University of California, Irvine

About: University of California, Irvine is a education organization based out in Irvine, California, United States. It is known for research contribution in the topics: Population & Galaxy. The organization has 47031 authors who have published 113602 publications receiving 5521832 citations. The organization is also known as: UC Irvine & UCI.

Degrees of Freedom Region of the MIMO $X$ Channel

Abstract: We provide achievability as well as converse results for the degrees of freedom region of a multiple-input multiple-output (MIMO) X channel, i.e., a system with two transmitters, two receivers, each equipped with multiple antennas, where independent messages need to be conveyed over fixed channels from each transmitter to each receiver. The inner and outer bounds on the degrees of freedom region are tight whenever integer degrees of freedom are optimal for each message. With M = 1 antennas at each node, we find that the total (sum rate) degrees of freedom are bounded above and below as 1 les eta*x les 4/3. If M > 1 and channel matrices are nondegenerate then the precise degrees of freedom eta*x = (4/3)M. Thus, the MIMO X channel has noninteger degrees of freedom when M is not a multiple of 3. Simple zero forcing without dirty paper encoding or successive decoding, suffices to achieve the (4/3)M degrees of freedom. If the channels vary with time/frequency then the channel with single antennas (M = 1) at all nodes has exactly 4/3 degrees of freedom. The key idea for the achievability of the degrees of freedom is interference alignment-i.e., signal spaces are aligned at receivers where they constitute interference while they are separable at receivers where they are desired. We also explore the increase in degrees of freedom when some of the messages are made available to a transmitter or receiver in the manner of cognitive radio.

e-Commerce Metrics for Net-Enhanced Organizations: Assessing the Value of e-Commerce to Firm Performance in the Manufacturing Sector

Abstract: In this study, we developed a set of constructs to measure e-commerce capability in Internet-enhanced organizations. The e-commerce capability metrics consist of four dimensions: information, transaction, customization, and supplier connection. These measures were empirically validated for reliability, content, and construct validity. Then we examined the nomological validity of these e-commerce metrics in terms of their relationships to firm performance, with data from 260 manufacturing companies divided into high IT-intensity and low IT-intensity sectors. Grounded in the dynamic capabilities perspective and the resource-based theory of the firm, a series of hypotheses were developed. After controlling for variations of industry effects and firm size, our empirical analysis found a significant relationship between e-commerce capability and some measures of firm performance (e.g., inventory turnover), indicating that the proposed metrics have demonstrated value for capturing e-commerce effects. However, our analysis showed that e-commerce tends to be associated with the increased cost of goods sold for traditional manufacturing companies, but there is an opposite relationship for technology companies. This result seems to highlight the role of resource complementarity for the business value of e-commerce--traditional companies need enhanced alignment between e-commerce capability and their existing IT infrastructure to reap the benefits of e-commerce.

Phylogeography: retrospect and prospect

Abstract: Phylogeography has grown explosively in the two decades since the word was coined and the discipline was outlined in 1987. Here I summarize the many achievements and novel perspectives that phylogeography has brought to population genetics, phylogenetic biology and biogeography. I also address future directions for the field. From the introduction of mitochondrial DNA assays in the late 1970s, to the key distinction between gene trees and species phylogenies, to the ongoing era of multi-locus coalescent theory, phylogeographic perspectives have consistently challenged conventional genetic and evolutionary paradigms, and they have forged empirical and conceptual bridges between the formerly separate disciplines of population genetics (microevolutionary analysis) and phylogenetic biology (in macroevolution).