Conversation partner paper for interculture communication class Essay - 1

Conversation partner paper for interculture communication class - Essay Example These are very important to them since they determine the success of the interpersonal relations amongst the members of the community (Benedict, R., 2005). The first cultural pattern he revealed to me is about the relationship between the young ones and their seniors. Upon thorough questioning, he told me that it is a common practice that the juniors should always respect their seniors. Even if such a respect is supposed to be reciprocated, the children are supposed to show a lot of respect to their elders. A part from listening to them, they should do whatever they are instructed to do without much questioning. This is a very crucial aspect of these people because it has enabled them to establish an obedient class of children who can effectively be guided to conform to the societal standards. The other important cultural pattern revealed to me by my partner is that in their society, there are norms which should be treated with respect. In order to promote cohesion and respect, there are certain things which should not be done by everyone. For instance, no body is allowed to discuss about topics such as sex in public. This regulation is specifically targeted towards children who are not permitted to engage in such discussions whatsoever (Ankerl, G., 2000). This, he told me, is meant to instill discipline in such children and make them believe that such are sacred activities which are exclusively meant for the married persons. As a matter of fact, I was able to notice my partner express these cultural patterns during our conversation. I learnt that he was keen on the way he was using both verbal and non-verbal communication to prove his point. When we first met, he treated me with lots of respect. Right from the beginning, he took me to be his senior. Therefore, he was the one who greeted me even before we started our interviews. Later, as we progressed with the discussions, I

Problem Assignment Example | Topics and Well Written Essays - 750 words

Problem - Assignment Example sponses including government’s actions on fiscal policy and the central bank’s actions on monetary policy to ensure stabilization of production over business cycle. Keynes, a supply-sider, encouraged governments to consider spending more money during economic depression (Arestis, 2011). This would create more employment opportunities; hence money would be spent to further employ more people. In the 1970s it was however ineffective because a kick start to the normal levels was all that was required. Human capital is simply an accumulation of experience, skills, and knowledge of labor force, though labor productivity may be significant. The productivity and incomes of workers tend to grow as their human capital advances. Unemployed individuals comprise those who have quit or lost their jobs, or have for the first time entered or re-entered the labor market. The employed comprise those under employment level that exhibits no cyclical unemployment. Often, discouraged workers belittle employment. Additionally, in cases where a worker is employed but is overqualified for the job, underemployment is still overlooked. Further, workers in underground economy normally do not consider themselves as employed. To measure unemployed rate, the participation rate of labor force is used (Arestis, 2011). GDP simply explains the economy’s size measured through use of output. Despite capturing productivity it considers population growth. Also, GDP measures per-capita, a factor that does not explain much on distribution or sustainability of the economy. An elevation in the net wealth lessens the possibility of saving thereby elevating consumption at each income. Greater price levels reduce money holdings value which decreases the net wealth and consumption at each income (Arestis, 2011). Savings become more rewarding while borrowing becomes more costly when the interest rates have been increased. This elevates savings and reduces consumption at each income. Consumption

Free Space Optical (FSO) Communication

Free Space Optical (FSO) Communication INTRODUCTION Free Space Optical (FSO) communication involves the transmission of data through a wireless medium using modulated near infrared light beam (with wavelength between 800 nm-1700 nm) [1] as carrier wave. FSO communication links can be used for satellite-to-satellite cross links [2] [3], up-and-down links between space platforms- aircraft, ships, and other ground platforms, and among mobile and stationary terminals within the atmosphere [3]. Light as a medium of communication is not a recent innovation as it was used in the Roman era, where polished metal plates where used as mirrors to reflect sunlight for long-range signaling. A similar sunlight-powered device was used by the U.S. military to send telegraph information between mountain tops in the early 1800s [4]. Additional optical communication developments occurred during the World War II, and the post-war era experienced further developments in this field fueled by electronic innovations such as the transistors, vacuum tubes and i ntegrated circuits, and most especially the invention of the laser in the early 1960s [1]. The unique characteristics of laser such as its powerful coherent light beam, the possibility of modulating it at high frequency and the low beam divergence has made it the preferred light source for enhanced FSO applications. FSO communication is considered to be one of the key technologies for realizing very-high-speed multi-gigabit-per-second large-capacity