Line Of Sight LOS Computer Science Essay

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System planning for a communication systems engineer is critical to the successful installation, operation and proper performance of any communication system. Wireless systems are no exception, and this is especially true for line-of-sight (microwave) wireless systems. Knowing that your proposed microwave link will be operating over a very long path, a systems engineer should be able to confirm whether a visible line-of-sight path exists between the two fixed antenna sites. This is only the first-step process, and is often accomplished by using a combination of strobe lights, mirrors (which reflect the sun), binoculars and spotting scopes. It should be noted that, being able to see one site from the other will not guarantee that the perceptible path is appropriate for a microwave signal, but at least you will know that the possibility of such a path exists. In many occasions there are obstacles which the systems engineer must prevail over such as buildings, trees, small hills and elevated roads which cause the transmitted signal to fade or be totally blocked. In most cases the engineer would need to use a very sophisticated equipment to confirm that a line-of-sight exists between the two antennas. (Allen, 1984, p. 26).

1.1 Line-of-Sight (LOS):

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As the term line-of-sight implies, this type of radio link communication system was invented based on the simple idea that two fixed terrestrial antennas (separated by a given distance "d") will only communicate when they see each other. Generally when we talk about the line-of-sight for a fixed microwave radio link operation, we are simply referring to the electromagnetic wave that is being generated and also propagated between the two antennas.

At low frequencies, these electromagnetic wave travel as ground waves. These waves are guided by the earth's curvature as a result of diffraction within the layers of the atmosphere which is above the earth's surface. It should be noted that due to the multiple diffraction of these electromagnetic signals by the earth's surface, would give rise to a time variation (or delay) of the signal, also known as quasi-curved path delay. (Feher, 1981, p. 173).

However, at higher frequencies and in lower levels of the atmosphere, neither of these effects applies. Thus any obstruction between the transmitting antenna and the receiving antenna will block the signal, just like the light that the eye may sense. Therefore, since the ability to visually see a transmitting antenna (disregarding the limitations of the eye's resolution) roughly corresponds to the ability to receive a radio signal from it, the transmission characteristic of high-frequency radio is called "line-of-sight". And the furthest possible point of propagation is referred to as the "radio horizon". In practice, (Feher, 1981, p. 176) the transmission characteristics of these high radio waves vary substantially depending on the exact frequency and the strength of the transmitted signal (which is a function of both the transmitter and the antenna characteristics). Unlike low frequency signals which easily propagates through buildings and forests.

Two antennas are shown below, each having the same height. The line-of-sight transmission means the transmitting and receiving antennae can "see" each other as shown. The maximum distance at which they can see each other will be given as dLOS, which occurs when the sighting line just grazes the earth's surface.

At the usual radio frequencies, propagating electromagnetic energy does not follow the earth's surface. Line-of-sight communication has the transmitter and receiver antennas in visual contact with each other. Assuming both antennas have height "h" above the earth's surface then the maximum line-of-sight distance is dLOS, would be given by the equation below;

Equation 1 [3]

Where R is the earth's radius (6.38Ã-106m)

Figure 1: Line-of-Sight Between Two Fixed Terrestrial Antennas [4]

1.2 Applications of Line-of-Sight:

There are quite a good number of applications that use the basic concept of fixed microwave line-of-sight as its mode of operation. I have been able to list a few of these applications as related to modern communication systems. The line-of-sight of a microwave radio link can be used in;

Base-Station (cell-site) communication; This involves the relay of information by the transmitting antenna trough the aid of repeater base-station to the receiving antenna.

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Military applications; In military situations transportable terminals are used to link a central command location, to a forward position.

Public communication; Towns and cities are linked together, and can transfer information directly with each other using the already exiting fixed terrestrial radio link.

Industrial applications; A central plant may be linked to Remote Terminal Unit (RTUs) at remote mines or bore sites, Hydro-graphic monitoring and control. (Allen, 1984, p. 38)

2. Fading in Fixed Terrestrial Radio Link:

Fading is defined as, "the variation with time of the intensity or relative phase, or both, of any of the frequency components of a received radio signal due to changes in the characteristics of the propagation path with time". (Dougherty, 1968, p. 70).

