Radio Wave Propagation
Radio Wave : Once a radio signal has been radiated by an antenna, it travels or propagates through space and ultimately reaches the receiving antenna. The energy level of the signal decreases rapidly with distance from the transmitting antenna. The electromagnetic wave is also affected by objects that it encounters along the way such as trees, buildings, and other large structures. In addition, the path that an electromagnetic signal takes to a receiving antenna depends upon many factors, including the frequency of the signal, atmospheric conditions, and time of day. All these factors can be taken into account to predict the propagation of radio waves from a transmitter to receiver.
Optical Characteristics of Radio Waves
Radio waves act very much as light waves do. Light waves can be reflected, refracted, diffracted, and focused on by other objects. The focusing of waves by antennas to make them more concentrated in the desired direction is comparable to a lens focusing light waves into a narrower beam. Understanding the optical nature of radio waves gives a better feel for how they are propagated over long distances.
Light waves are reflected by a mirror. Any conducting surface looks like a mirror to a radio wave, and so radio waves are reflected by any conducting surface they encounter along a propagation path. All metallic objects reflect radio waves, especially if the metallic object is at least one-half wavelength at the frequency of operation. Any metallic object on a transmission path, such as building parts, water towers, automobiles, airplanes, and even power lines, causes some reflection. Reflection is also produced by other partially conductive surfaces, such as the earth and bodies of water.
Radio wave reflection follows the principles of lightwave reflection. That is, the angle of reflection is equal to the angle of incidence, as shown in Fig. 14-37. The radio wave is shown as a wave front. To simplify the drawing, only the electrical lines of force, designated by arrows, are shown. The angle of incidence is the angle between the incoming line of the wave and a perpendicular line to the reflecting surface. The angle of reflection is the angle between the reflected wave and the perpendicular line.
A perfect conductor would cause total reflection: All the wave energy striking the surface would be reflected. Since there are no perfect conductors in the real world, the reflection is never complete. But if the reflecting surface is a good conductor, such as copper or aluminum, and is large enough, most of the wave is reflected. Poorer conductors simply absorb some of the wave energy. In some cases, the wave penetrates the reflecting surface completely.
As Fig. 14-37 shows, the direction of the electric field approaching the reflecting surface is reversed from that leaving the surface. The reflection process reverses the polarity of a wave. This is equivalent to a 180° phase shift.
Refraction is the bending of a wave due to the physical makeup of the medium through which the wave passes. The speed of a radio wave, like the speed of light, is approximately 300,000,000 m/s (186,400 mi/s) in free space, i.e., in a vacuum or air. When light passes through another medium, such as water or glass, it slows down. The slowing down as light enters or exits a different medium causes the light waves to bend.
The same thing happens to a radio wave. As a radio wave travels through free space, it encounters air of different densities, the density depending on the degree of ionization (caused by an overall gain or loss of electrons). This change of air density causes the wave to be bent.
The degree of bending depends on the index of refraction of a medium n, obtained by dividing the speed of a light (or radio) wave in a vacuum and the speed of a light (or radio) wave in the medium that causes the wave to be bent. Since the speed of a wave in a vacuum is almost the same as the speed of a wave in the air, the index of refraction for air is very close to 1. The index of refraction for any other medium will be greater than 1, with how much greater depending upon how much the wave speed is slowed.
Fig. 14-38 shows how a wave is refracted. The incident wave from a transmitter travels through air, where it meets a region of ionized air that causes the speed of propagation to slow. The incident wave has an angle of θ1 to a perpendicular to the boundary line between air and the ionized air. The bent (refracted) wave passes through the ionized air; it now takes a different direction, however, which has an angle of θ2 with respect to the perpendicular.
The relationship between the angles and the indices of refraction is given by a formula known as Snell’s law:
n1 sin θ1 = n2 sin θ2
where n1 = index of refraction of initial medium
n2 =index of refraction of medium into which wave passes
θ1 = angle of incidence
θ2 = angle of refraction
Note that there will also be some reflection from the boundary between the two media because the ionization causes the air to be a partial conductor. However, this reflection is not total; a great deal of the energy passes into the ionized region.
remember that light and radio waves travel in a straight line. If an obstacle appears between a transmitter and receiver, some of the signals are blocked, creating what is known as a shadow zone [Fig. 14-39(a)]. A receiver located in the shadow zone cannot receive a complete signal. However, some signal usually gets through due to the phenomenon of diffraction, the bending of waves around an object. Diffraction is explained by what is known as Huygens’ principle. Huygens’ principle is based on the assumption that all electromagnetic waves, light as well as radio waves, radiate as spherical wavefronts from a source. Each point on a wavefront at any given time can be considered as a point source for additional spherical waves. When the waves encounter an obstacle, they pass around it, above it, and on either side. As the wavefront passes the object, the point sources of waves at the edge of the obstacle create additional spherical waves that penetrate and fill in the shadow zone. This phenomenon, sometimes called knife-edge diffraction, is illustrated in Fig. 14-39(b).
