Achievable Rate and Modulation for Bandlimited Channels with Oversampling and 1-Bit Quantization at the Receiver

Sustainably realizing applications of the future with high performance demands requires that energy efficiency becomes a central design criterion for the entire system. For example, the power consumption of the analog-to-digital converter (ADC) can become a major factor when transmitting at large bandwidths and carrier frequencies, e.g., for ultra-short range high data rate communication. The consumed energy per conversion step increases with the sampling rate such that high resolution ADCs become unfeasible in the sub-THz regime at the very high sampling rates required. This makes signaling schemes adapted to 1-bit quantizers a promising alternative. We therefore quantify the performance of bandlimited 1-bit quantized wireless communication channels using techniques like oversampling and faster-than-Nyquist (FTN) signaling to compensate for the loss of achievable rate. As a limiting case, we provide bounds on the mutual information rate of the hard bandlimited 1-bit quantized continuous-time – i.e., infinitely oversampled – additive white Gaussian noise channel in the mid-to-high signal-to-noise ratio (SNR) regime. We derive analytic expressions using runlength encoded input signals. For real signals the maximum value of the lower bound on the spectral efficiency in the high-SNR limit was found to be approximately 1.63 bit/s/Hz. Since in practical scenarios the oversampling ratio remains finite, we derive bounds on the achievable rate of the bandlimited oversampled discrete-time channel. These bounds match the results of the continuous-time channel remarkably well. We observe spectral efficiencies up to 1.53 bit/s/Hz in the high-SNR limit given hard bandlimitation. When excess bandwidth is tolerable, spectral efficiencies above 2 bit/s/Hz per domain are achievable w.r.t. the 95 %-power containment bandwidth. Applying the obtained bounds to a bandlimited oversampled 1-bit quantized multiple-input multiple-output channel, we show the benefits when using appropriate power allocation schemes. As a constant envelope modulation scheme, continuous phase modulation is considered in order to relieve linearity requirements on the power amplifier. Noise-free performance limits are investigated for phase shift keying (PSK) and continuous phase frequency shift keying (CPFSK) using higher-order modulation alphabets and intermediate frequencies. Adapted waveforms are designed that can be described as FTN-CPFSK. With the same spectral efficiency in the high-SNR limit as PSK and CPFSK, these waveforms provide a significantly improved bit error rate (BER) performance. The gain in SNR required for achieving a certain BER can be up to 20 dB.Die nachhaltige Realisierung von zukünftigen Übertragungssystemen mit hohen Leistungsanforderungen erfordert, dass die Energieeffizienz zu einem zentralen Designkriterium für das gesamte System wird. Zum Beispiel kann die Leistungsaufnahme des Analog-Digital-Wandlers (ADC) zu einem wichtigen Faktor bei der Übertragung mit großen Bandbreiten und Trägerfrequenzen werden, z. B. für die Kommunikation mit hohen Datenraten über sehr kurze Entfernungen. Die verbrauchte Energie des ADCs steigt mit der Abtastrate, so dass hochauflösende ADCs im Sub-THz-Bereich bei den erforderlichen sehr hohen Abtastraten schwer einsetzbar sind. Dies macht Signalisierungsschemata, die an 1-Bit-Quantisierer angepasst sind, zu einer vielversprechenden Alternative. Wir quantifizieren daher die Leistungsfähigkeit von bandbegrenzten 1-Bit-quantisierten drahtlosen Kommunikationssystemen, wobei Techniken wie Oversampling und Faster-than-Nyquist (FTN) Signalisierung eingesetzt werden, um den durch Quantisierung verursachten Verlust der erreichbaren Rate auszugleichen. Wir geben Grenzen für die Transinformationsrate des Extremfalls eines strikt bandbegrenzten 1-Bit quantisierten zeitkontinuierlichen – d.h. unendlich überabgetasteten – Kanals mit additivem weißen Gauß’schen Rauschen bei mittlerem bis hohem Signal-Rausch-Verhältnis (SNR) an. Wir leiten analytische Ausdrücke basierend auf lauflängencodierten Eingangssignalen ab. Für reelle Signale ist der maximale Wert der unteren Grenze der spektralen Effizienz im Hoch-SNR-Bereich etwa 1,63 Bit/s/Hz. Da die Überabtastrate in praktischen Szenarien endlich bleibt, geben wir Grenzen für die erreichbare Rate eines bandbegrenzten, überabgetasteten zeitdiskreten Kanals an. Diese Grenzen stimmen mit den Ergebnissen des zeitkontinuierlichen Kanals bemerkenswert gut überein. Im Hoch-SNR-Bereich