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In quantum field theory, the quantum vacuum state (also called the quantum vacuum or vacuum state) is the quantum state with the lowest possible energy. Generally, it contains no physical particles. The word zero-point field is sometimes used as a synonym for the vacuum state of a quantized field which is completely individual.

According to present-day understanding of what is called the vacuum state or the quantum vacuum, it is "by no means a simple empty space". According to quantum mechanics, the vacuum state is not truly empty but instead contains fleeting electromagnetic waves and particles that pop into and out of the quantum field.

The QED vacuum of quantum electrodynamics (or QED) was the first vacuum of quantum field theory to be developed. QED originated in the 1930s, and in the late 1940s and early 1950s it was reformulated by Feynman, Tomonaga, and Schwinger, who jointly received the Nobel prize for this work in 1965. Today the electromagnetic interactions and the weak interactions are unified (at very high energies only) in the theory of the electroweak interaction.

The Standard Model is a generalization of the QED work to include all the known elementary particles and their interactions (except gravity). Quantum chromodynamics (or QCD) is the portion of the Standard Model that deals with strong interactions, and QCD vacuum is the vacuum of quantum chromodynamics. It is the object of study in the Large Hadron Collider and the Relativistic Heavy Ion Collider, and is related to the so-called vacuum structure of strong interactions.

Non-zero expectation value

If the quantum field theory can be accurately described through perturbation theory, then the properties of the vacuum are analogous to the properties of the ground state of a quantum mechanical harmonic oscillator, or more accurately, the ground state of a measurement problem. In this case the vacuum expectation value (VEV) of any field operator vanishes. For quantum field theories in which perturbation theory breaks down at low energies (for example, Quantum chromodynamics or the BCS theory of superconductivity) field operators may have non-vanishing vacuum expectation values called condensates. In the Standard Model, the non-zero vacuum expectation value of the Higgs field, arising from spontaneous symmetry breaking, is the mechanism by which the other fields in the theory acquire mass.

Energy

The vacuum state is associated with a zero-point energy, and this zero-point energy (equivalent to the lowest possible energy state) has measurable effects. In the laboratory, it may be detected as the Casimir effect. In physical cosmology, the energy of the cosmological vacuum appears as the cosmological constant. In fact, the energy of a cubic centimeter of empty space has been calculated figuratively to be one trillionth of an erg (or 0.6 eV). An outstanding requirement imposed on a potential Theory of Everything is that the energy of the quantum vacuum state must explain the physically observed cosmological constant.

Symmetry
For a relativistic field theory, the vacuum is Poincaré invariant, which follows from
Wightman axioms but can be also proved directly without these axioms. Poincaré invariance implies that only scalar combinations of field operators have non-vanishing VEV's. The VEV may break some of the internal symmetries of the Lagrangian of the field theory. In this case the vacuum has less symmetry than the theory allows, and one says that spontaneous symmetry breaking has occurred. See Higgs mechanism, standard model.

Non-linear permittivity

Quantum corrections to Maxwell's equations are expected to result in a tiny nonlinear electric polarization term in the vacuum, resulting in a field-dependent electrical permittivity ε deviating from the nominal value ε0 of vacuum permittivity. These theoretical developments are described, for example, in Dittrich and Gies.
The theory of quantum electrodynamics predicts that the QED vacuum should exhibit a slight nonlinearity so that in the presence of a very strong electric field, the permitivity is increased by a tiny amount with respect to ε0. Subject to ongoing experimental efforts is the effect that a strong electric field would modify the effective permeability of free space, becoming anisotropic with a value slightly below   μ0 in the direction of the electric field and slightly exceeding   μ0 in the perpendicular direction. The quantum vacuum exposed to an electric field thereby exhibits birefringence for an electromagnetic wave travelling in a direction other than that of the electric field. The effect is similar to the Kerr effect but without matter being present. This tiny nonlinearity can be interpreted in terms of virtual pair production A characteristic electric field strength for which the nonlinearities become sizable is predicted to be enormous, about V/m, known as the Schwinger limit; the equivalent Kerr constant has been estimated, being about 1020 times smaller than the Kerr constant of water. Explanations for dichroism from particle physics, outside quantum electrodynamics, also have been proposed. Experimentally measuring such an effect is very difficult, and has not yet been successful.

Virtual particles

The presence of virtual particles can be rigorously based upon the non-commutation of the quantized electromagnetic fields. Non-commutation means that although the average values of the fields vanish in a quantum vacuum, their variances do not. The term "vacuum fluctuations" refers to the variance of the field strength in the minimal energy state, and is described picturesquely as evidence of "virtual particles". It is sometimes attempted to provide an intuitive picture of virtual particles, or variances, based upon the Heisenberg energy-time uncertainty principle:

(with ΔE and Δt being the energy and time variations respectively; ΔE is the accuracy in the measurement of energy and Δt is the time taken in the measurement, and  is the Reduced Planck constant) arguing along the lines that the short lifetime of virtual particles allows the "borrowing" of large energies from the vacuum and thus permits particle generation for short times. Although the phenomenon of virtual particles is accepted, this interpretation of the energy-time uncertainty relation is not universal. One issue is the use of an uncertainty relation limiting measurement accuracy as though a time uncertainty Δt determines a "budget" for borrowing energy ΔE. Another issue is the meaning of "time" in this relation, because energy and time (unlike position  and momentum , for example) do not satisfy a canonical commutation relation (such as ). Various schemes have been advanced to construct an observable that has some kind of time interpretation, and yet does satisfy a canonical commutation relation with energy. The very many approaches to the energy-time uncertainty principle are a long and continuing subject.

Physical nature of the quantum vacuum
According to Astrid Lambrecht (2002): "When one empties out a space of all matter and lowers the temperature to absolute zero, one produces in a Gedankenexperiment [thought experiment] the quantum vacuum state." According to Fowler & Guggenheim (1939/1965), the third law of thermodynamics may be precisely enunciated as follows:
It is impossible by any procedure, no matter how idealized, to reduce any assembly to the absolute zero in a finite number of operations. (See also.Wilks, J. (1971). The Third Law of Thermodynamics, Chapter 6 in Thermodynamics, volume 1, ed. W. Jost, of H. Eyring, D. Henderson, W. Jost, Physical Chemistry. An Advanced Treatise, Academic Press, New York, page 477.)

Photon-photon interaction can occur only through interaction with the vacuum state of some other field, for example through the Dirac electron-positron vacuum field; this is associated with the concept of vacuum polarization. According to Milonni (1994): "... all quantum fields have zero-point energies and vacuum fluctuations." This means that there is a component of the quantum vacuum respectively for each component field (considered in the conceptual absence of the other fields), such as the electromagnetic field, the Dirac electron-positron field, and so on. According to Milonni (1994), some of the effects attributed to the vacuum electromagnetic field can have several physical interpretations, some more conventional than others. The Casimir attraction between uncharged conductive plates is often proposed as an example of an effect of the vacuum electromagnetic field. Schwinger, DeRaad, and Milton (1978) are cited by Milonni (1994) as validly, though unconventionally, explaining the Casimir effect with a model in which "the vacuum is regarded as truly a state with all physical properties equal to zero." In this model, the observed phenomena are explained as the effects of the electron motions on the electromagnetic field, called the source field effect. Milonni writes:

The basic idea here will be that the Casimir force may be derived from the source fields alone even in completely conventional QED, ... Milonni provides detailed argument that the measurable physical effects usually attributed to the vacuum electromagnetic field cannot be explained by that field alone, but require in addition a contribution from the self-energy of the electrons, or their radiation reaction. He writes: "The radiation reaction and the vacuum fields are two aspects of the same thing when it comes to physical interpretations of various QED processes including the Lamb shift, van der Waals forces, and Casimir effects."