communications when fibre optic cable is neither practical nor feasible [4]. FSO communication can be of crucial advantage particularly because of its wireless nature and several applications, making it a viable alternative to the laying of fiber cable underground which is expensive and has environmental consequences. Unlike radio and microwave systems, FSO has higher data rate due to its high carrier frequency, low power requirements, no frequency license required and much smaller packaging [4]. In FSO links, atmospheric turbulence is capable of degrading the wave-front quality of a signal-carrying laser beam, resulting in signal loss at the receiver and thereby impairing the link performance [4],[5]. In addition, fog, snow, rain, dust, smoke and other aerosol particles contribute to the attenuation of the signal-carrying beam and eventual degradation of the FSO link. Several studies have been done on the atmospheric turbulence channels of a FSO system [3], [5]. The real performance measure of an FSO communication system is provided by the binary error probability also referred to as Bit Error Rate (BER).FSO communication involve the use of optical amplifiers either as an optical booster or optical preamplification. Amplification is achieved by stimulated emission of photons from dopant ions in the dopant fibre by using a pump laser as used in Erbium Doped Fibre Amplifiers (EDFA) or electrically as used in semiconductor lasers, due to the excitation of ions from a higher ene rgy state to a lower energy state. The excited ions can also decay spontaneously (spontaneous emission) or even through non-radioactive processes involving interactions with phonons of the glass matrix. These last two decay mechanisms compete with stimulated emission reducing the efficiency of light amplification introducing what is known as the Amplified Spontaneous Emission (ASE) noise. Digital Pulse Position Modulation (DPPM) with direct-detection is the preferred modulation technique for FSO communication systems because of the lack of dispersion over the free space channel [2] , none requirement of threshold for detection [3] and the complexity associated with phase or frequency modulation [5]. At the moment, the benefits of FSO communications have not been fully exploited, hence more applied researches are needed at the laboratory to help transfer the performance of FSO optical systems into real life applications. CHAPTER TWO LITERATURE REVIEW 2.1 FSO – PRINCIPLES AND CLASSIFICATION A FSO system consists basically of a transmitter, usually a modulated laser or light emitting diode (LED) which produces light for conveying data through space, and a receiver such as a photodetector which receives close to collimated radiation independent of the transmitter pointing concentrated lens [1]. Other components include beam control optics, collection lens, optical amplifier, solar radiation filters and other electronics [6]. In general, FSO systems can be classified into indoor and outdoor FSO communication [4], [7] based on the distance covered for communication and the absence of environmental effects in the indoor FSO link. The basic operating principles of indoor point to point systems are not different from outdoor communication links but the designs are very different to accommodate various requirements. Some research on transmitter and receiver designs for long-range FSO communication systems, [8] like satellite and atmospheric optical communications, have already been reported, which might be beneficial to short-range systems. Recently, many indoor communication systems employ laser diode as light sources [8]. 2.1.1 INDOOR FSO COMMUNICATION The indoor FSO communication link can be further classified as point and shoot links which are subdivided into infrared data association (IRDA) and retro-reflect links; and networks which have two types namely the diffuse networks and line of sight networks [4],[7]. Indoor FSO applications are confined to short distances; hence it is appropriate to optical FSO systems that use wide divergence beams rather than narrow beams which are suitable for point to point systems. Such systems are sometimes referred to as optical telepoint systems [7]. Due to the fact that indoor FSO systems are not affected by atmospheric effects, the power budget depends solely on the transmitter launch power, free space loss, and receiver sensitivity. As reported in Mahdiraji and Zahedi [9], the use of infrared frequencies for short-range wireless communications has received extensive interest over the decade, and many potential applications of this technology have been suggested. Some of the applications inc lude portable device such as laptop computers, personal digital assistants, and portable telephones. Many indoor communication systems employing infrared LED wireless links have been reported in [7], [9]. Using pure diffuse link, a high speed and power efficient indoor wireless infrared communication using code combining was reported in Majumdar and Ricklin [4], where a multiple transmitter link design was used with a narrow field of view direction diversity receiver. The design goal was to eliminate the effect of inter symbol interference (ISI) so that power efficient signalling schemes such as DPPM can be employed at a very high data rate. A portable transceiver for indoor FSO link was reported by Jiang et al [8] .The system employs a transmitter of eye safe infrared LEDs and a receiver of photodiode arrays with multi channel trans-impedance-summer architecture. The received signal achieved a BER of 10-4 at a plane of 2 m away from the transmitter, even at a point 50 degree off the transmitters vertical axis. The bit rate of the transceiver was up to 40 Mbit/sec in an indoor non-directed infrared FSO link to be extended to 100 Mbit/s using LEDs with higher cut frequency. 