sind spektrale Effizienzen bis zu 1,53 Bit/s/Hz bei strikter Bandbegrenzung möglich. Wenn Energieanteile außerhalb des Frequenzbandes tolerierbar sind, können spektrale Effizienzen über 2 Bit/s/Hz pro Domäne – bezogen auf die Bandbreite, die 95 % der Energie enthält – erreichbar sein. Durch die Anwendung der erhaltenen Grenzen auf einen bandbegrenzten überabgetasteten 1-Bit quantisierten Multiple-Input Multiple-Output-Kanal zeigen wir Vorteile durch die Verwendung geeigneter Leistungsverteilungsschemata. Als Modulationsverfahren mit konstanter Hüllkurve betrachten wir kontinuierliche Phasenmodulation, um die Anforderungen an die Linearität des Leistungsverstärkers zu verringern. Beschränkungen für die erreichbare Datenrate bei rauschfreier Übertragung auf Zwischenfrequenzen mit Modulationsalphabeten höherer Ordnung werden für Phase-shift keying (PSK) and Continuous-phase frequency-shift keying (CPFSK) untersucht. Weiterhin werden angepasste Signalformen entworfen, die als FTN-CPFSK beschrieben werden können. Mit der gleichen spektralen Effizienz im Hoch-SNR-Bereich wie PSK und CPFSK bieten diese Signalformen eine deutlich verbesserte Bitfehlerrate (BER). Die Verringerung des erforderlichen SNRs zur Erreichung einer bestimmten BER kann bis zu 20 dB betragen

Two-Dimensional Electronic Materials and Devices: Opportunities and Challenges

The unprecedented growth of the Internet of Things (IoT) and the 4th Industrial Revolution (Industry 4.0) not only demands dimensional scaling of device technologies but also new types of applications beyond today’s electronics. Two-dimensional (2D) materials, a group of layered crystals (such as graphene and MoS2) with unique properties, have emerged as promising candidates for IoT and Industry 4.0 since they can, not only extend the scaling with unprecedented performance and energy efficiency but also exhibit high potential for novel electronic devices. However, such nanomaterials suffer from significant challenges in process integration, especially in the modules that involves the formation of interfaces between 2D materials and conventional bulk materials. Thus, realizing high-performance energy-efficient 2D electronic devices has been challenging. This dissertation focuses on understanding the fundamental issues in such 2D materials (such as contacts, interfaces and doping) and in identifying applications uniquely enabled by these materials.First, a comprehensive treatment of metal contacts to 2D semiconductors, which has been a huge hurdle for 2D electronic technologies, will be presented. As a pioneering study, new interface physics originating from the unique dimensionality and surface properties have been revealed [1]. Solutions to minimize contact resistance are described though techniques of interface hybridization [2] and seamless contacts [3], [4]. These techniques transform 2D semiconductors from solely scientifically-interesting materials into high-performance field-effect transistor (FET) technologies, such as MoS2 FETs with record-low contact resistances [5], [6] and WSe2 FETs with record-high drive current and mobility [7]. Beyond metal interfaces, dielectric interface is crucial for preserving the carrier mobility in 2D channels, for which a solution enabled by buffer layers has been proposed [8]. On the other hand, the vertical van der Waals interfaces between 2D and 3D semiconductors, which retain the advantages of pristine ultra-thin 2D films as well as maximized tunneling area/field, have been studied and exploited into a novel beyond-silicon transistor technology – the first 2D channel tunnel FET (TFET) [9], which beat the fundamental limitation in the switching behavior of transistors. Recent results from the engineering of such 2D-3D semiconductor interfaces by surface reduction/passivation are described, showing a significant boost of drive current. While conventional diffusion/ion implantation methods are infeasible for 2D materials, two efficient doping techniques that are specific for 2D materials – surface doping [10], [11] and intercalation doping [12] are presented. The theoretical study of surface doping using ab-initio methods helped develop a novel doping scheme that uniquely exploits the Lewis-base like pedigree of 2D semiconductors without disturbing the structural integrity of the 2D atomic layer configuration [13], as well as a novel electrocatalyst based on MoS2 that achieved record high hydrogen evolution reaction (HER) performance [14]. On the other hand, intercalation doping has been employed to demonstrate graphene based transparent electrodes