| 1. |
- Cheng, Hei Victor
(author)
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Aspects of Power Allocation in Massive MIMO
- 2016
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Licentiate thesis (other academic/artistic)abstract
- The past decades have seen a rapid growth of mobile data trac, both in terms of connected devices and data rate. To satisfy the ever growing data trac demand in wireless communication systems, the current cellular systems have to be redesigned to increase both spectral eciency and energy eciency. Massive MIMO (Multiple-Input-Multiple-Output) is one solution that satisfy both requirements. In massive MIMO systems, hundreds of antennas are employed at the base station to provide service to many users at the same time and frequency. This enables the system to serve the users with uniformly good quality of service simultaneously, with low-cost hardware and without using extra bandwidth and energy. To achieve this, proper resource allocation is needed. Among the available resources, transmit power is one of the most important degree of freedom to control the spectral eciency and energy eciency. Due to the use of excessive number of antennas and low-end hardware at the base station, new aspects of power allocation compared to current systems arises. In the rst part of the thesis, a new uplink power allocation schemes that based on long term channel statistics is proposed. Since quality of the channel estimates is crucial in massive MIMO, in addition to data power allocation, joint power allocation that includes the pilot power as additional variable should be considered. Therefore a new framework for power allocation that matches practical systems is developed, as the methods developed in the literature cannot be applied directly to massive MIMO systems. Simulation results conrm the advantages brought by the the proposed new framework. In the second part of the thesis, we investigate the eects of using low-end ampliers at the base stations. The non-linear behavior of power consumption in these ampliers changes the power consumption model at the base station, thereby changes the power allocation. Two dierent scenarios are investigated and both results show that a certain number of antennas can be turned o in low load scenarios.
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| 2. |
- Yanggratoke, Rerngvit, 1983-
(author)
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Data-driven Performance Prediction and Resource Allocation for Cloud Services
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Cloud services, which provide online entertainment, enterprise resource management, tax filing, etc., are becoming essential for consumers, businesses, and governments. The key functionalities of such services are provided by backend systems in data centers. This thesis focuses on three fundamental problems related to management of backend systems. We address these problems using data-driven approaches: triggering dynamic allocation by changes in the environment, obtaining configuration parameters from measurements, and learning from observations. The first problem relates to resource allocation for large clouds with potentially hundreds of thousands of machines and services. We developed and evaluated a generic gossip protocol for distributed resource allocation. Extensive simulation studies suggest that the quality of the allocation is independent of the system size for the management objectives considered.The second problem focuses on performance modeling of a distributed key-value store, and we study specifically the Spotify backend for streaming music. We developed analytical models for system capacity under different data allocation policies and for response time distribution. We evaluated the models by comparing model predictions with measurements from our lab testbed and from the Spotify operational environment. We found the prediction error to be below 12% for all investigated scenarios.The third problem relates to real-time prediction of service metrics, which we address through statistical learning. Service metrics are learned from observing device and network statistics. We performed experiments on a server cluster running video streaming and key-value store services. We showed that feature set reduction significantly improves the prediction accuracy, while simultaneously reducing model computation time. Finally, we designed and implemented a real-time analytics engine, which produces model predictions through online learning.