This point of view is also stated by Jaffe (2005): "The Casimir force can be calculated without reference to vacuum fluctuations, and like all other observable effects in QED, it vanishes as the fine structure constant, , goes to zero."

Notations
The vacuum state is written as  or . The vacuum expectation value (see also Expectation value) of any field  should be written as .

See also

 Pair production
 Vacuum energy
 Lamb shift
 False vacuum decay
 Squeezed coherent state
 Quantum fluctuation
 Scharnhorst effect
 Van der Waals force*
 Casimir effect

References and notes

Further reading
 Free pdf copy of The Structured Vacuum - thinking about nothing by Johann Rafelski and Berndt Muller (1985) .
 M.E. Peskin and D.V. Schroeder, An introduction to Quantum Field Theory.
 H. Genz,  Nothingness: The Science of Empty Space

 E. W. Davis, V. L. Teofilo, B. Haisch, H. E. Puthoff, L. J. Nickisch, A. Rueda and D. C. Cole(2006)"Review of Experimental Concepts for Studying the Quantum Vacuum Field"

External links
 Energy into Matter

Quantum field theory
Articles containing video clips
Vacuum


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Office Space Euston
Serviced Offices in Euston can benefit from the invaluable transport links available, with the Euston station being the gateway to London for rail travellers from the north of the UK, including major cities such as Liverpool, Glasgow and Birmingham. This is why many businesses search for serviced office space in Euston, as it allows easy physical access to both clients and suppliers.
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Euston itself is also a Tube hub, enabling easy access across London, including the central business district, via the Northern Line. Outside of the station, several London attractions are within walking distance, including Regent's Park, London Zoo and the British Museum.
Within just a short walk of the tube station, offices to rent in Euston are available in the period properties of Noho, such as Fitzroy Square, as well as more modern buildings on Tottenham Court Road. Typically, shared offices in Euston provide competitive serviced space, with a short walk to the West End.
Choosing an office space to rent in Euston places you in a prime London location. Here, you will operate from a managed office close to; West London, Euston Tower, Eversholt street, Euston Road, Mabledon place Kings Cross, a number of railway stations, Evergreen House, Chalton Street kings cross and many more.
When conducting office search, the office features of each of the office suites available to rent should be made clear. Some of our Euston office spaces are but a 5 minute walk from a leading underground station, come complete with bike racks, air conditioning, 24hour access, a business lounge, manned reception, excellent transport links and much more.
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A great option for those who don't yet need a work space, or for businesses that don't require a base. A virtual office will provide a prestigious registered address where mail can be sent and organised, a telephone number and call answering or forwarding services, plus offer business facilities should you need them.
Co-Working Office Space
Ideal for sole traders, start-ups and those who just thrive on collaboration; co-working/shared spaces provide you with a hot-desk or designated bank of desks within a building that incorporates other businesses. A straightforward option where you can rock up with a laptop and just get stuff done without the commitment of an entire office.
'The full package' option with negotiable contract length. A serviced office is your one-stop shop for a dedicated and private office for your company and come with internet and telecoms within the offering. For short or longer term arrangements, serviced offices offer a flexible but professional feel that allows growth and in an ever-changing business climate.
A managed office allows you to have a bespoke fit-out and arrange your own IT and telecoms, but in a fairly flexible manner in relation to contract commitment. One operator will take care of all elements including moving in and out, customisation and making sure the space fits your business needs.
A leased office is the business version of buying a house. Similar to a mortgage, you will have a long term loan on the property (leases are for contract terms of 5 years+) and be solely in charge of what goes on under its roof. A leased option is perfect for established companies who are relatively stable in terms of growth and location.
Kevin has been so helpful with the task of finding a great office for us in London. We have grown as a company over the last few years and he has supported our location search throughout this period and has helped us with new spaces on a number of occasions now. As a luxury brand we had a challenge as we needed office space that was befitting to our industry, but as a start-up we had to be monitoring our costs and Kevin delivered us with viable options that were on brand as well as in budget. Cornerstones knowledge of what is available and their approach will certainly help others find the spaces they need with the best advice, throughly recommend them
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Dealing with the guys at Cornerstone has been a straight forward and painless process. They handpicked specific offices for me to view and highlighted the best offers in the market whilst guiding me through the office search process with little effort on my part which has resulted in finding us our perfect office at an attractive rate. They didn't divulge my contact details which ensured that I wasn't hassled by sales calls which has happened in the past when using some of the larger brokers, it meant I had one point of contact throughout and could get on with my day to day.
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I have worked with Kevin in the past when he was part of a previous firm and have used him for a second time with Cornerstone. Both occasions I have used his services to be very professional and was able to find exactly what we needed to suit our budget for an office space. I found with other people there was more of a scatter gun approach, whereas Kevin listened to what we needed and provided with many good options which tailored to our company. I would certainly recommend them to anyone who is looking for offices within London
There are many different serviced office options available in London. However, finding the one that is perfect can take a lot of time and effort. Cornerstone does all of the hard work for you so that you can concentrate on other aspects of running your business.
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Home RANDOM Sports Today – J.J. Watt keeps making big plays, complicating Texans' decisions
Sports Today – J.J. Watt keeps making big plays, complicating Texans' decisions
With one season left on his contract, the Houston Texans have a decision to make on J.J. Watt this offseason.
In the Texans' 41-25 victory over the Detroit Lions on Thanksgiving, Watt showed why that is such a tough decision when he intercepted a pass by Lions quarterback Matthew Stafford and ran it back 19 yards to score, his first touchdown since 2014, when he caught a touchdown pass from Ryan Fitzpatrick when playing on offense.
This comes a game after Watt batted down a career-high four passes in Week 11 against the Patriots.
Everything you need this week:
• Full schedule » | Standings »
• Depth charts for every team »
• Transactions » | Injuries »
• Football Power Index rankings »
But the 31-year-old defensive end, the franchise's best-ever player, is owed $17.5 million (not guaranteed) in 2021 and despite having quarterback Deshaun Watson at quarterback, Houston doesn't have a roster that's ready to compete with the best in the AFC next season.
The Texans didn't really consider trading Watt at the trade deadline, but they could choose to move him after the season, a decision that will be left up to the new general manager the Texans will hire in January. With the respect that Watt has earned in Houston and within the organization, it wouldn't be a surprise to see both sides work together to find a solution that takes what Watt wants into account.
Quarterback breakdown: Watson showed everyone on Thursday that the Texans' 3-7 record is not due to his poor play. In one of his best games of the season, Watson completed 17 of 25 passes for 318 yards and four touchdowns without an interception. He again focused on getting the ball to Will Fuller (six catches for 171 yards and two touchdowns on seven targets) and Brandin Cooks (five catches for 85 yards). According to ESPN Stats & Information, Fuller is the fifth player since the merger to have 150 or more receiving yards and two touchdowns on Thanksgiving.
J.J. Watt has a big cap number next season — his last in his current deal — but is showing he's still got a lot of good football left in him at age 31. Tim Fuller/USA TODAY Sports
Promising trend: Defensive coordinator Anthony Weaver has been saying all season that turnovers come in bunches, but they hadn't come very often at all. That changed on Thursday when Houston forced turnovers on three straight drives. Entering the game, the Texans had just five takeaways this season and no more than two in a game.
Eye-popping stat: Watt joins Deion Sanders as the only players to debut in the Super Bowl era with at least three career receiving touchdowns and three pick-sixes, according to ESPN Stats & Information. Sanders had three receiving touchdowns and returned nine interceptions for touchdowns in his career.
Watt's touchdown on Thursday was his seventh career touchdown. Including the playoffs, he has three touchdown receptions, three interceptions and a fumble return for a touchdown. He scored five times in the 2014 season but entering Thursday had not scored in his past 62 games, including playoffs.
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Contains ledger of accounts from John McGrath's grocery in Virginia City, Nev., located on C Street. The second ledger with an inventory is assumed to be from John McGrath as C Street locations are specified.
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by THE ARTISTS FORUM April 4, 2021
NAME: Franco Volpi
ORIGIN: Buenos Aires, Argentina
ART: Film
MAIN GENRE: Fiction
TRAINING: BA in Film Directing & MA in Filmmaking
ARTS AFFILIATIONS: London Film School
BIO: In 2015, Franco Volpi moved to the UK to study at the prestigious London Film School, chaired by OBE Mike Leigh. He graduated with Distinction in 2018 with an MA in Filmmaking. His short film, August Sun, was named a Finalist at the 2018 Student Academy Awards® and shortlisted for the 2018 BAFTA Student Film Awards. It was officially selected at 35 film festivals, including THE ARTISTS FORUM FESTIVAL OF THE MOVING IMAGE: 2018, where it won 3 of its 7 nominations: Best Narrative Short, Best Screenplay, and Best Actor (Miguel Di Lemme).
CAREER HIGHLIGHTS: August Sun (Writer/Director)
LATEST WORKS: So What Did We Learn Today, Georgina?
WORK SAMPLE: Sol de Augusto (August Sun) – Full version with English subtitles
Regardless of genre or style, I think I'm mostly fascinated with themes of identity and family dynamics. The cinema of Ozu, for instance, resonates a lot with me. Another theme that I keep exploring is the one of perception; of how we perceive others, how we are constantly misjudging a book by its cover, and how that informs our daily decision-making – often for the worse.
It's a case of an idea finding its way into my consciousness, where it will stay lodged for a while, competing with other ideas. In time, one of these ideas will start cropping up more regularly, at which point I will start analyzing and exploring it. After that, from the moment I decide to sit down and put it in writing, I will have carefully thought about it, building it in my head, having done a bit of research. The actual writing is then quite straightforward and quick.
My long-term goal is to become feature film and television director. It's very hard at first, when you're just getting started. You can strive and toil for years, painstakingly going about building a tidy body of work before you get seriously noticed and given a break. Film funds, script labs, and film markets are all instrumental. But the cogs can move very slowly.
The industry is undergoing a profound transition in which nothing less than the moviegoing experience is at stake. The studios' focus on ever bigger franchises, sequels, and blockbusters; the rise of the streaming giants; the pandemic have all accelerated these changes. I fear the industry is getting lost in a maze of its own making and we could potentially be looking at a long, long period of creative stagnation and fewer and fewer quality films. It could be a generation or two before we start coming back around, if ever.
In a way, there has never been a better time to be a filmmaker. The available technology allows us to shoot cheaper and quicker than ever before. That's definitely a democratization in filmmaking. And, there are more and more outlets available. Maybe the film going experience is changing and something important is being lost, which is a great shame. But, on the other hand, there's an ever-growing global market hungry for quality material. My advice is to figure out what you're good at and what you really want to do as soon as possible and then work towards that, because there's a market and an outlet for it. But, there's an arms race of content and emerging filmmakers, so there is a lot of clutter, a lot of noise that can drown out the more original voices and interesting stories. It's loud and chaotic. But, I always think the same: go with your gut feeling. If it has served you well in the past, it's for a reason. Trust that.
WEBSITE: francovolpi.com
INSTAGRAM: @francvolpi
INSTAGRAM: @soldeagostofilm
TWITTER: @AugustSunFilm


