Hey dear friends , what’s up? Hop you are well by the grace of Almighty Allah . Alhamdulilah , I am also good .
Dear friends , Raise your hands who are using 4G network . How do you feel about its download speed ? I know , Maybe all of you , 4G is based on LTE technology . One of my practice post , I had discussed about LTE . But LTE has used two separate modulation techniques for downloading and uploading . Here I will discuss about the download technique which is OFDM .
Well , OFDM is not nothing but a general from Frequency division Multiplexing(FDM) . So OFDM refers Orthogonal Frequency Division Multiplexing. This is a modulation technique. Now you may ask me what is modulation .. right ? . Modulation is a process where the carrier signal is changed with the change of original signal. Simply you can think that it is way to send a normal thing with the help of a strong things. Like if you only throw a paper, it will not go longer distance but if you use a stone and throw the paper by folding with it.
OFDM is a multicarrier system which uses discrete fourier Transform /fast fourier transform with sin(x)/x for subcarriers . In this process , available bandwidth are divided into many narrow band and data stream in parallel on each of the separate band .
Then Why OFDM is chosen for 4G network … Is it not a question friends ? Yes . There is an effective reason for selecting this . In wireless system , most of the signal are propagated through more than one path which can be called as multipath propagation . Due to the multipath propagation , the received signal can not be accurate signals like original signal due to the attenuation or dispersions. But most of the broadband systems are subject to multipath transmission and there are used a conventional solution to multipath problem is an equalizer in the receiver part . But when the date rates increase , the equalizers become too complicated . But OFDM has several ways to minimize this problem .
Then the question is How does OFDM solve the multipath problem ? . Well , in OFDM data are transmitted in parallel . For this reason , we extend the available bandwidth and divided into narrow band . so each bands has longer symbol period . So the sum of parallel signal may be less affected by multipath propagation .
Another way of solving multipath problem is adding cyclic prefix . Cyclic prefix is a way to copy the last original signal and add that part into the first of the signal . If there is a problem after receiving signal , we can adjust the signal by cutting the first part . So there is no loss of signal .
How OFDM transmits signal in parallel without interference ? ok , for this reason , Each subcarrier has a different frequency . But the frequency chosen so that an integral number of cycles in a symbol period . Signals are mathematically orthogonal in this process i.e when the integral product of two signal is equal to zero is called orthogonal .
Does not have any disadvantages or problems of OFDM? . Yes , there is a some problem of using OFDM Such as high peak to average power ratio and very sensitivity to frequency errors . But A research also is ongoing to solve this problems.
OFDM is an interesting topic for a communication based student for research . So I am trying to clear my concept by writing this post . AS I think , I like to share my knowledge to help myself . Maybe some of you will not understand it properly due to my immature writing . But you can take it as your reading practice .
Thanks a lot for supporting me .
Happy learning.