30 research outputs found
System design and validation of multi-band OFDM wireless communications with multiple antennas
[no abstract
Adaptive equalizers for multipath compensation in digital microwave communications
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The theory of bootstrapped algorithms and their applications to cross polarization interference cancelation
Dual-polarized transmission has become an important method for frequency re-use, particularly in satellite and microwave radio communication. Nevertheless, cross-polarization interference, which is inherent to this method, may cause degradation in system performance.
Different canceler [sic] structures have been proposed to mitigate the effect of cross-polarization. Among these are the diagonalizer, the least mean square (LMS) canceler [sic] and the bootstrapped cancelers [sic]. Bootstrapped canceler [sic] schemes have been proposed and implemented in different applications, such as satellites, tactical communications, and quadrature amplitude madulation [sic] (QAM) dual polarized microwave radio. Nevertheless, no attempt was made in the past to quantify the probability of error of dual polarized transmission systems when such cancelers [sic] are used, nor were important issues such as stability and the dynamic behavior of algorithms controlling such cancelers [sic] studied.
In this thesis, the error probability performance of dual polarized QAM transmission, for nondispersive fading channels and different configurations of bootstrapped cross-pol cancelers [sic], is derived and compared to the performance for other cancelers [sic]. Stability analyses of different canceler [sic] configurations are investigated, and an application of orthogonal perturbation sequences in controlling the bootstrapped cancelers [sic] is considered.
It is shown that the error probability performance of the bootstrapped canceler [sic] is always better than that of other cancellers, such as the LMS canceler [sic]. It is also shown that, when the bootstrapped canceler [sic] is designed to meet certain conditions, it is asymptotically stable in converging to the calculated optimal points. Controlling the cancelers [sic] with adaptive algorithms using orthogonal dithering sequences is shown to be satisfactory; the canceler [sic] converges in the mean to the optimal condition.
The results indicate that bootstrapped algorithms are faster than other algorithms. Considering the fact such cancelers [sic] do not require decision feedback for their operation, we can conclude that bootstrapped algorithms are not only advantageous for cross polarization cancelation [sic], but perhaps suitable for other adaptive signal processing applications, as well
Adaptive bootstrap signal separators for BPSK/QAM-modulated wireless CDMA systems in a multipath environment
CDMA is an attractive multiple-access scheme, because of its potential capacity increase and its anti-multipath fading capability. For satisfactory performance, however, the effect of the near-far problem has to be resolved. This problem can be combated by using power-control, which, however, results in an overall reduction in communication ranges, and thus in a loss of capacity. Among other methods for mitigating the near-far problem is the use of decorrelating receivers, both of fixed type, which directly utilizes the cross-correlation of the users codes, and of adaptive type, which uses recursive algorithms that leads to signal decorrelation. Not to lessen the importance of other adaptive algorithms, the current research concentrates on what was termed in the literature bootstrap algorithm . Although the emphasis will be on applying the adaptive bootstrap decorrelator, the fixed type will be used primarily to provide comparison. Also used for comparison are both blind adaptive and training sequence based MMSE.
Most of the literature on multiuser detection has been assuming BPSK. However, a need for transferring wideband data demands using modulation schemes with high bits/cycle, such as QAM. Therefore, modification of the receiver is considered, so that QAM-modulation can be applied efficiently, using the complex signal approach of this modulation.
For the asynchronous channel, vast amounts of research have been devoted to using one-shot matched filter banks followed by conventional decorrelators which implement the inverse of some (partial) correlation matrix. In this work, an adaptive bootstrap version is presented, which is suitable for the one-shot structure shown previously to be more robust to errors in delay estimation. It has also been noted that such a correlation matrix can, depending on the channel characteristics, become ill-conditioned or even singular. Therefore, another matched filtering structure, followed by what is called a multishot conventional (fixed type) decorrelator, has been previously suggested to mitigate this singularity problem. However, the fixed type of the multishot decorrelator is expected to have similar non-robustness to errors in delay estimation as was previously shown for the one-shot. Therefore, the adaptive multishot bootstrap decorrelator is presented and evaluated. Also, by adding an adaptive canceler, an extension to the above matched filter-decorrelator combination, will be proposed and evaluated. A multipath time-variant fading environment will be used in some of these performance evaluations.