2.2.2 OUTDOOR FSO COMMUNICATION Unlike the indoor FSO communication link, the outdoor link covers a long distance over 500 m [7] using the atmosphere as its natural medium of communication. It is unlikely that long distance point-to-point systems will exceed the 4-5 km range due to atmospheric channel degradation. Outdoor point-to-point systems use high power lasers that operate in the Class 3B eye safety band to achieve optimum power link budget, particularly if high bit rate (e.g., 155 Mb/s) is required [7]. In order to achieve an improved power budget, an experiment was carried out by Heatley et al [8] to achieve a trial 155 Mb/s link that cover a 4 km distance between Imperial College and University College London. The aim of the experiment was to minimize the overall propagation loss focussing on the free space loss and the receiver sensitivity as little could be done to reduce the atmospheric losses. A low power laser coupled to an erbium doped fibre amplifier (EDFA) was used as the source; two astronomical t elescopes (Schmidt-Cassegrain, 20 cm aperture) were used at both the source and receiver ends, and an Avalanche photodiode (APD) was used as receiver. The result of the experiment was that the diameter at the receiver was reduced to 0.5 m which corresponds to 8 dB free space loss from a beam diameter of 2 m (20 dB free space loss). The major problem encountered was maintaining the beam alignment which depends on temperature changes. Although the APD receiver is relatively costly, it helped to improve the receiver sensitivity and they are generally used for long distance systems. FSO is well established for intersatellite and deep-space communications [3] but it can also be used in ground-to-space link, Unmanned-Aerial-Vehicle (UAV) to ground link, and among mobile and stationary terminals within the atmosphere [10]. New areas of application include quantum key, traffic and telematics [6]. It has been predicted that FSO can be fruitfully utilized as alternative for last-mile problem [3], [10], [11]. According to Majumdar and Ricklin [4], a group of researchers reported the design and development of acquisition, tracking and pointing subsystem for UAV to ground free space optical communications link. The communication link was developed from a UAV to stationary ground stations located at Wrightwood, California and Maui, Hawaii. The range of the UAV was 50 km. the downlink laser transmitter wavelength was 1550 nm and had power of 200 mW for developing a 2.5 Gbps data rate communication for a BER of 10-9. Furthermore researchers have presented results from exper imental demonstrations using a very lightweight optical wavelength communication without laser in space (LOWCAL) [4] between ground based telescope and a space shuttle. The uplink/downlink established at 852 nm wavelength and 852 nm signal beam. Some of the specifications for the experiment were: range of 640 km, data rate of 10 kbps, telescope diameter of 0.6 m, modular weight of 2-4 kg and retro-reflector area of 70-180 cm2. For the downlink, differential circular polarization keying (DCPK) format was used while Frequency Shift keying was used for the uplink. Both the downlink and uplink achieved a BER 2.2 FSO- TECHNOLOGICAL ADVANTAGES AND CHALLENGES Theoretically, the wavelength range of the near infrared electromagnetic spectrum (800 nm and 1700 nm [1]) used in optical communication implies it offers extremely high bandwidth, hereby providing higher data rates compared to other communication media such as the radio (1 mm-100 Mm ) and microwave (1 mm-1 m) systems [1]. Further, this technology requires no spectrum licensing requirements i.e. traffic free bands, no mutual interference between the FSO systems (high spatial selectivity of the beam), no Fresnel Zone requirement and difficult to eavesdrop on transmitted data [4], [6], [10]. The integration of the optical wireless and fibre is a research area been worked on by my many researchers [4]. This integration is possible because the two optical technologies offer high—speed optical bandwidth to meet market needs. They both use the same optical transmission wavelength (800 nm – 1700 nm) [1]. In addition the two optical technologies both share the same system compo nents and can transmit digital information using a range of protocols. The business advantages of FSO communications for network extensions include the reduction of cost incurred on fibre-optic cable and other associated costs, as well as time for deployments [4]. Despite its potentials, FSO communication link is affected by atmospheric attenuation owing to aerosol particles such as fog, haze, rain and snow which causes fluctuations in both the intensity and the phase of the received light signal hereby limiting the availability of FSO for a given transmission range and increasing the systems bit error rate [5]. These includes the fine mode with diameter, less than 2.5  µm, the accumulation mode with particle diameters 0.1  µm  µm and the coarse mode with diameter > 2.5  µm [12]. These particles can grow in size in regions of high humidity, and shrink by evaporation as humidity decreases. The effect of atmospheric aerosols in the channel on laser beam propagation can be determined using the Mie theory which depends critically on wavelength and particle size [13]. The dimensionless size parameter illustrates the nature of the Mie calculation as it gives the relationship between the particle size and radiation wavelength. This is given m athematically