The use of feedback to provide channel state information to the transmitter can greatly improve the performance of a communication system. However, the amount of information required to characterize a time-varying MIMO channel can exceed the capacity of the feedback channel. This paper surveys research in limited feedback systems, which employ a number of methods to reduce the information and improve performance in multi-antenna communication systems. This paper also presents a new method, Quantized Successor Pre-coding (QSP), that exploits time-correlation to implement spatial multiplexing in a MIMO system using very little feedback. QSP uses an ordered codebook of pre-coders and transmission modes to reduce the feedback to a single bit. Simulations of QSP demonstrate a substantial performance improvement relative to open-loop spatial multiplexing. / text

This dissertation focuses on downlink multi-antenna transmission with packet scheduling in a wireless packet data network. The topic is viewed as a critical system design problem for future high-speed packet networks requiring extremely high spectral efficiency. Our aim is to illustrate the interaction between transmission schemes at the physical layer and scheduling algorithms at the medium access control (MAC) layer from a sum-capacity perspective. Various roles of multiple antennas are studied under channel-aware scheduling, including diversity, beamforming and spatial multiplexing. At a system performance level, our work shows that downlink throughput can be optimized by joint precoding across multiple transmit antennas and exploiting small-scale fading of distributed multiple input and multiple output (MIMO) channels. There are three major results in this dissertation. First, it is shown that over a MIMO Gaussian broadcast channel, and under channel-aware scheduling, open-loop transmit antenna diversity actually reduces the achievable sum rate. This reveals a negative interaction between open-loop antenna diversity and the closed-loop multiuser diversity through scheduling. Second, a suboptimal dirty paper coding (DPC) approach benefits greatly from multiuser diversity by an efficient packet scheduling algorithm. Performance analysis of a suboptimal greedy scheduling algorithm indicates that, compared with the receiver-centric V-BLAST method, it can achieve a much larger scheduling gain over a distributed MIMO channel. Further, pre-interference cancellation allows for transmissions free of error propagation. A practical solution, termed Tomlinson-Harashima precoding (THP), is studied under this suboptimal scheduling algorithm. Similar to V-BLAST, a reordering is applied to minimize the average error rate, which introduces only a negligible sum-rate loss in the scenarios investigated. Third, for an orthogonal frequency division multiplexing (OFDM) system using MIMO precoding, it is shown that a DPC-based approach is readily applicable and can be easily generalized to reduce the peak-to-average power ratio (PAR) up to 5 dB without affecting the receiver design. Simulations show that in an interference-limited multi-cell scenario, greater performance improvement can be achieved by interference avoidance through adaptive packet scheduling, rather than by interference diversity or averaging alone. These findings suggest that, coordinated with channel-aware scheduling, adaptive multiplexing in both spatial and frequency domains provides an attractive downlink solution from a total capacity point of view. / Ph. D.

<p>In this thesis, analytical analysis and design techniques for wireless communications with diversity are studied. The impact of impairments such as correlated fading is analyzed using statistical models. Countermeasures designed to overcome, or even exploit, such effects are proposed and examined. In particular two applications are considered, satellite broadcast to vehicular terminals and communication using transmitters and receivers equipped with multiple antennas.</p><p>Mobile satellite broadcast systems offer the possibility of high data rate services with reliability and ubiquitous coverage. The design of system architectures providing such services requires complex trade-offs involving technical, economical, and regulatory aspects. A satisfactory availability can be ensured using space, terrestrial, and time diversity techniques. The amount of applied diversity affects the spectral efficiency and system performance. Also, dedicated satellite and terrestrial networks represent significant investments and regulatory limitations may further complicate system design.</p><p>The work presented in this thesis provides insights to the technical</p><p>aspects of the trade-offs above. This is done by deriving an efficient method for estimating what resources in terms of spectrum and delay are required for a broadcast service to reach a satisfactory number of end users using a well designed system. The results are based on statistical models of the mobile satellite channel for which efficient analytical design and error rate estimation methods are derived. We also provide insight to the achievable spectral efficiency using different transmitter and receiver configurations.</p><p>Multiple-element antenna communication is a promising technology for future high speed wireless infrastructures. By adding a spatial dimension, radio resources in terms of transmission power and spectrum can be used more efficiently. Much of the design and analysis work has focused on cases where the transmitter either has access to perfect channel state information or it is blind and the spatial channels are uncorrelated.</p><p>Herein, systems where the fading of the spatial channels is correlated and/or the transmitter has access to partial channel state information are considered. While maintaining perfect channel knowledge at the transmitter may prove difficult, updating parameters that change on a slower time scale could be realistic. Here we formulate analysis and design techniques based on statistical models of the multichannel propagation. Fundamental properties of the multi-element antenna channel and limitations given by information theory are investigated under an asymptotic assumption on the number of antennas on either side of the system. For example, limiting normal distributions are derived for the squared singular values of the channel matrix and the mutual information. We also propose and examine a practical scheme capable of exploiting partial channel state information.</p><p>In both applications outlined above, by using statistical models of the channel characteristics in the system design, performance can be improved. The main contribution of this thesis is the development of efficient techniques for estimating the system performance in different scenarios. Such techniques are vital to obtain insights to the impact of different impairments and how countermeasures against these should be designed.</p>