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The past year of debate and strife in cities nationwide has pushed law enforcement agencies to begin adopting body cameras. The result: shares of body camera makers are soaring.
Protesters on the street are demanding justice. Police are on TV pleading for peace. Taser is rolling in the dough.
The Scottsdale, Ariz., company that makes stun guns and body cameras for law enforcement officers has long been ignored by Wall Street for its still-debated controversial solutions to otherwise potentially deadly clashes between police and civilians.
Taser's stock began surging last summer as major protests ignited in Missouri and New York over alleged police brutality and the deaths of two unarmed black men at the hands of white police officers. As lawmakers from across the country continue searching for answers, Wall Street has found Taser.
The company's shares have risen more than 160 percent since the August shooting death of Michael Brown in Ferguson, Mo. -- better than some of the largest tech companies in Silicon Valley, including Apple, whose iPhones propelled its shares more than 36 percent over the same time, and video streaming service Netflix, whose shares have risen nearly 25 percent. Shares of a Taser competitor, Digital Ally, have risen nearly fourfold.
Wall Street isn't making a blind bet. Public perception and government money have shifted toward a seemingly inevitable future of placing a body camera on every police officer in the country. The sudden move has surprised veterans of law enforcement who have become accustomed to the ebbs and flows of police reform.
"If you had said to me 20 years ago that someday police would want to be wired up with body cameras, I would've said, 'Boy, that would've never happened,'" said Chuck Wexler, head of the Police Executive Research Forum (PERF), a Washington, DC-based think tank for police chiefs.
Now parking enforcement to wear body cameras?
On Friday alone, Taser's and Digital Ally's stocks each jumped almost 10 percent after the Justice Department announced a $20 million pilot program to fund police departments across the country with body cameras -- devices designed to be worn by law enforcement officers and record investigations and encounters. Wall Street investors are betting companies like Taser, which makes two of the most popular body cameras, will benefit from the national outcry for reform.
The latest protests are over the death of Freddie Gray, a man who died while in the custody of the Baltimore Police nearly two weeks ago. On Friday, six police officers were indicted on charges related to his death. When Gray's family members responded, they pleaded for law enforcement reforms across the country.
Taser said it's ready to respond.
"This is about positioning yourself to be at the right place at the right time," said company spokesman Steve Tuttle. "Sometimes it takes national, tragic incidents to bring this availability to the public's eye."
Los Angeles, San Francisco, San Diego, New Orleans, Fort Worth and Charlotte are some of the big-city police departments in the US that Tuttle rattles off that use Taser's "Axon" body cameras, which can be worn either on shirt lapels, or attached to the shoulder or sunglasses. He says departments in other major cities -- including New York and Chicago -- are among the hundreds currently testing its in-demand cameras, which cost between $400 to $600 apiece.
More than 3,000 law enforcement agencies globally use Taser, which counted 500 new accounts in the past year. That brings the number of cameras to more than 41,000 in the field from 30,000, and that's just in the past three months.
The tide appears to be turning. The Los Angeles Police Commission voted Tuesday to equip all officers with body cameras, making LA the largest US city to take this step. Mayor Eric Garcetti announced late last year that the LAPD will be outfitted with 7,000 body cameras to improve transparency.
There's also neighboring Rialto, Calif., which was the first police department in the country to equip all its officers with body cameras, Tuttle said. Following the rollout, the number of excessive-force complaints dropped from 24 in 2012 to just 3 in 2013 for the Southern California city of about 100,000 people. A study on the Rialto police's use of cameras by the University of Cambridge's Institute of Criminology last year gave the technology much-needed credibility.
"The cameras provide transparency and improve behavior on both sides of the badge," Tuttle said. "Law enforcement become more professional and it ratchets down a suspect's behavior dramatically."
Critics warn that no one technology can be a panacea to the country's crisis of confidence in law enforcement. Among concerns: when police will turn these devices on, how the public will get access to recordings, and the privacy of young people whose images could be recorded.
"Technology is just a tool," said Peter Bibring, head of police issues at the American Civil Liberties Union in California. He said the devices ultimately could help repair public trust of police, but only if they're used as a tool of transparency. "It's a question of how much it's used, as much as what it is."
Taser was founded in 1993, marketing a device that's designed to be held and used like a gun but offers a nonlethal alternative. The device shoots out two wires charged with an electrical signal that, when the wires embed in a person, gives the victim a significant jolt. The technology is often called a "stun gun."
The devices were hailed as a safer alternative than shooting a suspect, but Taser ultimately faced criticism that the device was turning into another weapon, like a baton, that could be overused.
In 2009, Taser began shipping a new device, called the "Axon" body camera, to law enforcement. A tiny police department in Aberdeen, S.D., was the first to use those cameras. At the time, the devices were criticized for their expense and potential for infringing civil liberties. Not so much anymore.
In many jurisdictions, officers are required to turn on their cameras, whose battery can last up to half a day, for traffic stops, possible searches and dangerous situations. The recorded data is protected by special algorithms and cannot be deleted, Taser said.
"We had to use technology to either protect life or protect truth," Tuttle said. "We're still trying to do both."
Taser devices represent about 95 percent of the market, the company says, competing with firms like Digital Ally and another called Vievu.
"What's driven this technology is citizens are now recording police activity, so the police feel the need to provide their own officers with cameras to record their own actions," said Wexler, whose organization submitted guidelines to the Department of Justice on how body cameras should be used.
One example: The ACLU on Thursday announced a smartphone app designed to help users record and upload encounters with the police to the Internet.
Merely buying body cameras won't be enough. Wexler and the ACLU's Bibring agree that for the devices to be effective, police departments must work with their communities to set expectations and guidelines for how they'll be used.
"It's a paradigm shift," Taser's Tuttle said, "and we are a part of that, as this could potentially become standard equipment for officers in the future."


