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They only tie us down and we would gladly do without them if we could still get 1 Chapter One reliable operation at an acceptable price. Cellular radio today is of lower quality, lower reliability, and higher price generally than wired telephone, but its acceptance by the public is nothing less than phenomenal. Imagine the consequences to lifestyle when electric power is able to be distributed without wires! Considering the ever-increasing influence of wireless systems in society, this book was written to give a basic but comprehensive understanding of radio communication to a wide base of technically oriented people who either have a curiosity to know how wireless works, or who will contribute to expanding its uses.
While most chapters of the book will be a gateway, or even a prerequisite, to understanding the basics of all forms of radio communication, including satellite and cellular systems, the emphasis and implementations are aimed at what are generally defined as short-range or low-power wireless applications. These applications are undergoing a fast rate of expansion, in large part due to the technological fall-out of the cellular radio revolution.
In addition, price and size limitations should restrict proliferation of wire replacement devices. However, technological developments defy these axioms. In particular, solid-state devices have recently been developed to amplify at millimeter wavelengths, or tens of gigahertz. Efficient, compact antennas are also available, such as planar antennas, which are often used in short-range devices. The development of surface acoustic wave SAW frequency-determining components allow generation of UHF frequencies with very simple circuits. Hybrid integrated circuits, combining analog and digital functions on one chip, and radio-frequency integrated circuits are to a large part responsible for the amazingly compact size of cellular telephone handsets.
This miniaturization is not only a question of convenience, but also a necessity for efficient design of very short-wavelength circuits. Hardly any of the applications that we will discuss will have all of these characteristics, but all of them will have some of the following features: This is being developed by the Bluetooth consortium of telecommunication and PC technology leaders for eliminating wiring between computers and peripherals, as well as wireless internet access through cellular phones. Mass production will eventually bring sophisticated communication technology to a price consumers can afford, and fallout from this development will surely reach many of the applications in the table above, improving their reliability and increasing their acceptance for replacing wiring.
Essentially all elements of this system will be described in detail in the later chapters of the book. A brief description of them is given below with special reference to short-range applications. Each of the devices listed in the table on page 4 has its own characteristic data source, which may be analog or digital. In this case, a change of state of the data will cause a message frame to be modulated on an RF carrier wave.
In its simplest form the message frame may look like Figure A parity bit or bits may be appended to allow detecting false messages. Message Frame Other digital devices have more complex messages. Computer accessories and WLANs send continuous digital data over the short-range link. These data are organized according to protocols that include sophisticated error detection and correction techniques see Chapter Audio devices such as wireless microphones and headsets send analog data to the modulator.
However, these data must be specially processed for best performance over a wireless channel. For FM transmission, which is universally used for these devices, a preemphasis filter increases the high frequencies before transmission so that, in the receiver, deemphasizing these frequencies will also reduce high-frequency noise. Similarly, dynamic range is increased by the use of a compandor. In the transmitter weak sounds are amplified more and strong signals are amplified less. The opposite procedure in the receiver reduces background noise while returning the weak sounds to their proper relative level, thus improving the dynamic range.
A quite different aspect of the data source is the case for RFIDs. Here, the data are not available in the transmitter but are added to the RF signal in an intermediate receptor, called a transducer. RFID mitted radio frequency is modified by the transducer and detected by a receiver that deciphers the data added and passes it to a host computer.
Radio frequency generating section This part of the transmitter consists of an RF source oscillator or synthesizer , a modulator, and an amplifier. In the simplest short-range devices, all three functions may be included in a circuit of only one transistor. Chapter 5 details some of the common configurations. Again RFIDs are different from the other applications in that the modulation is carried out remotely from the RF source. RF conduction and radiation Practically all short-range devices have built-in antennas, so their transmission lines are relatively short and simple.
However, particularly on the higher frequencies, their lengths are a high enough percentage of wavelength to affect the transmission efficiency of the transmitter. Chapter 3 discusses the transmission lines encountered in short-range systems and the importance and techniques of proper matching. The antennas of shortrange devices also distinguish them from other radio applications.
