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OFDM bandwidth.......

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sohiltri

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What will happen to OFDM bandwidth when the data rate is increased or decreased ???
Does it remains same or does it also changes and how???
 

A.Anand Srinivasan

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OFDM is a digital multi-carrier modulation scheme...
it uses a large number of closely-spaced orthogonal sub-carriers.

so it depends on the modulation scheme used.... mostly QAM is used in OFDM and so when the data rate increases i dont think the bandwidth is gonna ichange....
 

sohiltri

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Anand ,
U r partially right....but see when data rate increses the symbol forming rate of QAM should also increse.....so there should be some limit...beyond which increase in rate cannot be tolerated....
 

A.Anand Srinivasan

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in QAM both ASK and FSK are used.... when the data rate is increased then the number of bits per second only increased and how do you think the basic frequencies used in the QAM used is gonna change.... it is just that each bit will be scaled a little smaller so it just depends on the receiver and transmitter but i thought you were asking about the ideal case...

talkin in your specification i dont think it is possible to define something strictly like that because higher data rate compatible equipments are coming up everyday...
 

sohiltri

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I think u got my point.....so when it will be coded with less time period...the bandwidth should increase .....what do u think abt it.....

and QAM is MASK and MPSK....not FSK.....
 

A.Anand Srinivasan

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oh in a hurry i put FSK instead of PSK...

suppose the data rate increases but still we use 8QAM(for example) so now each symbol represents 3 bits.... if the data rate is made twice then effectively the time period for which a particular symbol is sent is halved but the frequency that the symbol consists doesnt vary...

hope u get what i'm saying...
 

sohiltri

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Anand,

I followed u what u said upto now....but see center freq will remains same as u say....it is rite....but then BW of each subcarriers will depend on symbol period...
do u reckon tht????
 

A.Anand Srinivasan

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the subcarriers are closely packed... so how much do you think that is going to affect the bandwidth...
also the same subcarriers are going to be used even if data rate increases and so no change in bandwidth.... so how do you expect a change in bandwidth when the subcarrier is not being changed...
 

amihomo

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sohiltri said:
What will happen to OFDM bandwidth when the data rate is increased or decreased ???
Does it remains same or does it also changes and how???

Hi

the data rate in OFDM is closely related to the
1) size of the Mapping Constellation used for each subcarrier, M
2) number of used subcarriers ,N
3) frequency spacing between subcarriers,Δf

and the resulting bandwith will be
BW=NΔf=N/Ts
=N/(N.(log2(M).Tb))=Rb/log2(M)

where Rb=1/Tb is the input bit rate of OFDM.

from the above formula if we increase the Rb , BW will increase. but we should notice that the OFDM hardware (i.e. the IFFT/FFT pair) are set up and designed for a certain Rb and they will not work correctly if the Rb is changed.

regards
 

    sohiltri

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niagara

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pls let get things straight. Are we talking about increasing User bandwith or system bandwidth? Keeping the system bandwidth fixed and increasing the user bandwidth increases the chance that some of the carriers being used will be in a null. On the other hand as the system bandwidth is increased(with user bandwidth fixed), it improves the chance of finding stronger carriers

Did I make any sense?
 

iTdl

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Increase in data rate can be accommodated to a certain level (depending on coding scheme and modulation. Higher the constellation number higher the data rate!). But after that B/W needs to be changed to have higher carriers accommodating in OFDM symbol.
 

niagara

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Yes I agree with iTdl. We shuld be aware that the data rate is at at a oint,a function of the modulation and coding formats. But from the IFFT part to the transmission, we consider bandwidth increament as more subcarriers will be needed. I am working on using IFWT, and hope this may take care of such reconfiguration problems
 

Naveed Alam

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Practically, every system is designed according to the requirements. I mean any system will strictly work on specified B.W and strict upper limit of data rate.


Naveed
 

bulx

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As "amihomo" has pointed out, the spread of each subcarrier depends onthe symbol rate, and so you cant arbitrarily increase data rate by increased symbol rate. This is because the sub-carriers will start interferfering with each other as "sohiltri" suspects.

You could increase the number of bits per subcarrier without increasing signal bandwidth, but soon you will not be able to distinguish between the ultra-close constellation points in presence of noise (signal power is limited in any practical system).

In a practical adaptive data rate system like DSL, increase in data rate is achieved by increasing bits/carrier when noise decreases due to some external reason. The symbol rate (and so the BW) is unchanged.
-b
 

    sohiltri

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