text:
Read the excerpt from the declaration of independence. governments are instituted among men, deriving their just powers from the consent of the governed, that whenever any form of government becomes destructive of these ends, it is the right of the people to alter or to abolish it, and to institute new government, laying its foundation on such principles and organizing its powers in such form, as to them shall seem most likely to effect their safety and happiness. what is the author's purpose in this excerpt? to inform readers about different systems of government to inform readers about recent governmental reforms to persuade readers of the need for governmental change to persuade readers to petition their local governments
i think it should be predicate nominative.
snowy, calm, beautiful soul.
i'm thinking its the second choice, sorry if i'm wrong!
parameters are sets of fixed limits or boundaries that are established to indicate the way in which something should be done. in other words, parameters determine the performance of a certain action by establishing limits. moreover, context clues are hints that a writer gives in order to the reader to understand the meaning of a term. the type of context clue used in this sentence is example. furthermore, this context clue is introduced by means of "such as", which is followed by different examples of parameters (rules, guidelines and instructions) in order to define that term.
Now pick a section from chapter 1 of the call of the wild in which buck is interacting with another character. imagine that section narrated in first person point of view from one of the character's perspective, and rewrite it in the space below, attempting to use a similar writing style as the original author. then, in a second paragraph, consider this: are you able to explore some of the same ideas and themes when viewing the situation from the character's first person point of view, as compared to when it is narrated from london's third-person limited point of view? explain. your response should be be about 150 words in total.
The president says last month electric lights and automobiles and airplanes become available to suggest
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From Facing Couples. Pass Thru; As Couples 1/4 toward the given direction.
A Travel Thru is a Travel Right. Also Travel Left/In/Out.
At one point I thought this would be a useful extension of Travel Thru (particularly since Left Travel Thru has historically been defined as a Right-shoulder Pass Thru then As Couples 1/4 Left). I no longer use this, and will probably remove it at some point from my lists.


text:
Home Living Health Minding the Gap
by Tina Eshleman
Supporters of Medicaid expansion demonstrated at the State Capitol in August. Photo courtesy Virgnia Organizing
Opponents of Medicaid expansion demonstrated at the State Capitol in August. Photo courtesy Chip Jones
The health-insurance marketplaces established by the federal Affordable Care Act are intended to make insurance accessible to those who don't have it. But when enrollment opens Oct. 1, thousands of uninsured Virginians won't be able to sign up. That's because the state has not yet expanded its Medicaid program to include up to 400,000 residents who don't currently qualify.
Jill Hanken, a health-law attorney with the Virginia Poverty Law Center, says that people whose incomes are between 100 percent and 400 percent of the federal poverty level — roughly $11,500 for an individual or $23,550 for a family of four — could qualify for premium tax credits or subsidies if they buy insurance through the exchange. Of the approximately 1 million Virginians who currently don't have health insurance, about 500,000 of them likely will be able to sign up through the marketplace, accessible online at healthcare.gov , says Kathy May, director of Virginia Consumer Voices for Healthcare.
Anyone who has an income below the poverty level and doesn't qualify for Medicaid would continue to use indigent-care services and safety-net providers.
"We don't know exactly how many will fall into the gap," Hanken says, adding, "Virginia's gap will leave hundreds of thousands of people out of luck."
As an incentive for states to expand the number of people who can receive Medicaid, the federal government has promised to pay the full cost of covering newly eligible adults for three years. The funding would start on Jan. 1, 2014, but it's uncertain when the General Assembly commission that's working on Medicaid expansion will complete its work. Made up of five state senators and five House of Delegates members, the Medicaid Innovation and Reform Commission (MIRC) is tasked with reviewing, recommending and approving reform proposals, and must meet a set of objectives outlined in the state budget for the expansion to take effect.
"I feel that we can accomplish it, but it's going to take significant effort," says Sen. Emmett W. Hanger Jr., R-Augusta, the commission chairman.
The clock is ticking. If Virginia doesn't expand Medicaid by Jan. 1, the state stands to lose $170 million per month — more than $5 million per day — in federal dollars available to cover 100 percent of the cost for new Medicaid recipients. After three years, that level of funding would gradually decrease to 90 percent by 2020.
"The revenue stream is already in place," Hanger says. "If we ultimately are going to expand [Medicaid], it's to our benefit to do it sooner rather than later."
Legislators are feeling political pressure as well. Opposition to "Obamacare" brought hundreds of protesters to the State Capitol during the commission's meetings, the next of which is set for Oct. 21. A public hearing is also being scheduled for October.
Americans for Prosperity, which organized the protests, is also rallying people who live in districts represented by Dels. Jimmie Massie and John M. O'Bannon III, two commission members, to urge them to oppose Medicaid expansion, says Dave Schwartz, the group's state director. Schwartz says his group does not believe "that a federal government that's trillions [of dollars] in debt is going to pay 100 percent of the expansion."
Citing a statistic that one in four physicians does not accept new Medicaid patients, Schwartz also argues that increasing the number of patients covered by Medicaid will mean lower-quality care for the very people who need the most help. "You're expanding a system that's already in shambles," he says. "It makes no sense."
Observers of the process also wonder about the effect of the gubernatorial election in November, given Republican nominee and Attorney General Ken Cuccinelli's strong opposition to the Affordable Care Act. Hanger says he is hopeful that if Cuccinelli is elected, his administration will be receptive to the reforms the commission is proposing.
And while state legislators consider Medicaid expansion, hospitals such as VCU Medical Center are facing a drop in both the federal and state money that compensates them for providing indigent care. To help pay for Medicaid expansion, the federal government will be decreasing Disproportionate Share Hospital funds to hospitals serving a large number of low-income patients, says Sheryl Garland, vice president for health policy and community relations with the VCU Health System. As the federal funds decrease, matching state funds also will drop, she says.
"We hope that state lawmakers will be sensitive to this issue," Garland says. "How will those folks who fall in that insurance gap receive care, and who will pay for it?"
Enrollment Help
If signing up for health insurance online through the federally run marketplace seems daunting, there are people ready to assist. The Virginia Poverty Law Center (VPLC) and Advanced Patient Advocacy (APA) have received funds to provide trained navigators to help. Jill Hanken of the VPLC says that her organization will have a website and toll-free support number. The APA will work with hospitals to enroll patients who need insurance. In addition to the navigators, the Virginia Community Healthcare Association is providing certified application counselors. The counselors will be available at community health centers such as those in Richmond's Capital Area Health Network, including the Vernon J. Harris Medical Center in Church Hill. People who have insurance through their jobs don't need to go through the marketplace enrollment process, but they may explore coverage options if they wish. The enrollment period opens Oct. 1 and continues through the end of March, but you must sign up by Dec. 15 for coverage to begin Jan. 1. Adults who do not get health insurance in 2014 would face a fine of $95 or 1 percent of taxable income — whichever is greater — an amount that increases to $695 or 2.5 percent of taxable income by 2016. However, there are exemptions for those with incomes below the tax-filing threshold and for financial hardship, religious beliefs and other factors. Resources:
Health insurance marketplace, healthcare.gov
Virginia Poverty Law Center, 782-9430 or vplc.org
Capital Area Health Network, cahealthnet.org
Virginia Consumer Voices for Healthcare, vaconsumervoices.org


