Industrial Wireless
Tutorials: Modulation is the process by which the bits comprising the
transmitted data are converted to the voltage levels that will be mixed with
the carrier, amplified, and sent to the antenna; it is a conversion and mixing
of raw data with a carrier wave that is oscillating at the channel center
frequency.
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Modulation should not be confused with encoding.
Encoding converts individual bits into symbols, which are essentially another
collection of bits. This is done to facilitate spreading, as in spread
spectrum; or to make data secure, as in CCMP (counter mode with cipher block
chaining message authentication code protocol); or to perform forward error
correction, a technique used in orthogonal frequency division multiplexing
(OFDM). Modulation is the process of converting these individual bits into a
form that can be put onto the physical layer. This form is a periodic variation
in the amplitude, frequency, phase, or position.
The most basic modulation is the keying of the
carrier, called CW (continuous wave). This was the first use of wireless,
utilizing Morse code to send information over great distances.
Amplitude modulation, or AM, varies the
amplitude of the carrier in proportion to the impressed frequency of the input
data. AM has a very low bandwidth and is not typically used for data
communication.
Another familiar method is frequency
modulation, or FM. FM varies the frequency of the carrier in proportion to the
frequency of the frequency of the input data. FM has a higher bandwidth and is
used for high-quality audio transmission, such as stereo radio broadcasting. It
is also used for video signals because of the wider bandwidth requirement.
These techniques have served the
entertainment industry well, but frequency spectrum usage has been very
inefficient. Very early in the development of data communication systems, it
was recognized that the nature of digital communication was such that it would
lend itself to multiplexing of several distinct data streams onto a shared
medium. Multiplexing is a technique that takes multiple data streams and
combines them into one stream, which is modulated and placed onto the shared
medium; this greatly increases the spectral efficiency of the medium, allowing it
to carry much more information than would be possible if limited to a single
stream.
Two ways to effectively divide the
transmitting channel efficiently are by using frequency and by using time
division. Two techniques widely used are frequency division multiplexing (FDM)
and time division multiplexing (TDM). In the first, separate
frequencies—usually subcarriers of the channel center frequency—are
individually modulated by another technique to allow multiple bit streams to be
transmitted independently. TDM uses time slots to transmit individual channels,
similar to, but not strictly analogous to frequency hopping; in TDM,
information is transmitted during a specific time period of the carrier wave
cycle. TDM requires fine synchronization of the transmitter and receiver in
order to work properly. Other types of multiplexing techniques are code
division multiplexing (CDM), which is widely used in cellular communication
as code division multiple access (CDMA). CDM is a technique by which
several information channels share the same frequency spectrum; you may
recognize this as spread spectrum.
Diving deeper, how does an individual bit
modulate the carrier? Regardless of the method of multiplexing, the carrier
voltage level still needs to be manipulated in such a fashion as to change its
characteristics before application to the antenna. Two common methods used in
Wi-Fi are frequency shift keying (FSK) and phase shift keying (PSK). In FSK,
the frequency is modulated to represent a 0 or a 1. In PSK, the phase is altered
to represent the two digital values. Note that these techniques use analog
values to represent the two digital states. Also note that this conversion
takes place at the physical layer.
FDM has issues and is not efficient in most
cases. PSK is more efficient in coding and spectral usage. Two forms of PSK are
used: binary phase shift keying (BPSK) and quadrature phase shift keying
(QPSK). Consider the sine wave as an alternating quantity that revolves through
360 deg of values; each method uses this variation as a means of encoding. In
BPSK, two binary values can be encoded per cycle. In QPSK (also called 4-QAM),
four values can be encoded per cycle. The underlying principle of PSK is the
abrupt shifting of the phase of the sine wave. The wave will be shifted N
degrees to represent a 0, and it will be shifted another value to represent a 1.
As can be seen by the constellation diagrams,
as the number of possible data points increases, so does the ability to
transmit increasing amounts of data during one cycle. The improvements in
throughput should be obvious. As one would expect, as more information is
stuffed into the available space, there is a real possibility of symbols
running into each other and corrupting the data. This is why a transmitter and
receiver negotiate a favorable modulation and coding scheme (MCS). Channel
conditions, traffic priority, and device characteristics and capabilities all
factor into what type of modulation can be used to most effectively transmit
error-free data at the highest possible speed.
The advantage of using QAM is that the technique
allows more bits per symbol and higher spectral efficiency: BPSK has a
theoretical bandwidth efficiency of 1 bit/second/Hz; 256QAM has a theoretical
efficiency of 8 bits/second/Hz. Higher orders of QAM translate into a higher
data rate. The following table gives a quick summary of the bit rates of the
various modulation techniques described above.
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Source: controleng
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