SUMMARY / RELATED TOPICS

Multipath propagation

In radio communication, multipath is the propagation phenomenon that results in radio signals reaching the receiving antenna by two or more paths. Causes of multipath include atmospheric ducting, ionospheric reflection and refraction, reflection from water bodies and terrestrial objects such as mountains and buildings. Multipath propagation causes multipath interference, including constructive and destructive interference, phase shifting of the signal; this may cause a radio signal to become too weak in certain areas to be received adequately, so multipath propagation can be detrimental in radio communication systems. Where the magnitudes of the signals arriving by the various paths have a distribution known as the Rayleigh distribution, this is known as Rayleigh fading. Where one component dominates, a Rician distribution provides a more accurate model, this is known as Rician fading. Multipath interference is a phenomenon in the physics of waves whereby a wave from a source travels to a detector via two or more paths and, under the right condition, the two components of the wave interfere.

Multipath interference is a common cause of "ghosting" in analog television broadcasts and of fading of radio waves. The condition necessary is that the components of the wave remain coherent throughout the whole extent of their travel; the interference will arise owing to the two components of the wave having, in general, travelled a different length, thus arriving at the detector out of phase with each other. The signal due to indirect paths interferes with the required signal in amplitude as well as phase, called multipath fading. In facsimile and television transmission, multipath causes jitter and ghosting, seen as a faded duplicate image to the right of the main image. Ghosts occur when transmissions bounce off a mountain or other large object, while arriving at the antenna by a shorter, direct route, with the receiver picking up two signals separated by a delay. In radar processing, multipath causes ghost targets to appear; these ghosts are bothersome since they move and behave like the normal targets, so the receiver has difficulty in isolating the correct target echo.

These problems can be minimized by incorporating a ground map of the radar's surroundings and eliminating all echoes which appear to originate below the ground or above a certain height. In digital radio communications multipath can affect the quality of communications; the errors are due to intersymbol interference. Equalizers are used to correct the ISI. Alternatively, techniques such as orthogonal frequency division modulation and rake receivers may be used. In a Global Positioning System receiver, Multipath Effect can cause a stationary receiver's output to indicate as if it were randomly jumping about or creeping; when the unit is moving the jumping or creeping may be hidden, but it still degrades the displayed accuracy of location and speed. Multipath propagation is similar in telephone local loops. In either case, impedance mismatch causes signal reflection. High-speed power line communication systems employ multi-carrier modulations to avoid the intersymbol interference that multipath propagation would cause.

The ITU-T G.hn standard provides a way to create a high-speed local area network using existing home wiring. G.hn uses OFDM with a cyclic prefix to avoid ISI. Because multipath propagation behaves differently in each kind of wire, G.hn uses different OFDM parameters for each media. DSL modems use Orthogonal frequency-division multiplexing to communicate with their DSLAM despite multipath. In this case the reflections may be caused by mixed wire gauges, but those from bridge taps are more intense and complex. Where OFDM training is unsatisfactory, bridge taps may be removed; the mathematical model of the multipath can be presented using the method of the impulse response used for studying linear systems. Suppose you want to transmit a signal, ideal Dirac pulse of electromagnetic power at time 0, i.e. x = δ At the receiver, due to the presence of the multiple electromagnetic paths, more than one pulse will be received, each one of them will arrive at different times. In fact, since the electromagnetic signals travel at the speed of light, since every path has a geometrical length different from that of the other ones, there are different air travelling times.

Thus, the received signal will be expressed by y = h = ∑ n = 0 N − 1 ρ n e j ϕ n δ where N is the number of received impulses, τ n is the time delay of the generic n t

Pyotr Kotlyarevsky

Pyotr Stepanovich Kotlyarevsky was a Russian military hero of the early 19th century. He was born in the village of Olkhovatka near Kharkiv into a cleric's family. Kotlyarevsky was brought up in an infantry regiment quartered near Mozdok, he was promoted officer for his exploits during Count Zubov's Persian Campaign in 1796. His leadership and boldness made him a national celebrity during the Russo-Turkish War #Caucasus Front and the Russo-Persian War. In 1810 he took hold of Meghri Citadel, withstood a siege by the Persian army and routed them on the Araks River. In 1812, he defeated Abbas Mirza in the Battle of Aslanduz and stormed Lankaran with as little as around 2,500 soldiers. Thereupon the Persians sued for peace, Kotlyarevsky was promoted General of Infantry. Suffering from wounds, he had to retire from active service and spent the rest of his life in Feodosiya, where he died as well. A local artist, Ivan Aivazovsky, devised Kotlyarevsky's mausoleum, while Prince Vorontsov ordered his statue to be erected in Ganja.

