01-12-2012, 02:07 PM
Digital Modulation in Communications Systems — An Introduction
[attachment=42076]
Introduction
This application note introduces the concepts of
digital modulation used in many communications
systems today. Emphasis is placed on explaining
the tradeoffs that are made to optimize efficiencies
in system design.
Most communications systems fall into one of three
categories: bandwidth efficient, power efficient, or
cost efficient. Bandwidth efficiency describes the
ability of a modulation scheme to accommodate
data within a limited bandwidth. Power efficiency
describes the ability of the system to reliably send
information at the lowest practical power level.
In most systems, there is a high priority on bandwidth
efficiency. The parameter to be optimized
depends on the demands of the particular system,
as can be seen in the following two examples.
For designers of digital terrestrial microwave
radios, their highest priority is good bandwidth
efficiency with low bit-error-rate. They have plenty
of power available and are not concerned with
power efficiency. They are not especially concerned
with receiver cost or complexity because
they do not have to build large numbers of them.
On the other hand, designers of hand-held cellular
phones put a high priority on power efficiency
because these phones need to run on a battery.
Cost is also a high priority because cellular phones
must be low-cost to encourage more users. Accordingly,
these systems sacrifice some bandwidth
efficiency to get power and cost efficiency.
Why Digital Modulation?
The move to digital modulation provides more
information capacity, compatibility with digital
data services, higher data security, better quality
communications, and quicker system availability.
Developers of communications systems face these
constraints:
• available bandwidth
• permissible power
• inherent noise level of the system
The RF spectrum must be shared, yet every day
there are more users for that spectrum as demand
for communications services increases. Digital
modulation schemes have greater capacity to convey
large amounts of information than analog modulation
schemes.
Trading off simplicity and bandwidth
There is a fundamental tradeoff in communication
systems. Simple hardware can be used in transmitters
and receivers to communicate information.
However, this uses a lot of spectrum which limits
the number of users. Alternatively, more complex
transmitters and receivers can be used to transmit
the same information over less bandwidth. The
transition to more and more spectrally efficient
transmission techniques requires more and more
complex hardware. Complex hardware is difficult
to design, test, and build. This tradeoff exists
whether communication is over air or wire, analog
or digital.
Polar display—magnitude and phase represented
together
A simple way to view amplitude and phase is with
the polar diagram. The carrier becomes a frequency
and phase reference and the signal is interpreted
relative to the carrier. The signal can be expressed
in polar form as a magnitude and a phase. The
phase is relative to a reference signal, the carrier
in most communication systems. The magnitude is
either an absolute or relative value. Both are used
in digital communication systems. Polar diagrams
are the basis of many displays used in digital communications,
although it is common to describe the
signal vector by its rectangular coordinates of I
(In-phase) and Q (Quadrature).
[attachment=42076]
Introduction
This application note introduces the concepts of
digital modulation used in many communications
systems today. Emphasis is placed on explaining
the tradeoffs that are made to optimize efficiencies
in system design.
Most communications systems fall into one of three
categories: bandwidth efficient, power efficient, or
cost efficient. Bandwidth efficiency describes the
ability of a modulation scheme to accommodate
data within a limited bandwidth. Power efficiency
describes the ability of the system to reliably send
information at the lowest practical power level.
In most systems, there is a high priority on bandwidth
efficiency. The parameter to be optimized
depends on the demands of the particular system,
as can be seen in the following two examples.
For designers of digital terrestrial microwave
radios, their highest priority is good bandwidth
efficiency with low bit-error-rate. They have plenty
of power available and are not concerned with
power efficiency. They are not especially concerned
with receiver cost or complexity because
they do not have to build large numbers of them.
On the other hand, designers of hand-held cellular
phones put a high priority on power efficiency
because these phones need to run on a battery.
Cost is also a high priority because cellular phones
must be low-cost to encourage more users. Accordingly,
these systems sacrifice some bandwidth
efficiency to get power and cost efficiency.
Why Digital Modulation?
The move to digital modulation provides more
information capacity, compatibility with digital
data services, higher data security, better quality
communications, and quicker system availability.
Developers of communications systems face these
constraints:
• available bandwidth
• permissible power
• inherent noise level of the system
The RF spectrum must be shared, yet every day
there are more users for that spectrum as demand
for communications services increases. Digital
modulation schemes have greater capacity to convey
large amounts of information than analog modulation
schemes.
Trading off simplicity and bandwidth
There is a fundamental tradeoff in communication
systems. Simple hardware can be used in transmitters
and receivers to communicate information.
However, this uses a lot of spectrum which limits
the number of users. Alternatively, more complex
transmitters and receivers can be used to transmit
the same information over less bandwidth. The
transition to more and more spectrally efficient
transmission techniques requires more and more
complex hardware. Complex hardware is difficult
to design, test, and build. This tradeoff exists
whether communication is over air or wire, analog
or digital.
Polar display—magnitude and phase represented
together
A simple way to view amplitude and phase is with
the polar diagram. The carrier becomes a frequency
and phase reference and the signal is interpreted
relative to the carrier. The signal can be expressed
in polar form as a magnitude and a phase. The
phase is relative to a reference signal, the carrier
in most communication systems. The magnitude is
either an absolute or relative value. Both are used
in digital communication systems. Polar diagrams
are the basis of many displays used in digital communications,
although it is common to describe the
signal vector by its rectangular coordinates of I
(In-phase) and Q (Quadrature).