Capacity of and Coding for Multiple-Aperture, Wireless, Optical Communications

Thesis Supervisor: Jeffrey H. Shapiro Title: Julius A. Stratton Professor of Electrical EngineeringRefractive index turbulence causes random power fluctuations in optical communication systems, making communication through the atmosphere diffcult. This same phenomenon makes the stars twinkle at night, and pavement shimmer on a hot sum- mer day. True to the old adage, "don't put all your eggs in one basket," we examine laser communication systems that use multiple transmit and receive apertures. These apertures provide redundant replicas of the transmitted message to the receiver, each corrupted separately by the atmosphere. Reliable communication occurs when not all of these paths are deeply faded. We quantify the maximum rate of reliable communication, or capacity, and study space-time coding techniques for both direct- and coherent-detection receivers. We also experimentally verify the performance of some simple techniques for optically-preamplified, direct-detection receivers.Defense Advanced Research Projects Agenc

Capacity of and coding for multiple-aperture, wireless, optical communications

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2003.Includes bibliographical references (p. 243-249).This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Refractive index turbulence causes random power fluctuations in optical communication systems, making communication through the atmosphere difficult. This same phenomenon makes the stars twinkle at night, and pavement shimmer on a hot summer day. True to the old adage, "don't put all your eggs in one basket," we examine laser communication systems that use multiple transmit and receive apertures. These apertures provide redundant replicas of the transmitted message to the receiver, each corrupted separately by the atmosphere. Reliable communication occurs when not all of these paths are deeply faded. We quantify the maximum rate of reliable communication, or capacity, and study space-time coding techniques for both direct- and coherent-detection receivers. We also experimentally verify the performance of some simple techniques for optically-preamplified, direct-detection receivers.by Shane M. Haas.Ph.D

Robust Optical Wireless Links over Turbulent Media using Diversity Solutions

Free-space optic (FSO) technology, i.e., optical wireless communication (OWC), is widely recognized as superior to radio frequency (RF) in many aspects. Visible and invisible optical wireless links solve first/last mile connectivity problems and provide secure, jam-free communication. FSO is license-free and delivers high-speed data rates in the order of Gigabits. Its advantages have fostered significant research efforts aimed at utilizing optical wireless communication, e.g. visible light communication (VLC), for high-speed, secure, indoor communication under the IEEE 802.15.7 standard. However, conventional optical wireless links demand precise optical alignment and suffer from atmospheric turbulence. When compared with RF, they suffer a low degree of reliability and lack robustness. Pointing errors cause optical transceiver misalignment, adversely affecting system reliability. Furthermore, atmospheric turbulence causes irradiance fluctuations and beam broadening of transmitted light. Innovative solutions to overcome limitations on the exploitation of high-speed optical wireless links are greatly needed.Spatial diversity is known to improve RF wireless communication systems. Similar diversity approaches can be adapted for FSO systems to improve its reliability and robustness; however, careful diversity design is needed since FSO apertures typically remain unbalanced as a result of FSO system sensitivity to misalignment. Conventional diversity combining schemes require persistent aperture monitoring and repetitive switching, thus increasing FSO implementation complexities. Furthermore, current RF diversity combining schemes may not be optimized to address the issue of unbalanced FSO receiving apertures.This dissertation investigates two efficient diversity combining schemes for multi-receiving FSO systems: switched diversity combining and generalized selection combining. Both can be exploited to reduce complexity and improve combining efficiency. Unlike maximum ratio combing, equal gain combining, and selective combining, switched diversity simplifies receiver design by avoiding unnecessary switching among receiving apertures. The most significant advantage of generalized combining is its ability to exclude apertures with low quality that could potentially affect the resultant output signal performance.This dissertation also investigates mobile FSO by considering a multi-receiving system in which all receiving FSO apertures are circularly placed on a platform. System mobility and performance are analyzed. Performance results confirm improvements when using angular diversity and generalized selection combining.The précis of this dissertation establishes the foundation of reliable FSO communications using efficient diversity-based solutions. Performance parameters are analyzed mathematically, and then evaluated using computer simulations. A testbed prototype is developed to facilitate the evaluation of optical wireless links via lab experiments

Vector perturbation technique

La “vector perturbation technique” è una tecnica di codifica che permette di avvicinarsi alla capacità teorica del canale in un sistema MIMO. Questa tecnica va a operare sul vettore dati da trasmettere e si articola in quattro punti fondamentali: Channel Inversion, Regolarizzazione, Perturbazione e Perturbazione Regolarizzata. Grazie ad essa è possibile ottenere una capacità che cresce linearmente con il numero minimo tra le antenne trasmittenti/riceventi del sistem

Optimizing communication performance of low-resolution ADC systems with hybrid beamforming

