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Radio_Propagation

VIEWS: 4 PAGES: 6

  • pg 1
									AN INTRODUCTION TO WIRELESS COMMUNICATIONS
BASIC RADIO PROPAGATION:

The most basic model of radio wave propagation involves so called "free space"
radio wave propagation. In this model, radio waves emanate from a point source of
radio energy, traveling in all directions in a straight line, filling the entire spherical
volume of space with radio energy that varies in strength with a 1/(range)^2 rule.
So, if you double the distance that you transmit, you will reduce the received power
by a factor of 4. Given below is a table of values that show the amount of power you
will receive when you transmit 1 Watt of power over the air.

d (transmission range)                        Average Power Received (wireless)
1 meter                                       0.00002 Watts
5 meters                                      0.0000013 Watts
10 meters                                     0.0000004 Watts
1000 meters                                   0.000000000158 Watts

To provide a comparison, if you were to transmit 1 Watt of power over a fiber optic
cable that is 1000 meters long, you would receive, on average, 0.933 Watts of power!

Real world radio propagation rarely follows a model quite as simple as above. The
three basic mechanisms of radio propagation are attributed to reflection, diffraction
and scattering. All three of these phenomenon cause radio signal distortions and
give rise to signal fades, as well as additional signal propagation losses. Below we
describe each of these factors and give some reasoning as to why each makes
wireless communication so difficult.

In the real world, wireless communication takes place between two antennas, which
may or may not be able to see each other. In the case that the two communicating
antennas can see each other, we have the situation given in the table above.
However, when there is no “line-of-sight” between the antennas, the signal can still
make it from one antenna to the other (after all, you can’t always see a cell tower
when you are placing a phone call). The reason for this is because you transmit a
signal in all directions since you do not know where the antenna is that you are
attempting to contact. In its simplest form, when a signal hits an object (the ground,
a wall, another person, etc.), two things happen. Part of the energy of the signal is
absorbed by that object (goes into the object), and part of it is reflected away from
that object (bounces off the object). As shown below there may actually be several
paths that exist between a transmit and a receive antenna (this phenomena is called
multi-path).




                                                                                        1
                                           Path 2

                                Path 1



            Antenna 1                                          Antenna 2
                                      Path 3




A brief description of each of the 3 paths shown in the figure above is given below:

      Path 1: Is a line-of-sight path. The amount of power we receive at antenna 2
       depends only on the distance between the antennas, a similar situation as
       given in the table on the previous page.
      Path 2: Is a path that is the result of a reflection off an object. Here we will
       lose power over the length of the path from antenna 1 to the object, and over
       the length of the path from the object to antenna 2. It is very important to
       note that, in this case, we are not propagating over the distance between
       antennas 1 and 2, but over the total length of Path 2. In addition, the object
       will also absorb some of the energy of the signal before it is reflected. The
       amount that it absorbs depends on the material of that object.
      Path 3: Similar to Path 2, we have the signal reflecting off an object on its
       way from antenna 1 to antenna 2. Notice also that it passes through another
       object on its way from the reflection to antenna 2 (remember that radio
       waves can pass through objects). The amount of power that is reduced as it
       passes through the object depends on the material of that object.

For all practical considerations, we will receive so much power from Path 1, since it
did not reflect or pass through any object, and because it was the shortest of the 3
paths, that we can ignore Paths 2 and 3. However, in many situations, we do not
have direct line-of-sight between the antennas and communication between them
can only be made possible through indirect paths like 2 and 3.

From the discussion above, it is a logical question to ask what does the signal look
like when it reaches antenna 2. The answer to this is that antenna 2 sees a signal that



                                                                                       2
is the sum of the 3 received signals. Sometimes this is a good thing and sometimes
this is a bad thing, as is shown by the example below.

Suppose we transmit a signal that looks like the following:


                1.5

                  1

                0.5
   Amplitu de




                  0
                       0   2   4   6     8         10      12        14   16        18        20
                -0.5

                 -1

                -1.5
                                               tim e (s)




And the signals received due to each of the 3 paths look like:


                0.8
                0.6
                0.4
   Amplitu de




                0.2                                                                      Path 1
                  0                                                                      Path 2
                -0.2 0         5          10                    15             20        Path 3
                -0.4
                -0.6
                -0.8
                                       tim e (s)




Then, adding the 3 signals gives the signal that antenna 2 sees. The plot below is this
resultant signal.




                                                                                                  3
Notice here that we transmitted a signal with amplitude 1, and each of the 3 received
signals, individually, have an amplitude less than 1. However, when the 3 are added,
we receive a signal that has amplitude greater than 1. In this instance, the fact that 3
paths are present helps to increase the received amplitude over any of the 3
individual paths. However, what if, instead of receiving the 3 signals as shown
above, we received the following 3 signals.


                0.8
                0.6
                0.4
   Amplitude




                0.2                                                                         Path 1
                  0                                                                         Path 2
                -0.2 0           5           10                    15             20        Path 3
                -0.4
                -0.6
                -0.8
                                         tim e (s)




Then, adding the 3 signals gives the following:

                  1
                0.8
                0.6
                0.4
   Amplitu de




                0.2
                   0
                -0.2 0   2   4       6   8           10       12        14   16        18     20
                -0.4
                -0.6
                -0.8
                  -1
                                                  tim e (s)




                                                                                                     4
In this case the addition of the 3 signals has a smaller amplitude than any of the 3
individual signals. This is widely referred to as a deep fade in terminology used by
wireless engineers.

So, what situation can we expect, the first case in which multi-path helped us by
increasing our received signal strength, or the second case where multi-path acted to
decrease our received signal strength? The answer depends on how many paths exist
between the transmitter and receiver and how long each path is. In real systems,
thousands of paths exist, each with a different length. Here, moving even a few
centimeters can move us from one situation to another. This is the reason that, when
we use our cellular telephones, we can get great reception at a given point, but move
1 foot away from that position and we lose our call.

OTHER FACTORS IN RADIO PROPAGATION:

In the previous section it was stated that when a radio wave hits an object, part of it
will reflect and part of it will pass into the object. However, the situation is actually
quite a bit more complex than this. Below I describe two other phenomena, known
as scattering and diffraction, which also affects how radio signals travel:

Scattering occurs when the medium through which the wave travels consists of
objects that are extremely small compared to the wavelength of the wave. Scattered
waves may be produced by rough surfaces, small objects, or by other irregularities
in the medium. In this phenomenon, the wave is not reflected in a signal path as
shown previously, but is scattered in many different directions. In practice, foliage,
street signs, and lampposts induce scattering in a mobile communications system.

Diffraction occurs when the radio path between the transmitter and receiver is
obstructed by a surface that has sharp irregularities (edges), such as the corners of
building or other objects. In this situation, the wave can actually bend around these
corners.

WHAT ELSE MAKES MOBILE COMMUNICATIONS DIFFICULT?

We have already discussed the fact that we lose power extremely quickly when
communicating over a wireless medium, even over short distances. Thus, in order to
communicate over large distances, we need to increase the amount of power that we
are transmitting so that the receiver may collect enough power to understand what
it is that we are sending. However, there are 3 major reasons why this is not
possible.
    1. Most wireless communication devices are mobile devices, meaning that they
        are designed to move around and not be fixed to a certain location (examples
        include cellular telephones, laptops with wireless internet services, PDAs,
        beepers, and many others). Because they are mobile devices, they are
        powered by batteries, and increasing the transmission power means



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      decreasing the life of the batteries. We could communicate much farther with
      wireless devices provided we were willing to accept the fact that our cell
      phone batteries would only last 10 minutes before needing to be recharged.
   2. Even if no one cared about battery life, we still could not use too much
      transmit power. The Federal Communications Commission (FCC) is a
      government agency that regulates how we communicate with each other. The
      FCC has laws that will only allow certain amounts of power to be used when
      communicating with wireless devices. This is both for the protection of
      people (too much radiating can be a health hazard) and so that others can
      communicate without being interfered with.
   3. The radio frequency spectrum is a scarce natural resource since we only have
      a certain amount of it that we can use. For this reason, we must all share it
      and, therefore, we can interfere with each other when we are using the same
      portion of it. So, when we are listening for data that we are trying to receive,
      we are also hearing data from other users using the same band as us, which
      can make things difficult to understand. It is similar to carrying on a
      conversation in a room in which many other conversations are taking place.
      The more conversations that are happening, and the louder the conversations
      are, the more difficult it is to understand our own conversation.


WHY WIRELESS?

With all of the difficulties that I have outlined previously in using wireless
communication, why bother even using it? We can communicate across continents
very easily using fiber optics or copper cables, but have difficulty communicating
over several miles using wireless. The answer, as simple as it is, is due to the
convenience that the lack of wires affords us. Being able to carry around a cellular
phone with us or using a mobile internet on our laptop is very convenient and,
therefore, very desirable. So, it is the job of wireless engineers to develop solutions
for these difficulties and make wireless communication possible.




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