Composite CDMA - A statistical mechanics analysis

Code Division Multiple Access (CDMA) in which the spreading code assignment to users contains a random element has recently become a cornerstone of CDMA research. The random element in the construction is particular attractive as it provides robustness and flexibility in utilising multi-access channels, whilst not making significant sacrifices in terms of transmission power. Random codes are generated from some ensemble, here we consider the possibility of combining two standard paradigms, sparsely and densely spread codes, in a single composite code ensemble. The composite code analysis includes a replica symmetric calculation of performance in the large system limit, and investigation of finite systems through a composite belief propagation algorithm. A variety of codes are examined with a focus on the high multi-access interference regime. In both the large size limit and finite systems we demonstrate scenarios in which the composite code has typical performance exceeding sparse and dense codes at equivalent signal to noise ratio.Comment: 23 pages, 11 figures, Sigma Phi 2008 conference submission - submitted to J.Stat.Mec

Reef Structure Alternatives for Restoration of Oyster (Crassostrea virginica) Populations in New Hampshire

Eastern oyster (Crassostrea virginica) populations in New Hampshire have experienced severe declines since the 1990s, and restoration of oyster populations has been a major goal for New Hampshire management agencies. The most widely used technique in recent years in New Hampshire has been spat seeding which involves setting larvae from disease-resistant and/or fast-growth broodstock onto cultch material in large shore-based tanks, then distributing the spat attached to cultch onto the bottom to initiate reef restoration. This approach has the dual potential of providing direct population enhancement as well as introduction to the local gene pool of disease-resistance and/or fast-growth potential. Although spat seeding has been shown to be an effective technique much remains to be learned about the overall restoration process, particularly specific design criteria, the most effective combinations of methods, and long-term viability of restored bottom areas. The present project was designed in part based on results of earlier experimental work (mainly the use of spat seeding) to address the general management question: How should reefs be structurally enhanced (if at all) to enhance oyster populations and improve spat set

Computing with Noise - Phase Transitions in Boolean Formulas

Computing circuits composed of noisy logical gates and their ability to represent arbitrary Boolean functions with a given level of error are investigated within a statistical mechanics setting. Bounds on their performance, derived in the information theory literature for specific gates, are straightforwardly retrieved, generalized and identified as the corresponding typical-case phase transitions. This framework paves the way for obtaining new results on error-rates, function-depth and sensitivity, and their dependence on the gate-type and noise model used.Comment: 10 pages, 2 figure

Social Support Networks of Gay Men

Although social workers and other helping professionals frequently stress the importance of social networks among gay men, there has been little empirical research to describe these networks. In the present study, the authors analyze data on perceived social support from 166 gay men recruited through gay community groups and social networks. Most gay men were found to have large and diverse social networks. Frequent communication occurs between respondents and network members, most of whom know of respondents\u27 homosexuality. The most frequent and supportive network member was a close friend, and the most common type of support received was emotional. Those not in a committed relationship, and those living alone, are more likely to report feelings of loneliness and to talk to network members more often

Water Delivery and Giant Impacts in the 'Grand Tack' Scenario

A new model for terrestrial planet formation (Hansen 2009, Walsh et al. 2011) has explored accretion in a truncated protoplanetary disk, and found that such a configuration is able to reproduce the distribution of mass among the planets in the Solar System, especially the Earth/Mars mass ratio, which earlier simulations have generally not been able to match. Walsh et al. tested a possible mechanism to truncate the disk--a two-stage, inward-then-outward migration of Jupiter and Saturn, as found in numerous hydrodynamical simulations of giant planet formation. In addition to truncating the disk and producing a more realistic Earth/Mars mass ratio, the migration of the giant planets also populates the asteroid belt with two distinct populations of bodies--the inner belt is filled by bodies originating inside of 3 AU, and the outer belt is filled with bodies originating from between and beyond the giant planets (which are hereafter referred to as `primitive' bodies). We find here that the planets will accrete on order 1-2% of their total mass from primitive planetesimals scattered onto planet-crossing orbits during the formation of the planets. For an assumed value of 10% for the water mass fraction of the primitive planetesimals, this model delivers a total amount of water comparable to that estimated to be on the Earth today. While the radial distribution of the planetary masses and the dynamical excitation of their orbits are a good match to the observed system, we find that the last giant impact is typically earlier than 20 Myr, and a substantial amount of mass is accreted after that event. However, 5 of the 27 planets larger than half an Earth mass formed in all simulations do experience large late impacts and subsequent accretion consistent with the dating of the Moon-forming impact and the estimated amount of mass accreted by Earth following that event