Hello, everyone!
Today, I would like to share an
article about OFDM Technique.
Orthogonal frequency division multiplexing (OFDM) is a particular frequency-division multiplexing (FDM) scheme utilised a digital multi-carrier modulation method. Every portion of input data is transmitted on one of the available closely-spaced orthogonal subcarriers.
OFDM is divided into several parallel data up and down streams or channels. Sub-carriers are modulated with conventional digital modulation schemes (such as Quadrature Amplitude Modulation (QAM) or Phase Shift Keying (PSK)) at a low symbol rate, which can maintain total data rates similar to conventional single-carrier modulation schemes at the same bandwidth.
The primary advantage of OFDM over single-carrier schemes is its ability to conquer severe channel conditions, especially the Multi-Path Effect. Channel equalisation is simplified because it could be viewed as using a lot of 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, which makes it possible to handle time-spreading and eliminates inter-symbol interference (ISI).
This mechanism also facilitates the design of single-carrier networks, where several adjacent transmitters send the same signal simultaneously at the same frequency, as the signals from multiple transmitters could be combined constructively, rather than interfering that would typically occur in a traditional single-carrier system.
The first OFDM experiments were presented is 1960s’ military radio link. At that time, actual civil use of OFDM was limited and the practicability of the concept was questioned. When the digital communication and integrated circuit chip have been tremendously promoted, OFDM are developed as a popular scheme for broadband digital communication, whether wireless or over copper wires, adopted in applications such as digital television and audio broadcasting.
Over the last decade, OFDM is exploited for wireless communication system, and also becoming a basic technique for next generation broadband wireless access network. Today it is proved in practice that traditional wireless communication systems, like TDMA, FDMA or even CDMA are not capable of meeting the required criteria because of their inherent limitations. This is the reason why a large amount of research effort in radio communications is focused on multicarrier transmission methods, and these techniques are now considered the only way to support future demands. OFDM/OFDMA is the criteria technique of Long Term Evolution (LTE) which is the extension of 3GPP (3rd Generation Partnership Project).
In recent years, seamless integrated wired and wireless access technology is becoming an interesting research subject. A significant solution of this subject is OFDM radio frequency signal transmission over fiber. Radio-Over-Fiber (ROF) has well known scheme for distributing RF and microwave signals, such as low transmission loss and wide bandwidth.
OFDM is more and more attractive in optical communication application, which appears in a large number of optical researches, such as long-haul transmission and WDM-PON. The combination of OFDM and ROF is increasing the high-speed wireless data transmission and video distribution in future broadband access network.
People are paying more attention to do channel optimisation in optical domain, such as dispersion shift fiber or dispersion compensating fiber. However, electrical optimisation is more effective to make received OFDM-ROF signal better. For instance, pilots that are padded into OFDM symbol can equalise noises and phase distortions of received signal.
Figure. QPSK constellations without (a) and with (b) equalisation.
OFDM-QPSK baseband signal transmitted over 50 km SSMF by 60-GHs ROF technique is used to prove the effect of electrical equalisation. The received QPSK constellations without and with equalisation are shown in the above Figure (a) and (b). In this demonstration, 60 of 64 sub- carriers are used to transmit data with QPSK modulation; pilots are carried by the other 4 sub-carriers. Coarse channel estimation is achieved from received pilots.
The above Figure (a) depicts QPSK constellation with severe noises and phase distortion caused by dispersion and nonlinear effect. Great impact of electrical equalisation which compensates signal‘s distortions is shown in Figure (b).
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