ITC/USA 2008 Conference Proceedings / The Forty-Fourth Annual International Telemetering Conference and Technical Exhibition / October 27-30, 2008 / Town and Country Resort & Convention Center, San Diego, California / Although multiple-input multiple-output (MIMO) systems have become increasingly popular, the existence of real time results to compare with those predicted by theory is still surprisingly limited. In this work the hardware description of a MIMO wireless communication system using orthogonal space time block codes (OSTBC) is discussed for two antennas at both the transmitter and receiver. A numerical example for a frequency flat time correlated channel is given to show the impact of channel estimation.

Due to the rapid development of wireless communications together with the inflexibility of the current spectrum allocation policy, radio spectrum becomes more and more exhausted. One of the critical challenges of wireless communication systems is to efficiently utilize the limited frequency resources to be able to support the growing demand of high data rate wireless services. As a promising solution, cognitive radios have been suggested to deal with the scarcity and under-utilization of radio spectrum. The basic idea behind cognitive radios is to allow unlicensed users, also called secondary users (SUs), to access the licensed spectrum of primary users (PUs) which improves spectrum utilization. In order to not degrade the performance of the primary networks, SUs have to deploy interference control, interference mitigating, or interference avoidance techniques to minimize the interference incurred at the PUs. Cognitive radio networks (CRNs) have stimulated a variety of studies on improving spectrum utilization. In this context, this thesis has two main objectives. Firstly, it investigates the performance of single hop CRNs with spectrum sharing and opportunistic spectrum access. Secondly, the thesis analyzes the performance improvements of two hop cognitive radio networks when incorporating advanced radio transmission techniques. The thesis is divided into three parts consisting of an introduction part and two research parts based on peer-reviewed publications. Fundamental background on radio propagation channels, cognitive radios, and advanced radio transmission techniques are discussed in the introduction. In the first research part, the performance of single hop CRNs is analyzed. Specifically, underlay spectrum access using M/G/1/K queueing approaches is presented in Part I-A while dynamic spectrum access with prioritized traffics is studied in Part I-B. In the second research part, the performance benefits of integrating advanced radio transmission techniques into cognitive cooperative radio networks (CCRNs) are investigated. In particular, opportunistic spectrum access for amplify-and-forward CCRNs is presented in Part II-A where collaborative spectrum sensing is deployed among the SUs to enhance the accuracy of spectrum sensing. In Part II-B, the effect of channel estimation error and feedback delay on the outage probability and symbol error rate (SER) of multiple-input multiple-output CCRNs is investigated. In Part II-C, adaptive modulation and coding is employed for decode-and-forward CCRNs to improve the spectrum efficiency and to avoid buffer overflow at the relay. Finally, a hybrid interweave-underlay spectrum access scheme for a CCRN is proposed in Part II-D. In this work, the dynamic spectrum access of the PUs and SUs is modeled as a Markov chain which then is utilized to evaluate the outage probability, SER, and outage capacity of the CCRN.

This thesis concerns communication across channels with multiple inputs and multiple outputs. Specifically, we consider the closed-loop scenario in which knowledge of the state of the multiple-input multiple-output (MIMO) channel is available at the transmitter. We show how this knowledge can be exploited to optimize performance, as measured by the zero-outage capacity, which is the capacity corresponding to zero outage probability. On at-fading channels, a closed-loop transmitter allocates different powers and rates to the multiple channel inputs so as to maximize zero-outage capacity. Frequency-selective fading channels call for a combination of orthogonal-frequency-division multiplexing (OFDM) and MIMO known as MIMO-OFDM. This exacerbates the allocation problem because it multiplies the number of allocation dimensions by the number of OFDM tones. Fortunately, this thesis demonstrates that simple allocations are sufficient to approach the zero-outage capacity. These simple strategies exploit the tendency for random MIMO channels to behave deterministically as the number of inputs becomes large.

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A wireless relay network consists of multiple source nodes, multiple destination nodes, and possibly many relay nodes in between to facilitate its transmission. It is clear that the performance of such networks highly depends on information for- warding strategies adopted at the relay nodes. This dissertation studies a particular information forwarding strategy called compute-and-forward. Compute-and-forward is a novel paradigm that tries to incorporate the idea of network coding within the physical layer and hence is often referred to as physical layer network coding. The main idea is to exploit the superposition nature of the wireless medium to directly compute or decode functions of transmitted signals at intermediate relays in a net- work. Thus, the coding performed at the physical layer serves the purpose of error correction as well as permits recovery of functions of transmitted signals. For the bidirectional relaying problem with Gaussian channels, it has been shown by Wilson et al. and Nam et al. that the compute-and-forward paradigm is asymptotically optimal and achieves the capacity region to within 1 bit; however, similar results beyond the memoryless case are still lacking. This is mainly because channels with memory would destroy the lattice structure that is most crucial for the compute-and-forward paradigm. Hence, how to extend compute-and-forward to such channels has been a challenging issue. This motivates this study of the extension of compute-and-forward to channels with memory, such as inter-symbol interference. The bidirectional relaying problem with parallel Gaussian channels is also studied, which is a relevant model for the Gaussian bidirectional channel with inter-symbol interference and that with multiple-input multiple-output channels. Motivated by the recent success of linear finite-field deterministic model, we first investigate the corresponding deterministic parallel bidirectional relay channel and fully characterize its capacity region. Two compute-and-forward schemes are then proposed for the Gaussian model and the capacity region is approximately characterized to within a constant gap. The design of coding schemes for the compute-and-forward paradigm with low decoding complexity is then considered. Based on the separation-based framework proposed previously by Tunali et al., this study proposes a family of constellations that are suitable for the compute-and-forward paradigm. Moreover, by using Chinese remainder theorem, it is shown that the proposed constellations are isomorphic to product fields and therefore can be put into a multilevel coding framework. This study then proposes multilevel coding for the proposed constellations and uses multistage decoding to further reduce decoding complexity.

The main topic of this thesis is based on multiple-input multiple-output (MIMO) wireless communications, which is a novel technology that has attracted great interest in the last twenty years. Conventional MIMO systems using up to eight antennas play a vital role in the urban cellular network, where the deployment of multiple antennas have significantly enhanced the throughput without taking extra spectrum or power resources. The massive MIMO systems “scales” up the benefits that offered by the conventional MIMO systems. Using sixty four or more antennas at the BS not only improves the spectrum efficiency significantly, but also provides additional link robustness. It is considered as a key technology in the fifth generation of mobile communication technology standards network, and the design of new algorithms for these two systems is the basis of the research in this thesis. Firstly, at the receiver side of the conventional MIMO systems, a general framework of bit error rate (BER) approximation for the detection algorithms is proposed, which aims to support an adaptive modulation scheme. The main idea is to utilize a simplified BER approximation scheme, which is based on the union bound of the maximum-likelihood detector (MLD), whereby the bit error rate (BER) performance of the detector for the varying channel qualities can be efficiently predicted. The K-best detector is utilized in the thesis because its quasi- MLD performance and the parallel computational structure. The simulation results have clearly shown the adaptive K-best algorithm, by applying the simplified approximation method, has much reduced computational complexity while still maintaining a promising BER performance. Secondly, in terms of the uplink channel estimation for the massive MIMO systems with the time-division-duplex operation, the performance of the Grassmannian line packing (GLP) based uplink pilot codebook design is investigated. It aims to eliminate the pilot contamination effect in order to increase the downlink achievable rate. In the case of a limited channel coherence interval, the uplink codebook design can be treated as a line packing problem in a Grassmannian manifold. The closed-form analytical expressions of downlink achievable rate for both the single-cell and multi-cell systems are proposed, which are intended for performance analysis and optimization. The numerical results validate the proposed analytical expressions and the rate gains by using the GLP-based uplink codebook design. Finally, the study is extended to the energy efficiency (EE) of the massive MIMO system, as the reduction carbon emissions from the information and communication technology is a long-term target for the researchers. An effective framework of maximizing the EE for the massive MIMO systems is proposed in this thesis. The optimization starts from the maximization of the minimum user rate, which is aiming to increase the quality-of-service and provide a feasible constraint for the EE maximization problem. Secondly, the EE problem is a non-concave problem and can not be solved directly, so the combination of fractional programming and the successive concave approximation based algorithm are proposed to find a good suboptimal solution. It has been shown that the proposed optimization algorithm provides a significant EE improvement compared to a baseline case.

