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.
It is actually true.
It is Adaptive Modulation in OFDM.
Naveed Alam said:hi,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, .........?
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
rajeshkumar123 said:Hi gayathri,
SC-FDMA is a type of OFDMA which reduces PAPR and this technique is used in LTE. Please refer the webinar given below
https://www.freescale.com/webapp/sp...on.jsp?code=WBNR_FTF2008_BASEBAND_LTE&fsrch=1
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.
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
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