Radio Wave Propagation Through Space
The three basic paths that a radio signal can take through space are the ground wave, the sky wave, and the space wave.
Ground or surface waves leave an antenna and remain close to the earth (see Fig. 14-40). Ground waves actually follow the curvature of the earth and can, therefore, travel at distances beyond the horizon. Ground waves must have vertical polarization to be propagated from an antenna. Horizontally polarized waves are absorbed or shorted by the earth.
Ground wave propagation is strongest at the low- and medium-frequency ranges. That is, ground waves are the main signal path for radio signals in the 30-kHz to the 3-MHz range. The signals can propagate for hundreds and sometimes thousands of miles at these low frequencies. AM broadcast signals are propagated primarily by ground waves during the day and by sky waves at night.
The conductivity of the earth determines how well ground waves are propagated. The better the conductivity, the less the attenuation and the greater the distance the waves can travel. The best propagation of ground waves occurs over saltwater because the water is an excellent conductor. Conductivity is usually lowest in low- moisture areas such as deserts.
At frequencies beyond 3 MHz, the earth begins to attenuate radio signals. Objects on the earth and features of the terrain become the same order of magnitude in size as the wavelength of the signal and thus absorb or adversely affect the signal. For this reason, the ground wave propagation of signals above 3 MHz is insignificant except within several miles of the transmitting antenna.
Skywave signals are radiated by the antenna into the upper atmosphere, where they are bent back to earth. This bending of the signal is caused by refraction in a region of the upper atmosphere known as the ionosphere (see Fig. 14-41). Ultraviolet radiation from the sun causes the upper atmosphere to ionize, i.e., to become electrically charged. The atoms take on or lose electrons, becoming positive or negative ions.
Free electrons are also present. At its lowest point, the ionosphere is approximately 30 mi (50 km) above the earth and extends as far as 250 mi (400 km) from the earth. The ionosphere is generally considered to be divided into three layers, the D layer, the E layer, and the F layer; the F layer is subdivided into the F1 and F2 layers.
The D and E layers, the farthest from the sun, are weakly ionized. They exist only during daylight hours, during which they tend to absorb radio signals in the medium-frequency range from 300 kHz to 3 MHz.
The F1 and F2 layers, the closest to the sun, are the most highly ionized and have the greatest effect on radio signals. The Flayers exist during both day and night. The primary effect of the F layer is to cause refraction of radio signals when they cross the boundaries between layers of the ionosphere with different levels of ionization.
When a radio signal goes into the ionosphere, the different levels of ionization cause the radio waves to be gradually bent. The direction of bending depends on the angle at which the radio wave enters the ionosphere and the different degrees of ionization of the layers, as determined by Snell’s law.
Fig. 14-41 shows the effects of refraction with different angles of radio signals entering the ionosphere. When the angle is large with respect to the earth, the radio signals are bent slightly, pass on through the ionosphere, and are lost in space. Radiation directly vertical from the antenna, or 90° with respect to the earth, passes through the ionosphere. As the angle of radiation decreases from the vertical, some signals continue to pass through the ionosphere. But at some critical angle, which varies with signal frequency, the waves begin to be refracted back to the earth. The smaller the angle with respect to the earth, the more likely it is that the waves will be refracted and sent back to earth. This effect is so pronounced that it actually appears as though the radio wave has been reflected by the ionosphere.
In general, the higher the frequency, the smaller the radiation angle required for refraction to occur. At very high frequencies, essentially those above about 50 MHz, refraction seldom occurs regardless of the angle. VHF, UHF, and microwave signals usually pass through the ionosphere without bending. However, during a period of sunspot activity, or other unusual electromagnetic phenomena, VHF and even UHF waves may be refracted by the ionosphere.
Refl ected radio waves are sent back to earth with minimum signal loss. The result is that the signal is propagated over an extremely long distance. This effect is most pronounced in the 3- to 30-MHz or shortwave range, which permits extremely long-distance communication.