Finally, when handling multipath channels, the question is raised whether path combining should be done before or after the signals are decorrelated. For the asynchronous case, a one-shot extension of the bootstrap algorithm is presented, which is capable of decorrelating the signals from resolved paths of different users, to facilitate the decorrelate before combining case
Pattern Diversity Characterization of Reconfigurable Antenna Arrays for Next Generation Wireless Systems
The use of multi-antenna technology in wireless radio communications has attracted tremendous attention due to its potential to increase data rates without requiring additional bandwidth and transmission power. This has been driven by the burgeoning demand for high data rates and the need for instantaneous and ubiquitous access to information. It is therefore no surprise that current and future generation wireless standards such as LTE and WiMAX have adopted the use of adaptive multi-antenna systems also known as adaptive Multiple Input and Multiple Output (MIMO) as their de facto transmission technology. In this thesis work, we focus on the design of a smart wireless antenna system, and the study of relevant techniques that enable us to reap the benefits of their deployment in small wireless devices with MIMO capability. Specifically, we employ a new class of adaptive antenna systems known as Reconfigurable Antenna Systems (RAS) for portable devices. These antennas are capable of dynamically changing their electrical and radiation characteristics to suit the conditions of the wireless channel. The changing radiation patterns lead to pattern diversity gains that improve system performance. This is in contrast to conventional non-reconfigurable arrays which depend on signal processing techniques such as antenna grouping and beamforming to achieve performance gains. However, despite the demonstrable system-level performance benefits of RAS in adaptive MIMO, few of these antennas have been adopted and integrated in state-of-the-art wireless standards. Their usage has been partly inhibited by the prohibitive costs of implementation and operation in a real wireless infrastructure. As part of this thesis research effort we attempt to integrate these new antennas into a cost-effective real wireless MIMO testbed for use in current generation technologies. The solution integration is carried-out through the use of readily available software-defined radio frameworks. We first design, analyze and characterize the pattern diversity in RAS antenna arrays that resonate at frequencies suitable for 4G applications. We then study the benefits of pattern diversity obtained from RAS arrays over conventional space diversity approaches such as antenna grouping and beamforming. This dissertation also presents low-complexity adaptive physical layer models and algorithms to exploit the benefits of RAS array integration in MIMO wireless systems. We implement these algorithms in software-defined radio frameworks, experimentally test, and benchmark them against other established approaches in literature. And finally, integrate and test these RAS array design prototypes as part of the MIMO wireless system that leverages a state-of-the-art wireless base station and mobile terminals.Ph.D., Electrical Engineering -- Drexel University, 201
Multi-user MIMO wireless communications
Mehrantennensysteme sind auf Grund der erhöhten Bandbreiteneffizienz und
Leistung eine SchlĂŒsselkomponente von Mobilfunksystemen der Zukunft. Diese
ermöglichen das gleichzeitige Senden von mehreren, rÀumlich getrennten
Datenströmen zu verschiedenen Nutzern. Die zentrale Fragestellung in der Praxis
ist, ob der ursprĂŒnglich vorausgesagte KapazitĂ€tsgewinn in realistischen
Szenarios erreicht wird und welche spezifischen Gewinne durch zusÀtzliche
Antennen und das Ausnutzen von Kanalkenntnis am Sender und EmpfÀnger erzielt
werden, was andererseits einen Zuwachs an Overhead oder nötiger Rechenleistung
bedeutet.
