Space-time autocoding

Prior treatments of space-time communications in Rayleigh flat fading generally assume that channel coding covers either one fading interval-in which case there is a nonzero “outage capacity”-or multiple fading intervals-in which case there is a nonzero Shannon capacity. However, we establish conditions under which channel codes span only one fading interval and yet are arbitrarily reliable. In short, space-time signals are their own channel codes. We call this phenomenon space-time autocoding, and the accompanying capacity the space-time autocapacity. Let an M-transmitter antenna, N-receiver antenna Rayleigh flat fading channel be characterized by an M×N matrix of independent propagation coefficients, distributed as zero-mean, unit-variance complex Gaussian random variables. This propagation matrix is unknown to the transmitter, it remains constant during a T-symbol coherence interval, and there is a fixed total transmit power. Let the coherence interval and number of transmitter antennas be related as T=βM for some constant β. A T×M matrix-valued signal, associated with R·T bits of information for some rate R is transmitted during the T-symbol coherence interval. Then there is a positive space-time autocapacity Ca such that for all R<Ca, the block probability of error goes to zero as the pair (T, M)→∞ such that T/M=β. The autocoding effect occurs whether or not the propagation matrix is known to the receiver, and Ca=Nlog(1+ρ) in either case, independently of β, where ρ is the expected signal-to-noise ratio (SNR) at each receiver antenna. Lower bounds on the cutoff rate derived from random unitary space-time signals suggest that the autocoding effect manifests itself for relatively small values of T and M. For example, within a single coherence interval of duration T=16, for M=7 transmitter antennas and N=4 receiver antennas, and an 18-dB expected SNR, a total of 80 bits (corresponding to rate R=5) can theoretically be transmitted with a block probability of error less than 10^-9, all without any training or knowledge of the propagation matrix

The academic and industrial embrace of space-time methods

[Guest Editors introduction to: Special issue on space-time transmission, reception, coding and signal processing] Every episode of the classic 1966–1969 television series Star Trek begins with Captain Kirk’s (played by William Shatner) famous words : “Space: The final frontier….” While space may not be the final frontier for the information and communication theory community, it is proving to be an important and fruitful one. In the information theory community, the notion of space can be broadly defined as the simultaneous use of multiple, possibly coupled, channels. The notions of space–time and multiple-input multiple-output (MIMO) channels are therefore often used interchangeably. The connection between space and MIMO is most transparent when we view the multiple channels as created by two or more spatially separated antennas at a wireless transmitter or receiver. A large component of the current interest in space–time methods can be attributed to discoveries in the late 1980s and early 1990s that a rich wireless scattering environment can be beneficial when multiple antennas are used on a point-to-point link. We now know that adding antennas in a rich environment provides proportional increases in point-to-point data rates, without extra transmitted power or bandwidth

Space-time autocoding constellations with pairwise-independent signals

The space-time autocoding effect implies that arbitrarily reliable communication is possible within a single coherence interval in Rayleigh flat fading as the symbol-duration of the coherence interval and the number of transmit antennas grow simultaneously. For relatively short (e.g., 16-symbol) coherence intervals, a codebook of isotropically random unitary space-time signals theoretically supports transmission rates that are a significant fraction of autocapacity with an extremely low probability of error. However a constellation of the required size (typically L = 280) is impossible to generate and store, and due to lack of structure there is little hope of finding a fast decoding scheme. We propose a random, but highly structured, constellation that is completely specified by log, L independent isotropically distributed unitary matrices. The distinguishing property of this construction is that any two signals in the constellation are pairwise statistically independent and isotropically distributed. Thus, the pairwise probability of error, and hence the union bound on the block probability of error, of the structured constellation is identical to that of a fully random constellation of independent signals

Space-time autocoding: arbitrarily reliable communication in a single fading interval

Prior treatments of space-time communications in Rayleigh flat fading generally assume that channel coding covers either one fading interval-in which case there is a nonzero “outage capacity”-or multiple fading intervals-in which case there is a nonzero Shannon capacity. However, we establish conditions under which channel codes span only one fading interval and yet are arbitrarily reliable. In short, space-time signals are their own channel codes. We call this phenomenon space-time autocoding, and the accompanying capacity the space-time autocapacity