In some cases, the signal reflected back from the ionosphere strikes the earth, is reflected back up to the ionosphere, and is re-reflected back to earth. This phenomenon is known as multiple-skip or multiple-hop transmission. For strong signals and ideal ionospheric conditions, as many as 20 hops are possible. Multiple-hop transmission can extend the communication range by many thousands of miles. The maximum distance of a single hop is about 2000 mi, but with multiple hops, transmissions all the way around the world are possible.
The distance from the transmitting antenna to the point on earth where the first refracted signal strikes the earth to be reflected is referred to as the skip distance (see Fig. 14-41). If a receiver lies in that area between the place where the ground wave is fully attenuated and the point of the first reflection from the earth, no signal will be received. This area is called the skip zone.
The third method of radio signal propagation is by direct waves or space waves. A direct wave travels in a straight line directly from the transmitting antenna to the receiving antenna. Direct wave radio signaling is often referred to as line-of-sight communication. Direct or space waves are not refracted, nor do they follow the curvature of the earth.
Because of their straight-line nature, direct wave signals travel horizontally from the transmitting antenna until they reach the horizon, at which point they are blocked, as shown in Fig. 14-42. If a direct wave signal is to be received beyond the horizon, the receiving must be high enough to intercept it.
Obviously, the practical transmitting distance with direct waves is a function of the height of the transmitting and receiving antennas. The formula for computing the distance between a transmitting antenna and the horizon is
d = √2ht
where ht = height of transmitting antenna, ft d = distance from the transmitter to horizon, mi
This is called the radio horizon.
To find the practical transmission distance D for straight-line wave transmissions, the height of the receiving antenna must be included in the calculations:
where hr = height of receiving antenna, ft. For example, if a transmitting antenna is 350 ft high and the receiving antenna is 25 ft high, the longest practical transmission distance is
Line-of-sight communication is characteristic of most radio signals with a frequency above approximately 30 MHz, particularly VHF, UHF, and microwave signals.
Such signals pass through the ionosphere and are not bent. Transmission distances at those frequencies are extremely limited, and it is obvious why very high transmitting antennas must be used for FM and TV broadcasts. The antennas for transmitters and receivers operating at the very high frequencies are typically located on top of tall buildings or on mountains, which greatly increases the range of transmission and reception.
To extend the communication distance at VHF, UHF, and microwave frequencies, special techniques have been adopted. The most important of these is the use of repeater stations (see Fig. 14-43). A repeater is a combination of a receiver and a transmitter operating on separate frequencies. The receiver picks up a signal from a remote transmitter, amplifies it, and retransmits it (on another frequency) to a remote receiver. Usually, the repeater is located between the transmitting and receiving stations, and therefore it extends the communication distance. Repeaters have extremely sensitive receivers and high-power transmitters, and their antennas are located at high points.
Repeaters are widely used to increase the communication range for mobile and handheld radio units, the antennas for which are naturally not very high off the ground. The limited transmitting and receiving a range of such units can be extended considerably by operating them through a repeater located at some high point.
In high-activity areas, a repeater used for mobile units will become overloaded when too many users try to access it at the same time. When that happens, some users have to wait until free time becomes available, continuing to call until they get through. Such access delays are only a nuisance in some cases but are clearly not acceptable when emergency services are unable to get through.
Although multiple repeaters can be used to ease overcrowding, they are often inadequate because communication activity is not equally distributed among them. This problem is solved by using trunked repeater systems in which two or more repeaters are under the control of a computer system that can transfer a user from an assigned but busy repeater to another, available repeater. Thus the communication load is spread around between several repeaters.
Repeaters can also be used in series, as shown in Fig. 14-44. Each repeater contains a receiver and a transmitter. The original signal is picked up, amplified, and retransmitted on a different frequency to a second repeater, which repeats the process. Typically, such relay stations are located 20 to 60 mi apart, mostly at high elevations to ensure reliable communication over very long distances. Microwave relay stations are used by many telephone companies for long-distance communication.
The “ultimate” repeater is, of course, a communication satellite. Most communication satellites are located in a geostationary orbit 22,500 mi above the equator. Since at that distance it takes exactly 24 h to rotate around the earth, communication satellites appear stationary. They act as fixed repeater stations. Signals sent to a satellite are amplified and retransmitted to earth long distances away. The receiver-transmitter combination within the satellite is known as a transponder. Most satellites have many transponders so that multiple signals can be relayed, making possible worldwide communication at microwave frequencies.