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| 3. |
- Chen, Kairang, 1986-
(author)
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Energy-Efficient Data Converters for Low-Power Sensors
- 2016
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Doctoral thesis (other academic/artistic)abstract
- Wireless sensor networks (WSNs) are employed in many applications, such as for monitoring bio-potential signals and environmental information. These applications require high-resolution (> 12-bit) analog-to-digital converters (ADCs) at low-sampling rates (several kS/s). Such sensor nodes are usually powered by batteries or energy-harvesting sources hence low power consumption is primary for such ADCs. Normally, tens or hundreds of autonomously powered sensor nodes are utilized to capture and transmit data to the central processor. Hence it is profitable to fabricate the relevant electronics, such as the ADCs, in a low-cost standard complementary metal-oxide-semiconductor (CMOS) process. The two-stage pipelined successive approximation register (SAR) ADC has shown to be an energy-efficient architecture for high resolution. This thesis further studies and explores the design limitations of the pipelined SAR ADC for high-resolution and low-speed applications.The first work is a 15-bit, 1 kS/s two-stage pipelined SAR ADC that has been implemented in 0.35-μm CMOS process. The use of aggressive gain reduction in the residue amplifier combined with a suitable capacitive array digital-to-analog converter (DAC) topology in the second-stage simplifies the design of the operational transconductance amplifier (OTA) while eliminating excessive capacitive load and consequent power consumption. A comprehensive power consumption analysis of the entire ADC is performed to determine the number of bits in each stage of the pipeline. Choice of a segmented capacitive array DAC and attenuation capacitorbased DAC for the first and second stages respectively enable significant reduction in power consumption and area. Fabricated in a low-cost 0.35-μm CMOS process, the prototype ADC achieves a peak signal-to-noise-and-distortion ratio (SNDR) of 78.9 dB corresponding to an effective number of bits (ENOB) of 12.8-bit at a sampling frequency of 1 kS/s and provides a Schreier figure-of-merit (FoM) of 157.6 dB. Without any form of calibration, the ADC maintains an ENOB > 12.1-bit up to the Nyquist bandwidth of 500 Hz while consuming 6.7 μW. Core area of the ADC is 0.679 mm2.The second work is a 14-bit, tunable bandwidth two-stage pipelined SAR ADC which is suitable for low-power, cost-effective sensor readout circuits. To overcome the high open-loop DC gain requirement of the OTA in the gain-stage, a 3-stage capacitive charge pump (CCP) is utilized to achieve the gain-stage instead of using the switch capacitor (SC) amplifier. Unity-gain OTAs have been used as the analog buffers to prevent the charge sharing between the CCP stages. The detailed design considerations are given in this work. The prototype ADC, designed and fabricated in a low-cost 0.35-μm CMOS process, achieves a peak SNDR of 75.6 dB at a sampling rate of 20 kS/s and 76.1 dB at 200 kS/s while consuming 7.68 μW and 96 μW, respectively. The corresponding Schreier FoM are 166.7 dB and 166.3 dB. Since the bandwidth of CCP is tunable, the ADC maintains a SNDR > 75 dB upto 260 kHz. The core area occupied by the ADC is 0.589 mm2.As the low-power sensors might be active only for very short time triggered by an external pulse to acquire the data, the third work is a 14-bit asynchronous two-stage pipelined SAR ADC which has been designed and simulated in 0.18-μm CMOS process. A self-synchronous loop based on an edge detector is utilized to generate an internal clock with variable phase. A tunable delay element enables to allocate the available time for the switch capacitor DACs and the gain-stage. Three separate asynchronous clock generators are implemented to create the control signals for two sub-ADCs and the gain-stage between. Aiming to reduce the power consumption of the gain-stage, simple source followers as the analog buffers are implemented in the 3-stage CCP gain-stage. Post-layout simulation results show that the ADC achieves a SNDR of 83.5 dB while consuming 2.39 μW with a sampling rate of 10 kS/s. The corresponding Schreier FoM is 176.7 dB.
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| 4. |
- Irukulapati, Naga Vishnukanth, 1987
(author)
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Towards the Limits of Nonlinearity Compensation for Fiber-Optic Channels
- 2016
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Doctoral thesis (other academic/artistic)abstract
- The performance of long-haul coherent optical systems is fundamentally limited by fiber nonlinearity and its interplay with chromatic dispersion and noise. Due to nonlinearity, the signal propagating through the fiber interacts with itself and with the noise generated from the inline amplifiers. This process results in nonlinearinter-symbol interference (NISI) and nonlinear signal–noise interaction (NSNI). The state-of-the-art algorithm for combating these impairments is digital backpropagation (DBP) and is typically used as a benchmark against other detectors. However, DBP compensates only for NISI, while studies have revealed that NSNI limits the capacity of the coherent optical communications. The goal of the thesis isto use a methodical approach to develop a near-optimal nonlinearity compensation algorithm that also accounts for NSNI. This allows us to identify the fundamental performance limits of the fiber-optic channel.Starting from the maximum a posteriori principle, we develop an algorithm called stochastic digital backpropagation (SDBP) using the framework of factor graphs. In contrast to DBP, SDBP accounts not only for NISI but also for NSNI. To account for the effects of pulse shaping, we propose three variants of SDBP inthis thesis. In the first variant, the output of SDBP is processed using a matched filter (MF) followed by sampling, and decisions are taken on a symbol-by-symbol (SBS) basis. In terms of symbol error rate (SER), SBS-SDBP has better performance than DBP. However, residual memory remains after performing the MF asthe MF operation need not be the optimal processing for the fiber-optic channel. This is accounted for in the second variant of SDBP, where the Viterbi algorithm is used after the MF to compensate for the residual memory. The SER of this variant is further improved compared to SBS-SDBP. In the third variant of SDBP,we use Gaussian message passing to account for the effect of pulse shaping, instead of using the MF. The SER of this third variant of SDBP is better than SBS-SDBP. For estimating the achievable throughput in a typical transmission system, mutual information is a better metric than error rate for soft-decision coded optical systems. We show that SDBP can be used as a tool to compute lower bounds on the mutual information, which are tighter than those obtained using DBP.
