In this blog, i am going to teach you some basics of OFDM(Orthogonal Frequency Division Multiplexing) and then Design OFDM System using Matlab under different Scenario:
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In my Next Page, I am going to explain Role of OFDM in Optical Communication
- Script-file(M file) and Simulink
- AWGN (Additive White Gaussian Noise) and Rayleigh Fading
- 4-QAM and 16-QAM
Introduction
Orthogonal
frequency-division multiplexing (OFDM), essentially identical to coded
OFDM (COFDM) and discrete multi-tone modulation (DMT),
is a frequency-division
multiplexing (FDM) scheme used as a digital multi-carrier modulation method. A large
number of closely-spaced orthogonal sub-carriers
are used to carry data. The
data is divided into several parallel data streams or channels, one for each
sub-carrier. Each sub-carrier is modulated with a conventional modulation
scheme (such as quadrature
amplitude modulation or phase-shift keying)
at a low symbol rate,
maintaining total data rates similar to conventional single-carrier
modulation schemes in the same bandwidth.
OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, wireless networking and broadband internet access.The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath) without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate intersymbol interference (ISI). This mechanism also facilitates the design of single frequency networks (SFNs), where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system.
DIGITAL MODULATION: Those used in OFDM are as follow:
Alternatively, instead of using the bit patterns to set the phase of the wave, it can instead be used to change it by a specified amount. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK since there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (it is a non-coherent scheme). In exchange, it produces more erroneous demodulations. The exact requirements of the particular scenario under consideration determine which scheme is used.
Dispersion: In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency,[1] or alternatively when the group velocity depends on the frequency. Media having such a property are termed dispersive media. Dispersion is sometimes called chromatic dispersion to emphasize its wavelength-dependent nature, or group-velocity dispersion (GVD) to emphasize the role of the group velocity.
The most familiar example of dispersion is probably a rainbow, in which dispersion causes the spatial separation of a white light into components of different wavelengths (different colors). However, dispersion also has an effect in many other circumstances: for example, GVD causes pulses to spread in optical fibers, degrading signals over long distances; also, a cancellation between group-velocity dispersion and nonlinear effects leads to soliton waves. Dispersion is most often described for light waves, but it may occur for any kind of wave that interacts with a medium or passes through an inhomogeneous geometry (e.g., a waveguide), such as sound waves.
There are generally two sources of dispersion: material dispersion and waveguide dispersion. Material dispersion comes from a frequency-dependent response of a material to waves. For example, material dispersion leads to undesired chromatic aberration in a lens or the separation of colors in a prism. Waveguide dispersion occurs when the speed of a wave in a waveguide (such as an optical fiber) depends on its frequency for geometric reasons, independent of any frequency dependence of the materials from which it is constructed. More generally, "waveguide" dispersion can occur for waves propagating through any inhomogeneous structure (e.g., a photonic crystal), whether or not the waves are confined to some region. In general, both types of dispersion may be present, although they are not strictly additive. Their combination leads to signal degradation in optical fibers for telecommunications, because the varying delay in arrival time between different components of a signal "smears out" the signal in time.
OFDM has developed into a popular scheme for wideband digital communication, whether wireless or over copper wires, used in applications such as digital television and audio broadcasting, wireless networking and broadband internet access.The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions (for example, attenuation of high frequencies in a long copper wire, narrowband interference and frequency-selective fading due to multipath) without complex equalization filters. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate intersymbol interference (ISI). This mechanism also facilitates the design of single frequency networks (SFNs), where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple distant transmitters may be combined constructively, rather than interfering as would typically occur in a traditional single-carrier system.
DIGITAL MODULATION: Those used in OFDM are as follow:
1.
Phase-shift keying (PSK) is a digital modulation scheme that
conveys data
by changing, or modulating, the phase
of a reference signal (the carrier wave).
Any digital modulation scheme uses a finite
number of distinct signals to represent digital data. PSK uses a finite number
of phases, each assigned a unique pattern of binary digits. Usually,
each phase encodes an equal number of bits. Each pattern of bits forms the symbol
that is represented by the particular phase. The demodulator,
which is designed specifically for the symbol-set used by the modulator,
determines the phase of the received signal and maps it back to the symbol it
represents, thus recovering the original data. This requires the receiver to be
able to compare the phase of the received signal to a reference signal — such a
system is termed coherent (and referred to as CPSK).Alternatively, instead of using the bit patterns to set the phase of the wave, it can instead be used to change it by a specified amount. The demodulator then determines the changes in the phase of the received signal rather than the phase itself. Since this scheme depends on the difference between successive phases, it is termed differential phase-shift keying (DPSK). DPSK can be significantly simpler to implement than ordinary PSK since there is no need for the demodulator to have a copy of the reference signal to determine the exact phase of the received signal (it is a non-coherent scheme). In exchange, it produces more erroneous demodulations. The exact requirements of the particular scenario under consideration determine which scheme is used.
2.