Calculating Received Power
A transmitted signal is radiated at a specific power level. The output power of a transmitter can be accurately determined by calculation or measurement. That power level is increased if the antenna has gain because of improved directivity. As a signal leaves an antenna, it immediately begins to become attenuated. Basically, the degree of attenuation is proportional to the square of the distance between the transmitter and receiver. As discussed previously, other factors also affect attenuation. Ground wave signals are greatly attenuated by objects on the earth, which block the signals and reduce their level at the receiver. In sky wave propagation, the ionospheric conditions and the number of hops determine the signal level at the receiver, with each hop further reducing the signal level. Space wave signals are simply absorbed and attenuated by objects in their path such as trees or walls.
Despite these factors, it is possible to predict the approximate power level at a receiver, and such calculations are quite accurate for the short distances characteristic of direct or space wave transmission.
In analyzing the transmission of radio waves, it is often useful to start with the concept of an isotropic radiator or point source of radio waves. That is, the signal radiates spherically in all directions. The power density at a given distance from an isotropic radiator is predicted by the formula
where Pd = power density of signal, W/m2
d = distance from point source, m
Pt = total transmitted power, W
The distance d is really the radius of an imaginary sphere enclosing the source, and 4πd2 is the area of that sphere at any given distance. However, since practical antennas are not purely isotropic sources, this formula must be modified somewhat. For example, if the transmitting antenna is a dipole, the dipole has a gain of 1.64 (or 2.15 dB) over an isotropic source, so the result must be multiplied by 1.64.
Suppose a transmitter puts a 50-W signal into a dipole antenna. The power density of the signal at a distance of 30 mi (48.3 km, or 48,300 m) is
Pd = 1.64Pt/4πd2 = 1.64(50)/4(3.1416)(48,300)2= 3x 10-9 = 3 nW/m2
Knowing the power density at a given distance is not a particularly useful thing. However, the formula for power at a given distance can be expanded to derive a general formula for computing the actual power value of a signal at a receiving antenna:
Pr = PtGtGrλ2/16π2d2
where λ = signal wavelength, m
d= distance from transmitter, m
Pr, Pt = received and transmitted power, respectively
Gr, Gt= receiver and transmitter antenna gains expressed as a power
ratio and referenced to an isotropic source
If the gains are those in reference to a dipole, each must be converted to a power ratio and multiplied by 1.64 before being used in the formula.
This formula is normally used only for ground wave, direct wave, or space wave calculations. It is not used for sky wave signal predictions because the refraction and reflection that occur make predictions highly inaccurate.
The expansive use of telephone networks has resulted in the integratively designed transmission of telephone and data communication. The antennas are covered with fiberglass material to protect them from the weather. The covering material does not impede the RF signals.
As an example, assume that a transmitter is operating at 150 MHz with a power of 3 W into a one-quarter wavelength vertical antenna. The receiver, which is 20 mi (32.2 km, or 32,200 m) away, has an antenna with a gain of 8 dB. What is the received power?
The wavelength at 150 MHz is λ = 300/f = 300/150 = 2 m. The gain of the quarter-wave vertical transmitting antenna is the same as that for a dipole. With a dipole gain of 1, we must multiply this by 1.64 to get the gain over an isotropic source.
The gain of the receiving antenna is 8 dB. The gain is usually expressed as the gain over a dipole. To convert to gain with respect to an isotropic source, we add 2.15 dB. This is the same as multiplying the power ratio represented by the gain in decibels by 1.64. The result is 8 +2.15 = 10.15 dB. This must now be converted to an actual power ratio. Since dB = 10 log (Pout /Pin), where Pin and Pout are the input and output power of the antenna, respectively,
If the receiver antenna, transmission line, and front-end input impedance are 50 Ω, we can calculate the input voltage, given this input power. Since P =V2/R, V = √PR. Substituting into V = √PR gives
This is a relatively strong signal; most good narrowband receivers can generate full intelligible output with 1 μV or less.
Path Attenuation. Another way to predict received power is to estimate the total power attenuation over a transmission path. This attenuation in decibels is given by
dB loss = 37 dB + 20 log f + 20 log d
where f = frequency of operation, MHz
d = distance traveled, mi
The distance can also be given in kilometers, in which case the 37-dB figure must be changed to 32.4 dB. Isotropic antennas are assumed.
The attenuation over a 20-mi path at a frequency of 150 MHz is
dB loss = 37 + 20 log 150 + 20 log 20 = 37 + 43.52 +26 = 106.52
The dB loss formula, then, tells us that for every doubling of the distance between transmitter and receiver, the attenuation increases about 6 dB.