They must be small—often a fraction of a wavelength—and omnidirectional for most uses. The allowed radio frequency power is relatively low and regulated by the telecommunication authorities. Also, the devices are often operated while close to or attached to a human or animal body, a fact which affects the communication performance. Reliable operating range is difficult to predict for these systems, and lack of knowledge of the special propagation characteristics of short-range radio by manufacturers, sellers, and users alike is a dominating reason for its reputation as being unreliable.
Short-range devices are often used to replace hard wiring, so when similar performance is expected, the limitations of radio propagation compared to wires must be accounted for in each application. Chapter 2 brings this problem into perspective. Receivers Receivers have many similar blocks to transmitters, but their operation is reversed. They have an antenna and transmission line, RF amplifiers, and use oscillators in their operation. Weak signal signals intercepted by the antenna are amplified above the circuit noise by a low noise amplifier LNA. The desired signal is separated from all the others and is shifted lower in frequency in a downconverter, where it may be more effectively amplified to the level required for demodulation, or detection.
The detector fulfulls the ultimate purpose of the receiver; conversion of the data source which was implanted on the RF wave in the transmitter back to its original form. While the transmitted power is limited by the authorities, receiver sensitivity is not, so the most obvious way to improve system performance is by improving the sensitivity and the selectivity to reduce interference from unwanted sources. This must be done under constraints of physics, cost, size, and often power consumption. Chapter 7 deals with these matters. In the case of the simpler systems—security and medical alarms, for example—the choice is between amplitude shift keying ASK , parallel to amplitude modulation in analog systems, and frequency shift keying FSK , analogous to frequency modulation FM.
When size is limited, as it is in hand-operated remote control transmitters and security detectors, battery size and therefore energy is limited. The need to change batteries often is not only highly inconvenient but also expensive, and this is an impediment to more widespread use of radio in place of wires. Thus, lowcurrent consumption is an important design aim for wireless devices.
This is usually harder to achieve for receivers than for transmitters. Many short-range applications call for intermittent transmitter operation, in security systems, for example. Transmitters can be kept in a very lowcurrent standby status until data needs to be sent. Another way to reduce receiver power consumption is to operate it in a reduced power standby mode, wherein operation goes to normal when the beginning of a signal is detected. This method often entails reduced sensitivity, however.
While its basic operating characteristics are the same as all radio systems, there are many features and specific problems that justify dealing with it as a separate field. Among them are low power, low cost, small size, battery operation, uncertainty of indoor propagation, and unlicensed operation on crowded bands. The rest of this book will delve into the operational and design specialties of short-range radio communication from the electromagnetic propagation environment through antennas, receivers and transmitters, regulations and standards, and a bit of relevant information theory.
The last chapter describes in detail current developments that are bringing wireless to the home at an unprecedented extent. Electronic worksheets contained on the accompanying CD-ROM and referred to throughout the book can be used to work out examples given in the text, and to help the reader solve his own specific design problems. If you want to design an efficient radio communication system, even for operation over relatively short distances, you should understand the behavior of the wireless channel in the various surroundings where this communication is to take place.
In , the British physicist James Clerk Maxwell published his Treatise on Electricity and Magnetism in which he presented a set of equations that describe the nature of electromagnetic fields in terms of space and time. Appendix 2-A gives a brief description of those equations. From them, you can derive the skin effect equation and the electric and magnetic field relationships very close to antennas of all kinds.
Ground waves exist only for vertical polarization, produced by vertical antennas, when the transmitting and receiving antennas are close to the surface of the earth see Polarization under Section 3. The reason that horizontal antennas are not effective for ground wave propagation is that the horizontal electric field that they create is short circuited by the earth.
Thus, in most short-range radio applications, a horizontal antenna will 44 Antennas and Transmission Lines receive transmissions from a vertical antenna, for example, albeit with some attenuation. Maximum ratio combining is known to be optimum as it gives the best statistical reduction of fading of any linear diversity combiner. Magnetic flux lines are continuous loops, so lines that enter a closed surface must also come out of it. The operating frequency is MHz. Effective area Another term often encountered is the effective area of an antenna. These data are organized according to protocols that include sophisticated error detection and correction techniques see Chapter If you are a seller for this product, would you like to suggest updates through seller support?