text:
Two-Hop Secure Communication Using an Untrusted Relay
Xiang He1 &
Aylin Yener1
We consider a source-destination pair that can only communicate through an untrusted intermediate relay node. The intermediate node is willing to employ a designated relaying scheme to facilitate reliable communication between the source and the destination. Yet, the information it relays needs to be kept secret from it. In this two-hop communication scenario, where the use of the untrusted relay node is essential, we find that a positive secrecy rate is achievable. The center piece of the achievability scheme is the help provided by either the destination node with transmission capability, or an external "good samaritan" node. In either case, the helper performs cooperative jamming that confuses the eavesdropping relay and disables it from being able to decipher what it is relaying. We next derive an upper bound on the secrecy rate for this system. We observe that the gap between the upper bound and the achievable rate vanishes as the power of the relay node goes to infinity. Overall, the paper presents a case for intentional interference, that is, cooperative jamming, as an enabler for secure communication.
Information theoretic security was proposed by Shannon [1]. The idea of measuring secrecy using mutual information lends itself naturally to the investigation of how the channel can influence secrecy and further to the characterization of the fundamental limit of secure transmission rate. Wyner, in [2], defined the wiretap channel, and showed that secure communication from a transmitter to a "legitimate" receiver is possible when the signal received by the wiretapper (eavesdropper) is degraded with respect to that received by the legitimate receiver. Reference [3] identified the secrecy capacity of the general discrete memoryless wiretap channel. The secrecy capacity of the Gaussian wiretap channel is found in [4].
Recent progress in this area has extended classical information theory channel models to include secrecy constraints. Examples are the multiple access channel, the broadcast channel, the two-way channel, the three-node relay channel and the two-user interference channel [5–13]. These studies are beginning to lead to insights for designing secure wireless communication systems from the physical layer up. Prominent such examples include using multiple antennas to steer the transmitted signal away from an eavesdropper [14–16], transmitting with the intention of jamming the eavesdropper [8, 10, 17], and taking advantage of variations in channel state to provide secrecy [18–20].
The focus of this work is on a class of relay networks where the source and the destination have no direct link and thus can only communicate utilizing an intermediate relay node. This models the practical scenario where direct communication between the source and the destination is too "expensive" in terms of power consumption: direct communication may be used to send some very low rate control packages, for example to initialize the communication, but it is infeasible to sustain a nontrivial reliable communication rate due to the power constraint.
In such a scenario, the source-destination pair needs the relay to communicate. On the other hand, more often than not, this relay node may be "untrusted" [11]. This does not mean the relay node is malicious, in fact quite the opposite, it may be part of the network and we will assume that it is willing to faithfully carry out the designated relaying scheme. The relay simply has a lower security clearance in the network and hence is not trusted with the confidential message it is relaying. Equivalently, we can assume the confidential message is one used for identification of the source node for authentication, which should never be revealed to a relay node in order not to be vulnerable to an impersonation attack. In all these cases, we must assume there is an eavesdropper colocated at the relay node when designing the system.
The "untrusted" relay model, or the eavesdropper being colocated with the relay node, was first studied in [9] for the general relay channel, with a rather pessimistic outlook, finding that for the degraded or the reversely degraded relay channel the relay node should not be deployed. More optimistic results for the relay channel with a colocated eavesdropper have been identified recently in [11, 21, 22]. Specifically, it has been shown that the cooperation from the relay may, in fact, be essential to achieving nonzero secrecy rate [11, 21]. The model is later extended to the more symmetric case in [23, 24] where the relay also has a confidential message of its own, which must be kept secret from the destination.
All these models assume that a direct link between the source and the destination is present including our previous work [11]. In contrast, when there is no direct link, it is impossible for this network to convey a confidential message from the source to the destination while keeping it secret from the relay [9]. This is because the destination can only receive signals from the relay resulting in a physically degraded relay channel [25]. Therefore, the relay knows everything the destination knows regarding the confidential message, and the secrecy capacity is zero.
The differentiating feature of the model studied in this work from those described above including [11] is that the destination has transmission capability. This opens the possibility of the destination node to actively participate in ensuring the secrecy of the information it wants to obtain. In an effort to address a practical two-hop communication scenario, we shall consider each node to be half-duplex, which leads to a two-phase communication model. In addition, feedback to the source is not considered in the channel model. Interestingly, in this model, the transmission capability of the destination proves to be the enabler of secure communication. By recruiting the help of the destination to do "cooperative jamming", positive secrecy rate can be achieved that would not have been possible otherwise. We also remark that in case the transmission by the destination is not possible or desired, the help from an external cooperative jammer will do as well.
The idea of using a helpful jammer goes back to [17, 26, 27] and has since been used in many different models. Besides the multiple access, two-way [8] and relay wiretap channels [10], other recent results that use "cooperative jamming" as the part of the achievability scheme, include [28–30]. In [28], a separate jammer is added to the classical Gaussian wiretap channel model. The jamming signal is revealed to the legitimate receiver via a wired link so that an advantage over the eavesdropper is gained. Reference [30] does not assume the wired connection, and employs a scheme tantamount to the two user multiple access channel with an external eavesdropper where one of the users perform cooperative jamming. Reference [29] considers the case where both the eavesdropper and the legitimate receiver observes a modulus channel and the destination carries out the jamming. We note that all these works deal with an external eavesdropper, in contrast to the focus of this work, which is an untrusted (but legitimate) node in the network.
In general, the optimality of recruiting a helpful jammer remains open as the converse results are limited. For the Gaussian case, the main difficulty is to find a upper bound for which the optimal input distribution can be found and evaluated. Doing so usually involves the introduction of genie information, as in the converse for the Gaussian wiretap channel [4], MIMO wiretap channel [14] and MAC wiretap channel [31]. The proofs then typically invoke the entropy power inequality, as in [4], or the generalized entropy power inequality, as in [32].
In this work, we derive a computable upper bound for the model in consideration by first introducing a second eavesdropper, an approach first used for a three-node relay channel in [11]. Next, after several steps of genie arguments, the channel is transformed into a wiretap channel with a helpful jammer, whose outer bound is then evaluated. The resulting bound is nontrivial in the sense that it is strictly tighter than the bound for the same channel without secrecy constraints. We also prove that it is tighter than an upper bound derived using the generalized entropy power inequality following a similar approach to [32], when the maximum sum received SNR at the relay is greater than dB. We show that the gap between the bound and the achievable rates converges to zero when the power of the relay goes to infinity.
The paper is organized as follows. Section 2 presents the channel model and the two-phase protocol that utilizes cooperative jamming. In Section 3, we derive the achievable rates. Section 4 presents our upper bound and compares with other known upper bounds. Section 5 presents the numerical results. Finally, Section 6 presents the conclusion.
The following notation is used throughout this work: We use to denote the entropy, to denote the differential entropy, and to denote any variable that goes to 0 when goes to . We define .
2. Channel Model
The system model is shown in Figure 1. We assume all nodes are half-duplex and the communication alternates between two phases, called phase one and phase two respectively. During phase one, shown with solid lines in Figure 1, the source transmits signal . At the same time, the destination node transmits jamming signal in order to confuse the relay node. The signal received by the relay in phase one, , is given by
where is a zero mean Gaussian random variable with unit variance. In an effort to reflect on the design of a practical system, we assume that the computation of , does not rely on the signals received by node in the past.
Two-hop communication using an untrusted relay.
In phase two, shown with dashed lines, the relay transmits signal , which is computed from the local randomness at the relay, the signal transmitted and received by the relay in the past.
The signal received by the destination in phase two is denoted by , which is given below:
where is a zero mean Gaussian random variable with unit variance.
The channel alternates between these two phases according to a random or deterministic schedule, which is generated by a global controller independently from the signals associated with the channel model. Hence here the term "schedule" is simply a finite number of channel uses which are either marked as phase one or phase two. We use to denote the number of channel uses marked as phase one, and to denote the number of channel uses marked as phase two. It should be noted that in general the channel uses of phase one are not consecutive. Neither are the channel uses of phase two. We assume the schedule is stable, in the sense that the following limit exists:
For a given , we use to denote a sequence of schedules with increasing number of channel uses such that (3) holds. According to this definition, becomes the limit of the time sharing factor of phase one in the schedule as .
When transmitting signals, the source, the destination, and the relay must satisfy certain power constraints. The average power constraints for the source, the jammer and the relay can be expressed as follows:
is the total number of channel uses.
For the purpose of completeness, we also introduce the notation to denote the average power of node during phase one. Since node and are only transmitting during phase one, and are related as
Similarly, we use to denote the average power of the relay node during phase two. Since the relay node only transmits during the second phase, is related to as follows:
After a number of phases, the destination node (node ) decodes a message from the signals it transmitted during the periods of phase one and the signals it received during the periods of phase two. For reliable communication, should equal the message from the source node with high probability. Hence we have the following requirement:
The message must also be kept secret from the eavesdropper at the relay node, who can infer the value of based on the following knowledges available to it.
The local randomness at the relay, denoted by .
The signals the relay transmitted during the periods of phase one, denoted by .
The signals the relay transmitted during the periods of phase two, denoted by .
The information on that the eavesdropper can extract from these knowledges should be limited. Hence we have the following secrecy constraint:
Since is a Markov chain, we have
Therefore, the secrecy constraint can be expressed as
For a given , and sequences of schedule , the secrecy rate is defined as
such that (9) and (12) are fulfilled. When deriving achievable rate, we will focus on a specific sequence of schedules , and maximize the secrecy rate over . When deriving the upper bound, we will consider all possible sequences of .
Remark 1.
Since the signals transmitted by node and do not depend on the signals they received in the past, is a Markov chain. Therefore,
Hence, the following secrecy constraint can be used instead:
We observe that in the system model shown in Figure 1, the destination, as the sender of the jamming signal during the periods of phase one, has perfect knowledge of this signal. This can be viewed as a special case of the model shown in Figure 2, where the destination only has a noisy copy of the jamming signal . If the jamming signal is corrupted by a noise sequence that is independent of the noise sequences at the other receivers, then the secrecy capacity of the model in Figure 2 can not be larger than the secrecy capacity of the model in Figure 1. This is because giving this noise sequence to the destination as genie information would simply reveal the jamming signal to it. Therefore, any upper bound we derive for Figure 1 is also an upper bound for Figure 2.
Two-hop network with an external cooperative jammer, CJ.
An apparent vulnerability of the described two phase protocol is that the destination may not be aware that the source has initiated its transmission. In this case, without the protection of the jamming signal from the destination, the message from the source would be revealed to the relay node and hence compromised. To prevent this from happening, proper initialization of the protocol is necessary.
3. Achievable Rate
In this section, we derive the achievable secrecy rate with the following sequence of deterministic periodic schedules.
The channel alternates between channel uses for phase one and channel uses for phase two, where and are two positive integers. The alternation takes times. Hence and . For a given , the sequence of schedules is obtained by letting and
With this sequence of schedules, we have the following theorem.
Theorem 1.
The following secrecy rate is achievable for the model in Figure 1:
where is the variance of the Gaussian quantization noise determined by:
where is defined in (7), is defined in (8).
The proof is given in the appendix.
It can be seen from (17) that, for any fixed time sharing factor the relay should always transmit at maximum power . However, the optimal transmission power of the source may be less than . This can be seen as follows: For a given jamming power , the achievable rate is not a monotonically increasing function of . This is because, if or , , indicating that even if the source power budget is , the optimal transmission power is actually finite. Let this value be . may or may not fall into the interval , which is the range of power consumption allowed for phase one. If it does, then the source should transmit with power rather than . If not, then the corresponding optimal value needs to be determined.
If the power constraint of the relay , then , . The achievable rate converges to
4. Upper Bound
In this section, we derive an upper bound for the secrecy rate.
We first need to determine the optimal schedule. It turns out that it is easy to find: We simply let the first channel uses be phase one, and the remaining channel uses be phase two. The optimality of this schedule can be proved as follows.