Common Propagation Problems
Although radio waves pass right through most objects on their way from transmitter to receiver, they are negatively influenced by these objects. The result is a common problem called fading. Good design of a communication system can minimize but not completely eliminate fading. One way to overcome fading is to use a diversity system.
One of the primary effects of radio wave propagation is called fading. Fading is the variation in signal amplitude at the receiver caused by the characteristics of the signal path and changes in it. Fading causes the received signal to vary in amplitude, typically making it smaller. Under some conditions, the received signal may actually be larger than a direct path signal depending upon the specific communication situation. Fading is caused by four factors: variation in distance between transmitter and receiver, changes in the environmental characteristics of the signal path, the presence of multiple signal paths, and relative motion between the transmitter and receiver.
As a receiver gets farther away from a transmitter, the signal gets weaker just because the path length is increasing. If the receiver moves closer to the transmitter, the signal strength increases. Both types of situations occur when one or perhaps both of the transceivers are moving with respect to the other. It is especially noticeable in airplanes and in cars. This type of fading is generally gradual and does not result in severe or rapid swings in signal amplitude.
Fading is also caused by objects coming between the transmitter and receiver. Known as shadow fading, this occurs if a vehicle containing a transceiver moves in such a way that a large building or a mountain comes between it and a base station transceiver. The obstacle causes the signal to be attenuated, resulting in fading. When a car enters a tunnel, the signal may be greatly attenuated so that fading occurs. Even the movement of a rainstorm or snowstorm between transmitter and receiver can cause fading. Weather-related effects are especially pronounced at the higher microwave frequencies, where the signal wavelengths are in the same size range as the raindrops or snowflakes that produce massive signal scattering by reflection.
One of the worst sources of fading is multipath interference. Sometimes called Rayleigh fading, this type of fading occurs when a transmitted signal takes multiple paths to the receiver because of reflections. The term Rayleigh refers to a particular type of statistical response curve that mathematically describes the variation of the received signal. As you saw earlier, radio signals are easily reflected by conducting objects. The signal is usually radiated by a nondirectional antenna over a wide horizontal range in such a way that it will strike the receiver antenna directly by way of the direct line-of-sight space wave, but it may also strike many obstacles along the way. Buildings, water towers, hills and mountains, and even moving vehicles all have reflected surfaces that will direct a signal along a separate path to the receiving antenna. The signal may also be reflected from the ground or water. The result is that multiple signals reach the receiver antenna at different times.
Reflected signals take a longer path than a direct signal, so they are delayed and arrive at the antenna later than the direct signal. This time delay is seen as a phase shift whose magnitude is a function of the total signal path distance and the wavelength (frequency) of the signal. Keep in mind that as you saw earlier in the chapter, reflections produce a 180° phase shift that worsens the problem. The receiver sees a composite of all the received signals. The phase shifts are usually such that they cancel the direct path signal, resulting in an overall weaker signal. But the delays could also be such that the reflected signals arrive in phase with the direct signal, thereby causing the signals to add in-phase and actually produce a higher-level signal.
Another type of fading is caused by the movement of either the transmitter or the receiver. When the transmitter is in a car, plane, or another vehicle, rapid movement toward or away from the receiver introduces a signal frequency change called a Doppler shift (to be described ). A movement that causes the transmitter and receiver to get closer to each other causes the signal frequency to increase. A movement that increases the distance causes a frequency decrease. Large signal-frequency changes produce lower-level signals because the signals are partially out of the passband of the receiver’s selective filters. In digital systems that predominantly use some form of phase-shift modulation, the Doppler shift confuses the demodulator and produces bit errors.
Example 14.5 A 275-ft-high transmitting antenna has a gain of 12 dB over a dipole. The receiving
antenna, which is 60 ft high, has a gain of 3 dB. The transmitter power is 100 W at 224 MHz. Calculate (a) the maximum transmitting distance and (b) the received power at the distance calculated in part (a). (There is 1.61 km/mi.)
In most cases, several types of fading occur simultaneously. Multipath fading and shadow fading are the worst offenders. If you have ever used a cell phone from a moving car in a changing environment, you know that fading can cause significant signal variations, including no service at all. Using a cell phone or radio in a large city with many tall buildings produces extreme multipath interference and shadow fading. Using cell phones or other wireless equipment inside a building essentially does the same.