From the definition above, I can then define fading as the difference or increase in attenuation that the information signal experiences when being transmitted over a wireless communication channel. This is as a result of many factors, some of which are random processes but then most of them are predictable. I will expand on these processes later during the course of this work.

At this point, we should note that for a fixed radio channel (Dougherty, 1968, p. 84) the "Bit Error Rate" (BER) decreases quickly as the signal-to-noise ratio is increased, therefore resulting to a fixed slope. If the BER is plotted on a log scale against the signal-to-noise ratio, then diversity or error correction can help to make the slope steeper, hence improve performance of the fixed radio channel.

2.1 Effects of Fading on Signal Propagation:

Multipath propagation basics

Multipath radio signal propagation is a phenomenon that occurs on all terrestrial radio links. The radio signals not only travel by the direct line of sight path, but as the transmitted signal does not leave the transmitting antenna in only the direction of the receiver, but over a range of angles even when a directive antenna is used. As a result, the transmitted signals spread out from the transmitter and they will reach other objects: hills, buildings, reflective surfaces such as the ground, water, etc. It only natural the signals may reflect off a variety of surfaces and reach the receiving antenna via paths other than the direct line of sight path (Greenstein & Czekaj, 1980, p. 94). This causes a delay in time of the reflected transmitted signal, thereby causing the phenomena of multipath fading.

Multipath fading

Signals are received in a terrestrial environment, i.e. where reflections are present and signals arrive at the receiver from the transmitter via a variety of paths. The overall signal received is the sum of all the signals appearing at the antenna. Sometimes these will be in phase with the main signal and will add to it, increasing its strength. At other times they will interfere with each other. This will result in the overall signal strength being reduced. (Greenstein & Czekaj, 1980, p. 89-112).

At times there will be changes in the relative path lengths. This could result from either the transmitter or receiver moving, or any of the objects that provides a reflective surface moving. This will result in the phases of the signals arriving at the receiver changing, and in turn this will result in the signal strength varying. It is this that causes the fading that is present on many signals.

It can also be found that the interference may be nonselective (flat), i.e. applied to all frequencies equally across a given channel, or it may be selective, i.e. applying to more to some frequencies across a channel than others.

Interference caused by multipath propagation

Multipath propagation can give rise to interference that can reduce the signal to noise ratio and reduce bit error rates for digital signals. One cause of a degradation of the signal quality is the multipath fading already described (Medard & Gallager, 2002, p.5). However there are other ways in which multipath propagation can degrade the signal and affect its integrity.

This arises from the fact that the signal is frequency modulated and at any given time, the frequency of the received signal provides the instantaneous voltage for the audio output. If multipath propagation occurs, then two or more signals will appear at the receiver. One is the direct or line of sight signal, and another is a reflected signal. As these will arrive at different times because of the different path lengths, they will have different frequencies, caused by the fact that the two signals have been transmitted by the transmitter at slightly different times. Accordingly when the two signals are received together, distortion can arise if they have similar signal strength levels.

2.2 Types of Fading Mechanism:

I) Selective fading:  

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Selective fading occurs when the multipath fading affects different frequencies across the channel to different degrees. It will mean that the phases and amplitudes of the signal will vary across the channel. Sometimes relatively deep signal fading may be experienced, and this can give rise to some reception problems. If we decide to simply maintain the overall amplitude of the received signal, we will not still be able to overcome the effects of selective fading, and some form of equalization may be needed. Some digital signal formats, e.g. OFDM are able to spread the data over a wide channel so that only a portion of the data is lost by any nulls. This can be reconstituted using forward error correction techniques and in this way it can mitigate the effects of selective multipath fading (Emsshwiller, 1991, p.5).

Selective multipath fading occurs because even though the path length will be change by the same physical length (e.g. the same number of meters, yards, miles, etc) this represents a different proportion of a wavelength. Accordingly the phase will change across the bandwidth used.