In this thesis, analytical analysis and design techniques for wireless communications with diversity are studied. The impact of impairments such as correlated fading is analyzed using statistical models. Countermeasures designed to overcome, or even exploit, such effects are proposed and examined. In particular two applications are considered, satellite broadcast to vehicular terminals and communication using transmitters and receivers equipped with multiple antennas. Mobile satellite broadcast systems offer the possibility of high data rate services with reliability and ubiquitous coverage. The design of system architectures providing such services requires complex trade-offs involving technical, economical, and regulatory aspects. A satisfactory availability can be ensured using space, terrestrial, and time diversity techniques. The amount of applied diversity affects the spectral efficiency and system performance. Also, dedicated satellite and terrestrial networks represent significant investments and regulatory limitations may further complicate system design. The work presented in this thesis provides insights to the technical aspects of the trade-offs above. This is done by deriving an efficient method for estimating what resources in terms of spectrum and delay are required for a broadcast service to reach a satisfactory number of end users using a well designed system. The results are based on statistical models of the mobile satellite channel for which efficient analytical design and error rate estimation methods are derived. We also provide insight to the achievable spectral efficiency using different transmitter and receiver configurations. Multiple-element antenna communication is a promising technology for future high speed wireless infrastructures. By adding a spatial dimension, radio resources in terms of transmission power and spectrum can be used more efficiently. Much of the design and analysis work has focused on cases where the transmitter either has access to perfect channel state information or it is blind and the spatial channels are uncorrelated. Herein, systems where the fading of the spatial channels is correlated and/or the transmitter has access to partial channel state information are considered. While maintaining perfect channel knowledge at the transmitter may prove difficult, updating parameters that change on a slower time scale could be realistic. Here we formulate analysis and design techniques based on statistical models of the multichannel propagation. Fundamental properties of the multi-element antenna channel and limitations given by information theory are investigated under an asymptotic assumption on the number of antennas on either side of the system. For example, limiting normal distributions are derived for the squared singular values of the channel matrix and the mutual information. We also propose and examine a practical scheme capable of exploiting partial channel state information. In both applications outlined above, by using statistical models of the channel characteristics in the system design, performance can be improved. The main contribution of this thesis is the development of efficient techniques for estimating the system performance in different scenarios. Such techniques are vital to obtain insights to the impact of different impairments and how countermeasures against these should be designed. / QC 20101019

With the rapid growth of wireless data demands and saturation of radio frequency (RF) capacity, visible light communication (VLC) has become a promising candidate to complement conventional RF communication, especially for indoor short range applications. However the performance of the system depends on the propagation and type of system used. An optical Orthogonal Frequency Division Multiplexing (O-OFDM) together with multiple input multiple output (MIMO) in different scenario and modulation techniques are studied in the thesis. A novel optical wireless communication (OWC) multi-cell system with narrow field of view (FOV) is studied. In this system the intensity modulated beam from four light sources are used for communication. The system allows beams to be concentrated in specific areas of the room to serve multiple mobile devices with low interference and hence increase system capacity. The performance of asymmetrically clipped optical orthogonal frequency division multiplexing (ACO-OFDM), direct current biased optical OFDM (DCO-OFDM) and single carrier (SC) modulation are then compared in this system considering single user and multiusers scenarios. The performance of the multi-cell is compared with single cell with wide FOV. It is shown that the capacity for multi-cell system increases with the number of users to 4 times the single user capacity. Also the findings show that multi-cell system with narrow beams can outperform a single wide beam system in terms of coverage area and hence average throughput of about 2.7 times the single wide beam system capacity. One of the impairments in line of sight (LOS) OWC systems is coverage which degrades the performance. A mobile receiver with angular diversity detectors in MIMO channels is studied. The objective is to improve the rank of the channel matrix and hence system throughput. Repetition coding (RC), spatial multiplexing (SMP) and spatial modulation (SM) concepts are used to evaluate throughput across multiple locations in a small room scenario. A novel adaptive spatial modulation (ASM) which is capable of combating channel rank deficiency is devised. Since the receiver is mobile, the channel gains are low in some locations of the room due to the lack of LOS paths between transmitters and receivers. To combat the situation adaptive modulation and per antenna rate control (PARC) is employed to maximise spectral efficiency. The throughputs for fixed transmitters and receivers are compared with the oriented/inclined detectors for different cases. Angular diversity detectors offer a better throughput improvement than the state of the art vertical detectors, for example in ASM angular diversity receiver gives throughput of about 1.6 times that of vertical detectors. Also in SMP the angular detectors offer throughput about 1.4 times that of vertical detectors. SMP gives the best performance compared to RC, SM and ASM, for example SMP gives throughput about 2.5 times that of RC in both vertical detectors and angular diversity receivers. Again SMP gives throughput about 6 times that of SM in both vertical detectors and angular diversity receivers. Also SMP provides throughput about 2 times that of ASM in both vertical detectors and angular diversity receivers. ASM exhibit improvement in throughput about average factor of 3.5 times SM performance in both vertical detectors and angular diversity detectors. As the performance of the system may be jeopardized by obstructions, specular and diffuse reflection models for indoor OWC systems using a mobile receiver with angular diversity detectors in MIMO channels are considered. The target is to improve the MIMO throughput compared to vertically oriented detectors by exploiting reflections from different reflecting surfaces in the room. The throughput across multiple locations in the small room by using RC, SMP and SM approaches is again evaluated. The results for LOS only channels against LOS with specular or diffuse reflection conditions, for both vertical and angular oriented receivers are then compared. The results show that exploiting specular and diffuse reflections provide significant improvements in link performance. For example the reflection coefficient (α) of 0.9 and the antenna separation of 0.6 m, RC diffuse model shows throughput improvement of about 1.8 times that of LOS for both vertical detectors and angular diversity receivers. SM diffuse model shows throughput improvement of about 3 times that of LOS for both vertical detectors and angular diversity receivers. ASM diffuse model shows throughput improvement of about 2 times that of LOS for both vertical detectors and angular diversity receivers. SMP diffuse model shows throughput improvement of about 1.5 times that of LOS for both vertical detectors and angular diversity receiver.