Suppose a different schedule is used. Since the signals received in the past are not used for encoding purposes at node and , we can always move the channel uses of phase one to the front without affecting the signals transmitted by these two nodes. On the other hand, we notice that the relay can only use signals received in the past to compute its transmission signals. However, during phase one, the relay only receives signals. Since moving phase one ahead only means the relay could receive signals sooner, doing so will not limit the capability of the relay to calculate its transmitted signals. Consequently, we observe that no matter what schedule is used to achieve a secrecy rate, we can always modify this schedule such that all channel uses of phase one are ahead of those of phase two and still achieve the same secrecy rate. Hence in the following we only consider the optimal schedule.
We also observe we can transform the channel into the one shown in Figure 3. The jammer and the receiver are now drawn separately, since the jammer does not use the signal received in the past to compute the jamming signal. Note that Figure 3 is similar to Figure 12 used in the achievability proof except that the dimension of the signals is changed from to .
Equivalent Channel Model for Deriving the Upper Bound.
We next leverage a technique first used in [11, 22] to derive the upper bound. Specifically, the upper bound is obtained via the following transformations.
( ) First, we add a second eavesdropper to the channel, as shown by Figure 4. Its received signal is denoted by and over channel uses is given by
Two-eavesdropper channel.
Here is a Gaussian noise with the same distribution as . can be arbitrarily correlated with . Since
From (15), this means
Hence the message is kept secret from the second eavesdropper. This means, for a given coding scheme that achieves secrecy rate in Figure 3, the same secrecy rate is achievable with the introduction of this additional eavesdropper.
( ) Next, we remove the first eavesdropper at the relay. Doing so will not decrease secrecy rate either, since we have one less secrecy constraint.
From (24), the secrecy rate can be upper bounded via . To do that, we provide the signal to the destination by a genie. Similarly, the signal is revealed to both the relay and the destination. is then bounded by
The genie information causes the signal to be useless to the relay, as shown by (25)-(26). Equation (28) is due to the fact that once the signal received by the relay is given, the signal transmitted by the relay , which is computed from , is independent from the jamming signal and the confidential message . Finally, revealing the genie information to the relay and the destination essentially removes the influence of the jamming signal from the relay link, as shown by (28)–(30). These are essentially a consequence of the link noises being independent. The resulting channel is equivalent to the one shown in Figure 5, and can be viewed as a special case of the channel in [8, 33]. Similar techniques to those in [31, 33] can be used here to bound the secrecy rate. Let . Then (30) becomes
Channel model after transformation.
Here (34) follows from the fact that determines . The first term in (37) is maximized when and are i.i.d. Gaussian sequences [14]. Let the variance of each component of be , . Let be the correlation factor between and . Then (37) is equal to
It can be verified that, for any fixed , equation (37) is an increasing function of and . Therefore, the upper bound is maximized with maximum average power. Equation (38) can then be tightened by minimizing it over . The optimal is given below:
As a result, we have the following theorem.
The secrecy rate of the channel in Figure 12 is upper bounded by
where is given by (39). , , and are the average power constraints of node and the relay for the time sharing factor .
If we further fix and let , then . converges to given by
The difference of the upper bound and the achievable rate converges to
We observe that the difference is only a function of . By comparison, the gap between the achievable rate and the trivial upper bound is , which is unbounded.
If we instead fix , and let , then . The achievable rate converges to (19). In this case, if we further let , the upper bound given by (41) converges to
Comparing it with (19), we observe the difference of the upper bound and the achievable rate converges to . Hence, in this case, our upper bound is asymptotically tight.
The first term in the bound (41) is strictly smaller than the trivial bound obtained by removing the secrecy constraints. To show that, simply let . Equation (41) becomes
The second term in (45) is always negative.
4.1. Comparison with the Bound Derived with Generalized Entropy Power Inequality
Recently the generalized entropy power inequality [34] was used to derive a computable upper bound for the Gaussian multiple access channel with secrecy constraints [32]. Here the same technique is applicable and another computable upper bound for the model in Figure 1 can be derived. It is of interest to know which bound is tighter. Next, we prove that as long as , this upper bound is always looser than the bound given by (38)-(39).
First, we briefly describe the derivation of the bound based on the approach in [32]:
Here step follows from Fano's inequality. (49) can be written as:
Step follows from being independent.
Next, like [32, equation ( 76)], we invoke the inequality from [34] and obtain
Hence (52) can be upper bounded with
This expression is maximized when are chosen to be i.i.d. Gaussian sequences. Dividing by the total number of channel uses , the final expression of the upper bound is given by
Note that (55) is also tighter than the bound when . Hence it is a nontrivial bound when .
Remark 10.
When , then , . Comparing (19) with (55), the gap between the achievable rates and the bound given by (55) is
which is smaller than bit/channel use.
We next show that for any given , if , (55) is always bigger than the first term in (41). We omit the time sharing factor in the front since they are present in both expressions. Then we pick such that
Note that this is a valid choice for since the right hand side is within the interval . Then (41), after canceling in the front, becomes
Hence we only need to verify that (55) is greater than (58) when . This is equivalent to verifying
or . A sufficient condition for this to hold is to require . Substitute (57) into this requirement we get .
Since the gap between the achievable rates and the bound given by (55) is bounded by bit/channel use when , the gap between the achievable rates and the bound given by (41) is also bounded by bit/channel use when and . Note that since when we have , the condition is equivalent to .
For the case that , it is not clear between (55) and (41) which bound is tighter. However, for these cases, the secrecy capacity is so small that the bounds are of no consequence.
5. Numerical Results
Shown in Table 1 are the six cases of interest, corresponding to different power budgets of the relay and the jammer and whether time sharing factor is fixed. We included the cases with fixed time sharing factor because in a real system, for simplicity the time sharing factor may not be dynamically adjusted according to power budgets. The numerical result of each case is shown in the figures listed in the table. We stress that, though not explicitly considered in the numerical results, for the more general case where the cooperative jammer is external, as shown in Figure 2, the upper bound still holds, but the gap between the upper bound and achievable rates would be wider.
Table 1 Scenarios considered in the numerical results.
Figures 6 and 7 demonstrate the asymptotic behavior described in Remark 6 when the power of relay goes to . Note that in this case the optimal time sharing factor converges to 1. Figures 8 and 9 demonstrate the case where the power the relay is finite, and the time sharing factor is fixed. In all four cases, we observe the upper bound is close to the achievable rate when relay's power is larger than the power of the source and the jammer. In this region, typically, the achievable rate increases linearly with the source SNR. In Figure 6, the gap between the upper bound and achievable rate goes to zero as . In Figure 7, the upper bound almost coincides with the achievable rate. The gap, given by (43), equals bits/channel use.
Secrecy Rate, , , optimal
Secrecy Rate, , dB, optimal
Secrecy Rate, dB, ,
Secrecy Rate, dB, dB,
Also shown in each figure is the cut-set bound without secrecy constraints. The improvement provided by the new bounds depends on the power budget. In general, the improvement is small if the power of the jammer is large. Note that since we have normalized all channel gains and included them into the power constraint, the power budget difference can be considered a consequence of the difference in link gains.
Figure 9 also illustrates the power control problem described in Remark 4. Without power control at the source node, the achievable rate will eventually decrease to zero. Note that this behavior crystallizes only when the relay's power is limited.
Finally, in Figures 10 and 11, we compare the achievable rates and the upper bound when each are maximized over the time sharing factor . The gap between the upper bound and the achievable rate is now wider because the second term in the upper bound (41) is the same as the upper bound without secrecy constraint. The role played by the second term (41) becomes significant when the bound is optimized over the time sharing factor, which as pointed out in [35], has a tendency to balance the two terms in the bound (41). However, as shown in these figures, compared to the upper bound without secrecy constraints, the new bound still offers significant improvement.
Secrecy Rate, dB, , optimized over
Secrecy Rate, dB, dB, optimized over , power control at the source node enabled.
Equivalent channel model.
In this paper, we have considered a relay network without a direct link, where relaying is essential for the source and the destination to communicate despite the fact that the relay node is untrusted. Imposing secrecy constraints at the relay node, contrary to the previous work, we have shown that a nonzero secrecy rate is indeed achievable. This is accomplished by enlisting the help of the destination (or another dedicated node) who transmits to jam the relay, and uses the jamming signal as side information. We have derived an upper bound for the secrecy rate with the assumption that no feedback is used for encoding at the source or destination. The new upper bound is strictly tighter than the upper bound without secrecy constraints. We have also proved that it is tighter than an upper bound derived from generalized entropy power inequality when the maximum sum received SNR at the relay is greater than dB. The gap between the bound and the achievable rates converges to when the power of the transmitter, the relay and the jammer goes to . Numerical results show that our upper bound is in general close to the achievable rate, and is indistinguishable from it for a fixed time sharing factor with a relay whose power is in abundance.
In this work, we considered the case where the source or the jammer does not make use of the relay transmission for encoding purposes. An upper bound for the secrecy rate when feedback is used is recently found in [36]. A gap exists between the upper bound and the achievable rate in [36], which is bounded by bit per channel use but does not vanish when the power of the transmitter, the relay and the jammer goes to infinity. By comparison, the bound presented in this work is asymptotically tighter in this case.
We conclude by reiterating that our findings in this paper presents cooperative jamming as an enabler for secrecy from an internal eavesdropper, and motivates further investigation of such cooperation ideas in more general settings including those in larger networks. We also comment whether and when cooperative jamming actually yields the secrecy capacity (region) for various multiuser channels remain open problems in information theory.
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This work was presented in part at the IEEE Globecom Conference, December 2008. This work is supported in part by the National Science Foundation with Grants CCR-0237727, CCF-051483, CNS-0716325, CNS-0721445, and the DARPA ITMANET Program with Grant W911NF-07-1-0028.
Wireless Communications and Networking Laboratory, Electrical Engineering Department, The Pennsylvania State University, University Park, PA, 16802, USA
Xiang He
& Aylin Yener
Search for Xiang He in:
Search for Aylin Yener in:
Correspondence to Aylin Yener.
Proof of Theorem 1
We first introduce several supporting results used in proving Theorem 1.
In [11, 21], we presented the following achievable secrecy rate for a general relay channel.
Consider a relay network with conditional distribution , with , being the input from the source and the relay respectively, and , being the signals received by the relay and the destination, respectively. For the distribution
the following range of rates is achievable:
Theorem 3 follows from the achievable equivocation region given in [11, Theorem ] by simply considering rates that equal the equivocation rate . The proof of Theorem 3 is given in [11]. The outline of the achievable scheme is as follows: The relay does compress-and-forward as described in [25]. Therefore, as in [25], is independent from in the input distribution expression (A.1). The same decoder in [25] is used at the destination. The same codebook as [25] is used at the source node. However, instead of mapping the message to the codeword deterministically as in [25], a stochastic encoder is used at the source node. In this encoder, the codewords are randomly binned into several bins. The size of each bin is where is the total number of channel uses. The message determines which bin to use by the encoder. The actual transmitted codeword is then randomly chosen from the bin according to a uniform distribution. This randomness serves to confuse the eavesdropper at the relay node at the cost of the rate as shown by the term in (A.2).
We next extend this result by considering a relay channel with a jammer defined by
where is the signal transmitted by the jammer and the notation follows the definition above. Then, if the jammer transmits an i.i.d. signal according to distribution and , the induced channel is given below:
and it is also a memoryless relay channel. Hence, we can use Theorem 3 and obtain the following corollary.
Corollary 1.
The following secrecy rate is achievable:
We next reformulate our channel in Figure 1 in a way such that Corollary 1 can be applied. This is shown in Figure 12. Here we can draw the jammer and the receiver separately, since the jammer does not use the signal received in the past to compute the jamming signal. The and are the parameters of the schedule described in Section 3. We then observe Figure 12 can be viewed as a three node relay network with a jammer, defined as follows:
The input distributions to this vector input channel are chosen as
where , and are given below.
(1)Let , where is the average power consumption of node during the periods of phase one. Hence .
(2)Let the auxiliary random variable in compress-and-forward be . Let , where ,
(3)Let and ,
where denotes an identity matrix. With ( )–( ), we have
From (A.10), . Therefore (A.13) equals
(A.17) equals:
In (A.14), (A.20), (A.26), (A.30), (A.36), and (A.37), we use (A.10) repeatedly, which says that with compress-and-forward, the input distribution are chosen such that are independent.
Substituting the values of , , and into Corollary 1, dividing both sides by , and taking the limit , we proved the theorem.
He, X., Yener, A. Two-Hop Secure Communication Using an Untrusted Relay. J Wireless Com Network 2009, 305146 (2009) doi:10.1155/2009/305146
Revised: 08 August 2009
Relay Node
Power Constraint
Achievable Rate
Power Budget
Secrecy Rate
Wireless Physical Layer Security