Selective fading can occur over many frequencies. It can often be noticed when medium wave broadcast stations are received in the evening via ground wave and sky wave. The phases of the signals received via the two means of propagation change with time and this causes the overall received signal to change (Rana, Webster & Sylvain, 1993, p.11). It should be noted that selective multipath fading are also experienced at higher frequencies signals and can be resolved by employing other modern communication schemes.

II) Non-selective fading:  

Flat fading:

By the term "flat fading", I simply mean the type of fading in a communications channel that attenuates or fades all frequencies in the channel in the same amount. This form of multipath fading (Freeman, 1981, p. 191) affects all the frequencies across a given channel either equally or almost equally. When flat multipath fading is experienced, the signal will just change in amplitude, rising and falling over a period of time, or with movement from one position to another.

Fast fading:

In a fast-fading channel, the transmitting antenna may take advantage of the variations in the channel conditions using time diversity to help increase robustness of the communication to a temporary deep fade. Although a deep fade may temporarily erase some of the information transmitted, use of an error-correcting code together with a successfully transmitted bit during other time instances can allow for the erased bits to be recovered. While in a slow-fading channel, it is not possible to use time diversity because the transmitter sees only a single realization of the channel within its delay limitation. A deep fade (David & Pramod, 2005, p. 18) therefore lasts the entire duration of transmission and cannot be mitigated using coding.

Type of Physical Phenomenon:

Rain Fading

Rain fading is the most dominant factor. Attenuation occurs due to absorption and scattering in rain. Fading due to rain attenuation is described empirically from link tests and point rainfall data. Location variation (Crane, 1996, p. 49) is based on selected point rainfall data and radar reflectivity data accumulated around the world. Because raindrops are oblate rather than spherical, attenuation tends to be greater for horizontally polarized signals than for vertical polarized signals.

Clear Air Fading Mechanism

Clear air fading can occur in three different situations.

The refractive index gradient of the atmosphere near the earth's surface (0 - 100 meters) varies over time and can cause decoupling of the beam.

ii) If the antenna beam might be too narrow, the link can be lost when this condition is severe. If the antenna beam is too wide, surface multipath can occur.

iii) Another effect is scintillation fading due to turbulent irregularities in the atmosphere.

2.3 Diversity Scheme:

This refers to a method for increasing the reliability of a transmitted signal by using two or more communication links with different characteristics. Diversity (Jakes, 1974, p. 67) in wireless communication is important for fighting fading and co-channel interference while avoiding error bursts. Below are several types of diversity scheme;

Frequency diversity: The signal is transmitted along the communication link using several frequency channels or spread over a wide spectrum which is affected by frequency-selective fading.

Polarization diversity: A multiple level of signals are transmitted and received by means of antennas with different polarization.

Time diversity: This caused when a series of the same signal are transmitted at different time instants.

Space diversity: The signal is transmitted over numerous but diverse propagation paths.

2.4 Techniques Used to Mitigate Fading:

Microwave links are subjected to momentary deteriorations of theirs conditions of propagation in the form of periodic and selective fading effects, distortions of the group delay, field enhancements or polarisation decoupling phenomena. This (Alamouti, 1998, p. 1453) results in the appearance of errors in the transmitted numerical data and may even leads to the total cut-off of the transmissions if a given error rate threshold has been exceeded. One way of overcoming this is to transmit the data at a rate the signal is sampled, only when all the reflections have arrived and the data is stable. This naturally limits the rate at which data can be transmitted, but ensures that data is not corrupted and the bit error rate is minimized.

Using the latest signal processing techniques, a variety of methods can be used to overcome the problems with multipath propagation and the possibilities of interference.