in equation 2.1[13] as where is wave number, is particle radius and is particle wavelength. Table 2.1 adapted from Ricklin et al [13] show some sources of aerosol in the atmosphere. Aerosol particles can be classified into three modes based on the diameter of the particles. Table 2.2 adapted from OBrien et al [6] shows the attenuations caused by rain, snow and fog. Table 2.1 Estimates of particles smaller than 20 micron radius emitted into or formed in the atmosphere (106 metric tons/year) (adapted from [13]) *Includes unknown amounts of indirect man-made contributions. Table 2.2 Attenuations caused by rain, snow and fog (adapted from [6]) Extinction is a term which describes the attenuation of a laser beam as it passes through a medium containing atoms, molecules, and particles. As reported in Ricklin et al [13], Goody and Yung defined the fundamental law of extinction as that of Lambert, which states that â€Å"the extinction is linear in both intensity of radiation and in the amount of matter, provided that the physical state (i.e., temperature, pressure, composition) is held constant†. Intensive researches [3], [4], [5], [13] have been conducted on the effects of atmospheric losses, most especially atmospheric scintillation, on FSO communication and some solutions have been proffered to reduce these effects. 2.2.1 PROPAGATION LOSS This atmospheric loss is associated with the distance covered by the laser beam. According to Prokes [12], free space propagation loss can be expressed as shown in equation 2.2 [dB] (2.2) where L is the link path distance, is the beam divergence full angle and is the diameter of transmitting circular aperture. For the Gaussian beam and a sufficiently long link distance ( >>) the additional gain is = 3.7 dB. In Heatley et al [7], it was reported that for a point-to-point system that operate with a slightly diverging beam, the free space propagation loss would be 20 dB whereas in an indoor system using wide angle beam, the free space loss would be 40 dB or more. At very short link distances, the total transmitted power is detected at the receiver because the beam spot diameter at the receiver position is lower than the diameter of the receiver lens [12]. 2.2.2 PHYSICAL OBSTACLES Physical obstructions such as birds, insects, tree limbs, buildings or other factors can temporarily or permanently block the laser line-of-sight [6], [13]. Platform/building motion due to wind, differential heating and cooling, or ground motion over time can result in serious misalignment of fixed-position laser communication systems [13]. Proper planning and site measurements are ways of avoiding this effect [6]. 2.2.3 ABSORPTION AND SCATTERING Molecular absorption process which is wavelength dependent is a major factor in beam attenuation. At wavelengths greater than 1 µm, the effect of molecular extinction can be negligible as atoms couple weakly with electromagnetic field [4]. Furthermore, molecular absorptions at these wavelengths are due to absorption of incident radiation with only minor scattering contributions [4]. Aerosol scattering effect is caused by rain, fog, mist and snow. This effect accounts for the degradation in quality of service experienced during snow, rain, fog and mist as reported in a detailed measurement take by [7] over a period of one year in both rural and urban areas. The results also reported in Heatley et al [7], showed a similar trend but with rather less variability between seasons and higher average attenuations. For many molecules, the absorption spectra have been measured experimentally in the laboratory and the respective extinction ratios of specific molecules have been made available for evaluation [4]. Molecular absorption can be minimized by appropriate selection of the optical wavelength [4]. It has also been suggested in [7] that the attenuation effects due to scattering can be minimized by reducing the link range and/or reducing the optical power budget. The attenuation caused by scattering in decibel scale A10, scat is given by the product of the atmospheric attenuation coefficient ÃŽ ±10, scat and the link distance in kilometres. The mathematical expressions are shown in equations (2.3), (2.4) and (2.5) [12]. [dB/km] (2.3) Where is the particle size distribution coefficient defined as: [dB] (2.5) 2.2.4 ATMOSPHERIC SCINTILLATION Scintillation is caused by solar energy heating up small air pockets inhomogenously, thereby creating varying refractive index along the FSO link [7]. This results in the scattering of laser beams at various angles along the propagation path and a resultant fluctuation in both the intensity and phase of the received light [5], [7].Atmospheric scintillation is less significant at distances less than 500 m [7], [12], but degrades performance of a FSO link at ranges of the order of 1km or longer [5]. The intensity I of an optical wave propagating through turbulent atmosphere is a random variable. The normalized variance of optical wave intensity, referred to as the scintillation index, is defined by equation (2.6) [12] where the angular brackets denote an ensemble average. The scintillation index indicates the strength of intensity fluctuations. For weak fluctuations, it is proportional and, for strong fluctuations, it is inversely proportional to the Rytov variance for a plane which is shown in equation (2.7) [12] where is the refractive-index structure parameter. This parameter is dependent on temperature, humidity, atmospheric pressure, altitude and wind strength [12]. Beam wander is another occurrence in an atmospheric channel which causes similar effects as atmospheric scintillation. This is characterized by the