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Tag: dna lab
Quest Sells OralDNA, HemoCue To Clear Its Decks for 2013
By Robert Michel, from the Volume XX No. 1 - January 22, 2013 issue
THIS WEEK, Quest Diagnostics Incorporated will issue its fourth quarter and full year 2012 financial report. In anticipation of this, the nation's largest lab company has been cleaning out its closets, so to speak.
With its new CEO finishing out his first eight months of service, Quest Diagnostics is taking the opportunity to dispose of some
New Lab Company Intends to Buy Outreach Labs
By Joseph Burns, from the Volume XIX No. 9 - June 25, 2012 issue
CEO SUMMARY: Based in Brentwood, Tennessee, Regional Diagnostic Laboratories (RDX) says it wants to purchase the laboratory outreach programs of nonprofit community hospitals. RDX is a partnership with Warburg Pincus, a New York investment company. It is a credible buyer and has $250 million available to fund these acquisitions. Brian Carr, RDX's CEO, predicts that


My overall budget for upgrade is around £600, i'm hoping to keep my PSU to save some money. I live in the UK so PC components tend to me mor expensive I was only looking to spend a max of £250 on a new GPU, given that I need to get a processor, motherboard and RAM as well. The cheapest Vega 56 I can get here is around £260.
Hi all, I am going to be upgrading my PC this year, but due to limited funds I am going to be spreading th cost out by buying components over a few months. Current PC specs: Intel® Core i5 Quad Core Processor i5-4690 (3.5GHz) 6MB Cache ASUS® Z97-A: ATX 8 GB KINGSTON HYPER-X FURY DUAL-DDR3 1600MHz 4GB GTX 960 CORSAIR 650W CS SERIES™ MODULAR 80 PLUS® GOLD, ULTRA QUIET In terms of processor I am going to wait until Ryzen 3000 series processors are announced, either to get one of get a 2000 series if the cost goes down. I am thinking of upgrading the GPU first as this is really due for an update. I'm looking at either GTX 1660 or 1660ti. I have seen the Gigabyte NVIDIA GeForce GTX 1660 6GB for £200, but I have also found a deal on the Zotac NVIDIA GeForce GTX 1070 Mini 8GB for only £230, is it worth the extra £30, or would I be better off looking for a deal on a 1660ti?
Yeah thats the sort of thing that I was thinking of for a new build. How does the Radeon RX 580 compare to something like a gtx 1060? I might just be being old fashioned but I've never strayed for an AMD CPU Also, might be a stupid question but when switching storage between builds do I need to wipe the SSD cos it will have drivers for old GPU, CPU etc. on it, or does it not matter?