Structured unitary space-time autocoding constellations

We previously showed that arbitrarily reliable communication is possible within a single coherence interval in Rayleigh flat fading as the symbol duration of the coherence interval and the number of transmit antennas grow simultaneously. This effect, where the space-time signals act as their own channel codes, is called autocoding. For relatively short (e.g., 16-symbol) coherence intervals, a codebook of independent isotropically random unitary space-time signals theoretically supports transmission rates that are a significant fraction of autocapacity with an extremely low probability of error. The exploitation of space-time autocoding requires the creation and decoding of extraordinarily large constellations-typically L = 2^80. We make progress on the first part of the problem through a random, but highly structured, constellation that is completely specified by log2 L independent isotropically distributed unitary matrices. The distinguishing property of this construction is that any two signals in the constellation are pairwise statistically independent and isotropically distributed. Thus, the pairwise probability of error, and hence the union bound on the block probability of error, of the structured constellation is identical to that of a fully random constellation of independent signals. We establish the limitations of an earlier construction through a subsidiary result that is interesting in its own right: the square (or for that matter, any integer power greater than one) of an isotropically random unitary matrix is not isotropically random, with the sole exception of the one-by-one unitary matrix

Emergent Orthotopic Liver Transplantation for Hemorrhage from a Giant Cavernous Hepatic Hemangioma: Case Report and Review

IntroductionCavernous hemangiomas represent the most common benign primary hepatic neoplasm, often being incidentally detected. Although the majority of hepatic hemangiomas remain asymptomatic, symptomatic hepatic hemangiomas can present with abdominal pain, hemorrhage, biliary compression, or a consumptive coagulopathy. The optimal surgical management of symptomatic hepatic hemangiomas remains controversial, with resection, enucleation, and both deceased donor and living donor liver transplantation having been reported.Case reportWe report the case of a patient found to have a unique syndrome of multiorgan cavernous hemangiomatosis involving the liver, lung, omentum, and spleen without cutaneous involvement. Sixteen years following her initial diagnosis, the patient suffered from intra-abdominal hemorrhage due to her giant cavernous hepatic hemangioma. Evidence of continued bleeding, in the setting of Kasabach-Merritt Syndrome and worsening abdominal compartment syndrome, prompted MELD exemption listing. The patient subsequently underwent emergent liver transplantation without complication.ConclusionAlthough cavernous hemangiomas represent the most common benign primary hepatic neoplasm, hepatic hemangioma rupture remains a rare presentation in these patients. Management at a center with expertise in liver transplantation is warranted for those patients presenting with worsening DIC or hemorrhage, given the potential for rapid clinical decompensation

Vitamin C and infectious diseases

Non peer reviewe

Microvascular density and hypoxia-inducible factor pathway in pancreatic endocrine tumours: negative correlation of microvascular density and VEGF expression with tumour progression

Tumour-associated angiogenesis is partly regulated by the hypoxia-inducible factor (HIF) pathway. Endocrine tumours are highly vascularised and the molecular mechanisms of their angiogenesis are not fully delineated. The aim of this study is to evaluate angiogenesis and expression of HIF-related molecules in a series of patients with pancreatic endocrine tumours (PETs). The expression of vascular endothelial growth factor (VEGF), HIF-1α, HIF-2α and carbonic anhydrase 9 (CA9) was examined by immunohistochemistry in 45 patients with PETs and compared to microvascular density (MVD), endothelial proliferation, tumour stage and survival. Microvascular density was very high in PETs and associated with a low endothelial index of proliferation. Microvascular density was significantly higher in benign PETs than in PETs of uncertain prognosis, well-differentiated and poorly differentiated carcinomas (mean values: 535, 436, 252 and 45 vessels mm−2, respectively, P<0.0001). Well-differentiated tumours had high cytoplasmic VEGF and HIF-1α expression. Poorly differentiated carcinomas were associated with nuclear HIF-1α and membranous CA9 expression. Low MVD (P=0.0001) and membranous CA9 expression (P=0.0004) were associated with a poorer survival. Contrary to other types of cancer, PETs are highly vascularised, but poorly angiogenic tumours. As they progress, VEGF expression is lost and MVD significantly decreases. The regulation of HIF signalling appears to be specific in pancreatic endocrine tumours

Glutathione in Cancer Cell Death

Glutathione (L-γ-glutamyl-L-cysteinyl-glycine; GSH) in cancer cells is particularly relevant in the regulation of carcinogenic mechanisms; sensitivity against cytotoxic drugs, ionizing radiations, and some cytokines; DNA synthesis; and cell proliferation and death. The intracellular thiol redox state (controlled by GSH) is one of the endogenous effectors involved in regulating the mitochondrial permeability transition pore complex and, in consequence, thiol oxidation can be a causal factor in the mitochondrion-based mechanism that leads to cell death. Nevertheless GSH depletion is a common feature not only of apoptosis but also of other types of cell death. Indeed rates of GSH synthesis and fluxes regulate its levels in cellular compartments, and potentially influence switches among different mechanisms of death. How changes in gene expression, post-translational modifications of proteins, and signaling cascades are implicated will be discussed. Furthermore, this review will finally analyze whether GSH depletion may facilitate cancer cell death under in vivo conditions, and how this can be applied to cancer therapy