deflection of the entire laser beam by optical tubules of larger diameter than the beam, resulting in a random movement of the light beam about the target point. Atmospheric scintillation is a major impairment of FSO communications systems, as it can produce large transient dips in the optical signal. It has been studied extensively with various theoretical models already proposed to describe the signal fading [3], [5], [11]. In addition, several communication techniques have been described to mitigate the signal fading effect [5], [6], [12], [13]. In [3], the error performance of terrestrial FSO links were modelled as PPM/Poisson channels in turbulent atmosphere. The scintillation effects were modelled as lognormal for weak turbulence and as exponential for heavy turbulence. In Kiasaleh [11], the performance of a direct-detection, APD-based PPM FSO communication system in atmospheric turbulence was characterized. Here the weak turbulence link was investigated by modelling the received signal as a log-normal random process and also as a negative exponentially distributed received signal intensity. The binary PPM was used as the modulation sch eme for the system. It was assumed that the receiver thermal noise is non-negligible and the average signal intensity was large enough to justify as Gaussian approximation. It was concluded that the performance of the APD-based PPM FSO system was severely affected by turbulence and that the optimum APD gain must be used to avoid excessive APD noise at the receiver. It was also concluded that the negative exponential channel scintillation affected the systems performance with only large signal power capable of influencing the performance. Zhu and Kahn [5] employed the statistical properties of signal fading, as a function of both temporal and spatial coordinates as an approach of mitigating turbulence-induced intensity fluctuation. In spatial domain technique, at least two receivers are used to collect the signal light at different angles. In temporal domain techniques, only one receiver is used. Here signal-by-signal maximum likelihood detector (ML) is used to optimize performance w hen the receiver knows only the marginal statistics of the fading while maximum-likelihood sequence detection (MLSD) is used when the receiver knows the temporal correlation of the fading. The investigation showed that BER has greater degradation when the standard deviation of the turbulence induced fading is large. Furthermore, the diversity reception with the two receivers can improve the performance than a single receiver. According to Prokes [12], the receiver lens area causes an integration of various intensities incident on particular parts of the lens. It was reported in [12] that optical scintillations can be reduced by increasing the collection area of the receiver lens. This phenomenon is known as aperture averaging and the aperture averaging factor for a spherical wave is shown in equation (2.8) [12] where is the power scintillation index and is the Rytov variance for the spherical wave. An experiment was carried out by Prokes [12] to investigate the effect of the aperture averaging factor on the power scintillation index using two different refractive index structure parameters for the calculation. The result showed that the influence of both the lens diameter and refractive-index structure parameter on the scintillation level was relatively large. The power link budget of a FSO communication link was given in [12] based on statistical analysis of the atmospheric attenuation. Figure 2.1 shows a power level diagram of FSO deployed at a distance of about 1 km. According to figure 1, the total received optical power, was given by Prokes [12] in equation (2.9) as where is the mean optical power of a laser diode, includes the coupling loss between the laser and the transmitter lens and the attenuation loss in the lens, is the beam attenuation due to propagation loss, includes random losses caused by atmospheric phenomena (scattering and turbulence), and represents the coupling lens between the receiver lens and photodiode and the attenuation and reflection at the lens. 2.2.5 AMPLIFIED SPONTANEOUS EMISSION (ASE) Optical amplifiers are important components in communication links, as they are used to boost output power levels in the order of one Watt and higher. Due to the atmosphere medium used in FSO, only optical booster and pre-amplification schemes can be used [14]. Apart from the desired optical signal amplification, the optical booster amplifier constitutes a source of background radiation that can exceed the background radiation from the sun. This radiation is called amplified spontaneous emission (ASE). The ASE power spectral density is represented in equation (2.10) [14] as where is one photon energy, denotes the optical amplifier gain, and is the amplifiers noise figure which ideally should be 3 dB. Equation (2.10) is valid for a single spatial mode (including polarization modes); in multimode applications has to be multiplied by the number of emitted modes to arrive at the total ASE power spectral density [14]. The effect of ASE radiation in optical booster and preamplification in FSO communication link was investigated by [14] using on/off keying (OOK) as the modulation technique. In the case of the booster amplifier, considering the fact that the maximum on-axis gain of a central unobscured optical transmit antenna with respect to an isolator radiator equals where is the diameter of the telescope optics and is the optical transmit wavelength .The value 0.81 is the optimum value of the fraction of incident power that can be coupled to an optical fibre from