with the best combination of transmittance and sheet resistance [12], and also the first graphene interconnects with excellent performance, reliability and energy-efficiency [15], [16]. Moreover, by uniquely exploiting the high kinetic inductance and conductivity of intercalation doped graphene, a fundamentally different on-chip inductor has been demonstrated [17], [18], with both small form-factors and high inductance values, that were once thought unachievable in tandem. This 2D technique provides an attractive solution to the longstanding scaling problem of analog/radio-frequency electronics and opens up an unconventional pathway for the development of future ultra-compact wireless communication systems. Finally, a novel dissipative quantum transport methodology based on Büttiker probes with band-to-band tunneling capability is developed for 2D FETs [19]. Subsequently, gate-induced-drain-leakage (GIDL), one of the main leakage mechanisms in FETs especially access transistors, is evaluated for the first time for 2D FETs. The results establish the advantages of certain 2D semiconductors in greatly reducing GIDL and thereby support use of such materials in future memory technologies.The dissertation concludes with a vision for how a smart life can be realized in the future by harnessing the capabilities of various 2D technologies in the era of IoT and Industry 4.0.[1] J. Kang, D. Sarkar, W. Liu, D. Jena, and K. Banerjee, “A computational study of metal-contacts to beyond-graphene 2D semiconductor materials,” in IEEE International Electron Devices Meeting, 2012, pp. 407–410.[2] J. Kang, W. Liu, D. Sarkar, D. Jena, and K. Banerjee, “Computational Study of Metal Contacts to Monolayer Transition-Metal Dichalcogenide Semiconductors,” Phys. Rev. X, vol. 4, no. 3, p. 31005, Jul. 2014.[3] J. Kang, D. Sarkar, Y. Khatami, and K. Banerjee, “Proposal for all-graphene monolithic logic circuits,” Appl. Phys. Lett., vol. 103, no. 8, p. 83113, 2013.[4] A. Allain, J. Kang, K. Banerjee, and A. Kis, “Electrical contacts to two-dimensional semiconductors,” Nat. Mater., vol. 14, no. 12, pp. 1195–1205, 2015.[5] W. Liu et al., “High-performance few-layer-MoS2 field-effect-transistor with record low contact-resistance,” in IEEE International Electron Devices Meeting, 2013, pp. 499–502.[6] J. Kang, W. Liu, and K. Banerjee, “High-performance MoS2 transistors with low-resistance molybdenum contacts,” Appl. Phys. Lett., vol. 104, no. 9, p. 93106, Mar. 2014.[7] W. Liu, J. Kang, D. Sarkar, Y. Khatami, D. Jena, and K. Banerjee, “Role of metal contacts in designing high-performance monolayer n-type WSe2 field effect transistors.,” Nano Lett., vol. 13, no. 5, pp. 1983–90, May 2013.[8] J. Kang, W. Liu, and K. Banerjee, “Computational Study of Interfaces between 2D MoS2 and Surroundings,” in 45th IEEE Semiconductor Interface Specialists Conference, 2014.[9] D. Sarkar et al., “A subthermionic tunnel field-effect transistor with an atomically thin channel,” Nature, vol. 526, no. 7571, pp. 91–95, Sep. 2015.[10] Y. Khatami, W. Liu, J. Kang, and K. Banerjee, “Prospects of graphene electrodes in photovoltaics,” in Proceedings of SPIE, 2013, vol. 8824, p. 88240T–88240T–6.[11] D. Sarkar et al., “Functionalization of Transition Metal Dichalcogenides with Metallic Nanoparticles: Implications for Doping and Gas-Sensing,” Nano Lett., vol. 15, no. 5, pp. 2852–2862, May 2015.[12] W. Liu, J. Kang, and K. Banerjee, “Characterization of FeCl3 intercalation doped CVD few-layer graphene,” IEEE Electron Device Lett., vol. 37, no. 9, pp. 1246–1249, Sep. 2016.[13] S. Lei et al., “Surface functionalization of two-dimensional metal chalcogenides by Lewis acid–base chemistry,” Nat. Nanotechnol., vol. 11, no. 5, pp. 465–471, Feb. 2016.[14] J. Li, J. Kang, Q. Cai, W. Hong, C. Jian, and W. Liu, “Boosting Hydrogen Evolution Performance of MoS2 by Band Structure Engineering,” Adv. Mater. Interfaces, vol. 1700303, 2017.[15] J. Jiang et al., “Intercalation doped multilayer-graphene-nanoribbons for next-generation interconnects,” Nano Lett., vol. 17, no. 3, pp. 1482–1488, Mar. 2017.[16] J. Jiang, J. Kang, and K. Banerjee, “Characterization of Self - Heating and Current - Carrying Capacity of Intercalation Doped Graphene - Nanoribbon Interconnects,” in IEEE International Reliability Physics Symposium, 2017, p. 6B.1.1-6B.1.6.[17] X. Li et al., “Graphene inductors for high-frequency applications - design, fabrication, characterization, and study of skin effect,” in IEEE International Electron Devices Meeting, 2014, p. 5.4.1-5.4.4.[18] J. Kang et al., under review.[19] J. Kang et al., under review