Someone told me that it can be used as a modulator and demodulator but I do not believe this because FFT and IFFT formula in the subject called signal and systems or DSP are used to change frequency domain data into time domain data or time domain to frequency domain data. It should not be related to the modulator or demodulator which has a definition like multiplied amplitude and generate modulation frequencies and intermodulation frequencies like basic AM and FM.
Dear Kittipong,
Yes, FFT and IFFT can be applied in the digital signal processing of OFDM symbols since they can transform these symbols from frequency domain to time domain in the modulator and vice verse in the demodulator. They are tools to synthesize and analyse the wavefroms from the frequency domain to time domain and back to the frequency domain
Conceptually,the Fourier transform is used to convert the signals from time domain to frequency domain and the inverse Fourier transform is used to convert the signal back from the frequency domain to the time domain.The Fourier transform is a powerful tool to analyse the signals and construct them to and from their frequency components. If the signal is discrete in time that is sampled, one uses the discrete Fourier transform to convert them to the discrete frequency form DFT, and vice verse, the inverse discrete transform IDFT is used to back convert the discrete frequency form into the discrete time form.
To reduce the mathematical operations used in the calculation of DFT and IDFT one uses the fast Fourier transform algorithm FFT and IFFT which corresponds to DFT and IDFT, respectively.
In transmitters using OFDM as a multicarrier modulation technology, the OFDM symbol is constructed in the frequency domain by mapping the input bits on the I- and Q- components of the QAM symbols and then ordering them in a sequence with specific length according to the number of subcarriers in the OFDM symbol. That is by the mapping and ordering process, one constructs the frequency components of the OFDM symbol. To transmit them, the signal must be represented in time domain. This is accomplished by the inverse fast Fourier transform IFFT.
The alternative for using the the IFFT is to generate the subcarriers by an a number of oscillators equals the number of the subcarriers in the OFDM symbol
which may be exceedingly large according to the OFDM standards. Say you want to generate 64 sub carriers, with stable and synchronized frequencies, how much hardware and power will be consumed.
In fact the multicarrier OFDM modulation technique remained just a theoretical idea till the advent of FFT and IFFT and presence of powerful computing machines for them. It can be said without exaggeration, that IFFT and FFT are the enablers of the OFDM.
So, in summary the OFDM signal is easier synthesized in discrete frequency domain in the transmitter and to transmit it must be converted to discrete time domain by IFFT.
wish you success
By using FFT/IFFT processor in LTE-OFDM channel is divided into many narrower channels with lower data rates and hence longer symbol duration, to reduce problems with multipath/reflections.
OFDM is the technique used in DRM broadcast also.The AM transmitter can transmit near FM quality audio with coverage of AM. No of OFDM carriers are modulated by performing IFFT operation for modulation and FFT operation at receiving end .For more details pl. follow the link www.drm.org
Thank you prof. dr. Zekry for detail answer of my question. So if I want to transmit 64 subcarriers at the same time, Do I must multiply 64 times the current consumption with oscillator which are existed in parallel in the architecture of PLL ?
Kittipong.
May be the most suitable generation of synchronized 64 subcarriers is to use a Frequency synthesizer issuing the 64 subcarriers simultaneously. The divider chain of the pll is to be adjusted to output the required frequencies. Then one has to filter the output square waveform to get it fundamental. One can use also a digital frequency synthesizer. In order to perform the modulators you need 64 multiplying circuit. Analog multiplying circuits are also complex and power consuming.
If you use 64 oscillators based on separate plls with common reference for synchronization, then the answer is yes.
wish you success
Each carrier represents an "element" of ifft, so the multi-carriers of OFDM is the ifft of the transmitted sample. Even if the OFDM is very old, the recent development of the processors allows the use the IFFT ...
The main part of OFDM is FFT and IFFT sections. For more details you can see the block diagram of modulator and demodulator of OFDM.
I see a parallel in how stereo FM and OFDM work. In both cases, we first think of the spectrum occupied by the signal to be transmitted, then go through a process that converts the information into a single stream, then back to the initial spectrum differentiation at the receiver, to parse the information as required.
In OFDM, you create a signal that consists of multiple subcarriers across the spectrum, each subcarrier carrying some of the data, in long duration (mutually orthogonal) symbols. That spectrum-divided signal is sent through the IFFT, to create the symbol stream that gets transmitted over the medium. At the receiver, an FFT is used to recreate the multiple subchannels, with long duration symbols. (The FFT recreates a frequency domain signal, with its multiple subcarriers, from the time domain signal received over the air.)
In stereo FM, much the same is happening. We first think of the analog AM signal consisting of L+R audio channels within the lower 15 kHz of spectrum, the L-R audio in the spectrum above 19 kHz to 38 kHz, then possibly more audio above that 38 kHz internal carrier. This nicely organized AM signal is sent through an FM modulator, and from there, it is transmitted over the medium, as a frequency modulated signal. It's no longer AM. At the receiver, the process is reversed, where the receiver demodulates the FM signal, to restore the AM signal organized in different spectrum bands. Now the receiver can reconstruct either monoaural or stereo audio, using either only the L+R information below the 19 kHz pilot, or adding and subtracting L+R to/from the L-R signal, to form 2L and 2R, or extract that other audio content up beyond the 38 kHz carrier.
So in short, the nicely explained spectrum we see in the literature, to explain OFDM or stereo FM, is not how the signal actually gets transmitted over the air. It's only the first step in the process. The IFFT, or the FM modulator, follows this first step. What is transmitted of the air does not look anything like those well laid out, frequency divided baseband signals.