QPSK
(Quadrature Phase Shift Keying) is a phase modulation algorithm.
Phase modulation is a version of
frequency modulation where the phase of the carrier wave is modulated to encode
bits of digital information in each phase change.
The “PSK” in QPSK refers to the use of Phased Shift Keying. Phased
Shift Keying is a form of phase modulation which is accomplished by the use of
a discrete number of states. QPSK refers to PSK with 4 states. With half that
number of states, you will have BPSK (Binary Phased Shift Keying). With twice the number of states as
QPSK, you will have 8PSK.
The “Quad” in QPSK refers to four
phases in which a carrier is sent in QPSK: 45, 135, 225, and 315 degrees.
QPSK
Encoding
Phase
|
Data
|
45 degrees
|
Binary 00
|
135 degrees
|
Binary 01
|
225 degrees
|
Binary 11
|
315 degrees
|
Binary 10
|
3.
Quadrature
amplitude modulation (QAM) is both an analog and a digital modulation scheme. It
conveys two analog message signals or two digital bit streams,
by changing (modulating) the amplitudes of
two carrier
waves, using the amplitude-shift
keying (ASK) digital modulation scheme or amplitude modulation
(AM) analog modulation scheme. These two waves, usually sinusoids, are out of
phase with each other by 90°
and are thus called quadrature carriers or quadrature components —
hence the name of the scheme. The modulated waves are summed, and the resulting
waveform is a combination of both phase-shift keying
(PSK) and amplitude-shift
keying (ASK), or in the analog case of phase modulation (PM) and amplitude
modulation. In the digital QAM case, a finite number of at least two phases,
and at least two amplitudes are used. PSK modulators are often designed using
the QAM principle, but are not considered as QAM since the amplitude of the
modulated carrier signal is constant. QAM is used extensively as a modulation
scheme for digital telecommunication
systems.
4.
FFT IN OFDM: process of a typical FFT-based OFDM
system. The incoming serial data is first converted form serial to parallel and
grouped into x bits each to form a complex number. The number x
determines the signal constellation of the corresponding subcarrier, such as 16
QAM or 32QAM. The complex numbers are modulated in a baseband fashion by the
inverse FFT (IFFT) and converted back to serial data for transmission. A guard
interval is inserted between symbols to avoid intersymbol interference (ISI)
caused by multipath distortion. The discrete symbols are converted to analog
and low-pass filtered for RF upconversion. The receiver performs the inverse
process of the transmitter. One-tap equalizer is used to correct channel
distortion. The tap-coefficients of the filter are calculated based on the
channel information.
WAVELENGTH DIVISON
MULTIPLEXING: In fiber-optic communications, wavelength-division
multiplexing (WDM) is a technology which multiplexes
a number of optical carrier signals onto a single optical fiber by using
different wavelengths
(colours) of laser light. This technique enables bidirectional
communications over one strand of fiber, as well as multiplication of
capacity.The term wavelength-division multiplexing is commonly applied
to an optical carrier (which is typically described by its wavelength), whereas
frequency-division
multiplexing typically applies to a radio carrier (which is more often
described by frequency).
Since wavelength and frequency are tied together through a simple
directly inverse relationship, the two terms actually describe the same
concept.Dispersion: In optics, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency,[1] or alternatively when the group velocity depends on the frequency. Media having such a property are termed dispersive media. Dispersion is sometimes called chromatic dispersion to emphasize its wavelength-dependent nature, or group-velocity dispersion (GVD) to emphasize the role of the group velocity.
The most familiar example of dispersion is probably a rainbow, in which dispersion causes the spatial separation of a white light into components of different wavelengths (different colors). However, dispersion also has an effect in many other circumstances: for example, GVD causes pulses to spread in optical fibers, degrading signals over long distances; also, a cancellation between group-velocity dispersion and nonlinear effects leads to soliton waves. Dispersion is most often described for light waves, but it may occur for any kind of wave that interacts with a medium or passes through an inhomogeneous geometry (e.g., a waveguide), such as sound waves.
There are generally two sources of dispersion: material dispersion and waveguide dispersion. Material dispersion comes from a frequency-dependent response of a material to waves. For example, material dispersion leads to undesired chromatic aberration in a lens or the separation of colors in a prism. Waveguide dispersion occurs when the speed of a wave in a waveguide (such as an optical fiber) depends on its frequency for geometric reasons, independent of any frequency dependence of the materials from which it is constructed. More generally, "waveguide" dispersion can occur for waves propagating through any inhomogeneous structure (e.g., a photonic crystal), whether or not the waves are confined to some region. In general, both types of dispersion may be present, although they are not strictly additive. Their combination leads to signal degradation in optical fibers for telecommunications, because the varying delay in arrival time between different components of a signal "smears out" the signal in time.
Design OFDM System
Related Programming Files
I hope this material is useful for you and if you need further help then click on above facebook page link and LIKE it and put your queries.
In my Next Page, I am going to explain Role of OFDM in Optical Communication