When digital communication is involved, multipath fading can cause intersymbol interference. If high-speed data transmission is used, the symbols are short, and multipath delays may be of the same order of magnitude. A symbol received directly may be different from a symbol received from a reflected source. The result is severe bit errors.
Although fading can occur on signals of any frequency, it is most pronounced in UHF and microwave communication, where the signal wavelengths are very short compared to the path distances and size of refl ective surfaces. Fading is a signifi cant problem with cell phones and other radio equipment especially when one or more of the transceivers are in motion. Fading is also a problem in long-distance shortwave communication when the signal can take several bounces off the ionosphere and the earth to produce canceling or reinforcing signals. Fading signal variations can be only a few decibels or as much as 20 to 30 dB. A signifi cant amount of fading can make communication unreliable.
To overcome fading, most communication systems have a built-in fading margin. That is, they have a high enough transmitter power and sufficient receiver sensitivity to ensure that the weaker reflective signals do not degrade the direct signal as much. A fade margin of at least 5 dB is built into most systems.
Multipath fading can also be greatly minimized by using highly directive antennas, either at the transmitter or at the receiver or at both. Narrow transmit and receive beams virtually eliminate multiple paths and the related fading. However, in most communication systems, nondirectional (i.e., omnidirectional) antennas are the norm. Cell phones, two-way radios, and even base stations must have broad azimuth coverage to receive or transmit signals over a wide area.
Broadband signals are much less sensitive to multipath fading than narrowband signals are. If broadband methods such as spread spectrum (CDMA) or OFDM are used, the signals are spread over a very wide frequency range. Multiple reflections of signals over a wide range of frequencies are received in such a way that less cancellation or intersymbol interference occurs. This is a major factor in designing new communication systems where fading is expected.
Fading can also be minimized by using what is called a diversity system. A diversity system uses multiple transmitters, receivers, or antennas to mitigate the problems caused by multipath signals. Two common types of diversity are frequency and spatial. With frequency diversity, two separate sets of transmitters and receivers operating on different frequencies are used to transmit the same information simultaneously. The theory is that signals on different frequencies will react differently to the various fading mechanisms, thereby resulting in the least one reliably received signal. To be effective, the frequencies should be widely spaced from one another. Of course, such systems are very expensive since they require two transmitters, receivers, and antennas, all on different frequencies. The scarcity of frequency spectrum also makes this type of system impractical. It is rarely used except in cases where extreme reliability is a must.
Another more widely used form of diversity is called space or spatial diversity. It uses two receiver antennas spaced as far apart as possible to receive the signals. Diversity systems are used mainly at base stations rather than in portable or handheld units. The basic idea is that antennas at slightly different locations will receive different variations of the signals, with one being better than another. The spacing may be horizontal or vertical, whichever is the more convenient. However, in some cases, one arrangement will be superior to the other.
Diversity reception is particularly difficult at shortwave frequencies, where the spacing will typically be many hundreds or even thousands of feet. Only horizontal spacing is used. At UHF and microwave frequencies, wide antenna spacing is relatively simple because the wavelengths are short. In general, the wider the spacing (10 or 20 wavelengths or more), the better. Many systems use the relationship h/d = 11 to determine a minimum and, as it turns out, optimum spacing for antennas. In this relationship, h is the height of the antenna and d is the spacing distance. For antennas that are 100 ft high, the minimum spacing would be
d = 100/11 = 9.09 ft
Fig. 14-45(a) shows a typical spatial diversity system. The two antennas feed a combiner network, where the two signals are linearly added. The result is a larger signal and minimized fading effects. The signals may be combined at the antenna or almost anywhere in the receiver. Some systems combine after the LNAs. Others combine after the IF. In still other systems, two completely separate receivers are used and the signals are combined after demodulation. Experimentation is required to determine the best results.
Another form of spatial diversity is selective or switched diversity. In this system, shown in Fig. 14-45(b), the two antennas feed separate LNAs whose outputs are monitored by circuits that measure received signal strength. In cell phone systems, these circuits are called received signal strength indicators (RSSIs). They determine the signal with the greatest strength and switch that signal to the remainder of the receiver circuits. All this is done automatically at high speed, ensuring that the receiver always has the strongest signal.
Diversity systems are widely used in the newer cell phone systems and in wireless LANs that work indoors and, in some cases, with mobile wireless units (laptop computers, PDAs, etc.) that are frequently in motion. New techniques such as multiple-input, multiple-output (MIMO), and smart (adaptive) antennas are now being used to further improve transmission in multipath environments. These techniques will be covered.
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