Low-resolution analog-to-digital converter (ADC) systems and hybrid analog-and-digital beamforming systems have drawn extensive attention as a promising receiver architecture for millimeter wave (mmWave) communications by reducing hardware cost and power consumption. In this dissertation, hybrid beamforming systems that employ low-resolution ADCs are considered to achieve a better trade-off between communication performance and power consumption. Due to non-negligible quantization errors, however, existing state-of-the-art hybrid beamforming techniques cannot be directly applied to such systems as they ignore the impact of the quantization error. In this regard, I propose new receiver architectures and algorithms for hybrid beamforming with low-resolution ADC systems to enhance spectral efficiency under coarse quantization in different layers of the network stack, and provide subsequent analyses. First, problems of optimizing the number of ADC bits and designing analog combiners with fixed-resolution ADCs are tackled to design an energy-efficient receiver architecture with phase shifter-based hybrid beamforming. A hybrid receiver architecture with resolution-adaptive ADCs for mmWave communications is proposed to optimize the power distribution over ADCs. For the proposed architecture, a near-optimal bit-allocation solution is derived in closed form. In addition, the performance lower bound of the proposed receiver architecture is derived in ergodic rate. For a fixed-resolution ADC system, a new analog combining architecture is proposed for mmWave communications. The proposed analog combiner consists of two consecutive analog combiners that maximize channel gain and minimize effective quantization error. An approximated ergodic rate of the proposed receiver is also derived in closed form. Next, considering switch-based analog beamforming, antenna selection at a base station is investigated for low-resolution ADC systems. Unlike downlink transmit antenna selection problems, a quantization-aware antenna selection criterion is necessary and derived to incorporate quantization error for uplink receive antenna selection problems. Leveraging the criterion, a quantization-aware antenna selection algorithm is proposed and analyzed for uplink. Last, in a higher layer of the network stack, a user scheduling problem is investigated for hybrid beamforming systems with low-resolution ADCs. New user scheduling criteria are derived to maximize scheduling gain under coarse quantization and efficient scheduling algorithms are proposed accordingly. Subsequent analysis for the proposed algorithm provides closed-form ergodic ratesElectrical and Computer Engineerin

Multi-user Diversity Systems with Application to Cognitive Radio

abstract: This thesis aims to investigate the capacity and bit error rate (BER) performance of multi-user diversity systems with random number of users and considers its application to cognitive radio systems. Ergodic capacity, normalized capacity, outage capacity, and average bit error rate metrics are studied. It has been found that the randomization of the number of users will reduce the ergodic capacity. A stochastic ordering framework is adopted to order user distributions, for example, Laplace transform ordering. The ergodic capacity under different user distributions will follow their corresponding Laplace transform order. The scaling law of ergodic capacity with mean number of users under Poisson and negative binomial user distributions are studied for large mean number of users and these two random distributions are ordered in Laplace transform ordering sense. The ergodic capacity per user is defined and is shown to increase when the total number of users is randomized, which is the opposite to the case of unnormalized ergodic capacity metric. Outage probability under slow fading is also considered and shown to decrease when the total number of users is randomized. The bit error rate (BER) in a general multi-user diversity system has a completely monotonic derivative, which implies that, according to the Jensen's inequality, the randomization of the total number of users will decrease the average BER performance. The special case of Poisson number of users and Rayleigh fading is studied. Combining with the knowledge of regular variation, the average BER is shown to achieve tightness in the Jensen's inequality. This is followed by the extension to the negative binomial number of users, for which the BER is derived and shown to be decreasing in the number of users. A single primary user cognitive radio system with multi-user diversity at the secondary users is proposed. Comparing to the general multi-user diversity system, there exists an interference constraint between secondary and primary users, which is independent of the secondary users' transmission. The secondary user with high- est transmitted SNR which also satisfies the interference constraint is selected to communicate. The active number of secondary users is a binomial random variable. This is then followed by a derivation of the scaling law of the ergodic capacity with mean number of users and the closed form expression of average BER under this situation. The ergodic capacity under binomial user distribution is shown to outperform the Poisson case. Monte-Carlo simulations are used to supplement our analytical results and compare the performance of different user distributions.Dissertation/ThesisM.S. Electrical Engineering 201

A Data-Aided Channel Estimation Scheme for Decoupled Systems in Heterogeneous Networks

Uplink/downlink (UL/DL) decoupling promises more flexible cell association and higher throughput in heterogeneous networks (HetNets), however, it hampers the acquisition of DL channel state information (CSI) in time-division-duplex (TDD) systems due to different base stations (BSs) connected in UL/DL. In this paper, we propose a novel data-aided (DA) channel estimation scheme to address this problem by utilizing decoded UL data to exploit CSI from received UL data signal in decoupled HetNets where a massive multiple-input multiple-output BS and dense small cell BSs are deployed. We analytically estimate BER performance of UL decoded data, which are used to derive an approximated normalized mean square error (NMSE) expression of the DA minimum mean square error (MMSE) estimator. Compared with the conventional least square (LS) and MMSE, it is shown that NMSE performances of all estimators are determined by their signal-to-noise ratio (SNR)-like terms and there is an increment consisting of UL data power, UL data length and BER values in the SNR-like term of DA method, which suggests DA method outperforms the conventional ones in any scenarios. Higher UL data power, longer UL data length and better BER performance lead to more accurate estimated channels with DA method. Numerical results verify that the analytical BER and NMSE results are close to the simulated ones and a remarkable gain in both NMSE and DL rate can be achieved by DA method in multiple scenarios with different modulations