The ionosphere is responsible for long-distance communication in the high-frequency bands between 3 and 30 MHz. It is very dependent on time of day, season, longitude on the earth, and the multi-year cyclic production of sunspots on the sun. It makes possible long-range communication using very low-power transmitters. There are times when ionospheric reflection occurs at the low end of this range, and then sky wave propagation can be responsible for interference from signals originating hundreds of kilometers away.
However, in general, sky wave propagation does not affect the short-range radio applications that we are interested in. We discuss below the relationship between signal strength and range in lineof-sight and open-field topographies. The range of line-of-sight signals, when there are no reflections from the earth or ionosphere, is a function of the dispersion of the waves from the transmitter antenna. In this free-space case the signal strength decreases in inverse proportion to the distance away from the transmitter antenna.
When the radiated power is known, the field strength is given by equation To find the power at the receiver Pr when the power into the transmitter antenna is known, use Range can be calculated on this basis at high UHF and microwave frequencies when high-gain antennas are used, located many wavelengths above the ground. At microwave frequencies, signal strength is also reduced by atmospheric absorption caused by water vapor and other gases that constitute the air. The path lengths of the reflected signals differ from that of the line-of-sight signal, so the receiver sees a combined signal with components having different amplitudes and phases.
The reflection causes a phase reversal. In an open field with flat terrain there will be no reflections except the unavoidable one from the ground.
It is instructive and useful to examine in depth the field strength versus distance in this case. Using trigonometry, we can find the line of sight and reflected signal path lengths d1 and d2. We get the relative strength of the direct signal and reflected signal using the inverse path length relationship. If the ground were a perfect mirror, the relative reflected signal strength would exactly equal the inverse of d2. In this case, the reflected signal phase would shift degrees at the point of reflection.
However, the ground is not a perfect reflector. Its characteristics as a reflector depend on its conductivity, permittivity, the polarization of the signal and its angle of incidence. In the Mathcad worksheet we have accounted for polarization, angle of incidence, and permittivity to find the reflection coefficient, which approaches —1 as the distance from the transmitter increases.
The signals reaching the receiver are represented as complex numbers since they have both phase and amplitude. Range at MHz Figure gives a plot of relative open field signal strength versus distance using the following parameters: Polarity — horizontal Frequency — MHz Antenna heights — both 3 meters Relative ground permittivity — 15 Also shown is a plot of free space field strength versus distance dotted line.
In both plots, signal strength is referenced to the free space field strength at a range of 3 meters. Beyond meters, signal strength decreases more rapidly than for the free space model. Whereas there is an inverse distance law for free space, in the open field beyond meters for these parameters the signal strength follows an inverse square law. Increasing the antenna heights extends the distance at which the inverse square law starts to take effect. In plotting Figure , we assumed horizontal polarization.
Both antenna heights, h1 and h2, are 3 meters. When vertical polarization is used, the extreme local variations of signal strengths up to around 50 meters are reduced, because the ground reflection coefficient is less at larger reflection angles. However, for both polarizations, the inverse square law comes into effect at approximately the same distance.
In Figure we see that this is approximately the distance where the open-field field strength falls below the free-space field strength. It also fills in the spaces around obstacles when short-range radio is used inside buildings. Figure is an illustration of diffraction geometries, showing an obstacle whose extremity has the shape of a knife edge. The obstacle should be seen as a half plane whose dimension is infinite into and out of the paper. The field strength at a receiving point relative to the free-space field strength without the obstacle is the diffraction gain. The phenomenon of diffraction is due to the fact that each point 16 Radio Propagation Figure Knife-edge Diffraction Geometry d1 d2 h R T a R is in shadow—h is positive T d1 R h d2 b T and R are line of sight—h is negative on the wave front emanating from the transmitter is a source of a secondary wave emission.
Thus, at the knife edge of the obstacle, as shown in Figure a, there is radiation in all directions, including into the shadow. The diffraction gain depends in a rather complicated way on a parameter that is a function of transmitter and receiver distances from the obstacle, d1 and d2, the obstacle dimension h, and the wavelength.
Think of the effect of diffraction in an open space in a building where a wide metal barrier extending from floor to ceiling exists between the transmitter and the receiver. In our example, the space is 12 meters wide and the barrier is 6 meters wide, extending to the right side.
