LINE-OF-SIGHT TRANSMISSION
With any communications system, the signal that is received will differ from the signal
that is transmitted, due to various transmission impairments. For analog signals,
these impairments introduce various random modifications that degrade the signal
quality. For digital data, bit errors are introduced: A binary 1 is transformed into a
binary 0, and vice versa. In this section we examine the various impairments and
comment on their effect on the information-carrying capacity of a communications
link. Our concern in this book is with LOS wireless transmission, and in this context,
the most significant impairments are
• Attenuation and attenuation distortion
• Free space loss
• Noise
• Atmospheric absorption
• Multipath
• Refraction
Attenuation
The strength of a signal falls off with distance over any transmission medium. For
guided media, this reduction in strength, or attenuation, is generally exponential and
thus is typically expressed as a constant number of decibels per unit distance. For
unguided media, attenuation is a more complex function of distance and the makeup of
the atmosphere. Attenuation introduces three factors for the transmission engineer.
1. A received signal must have sufficient strength so that the electronic circuitry
in the receiver can detect and interpret the signal.
2. The signal must maintain a level sufficiently higher than noise to be received
without error.
3. Attenuation is greater at higher frequencies, causing distortion.
The first and second factors are dealt with by attention to signal strength and
the use of amplifiers or repeaters. For a point-to-point transmission (one transmitter
and one receiver), the signal strength of the transmitter must be strong enough to be
received intelligibly, but not so strong as to overload the circuitry of the transmitter
or receiver, which would cause distortion. Beyond a certain distance, the attenuation
becomes unacceptably great, and repeaters or amplifiers are used to boost the signal
at regular intervals. These problems are more complex when there are multiple
receivers, where the distance from transmitter to receiver is variable.
The third factor is known as attenuation distortion. Because the attenuation varies
as a function of frequency, the received signal is distorted, reducing intelligibility. Specifically,
the frequency components of the received signal have different relative strengths
than the frequency components of the transmitted signal.To overcome this problem, techniques
are available for equalizing attenuation across a band of frequencies. One
approach is to use amplifiers that amplify high frequencies more than lower frequencies.
With any communications system, the signal that is received will differ from the signal
that is transmitted, due to various transmission impairments. For analog signals,
these impairments introduce various random modifications that degrade the signal
quality. For digital data, bit errors are introduced: A binary 1 is transformed into a
binary 0, and vice versa. In this section we examine the various impairments and
comment on their effect on the information-carrying capacity of a communications
link. Our concern in this book is with LOS wireless transmission, and in this context,
the most significant impairments are
• Attenuation and attenuation distortion
• Free space loss
• Noise
• Atmospheric absorption
• Multipath
• Refraction
Attenuation
The strength of a signal falls off with distance over any transmission medium. For
guided media, this reduction in strength, or attenuation, is generally exponential and
thus is typically expressed as a constant number of decibels per unit distance. For
unguided media, attenuation is a more complex function of distance and the makeup of
the atmosphere. Attenuation introduces three factors for the transmission engineer.
1. A received signal must have sufficient strength so that the electronic circuitry
in the receiver can detect and interpret the signal.
2. The signal must maintain a level sufficiently higher than noise to be received
without error.
3. Attenuation is greater at higher frequencies, causing distortion.
The first and second factors are dealt with by attention to signal strength and
the use of amplifiers or repeaters. For a point-to-point transmission (one transmitter
and one receiver), the signal strength of the transmitter must be strong enough to be
received intelligibly, but not so strong as to overload the circuitry of the transmitter
or receiver, which would cause distortion. Beyond a certain distance, the attenuation
becomes unacceptably great, and repeaters or amplifiers are used to boost the signal
at regular intervals. These problems are more complex when there are multiple
receivers, where the distance from transmitter to receiver is variable.
The third factor is known as attenuation distortion. Because the attenuation varies
as a function of frequency, the received signal is distorted, reducing intelligibility. Specifically,
the frequency components of the received signal have different relative strengths
than the frequency components of the transmitted signal.To overcome this problem, techniques
are available for equalizing attenuation across a band of frequencies. One
approach is to use amplifiers that amplify high frequencies more than lower frequencies.
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