# Ofdm

Status
Not open for further replies.

#### tech_vaibhav_eee

##### New Member
OFDM(orthogonal frequency division technique) is used to provide different channels to different users.I have some doubt regarding ho these channels are allocated.I would try to ask my my question with the help of an example:

Suppose, there are 3 users and total bandwidth is 9 mhz.\
In traditional FDM technique , each user is given equal bandwith of 3mhz each and each band having a carrier frequency (neglecting guard band and considering requirements of each user as equal).
I am not sure what happens in OFDM.This is what i think happens but correct me if i am wrong:
Instead of using three carrier frequencies as in FDM, Subcarrier frequencies are calculated in each band (orthogonal to each other) and data is sent at these subcarrier frequencies.
Now , the part where i am confused is that:
suppose , the total freq. spectrum is :F1 to F2 --> bandwidth = F2-F1
Bandwidth of first user: F1 to F1+(F2-F1)/3...is this true?? what i want to know is :are the subcarrier frequencies given to user 1 lies between this frequency range or is it distributed all over the spectrum, i.e F1 to F2.

#### RCinFLA

##### Well-Known Member
OFDM is used for wlan and is the modulation for NextGen 4G cellular.

It uses pulse shaping to provide some of the selectivity by placing adjacient carriers in the nulls of the sinx/x modulation spectrum.

There is physical channels and virtual channels involved. All involve digital modulation with bits spread across multiple physical channels in time slots. The spreading of virtual channels across physical channels, along with error correction, provides better performance in multipath environments.

It has a very high spectrum utilization compared to older FDM or TDMA systems.

On the downside, OFDM has a high peak to average power envelope that has tough RF power amp linearity requirements. For cellphones this means PA's are less efficient resulting in potentially worse battery life.

Digital signal processing is several orders of magnitude higher then WCDMA 3G systems. Specialized digital signal processing hardware assist circuits are used to do the high level DSP computations involved. (in range of equivalent to 5 giga-ops per second)

Hi