Figure is a plot of diffraction gain when transmitter and receiver are each 10 meters from the edge of the 17 Chapter Two Figure Example Plot of Diffraction Gain vs. Transmission frequency for the plot is MHz. When the barrier edge is on the line of sight, diffraction gain is approximately —6 dB, and as the line-of-sight path gets farther from the barrier to the left in this example , the signal strength varies in a cyclic manner around 0 dB gain.
As the path from transmitter to receiver gets farther from the barrier edge into the shadow, the signal attenuation increases progressively. Admittedly, the situation depicted in Figure is idealistic, since it deals with only one barrier of very large extent. Normally there are several partitions and other obstacles near or between the line of sight path and a calculation of the diffraction gain would be very complicated, if not impossible. However, a knowledge of the idealistic behavior of the defraction gain and its dependence on distance and frequency can give qualitative insight.
Rough surfaces in the vicinity of the transmitter do not reflect the signal cleanly in the direction determined by the incident angle, but diffuse it, or scatter it in all directions. As a result, the receiver has access to additional radiation and path loss may be less than it would be from considering reflection and diffraction alone. The degree of roughness of a surface and the amount of scattering it produces depends on the height of the protuberances on the surface compared to a function of the wavelength and the angle of incidence. The critical surface height hc is given by [Gibson, p.
It is the dividing line between smooth and rough surfaces when applied to the difference between the maximum and the minimum protuberances. This is the ratio of the transmitter power delivered to a lossless antenna with numerical gain of 1 0 dB to that at the output of a 0 dB gain receiver antenna.
Sometimes, for clarity, the ratio is called the isotropic path loss. An isotropic radiator is an ideal antenna that radiates equally in all directions and therefore has a gain of 0 dB. The inverse path loss ratio is sometimes more convenient to use. It is called the path gain and when expressed in decibels is a negative quantity. For a given site, it would be very difficult to calculate the path loss between transmitters and receivers, but empirical observations have allowed some general conclusions to be drawn for different physical environments.
These conclusions involve determining the exponent, or range of exponents, for the distance d related to a short reference distance d0. We then can write the path gain as dependent on the exponent n: Table shows path loss for different environments.
What range can we expect for their installation in a building? The reference distance is 10 meters, and for all three curves the path gain at 10 meters is taken to be that of free space. For an 20 Radio Propagation Path Gain vs. Path Gain open field distance of meters, the path gain is —83 dB.
Thus, a wireless system that has an outdoor range of meters may be effective only over a range of 34 meters, on the average, in an indoor installation. The use of an empirically derived relative path loss exponent gives an estimate for average range, but fluctuations around this value should be expected. The next section shows the spread of values around the mean that occurs because of multipath radiation. The nature of short- range radio links, which are very often installed indoors and use omnidirectional antennas, makes them accessible to a multitude of reflected rays, from floors, ceilings, walls, and the various furnishings and people that are 21 Chapter Two invariably present near the transmitter and receiver.
Thus, the total signal strength at the receiver is the vector sum of not just two signals, as we studied in Section 2. In most cases indoors, there is no direct line-of-sight path, and all signals are the result of reflection, diffraction and scattering. From the point of view of the receiver, there are several consequences of the multipath phenomena.
Phase cancellation and strengthening of the resultant received signal causes an uncertainty in signal strength as the range changes, and even at a fixed range when there are changes in furnishings or movement of people. The receiver must be able to handle the considerable variations in signal strength. If the bandwidth of the signal is wide enough so that its various frequency components have different phase shifts on the various signal paths, then the resultant signal amplitude and phase will be a function of sideband frequencies. This is called frequency selective fading.
The differences in the path lengths of the various reflected signals causes a time delay spread between the shortest path and the longest path. The resulting distortion can be significant if the delay spread time is of the order of magnitude of the minimum pulse width contained in the transmitted digital signal. There is a close connection between frequency selective fading and time-delay distortion, since the shorter the pulses, the wider the signal bandwidth.
