Monday, October 13, 2014

OFDM Basic

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:

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:
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
Because QPSK has 4 possible states, QPSK is able to encode two bits per symbol.
Phase
Data
45 degrees
Binary 00
135 degrees
Binary 01
225 degrees
Binary 11
315 degrees
Binary 10
QPSK is more tolerant of link degradation than 8PSK, but does not provide as much datahttp://images.intellitxt.com/ast/adTypes/mag-glass_10x10.gif capacity.
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




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In my Next Page, I am going to explain Role of OFDM in Optical Communication

Sunday, October 12, 2014

OFDM in Optical Fibre

In this blog, i am going to explain you advantages of OFDM(Orthogonal Frequency Division Multiplexing)  in Optical Fiber Communication and how combination of two will construct Next Generation Network.I am going to explain my views with the help of Presentation:
Few Years Ago, I upload an video based on this this topic and this video was very popular. A large number of Students/Professionals/Teachers was asking me about that Presentation. On the Public Demand, I am going to put all the related contents in this blog. I hope it will be helpful for you.

Video Links of OFDM in Optical 

These are the video where i explained details of OFDM(Orthogonal Frequency Division Multiplexing) in Optical Fiber Communication. I got very good response from these videos

OFDM in Opticals Part I



OFDM in Opticals Part II




Introduction


Orthogonal frequency division multiplexing (OFDM) is widely adopted in communication standards such as Wi-Fi and WiMAX. It offers essential benefits like spectral efficient transmission in combination with robustness against channel impairments. In order to meet the growing demand
for bandwidth, novel robust and efficient modulation techniques are required not only in the backbone but also for home-networks. OFDM has recently been proposed as an attractive format for communication over various types of optical channels making use of the remarkable progress in
the field of digital signal processing.

Related Presentation



Related Book


I take reference for book "Orthogonal Frequency Division Multiplexing for Optical Communications by William Shieh". This is one of the BEST BOOK for beginners/Intermediates/Professionals who want to start their career in Optical Fiber Communication/Next Generation Networks/OFDM for Optical fiber.

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