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The interest in wireless communications among consumers has exploded since the introduction of the "3G" cell phone standards. One reason for their success is the increasingly higher data rates achievable through the networks. A further increase in data rates is possible through the use of multiple antennas at either or both sides of the wireless links. Precoding and receive filtering using matrices obtained from a singular value decomposition (SVD) of the channel matrix is a transmission strategy for achieving the channel capacity of a deterministic narrowband multiple-input multiple-output (MIMO) communications channel. When signalling over wideband channels using orthogonal frequency-division multiplexing (OFDM), an SVD must be performed for every sub-carrier. As the number of sub-carriers of this traditional approach grow large, so does the computational load. It is therefore interesting to study alternate means for obtaining the decomposition. A wideband MIMO channel can be modeled as a matrix filter with a finite impulse response, represented by a polynomial matrix. This thesis is concerned with investigating algorithms which decompose the polynomial channel matrix directly. The resulting decomposition factors can then be used to obtain the sub-carrier based precoding and receive filtering matrices. Existing approximative polynomial matrix QR and singular value decomposition algorithms were modified, and studied in terms of decomposition quality and computational complexity. The decomposition algorithms were shown to give decompositions of good quality, but if the goal is to obtain precoding and receive filtering matrices, the computational load is prohibitive for channels with long impulse responses. Two algorithms for performing exact rational decompositions (QRD/SVD) of polynomial matrices were proposed and analyzed. Although they for simple cases resulted in excellent decompositions, issues with numerical stability of a spectral factorization step renders the algorithms in their current form purposeless. For a MIMO channel with exponentially decaying power-delay profile, the sum rates achieved by employing the filters given from the approximative polynomial SVD algorithm were compared to the channel capacity. It was shown that if the symbol streams were decoded independently, as done in the traditional approach, the sum rates were sensitive to errors in the decomposition. A receiver with a spatially joint detector achieved sum rates close to the channel capacity, but with such a receiver the low complexity detector set-up of the traditional approach is lost. Summarizing, this thesis has shown that a wideband MIMO channel can be diagonalized in space and frequency using OFDM in conjunction with an approximative polynomial SVD algorithm. In order to reach sum rates close to the capacity of a simple channel, the computational load becomes restraining compared to the traditional approach, for channels with long impulse responses.

The evolution to ultra-dense next generation networks requires a massive increase in throughput and deployment flexibility. Therefore, novel wireless backhaul solutions that can support these demands are needed. In this work we present an approach for a millimeter wave line-of-sight MIMO backhaul design, targeting transmission rates in the order of 100 Gbit/s. We provide theoretical foundations for the concept showcasing its potential, which are confirmed through channel measurements. Furthermore, we provide insights into the system design with respect to antenna array setup, baseband processing, synchronization, and channel equalization. Implementation in a 60 GHz demonstrator setup proves the feasibility of the system concept for high throughput backhauling in next generation networks.

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