time domain orthogonality and frequency domain orthogonality are two important aspects, in ofdm if we go to frequency domain the fourier transform of pulse is sinc function, so there will be lot of side lobes, and if there is any problem in terms of synchronization , suppose if you take any sampling then you may be ending at sampling at side lobe, this leads to leakage energy, this leads to ICI, mostly occur in W dispersive channel, where you have both time dispersion and time variation takes place, , time variation leads to doopler shift, there is guard band between each and every symbol this is again loss of spectral efficiency, advantage is Equalization because of H matrix and it can be decomposed into eigen matrix, PAPR is very high in OFDM two alternatives are there to overcome this OFDM GFDM, OQAM GFDM has many advantages here the pulse is no more rectangle, pulse duration in ofdm is o toT, but in GFDM the pulse duration is not 0 to T. we use long duration pulse in GFDM we have to long pulse , the frequency domain is flat. COMBINATION of subcarriers and sub symbol make one symbol in GFDM. we have to consider a time vs frequency grid, to analyze the subcarriers we have to shift the pulse and rotate it M subsymbols each having pulse M subsymbols each modulated on K subcarriers summed together to get total symbol we can reduce M, bandwidth is same 1)M=1, K increases to become N.(no of subcariers goes to N, bandwidth is same) then the spacing between the subcarriers is present, becaues to accomodate this subcarriers into this bandwidth, this becomes OFDM 2)WHEN K=0, then M=N, it becomes SC-MODULATION, (NO ORTHOGOANLTY BETWWEN THE SUBCARRIERS) but all M subsymbols in a series all thes M have to modulated on a single pulse , this leads to single carrier FDE(SC-FDE) 3) by varying the values of M&T u can vary the bandwidth associated with these pulses, when M=1. subcarrier spacing is 1/T which is very small value, if you increase M the bandwidth is increase. some applications the subcarriers need more bandwidth, in GFDM, for a combination of M subsymbols and K subcarriers all combine together you are going to put one guard interval. IFFT or IDFT can be used in tx