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Orthogonal frequency-division multiplexing

In telecommunications, orthogonal frequency-division multiplexing (OFDM) is a type of digital transmission used in digital modulation for encoding digital (binary) data on multiple carrier frequencies. OFDM has developed into a popular scheme for wideband digital communication, used in applications such as digital television and audio broadcasting, DSL internet access, wireless networks, power line networks, and 4G/5G mobile communications.[1]

OFDM is a frequency-division multiplexing (FDM) scheme that was introduced by Robert W. Chang of Bell Labs in 1966.[2][3][4] In OFDM, the incoming bitstream representing the data to be sent is divided into multiple streams. Multiple closely spaced orthogonal subcarrier signals with overlapping spectra are transmitted, with each carrier modulated with bits from the incoming stream so multiple bits are being transmitted in parallel.[5] Demodulation is based on fast Fourier transform algorithms. OFDM was improved by Weinstein and Ebert in 1971 with the introduction of a guard interval, providing better orthogonality in transmission channels affected by multipath propagation.[6] Each subcarrier (signal) is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase-shift keying) at a low symbol rate. This maintains total data rates similar to conventional single-carrier modulation schemes in the same bandwidth.[7]

The main 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 the need for 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 eliminate intersymbol interference (ISI) and use echoes and time-spreading (in analog television visible as ghosting and blurring, respectively) to achieve a diversity gain, i.e. a signal-to-noise ratio improvement. 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 re-combined constructively, sparing interference of a traditional single-carrier system.

In coded orthogonal frequency-division multiplexing (COFDM), forward error correction (convolutional coding) and time/frequency interleaving are applied to the signal being transmitted. This is done to overcome errors in mobile communication channels affected by multipath propagation and Doppler effects. COFDM was introduced by Alard in 1986[8][9][10] for Digital Audio Broadcasting for Eureka Project 147. In practice, OFDM has become used in combination with such coding and interleaving, so that the terms COFDM and OFDM co-apply to common applications.[11][12]

Example of applications

The following list is a summary of existing OFDM-based standards and products. For further details, see the Usage section at the end of the article.

Wired version mostly known as Discrete Multi-tone Transmission (DMT)

ADSL and VDSL broadband access via POTS copper wiring

DVB-C2, an enhanced version of the DVB-C digital cable TV standard

Power line communication (PLC)

ITU-T G.hn, a standard which provides high-speed local area networking of existing home wiring (power lines, phone lines and coaxial cables)[13]

TrailBlazer telephone line modems

Multimedia over Coax Alliance (MoCA) home networking

DOCSIS 3.1 Broadband delivery

Wireless

The wireless LAN (WLAN) radio interfaces IEEE 802.11a, g, n, ac, ah and HIPERLAN/2

The digital radio systems DAB/EUREKA 147, DAB+, Digital Radio Mondiale, HD Radio, T-DMB and ISDB-TSB

The terrestrial digital TV systems DVB-T and ISDB-T

The terrestrial mobile TV systems DVB-H, T-DMB, ISDB-T and MediaFLO forward link

The wireless personal area network (PAN) ultra-wideband (UWB) IEEE 802.15.3a implementation suggested by WiMedia Alliance

Wi-SUN (Smart Ubiquitous Network)

The OFDM-based multiple access technology OFDMA is also used in several 4G and pre-4G cellular networks, mobile broadband standards and the next generation WLAN:

The mobility mode of the wireless MAN/broadband wireless access (BWA) standard IEEE 802.16e (or Mobile-WiMAX)

The mobile broadband wireless access (MBWA) standard IEEE 802.20

The downlink of the 3GPP Long Term Evolution (LTE) fourth generation mobile broadband standard. The radio interface was formerly named High Speed OFDM Packet Access (HSOPA), now named Evolved UMTS Terrestrial Radio Access (E-UTRA)

WLAN IEEE 802.11ax

A DVB-T platformon a mindigTV és a mindigTV PRÉMIUM szolgáltatásnál alkalmazandó beállítások:

Vivők száma: IFFT 8k

Moduláció: 64QAM

Kódarány: 3/4

Védelmi intervallum: GI 1/4

IFFT (Inverse Fast Fourier Transformation) alapján két értéket definiáltak, 2k esetén 1705, míg 8k esetén 6817 vivő van.

KÉRDÉSEM az lenne: akkor a 8MHz sáv a 498MHZ-es frekvencia (alatt vagy felett) felvan osztva 6817 kisebb sávra ? Hasonlóan az ADSL-hez ahol 2kHz-es sávok vannak?

Egy DAC kirajzolja avagy szintetizálja azt a jelformát ami elvileg ezeknek a modulált 1kHz, 2kHz, ...,6817kHz rezgéseknek az összege? és ezt hozzáadják (hozzákeverik)egy vivőfrekvenciához (494MHz+)? Vevőoldalon levágják a vivőt marad egy kb 8MHz-es jelalak amiből digitális mintavételezéssel ki lehet olvasni az eredeti bitstream-et?

nagyon messze járok ezzel az elképzeléssel a valóságtól?

[link] Link !!!

RF Down Converting to I/Q Data

There is one fundamental difference between a baseband and modulated RF signal. The modulated signal rides on a carrier of a given frequency, but the base band signal got no fixed frequency at all. Because of this, we have the possibility to encode the two-dimensional I/Q signal onto the one-dimensional RF signal without losing anything. Magic!

https://www.gyakorikerdesek.hu/elektronikus-eszkozok__szorak..

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