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Questions about the disadvantages of OFDM

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BigDogBear

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Hello, all, I just studied the OFDM, and now some confused concepts occured in my brain, so your help is warmly welcomed, thanks a lot:
1. Is IFFT itself can be treated as a modulation type in OFDM scheme?
2. For each sub-carrier, can it be "modulated" by different bit pattern? I mean, for example, for sub-carrier 1, we use BPSK; for sub-carrier 2, we use QPSK, and for sub-carrier 3, use 16QAM, .........?

Best Regards!
 

adaptive loading algorithms in ofdma and sc-fdma

1. Is IFFT itself can be treated as a modulation type in OFDM scheme?

Taking IFFT is modualting using QAM, since it is generating the sine wave with the phase and amplitude you specify at the input of the IFFT.


2. For each sub-carrier, can it be "modulated" by different bit pattern? I mean, for example, for sub-carrier 1, we use BPSK; for sub-carrier 2, we use QPSK, and for sub-carrier 3, use 16QAM, .........?

May be it can be theoretically done, but it wouldnt be practical, since OFDM invovles may be hunderds of carriers and modulating each differently would be very difficult. In addition, if you use any modulation scheme other than QAM for any carrier, FFT/IFFT wouldnt work.

-b
 

ofdm based questions

Hi
About your 2nd question:
It is actually true.
It is Adaptive Modulation in OFDM.
 

fft mathematical representation in ofdm

hai
i am doing project in ofdm

is it true that we can generrate different bit pattern (modulation tech) for each sub carrier.

this is my question

how to generate subcarriers from base band signal if any codes available in matlab help me.....

help me please
 

ofdm matlab for papr

Hi venkateshkumar
I think your answer is IFFT.
Do u mean any thing else?
 

some ofdm questions

2. For each sub-carrier, can it be "modulated" by different bit pattern? I mean, for example, for sub-carrier 1, we use BPSK; for sub-carrier 2, we use QPSK, and for sub-carrier 3, use 16QAM, .........?
Click to expand...
May be it can be theoretically done, but it wouldnt be practical, since OFDM invovles may be hunderds of carriers and modulating each differently would be very difficult.
Click to expand...
It is actually true.
It is Adaptive Modulation in OFDM.
Click to expand...

Practically, the total number of subcarriers are divided into groups of subcarriers and adpative modulation is usually done per group i.e each group of subcarriers can use different modulation scheme

 

adaptive bit loading+ofdm+matlab

Naveed Alam said:
BigDogBear said:
1. Is IFFT itself can be treated as a modulation type in OFDM scheme?
2. For each sub-carrier, can it be "modulated" by different bit pattern? I mean, for example, for sub-carrier 1, we use BPSK; for sub-carrier 2, we use QPSK, and for sub-carrier 3, use 16QAM, .........?
Click to expand...
hi,
1. yes (when using OFDMA)
2. i don't think so. because i have seen many system diagrams, and before ifft operation they take QAM/PSK. but the order has to be taken once. it is the ifft block which makes sub-carriers, and these subcarriers carry the QAM/PSK mapped data,.. so, there isn't any choice..

if some one have recent knowledge which say that it is possible, then tell me also..


Regards
Naveed
Click to expand...
 

probable questions in ofdm

1. Yes
2. Adaptive bit-loading for different subcarriers based on S/N for each subcarrier
 

ofdm questions

i need know how to reduce PAPR in OFDM . some basic ideas are there i need to have some algorithm and coding in MATLAB
 


ofdm qpsk lte

no naveed,
this is one of the disadvantages in OFDM .i like to some algorithm or MATLAB coding to reduce PAPR.
 

viva questions on ofdm

yes it is a disadvantage in OFDM.
 

zumbeel

technique used to reduce PAPR and MATLAB coding for the systems.
 

matlab codeto compute papr in ofdm

i need to start in the digital communications please advise me
 

techniques reduce papr in matlab

refer to the digital communications by barry,a.lee,david g.m or proakis
 

Re: OFDM Questions?

Short summarize:

LTE is using SC-FDMA, that means ther PAPR is 0dB. That favor 50% efficient PA's in the mobile versus the basestation should use more linear and less efficient PA's

You can assign to each subcarrier a modulation but which must be constant over one OFDM symbol. BSPK-QPSK-16QAM-64QAM-256QAM are typical modulations which are used in commercial systems only for all carriers together. For total channel efficiency you can code less noisy subcarriers with more bits and less bits or setting carriers to zero which being in a fade position or being more noisy.

Because this information is generated at the receiver but must be used at the transceiver there are much investigations to get to clever and robust protocols.
 

Re: OFDM Questions?

rfsystem said:
Short summarize:

LTE is using SC-FDMA, that means ther PAPR is 0dB. That favor 50% efficient PA's in the mobile versus the basestation should use more linear and less efficient PA's

You can assign to each subcarrier a modulation but which must be constant over one OFDM symbol. BSPK-QPSK-16QAM-64QAM-256QAM are typical modulations which are used in commercial systems only for all carriers together. For total channel efficiency you can code less noisy subcarriers with more bits and less bits or setting carriers to zero which being in a fade position or being more noisy.

Because this information is generated at the receiver but must be used at the transceiver there are much investigations to get to clever and robust protocols.
Click to expand...




Yes but SC-FDMA has its dis-advantages also.

I requires a serious equalization at the base station since it used at uplink. And if we look really hard the physical layer model of OFDMA and SC-FDMA you'll find that SC-FDMA would require more computational power as compared to OFDMA because at the transmitter and at the receiver we perform FFT/IFFT two times, unlike OFDMA where we do it only once, not to mention the intense equalization of a frequency-selective faded signal that reaches the base station.

So, we are going into all this just to avoid the PAPR problem. And really it is that serious. Just imagine 1024 sub-carriers, all adding constructively, how much power would be utilized!

So I think the person who is working on reducing PAPR problem should share his/her knowledge with us here, so that we also understand it



By the way, I've heard 3gpp has frozen its LTE standard. Is it true? Any comments?
 

Re: OFDM Questions?

Yes, i have heard it too. But it means that LTE system is matured now and now 3GPP will not make drastic changes in it. So major changes are frozen now.


Naveed
 

Re: OFDM Questions?

Naveed Alam said:
Yes, i have heard it too. But it means that LTE system is matured now and now 3GPP will not make drastic changes in it. So major changes are frozen now.


Naveed
Click to expand...

I think you have also asked this at the Zumbeel forum, lets see if some industry people can answer it
 

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