an incident plane wave. On inserting into equation (2.10), the ASE power spectral density generated by the booster amplifier and coupled to the receiver is given in equation (2.11) [14] as [W/Hz] (2.11) where is the transmitter gain, is the transmitter noise figure and and are the transmitter and receiver telescope diameter respectively. Also the signal power coupled to the receiver was given in equation (2.12) [14] as [W] (2.12) where is the transmit optical power at the output of the booster amplifier. Figure 2.2 [14] shows the general setup of an optically boosted FSO system. Modulated TX data Optical Receiver Optical booster amplifier GTX, FTX Telescope optics DTX Amplified data signal transmit booster ASE DRX R Using Erbium-doped booster amplifiers with specifications, telescope diameters, and a communication distance of , Winzer et al [14] arrived at which is of the order of the magnitude of the background radiation per mode produced by the sun. It also was reported that the booster ASE constitutes the dominating background radiation term up to communication distances of 600 000 km. In the optically preamplified FSO communication system, (see Figure 2.3) the received signal passes through polarization filter and optical bandpass which reduces the detected preamplifier ASE power. The preamplification introduces additional noise terms such as the shot noise, ASE-shot beat noise, signal-background beat noise, signal-ASE beat noise, background-preamplifier ASE beat noise, background-background beat noise and the ASE-ASE beat noise. It was reported by Winzer et al [14] that up to a link distance of 1000 km the beating of the signal and the transmit booster ASE dominates all other noise terms, c ausing the signal-to-noise ratio (SNR) to become independent of R. The communication quality does not in this case increase with decreasing communication distance. It was thus concluded that the optical booster ASE has significant impact on the performance of a FSO communication link, especially at short link distances [14]. Optical preamplifier POL- filter (optional) Optical bandpass Decision logic RX data In Winzer et al [14], the use of the pointing, acquisition and tracking (PAT) system (see figure 2.4) to reduce the optical booster ASE at the receiver was presented. The proposed patent-pending PAT system eliminates the need for the using separate power or hardware for beacon lasers, taking care of alignment procedures between the beacon-laser and the transmit or receive telescopes, and splitting off a certain fraction of the information carrying data signal for PAT purposes [14].The booster ASE is applicable to the PAT system because the ASE spectrum exceeds that of the data signal by orders of magnitude and also the ASE has the same spatial modes as the data signal. Figure 2.4 The PAT system retrieving pointing/tracking information from ASE emitted by the booster amplifier at the transmitter [14] Phillips et al[2] carried out an analysis of the optically preamplified intersatellite pulse position modulation (PPM) receiver employing maximum likelihood detection (MLD) using Gaussian approximation (GA) and Chernoff bound (CB) techniques. The results from the calculations carried out at a wavelength of and bit rate of 25 Mb/s showed that this mentioned receiver configuration is approximately 1.5 dB more sensitive than the optically preamplified OOK non-return-to zero (NRZ) signalling. This method is proposed to have future implementation in future laser intersatellite communication systems. This paper has been reviewed in my report, with special interest in the BER evaluation. 2.2.6 EYE SAFETY FSO systems involve the emission of high power optical power which can be unsafe, especially if operated incorrectly [7]. As a result of this, laser safety standards have been established and classified based on the amount of power emitted by the transmitter source. Table 2.3 (adapted from [7]) summarizes the principal classifications. Table 2.3 Laser safety classifications for point-to-source emitter (adapted from [7]) 650 nm (visible) 880 nm (infrared) 1310 nm (infrared) 1550 nm (infrared)

Perfect Storm Essay -- essays research papers

  Ã‚  Ã‚  Ã‚  Ã‚  The Perfect Storm by Sebastian Junger was an account of an immense storm and its destructive path through the North Atlantic. In late October of 1991, many a crew of several different fishing ships left their port for their final haul. Little did they know that they would soon cross paths with one of the greatest storms ever recorded. This particular storm would create huge swells, high winds, and hard rain. The system was said to be a â€Å"perfect storm† because all of the elements were just right to create the worst imaginable storm ever seen claimed some meteorologists. Such a storm left little room for rescue if one’s boat got into trouble. But there were those daring rescues from the Coast Guard during the storm that saved the lives of many and cost the life of one. These men risked their lives for the safety of others and have earned the title of â€Å"hero.† The Para rescue jumpers and pilots, our heroes in The Perfect Storm, had a re sponsibility to the well being of everyone at sea. When a distress call was made, it was their duty to answer it to the best of their ability. It seemed â€Å"to the best of their ability† meant do the job until they died trying. These men had undergone extensive training in their fields to prepare for such situations as the perfect storm presented them. The PJ’s had undergone a highly selective process that many Navy SEALS could not complete. And the pilots of the helicopters and jets showed tremendous skill as they navig...

