Dear sir / Madam, I am doing M.E in Communication system, i have gone through the concept and fundamentals of OFDM first time , but i could not understand the process of IFFT in OFDM modulation, why FFT is not taken at the first stage? if any kind personality explains the concepts behind the IIFT in OFDM, which will helpful for me to understand better. How can i get e-version of any OFDM text book in the net- kindly suggest any suitable website to download . With regards, Velan

# IFFT in OFDM

Started by ●December 28, 2008

Reply by ●December 28, 20082008-12-28

velan wrote:> Dear sir / Madam, I am doing M.E in Communication system, i have gone > through the concept and fundamentals of OFDM first time , but i could not > understand the process of IFFT in OFDM modulation, why FFT is not taken at > the first stage? if any kind personality explains the concepts behind the > IIFT in OFDM, which will helpful for me to understand better. >Because each symbol controls the amplitude/phase of a sub-carrier in the frequency-domain. However, the transmitted waveform is a time-domain waveform, therefore one needs an IFFT to convert from frequency to time-domain. -- Oli

Reply by ●December 28, 20082008-12-28

On Dec 29, 1:43�am, "velan" <velan.resea...@gmail.com> wrote:> �Dear sir / Madam, I am doing M.E in Communication �system, i have gone > through the concept and fundamentals of OFDM first time , but i could not > understand the process of IFFT in OFDM modulation, why FFT is not taken at > the first stage? if any kind personality explains the concepts behind the > IIFT in OFDM, which will helpful for me to understand better. > > How can i get e-version of any OFDM text book in the net- kindly suggest > any suitable website to download . > > With regards, > VelanThere's lots of ways to think of this, but here is one... An OFDM symbol comprises the sum of a set of carriers (over the finite symbol period) at different frequencies, each of which has a complex amplitude given by one of a series of modulated (e.g. QPSK) data. The frequencies of these carriers are carefully chosen so that they can be easily separated at the receiver without any interference between them (orthogonality). In turns out this is what happens if you perform an IFFT on the set of modulated data at the transmitter (to get the transmitted time domain signal), and perform an FFT on the received signal at the receiver (in order to separate the received signals in the frequency domain). So basically in the transmitter you are assigning modulated data to carrier frequencies in the frequency domain. You could literally create each carrier frequency one at a time in the time domain, multiply each by the corresponding complex amplitude (equal to the modulated data), and then sum them together, then transmit; but simply doing an IFFT on the set of modulated data does exactly the same thing, usually with a lot less complexity. Note data is just data - it is neither a time domain nor a frequency domain phenomenon. You can do what you want with it to communicate it in the most efficient / accurate way possible. The only requirement is that the physical signal you transmit must be in the time domain! If you're a student your university has a library - try it out! They probably also have a subscription to IEEE Xplore too, although a simple textbook would be the best place to start. -T

Reply by ●December 29, 20082008-12-29

On Dec 28, 9:43�pm, "velan" <velan.resea...@gmail.com> wrote:> �Dear sir / Madam, I am doing M.E in Communication �system, i have gone > through the concept and fundamentals of OFDM first time , but i could not > understand the process of IFFT in OFDM modulation, why FFT is not taken at > the first stage? if any kind personality explains the concepts behind the > IIFT in OFDM, which will helpful for me to understand better. > > How can i get e-version of any OFDM text book in the net- kindly suggest > any suitable website to download . > > With regards, > Velanhope u know the basics behind the ofdm well, here small intro, u know that the high speed data is trannsmitted through several low data rate subcarriers. the carrier frequency will be in GHz range. in a single carrier system we need single local oscillator in GHz range. but in ofdm we r using several subcarriers. it is too complex to design te circuits with lot of subcarriers. because all the subcarriers are closely placed. the oscilator stability also the issue here. so instead of generating all the subcarriers in the ghz range, we generate the carrriers in baseband level. then upconverting to ghz range. in baseband it is easy to generate the subcarriers orthogonoly using IFFT. by using single local oscillator u can generate several orthogonal subcarriers in rf(high freq) range. Usinfg IFFT we can generate sinusoidal carriers ortogonoly. for example, let try this in matlab, input_ifft=zeros(1,64); input_ifft(2)=1; output_ifft=ifft(input_ifft); plot(real(output_ifft)) ; here u can see one complete cycle.then plot(imag(output_ifft)); here index 2 is fundamental frequency let Ff. if u use input_ifft(3) means the frequency is 2*Ff.i.e u can see 2 cycles it is orthogonal to previous one. now u can upconvert, i.e multiply with high frequency carrrier to understand ofdm first u should know how FFT works. for FFT basics gothrough http://www.complextoreal.com/tutorial.htm in that read tutorials 4,5 and 6. for ofdm tutorial read 22. hope u will get the ans