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| 5. |
- Navarro, Jaime Rodrigo, et al.
(author)
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Carrier Phase Recovery Algorithms for Coherent Optical Circular mQAM Systems
- 2016
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In: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 34:11, s. 2717-2723
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Journal article (peer-reviewed)abstract
- The phase noise tolerance of circular multilevel quadrature amplitude modulation (C-mQAM) constellations employing different carrier phase recovery (CPR) algorithms is studied. A differential decoding scheme and a bit mapping for this type of constellations are proposed. A novel CPR scheme for C-mQAM constellations is also presented. The particular distribution of the constellation points in a C-mQAM signal is exploited to reduce the required Nth power for the removal of the modulation component by a factor of two. Hence, the computational complexity of the proposed algorithm is drastically reduced. The combined linewidth symbol duration product (Delta nu T-s) tolerance of different CPR algorithms for C-mQAM constellations is studied and compared with the proposed CPR scheme. The results are analyzed at 3.8e-3 and 1e-2 bit error rate forward error correction limits. The proposed CPR scheme achieves similar Delta nu Ts tolerance compared to single stage BPS algorithm while its computational complexity is reduced by group factors of 27.2 vertical bar 32.3, and 30.5 vertical bar 32.6 (in the form of multipliers vertical bar adders) for C-16QAM and C-64QAM, respectively.
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| 6. |
- Ström, Erik, 1965
(author)
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Optimal detection of digital modulations in AWGN
- 2016
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In: Academic Press Library in Mobile and Wireless Communications: Transmission Techniques for Digital Communications. - 9780123982810 ; , s. 121-169
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Book chapter (other academic/artistic)abstract
- The material developed in this chapter can be found in many other books, but perhaps presented in a different manner. The approach here is inspired by several classical books and countless other sources, including discussions with teachers, students, and colleagues. It would be impossible to list all of them. However, the choice to start the exposition with the minimum-distance detector is inspired by Lee and Messerschmidt [1]. Another important source of inspiration is the classical book from1965 by Wozencraft and Jacobs [2], which is very readable even today. An accessible introduction to estimation and detection theory is found in Kay’s two books [3,4], and a more advanced text is by Poor [5].Authors are always faced with a dilemma when selecting notation and the level of mathematical rigor to employ. Too little rigor leaves the reader unsure about what exactly is claimed, while too much rigor will obscure the big picture. For readers who would enjoy a more precise and rigorous approach than the one taken here, we recommend the excellent books by Lapidoth [6] and by Gallager [7]. In particular, the shortcuts taken here (including the use of white noise and Dirac delta functions, the fearless faith in that all integrals and sums converge, and that it is always allowed to change the order of expectation, integration, and summation) are criticized, explained, and (sometimes) justified in these books. Readers can, however, rely on the fact that the developed detectors and analysis methods in this chapter are indeed the same as would have been obtained with a more rigorous approach.