Dt Gcse Revision: Smart Materials

SMART Materials A SMART material is a material that is affected by outside influence. Polymorph * Type of thermoplastic (can be reshaped many times). * Normally supplied in form of granules or small plastic beads. * Low fusing temperature of around 60Â °C. This means it can be reduced to a mouldable condition by immersion in hot water. * Once fused turns from opaque to clear. * Heating above 65Â °C is dangerous as it will become a sticky-adherent mass, similar to hot-melt glue. Polymorph has high molecular weight and great tensile strength, and therefore suited for applications where mechanical strength is important. * Some uses are: Mould-making (i. e Gum-shields), general DIY materials, making a polymorph skeleton to support other materials, trainer/shoe components. QTC (Quantum Tunnelling Composite)* QTC is a composite material made from conductive filler particles combined with an elastomeric binder, typically silicone rubber. QTC Material has the unique ability to smoothly cha nge from an electrical insulator to a metal-like conductor when placed under pressure. * While in an unstressed state the QTC Material is a near-perfect insulator; with any form of deformation the material starts to conduct and with sufficient pressure metallic conductivity levels can be achieved. * QTCâ„ ¢ Material can be formed or moulded into virtually any size, thickness or shape. * QTCâ„ ¢ Material is mechanically strong. QTCâ„ ¢ Material can be made to withstand extreme temperatures limits. * Used for switches and keyboards. Thermochromic Ink * These materials change colour when there is a change in temperature. * Thermochromic ink comes in two forms: Liquid crystals and Leuco dyes. * Leuco dyes are easier to work with and allow for a greater range of applications. * Applications of thermochromic ink include: flat thermometers, battery testers, clothing. Mr Tsiang

Literary Analysis on Death of a Salesman Essay

In Death of a Salesman by Arthur Miller, the conflict between a father and son shapes the overall meaning of the work and explains all of the adverse events that occur throughout. The sources of Willy and Biff’s conflicts, which include Biff’s delusional perception of the world as a result of ideas planted in him by his father, Biff’s discovery of his father’s affair, and Biff’s lack of business success all accumulate and result in the ultimate rivalry between the father and son. Altogether, these contribute greatly to the formation of the concept that personal dreams and desire to achieve success can often negatively interfere with personal relationships, and causing people to loose sight of what is important in our lives, as Willy and Biff exemplify. Throughout the play, there are flashbacks to Biff’s childhood as a successful athlete and motivated individual. Willy’s pride in his son’s accomplishments is apparent, as he constantly praises him saying, â€Å"Good work Biff!† (1561), yet Willy’s lack of acceptance of reality are as well. Frequently Bernard, a studious young boy, appears and reminds Willy of Biff’s unsatisfactory grades, yet Willy refuses to admit these downfalls and does not accept the reality of his son’s situation. Willy merely tells Bernard, â€Å"Don’t be a pest, Bernard! What an anemic!† (1560), and dismisses the negative statements made about Biff. Bernard constantly reappears almost as a symbol of Biff’s conscience, telling him to study or else he will not graduate. Willy does not help the situation and completely combats Bernard’s efforts by filling Biff’s head with lies and selling him on the idea of the American Dream as s omething that is easily achieved, by giving simple advice such as, â€Å"Be liked and you will never want† (1561). It is apparent that Willy weighs the importance of being well-liked and socially accepted more heavily than actual hard work and success, a negative reflection of his character. Willy preaches his philosophy that, â€Å"the man who makes an appearance in the business world, the man who creates personal interest, is the man who gets ahead† (1561). This is purely ironic due to the fact that Willy is the man who creates a personal interest in the business world with men of high status, but when all of his friends pass away he is left with nothing but a glorified past to remember. This false reality that Willy paints for Biff fosters the conflict between father and  son due to the fact that Biff fails as a result of the way he was raised. Biff follows his fathers ways and words, and by the time he takes his first job he has been raised to think that success and happiness will just come to him without excessive effort on his part. As any son would look up to and admire his father, Biff took his father’s advice and therefore makes no excessive efforts and put forth minimal work expecting to become successful merely because of his personality. This sense of entitlement is clearly diminished when Biff fails to keep a job and ends up at home. Willy never takes the time to teach Biff a good work ethic, good values, and strong morals, because Willy himself has not even established these within his own character. Therefore Biff steals, does not work hard, and finds it hard to make it in the real world. Willy himself does not know what is important in life, does not have morals, and does not value his family relationships, therefore he has no way of teaching Biff these vital tools for success and happiness. The resentment Willy feels because of Biff’s lack of success becomes the main conflict throughout the play ultimately reflects negatively upon Willy’s lack of ability to achieve the American dr eam himself, displaying Willy’s overall weak character. Biff’s discovery of his father’s affair serves as a main turning point for him as a character, a turning point that sends him downward into a life of struggle and lack of achievement. It is at this point that Biff loses respect for his father and begins to recognize the lie that he is living, thus making it a main source of conflict. Willy is in denial about his involvement with Biff’s failure in life, and when indirectly confronted by Bernard about the incident in Boston asking â€Å"What happened in Boston, Willy?