Pilot Decontamination Through Pilot Sequence Hopping in Massive MIMO Systems

This work concerns wireless cellular networks applying massive multiple-input multiple-output (MIMO) technology. In such a system, the base station in a given cell is equipped with a very large number (hundreds or even thousands) of antennas and serves multiple users. Estimation of the channel from the base station to each user is performed at the base station using an uplink pilot sequence. Such a channel estimation procedure suffers from pilot contamination. Orthogonal pilot sequences are used in a given cell but, due to the shortage of orthogonal sequences, the same pilot sequences must be reused in neighboring cells, causing pilot contamination. The solution presented in this paper suppresses pilot contamination, without the need for coordination among cells. Pilot sequence hopping is performed at each transmission slot, which provides a randomization of the pilot contamination. Using a modified Kalman filter, it is shown that such randomized contamination can be significantly suppressed. Comparisons with conventional estimation methods show that the mean squared error can be lowered as much as an order of magnitude at low mobility

The question of ritual: a cognitive approach

Why does ritual continue to be an issue in religious studies and in anthropology? In this paper the author proposes a cognitive approach to rituals, focusing those aspects of rituals that are distinct from other types of actions, together with what cognitive responses these differences provoke. It will be argued that rituals violate basic causal assumptions and by doing so, trigger off cognitive processes in order to ascribe purpose and meaning to the action. In conclusion, this will be related to findings in ethology and evolutionary theory, arguing that ritual as a behavioural category plays an important role in the formation of symbolic thinking

Cognition and Religious Phenomena - A Response to Håkan Rydving

No abstract available

Design and Analysis of LT Codes with Decreasing Ripple Size

In this paper we propose a new design of LT codes, which decreases the amount of necessary overhead in comparison to existing designs. The design focuses on a parameter of the LT decoding process called the ripple size. This parameter was also a key element in the design proposed in the original work by Luby. Specifically, Luby argued that an LT code should provide a constant ripple size during decoding. In this work we show that the ripple size should decrease during decoding, in order to reduce the necessary overhead. Initially we motivate this claim by analytical results related to the redundancy within an LT code. We then propose a new design procedure, which can provide any desired achievable decreasing ripple size. The new design procedure is evaluated and compared to the current state of the art through simulations. This reveals a significant increase in performance with respect to both average overhead and error probability at any fixed overhead

Reduction of Oscillations in Hydraulically Actuated Knuckle Boom Cranes

Doktorgradsavhandling ved Universitetet i Agder, Institutt for ingeniørvitenskap, 2016A knuckle boom crane is characterized by being a versatile machine that during operation experiences large load variations caused by the changes in position and payload. Common uses are as a mobile loader crane mounted on trucks and in offshore applications. Since their introduction the use of counterbalance valves (CBV) have been the de facto standard on load-carrying hydraulically actuated applications like the knuckle boom crane. It offers a simple and practical solution to one of the issues of mobile cranes: Controlling the load safely when lowering. By law (e.g. European Standard) the hydraulic circuit of load-carrying applications is required to contain a load holding protection device. The classical way of actuating such a crane is to use a circuit containing a pressure compensator valve and a directional control valve (DCV) in series with a CBV. This circuit is referred to as the base circuit. It is well known that this combination of valve components tends to introduce instability in the base circuit. This is mainly a problem when the controlled actuator is subjected to a negative load, because this will require the CBV to throttle the return flow. The instability presents itself as pressure oscillations in the hydraulic circuit which cause the mechanical structure to oscillate. The consequence of the oscillations is a decreased accuracy of the boom motion which create a safety risk, reduces productivity and introduces an undesirable extra fatigue load. The objective of this project was to investigate the oscillations created in the hydraulic circuit of knuckle boom cranes and reduce their severity. The effort has mainly been split in two: First, was looked into existing solutions with the focus on the ones not requiring control systems to function. This was done with reliability and robustness in mind. The investigation identified the pressure control valve (PCV) as the best commercially available solution. The use of a PCV to control the inlet flow in crane applications was rather uncharted territory. The valve, a DCV with a pressure control spool manufactured by Danfoss, has been investigated both theoretically and experimentally. A linear stability analysis has been performed with the Routh-Hurwitz stability criterion. This analysis of the valve used together with a hydraulically actuated experimental setup indicates that the combination is stable in all situations. The use of the pressure control spool in the DCV is a simple and robust solution to the stability problem of the base circuit. Not related to the PCV’s ability to reduce the oscillations the use of it in knuckle boom cranes, however, comes with certain drawbacks. The drawbacks include a load dependent dead band and a load dependent inlet flow. In order to achieve similar behavior as the normal pressure compensated DCV a closed loop control system is required. These issues are addressed in this project, where control schemes are proposed to handle them. In the second part the perspective of the search was broadened to include solutions using control systems. This has lead to the development of a novel, patent pending, concept that significantly reduces the oscillations of the base circuit. It introduces a secondary circuit where a low-pass filtered value of the load pressure is generated and fed back to the compensator of the flow supply valve. The work has demonstrated a significant improvement of stability obtained for a system with the novel concept implemented both theoretically and experimentally. The stability has been investigated both using a linear and a nonlinear model of a hydraulically actuated experimental setup. The presented novel concept circuit has the same steady state characteristics as the base circuit but without the corresponding oscillatory nature. Because the main spool of the DCV is not used for stabilising the system, the novel concept can be combined with any feedback control strategy. In this project, the novel concept is presented with linear actuators only. However,its use covers circuits with rotational actuators and CBV’s as well. The base circuit is used as a reference for comparison. Therefore, the stability of the base circuit is also investigated with a linear model