Measurements in factories and other buildings have shown multipath delays ranging from 40 to ns Gibson, p. When the transmitter or receiver is in motion, or when the physical environment is changing tree leaves fluttering in the wind, people moving around , there will be slow or rapid fading, which can contain amplitude and frequency distortion, and time delay fluctuations. The receiver AGC and demodulation circuits must deal properly with these effects.
The multipath effect in this case is classified as flat fading.
Communication. Fundamentals of RF System Design and Application reference for anyone designing short-range wireless devices or integrating them into communications applications of UWB are detailed in this edition. Chapter 9 also. Short-range Wireless Communication: Fundamentals of RF System Design and Application (Communications Engineering Series) [Alan Bensky] on.
In describing the variation of the resultant signal amplitude in a multipath environment, we distinguish two cases: Short-range radio systems that are installed indoors or outdoors in built-up areas are subject to multipath fading essentially of the first case. Our aim in this section is to determine the signal strength margin that is needed to ensure that reliable communication can take place at a given probability. While in many situations there will be a dominant signal path in addition to the multipath fading, restricting ourselves to an analysis of the case where all paths are the result of random reflections gives us an upper bound on the required margin.
Their values may vary with time, when various reflecting objects are moving people in a room, for example , or with changes in position of the transmitter or receiver which are small in respect to the distance between them. We are not dealing here with the large-scale path gain that is expressed in Eq. This function is plotted in Figure In this plot, the average value of the signal envelope, shown by a dotted vertical line, is 1.
The area of the curve between any two values of signal strength r represents the probability that the signal strength will be in that range. The average for the Rayleigh distribution, which is not symmetric, does not divide the curve area in half. The parameter that does this is the median, which in this case equals 1. Rayleigh Probability Density Function As stated above, the Rayleigh distribution is used to determine the signal margin required to give a desired communication reliability over a fading channel with no line of sight.
For any point on the curve, the probability of fading below the margin indicated on the abscissa is given as the ordinate. Thus, you need a signal strength 20 dB larger than the required signal if there was no fading. Fading Margins 25 0 10 Chapter Two The following table shows signal margins for different reliabilities.
Reliability, Percent Fading Margin, dB 90 10 99 20 If signals are received over multiple, independent channels, the largest signal can be selected for subsequent processing and use. The key to this solution is the independence of the channels. The multipath effect of nulling and of strengthening a signal is dependent on transmitter and receiver spatial positions, on wavelength or frequency and on polarity.
Space diversity A signal that is transmitted over slightly different distances to a receiver may be received at very different signal strengths. For example, in Figure the signal at 17 meters is at a null and at 11 meters at a peak. If we had two receivers, each located at one of those distances, we could choose the strongest signal and use it. In a true multipath environment, the source, receiver, or the reflectors may be constantly in motion, so the nulls and the peaks would occur at different times on each channel.
Sometimes Receiver 1 has the strongest signal, at other times Receiver 2. Figure illustrates the paths to two receivers from several reflectors. By selecting the strongest output, the average output after selection will be greater than the average output of one channel alone. To increase even more the probability of getting a higher average output, we could use 26 Radio Propagation three or more receivers. From Figure you can find the required fading margin using diversity reception having 2, 3, or 4 channels.
Note that the plots in Figure are based on completely independent channels. When the channels are not completely independent, the results will not be as good as indicated by the plots. Space Diversity Frequency diversity You can get a similar differential in signal strength over two or more signal channels by transmitting on separate frequencies.
For the same location of transmitting and receiving antennas, the occurrences of peaks and nulls will differ on the different frequency channels. As in the case of space diversity, choosing the strongest channel will give a higher average signal-to-noise ratio than on either one of the channels. The required frequency difference to get near independent fading on the different channels depends on the diversity of path lengths or signal delays.
The larger the difference in path lengths, the smaller the required frequency difference of the channels. A signal can be transmitted and received separately on horizontal and vertical antennas to create two diversity channels. Reflections can cause changes in the direction of polarization of a radio wave, so this characteristic of a signal can be used to create two separate signal channels.
Thus, cross-polarized antennas can be used at the receiver only. Polarization diversity can be particularly advantageous in a portable handheld transmitter, since the orientation of its antenna will not be rigidly defined. However, it may be simpler and less expensive to implement and may give enough improvement to justify its use, although performance will be less than can be achieved with space or frequency diversity.