† (1600), Willy becomes defensive, saying, â€Å"What are you trying to do, blame it on me? Don’t talk to me that way!† (1600). After being told about Biff’s reaction upon his return from Boston and the burning of his favorite University of Virginia shoes that symbolize Biff’s dreams and hopes for the future, Willy realizes the extent of impact that Biff’s discovery of the affair had. Willy’s lack of acc eptance of reality adversely affects his relationship with Biff because he never takes responsibility for his affair or even has the courage to admit it to Biff. As a result, when Biff discovers a woman in his father’s hotel room, he confronts his father, â€Å"You  fake! You phony little fake! You fake!† (1618) and all Willy can do is attempt to exercise his authority as a father which ultimately fails. Frequently throughout the play, Happy makes references to the man Biff used to be, asking him, â€Å"What happened, Biff? Where’s the old humor, the old confidence?† (1552). Learning about his father’s affair and seeing it firsthand that day in Boston was the turning point for Biff, the point where he grew up and realized that his father was a broken and defeated man, not the successful business man he portrayed himself as and used to be. As a result of this, Biff loses all respect for his father, and alternatively Willy begins to loathe Biff as well. Due to his discovery of the affair, Biff not only sees his father as a failed businessman, but a failed man. A man without money does not make him a bad man, but an adulterer who betrayed a woman who gave him everything cannot be forgiven in the eyes of a son. Throughout Willy’s continuous failures and defeats, his wife still remains supportive of him and loving, constantly reminding him of her affection for him. Despite this, Willy still yearns to have what he does not and thus pursues an extramarital relationship with â€Å"the other woman.† It is clear that Willy finds some kind of comfort and validation in this affair with a woman who makes him feel wanted, yet his wife does the same therefore it is clearly a matter of greed. â€Å"Willy’s sense of failure, his belief that he has no right to his wife, despite Linda’s love for him, is what motivates Willy’s deceptions, and those of his sons after him† (Bloom, Bloom’s Modern Critical Interpretations: Death of a Salesman). This event contributes to the overall meaning of the work as a symbol of the failure of the American Dream by Willy, not only in terms of personal success but also in terms of family relationship and his family’s success. Not only does Willy cheat on his wife, loathe his son, and struggle to keep a job, but he has let his values go and seems to have no moral compass of right and wrong. It shows that he has failed in the business aspect of his life, and also in his morals. Finally, Biff’s lack of success in the real world contributes largely to the conflict between him and his father. After having countless jobs over a period of several years, Biff returns home with loss of all hope of finding a steady job to support himself. Willy is disappointed by Biff’s lack of  ability to succeed, and, â€Å"It is to Biff, the returning son, to whom Willy relates most affectively.† (Hadomi, Rhythm Between Father and Son.) It is because Willy can see so much of himself in Biff and relates so heavily to him that these resentful feelings arise. Biff reflects his father’s failed ideals and expectations for himself, which are represented in Willy’s fantasies and flashbacks regarding Biff’s successful and glorious childhood, as well as expectations that Willy originally had for himself. Willy sees his failed life and career as a middle-aged man, and recognizes similar traits and qualities in Biff. Although he never expresses these, it is apparent that Willy largely sees himself in his son and thus takes out his anger for himself on Biff, resulting in constant fighting and conflict. The conflicted relationship between Willy and Biff exemplifies the theme of the work that in one’s pursuit of professional and material success, it is easy to become preoccupied with superficial aspects of life while simultaneously losing sight of what matters most. Willy’s preoccupation with his quest for material fulfillment ultimately results in a flawed relationship with his family, and ultimately with his son Biff when Willy sees him following in his footsteps. This conflict between father and son is what shapes the theme of the work and serves to highlight Miller’s purpose and the greater meaning of the play; that nothing is more important than family. (Word Count: 1517)