Les systèmes de communication à intensité optique en espace libre (FSOI) sont largement utilisés dans les communications à courte portée, telles que les communications infrarouges entre des dispositifs électroniques portables. L’émetteur de ces systèmes module sur l’intensité des signaux optiques émis par des diodes électroluminescentes (LEDs) ou des diodes laser (LDs), et le récepteur mesure les intensités optiques entrantes au moyen de photodétecteurs. Les entrées ne sont pas négatives car elles représentent des intensités. En outre, ils sont généralement soumis à des contraintes de puissance de pointe et moyenne, la contrainte de puissance de pointe étant principalement dû aux limitations techniques des composants utilisés, alors que la contrainte de puissance moyenne est imposée par des limitations de batterie et des considérations de sécurité. En première approximation, le bruit dans de tels systèmes peut être supposé être gaussien et indépendant du signal transmis. Cette thèse porte sur les limites fondamentales des systèmes de communication FSOI, plus précisément sur leur capacité. L’objectif principal de notre travail est d’étudier la capacité d’un canal FSOI général à entrées multiples et sorties multiples (MIMO) avec une contrainte de puissance de crête par entrée et une contrainte de puissance moyenne totale sur toutes les antennes d’entrée. Nous présentons plusieurs résultats de capacité sur le scénario quand il y a plus d’antennes d’émission que d’antennes de réception, c’est à-dire, nT > nR > 1. Dans ce scénario, différents vecteurs d’entrée peuvent donner des distributions identiques à la sortie, lorsqu’ils aboutissent au même vecteur d’image multiplié par la matrice de canal. Nous déterminons d’abord les vecteurs d’entrée d’énergie minimale permettant d’atteindre chacun de ces vecteurs d’image. Il définit à chaque instant dans le temps un sous-ensemble de nT − nR antennes à zéro ou à pleine puissance et utilise uniquement les nR antennes restantes pour la signalisation. Sur cette base, nous obtenons une expression de capacité équivalente en termes de vecteur d’image, ce qui permet de décomposer le canal d’origine en un ensemble de canaux presque parallèles. Chacun des canaux parallèles est un canal MIMO nR x nR à contrainte d’amplitude, avec une contrainte de puissance linéaire, pour laquelle des limites de capacité sont connues. Avec cette décomposition, nous établissons de nouvelles limites supérieures en utilisant une technique de limite supérieure basée sur la dualité, et des limites inférieures en utilisant l’inégalité de puissance d’entropie (EPI). Les limites supérieure et inférieure dérivées correspondent lorsque le rapport signal sur bruit (SNR) tend vers l’infini, établissant la capacité asymptotique à haut SNR. À faible SNR, il est connu que la pente de capacité est déterminée par la trace maximale de la matrice de covariance du vecteur image. Nous avons trouvé une caractérisation de cette trace maximale qui est plus facile à évaluer en calcul que les formes précédentes. / Free-space optical intensity (FSOI) communication systems are widely used in short-range communication such as the infrared communication between electronic handheld devices. The transmitter in these systems modulates on the intensity of optical signals emitted by light emitting diodes (LEDs) or laser diodes (LDs), and the receiver measures incoming optical intensities by means of photodetectors. Inputs are nonnegative because they represent intensities. Moreover, they are typically subject to both peak- and average-power constraints, where the peak-power constraint is mainly due to technical limitations of the used components, whereas the average-power constraint is imposed by battery limitations and safety considerations. As a first approximation, the noise in such systems can be assumed to be Gaussian and independent of the transmitted signal. This thesis focuses on the fundamental limits of FSOI communication systems, more precisely on their capacity. The major aim of our work is to study the capacity of a general multiple-input multiple-output (MIMO) FSOI channel under a per-input-antenna peak-power constraint and a total average-power constraint over all input antennas. We present several capacity results on the scenario when there are more transmit than receive antennas, i.e., nT > nR > 1. In this scenario, different input vectors can yield identical distributions at the output, when they result in the same image vector under multiplication by the channel matrix. We first determine the minimum-energy input vectors that attain each of these image vectors. It sets at each instant in time a subset of nT − nR antennas to zero or to full power, and uses only the remaining nR antennas for signaling. Based on this, we derive an equivalent capacity expression in terms of the image vector, which helps to decompose the original channel into a set of almost parallel channels. Each of the parallel channels is an amplitude-constrained nR⇥nR MIMO channel, with a linear power constraint, for which bounds on the capacity are known. With this decomposition, we establish new upper bounds by using a duality-based upper-bounding technique, and lower bounds by using the Entropy Power Inequality (EPI). The derived upper and lower bounds match when the signal-to-noise ratio (SNR) tends to infinity, establishing the high-SNR asymptotic capacity. At low SNR, it is known that the capacity slope is determined by the maximum trace of of the covariance matrix of the image vector. We found a characterization to this maximum trace that is computationally easier to evaluate than previous forms.