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| 7. |
- Karlsson, Marcus, 1988-
(author)
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Aspects of Massive MIMO
- 2016
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Licentiate thesis (other academic/artistic)abstract
- Next generation cellular wireless technology faces tough demands: increasing the throughput and reliability without consuming more resources, be it spectrum or energy. Massive mimo (Multiple-Input Multiple-Output) has proven, both in theory and practice, that it is up for the challenge. Massive mimo can offer uniformly good service to many users using low-end hardware, simultaneously, without increasing the radiated power compared to contemporary system. In Massive mimo, the base stations are equipped with hundreds of antennas. This abundance of antennas brings many new, interesting aspects compared to single-user mimo and multi-user mimo. Some issues of older technologies are nonexistent in massive mimo, while new issues in need of solutions arise. This thesis considers two aspects, and how these aspects differ in a massive mimo context: physical layer security and transmission of system information. First, it is shown that a jammer with a large number of antennas can outperform a traditional, single-antenna jammer in degrading the legitimate link. The excess of antennas gives the jammer opportunity to find and exploit structure in signals to improve its jamming capability. Second, for transmission of system information, the vast number of antennas prove useful even when the base station does not have any channel state information, because of the increased availability of space-time coding. We show how transmission without channel state information can be done in massive mimo by using a fixed precoding matrix to reduce the pilot overhead and simultaneously apply space-time block coding to use the excess of antennas for spatial diversity.
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| 8. |
- Aramrattana, Maytheewat
(author)
-
Modelling and Simulation for Evaluation of Cooperative Intelligent Transport System Functions
- 2016
-
Licentiate thesis (other academic/artistic)abstract
- Future vehicles are expected to be equipped with wireless communication technology, that enables them to be “connected” to each others and road infrastructures. Complementing current autonomous vehicles and automated driving systems, the wireless communication allows the vehicles to interact, cooperate, and be aware of its surroundings beyond their own sensors’ range. Such sys- tems are often referred to as Cooperative Intelligent Transport Systems (C-ITS), which aims to provide extra safety, efficiency, and sustainability to transporta- tion systems. Several C-ITS applications are under development and will require thorough testing and evaluation before their deployment in the real-world. C- ITS depend on several sub-systems, which increase their complexity, and makes them difficult to evaluate.Simulations are often used to evaluate many different automotive applications, including C-ITS. Although they have been used extensively, simulation tools dedicated to determine all aspects of C-ITS are rare, especially human factors aspects, which are often ignored. The majority of the simulation tools for C-ITS rely heavily on different combinations of network and traffic simulators. The human factors issues have been covered in only a few C-ITS simulation tools, that involve a driving simulator. Therefore, in this thesis, a C-ITS simulation framework that combines driving, network, and traffic simulators is presented. The simulation framework is able to evaluate C-ITS applications from three perspectives; a) human driver; b) wireless communication; and c) traffic systems.Cooperative Adaptive Cruise Control (CACC) and its applications are chosen as the first set of C-ITS functions to be evaluated. Example scenarios from CACC and platoon merging applications are presented, and used as test cases for the simulation framework, as well as to elaborate potential usages of it. Moreover, approaches, results, and challenges from composing the simulation framework are presented and discussed. The results shows the usefulness of the proposed simulation framework.
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| 9. |
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| 10. |
- Björklund, Svante, et al.
(author)
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Measurement of Rank and Other Properties of Direct and Scattered Signals
- 2016
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In: International Journal of Antennas and Propagation. - : Hindawi Publishing Corporation. - 1687-5869 .- 1687-5877. ; 2016
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Journal article (peer-reviewed)abstract
- We have designed an experiment for low-cost indoor measurements of rank and other properties of direct and scattered signals with radar interference suppression in mind. The signal rank is important also in many other applications, for example, DOA (Direction of Arrival) estimation, estimation of the number of and location of transmitters in electronic warfare, and increasing the capacity in wireless communications. In real radar applications, such measurements can be very expensive, for example, involving airborne radars with array antennas. We have performed the measurements in an anechoic chamber with several transmitters, a receiving array antenna, and a moving reflector. Our experiment takes several aspects into account: transmitted signals with different correlation, decorrelation of the signals during the acquisition interval, covariance matrix estimation, noise eigenvalue spread, calibration, near-field compensation, scattering in a rough surface, and good control of the influencing factors. With our measurements we have observed rank, DOA spectrum, and eigenpatterns of direct and scattered signals. The agreement of our measured properties with theoretic and simulated results in the literature shows that our experiment is realistic and sound. The detailed description of our experiment could serve as help for conducting other well-controlled experiments. © Copyright 2016 Svante Björklund et al.
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