Biography

Homocitrulline

L-Homocitrulline is an amino acid and a metabolite of ornithine in mammalian metabolism. The amino acid can be detected in larger amounts in the urine of individuals with urea cycle disorders. At present, it is thought that the depletion of the ornithine supply causes the accumulation of carbamyl-phosphate in the urea cycle which may be responsible for the enhanced synthesis of homocitrulline and homoarginine. Both amino acids can be detected in urine. Amino acid analysis allows for the quantitative analysis of these amino acid metabolites in biological fluids such as urine or blood. Homocitrulline is one methylene group longer than citrulline, but similar in structure; the metabolite is generated from a lysine residue. Cyanate is present in the human body in equilibrium with urea. Under physiological conditions the urea concentration may be too low to allow extensive carbamylation. However, the conversion process leading to the formation of homocitrulline from lysine in proteins is known to occur in vivo.

During renal failure conditions, the urea concentration increases and carbamylation of many proteins can occur, which can be detected. It is believed that most carbamylation takes place during inflammation when the enzyme myeloperoxidase is released from neutrophils; this enzyme converts thiocyanate to cyanate. Increased levels of cyanate can now carbamylate lysine residues. Myeloperoxidase released from neutrophils converts thiocyanate to cyanate which carbamylates lysine residues to form homocitrulline. Thiocyanate is a occurring pseudohalide found in dietary sources. Myeloperoxidase can use SCN− as a cosubstrate together with hydrogen peroxide to form cyanate. In patients with kidney dysfunction urea is elevated. Urea is in equilibrium with isocyanate. Carbamylation of nucleophilic amino groups, for example lysine residues, can modify protein structures and cause metabolic dysfunctions. Homocitrulline has been suggested as a confounding antigen for rheumatoid arthritis antibodies targeting citrullinated proteins/peptides.

Antibodies binding to homocitrulline-containing sequences have been found in rheumatoid arthritis patients' sera More it has been shown that homocitrulline-containing proteins are present in rheumatoid arthritis joints. In rodents they may affect T-cell triggering and autoantibody formation, also in humans. In another metabolic disorder, in the hyperornithinemia-hyperammonemia-homocitrullinuria syndrome, first described in 1969, ornithine levels maybe elevated five to ten times in comparison to normal levels. In addition, in this syndrome, levels of alanine, orotic acid and homocitrulline may be elevated as well. In people with hyperammonemia orotic acid and homocitrulline appear to be chronically elevated after a high protein diet, but may be normal when fasting; the metabolic disorder, lysinuric protein intolerance is caused by the body's inability to digest and use certain protein building blocks or amino acids. These are lysine and ornithine; these amino acids are found in many protein-rich foods.

Since in this disorder the human body cannot break down these amino acids people experience nausea and vomiting after ingesting protein rich foods. Associated features of this protein intolerance may include an enlarged liver and spleen, short stature, muscle weakness, impaired immune function, progressively brittle bones that are prone to fracture and a lung disorder called pulmonary alveolar proteinosis may develop. In addition, the accumulation of amino acids in the kidneys can cause end-stage renal disease. In ESRD the kidneys are no longer able to filter fluids and waste products from the body effectively. Dionisi Vici C, Bachmann C, Gambarara M, Colombo JP, Sabetta G: Hyperornithinemia-hyperammonemia-homocitrullinuria syndrome: low creatine excretion and effect of citrulline, arginine, or ornithine supplement. Pediatr Res. 1987 Sep. Evered DF, Vadgama JV: Absorption of homocitrulline from the gastrointestinal tract. Br J Nutr. 1983 Jan. Hommes FA, Roesel RA, Metoki K, Hartlage PL, Dyken PR: Studies on a case of HHH-syndrome.

Neuropediatrics. 1986 Feb. Kato T, Sano M, Mizutani N.. J Inherit Metab Dis. 1989. Kato T, Sano M, Mizutani N: Inhibitory effect of intravenous lysine infusion on urea cycle metabolism. Eur J Pediatr. 1987 Jan. Pubmed: 3107993. Kato T, Sano M, Mizutani N, Hayakawa C: Homocitrullinuria and homoargininuria in hyperargininaemia. J Inherit Metab Dis. 1988. Kato T, Sano M: Effect of ammonium chloride on homocitrulline and homoarginine synthesis from lysine. J Inherit Metab Dis. 1993. Kato T, Sano M, Mizutani N: Homocitrullinuria and homoargininuria in lysinuric protein intolerance. J Inherit Metab Dis. 1989. Koshiishi I, Kobori Y, Imanari T: Determination of citrulline and homocitrulline by high-performance liquid chromatography with post-column derivatization. J Chromatogr. 1990 Oct 26. Kraus LM, Gaber L, Handorf CR, Marti HP, Kraus AP Jr: Carbamoylation of glomerular and tubular proteins in patients with kidney failure: a potential mechanism of ongoing renal damage. Swiss Med Wkly. 2001 Mar 24. Kraus LM, Elberger AJ, Handorf CR, Pabst MJ, Kraus AP Jr: Urea-derived cyanate forms epsilon-amino-carbamoyl-lysine in leukocyte proteins in patients with end-stage renal disease on peritoneal dialysis.

J Lab Clin Med. 1994 Jun. Rajantie J, Simell O, Perheentupa J: Oral administration of epsilon N-acetyllysine and homocitrulline in lysinuric protein intolerance. J Pediatr. 1983 Mar.