In dieser Arbeit werden neue lineare und nicht-lineare MU-MIMO Precoding-
Verfahren vorgestellt. Der verfolgte Ansatz zur Bestimmung der Precoding-
Matrizen ist allgemein anwendbar und die entstandenen Algorithmen können zur
Optimierung von verschiedenen Kriterien mit beliebig vielen Antennen an der
Mobilstation eingesetzt werden. Das wurde durch die Berechnung der Precoding-
Matrix in zwei Schritten erreicht. Im ersten Schritt wird die Ăberschneidung der
ZeilenrÀume minimiert, die durch die effektiven Kanalmatrizen verschiedener
Nutzer aufgespannt werden. Basierend auf mehreren parallelen Einzelnutzer-MIMO-
KanĂ€len wird im zweiten Schritt die Systemperformanz bezĂŒglich bestimmter
Kriterien optimiert.
Aus der gĂ€ngigen Literatur ist bereits bekannt, dass fĂŒr Nutzer mit nur einer
Antenne das MMSE Kriterium beim precoding optimal aber nicht bei Nutzern mit
mehreren Antennen. Deshalb werden in dieser Arbeit zwei neue Mehrnutzer MIMO
Strategien vorgestellt, die vom MSE Kriterium abgeleitet sind, nÀmlich
sukzessives MMSE und RBD. Bei der sukzessiven Verarbeitung mit einer
entsprechenden Anpassung der Sendeleistungsverteilung kann die volle DiversitÀt
des Systems ausgeschöpft werden. Die KapazitÀt nÀhert sich dabei der maximalen
Summenrate des Systems an. Bei gemeinsamer Verarbeitung der MIMO KanÀle wird
unabhÀngig vom Grad der Mehrnutzerinterferenz die maximale DiversitÀt erreicht.
Die genannten Techniken setzen entweder eine aktuelle oder eine ĂŒber einen
lĂ€ngeren Zeitraum gemittelte Kanalkenntnis voraus. Aus diesem Grund mĂŒssen die
Auswirkungen von Kanal-SchĂ€tzfehlern und EinflĂŒsse des Transceiver Front-Ends
auf die Verfahren nÀher untersucht werden.
FĂŒr eine weitergehende AbschĂ€tzung der Mehrantennensysteme muss die Performanz
des Gesamtsystems untersucht werden, da viele EinflĂŒsse auf die rĂ€umliche
Signalverarbeitung bei Betrachtung eines einzelnen Links nicht erkennbar sind.
Es wurde gezeigt, dass mit MIMO Precoding Strategien ein Vielfaches der
Datenrate eines Systems mit nur einer Antenne erzielt werden kann, wÀhrend der
Overhead durch Pilotsymbole und Steuersignale nur geringfĂŒgig zunimmt.Multiple-input, multiple-output (MIMO) systems are a key component of future
wireless communication systems, because of their promising improvement in terms
of performance and bandwidth efficiency. An important research topic is the
study of multi-user (MU) MIMO systems. Such systems have the potential to
combine the high throughput achievable with MIMO processing with the benefits of
space division multiple access (SDMA). The main question from a practical
standpoint is whether the initially predicted capacity gains can be obtained in
more realistic scenarios and what specific gains result from adding more
antennas and overhead or computational power to obtain channel state information
(CSI) at the transceivers.
In this thesis we introduce new linear and non-linear MU MIMO processing
techniques. The approach used for the design of the precoding matrix is general
and the resulting algorithms can address several optimization criteria with an
arbitrary number of antennas at the user terminals (UTs). This is achieved by
designing the precoding matrices in two steps. In the first step we minimize the
overlap of the row spaces spanned by the effective channel matrices of different
users. In the next step, we optimize the system performance with respect to the
specific optimization criterion assuming a set of parallel single-user MIMO
channels.
As it was previously reported in the literature, minimum mean-squared-error
(MMSE) processing is optimum for single-antenna UTs. However, MMSE suffers from
a performance loss when users are equipped with more than one antenna. The two
MU MIMO processing techniques that result from the two different MSE criteria
that are proposed in this thesis are successive MMSE and regularized block
diagonalization. By iterating the closed form solution with appropriate power
loading we are able to extract the full diversity in the system and empirically
approach the maximum sum-rate capacity in case of high multi-user interference.
