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In single carrier system, N number of symbols are transmitted using T time in which each of this symbol occupied T/N duration. And the bandwidth wise, each of transmitted symbol occupied the whole bandwidth B.
In multi carrier system, N number of symbols are transmitted using T time in which each of this symbol occupied T duration (All the symbols transmitted in the same time and the sub carrier of each symbol is overlapping between each other.). And the bandwidth wise, each of the transmitted symbol occupied B/N bandwidth.
Why we want to transmit all the symbols in the same time? Easy! Imagine that there is a very deep fading in particular frequency (frequency selective fading channel). For the single carrier system, all of the transmitted symbols suffered deep fade at that frequency. However, for multicarrier system, only a small amount of the symbols suffered from deep fade. Therefore, OFDM is so useful in frequency selective fading channel.
Simply speaking OFDM is that you divide your available
bandwidth into smaller independent frequency band.
For example ... say your BW is 100MHz and you divide
that into 10 - 10MHz Channels then you have divided
one channel into 10 channels and they are orthogonal i.e. independent of each other. An example of this is ADSL.
In OFDM the total bandwidth is divided into small subcarriers and the information is transmitted over these carriers. There is no need of guard band. eavh carrier is orthogonal to each other.
essentially identical to Coded OFDM (COFDM) and Discrete multi-tone modulation (DMT) — is a frequency-division multiplexing (FDM) scheme utilized as a digital multi-carrier modulation method. A large number of closely-spaced orthogonal sub-carriers are used to carry data. The data are 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, 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.
Orthogonal means that there are mathematical relationships between the frequencies of the carriers of the systems. Why don’t we just use the normal FDM?
In the normal FDM, the frequency of each carriers are spaced apart so that is can use conventional filters and demodulators and in such receiver, guard band have to be introduced between different carriers. It results in taking a lot of bandwidth space in frequency domain which lowers down the frequency spectral efficiency.
By using OFDM, each carrier will overlap at its sideband without any adjacent interference with the pre-requisite of that each carriers must be orthogonal. (it is orthogonal if the carriers are spaced with the multiplication of 1 / Ts (reciprocal of symbol period)
You have to go through a mathematical relation in order to understand. But dont worry its not that complicated
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