Figure 2: How intersymbol interference can be avoided [13]

OFDM: (Orthogonal Frequency-Division Multiplexing)

In order to meet the requirements to transmit large amounts of data over a radio channel, it is required that a communication engineer choose the most appropriate form of signal bearer format. One form of signal lends itself to radio data transmissions in an environment where reflections may be present is Orthogonal Frequency Division Multiplex, OFDM. An OFDM signal comprises a large number of carriers, each of which are modulated with a low bit rate data stream. In this way (Robertson & Kaiser, 1999, p.8) the two contracting requirements for high data rate transmission, to meet the capacity requirements, and low bit rate to meet the inter-symbol interference requirements can be met.

Orthogonal Frequency Division Multiplex (OFDM) is a form of transmission that uses a large number of close spaced carriers that are modulated with low data rate. More often than not, these signals would be expected to interfere with each other, but by making the signals orthogonal to each another there is no reciprocal interference. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period. This means that when the signals are demodulated they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution. The data to (Brodhage & Hormuth, 1978, p. 87) be transmitted is split across all the carriers and this means that by using error correction techniques, if some of the carriers are lost due to multi-path effects, then the data can be reconstructed. Additionally having data carried at a low rate across all the carriers means that the effects of reflections and inter-symbol interference can be overcome. It also means that single frequency networks, where all transmitters can transmit on the same channel can be implemented.

OFDM is the modulation format that is used for many of today's data transmission formats. The fact that OFDM is being widely used demonstrates that it is an ideal format to overcome multipath propagation problems.

MIMO: (Multiple-Input and Multiple-Output)

While multipath propagation creates interference for many radio communications systems, it can also be sued to advantage to provide additional capacity on a given channel. Using a scheme known as MIMO, multiple input multiple outputs; it is possible to multiple the data capacity of a given channel several times by using the multipath propagation that exists.

Two major limitations in communications channels can be multipath interference, and the data throughput limitations as a result of Shannon's Law. MIMO (Tarokh & Jafarkhani, 2000, p. 1171) provides a way of utilizing the multiple signal paths that exist between a transmitter and receiver to significantly improve the data throughput available on a given channel with its defined bandwidth. By using multiple antennas at the transmitter and receiver along with some complex digital signal processing, MIMO technology enables the system to set up multiple data streams on the same channel, thereby increasing the data capacity of a channel.

In view of the advantages that MIMO offers, (Claude & Bruno, 2007, p. 448) many current wireless and radio communications schemes are using it to make far more efficient use of the available spectrum. The disadvantage to MIMO is that it requires the use of multiple antennas, and with modern portable equipment such as cell phones being increasingly small, it can be difficult to place tow sufficiently spaced antennas onto them.

Figure 3: MIMO Channel Scheme [17]

C SPACE DIVERSITY (SPACE-TIME CODING)

A space-time code (STC) is a method employed to improve the reliability of data transmission in wireless communication systems using multiple transmit antennas. STC's (Vahid, Nambi & Calderbank, 1998, p. 748) rely on transmitting multiple, redundant copies of a data stream to the receiver in the hope that at least some of them may survive the physical path between transmission and reception in a good enough state to allow reliable decoding. In coherent STC, the receiver knows the channel impairments through training or some other form of estimation. These codes have been studied more widely because they are less complex than other non-coherent schemes. In non-coherent STC the receiver does not know the channel impairments but knows the statistics of the channel. In differential space-time codes neither the channel nor the statistics of the channel are available.

Fig4: Space-Time Code (STC) Scheme [18]

3. Types of ITU-R Models:

Fading channel models are often used to model the effects of electromagnetic transmission of information over the air in cellular networks and broadcast communication. Mathematically, fading (Crane, 1996, p.205-217) is usually modeled as a time-varying random change in the amplitude and phase of the transmitted signal.

A prediction method which takes into account atmospheric multipath, associated with abnormal refractive layers, beam spreading (defocusing), antenna decoupling and surface multipath. These phenomena are the result of abnormal atmospheric conditions for few tens of seconds and can cause deep fade in the received signal level (fast fading).

3.2: NBS-101 [2],

A prediction method which takes into account topographical and climatic conditions - temperature, barometric pressure and water vapour pressure. These parameters slowly change during day time and cause slow changes in the received signal level (slow fading)

4. Conclusion: