CONCEPTS IN TRANSMISSION TRANSPORT
ray beam from the transmitting antenna will graze the rounded surface of the horizon can
be calculated using one of the formulas given below:
To the optical horizon
(k = 1):
and to the radio horizon
(k = 4/3):
d = 2.9(2h )
k expresses the bending characteristic of the path.
These formulas should only be used for rough estimates of distance to the horizon
under smooth earth conditions. As we will find out later, the horizon clearance must be
something greater (
n feet or meters) of grazing. The difference between formulas (9.1a)
and (9.1b) and (9.1c) is that formula (9.1a) is "true" line-of-sight and expresses the optical
distance. Here the radio ray beam follows a straight line. Under most circumstances the
microwave ray beam is bent toward the earth because of characteristics of the atmosphere.
This is expressed in formulas (9.1b) and (9.1c), and it assumes the most common bending
characteristic. Figure 9.3 is a model that may be used for formulas (9.1). It also shows
the difference between the optical distance to the horizon and the radio distance to
The design of an LOS microwave link involves five basic steps:
1. Setting performance requirements.
2. Site selection and the preparation of a path profile to determine antenna tower
3. Carrying out a path analysis, often called a link budget. Here is where we dimension
equipment to meet the performance requirements set in step 1.
4. Physically running a path/site survey.
5. Installation of equipment and test of the system prior to cutting it over to carry traffic.
In the following paragraphs we review the first four steps.
Setting Performance Requirements
. As we remember from Chapter 6, the
performance of a digital system is expressed in a bit error rate (bit error ratio) (BER).
In our case here, it will be expressed as a BER with a given time distribution. A time
Radio and optical horizon (smooth earth).