京都大学 / 0048 / 新制・課程博士 / 博士(情報学) / 甲第20031号 / 情博第626号 / 新制||情||109(附属図書館) / 33127 / 京都大学大学院情報学研究科通信情報システム専攻 / (主査)教授 守倉 正博, 教授 原田 博司, 教授 梅野 健 / 学位規則第4条第1項該当 / Doctor of Informatics / Kyoto University / DGAM

Multi-antenna communication systems are attracting research interest as a means to increase the information capacity, reliability, and spectral efficiency of wireless information transfer. Ray-tracing methods predict the behavior of wireless channels using a model of the propagation environment and are a low-cost alternative to direct measurements. We use ray tracing simulations to validate the statistical time and angle of arrival characteristics of an indoor multipath channel and compare model parameter estimates with estimates derived from channel sounding measurements. Ray tracing predicts the time and angle clustering of multipaths observed in the measurements and provides model parameter estimates which are closely correlated with measured estimates. The ray tracing parameters relating to power characteristics show more deviation from measurements than the time and angle related parameters. Our results also indicate that the description of reflective scatterers in the propagation environment is more important to the quality of the predicted statistical behavior than the description of bulk materials. We use a ray synthesis model to investigate means of efficiently representing the channel for feedback information to the transmitter as a means to increase the information capacity. Several methods of selecting the ray-model feedback information are demonstrated with results from simulated and measured channels. These results indicate that an ESPRIT algorithm coupled with ad hoc transmit/receive pairing can yield better than 90% of the ideal waterfilling capacity when adequate training-based channel estimates are available. Additionally, we investigate a covariance feedback method for providing channel feedback for increased capacity. Both the ray-based and covariance-based feedback methods yield their highest capacity improvements when the signal to noise ratio is low. This results because of the larger benefit of focusing transmit power into the most advantageous eigenmodes of the channel when fewer eigenmodes have power allocated to them by the waterfilling capacity solution. In higher signal to noise ratio cases, more eigenmodes of the channel receive power when waterfilling, and the capacity improvement from feedback information decreases relative to a uniform power allocation. In general, ray model feedback methods are preferable because the covariance feedback quickly requires higher computational effort as the array sizes increase and typically results in lower capacity for a given amount of feedback information.

Yes / The paper presents a technique to enhance the isolation between adjacent radiating elements which is common in densely packed antenna arrays. Such antennas provide frequency beam-scanning capability needed in Multiple-Input Multiple-Output (MIMO) systems and Synthetic Aperture Radars (SARs). The method proposed here uses a metamaterial decoupling slab (MTMDS), which is located between radiating elements, to suppress mutual-coupling between the elements that would otherwise degrade the antenna efficiency and performance in both the transmit and receive mode. The proposed MTM-DS consists of mirror imaged Eshaped slits engraved on a microstrip patch with inductive stub. Measured results confirm over 9–11 GHz with no MTM-DS the average isolation (S12) is -27 dB; however, with MTM-DS the average isolation improves to -38 dB. With this technique the separation between the radiating element can be reduced to 0.66λo, where λ0 is free space wavelength at 10 GHz. In addition, with this technique there is 15% improvement in operating bandwidth. At frequencies of high impedance match of 9.95 GHz and 10.63 GHz the gain is 4.52 dBi and 5.40 dBi, respectively. Furthermore, the technique eliminates poor front-to-back ratio encountered in other decoupling methods. MTM-DS is also relatively simple to implement. Assuming adequate space is available between adjacent radiators the MTM-DS can be fixed retrospectively on existing antenna arrays, which makes the proposed method versatile. / Partially supported by innovation programme under grant agreement H2020-MSCA-ITN-2016 SECRET- 722424 and the financial support from the UK Engineering and Physical Sciences Research Council (EPSRC) under grant EP/E022936/1.