Joint processing of MIMO channels yields maximum diversity regardless of the
level of multi-user interference.
As these techniques rely on the fact that there is either instantaneous or long-
term CSI available at the base station to perform precoding and decoding, it was
very important to investigate the influence of the transceiver front-end
imperfections and channel estimation errors on their performance.
For a comprehensive assessment of multi-antenna techniques, it is mandatory to
consider the performance at system level, since many effects of spatial
processing are not tractable at the link level. System level investigations have
shown that MU MIMO precoding techniques provide several times higher data rates
than single-input single-output systems with only slightly increased pilot and
control overhead
Adaptive interference cancelation techniques for multicarrier modulated systems
Current wireline systems and wireless broadcasting systems employ multicarrier modulation (MCM). This includes the high-rate digital subscriber line (HDSL), digital audio broadcasting system (DAB) and the digital terrestrial television broadcasting system (dTTb). Multicarrier modulation is also envisioned for high-speed indoor wireless local area networks (WLAN). Additionally, multicarrier code division multiple access (MC-CDMA), a hybrid of orthogonal frequency division multiplexing (OFDM) and CDMA, is proposed for the downlink (base-to-mobile) of a 3rd generation wireless system as part of the IMT-2000 standardization process.
The performance of an MC-CDMA system--similar to a direct sequence CDMA (DS-CDMA) system--is limited by the presence of multiple access interference (MAI) . Downlink communications also suffers from MAI as a result of the multipath channel effect, even if it implements orthogonal code multiplexing. Additionally, transmissions aimed at different mobile users may be assigned different powers in order to increase the system capacity, essentially creating a near-far problem for some users.
Due to the MC-CDMA signal structure the conventional decorrelator (based on the inverse of the correlation matrix) is dependent on the channel coefficients, suggesting the use of an adaptive multiuser detector, which can track a time-variant channel. The performance of a blind adaptive multiuser detector for MC-CDMA, based on the bootstrap algorithm, is investigated and compared to the performance of the conventional decorrelator. Additionally, the performance is investigated for different channel conditions. First, for a non-faded flat additive white Gaussian noise (AWGN) channel. Second, for a frequency selective channel with and without correlation between the channel coefficients at the different subcarriers.
In general, the mobile terminal suffers from limited available resources such as computing power or battery life and, therefore, cannot accommodate the same level of receiver complexity as the base station. For the downlink, however, the received signal structure is less complex due to the assumed synchronized transmission. Moreover, the mobile receiver is merely required to detect the desired user\u27s data stream. To reduce the complexity, detectors are proposed that do not require knowledge of the active users nor their respective codes, but rather use a combined code to represent all the interfering users at once. The performance of the reduced complexity conventional decorrelator is compared to the performance of an adaptive reduced complexity detector using the bootstrap algorithm. The performance of these detectors is also investigated for the aforementioned channel types.
For spectral-efficiency, closely spaced subcarriers are used in a multicarrier modulated system. A resulting drawback is a high sensitivity of the performance to a frequency offset. This results from a Doppler shift, due to mobile movement, as well as from a mismatch between the carrier frequencies at the transmitter and receiver. To mitigate this problem an adaptive decorrelator based frequency offset correction scheme is developed for OFDM and its performance is investigated. Additionally, a blind frequency offset estimation and correction structure is proposed based on a stochastic gradient method. The convergence and statistical properties of this estimator are investigated.
A blind adaptive joint multiuser detection and frequency offset correction structure for downlink MC-CDMA is developed. This detector is a combination of the structures for multiuser detection for MC-CDMA and frequency offset correction for OFDM. Moreover, the performance of this detector is investigated and compared to a joint detector based on a minimum mean square error (MMSE) criterion