Diversity implementation In the descriptions above, we talked about selecting or switching to the channel having the highest signal level. Maximum ratio combining is known to be optimum as it gives the best statistical reduction of fading of any linear diversity combiner. In applications where accurate amplitude estimation is difficult, the channel phases only may be equalized and the outputs added without weighting the gains. Performance in this case is almost as good as in maximum ratio combining.
In the case where multipath signals arrive from all directions, antenna spacing on the order of. This is at least one-half meter at MHz. When the multipath angle spread is small—for example, when directional antennas are used—much larger separations are required. Frequency separation must be adequate to create decorrelated channels. The bandwidths allocated for unlicensed short-range use are rarely adequate, particularly in the VHF and UHF ranges transmitting simultaneously on two separate bands can and has been done.
Frequency diversity to reduce the effects of time delay spread is achieved with frequency hopping or direct sequence spread spectrum modulation, but for the spreads encountered in indoor applications, the pulse rate must be relatively high—of the order of several megabits per second—in order to be effective. For long pulse widths, the delay spread will not be a problem anyway, but multipath fading will still occur and the amount of frequency spread normally used in these cases is not likely to solve it.
When polarity diversity is used, the orthogonally oriented antennas can be close together, giving an advantage over space diversity when housing dimensions relative to wavelength are small. Performance may not be quite as good, but may very well be adequate, particularly when used in a system having portable hand-held transmitters, which have essentially random polarization. Although we have stressed that at least two independent decorrelated channels are needed for diversity reception, sometimes shortcuts are taken.
In some low-cost security systems, for example, two receiver antennas— space diverse or polarization diverse—are commutated directly, usually by diode switches, before the front end or mixer circuits. Thus, a minimum of circuit duplication is required. In such applications the message frame is repeated many times, so if there happens to be a multipath null when the front end is switched to one antenna and the message frames are lost, at least one or more complete frames will be correctly received when the switch is on the other antenna, which is less affected by the null.
In that case, a receiver with one antenna will have a better chance of decoding at least one of many frames than when 29 Chapter Two switched antennas are used and only half the total number of frame repetitions are available for each. In a worst-case situation with fast fading, each antenna in turn could experience a signal null. Statistical performance measure We can estimate the performance advantage due to diversity reception with the help of Figure From probability theory see Chapter 10 the probability that two independent channels would both have communication errors is the product of the error probabilities of each channel.
Thus, if each of two channels has an error probability of 10 percent, the probability that both channels will have signals below the sensitivity threshold level when selection is made is. We see that the signal margin needed for 99 percent reliability 1 percent error is 10 dB. Using diversity reception with selection from two channels allows a reliability margin of only 10 dB instead of 20 dB, which is required if there is no diversity. Continuing the previous example, we need to transmit only 40 mW for 99 percent reliability instead of mW.
Required margins by selection among three channels and four channels is even less—6 dB and 4 dB, respectively. Remember that the reliability margins using selection combining diversity as shown in Figure are ideal cases, based on the Rayleigh fading probability distribution and independently fading channels.
However, even if these premises are not realized in practice, the curves still give us approximations of the improvement that diversity reception can bring. Weak signals can be amplified to practically any extent, but it is the noise that bounds the range we can get or the communication reliability that we can expect from our radio system. There are two sources of receiver 30 Radio Propagation noise—interfering radiation that the antenna captures along with the desired signal, and the electrical noise that originates in the receiver circuits.
In either case, the best signal-to-noise ratio will be obtained by limiting the bandwidth to what is necessary to pass the information contained in the signal. A further improvement can be had by reducing the receiver noise figure, which decreases the internal receiver noise, but this measure is effective only as far as the noise received through the antenna is no more than about the same level as the receiver noise.
Finally, if the noise can be reduced no further, performance of digital receivers can be improved by using error correction coding up to a point, which is designated as channel capacity. The capacity is the maximum information rate that the specific channel can support, and above this rate communication is impossible. The channel capacity is limited by the noise density noise power per hertz and the bandwidth.
