The frequency of each subcarrier is selected to form an orthogonal signal set. These frequencies are also known at the receiver for signal recovery. Note that the output is updated at a periodic interval T that forms the symbol period. To maintain orthogonality, T must be the reciprocal of the subcarrier spacing. In the frequency domain, each transmitted subcarrier results in a sinc function spectrum with side lobes that produce overlapping spectra between subcarriers, see "OFDM Signal Frequency Spectra" figure below.
This results in subcarrier interference except at orthogonally spaced frequencies. At orthogonal frequencies, the individual peaks of subcarriers all line up with the nulls of the other subcarriers. The receiver multiplies i. The use of orthogonal subcarriers allows more subcarriers per bandwidth resulting in an increase in spectral efficiency. In FDM systems, any overlap in the spectrums of adjacent signals will result in interference.
In OFDM systems, the subcarriers will interfere with each other only if there is a loss of orthogonality. For example, frequency error will cause the subcarrier frequencies to shift so that the spectral nulls will no longer be aligned resulting in inter-subcarrier-interference.
These transforms are important from the OFDM perspective because they can be viewed as mapping digitally modulated input data data symbols onto orthogonal subcarriers. In principle, the IFFT takes frequency-domain input data complex numbers representing the modulated subcarriers and converts it to the time-domain output data analog OFDM symbol waveform. In a digitally implemented OFDM system, the input bits are grouped and mapped to source data symbols that are a complex number representing the modulation constellation point e.
These complex source symbols are treated by the transmitter as though they are in the frequency-domain and are the inputs to an IFFT block that transforms the data into the time-domain. Each of these N input symbols has a symbol period of T seconds. These orthogonal sinusoids each have a different frequency and the lowest frequency is DC. The input symbols are complex values representing the mapped constellation point and therefore specify both the amplitude and phase of the sinusoid for that subcarrier.
After some additional processing, the time-domain signal that results from the IFFT is transmitted across the radio channel. At the receiver, an FFT block is used to process the received signal and bring it into the frequency domain which is used to recover the original data bits. An To begin the OFDM signal creation process, the input data bit stream is encoded with convolutional coding and Interleaving.
Learn more. Asked 8 years, 9 months ago. Active 6 years ago. Viewed 10k times. Improve this question. Chethan Mantaiah Chethan Mantaiah 31 2 2 silver badges 5 5 bronze badges. If you have more specific questions about it, I'll be glad to help. Add a comment. Active Oldest Votes. But how you can have this? Improve this answer. For the sake of simplicity, I'm ignoring phase here.. Let's start with some basic questions. What is the output of the IDFT section?
What does this output represent? What is the frequency content of the output of the IDFT stage? How would you verify this frequency content? Keep in mind that the subcarriers are not explicitly represented in the time domain, but they are explicitly represented in the frequency domain. Dave C Dave C 4 4 silver badges 8 8 bronze badges. The Overflow Blog. Podcast Explaining the semiconductor shortage, and how it might end. Does ES6 make JavaScript frameworks obsolete?
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