for three scenarios: the single-user MIMO system and the corresponding adaptive modulation scheme as well as the multi-user MIMO system. In a single-user MIMO system, the transmit beamformer provides the maximum average received SNR and ensures the robustness to the CSIT errors by introducing probabilistic constraint on the instantaneous SNR. Beside the robustness against channel imperfections, the outage probability-based approach also provides a tight BER bound for adaptive modulation scheme, so that the maximum transmission rate can be achieved by taking advantage of transmit beamforming. Moreover, in multi-user MIMO (MU-MIMO) systems, the leakage power is accounted by probability measurement. The resulting transmit beamformer is designed based on signal-to-leakage-plus-noise ratio (SLNR) criteria, which maximizes the average received SNR and guarantees the least leakage energy from the desired user. In such a setting, an outstanding BER performance can be achieved as well as high reliability of signal-to-interference-plus-noise ratio (SINR). Given the superior overall performances and significantly improved robustness, the probabilistic approach provides an attractive alternative to existing robust techniques under imperfect channel information at transmitter.

Swath bathymetry sidescan (SBS) sonar and the angle-of-arrival processing that underlies these systems has the capability to produce much higher resolution three dimensional imagery and bathymetry than traditional beamformed approaches. However, the performance of these high resolution systems is limited by signal-to-noise ratio (SNR) and they are also susceptible to multipath interference. This thesis explores two methods for increasing SNR and mitigating multipath interference for SBS systems. The first, binary coded pulse transmission and pulse compression is shown to increase the SNR and in turn provide reduced angle variance in SBS systems. The second, transmit beamforming, and more specifically steering and shading, is shown to increase both acoustic power in the water and directivity of the transmitted acoustic radiation. The transmit beamforming benefits are achieved by making use of the 8-element linear angle-of-arrival array typical in SBS sonars, but previously not utilized for transmit. Both simulations and real world SBS experiments are devised and conducted and it is shown that in practice pulse compression increases the SNR, and that transmit beamforming increases backscatter intensity and reduces the intensity of interfering multipaths. The improvement in achievable SNR and the reduction in multipath interference provided by the contributions in this thesis further strengthens the importance of SBS systems and angle-of-arrival based processing, as an alternative to beamforming, in underwater three dimensional imaging and mapping. / Graduate / 2015-04-15 / 0544 / 0547 / mark.butowski@gmail.com

Swath bathymetry sidescan (SBS) sonar and the angle-of-arrival processing that underlies these systems has the capability to produce much higher resolution three dimensional imagery and bathymetry than traditional beamformed approaches. However, the performance of these high resolution systems is limited by signal-to-noise ratio (SNR) and they are also susceptible to multipath interference. This thesis explores two methods for increasing SNR and mitigating multipath interference for SBS systems. The first, binary coded pulse transmission and pulse compression is shown to increase the SNR and in turn provide reduced angle variance in SBS systems. The second, transmit beamforming, and more specifically steering and shading, is shown to increase both acoustic power in the water and directivity of the transmitted acoustic radiation. The transmit beamforming benefits are achieved by making use of the 8-element linear angle-of-arrival array typical in SBS sonars, but previously not utilized for transmit. Both simulations and real world SBS experiments are devised and conducted and it is shown that in practice pulse compression increases the SNR, and that transmit beamforming increases backscatter intensity and reduces the intensity of interfering multipaths. The improvement in achievable SNR and the reduction in multipath interference provided by the contributions in this thesis further strengthens the importance of SBS systems and angle-of-arrival based processing, as an alternative to beamforming, in underwater three dimensional imaging and mapping. / Graduate / 0544 / 0547 / mark.butowski@gmail.com

Wireless sensor networks has been one of the major research topics in recent years because of its great potential for a wide range of applications. In some application scenarios, sensor nodes intend to report the sensing data to a far-field destination, which cannot be realized by traditional transmission techniques. Due to the energy limitations and the hardware constraints of sensor nodes, distributed transmit beamforming is considered as an attractive candidate for long-range communications in such scenarios as it can reduce energy requirement of each sensor node and extend the communication range. However, unlike conventional beamforming, which is performed by a centralized antenna array, distributed beamforming is performed by a virtual antenna array composed of randomly located sensor nodes, each of which has an independent oscillator. Sensor nodes have to coordinate with each other and adjust their transmitting signals to collaboratively act as a distributed beamformer. The most crucial problem of realizing distributed beamforming is to achieve carrier phase alignment at the destination. This thesis will investigate distributed beamforming from both theoretical and practical aspects. First, the bit error ratio performance of distributed beamforming with phase errors is analyzed, which is a key metric to measure the system performance in practice. We derive two distinct expressions to approximate the error probability over Rayleigh fading channels corresponding to small numbers of nodes and large numbers of nodes respectively. The accuracy of both expressions is demonstrated by simulation results. The impact of phase errors on the system performance is examined for various numbers of nodes and different levels of transmit power. Second, a novel iterative algorithm is proposed to achieve carrier phase alignment at the destination in static channels, which only requires one-bit feedback from the destination. This algorithm is obtained by combining two novel schemes, both of which can greatly improve the convergence speed of phase alignment. The advantages in the convergence speed are obtained by exploiting the feedback information more efficiently compared to existing solutions. Third, the proposed phase alignment algorithm is modified to track time-varying channels. The modified algorithm has the ability to detect channel amplitude and phase changes that arise over time due to motion of the sensors or the destination. The algorithm can adjust key parameters adaptively according to the changes, which makes it more robust in practical implementation.