Figure shows various sources of noise over a frequency range of 20 kHz up to 10 GHz. The data in Figure are only a representative example of radiation and receiver noise, taken at a particular time and place. Note that all of the noise sources shown in Figure are dependent on frequency.
The relative importance of the various noise sources to receiver sensitivity depends on their strength relative to the receiver noise. Atmospheric noise is dominant on the low radio frequencies but is not significant on the bands used for short-range communication—above around 40 MHz. Cosmic noise comes principally from the sun and from the center of our galaxy. In the figure, it is masked out by man-made noise, but in locations where man-made noise is a less significant factor, cosmic noise affects sensitivity up to 1 GHz. Man-made noise is dominant in the range of frequencies widely used for short-range radio systems—VHF and low to middle UHF bands.
It is caused by a wide range of ubiquitous electrical and electronic equipment, 31 Chapter Two Figure External Noise Sources Reference Data for Radio Engineers, Fourth Edition including automobile ignition systems, electrical machinery, computing devices and monitors. While we tend to place much importance on the receiver sensitivity data presented in equipment specifications, high ambient noise levels can make the sensitivity irrelevant in comparing different devices.
For example, a receiver may have a laboratory measured sensitivity of — dBm for a signal-to-noise ratio of 10 dB. However, when measured with its antenna in a known electric field and accounting for the antenna gain, —95 dBm may be required to give the same signal-tonoise ratio. Improved low-noise amplifier discrete components and integrated circuit blocks produce much better noise figures than those shown in Figure for high UHF and microwave frequencies.
Improving the noise figure must not be at the expense of other characteristics—intermodulation distortion, for example, which can be degraded by using a very high-gain amplifier in front of a mixer to improve the noise figure. Intermodulation distortion causes the production of inband interfering signals from strong signals on frequencies outside of the receiving bandwidth. External noise will be reduced when a directional antenna is used. Regulations on unlicensed transmitters limit the peak radiated power.
When possible, it is better to use a high-gain antenna and lower transmitter power to achieve the same peak radiated power as with a lower gain antenna. The result is higher sensitivity through reduction of external noise. Manmade noise is usually less with a horizontal antenna than with a vertical antenna. Propagation of electromagnetic waves is influenced by physical objects in and near the path of line-of-sight between transmitter and receiver. We can get a first rough approximation of communication distance by considering only the reflection of the transmitted signal from the earth.
If the communication system site can be classified, an empirically determined exponent can be used to estimate the path loss, and thus the range. When the transmitter or receiver is in motion, or surrounding objects are not static, the path loss varies and must be estimated statistically. We described several techniques of diversity reception that can reduce the required power for a given reliability when the communication link is subject to fading. This pioneering consultant offers essentially with frontier antenna designs and frontier numerical tools.
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This e-book offers a concise creation to either photo and video processing, supplying a balanced assurance among thought, purposes and criteria. Mostly I focus on entry-level content for pages I create and provide hyperlinks to sites with all the in-depth material. The content of "Short-range Wireless Communication" represents precisely the type of content I would include on RF Cafe, so that explains why I think highly of the book ;-.
Short range wireless or "MicroRadio" is a field we are all familiar with. Garage door openers and keyless entry systems are common examples, and the field is now moving into the realm of sophisticated two way links, such as Bluetooth. Though many tens of millions of these links are sold per year, I believe this to be the first book devoted to the subject. It provides an excellent overview of the world wide regulations governing operation, which is particularly helpful for the convoluted European rules.
The light mathematical introduction to communications systems and electronics is a good start, though serious practitioners will need more advanced references for detailed product development. This book is clearly written and is a good introduction to the field of wireless communication. I am giving it only 3 stars because the CD-ROM does not work as advertised it boasts an e-book version of the book, which cannot be unlocked because [ If you're not concerned about the CD-ROM you should be in good shape, but consider paying a lower price.
I found this book very useful and I often refer to it for reference purposes, however the mathcad worksheets on the CD were not correct. I obtained the correct updated worksheets and they work great. The worksheets are downloadable from the WEB Site [ One person found this helpful. I too agree that this book is overpriced without a working CD. Does a good job explaining things in simpler terms though. Buyer beware about the cd. One person found this helpful 2 people found this helpful. See all 5 reviews.
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