Massive MIMO for Crowd Scenarios: A Solution Based on Random Access

This paper presents a new approach to intra-cell pilot contamination in crowded massive MIMO scenarios. The approach relies on two essential properties of a massive MIMO system, namely near-orthogonality between user channels and near-stability of channel powers. Signal processing techniques that take advantage of these properties allow us to view a set of contaminated pilot signals as a graph code on which iterative belief propagation can be performed. This makes it possible to decontaminate pilot signals and increase the throughput of the system. The proposed solution exhibits high performance with large improvements over the conventional method. The improvements come at the price of an increased error rate, although this effect is shown to decrease significantly for increasing number of antennas at the base station

Are the Intraday Effects of Central Bank Intervention on Exchange Rate Spreads Asymmetric and State Dependent?

This paper investigates the intraday effects of unannounced foreign exchange intervention on bid-ask exchange rate spreads using official intraday intervention data provided by the Danish central bank. Our starting point is a simple theoretical model of the bid-ask spread which we use to formulate testable hypotheses regarding how unannounced intervention purchases and intervention sales influence the market asymmetrically. To test these hypotheses we estimate weighted least squares (WLS) time-series models of the intraday bid-ask spread. Our main result is that intervention purchases and sales both exert a significant influence on the exchange rate spread, but in opposite directions: intervention purchases of the smaller currency, on average, reduce the spread while intervention sales, on average, increase the spread. We also show that intervention only affects the exchange rate spread when the state of the market is not abnormally volatile. Our results are consistent with the notion that illiquidity arises when traders fear speculative pressure against the smaller currency and confirms the asymmetry hypothesis of our theoretical model.Foreign Exchange Intervention; Exchange Rate Spreads; Intraday Data

Are the intraday effects of central bank intervention on exchange rate spreads asymmetric and state dependent?

This paper investigates the intraday effects of unannounced foreign exchange intervention on bid-ask exchange rate spreads using official intraday intervention data provided by the Danish central bank. Our starting point is a simple theoretical model of the bid-ask spread which we use to formulate testable hypotheses regarding how unannounced intervention purchases and intervention sales influence the market asymmetrically. To test these hypotheses we estimate weighted least squares (WLS) time-series models of the intraday bid-ask spread. Our main result is that intervention purchases and sales both exert a significant influence on the exchange rate spread, but in opposite directions: intervention purchases of the smaller currency, on average, reduce the spread while intervention sales, on average, increase the spread. We also show that intervention only affects the exchange rate spread when the state of the market is not abnormally volatile. Our results are consistent with the notion that illiquidity arises when traders fear speculative pressure against the smaller currency and confirms the asymmetry hypothesis of our theoretical model.Financial markets ; Banks and banking, Central ; Monetary policy ; Foreign exchange rates ; International finance

Coming From Good Stock: Career Histories and New Venture Formation

We examine how the social structure of existing organizations influences entrepreneurship and suggest that resources accrue to entrepreneurs based on the structural position of their prior employers. We argue that information advantages allow individuals from entrepreneurially prominent prior firms to identify new opportunities. Entrepreneurial prominence also reduces the perceived uncertainty of a new venture. Using a sample of Silicon Valley start-ups, we demonstrate that entrepreneurial prominence is associated with initial strategy and the probability of attracting external financing. New ventures with high prominence are more likely to be innovators; furthermore, innovators with high prominence are more likely to obtain financing