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Abstract This thesis considers data transmission in a full-duplex (FD) multiuser multiple-input multiple-output (MU-MIMO) system, where a FD capable base station (BS) bidirectionally communicates with multiple half-duplex (HD) users in downlink (DL) and uplink (UL) channels using the same radio resources. The main challenge in FD communications is how to deal with the self-interference (SI) between transmit and receive antennas at the BS. The work carried out in the thesis is motivated by recent advanced techniques in hardware design demonstrating that the SI can be suppressed to a degree that possibly allows for FD transmission in cellular networks. In particular, this thesis attempts to explore the potential gains in terms of the spectral efficiency (SE) and energy efficiency (EE) that can be brought by the FD MU-MIMO model. As the first of its kinds, the thesis aims to present a solid mathematical framework and report interesting results that foster research on wireless communications in general and FD communications in particular. For the FD system of interest the major challenge is due to the SI and co-channel interference from users in the UL channel to the ones in the DL channel, resulting in the coupling between the two channels. As a result we are concerned with the problem of joint transmit processing design to maximize the SE and EE subject to certain power constraints. Since the design problems are natually non-convex, it is difficult to find the globally optimal solutions or even when possible it is not practically appealing. Our contributions to solving these design problems are on the development of several iterative algorithms that can obtain locally optimal solutions. The proposed algorithms are built upon a framework of local optimization strategies such as the sequential parametric convex approximation and the Frank-Wolfe methods. In special cases closed-form designs are also presented. The reported results show that when the SI is sufficiently suppressed the considered FD MU-MIMO system with the proposed SE designs achieves a significantly better SE but consumes more energy, compared to the HD counterpart. In terms of EE the proposed EE scheme is superior to the proposed SE design. Moreover, in the low transmit power region, the EE design achieves a worse EE than the HD system but a better one in the high trasmit power regime when the SI power is low. / Tiivistelmä Tämä väitöskirja käsittelee datansiirtoa samanaikaisesti kaksisuuntaisessa (full-duplex, FD) usean käyttäjän moniantennijärjestelmässä (MU-MIMO), jossa FD-kykyinen tukiasema on yhtä aikaa yhteydessä vuorosuuntaisten (half-duplex, HD) käyttäjien kanssa laskevalla (DL) ja nousevalla (UL) siirtotiellä käyttäen samoja radioresursseja. FD-kommunikaation suurin haaste liittyy lähetys- ja vastaanottoantennien välisen omahäiriön (SI) hallintaan. Tässä työssä hyödynnetään tuoreita tutkimustuloksia, joissa edistyneillä häiriönvaimennustekniikoilla on kyetty vaimentamaan omahäiriö tasolle, jolla FD-lähetys solukkoverkoissa on toteutuskelpoista. Tässä työssä tutkitaan etenkin mahdollisia FD MU-MIMO –järjestelmän tuomia suorituskykyparannuksia spektrinkäytön tehokkuudessa (SE) ja energiatehokkuudessa (EE). Väitöskirjalla on uutuusarvoa matemaattisessa suorituskykyarvioinnissa ja työn mielenkiintoiset tulokset edistävät jatkotutkimusta aiheen ympärillä. Tutkittavan FD-järjestelmän merkittävänä haasteena on omahäiriön ja muiden käyttäjien siirtosuuntien välisen samankanavan häiriön yhteisvaikutus, jonka johdosta siirtosuunnat kytkeytyvät toisiinsa. Tämä johtaa lähetysprosessoinnin yhteisoptimointiin, jossa spektri- ja energiatehokkuus pyritään maksimoimaan määritetyillä tehorajoituksilla. Nämä suunnitteluongelmat eivät ole luonteeltaan konvekseja, joten niihin on vaikeaa löytää globaalisti optimaalisia ratkaisuja ja vaikka onnistuisikin niin ne eivät yleensä ole käytännöllisiä. Työssä esitetään useita iteratiivisia algoritmejä, joilla saavutetaan paikallisesti optimaalisia ratkaisuja. Ehdotetut algoritmit pohjautuvat paikallisten optimointistrategioiden viitekehykseen, jossa käytetään esimerkiksi peräkkäistä parametristä konveksiapproksimaatiota ja Frank-Wolfe –menetelmiä. Erityistapauksissa suljetun muodon ratkaisut on myös esitetty. Raportoidut tulokset osoittavat, että omahäiriön ollessa riittävästi vaimennettu mallinnetulla järjestelmällä saavutetaan spektrinkäytön optimointimielessä huomattavaa etua HD-verrokkiin lisääntyneen energian kulutuksen kustannuksella. Energiatehokkuuden optimointiin pohjautuvalla strategialla puolestaan päästään suurempiin suorituskykyetuihin. Pienillä lähetystehoilla energiatehokkuus voi kuitenkin olla HD-järjestelmää alempi, mutta vastaavasti suurten lähetystehojen alueella tilanne on päinvastainen kunhan omahäiriön teho on tarpeeksi alhainen.