In electronics
and
telecommunications,
modulation is the
process of varying one or more properties of a high-frequency periodic
waveform,
called the
carrier signal, with a
modulating signal
which typically contains information to be transmitted. This is done in a
similar fashion to a
musician modulating a tone (a periodic waveform) from a
musical instrument by varying its
volume, timing and
pitch.
The three key parameters of a periodic waveform are its
amplitude
("volume"), its
phase
("timing") and its
frequency ("pitch"). Any of these properties can be
modified in accordance with a low frequency signal to obtain the modulated
signal. Typically a
high-frequency sinusoid
waveform is used as
carrier signal, but a square wave pulse train
may also be used.
In telecommunications, modulation is the process
of conveying a message signal, for example a digital bit stream or an
analog
audio signal, inside another signal that can be physically transmitted.
Modulation of a sine waveform is used to transform a
baseband
message signal into a
passband signal, for example low-frequency audio signal into a
radio-frequency signal (RF signal). In radio communications, cable TV systems
or the
public switched telephone network
for instance, electrical signals can only be transferred over a limited
passband frequency spectrum, with specific (non-zero) lower and upper cutoff
frequencies. Modulating a sine-wave carrier makes it possible to keep the
frequency content of the transferred signal as close as possible to the centre
frequency (typically the carrier frequency) of the passband.
In analog
modulation, the modulation is applied continuously in response to the analog
information signal. Common analog modulation techniques are:
[1]
- Amplitude modulation (AM) (here the
amplitude of the carrier signal is varied in accordance to the
instantaneous amplitude of the modulating signal)
- Angle modulation, which is
approximately constant envelope
- Frequency modulation (FM) (here the
frequency of the carrier signal is varied in accordance to the
instantaneous amplitude of the modulating signal)
- Phase modulation (PM) (here the
phase shift of the carrier signal is varied in accordance to the
instantaneous amplitude of the modulating signal)
In digital
modulation, an analog carrier signal is modulated by a discrete signal. Digital
modulation methods can be considered as digital-to-analog conversion, and the
corresponding
demodulation or detection as analog-to-digital
conversion. The changes in the carrier signal are chosen from a finite number
of M alternative symbols (the
modulation alphabet).
A simple
example: A telephone
line is designed for transferring audible sounds, for example tones, and not
digital bits (zeros and ones). Computers may however communicate over a
telephone line by means of modems, which are representing the digital bits by
tones, called symbols. If there are four alternative symbols (corresponding to
a musical instrument that can generate four different tones, one at a time),
the first symbol may represent the bit sequence 00, the second 01, the third 10
and the fourth 11. If the modem plays a melody consisting of 1000 tones per
second, the symbol rate is 1000 symbols/second, or
baud. Since each tone
(i.e., symbol) represents a message consisting of two digital bits in this
example, the
bit rate
is twice the symbol rate, i.e. 2000 bits per second. This is similar to the
technique used by dialup modems as opposed to
DSL modems.
- Group the
incoming data bits into codewords, one for each symbol that will be
transmitted.
- Map the
codewords to attributes, for example amplitudes of the I and Q signals
(the equivalent low pass signal), or frequency or phase values.
- Adapt pulse
shaping or some other filtering to limit the bandwidth and form
the spectrum of the equivalent low pass signal, typically using digital
signal processing.
- Perform
digital to analog conversion (DAC) of the I and Q signals (since today all
of the above is normally achieved using digital signal processing,
DSP).
- Generate a
high frequency sine carrier waveform, and perhaps also a cosine quadrature
component. Carry out the modulation, for example by multiplying the sine
and cosine waveform with the I and Q signals, resulting in the equivalent
low pass signal being frequency shifted to the modulated passband
signal or RF signal. Sometimes this is achieved
using DSP technology, for example direct digital synthesis
using a waveform table,
instead of analog signal processing. In that case the above DAC step
should be done after this step.
- Amplification
and analog bandpass filtering to avoid harmonic distortion and periodic
spectrum
- Bandpass
filtering.
- Automatic gain control, AGC (to
compensate for attenuation, for example fading).
- Frequency
shifting of the RF signal to the equivalent baseband I and Q signals, or
to an intermediate frequency (IF) signal, by multiplying the RF signal
with a local oscillator sinewave and cosine wave frequency (see the superheterodyne receiver principle).
- Sampling
and analog-to-digital conversion (ADC) (Sometimes before or instead of the
above point, for example by means of undersampling).
- Equalization
filtering, for example a matched
filter, compensation for multipath propagation, time spreading,
phase distortion and frequency selective fading, to avoid intersymbol interference and symbol
distortion.
- Detection
of the amplitudes of the I and Q signals, or the frequency or phase of the
IF signal.
- Quantization
of the amplitudes, frequencies or phases to the nearest allowed symbol
values.
- Mapping of
the quantized amplitudes, frequencies or phases to codewords (bit groups).
- Parallel-to-serial
conversion of the codewords into a bit stream.
- Pass the
resultant bit stream on for further processing such as removal of any
error-correcting codes.
MSK and
GMSK are particular cases
of continuous phase modulation. Indeed, MSK is a particular case of the
sub-family of CPM known as
continuous-phase frequency-shift
keying (CPFSK) which is defined by a rectangular frequency pulse
(i.e. a linearly increasing phase pulse) of one symbol-time duration (total
response signaling).
OFDM is based on the idea
of
frequency-division multiplexing
(FDM), but the multiplexed streams are all parts of a single original stream.
The bit stream is split into several parallel data streams, each transferred
over its own sub-carrier using some conventional digital modulation scheme. The
modulated sub-carriers are summed to form an OFDM signal. This dividing and
recombining helps with handling channel impairments. OFDM is considered as a
modulation technique rather than a multiplex technique, since it transfers one
bit stream over one communication channel using one sequence of so-called OFDM
symbols. OFDM can be extended to multi-user
channel access method in the
orthogonal frequency-division
multiple access (OFDMA) and
multi-carrier code division multiple
access (MC-CDMA) schemes, allowing several users to share the same
physical medium by giving different sub-carriers or
spreading codes to different
users.
Of the two
kinds of RF power amplifier,
switching amplifiers (
Class D
amplifiers) cost less and use less battery power than
linear
amplifiers of the same output power. However, they only work with
relatively constant-amplitude-modulation signals such as angle modulation (FSK
or PSK) and
CDMA, but not with QAM and OFDM.
Nevertheless, even though switching amplifiers are completely unsuitable for
normal QAM constellations, often the QAM modulation principle are used to drive
switching amplifiers with these FM and other waveforms, and sometimes QAM demodulators
are used to receive the signals put out by these switching amplifiers.