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Haji, Kinoti praise Catholic bishops for banning huge cash donations
By Amina Wako October 9th, 2019 2 min read
The Director of Public Prosecutions, Noordin Haji (left) and the Director of Criminal Investigations George Kinoti during a past appearance before the Senate Committee on Justice and Legal. PHOTO | JEFF ANGOTE
The Director Public Prosecutions Noordin Haji and his counterpart from Director of Criminal Investigations, George Kinoti, have lauded Catholic bishops for banning huge cash donations in churches.
In a joint press statement, the two welcomed a declaration by Catholic bishops in the fight against corruption.
This is after the Kenya Conference of Catholic Bishops outlined measures to be taken in a six-month national campaign by the Catholic faithful in Kenya in support of the fight against corruption.
"We sincerely thank the Kenya Conference of Catholic Bishops for taking a resolute stand and offering much needed spiritual and moral leadership in the country's fight against corruption. This is a commitment worth emulating," read the statement.
They noted that the stand by the bishops has provided a clear direction though which their faithful can back their faith claims with corresponding practice of morality and accountability.
On Sunday, just a day after the declaration by the bishop, the Ethics and Anti-Corruption Commission (EACC) CEO, Twalib Mbarak, commended the move calling for proper accountability and transparency in handling of all financial donations to the church.
"The decision by the Catholic Church to rally its faithful to strongly support the fight against corruption and ban all donations without proper source is welcomed,"said Mbarak.
On Saturday the Catholic Bishops led by their chairperson, Archbishop Phillip Anyolo, said contributions to the church by leaders, politicians and well-wishers will only be made electronically through banks and mobile money transfer technology.
This, the church leaders said, will help regulate the monies that come into the church.
They also agreed that no politician will address worshipers in all the Catholic Churches across the country.
"Our churches will not be used as political platforms or for any other motive other than for the worship of God," said Bishop Anyolo.
Catholic bishops
George Kinoti
Noordin Haji
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