What is OFDM Technology?

OFDM

Frequency division multiplexing (FDM) is a technology that transmits multiple signals simultaneously over a single transmission path, such as a cable or wireless system. Each signal travels within its own unique frequency range (carrier), which is modulated by the data (text, voice, video, etc.).

Orthogonal FDM's (OFDM) spread spectrum technique distributes the data over a large number of carriers that are spaced apart at precise frequencies. This spacing provides the "orthogonality" in this technique which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to RF interference, and lower multi-path distortion. This is useful because in a typical terrestrial broadcasting scenario there are multipath-channels (i.e. the transmitted signal arrives at the receiver using various paths of different length). Since multiple versions of the signal interfere with each other (inter symbol interference (ISI)) it becomes very hard to extract the original information.

OFDM is sometimes called multi-carrier or discrete multi-tone modulation. It is the modulation technique used for digital TV in Europe, Japan and Australia.

Uses

DAB - OFDM forms the basis for the Digital Audio Broadcasting (DAB) standard in the European market.

ADSL - OFDM forms the basis for the global ADSL (asymmetric digital subscriber line) standard.

Wireless Local Area Networks - development is ongoing for wireless point-to-point and point-to-multipoint configurations using OFDM technology.

In a supplement to the IEEE 802.11 standard, the IEEE 802.11 working group published IEEE 802.11a, which outlines the use of OFDM in the 5.8-GHz band.

DSSS

Acronym for direct-sequence spread spectrum. DSSS is one of two types of spread spectrum radio, the other being frequency-hopping spread spectrum. DSSS is a transmission technology used in LAWN transmissions where a data signal at the sending station is combined with a higher data rate bit sequence, or chipping code, that divides the user data according to a spreading ratio. The chipping code is a redundant bit pattern for each bit that is transmitted, which increases the signal's resistance to interference. If one or more bits in the pattern are damaged during transmission, the original data can be recovered due to the redundancy of the transmission

OFDM stands for Orthogonal Frequency Division Multiplexing. This is reasonably new mulitplexing technology which will be used on the 4-G New Generation Networks (NGN) - mostly for wireless transmission. It is better than any other transmission method over long and short ranges.

Orthogonal Frequency-Division Multiplexing (OFDM) — essentially identical to Coded OFDM (COFDM) — is a digital multi-carrier modulation scheme, which uses a large number of closely-spaced orthogonal sub-carriers. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation) at a low symbol rate, maintaining data rates similar to conventional single-carrier modulation schemes in the same bandwidth. In practice, OFDM signals are generated using the Fast Fourier transform algorithm. The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions — for example, multipath and narrowband interference — 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 orthogonality of the sub-carriers results in zero cross-talk, even though they are so close that their spectra overlap. Low symbol rate helps manage time-domain spreading of the signal (such as multipath propagation) by allowing the use of a guard interval between symbols. The guard interval also eliminates the need for a pulse-shaping filter. Applications : * ADSL, SDSL and VDSL broadband access via POTS copper wiring. * Wi-Fi (IEEE 802.11a/g) Wireless LANs. * DAB systems EUREKA 147, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB. * DVB terrestrial digital TV systems DVB-T, DVB-H, T-DMB and ISDB-T. * IEEE 802.16 or WiMAX Wireless MANs. * IEEE 802.20 or Mobile Broadband Wireless Access (MBWA) systems. * Flash-OFDM cellular systems. * Ultra wideband (UWB) systems. * Power line communication (PLC). * MoCA home networking. Advantages : * Can easily adapt to severe channel conditions without complex equalization. * Robust against narrow-band co-channel interference. * Robust against Intersymbol interference (ISI) and fading caused by multipath propagation. * High spectral efficiency. * Efficient implementation using FFT. * Low sensitivity to time synchronization errors. * Tuned sub-channel receiver filters are not required (unlike conventional FDM) * Facilitates Single Frequency Networks, i.e. transmitter macrodiversity. Disadvantages: * Sensitive to Doppler shift. * Sensitive to frequency synchronization problems. * Inefficient transmitter power consumption, due to linear power amplifier requirement.

oxygen free dense material